Power Topics for Power Supply Users

What Size Heatsink Do I Need?

2013-05-10T11:00:00.002-07:00

Today there are a large array of high power modules in the range of 300 to 1,000-watts, both DC-DC converters as well as AC-DC power modules, which are commonly referred to as “bricks.” Even though these devices feature high conversion efficiencies in the area of 85 to 90% (or higher), some power is lost in the form of heat that must be dealt with in order to maximize the lifespan of the end product. For example, a 500-watt power module with 90% conversion efficiency would generate over 55-watts of wasted heat within the module that must be removed to maximize its reliability.

The concentrated power density (watts per cubic inch) within these power modules make them a challenge to cool in real world applications. Most high power bricks are packaged in thermally conductive plastic or epoxy cases with integral metal baseplates. The high power components within the bricks (i.e., semiconductors, inductors, transformers, etc.) are thermally coupled to these baseplates, which in turn can be attached to external heatsinks or liquid-cooled cold plates in order to keep the baseplate at or below its maximum operating temperature (typically 85 to 100°C).

Half Brick – DC-DC Converter

Full Brick – AC-DC Power Module

The maximum baseplate temperature is primarily determined by the maximum internal junction temperature of the semiconductors within the power bricks. The term “thermal management” refers to the designer’s challenge of cooling these power bricks by considering the many levels for heat transfers via conduction (direct contact between solids), convection (contact with air or a fluid) and thermal radiation (electromagnetic infrared energy), both internal and external to the power module.

The diagram above shows the series-connected thermal resistances that impede the flow of heat from one level to the next. These impedances need to be considered, beginning with the internal semiconductor’s junction temperature relative to its case, the thermoplastic module case and its metal baseplate, and ending with a mechanically attached heatsink that conducts away the heat from the baseplate to the surrounding ambient air via natural or forced air convection cooling. Heatsinks are designed to cross thermal barriers primarily by substantially increasing the surface area that comes in contact with the ambient air, thereby providing enhanced convection cooling. Because the mating surfaces of the power module’s baseplates and heatsinks are not perfectly flat, some type of thermally conductive interface material is required to fill the tiny voids. This interface material can range from a thin layer of thermal grease to a custom designed silicon pad.

AC-DC Power Module with Heatsink & Other Components

Selecting the proper size and shape of a heatsink and determining if forced air cooling is required are among the tradeoffs the designer needs to consider. This process begins with a detailed review of the power module’s specifications and knowledge of the end product’s heat loads, internal and external operating temperatures, space constraints, and available air flow sources, paths and restrictions.

The next step in this process is to determine the amount of power that will be lost (wasted) within the power module, based on its efficiency. This information for computing this is usually listed on the power module’s datasheet or installation manual, but it can also be determined by actual measurements of the input and output powers. For this example, we will use a typical AC-DC power module with a 48V/10.5A, 504W output rating, and a typical efficiency of 85% with a 120VAC input. By the way, the 85% efficiency rating is very good considering the fact that this module contains full-bridge rectification and active power factor correction AC front-end circuits as well as an integral DC to DC converter. In addition, this module has a maximum operating baseplate temperature, as measured at its center point, of 100°C.

Based on the above information, to compute the internal power dissipated (wasted heat); we can use the following formula:

Pd = (Pout / η) – Pout
Definitions & Calculation Example:
Pd : Internal Power Dissipated (W)

Pout : Output Power (504W)

η : Efficiency (85%)

Pd = (504W / 0.85) - 504W = 88.9W
To calculate the required baseplate to ambient air thermal resistance that would be needed for this application, the following formula would apply:
θba = Tb - Ta / Pd
Definitions & Calculation Example:
θba : Baseplate to Ambient Air Thermal Resistance (°C/W)

Tb : Baseplate Temperature (100°C)

Ta : Ambient Air Temperature (40°C)

Pd : Internal Power Dissipated (88.9W)
θba = 100°C - 40°C / 88.9W = 0.67°C/W

In this example, we would need a heatsink (with or without air flow) that provided a thermal resistance of 0.67°C/W. However, unless the heatsink includes a thermal interface material like thermal grease or a pad in its rating, we need to account for this additional thermal contact resistance (θbs), which can be on the order of 0.1°C/W. Therefore, the required thermal resistance of the heatsink itself, with the thermal interface material included, can be calculated per this formula and example:

θba-bs = θba – θbs

θba – θbs = 0.67°C/W - 0.1°C/W = 0.57°C/W

The next step in this process is to review specifications for potential heatsinks that have a thermal resistance of 0.57°C/W. In this case, the power module has three optional heatsinks to choose from as shown in the chart below.

The Y axis of this chart shows the thermal resistance between the heatsink and the air (°C/W) and the X axis shows the required airflow velocity for the three heatsinks. In this example, we need to find 0.57°C/W along the Y axis and then move to right along the X axis to where it intersects a heatsink curve. In this example, 0.57°C/W intersects with the HAF-15T heatsink curve at about the 1 m/s airflow velocity point. Therefore, for this application we would select the model HAF-15T heatsink and would have to provide forced air cooling with an air velocity of 1 m/s. To translate m/s (meters/second) into LFM (linear feet/second), use this general conversion factor: 1 m/s = 200 LFM. In this example, since 1 m/s = 200 LFM of forced-air velocity, the fan required for this application must provide 200 LFM.
Based on the above, we have now determined the requirements for cooling this power module with a heatsink, thermal compound and forced air flow. If we wanted to be more conservative and improve the MTBF of the module, we would recalculate the required heatsink with the assumption that we wanted to keep the baseplate temperature at 85°C.

A word or caution should to be injected here. After going through the thermal calculations and selecting a heatsink, air flow, etc., the next step is to confirm the “paper-design” by running actual tests on a sample unit. The tricky part is to get access to the center point of the power module’s baseplate so you can measure the temperature at that point while the module is operating under load. One way to do this is to drill a hole in the center of the heatsink so the leads from a thermocouple can be mounted on the module’s baseplate and routed to your temperature measurement device.
In summary, we have shown how to determine the correct heatsink for power module applications. As the efficiencies of these devices improve the need for cooling will reduce, but the designer should always be aware of the heating effects from not only from the power module, but also from nearby devices. Therefore, it’s always best to run actual thermal tests with thermocouples attached to the power module and inside the end product to insure the design will be as reliable as possible.

Power Supply "Remote Sense" Mistakes and Remedies

2013-04-02T11:18:00.001-07:00

Most medium to high power AC-DC power supplies and some DC-DC converters include "Remote Sense" connection points (+ and - Sense) that are used to tightly regulate the supply's output voltage at the load. Since the output cables that connect a power supply's output to its load have some resistance, as current flow increases, so will the voltage drop across the cables (I x R = Voltage Drop). Moreover, since it's best to regulate the voltage directly at the load, the use of the two Remote Sense wires connected from the supply to the load will compensate for these unwanted voltage drops. Refer to Fig. 1 which shows the typical connections when the Remote Sense function is used.

Fig. 1: Power Supply with Twisted "Remote Sense" Wires Connected to the Load

Typical "Remote Sense" Problems & Remedies

Most remote sensing circuits are capable of compensating for from 0.25V to 0.75V of voltage-drops across the output cables. However, to be sure, always check your power supply's instruction manual to determine its maximum remote sense compensating range. If the voltage drop across the output cables exceeds the compensating range of the remote sense circuits, the voltage at the load will no longer be regulated. This problem can be remedied by either reducing the length of the output cables or increasing the size (heavier wire gauge) of the output cable's to reduce the excessive voltage drop. Voltage drops across the output cables should be minimized since this is a source of wasted power. For example, with just a 0.5V cable drop with a 100A load, the lost power amounts to 50W in each cable or 100W total.
The remote sense function automatically increases the output voltage at the output terminals of the supply to compensate for any unwanted voltage drop in the output cables with heavy load currents. Likewise, the remote sense function decreases the output voltage of the supply when the required load current is reduced. In some applications, the power supply's output needs to be adjusted by the user to voltage higher than its nominal (e.g. 5V nominal, adjusted to 5.5V). Always adjust the power supply's output while measuring the voltage at the load. In addition, care should be taken to assure that under full load that the remote sense function does not push the Vout to a higher voltage that could possibly trip the OVP set-point and shutdown the supply. Therefore, always read the power supply's instruction manual to be aware of the supply's adjustment range and OVP set-point.
The remote sense leads carry very little current so light gauge wires can be used. However, steps should be taken to ensure that the remote sense wires do not pick up radiated noise by either twisting the + and - Sense wires together and/or by shielding the wires from the noise (refer to Fig 1). It is best to use different colored sense wires (e.g., black and red) so that after they are twisted it is easy to determine which wire is the + and – Sense.
Refer to Fig. 2 below for a simplified schematic of a power supply's remote sense circuits. It is important to observe the correct polarities, i.e., the +Sense wire should connect at the load near the +Vload connection and the –Sense wire should connect at the load near to the – Vload connection. If by mistake the remote sense wires are crossed-connected (+Sense to –Vload and – Sense to +Vload) current will flow in the Sense lines and burn out the internal Rsense resistors, causing a malfunction of the supply. Typically, these internal Rsense resistors are around 10 to 100 Ohms with a maximum rating of 0.5W.

Fig. 2: Simplified Schematic of Remote Sense Circuit with External Output & Sense Wires

We have seen applications where the user has installed a switch or fuse in series with one or both output wires. This can cause a serious problem if the remote sense lines remain connected to the load, because if the output cable switch or fuse opens, current will flow in the sense lines and cause the internal Rsense resistors to burn up. System debugging can cause similar problems, for example, where the power and sense cables are located on separate connectors and if by error, only the power cable connector is disconnected.
There are applications where the user may not want to use the remote sense feature. In these cases, the remote sense lines should not be left open for optimum load regulation; instead, a local sense configuration must be used. Referring to Fig. 3, to use a local sense set up the + and -Sense lines should be connected to either their corresponding local sense (LS) terminals, which are provided on many power supplies, or connected to the corresponding +Vout and –Vout terminals. Most power supplies are shipped from the factory with these "Local Sense" jumpers installed on the power supply (see photos below).

Fig. 3: Schematic of Power Supply with "Local Sense" Jumpers Installed

Photo of Power Supply with Local Sense Wires Connected (see Red & Black jumper wires)

Photo of PSU with Sense Screw Terminals Connected to Output Screw Terminals with Metal Jumpers

In summary, the "Remote Sense" feature automatically compensates for unwanted output cable drops, which vary as the output current increases and decreases. This feature is advantageous to the user, but is subject to mistakes that should be avoided to insure the proper operation of the power supply and the end-product.

What does SELV mean for Power Supplies?

2013-03-05T14:35:00.001-08:00

SELV stands for Safety Extra Low Voltage. Some AC-DC power supply installation manuals contain warnings concerning SELV. For example, there may be a warning about connecting two outputs in series because the resulting higher voltage may exceed the defined SELV safe level, which is less than or equal to 60VDC. In addition, there may be warnings about protecting the output terminals and other accessible conductors in the power supply with covers to prevent them from being touched by operating personnel or accidently shorted by a dropped tool, etc.

UL 60950-1 states that a SELV circuit is a “secondary circuit which is so designed and protected that under normal and single fault conditions, its voltages do not exceed a safe value.” A “secondary circuit” has no direct connection to the primary power (AC mains) and derives its power via a transformer, converter or equivalent isolation device.

Most switchmode low voltage AC-DC power supplies with outputs up to 48VDC meet the SELV requirements. With a 48V output the OVP setting can be up to 120% of nominal, which would allow the output to reach 57.6V before the power supply shuts down; this would still conform to the maximum 60VDC for SELV power.

In addition, an SELV output is achieved through electrical isolation with double or reinforced insulation between the primary and secondary side of the transformers. Moreover, to meet SELV specifications, the voltage between any two accessible parts/conductors or between a single accessible part/conductor and earth must not exceed a safe value, which is defined as 42.4 VAC peak or 60VDC for no longer than 200 ms during normal operation. Under a single fault condition, these limits are allowed to go higher to 71VAC peak or 120VDC for no longer than 20 ms.

Don’t be surprised if you find other electrical specs that define SELV differently. The above definitions/descriptions refer to SELV as defined by UL 60950-1 and other associated specs regarding low voltage power supplies.

Power Supply Rise and Fall Output Characteristics

2013-02-07T09:30:00.000-08:00

A common question asked by our customers is “what are the power supply’s rise and fall output characteristics?” Our usual answer “it depends” sometimes raises an eyebrow. Let’s have a look at why.
As you know, there are two ways a power supply can be turned-on and off. One method is to apply or remove the AC input power via a switch or circuit breaker to the supply. Another method is use the Remote On/Off control of the supply, if the unit has this feature. Let’s examine both methods.

The curves below (Fig. 1) show the typical delay between when the AC input power (Vin) is applied and the power supply’s output reaches its rated voltage of 12V under full load conditions. As can be seen (in this example) with a low AC input of 85VAC the output turn-on delay is slightly more than with higher input voltages. However, in this example, the typical turn-on delay is about 250ms and the output rise time is about 25ms.

Conditions:

Vin	: 85VAC (A)
	: 115VAC (B)
	: 230VAC (C)
	: 264VAC (D)
lout	: 100%
Ta	: 25°C

Fig. 1

The next curves (Fig. 2) show the same power supply’s output (with 100% load) when the AC input is removed. The time delay from when the AC power is removed (or lost) is referred to as the Hold-Up time spec. For this supply, the specified minimum hold-up time is 16ms. As shown below, the measured hold-up time is about 30ms, which meets the spec. The output fall time is approximately 10ms, when measured from 90% to 10%. Note that the energy is pulled from the power supply very quickly due to the heavy load on its output.

Fig. 2

By comparison, without a load (zero load), the curves below (Fig. 3) show that the Hold-Up time of the power supply increases substantially to about 1.8 seconds, and the output drops to zero in about 6 seconds!

Fig. 3

Below is a graph (Fig. 4) for this power supply that shows how minimum Hold-Up time varies as the load changes between 10% and 100% (maximum load).

Fig. 4

Next, we shall review how the remote on/off control (if used) can affect the power supply’s output rise/fall, and delay times.

Conditions:

Vin	: 115VAC (A)
lout	: 100%
Ta	: 25°C

Fig. 5

In the above curves (Fig. 5), the remote On/Off control is the bottom trace and above that is the power supply’s DC OK signal level. This situation assumes the AC input in always on. As can be seen, from the time the Remote On is activated (goes from high to low logic level); it takes the output about 150ms to reach its full rated voltage. And, the DC OK signal changes state when the output level is about 75% of its rated voltage, which in this case is about 125ms after the Remote On signal is activated.

The next set of curves (Fig. 6) show the reverse situation, where the Remote On/Off signal is used to turn the power supply off.

Fig. 6

There is about 25ms delay after the Remote On/Off changes state to when the Vout reduces to approximately 75% its nominal. In addition, at that time the DC OK signal changes state. The output fall time, from 90% to 10% of nominal, is approximately 15ms.

From the above curves and explanations, I believe the reader can see why our answer at the beginning was "it depends." In these types of measurements, delay times are in many cases more important than rise and fall times. Other considerations include the AC input voltage, the load, operating temperatures, measurement criteria and the design specifications for the power supply.

How to Avoid Being Ripped Off When Buying Power Supplies

2013-01-09T16:28:00.001-08:00

When shopping for power supplies, remember these wise words, “If it seems too good to be true, it probably isn’t (true).” Time and again, we hear or read about cases where counterfeit and/or fake power supplies are sold to those who are enticed by ultra low prices, but end up with big headaches instead. Here are some examples:

700W ATX Power Supply with ‘PFC Coil Made from Cement’: http://www.hardwaresecrets.com/blog/Hantol-PSU-Fake-PFC-Made-of-Cement/220

The yellow PFC coil is a fake that is actually made of cement

Fake iPhone Chargers; Raspberry Pi foundation looks at counterfeit Apple power supplies: http://hackaday.com/2012/10/10/raspberry-pi-foundation-looks-a-counterfeit-apple-power-supplies/
Exposing some fake electronics with too-good-to-be-true prices: http://hackaday.com/2012/01/04/exposing-some-fake-electronics-with-too-good-to-be-true-prices/
Low Cost Power Supplies Can Be Expensive: http://power-topics.blogspot.com/search/label/Power%20Supply%20Failure
Don't buy a cheap replacement power supply!: http://www.youtube.com/watch?v=7nTRIxloDcI
UL Warns of Power Supplies with Counterfeit UL Listing Mark and UL Energy Verified Mark: http://www.wric.com/story/20226384/ul-warns-of-power-supplies-with-counterfeit-ul-listing-mark-and-ul-energy-verified-mark
Fake! Counterfeit UL mark found on LED power supply: http://www.edn.com/electronics-blogs/led-zone/4395270/Fake--Counterfeit-UL-mark-found-on-LED-power-supply

Because there are bad guys out there who are dishonest, we come back to the question of ‘how to avoid being ripped off when buying power supplies.’ The answer is simple. Buy from reputable and well established companies who have been providing power supplies for at least ten (10) years. For example, TDK-Lambda has been designing and manufacturing reliable power supplies for well over 50 years and offers limited lifetime warranties on some models http://www.us.tdk-lambda.com/lp/ . Avoid buying ‘seemingly’ bargain priced power supplies from eBay or other online discount websites. Fake chargers can kill your expensive cameras, smart phones and other devices.

If you work for an electronics manufacturer, remember that the switchmode power supply is the heart of your end-product or system and your company requires power supplies that will provide excellent performance and trouble free operation for many years. The last thing an OEM needs is a lawsuit or the need to recall thousands of products from the field.

If in doubt, ask the power supply vendor for the UL listed file number for the supply you are interested in using. For UL Listed and UL Recognized products, the file number is usually printed near the UL mark and typically begins with an E prefix followed by a 6 character number, e.g., E133400. If the file number is not shown, most reputable power supply vendors/manufacturers will provide the end-user with this information. Anyone can go to the UL website to check on a UL file number to make sure the supply is actually listed (approved) and to confirm the name of the manufacturer. The ‘UL Online Certification Directory’ is available at http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html. At this website you can enter the ‘UL File Number’ in the box provided (e.g., E133400) and then click the SEARCH button. If the power supply is in the UL database, information about the manufacturer will be shown. If you then click one of the items shown, the associated model number(s) that conform to the specific UL safety specification(s) will be listed.

By following the above recommendations, you’ll become a wise power supply consumer and thereby avoid costly mistakes in your next power supply selection and purchase.

How can I use my power supply’s alarm signals?

2012-12-05T09:16:00.004-08:00

Many power supply alarm signals, such as AC Fail, DC Good, etc., utilize optocouplers or optical isolators as a means of transferring alarm signals from the power supply to the end-users equipment without direct connections. The main purpose of an optocoupler is to prevent noise, ground loops, and/or high voltages from the power supply from damaging the end-equipment to which the signals connect. Below is a typical schematic diagram of an optocoupler that consists of an LED on the input side and a phototransistor on the output side. Signals from power supply activate the LED, which in turn activates the electrically isolated phototransistor.

Typical power supply alarm signals may include AC Fail, DC Good, Over-Temp, and Inverter OK.

Fig A: These opto-isolated, open-collector alarm signals share a common ground

Fig. B: This opto-isolated alarm signal has a separate output and ground

Pull-Up Resistors - When using open-collector alarm signals an external pull-up resistor is required. This pull-up resistor needs to be selected and connected between the alarm output (collector) and an external voltage source (+VCC). The purpose of the resistor is to limit the amount of current that flows through the open-collector transistor. For example, in some applications the current should not exceed 10mA, however, always check your power supply manual to confirm the maximum allowable current and maximum +VCC voltage.

External +VCC – The +VCC voltage that is connected to the pull-up resistor(s) for the alarm signal(s) should come from an external voltage source if maximum isolation is desired. However, in some cases power supplies come with an Auxiliary DC Output, which is always present as long as the AC input voltage is present. In some applications, this Auxiliary DC Output can be used as the +VCC for the alarm signals except for “AC Fail.”

Logic Ground – If an isolated logic ground is present, it usually needs to be tied to either the (-) Vout of the power supply or to the ground of the end-equipment system.

Alarm Signal Levels - In most cases, when an alarm condition is Not present the open-collector transistor output(s) will be On (or logic Low). If an alarm condition should occur, the open-collector output(s) will turn Off (or logic High). But, different supplies can have different alarm logic levels, so you should always check your power supply’s instruction manual to determine the supply’s alarm logic levels, with and without alarm conditions.

Combining Alarm Signals – The diagram below shows an example of how “DC-OK” signals from 3 different power supplies that are mounted in a power system rack can be combined (OR’d) to form a single signal In this example, the +VCC comes from the Aux DC Output of the power system is connected to 3 separate pull-up resistors. These pull-up resistors (10K ohm) connect to an open-collector “DC-OK” output from each of the 3 power supplies. And, all open-collector transistors have a common ground connection (similar to Fig A above).

Normally, if all supplies are OK, their DC-OK signals will be in the low state (approx. +0.6Vdc or lower). Should one of the power supply’s output’s fail, its “DC-OK” open-collector transistor will turn off and that output will go high via the pull-up resistor to the +12 to +15Vdc aux supply output. This positive “high” signal will forward bias the diode and cause the combined “DC-OK” alarm output to go high (relative to the Return or Ground line), which indicates that one of the 3 supplies have failed “DC Not OK”. The indicator light on the failed supply will show which supply has failed.

Alternatively, each of the “DC-OK” signals from the individual supplies in this power system could have been connected separately to a monitoring system (without combining them). The advantage of doing this is that the specific failed supply could be identified remotely without viewing the front panel mounted indicators.

Review Hot Swap/Rack Mount Front End power supplies from TDK-Lambda

New Requirements for CE Marking on Power Supplies

2012-11-06T08:31:00.000-08:00

I thought I would share some upcoming legislation changes regarding the CE mark for power supplies.
Currently the CE mark is applied to “embedded power supplies” (supplies installed in end-users equipment) that meet the Low Voltage Directive 2006/95/EC. Assurance of conformance is usually backed by compliance to EN60950-1.

Non-embedded units like external power adapters or DIN rail power supplies have to comply with the EMC Directive 2004/108/EC in addition to the Low Voltage Directive. Compliance is backed by testing to the immunity standards described in EN61000.

DC-DC converters that operate with a DC input of less than 75V were exempt from CE marking, a subject of my 2009 blog post.

On January 2nd, 2013, the CE legislation changes! In July 2011, the Recast RoHS Directive was published, and next year becomes enforceable. The six hazardous and restricted substances originally covered by RoHS remain the same, which is good for manufacturers.
Prior to the recast, this was applicable to 8 product categories:

Large household appliances
Small household appliances
IT & Communications equipment
Consumer equipment
Lighting equipment
Electrical & electronic tools
Toys, leisure and sports equipment
Automatic dispensers

Post recast, 5 more categories have been added (but not immediately):

Medical devices	July 2014
In-Vitro Diagnostic devices	July 2016
Monitoring & Control Instruments	July 2014
Industrial Monitoring & Control instruments	July 2017
All other electrical & electronic equipment	July 2019

To apply a CE mark to a product, one must have proof of compliance available; it is insufficient to say, “to the best of our knowledge our products are compliant”. I have read articles in the press that random testing for RoHS compliance has uncovered that a large percentage of consumer products failed to meet the standard. By including RoHS into the CE mark gives the EU the ability to severely penalize importers of non-compliant product.

Power supply manufacturers are now updating their Declaration of Conformance documents (D of Cs) to include RoHS.

One other impact is that the previously exempt low voltage input DC-DC converters (<75VDC) will now have to have the CE mark applied to indicate compliance (just for the RoHS). This will not be a problem as most manufacturers have already converted their manufacturing over for current products.

How Does Altitude Affect AC-DC Power Supplies?

2012-10-09T14:41:00.000-07:00

Most AC-DC power supplies that meet the safety standards per UL/EN 60950-1 for ITE (Information Technology Equipment) applications are designed to operate at typical office and factory altitudes, which can vary from slightly above sea level to as high as 2,000 meters (6,562 feet). And, many power supply manufacturers provide units that are designed and rated for operation at higher altitudes, up to 3,000 meters (9,843 feet) so their supplies can be used in major cities located at higher elevations (e.g., Denver, Santa Fe, Mexico City, Bogota). Many broadcasting/communications stations/towers are located at altitudes up to 3,000 meters or higher in order to maximize their range.

Altitude affects the design of power supplies since ‘air’ is used as an electric insulating medium (aka, dielectric) in the construction of power supplies, as well as most electronic devices. The density and dielectric strength (insulating property) of air is very good at sea level, but at higher altitudes, the thinner air loses some of its dielectric strength, which needs to be compensated for. Switchmode power supplies operate off of high voltages (inputs of 90 to 265Vac) and internally generate even higher voltages (400Vdc or more), which need to be insulated and contained to prevent high voltage arcing or breakdown within the supply, and to protect the end-equipment and operating personnel.

The drawing below shows a cross section of a typical printed circuit board (PCB), which is comprised of copper electric conduction paths that our chemically etched on an insulated (dielectric) fiber board material (e.g., FR4, woven fiberglass cloth with epoxy resin), plus electronic components that are not shown in this drawing. As can be seen, the fiber board and air, combined with the distances between the etched conductive traces are the primary insulation mediums for the circuit board.

Drawing Credits: Mammano B, ‘Safety Considerations in Power Supply Design, Underwriters Laboratory / TI

The term ‘Clearance’ refers to shortage path between the two conductive parts (circuit traces, components, etc.), measured through air.
The term ‘Creepage’ refers to the shortage path between two conductive parts measured along the surface of the insulation (PCB, insulating materials/barriers, etc.).

What does this have to do with altitude? Since ‘air’ gets thinner (reduced barometric pressure) at higher altitudes and becomes less of an insulator, the PCB and component layouts have to be designed with sufficient safety spacing distances to prevent high voltage arcs or breakdowns between conductors and/or electronic components.

For example, typical power supply design practice may allow 8 mm spacing distance between primary and secondary circuits and 4 mm spacing distance between primary and ground. These spacing distances will vary depending upon the voltage levels between conductors and components and the expected humidity, temperatures, pollution levels, and attitudes.

For those power products that must be approved per the Chinese CCC organization (required to export supplies into China), the new Chinese Safety Standard GB 4943.1-2011, which is similar to UL/EN 60950, requires strict specs for creepage and clearance distances. As of December 1, 2012, the primary-to-secondary clearances must increase by a factor of 1.48 to qualify the supply for operation up to 5,000 meters, since many regions in China are located at high altitudes. The alternative for CCC certified power supplies is that they must clearly marked with a warning label that states that the power supply must be used below 2,000 meters (see table below).

The base design altitude for ITE power supplies is 2,000 meters. However, as mentioned before, as the altitude increases, the air becomes a poorer insulator and the spacing distances have to be increased per the following table (assuming an 8 mm clearance at 2000m).

Altitude (meters)	Barometric Pressure (kPa)	Multiplication Factor for Clearance	Resulting Clearance (mm)
2000	80.0	1.00	8.00
3000	70.0	1,14	9.12
4000	62.0	1.29	10.32
5000	54.0	1.48	11.84

As can be seen from this table, if a power supply is to be operated at 5,000 meters, its conductor/components clearances must be increased by 48% compared to a supply designed for 2,000 meters.

The other major effect of high altitudes on power supplies is that the less dense air does not conduct heat as well. To compensate for higher altitudes, power supplies need to be derated, or employ larger heat sinks, or have increased forced air flow, or a combination of these to insure proper cooling. In addition, the power supply must be designed with the proper conductor and component clearances as discussed above.

In summary, whenever an application requires that a power supply must operate at altitudes above 2,000 meters (6,562 feet), always check with the manufacturer to determine if this is acceptable, or if an alternate model that is designed for higher altitudes is required.

Efficiency Calculations for Power Converters

2012-09-05T10:32:00.000-07:00

A power converter’s efficiency (AC-DC or DC-DC) is determined by comparing its input power to its output power. More precisely, the efficiency of the converter is calculated by dividing the output power (Pout) by its input power (Pin). The Greek symbol Eta “η” is usually used to represent “Efficiency.” Here is the formula for determining a power converter’s Efficiency (η).

η = Pout / Pin

For example, the efficiency of a converter that provides 500W of output power (Pout) and requires 625W for the input power (Pin), would be 80% (500W/625W=0.80). In this case, the input power exceeds the output power by 125W or 20%, which is lost/wasted power. Therefore, 20% of the input power is converted to heat energy that must be removed from the converter by some means of cooling (conduction, convection, and/or radiation).

Since all power converters have inherent conversion losses, the output power is always less than the input power. Most often, the manufacturer of the power converter specifies its efficiency and maximum output power on the product’s datasheet. When the efficiency (η) and output power (Pout) is known, the end-user can determine how much input power (Pin) will be required and how much power will be wasted (Pwaste) and converted to heat energy under full load conditions.

Here are the formulas to determine Pwaste and Pin with sample calculations using the examples listed above.

Pwaste = (Pout/η) – Pout
Pwaste = (500W/0.80) – 500W = 625W - 500W = 125W

Pin = Pout + Pwaste
Pin = 500W + 125W = 625W

Obviously, with a higher efficiency converter, Pwaste is reduced. Using the example above, but with an improved efficiency of 90% (instead of 80%), here are the revised calculations:

Pwaste = (Pout/η) – Pout
Pwaste = (500W/0.90) – 500W = 555.5W - 500W = 55.5W

Per the examples above, by employing a more efficient power converter it reduces Pwaste from 125W to 55.5W, which provides a substantial savings to the user in both electric energy and cooling costs.

Here are alternate formulas for calculating the factors associated with power converter efficiencies:

Pin = Pout/η
Pwaste = Pin – Pout
Pwaste = Pout (1/η - 1)

In some formulas, Pwaste is referred to as Pd, where “Pd” means the power dissipated (in the form of heat) within the power converter. Pwaste = Pd.

When dealing with AC-DC power supplies, not only is “Efficiency” important, but so is the power supply’s “Power Factor.” Information about the effect and importance of the power factor in power supplies is covered in the following article.

Power Factor Correction

http://power-topics.blogspot.com/search/label/Power%20Factor%20Correction

More information about power converter efficiencies and cooling methods/techniques can be found at these web links:

Power Converter Efficiencies

http://power-topics.blogspot.com/2011/06/power-supply-losses-and-impact-of.html
http://us.tdk-lambda.com/lp/ftp/other/cost-savings-high-efficiency.pdf
http://us.tdk-lambda.com/lp/news/2012_release05.htm

Cooling Methods

http://power-topics.blogspot.com/2009/01/what-size-fan-do-i-need.html
http://us.tdk-lambda.com/lp/ftp/Other/cooling_bricks_ecn.pdf

Encoders vs. Potentiometers: Which one is best for programmable power supplies?

2012-08-09T08:46:00.000-07:00

Recently I asked the product manager of TDK-Lambda’s new Z+ Series of programmable power supplies a question, “Why use rotary encoders to adjust the output voltage and current limit in preference to potentiometers?”

“The answer is relatively simple,” he said; it’s all about digital circuitry compatibility and resolution”. The key advantage to using a rotary encoder over a potentiometer is that that the digital signal produced by the encoder eliminates the need for the analog to digital (A/D) conversion that is required when potentiometers are used. In addition, multi-turn encoders can provide more accurate and higher resolution set-points than potentiometers.

Let’s put this into context: if a single turn potentiometer were to be used to adjust the output voltage on a 20V rated model, one full turn would typically represent 20V; this resolution is relatively low and therefore not precise enough for a programmable power supply. Of course, multi-turn potentiometers could be employed but these are bulkier than and not as reliable as encoders are.

Digital rotary encoders are available with very high resolutions and can operate in dual modes, including a coarse and fine mode. In the coarse mode, the encoder operates with a lower resolution; for example, a 20V rated model would require about six turns of the encoder knob to adjust the output from zero to 20 volts. In the fine mode, one turn equates to an approximately 40 millivolts, thereby providing a much higher resolution and more precise set-point.

To avoid an inadvertent change to the voltage setting during use, a front panel locking feature is needed. With an encoder, this locking function is done via software and internal memory, whereas a potentiometer requires a less reliable mechanical locking mechanism.

The Z+ programmable power supply employs two encoders as set-point controls for the output voltage and current. The encoder’s digital output feeds the unit’s memory, which retains the last set-points prior to the AC power being turned off and on, either intentionally or due to an unplanned power outage, and keeps the output disabled until the user enables it (aka “safe-start” mode). In the “auto-restart” mode, the Z+ power supply remembers its last output set-points and when the power is turned back on, it resumes its normal operation, which is handy for unattended applications.

To see more details including videos about the encoder driven Z+ programmable power supplies, please use this link: http://www.us.tdk-lambda.com/lp/products/zplus-series.htm

Low Cost Power Supplies Can Be Expensive

2012-07-03T13:23:00.000-07:00

Recently, one of our Field Application Engineers came into my office to show me an AC-DC adapter/power supply he had purchased on-line.

“I paid $6.00 for this power supply. It has selectable outputs and multiple connectors, he proudly announced. How do they build it for such a low price?” he asked.

I was a little suspicious as it did not seem to have a UL logo, and judging by the weight of the adapter, it was a linear (not switch-mode), which may violate the off load power draw requirements in the US.

“Don’t leave it plugged-in when you leave the house” was my advice.

The next day he came back into my office and said:

“I set the output voltage to 4.5V and it powered up my digital camera just fine. After I finished down loading my photographs, the camera went into standby mode and stopped working (permanently).”

When tested, the adapter would regulate under load, but at light loads, the output rose to from 4.5V to 12V. Of course, when the camera went into sleep mode it drew very little power and the resulting unforeseen 12V output fried the camera’s circuitry.

Now, looking at the attached photograph, I see that the manufacturer misspelled “Adapter”, which should have been a clue regarding the low quality of this unit.

“Sorry, you got what you paid for my friend” I commented.

What is a Limited Power Source?

2012-06-13T11:24:00.000-07:00

There are many technical details regarding Limited Power Sources (often referred to as an LPS) covered in the IEC60950-1 safety standard, involving a variety of applications, but I will cover just the basic aspects regarding AC-DC power supplies in this blog article.

Several safety standards refer to IEC60950-1 for a wide range of applications, and one of those referrals involves the use of Limited Power Sources.

What is, and why are Limited Power Sources important? Simply put, if a piece of electrical or electronic equipment supplying DC power to external devices is to be installed by a third party, such as an electrician, the risk of wiring fires & electrical shock needs to be minimized. That electrician will not be expected to know all the potential fault scenarios and use the appropriate cable thicknesses and insulation to cover those hazards. By using a Limited Power Source, the system wiring can also be reduced, saving cost.

If a Limited Power Source is used, then the electrician’s job is simplified, even if there is a (single) fault inside of the power supply.

Some examples of testing that the power supply manufacturer will do, to determine the maximum output current and power meet the “Limited Power Source” requirements are:

Check the power supply at maximum rated load
Increase the load of the power supply until it is on the verge of overload
Simulate an internal fault by shorting the current-limiting resistor
Short out the opto-coupler that provides feedback to the control loop

The conditions for a “Limited Power Source” AC-DC power supply are:

1. For a power supply rated at 30V or less, the following must be met even with a single fault condition:
    a. The output current must not exceed 8A
    b. The output power must not exceed 100W
2. For a power supply rated above 30V, but not exceeding 60V:
    a. The output current must not exceed 150 ÷ Vout
    b. The output power must not exceed 100W

This graph shows the limits for Limited Power Sources:

Taking some examples from TDK-Lambda’s DSP series of low profile DIN rail power supplies:

The DSP60-5 is rated at 5V 7A – That would meet (1b), but when put into overload it would certainly not meet the (1a) limit of 8A. This model is not listed as being a Limited Power Source.

The DSP60-12 is rated at 12V 4.5A – Overload current would be around 7A which meets (1a) and the maximum power is ~ 84W (12V x 7A) which also meets (1b). This model is listed as being a Limited Power Source.

The DSP100-24 is rated at 24V 4.2A – Right out of gate this unit exceeds 100W, and so it is not listed as a Limited Power Source.

The DSP100-24/C2 is rated at 24V 3.8A – This model actually has a special current limit & over power circuit which strictly limits the output current and power under a fault condition, so it meets the <8A requirement of (b) and the limit of <100W. Interestingly though, a single fault on the control circuit made the output rise to 30.8V and so it now falls under the 30-60V limit of 150 ÷ Vout = 4.8A maximum current (2b), which it passed. Therefore, this model is listed as a Limited Power Source.

Test results performed by the safety bodies, such as UL, CSA or TUV, are normally found in the CB report for the power supply.

What is the difference between Bit Rate and Baud Rate in Power Supply Remote Communications?

2012-05-08T11:27:00.000-07:00

Many modern power supplies, such as TDK-Lambda's new Z+ programmable supplies and the HFE series of rack mount supplies, now feature serial data interfaces such as I²C, USB 2.0, and RS485, to name a few. The transmission rates of these serial data interfaces are specified as bits-per-second (bits/s) and/or baud rate. As a refresher, here is a brief review of the difference between Bit rates and Baud rates.

The Bit rate is the number of bits (binary zeros and ones) that are transmitted during one second (bits/s). The Baud rate refers to the number of signal units (symbols or characters) that are transmitted per second.

Equation for Baud Rate:

baud rate = bit rate (bits/s) ÷ N, where N is the number of bits represented by each signal shift (symbol or character).

For example, if the bit rate is 9,600 bits/s and the communications format require 8 bits per symbol, the baud rate would be 9,600 bits/s divided by 8 bits, which would equal 1200 baud.

Equation for Bit Rate:

Bits/s = baud per second x the number of bits per symbol

Therefore, from the above example:

Bits/s = 1200 baud x 8 bits = 9,600 bits/s

Minimum loads and cross-regulation on multiple output power supplies

2012-04-09T14:57:00.001-07:00

One subject that our Technical Support team frequently gets asked about is minimum loads on multiple output power supplies, so I thought this would be a good subject to write about.

On a low cost, low power, multiple output power supply, the datasheet will often state that to maintain regulation, a minimum load has to be applied to one or more of the outputs.

To explain why, here is a block diagram of one such simple triple output power supply.

On the middle right hand side of the diagram are the three output windings from the transformer.

On Channel 1 (+5V), the output from the transformer is rectified and filtered to provide a smooth, DC output. If that output voltage is not at the set voltage, say due to a load change, the power supply will automatically correct itself. It does this by sensing the output voltage, comparing it to an internal reference, and feeding back a signal to the control circuit via the opto-coupler. The control circuit will then adjust the pulse width of the converter accordingly. The regulation on this output is typically 1 to 2%.

On Channels 2 & 3 (+V and –V) though, it can be seen that there is no feed back to the control circuit. These outputs are referred to as “semi-regulated”. If Channel 2’s load were to increase for example, the output would drop slightly, but there would not be any automatic correction. That voltage drop is specified by the load regulation specification, typically 3 to 5%.

With respect to minimum loading, if there is little or no load on Channel 1, the output will still be at the set voltage, but the switching converter pulse width will be very narrow. The output voltage on Channels 2 & 3 drops fairly dramatically at those narrow pulse widths, particularly if the outputs are supplying their full rated load. An output voltage of 12V may drop to 8V.

Conversely, if the full rated load is applied to Channel 1, but Channels 2 & 3 are not loaded, the voltages on 2 & 3 will rise, and a 12V output could deliver over 14V.

The effect that varying loads on Channel 1 has on the “semi-regulated,” Channels 2 & 3, is many times referred to as the “cross regulation” specification.

Manufacturers of power supplies specify a minimum load requirement on Channel 1, usually 10%, to warn the user. Minimum loads may also be specified on Channels 2 & 3 to promote a better regulation specification.

Operating without a minimum load will not normally cause a power supply to fail, but can stress the user’s equipment.

Some products like TDK-Lambda’s MTW series employ two converters to improve power supply regulation, one to supply Channel 1 & one to supply Channels 2 & 3. Note that both V2 and V3 are sensed by the control circuit.

Fig. 2: Two converters (click to enlarge image)

Here is a link for more details about the MTW series: http://www.us.tdk-lambda.com/lp/products/mtw-series.htm

Other products like the NV175 series employ post regulators on each output, completely eliminating the minimum load requirement. Although this does add cost to the power supply, it removes any concern for the user and helps with system flexibility. Here is a link to the datasheet for NV175 series: http://www.us.tdk-lambda.com/lp/ftp/Specs/nv175.pdf

Application Solutions: Displays and Kiosks

2012-03-15T09:34:00.000-07:00

I sometimes get asked by our customers "so what power supply should I use for my application?" This month we are going to start interlacing our educational articles with some real life examples, and will sort them by the application.

Next time you're going through the drive thru, you may be closer to a TDK-Lambda power supply than you think. As most drive thru restaurants operate 24 hours a day, 7 days a week, display reliability is paramount. One recent customer chose the HWS150 series for their application, with the PCB coating "HD" option. The HWS has very conservative component ratings and tested extremely well at ambient temperatures far exceeding the power supply's specifications, simulating abnormal heat wave conditions in desert locations. Convection cooling was also a big plus.

Wide range adjustable power supplies

2012-03-14T09:50:00.000-07:00

SWS1000L Series

We had a customer that wanted a 1000-watt power supply to drive a medical centrifuge. The only catch was that the speed of the DC motor that drives the centrifuge had to be varied and they were going to do this by using a step down dc-dc converter to change the voltage applied to the motor from 12V to 60V.

We suggested TDK-Lambda’s SWS1000L-60 power supply, which has, as standard, a Programming Voltage (PV) Input. By applying from 1 to 5.5 volts to this input, the output of the power supply can be varied from 12 to 66V.

The connections are very simple, using the PV input and common terminals on the front panel of the power supply (see diagram).

Alternatively, the Programming Voltage could have been derived from the SWS1000L’s 12V auxiliary output, which in this case could be connected to an external potentiometer to provide the variable dc input.

This simple solution saved the customer both design time and money. More information about the SWS1000L power supplies can be found at this web link http://www.us.tdk-lambda.com/lp/ftp/specs/sws600_1000l.pdf

The PV function is also available as on option on TDK-Lambda’s HWS series 300-600W models and is standard on the 1000-1500W models. More info can be found here http://www.us.tdk-lambda.com/lp/ftp/specs/hws1500.pdf

Advantages of Conduction-Cooled Power Supplies

2012-02-02T10:27:00.000-08:00

Most mid- to high-power supplies use fans to help dissipate the internal heat that is generated as a result of imperfect AC to DC conversion efficiencies. Since fans are electromechanical devices, they reduce the system’s MTBF and add to the required maintenance expenses.

Attached is a photo of a power supply that operated for many years at a postal depot where mail is handled and sorted automatically. As can be seen (after the fan was removed) paper fragments and airborne dust contaminants were pulled into the supply by the fan and eventually caused a blown fuse.

As might be expected, the proper maintenance program for any fan-cooled power supply calls for the periodic inspections of the supply, with the fan removed, and the replacement of the fan with a new one.

A new breed of conduction-cooled power supplies has been developed that do not depend on fans for cooling. Instead, the required cooling is accomplished by conducting the internal heat loads to an external metal structure or enclosure, which act as a large heat sink surface.

The second attached photo shows TDK-Lambda’s new CPFE1000F series, which are conduction-cooled, 1,000 watt AC-DC power supplies. (A 500 watt version is also available.) All heat is conducted to the supply’s aluminum plate, which is designed to easily mount to a metal enclosure or cold plate for cooling. More details and specifications for these power supplies are at this web link: http://www.us.tdk-lambda.com/lp/products/cpfe-series.htm

In some applications, these conduction-cooled devices are mounted to liquid cooled cold plates that are made of metal with internal serpentine channels through which a liquid circulates while removing the unwanted heat. The net result is that the system operates with a substantial reduction in audible noise, reduced maintenance costs (no dust build-up and fan wear-out), and an enhanced MTBF.

Recently, I visited a Television Broadcasting Station that consumes about 100 kilowatts of power. At this location, in separate areas, was a traditional fan-cooled system as well as the latest generation system, which uses conduction-cooled power supplies and RF amplifiers that are cooled via liquid flow cold plates. During the operation of the traditional system with fan cooling, the audible noise was so loud that personnel within 100 feet of the system had to wear hearing protection devices. By comparison, in the other area where the new system with liquid cooling was operating, the noise level was so low (similar to an office environment) that no hearing protection was required.

Simplify Power Supply Decisions via Free Mobile App

2012-01-17T13:49:00.000-08:00

It seems endemic that the final specifications for a power supply to run most new product designs tend to be firmed-up near the final stages of product development. Too often, this occurs when schedules have already slipped. As a result, there tends to be little time allocated to this important task. This happens so frequently that in the trade it has nicknamed ‘the tailpipe syndrome.’

Now, a fast and easy method for the selection of a power supply for your latest new product is as close as your smartphone. TDK-Lambda’s new mobile app works with Android and iPhones to greatly simplify this process. Say ‘goodbye’ to turning catalog pages or running tedious internet searches. The app has been shown to quickly guide the user to a power supply that meets their exact needs. To access this app, go to wbxapp.com/tdk-lambda. After the app loads, just press the AC power plug symbol. This app makes the selection process as easy as 1-2-3:

Step 1. Select Your Application:
LEDs, Medical, General Industrial, Comms, Test & Measure, or Military/COTS

Step 2. Select Your Power Range:
1-25W, 26-50W, 51-150W, 151-600W, or 600W & Higher

Step 3. Select from Available Products:
A number of power supply series that meet your Application and Power Range will be offered. Just click on any of these to view the specs and even detailed datasheets.

Quick Results:
In this example, we needed a Medical power supply with three (3) outputs and a total output power of 600-watts. The NV Series meets these requirements.

More Features:
This unique app includes quick links to YouTube (for power supply demo videos) and to Twitter (for new product info).

Additional information about this handy mobile app and other AC-DC power supplies, power racks, customized solutions, and DC-DC converters, is available at http://www.us.tdk-lambda.com/lp/

What is a Power Supply’s IP Rating?

2011-12-05T13:59:00.000-08:00

The popularity of outdoor electronics has brought the subject of a power supply’s IP rating from almost obscurity to an everyday question. I frequently get asked about it by our sales people now, so I thought it would be nice subject to cover in our blog. In researching this blog article I even discovered something new myself.

IP is the acronym for Ingress Protection and for power supplies the IP Rating Code consists of the letters “IP” and two numbers as defined below.

The first number indicates the power supply’s protection level against the ingress of solid objects or dust.

First Number for Solids or Dust

Level	Size of Object	Type of Object
X	Test not made	Test not made
0	N/A	No protection
1	50mm or larger	Large body surfaces*
2	12.5mm or larger	Fingers
3	2.5mm or larger	Small tools
4	1mm or larger	Screws
5	Dust protected	-
6	Dust tight	-

* Does not include deliberate body part contact

The second number indicates the power supply’s protection against the ingress of water or other liquids.

Second Number for Liquids

Level	Protected against
X	Test not made
0	No protection
1	Water dripping vertically
2	Water dripping at an angle
3	Spray water up to 60° from vertical
4	Splashing water from any angle
5	Low pressure water jets
6	Strong spray jets, heavy seas (ship decks)
7	Temporary immersion (up to 1m)
8	Permanent immersion (deeper than 1m)

Most recently LED power supplies, or drivers as they are often referred to, have ratings of IP66 or higher. Referring to the charts above, an IP66 rating means the unit has ingress protection from Dust and Strong Jet Sprays of Water.

These IP ratings also apply to the end system of course, and many of our customers utilize a NEMA enclosure to make their products meet a higher rating.

TDK-Lambda's IP66 rated LED Driver (ALC/ALV series)

Mounting Precautions for Power Supplies

2011-11-03T10:00:00.000-07:00

Before mounting your power supply, be sure to read its installation manual if you intend to mount it in an orientation other than along the horizontal plane (Fig. A). Many power supplies have restrictions regarding mounting. For example, since heat rises, if you mount some power supplies on a vertical plane (Fig. B, C, & D), the heat from the lower section of the power supply will rise and further heat the upper part of the supply, which may cause over heating problems. Likewise, with some power supplies you are not allowed to mount them upside down (Fig. E) because this traps the heat and restricts the normal convection air cooling around the power supply.

In some cases, vertical mounting of power supplies is permitted as long as you reduce the amount of power that will be drawn from the supply. This is referred to as “Derating” the power supply. Below are the derating curves for the TDK-Lambda’s model LS150-12, a convection cooled 150-Watt, 12V output, AC-DC power supply.

This graph shows the percentage of rated output power on the vertical axis and the operating ambient temperatures on the horizontal axis. Notice when mounting this power supply on the horizontal plane (Fig. A), the power supply is rated at 100% output power from -25°C up to +50°C. However, if you mount this supply on a vertical plane (Fig. B, C, & D), the maximum ambient temperature is reduced to +40°C before the power must be derated.

It is worth mentioning that many low cost competitors do not mention the preferred mounting orientation, and some do not even have an installation manual on their website!

An incorrectly mounted power supply will get too hot resulting in premature electrolytic capacitor degradation, catastrophic semiconductor failure or even a fire due to transformers overheating.

Other general power supply mounting considerations include the following:

Make sure there is adequate space around the power supply to allow air to circulate.
Do not block off vent holes on convection cooled supplies or restrict air inlet or outlet ports on fan cooled supplies.
In the event fans are employed within power supply, a system, or an enclosure make sure the airflow direction for all fans are the same

How to safely power LEDs

2011-10-04T14:33:00.000-07:00

For well over 25 years, LEDs (Light Emitting Diodes) have been used in TV remote controls. These specific LEDs emit invisible light pulses in the infrared (IR) light spectrum. Because the LED can be turned on and off very rapidly, it easily transmits pulses of binary-coded messages to the receiver built into the TV.

In addition, early applications of LEDs included red-segment clocks, calculators and even digital watches that have now been replaced by more modern display technologies, such as LCDs (Liquid Crystal Displays).

Today, white or multi-colored LEDs are rapidly being employed in modern home/street lighting, signage, traffic signals, large screen displays and backlit LCD monitors, etc. In these applications, multiple LEDs are placed in either clusters or connected as strings to provide the required light intensity or light distribution.

LEDs are similar to conventional diodes in that they are designed to conduct current in one direction and when doing so, in most cases, they emit visible light. A basic LED circuit consists of a voltage source, a current limiting resistor and the LED as shown below.

The current limiting resistor (R) is required to maintain the current flowing through the LED at a safe operating level. When conducting current LEDs have an inherent “voltage drop” that can vary from 1.2V to 4.0V, depending upon the model. Referring the circuit diagram, if the LED has a voltage drop of say 2V (Vd) with a safe operating current of 20mA (I), and the voltage source (Vs) is 5VDC, the value of the current limiting resistor can be calculated as follows:

R = (Vs –Vd) ÷ I, therefore, R = (5V – 2V) ÷ 0.02A = 150 ohms

The voltage drop across an LED and its light output will vary with the current flowing through it. Below are curves that show the forward voltage drop (Vd) versus the current (I) flowing through two sample LEDs.

In viewing the white curve above, it’s important to notice that the forward voltage drop across the LED between 3.2V and 3.6V (a 0.4V change), results in a current increase of over five times (from 10mA to 60mA). In this example, if the maximum allowable LED current is 40mA and if 60mA or more current is allowed to flow through it, the LED could be destroyed or its operational life substantially reduced.

As current flows through an LED its forward voltage drop times the current results in wasted power (e.g., 3.3V x 40mA = 132mW). This wasted power, in the form of heat, becomes a real problem when high brightness LEDs is employed in lighting applications. The internal LED heat must be dissipated by either its design, the substrate it’s mounted on, or via added heat sinks. As the internal junction of an LED gets warmer, the current through it at a given voltage increases. If not controlled, this can result in thermal runaway, where the LED self-destructs.

The main point here is that LEDs are “current driven” devices and that this current must be carefully controlled. In the circuit above, the resistor is used to control the current though the LED. However, the resistor also causes a voltage drop which contributes further to wasted power. As a result “constant-current” LED drivers have been developed that maintain the current flowing through the LED (or strings/clusters of multiple LEDs) at a safe level with improved efficiency.

For more information about selecting power supplies and drivers for LEDs, see the article at this web link: http://power-topics.blogspot.com/search/label/LED%20lights

References:

http://en.wikipedia.org/wiki/LED_circuit

http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/

http://us.tdk-lambda.com/lp/products/ledsigns.htm

Using Power Supplies with DC Motors

2011-09-09T09:28:00.000-07:00

There is often confusion regarding the use of external diodes when power supplies are used to power DC motors. Most people know that a diode has to be used, but are unsure where to place them or what their purpose is.

From a power supply concern there are two types of DC motors; a brushed DC motor and a brushless DC motor.

Brushed DC motors

With this type of motor, the magnets are stationary and the coil spins. Electricity is transferred to the spinning coil by the use of “brushes”.

The advantages of this type of motor are low initial cost and easy speed control.

When the power is interrupted, the motor coil will act like an inductor and will try to continue to produce current, effectively becoming an inverted voltage source. This will apply a reverse polarity to the power supply and can cause damage. (Back EMF – Electro-Magnetic Flux)

By using a diode, as shown below, the diode provides a current path for the reverse motor current and will clamp the reverse voltage to a level no greater than the forward voltage drop of the diode. This protects the power supply’s output capacitors and other components from being stressed by the reverse voltage.

Brushless DC motors

Brushless DC motors, often referred to as BDCMs or BLDC motors, have permanent magnets that rotate and the armature is fixed.

Although more expensive, they are more reliable in the long term as there is no brush or commutator wear and position control is more accurate.

When the motor is turned off or reversed, it will act as a generator and produce a high voltage spike. This spike can cause the power supply’s overvoltage protection to trip, shutting down the unit.

By using a diode in series with the output, as shown below, the spike will be blocked from interfering with the power supply.

In both cases a general purpose diode can be used, providing that the voltage and current ratings for the diode are correctly calculated.

LED Drivers for LCD panel backlighting (ALD Series)

2011-08-21T10:50:00.000-07:00

What type of LED driver or power supply do I need?

2011-08-02T14:30:00.000-07:00

Conventional AC-DC power supplies and DC-DC converters provide an output that is regulated to provide a “constant-voltage.” However, LEDs work most efficiently and safest with a “constant-current” drive. As a result, many new devices have been developed to provide this type of LED drive. LED power sources that provide a “constant-current” output have typically been referred to as LED drivers. In the past, AC-DC power supplies that provided a regulated “constant-voltage” to LEDs were referred to as LED power supplies. Today, the terms “LED driver” and “LED Power Supply” are used interchangeably. The important thing to keep in mind is whether the output of the power device provides a “constant-voltage” or a “constant-current.”

When do I need a “constant-voltage” LED driver?
Most commercially available LED “light modules” are constructed by connecting a number of LEDs in series or parallel to form cluster or string configurations. In cases where these light modules include a “constant-current” driver as part of the assembly, an external “constant-voltage” driver or power supply is required. Some LED circuits control the current flowing through the LED with a simple resistor. This is another case where a constant-voltage power source is required. Other examples where external “constant-voltage” supplies have been employed include backlit ad signs, traffic information signs and large screen high definition LED displays, such as those described in this article: http://www.ledsmagazine.com/products/20877. Constant-voltage drivers come in many different forms. They can look like a conventional power supply or they can be enclosed for moisture/environmental protection.

When do I need a “constant-current” LED driver?
In cases where a manufactured cluster or string of LEDs does not include an internal “constant-current” driver, an external LED driver or power supply that provides a “constant-current” is required. Constant current LED drivers are available in many different package configurations, ranging from integrated circuits to enclosed moisture-proof packages, depending on the application and the required output power.

Series and Parallel LED Configurations
Depending on the application, LEDs can be connected in series and/or parallel configurations. Obviously, when LEDs are connected in series the forward voltage drop of each LED in the string are additive. For example, if you put 15 LEDs in series and each one has a voltage drop of 3V (at its nominal current), you need to provide a voltage source of 45V (15 x 3V = 45V) to drive the required current. This is why “constant-current” drivers always include in their specs the output voltage range that it is capable of providing to overcome the LED voltage drops. In order to limit the drive voltage to reasonable levels, multiple strings of series-connected LEDs can be placed in parallel and driven by multi-output constant-current drivers.

Below is an excerpt from the datasheet for TDK-Lambda’s ALD6 series of LED drivers. As you can see from the diagram, this driver contains up to 6 independent “constant-current” LED drivers. The 38V output corresponds to combined forward voltage drop of 10 typical white LEDs connected in series. For high-current applications, up to 300mA is available to power one series-string of high brightness LEDs. For applications where the LEDs require up to 50mA, this device can power up to 6 strings of LEDs via its multi-output drivers. These drivers are ideal for LCD display backlighting and general LED lighting applications.

Click to enlarge

How is LED dimming accomplished?
The light output of LEDs can be controlled by varying the amount of current flowing through the LED (within defined limits) or by turning the LED on and off via pulse width modulation (PWM). LED drivers like the ALD6 series have the capability of providing “dimming” by both of these popular methods.

The drawing above shows the two methods of light dimming that are included in the ALD6 LED driver. It is permitted to use a combination of both of these methods simultaneously.

The “Rbr” is an external variable 10kohm resistor input. By varying this potentiometer from 1k to 10kohms, an analog dimming control is achieved. In this case, the maximum LED brightness occurs when the pot is set to 10k ohms. This same input can operate with variable analog voltage ranging from 1.6 to 3.8-volts. In some applications this input can be connected to a temperature sensing device which could reduce the current flow through the LEDs as the temperature rises, thus providing a means for temperature compensation.
The “Vpwm” is a “Pulse Width Modulation” input that controls the LED brightness by varying the duty-cycle of the input signal from 1% to 100%. Typical PWM frequencies can range from 180 to 270 Hz.

More information about LED drivers/supplies can be found at these web links:
http://us.tdk-lambda.com/lp/products/ledsigns.htm
http://us.tdk-lambda.com/lp/products/ald-series.htm
http://power-topics.blogspot.com/search/label/LED%20lights

What does a BF rating on a power supply mean?

2011-07-06T13:15:00.000-07:00

TDK-Lambda recently launched the EFE-M series, a medically BF rated power supply. It immediately sparked the question from my colleagues – “What is a BF rating?” To answer this question we need to start with the term “Applied Part.”

IEC 60601-1 is the international medical electric safety standard that uses the term “Applied Part” to refer to a part of a medical device which may come in physical contact with the patient during its normal operation.

Applied Parts fall into three classifications according to the nature of the medical device and the type of contact. Each classification must have a different protection level against electrical shock.

Type CF (“Cardiac Floating”) is the most stringent classification, and is used for applied parts that may come in direct contact with the heart, such as dialysis machines.

Type BF (“Body Floating”) is less stringent than Type CF, and is generally used for applied parts that have conductive contact with the patient, or having medium or long term contact with the patient. Examples of this type of equipment are blood pressure monitors, incubators and ultrasound equipment.

Type B (“Body”) is the least stringent classification, and is used for applied parts that are normally not conductive and can be immediately released from the patient. Examples of that would be LED operating lighting, medical lasers, MRI body scanners, hospital beds and phototherapy equipment.

Type B applied parts may be connected to earth ground, but Type BF & CF are separated from earth – hence the term “floating”.

Power supply Isolation Voltages vary according to the type rating.

Type	Input to Output Isolation	Input to Ground Isolation	Output to Ground Isolation
B rated	4000VAC	1500VAC	500VAC
BF/CF rated	4000VAC	1500VAC	1500VAC

Please note: power supplies are not medical devices or applied parts, and the outputs of power supplies should never be connected directly to a patient.

Many medical devices contain medical-rated power supplies. However, only the part of these “medical devices” that may come in contact with a patient during normal operation is classified as an “Applied Part.”