Power Topics for Power Supply Users

Why Use DIN Rail Mount Power Supplies?

2009-11-03T10:35:00.001-08:00

DIN Rails are metal strips that provide a convenient means for mounting electric and electronic devices in a compact and neat manner. For example, DIN Rails are frequently used for mounting circuit breakers (Fig #4), terminal strips (Fig #3), power supplies (see photo above) and all sorts of industrial control equipment within racks/enclosures or attached to backboards. In this way, any combination of devices can be mounted next to each other to meet the system requirements.

Standard DIN rails are shaped as shown in Figure #1 (end view) and #2 (photo). They typically measure 35mm from edge to edge. The distance from the back to the rail to the front bends can be either 7.5mm or 15mm. These metal DIN Rail strips can be provided in any length to suit the application and multiple rows of rails can be used.

Figure #1 – End view of typical DIN Rail

Figure #2 – DIN Rail with slotted mounting holes

Figure #3- Terminals Strips mounted on DIN Rail

Figure #4- Circuit Breaker mounted on DIN Rail

The use of DIN Rail mounting systems saves installation time since all devices just snap onto the metal rails. A complete system can be quickly put together in an organized configuration that provides high density, flexibility, safety and design time savings. Associated devices can be mounted adjacent to each other, thus reducing the length of interconnect wiring.

The DIN Rail concept is widely used in industrial control, instrumentation and automation applications. Today, even DIN Rail mountable micro-computers are available and being used.
DIN rail mounted AC-DC power supplies provide a convenient means for powering DC operated devices including sensors, transmitters/receivers, analyzers, programmable controllers, motors, actuators, solenoids, relays, etc., to mention a few. Since these power supplies are convection cooled, no cooling fans are needed. Output voltages from these supplies range from 5V up to 56V with power ratings from 7.5W up to 480W. Many of these supplies can be connected in parallel for higher power applications.

In some cases, conventional power supplies can be utilized in DIN Rail systems by means of “DIN Rail Mounting Kits/Adapters”. See Figure #5 below and more details at this web site: http://www.us.tdk-lambda.com/lp/products/ldin-series.htm

Figure #5 - DIN Rail Mounting Adapter Kit for conventional power supplies

Detailed information about TDK-Lambda’s wide range of DIN Rail mount power supplies is available at this web link: http://www.us.tdk-lambda.com/lp/products/finder6.htm

Is there a wave of power supply obsolescence coming?

2009-10-09T08:46:00.001-07:00

The old 2001 version of the safety agency specification UL60950-1 for power supplies expires in December 2010. Usually revision changes are minor for industrial power supplies, but one key component is affected this time. The 2005 version stipulates that any surge suppressor on the primary side shall be a VDR (Voltage Dependant Resistor) which must be comply with Annex Q and be approved to IEC 61051-2.

To comply, power supply manufacturers will have to modify their designs and retest to demonstrate compliance to the immunity specification EN61000-4. This is going to be costly in either Engineering labor costs (opportunity cost) or external test house fees.

Manufacturers may now decide to obsolete slow moving products before December 2010 and continue investing in new products.

TDK-Lambda's 100-150W External Power Supplies Meet New Energy Efficiency Standards

2009-08-18T09:32:00.000-07:00

TDK-Lambda has released a new range of AC-DC external power supplies with models rated from 100 to 150-watts that meet the latest Energy Star, EISA, and CEC standards. The DT100-C and DT150-C series feature active PFC (meets EN61000-3-2) and operate from a universal AC input of 90 to 264Vac (47-63Hz). Available output voltages include 12V, 16V, 19V, 24V, 36V and 48V.

These external power supplies are packaged in an insulated compact and lightweight enclosure measuring 3.35" wide by 6.7" long by only 1.73" high and are convection cooled (no fans needed). The operating temperature range is 0 to +40°C with no derating required. All models are fully isolated (3kVac, input to output) and meet the Energy Star 1.1 and the California Energy Commission (CEC) level IV efficiency standards. Plus, models with outputs of 24V to 48V meet the Energy Star 2.0 version level V standards.

These units include overvoltage and short-circuit protections and off/no-load standby power consumption of less than 0.50 watt as required by the green energy initiatives. In addition, these series include UL/EN/IEC60950-1 international safety agency certifications and meet EN55022-B and FCC Class B conducted and radiated EMI standards.

Moreover, these external power supplies feature a recessed IEC320-C14 AC input receptacle and a 3.4 foot long output cable with a molded 4-pin connector. Other output connector types can be special ordered. These 100-150W supplies are ideal for powering instrumentation, industrial devices and a wide range of other general purpose applications.

The DT100-C and DT150-C series are available now and priced from $40.50 each in OEM quantities of 1,000 units. For more information, please call TDK-Lambda directly at 1-800-LAMBDA-4 or see website: http://www.us.tdk-lambda.com/lp/products/dt-c-series.htm

About TDK-Lambda:

TDK-Lambda has been a major provider of power solutions for over 60 years. The company designs and manufactures a wide range of AC-DC and DC-DC power products and EMI filters for Industrial, Medical, Telecom, Datacom and Test & Measurement applications. TDK-Lambda is a subsidiary of the TDK Corporation (TSE, LSE: TDK), a leading global electronics company (www.tdk.com). For more information, please call TDK-Lambda at 619-575-4400 or 1-800-LAMBDA-4 (toll free), or visit our website at: www.us.tdk-lambda.com/lp

Operating Power Supplies in Series

2009-07-22T14:24:00.000-07:00

Although some users are nervous about operating power supplies in series, it is common practice in the industry. The benefit is that voltages greater than 60V can be obtained using off-the-shelf products.

It is possible to connect several power supplies in series, but please read the precautionary notes below:

Connect back-biased diodes across the power supply terminals as shown below.

Rate these diodes at the same output current as the power supplies.

In the event both power supplies do not turn on at the same time, or if the load becomes a short circuit, then the diodes will protect the power supplies from any applied reverse voltage.

Do not exceed the output to ground/chassis voltage rating. Inside most power supplies are noise filter capacitors connected from the output to ground. It is possible to exceed the operating voltage of those capacitors, particularly when configuring several units in series.

Avoid using “fold-back style” current limited power supplies as these may lock up the power supply during initial switch on.

TDK-Lambda Packs 15W into 1 x 1 Inch DC-DC Converter

2009-07-21T09:44:00.000-07:00

San Diego, CA – July 2009 – TDK-Lambda has added two new series to its already successful PX family of DC-DC converters. The fully-isolated PXA and PXB Series have a tiny 1 x 1 inch footprint and are designed for applications ranging from communications to factory automation equipment. They are available in open frame (PXA) and shielded metal case (PXB) versions.

The PXA Series offers single-output models with nominal inputs of 24V and 48VDC in 2:1 and wide 4:1 versions. The PXB Series offers both single and dual-output models with nominal inputs of 12VDC in a 2:1 version and inputs of 24V and 48VDC in 2:1 and wide 4:1 versions.

Available single-output voltages for the PXA and PXB series include 3.3V, 5V, 12V and 15VDC. In addition, the PXB series offers dual-output models that provide ± 5V, ± 12V and ± 15VDC outputs. Efficiency is up to 88%. Remote on/off and output adjustment (trim - single output models) features are standard, plus all models include overvoltage and overcurrent/short circuit protection. Operating temperature range is -40°C to +85°C.

The PXA is available in through-hole or SMD mounting formats and the PXB in a through-hole mounting format. All models are CE-Marked and are certified to UL/CSA/EN60950-1 safety standards as well as meeting the rigorous MIL-STD-810F thermal shock and vibration specifications.

The PXA and PXB series are available now and priced from $28.00 each in 500-unit quantities. For more information, please call TDK-Lambda directly at 1-800-LAMBDA-4 or visit the website at: www.us.tdk-lambda.com/lp.

About TDK-Lambda:
TDK-Lambda has been a major provider of power solutions for over 60 years. The company designs and manufactures a wide range of AC-DC and DC-DC power products and EMI filters for Industrial, Medical, Telecom, Datacom and Test & Measurement applications. TDK-Lambda is a subsidiary of the TDK Corporation (TSE, LSE: TDK), a leading global electronics company (www.tdk.com). For more information, please call TDK-Lambda at 619-575-4400 or 1-800-LAMBDA-4 (toll free), or visit our website at: www.us.tdk-lambda.com/lp

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NV Series Configurable Power Supply

2009-06-10T14:18:00.001-07:00

Comprehensive review of the powerful and adaptable NV Series of power supplies from TDK-Lambda Americas. These power supplies are 1U high, have up to 90% efficiency, active power factor correction, up to 8 outputs, no minimum loads and medical approvals.

PFE Series Brick Power Supply

2009-06-10T14:17:00.003-07:00

This video describes an innovative new AC-DC power module (PFE Series) that combines in a single pcb-mountable brick the major functions of an AC-DC power supply including input rectifiers, PFC circuits, switchmode converter, and provides a tightly regulated DC output.

Redundant Power Systems (FPS Series)

2009-06-10T14:17:00.001-07:00

Provide increased output power by connecting two or more power supplies in parallel. We have also covered how to construct an N+1 Redundant and Fault-Tolerant Power System and the features required of the supplies in order to accomplish this with the greatest amount of reliability and ease of system maintenance.

CE Mark and DC-DC converters

2009-05-03T11:47:00.000-07:00

The CE identification mark is accepted across the entire European Community. It is an indication that the product to which it is applied to conforms with the minimum requirements of all the applicable European Directives for that product, and that duly authorized assessment procedures (technical files) have been carried out on that product.

The most common power supply Directive is the Low Voltage Directive (LVD), EC number 73/23/EEC, which came into force in 1973. This applies to all electrical equipment with an AC input voltage of between 50 and 1000 V for alternating current and between 75 and 1500 V for direct current.

Many DC-DC board mount converters are hence exempt from this directive if they have a nominal input voltage of 5VDC, 12VDC or 24VDC provided the input range is not 75VDC or higher.

Most 48V input DC-DC converters have a DC input range of 36V to 75VDC and so fall under the Directive. 4 : 1 input range models (18 - 75VDC) would also be covered.

A link to the LVD can be found here http://ec.europa.eu/enterprise/electr_equipment/lv/index.htm

Nagaoka Fire Works 2008/08/02 TDK-Lambda

2009-04-24T16:22:00.001-07:00

Fireworks display from our sister company's fall festival. Fun to watch if you like fireworks.

Maximizing the Life of Power Supply Fans

2009-04-22T14:57:00.000-07:00

The vast majority of medium to high-power AC-DC power supplies have integral fans that are required to keep their internal components at safe operating temperatures. Since fans are electro-mechanical devices they are subject to wear out faster than any other component in the power supply.

The chart and diagram below illustrate this very well. As can be seen from the chart, if a power supply’s fan is operated with a high exhaust air temperature at perhaps 80°C (176°F) its life expectancy may be a short as 1.5 years. However, by reducing the exhaust air temperature (as measured 2-inches away) to perhaps 40°C (104°F) the fan’s life expectancy may now exceed 5 years.

Obviously the requirements of a specific application may require different operating temperatures. However, whenever possible, lowering the operating temperature of the power supply will increase the life of the fan as well as the components within the supply. Also, by derating a power supply below its maximum power rating will have a direct effect on its internally generated heat and, therefore, its exhausted air temperature, which will extend the life of its fan.

Positioning the power supply so cooler air is drawn in through the power supply from outside of the system will also help.

Also, a power supply fan’s life will naturally be extended if the supply is turned off when not needed. Some of the newer fans are thermally controlled so they turn on and off automatically. There are also variable-speed fans that increase or decrease the fan’s speed depending upon the sensed ambient temperature or the load required of the power supply. These have the advantage of extending the fan’s life as well as reducing the audible noise when the load current is low.

Another important factor for fan life maximization is to keep the area around the power supply (inlet and outlet) as free of dust and dirt as possible. Dust, metal and chemical particulates can sometimes kill a fan quicker than high temperatures.

If a fan starts making squeaking sounds, it’s a good indication that it should be replaced very soon, before it freezes up. Fan replacements should only be done by qualified electronic technicians who are familiar with the high voltages that can exist within power supplies even after the AC power is removed.

Why pay more for a power supply with a longer warranty?

2009-03-28T07:41:00.000-07:00

Since all power supplies contain similar electronic components such as capacitors, semiconductors, resistors, transformers, inductors, etc., why pay more for one with a longer warranty period? In today’s cost sensitive world, questions like this come up all the time. It’s easy to get caught up in the idea of buying a power supply with the lowest price rather than its warranty time-span.

It’s interesting to note that over 50% of TDK-Lambda’s standard power supplies that are sold each year carry a five-year or longer warranty. Is it that these customers have lots of money to fritter away on this luxury, or do they realize some hidden benefits?

One of the major cost drivers in power supplies is, not surprisingly, the component costs. For example, all power supplies use electrolytic capacitors, which are available with various capacitance, voltage and operating temperature ratings.

Electrolytic capacitors contain a paste-like electrolyte which will eventually dry out and cause the capacitor to fail. How quickly this process occurs depends heavily upon what materials are used to make these capacitors and how close to their maximum ratings these components are utilized.

Electrolytic capacitors used in industrial-rated power supplies are more costly than those used in light commercial applications, but they are made to last for many, many years without failing. It’s like comparing a professional mechanic’s tools to those sold in variety stores. You get what you pay for when it comes to high quality tools; the same holds true when buying power supplies.

Furthermore, the power supply designer can choose to operate the capacitors at or near their maximum ratings, which will result in a low-cost product, but with a shorter life. Or, if a longer field life is a consideration, the designer will “derate” the capacitors, which means he will make sure the capacitors are running at a lower voltage and operating at temperatures that are well below its maximum. In this way the designer can achieve a much more reliable and longer life design at a somewhat higher cost. The same trade-offs in design are made for the semiconductors, resistors and other components that comprise the power supply.

In addition to the above, the life span of a power supply depends a great deal on the operating environment. In an industrial environment where a manufacturing plant is running multiple shifts, the power supply may be operating 24 hours a day, 360 days a year, with an ambient temperature within the equipment of perhaps +50°C (+122°F) or higher. Compare this to an office or medical environment where the ambient temperature might be typically +30°C (+86°F) and the equipment is running 8 hours/day, 5 days a week. Obviously, in the industrial application a more robust and higher quality power supply would be required to handle the rigors of these applications.

Power supply manufacturers want to avoid paying the high costs associated with repairing a failed unit within its warranty period. Therefore, based on their predicted life calculations and field return data, they set the warranty period such that the power supply will, in the vast majority of cases, not fail within the warranty period. And, they usually ensure that their supplies have a buffer life-time of 6-months to a year or so beyond their warranty period. So, it turns out that the warranty period is a fairly good indicator of how long you can expect the power supply in your equipment to run without failing. If you purchase a low cost commercial power supply with perhaps a one year warranty and install it in your industrial equipment that may carry a 3 year warranty, that would be a big mistake. Your low-cost power supply would quickly lose its cost advantage when it fails prior to your OEM warranty expiring.

So, we now come to the answer of our headline question:
Why pay more for a power supply with a longer warranty?

Answer: Because it’s the most cost effective way for the OEM to avoid premature field failures, trouble calls, unhappy customers, and high field service/product repair costs.

HWS Series power supplies from TDK-Lambda come with a Limited Lifetime Warranty -- an industry first

Power Supply Considerations for Industrial Applications

2009-02-20T12:02:00.000-08:00

Although power supplies are among the most important components of any industrial application, they seldom receive any significant attention. Engineers often do not fully understand all of the variables that go into choosing the correct power supply, and may select a product that is insufficient or more costly than what is needed.

When considering a power supply for an industrial application, it's helpful if a designer has an understanding of the steady state output parameters of the product, as well as the electrical and physical environment that the equipment will operate in. Here are some critical considerations.

Unique Load Requirements
Motors, solenoids and relay controls require higher levels of current when they are turned on than they do for continuous operation. It is necessary for the designer to examine the magnitude of the pulses and either specify a power supply that is capable of providing the surge currents continuously, or use a product that can provide peak power for a limited time. Certain models, for example, can deliver up to 200% of the nominal rated current for up to 30 seconds. This enables the user to purchase a 240W unit to meet a 480W surge load, saving both money and space.

The designer should also anticipate potential mechanical failure of factory equipment. If a motor stalls or a relay "sticks", the current draw can rise dramatically. Using a power supply that is capable of protecting itself in overload conditions will both protect the unit and the system.

Input Line Disturbances
In most industrial environments the AC line is far from clean. This is because the same line that feeds a power supply is also being used to drive larger equipment. Large disturbances such as power sags and surges are commonplace.

High spikes on the AC line can damage a power supply in a similar way that ESD can damage semiconductors. On the surface, the unit can pass bench testing but long-term damage may occur to capacitors and power semiconductors, which leads to failure after just a few months of operation in the field. Industrial power supplies should meet EN61000-4 standards for immunity to line transients, and for extremely dirty AC line conditions the designer should consider using an external AC line EMI filter with high voltage pulse attenuation specs.

To prevent loss of DC power during sags, which is typical when a large piece of equipment is switched on that is in close proximity to our designer's system, it will help to specify a power supply that has a wide AC input range. If the AC line is 208VAC nominal, and sags down to 140VAC occur, utilizing a product that has an input range of 85 – 264VAC will allow DC power to be supplied without interruption. Even a short dip in the DC output can cause microcontrollers to reset and the host equipment to run through a reboot sequence.

Mounting Considerations
Most power supplies typically use electrolytic capacitors for filtering and energy storage. The higher the operating temperature of these capacitors, the shorter the life. As these parts age, the output ripple of the power supply increases, causing functional problems with the load equipment.

When mounting the power supply, ensure that adequate space is provided around the product to allow air to circulate. Do not block off heatsink fins with mounting brackets, restrict air inlet or exit from fan cooled units (1.5 to 2" clear space is a good rule of thumb), or mount the supply in a plane other than its standard-mounting orientations without consulting the installation manual.

In the event that other fans are in the enclosure, take note of the general system airflow direction, and be aware of any potential backpressure issues that may occur.

Operating Temperature and Life Effects
In addition to mounting considerations, the operating ambient temperature also plays a key part in the life of the power supply. The life of an electrolytic capacitor doubles for every 10°C reduction in temperature. The designer should be aware of the derating characteristics of the proposed power supply. Most AC/DC power supplies start to derate from 40°C or 50°C, and can only operate at 50% of its rated load at 70°C.

The derating calculations may indicate that a higher power unit is needed. Using a manufacturer with a broad base of products and a large number of models within a series will simplify this choice.

As a note, the ambient temperature is specified at the inlet of the fan or close proximity to the power supply. Designers should take into account any internal temperature rises in their system when considering potential derating.

To make an "apples-to-apples" comparison on competing products, also consider the warranty of the power supply. A product with a five-year warranty will have greater component deratings and higher quality components (use of 105°C rather than 85°C rated capacitors) for a longer field life than a product with a one-year warranty.

Operating Environment
Vibration and shock will also heavily influence the life of a power supply. A more rugged power supply will meet more stringent MIL-STD specifications. When considering the specifications, remember that how the power supply is mounted can cause mechanical resonance in the system. When the entire system is subjected to shock and vibration, a power shelf containing one or two supplies may start to vibrate at amplitudes greater than the system itself.

Think Ahead
While it is true that the power supply is only a small fraction of the size, complexity, and cost of industrial equipment, it is a key component that can have a disproportionate impact when the role in the system is not carefully considered. Because of the power supply's high unit cost compared to other electrical and electronic components, it is often targeted as an item for cost reduction. In the world of power supplies, you truly get what you pay for. Bargain-priced power supplies are not a bargain when the costs of field-failures, customer complaints, warranty repairs and potential damage to your company’s brand name are included in the equation.

Designers who consider their power applications carefully and early in the project are more likely to see their project go more smoothly, faster and most importantly protect their company's name and reputation with greater field reliability.

What Size Fan Do I Need?

2009-01-29T08:57:00.000-08:00

There are many AC-DC power supplies and DC-DC converters with output power ratings that can vary dependant upon the type of air cooling provided. “Convection air cooling” usually refers to situations where a power supply or converter is cooled by the prevailing ambient air temperature, adjacent to the power device, without forced-air-flow from fans or blowers. If the power device has two output power ratings, the “convection cooled” (still-air) power rating is lower than the “forced-air convection cooled” rating.

The power supply pictured above is an open frame switchmode supply with two output power ratings. For “convection cooled” applications, this supply can provide up to 151W of output power. However, with “forced-air-cooling” it can provide up to 201W of output power. The datasheet for this power supply indicates that for “forced-air-cooled” applications, 1.5 m/s (Meters per Second) must be provided by the user. 1.5 m/s equals 295 LFM (Linear Feet per Minute). Refer to conversion factors shown below.

Most fans are rated in CFM or Cubic Feet per Minute of air “Volume” flow. So, what size fan do you need to provide 295 LFM of air “Velocity” flow for the above application?

Most times the power supply is cooled by directing the air flow along its longest dimension; for example, from the input connector end to the output connector end. However, always read the power supply’s instruction manual to determine the manufacturer’s recommended axis for the cooling air-flow. The usual method for determining the required fan size is to first determine the height and width for the opening or port through which the air will flow around and through the power supply. In this instance the power supply is 3.15” wide and 1.46” high (and 8.2” long). We can consider the supply’s width times its height as the minimum area of the inlet port for forced air cooling of the supply. Then, we need to convert these dimensions from inches to feet by dividing by 12”. 3.15” = 0.26’ and 1.46” = 0.12’. So, the minimum “Area” of the port through which the air must flow to cool the power supply is 0.26’ x 0.12’ = 0.0312 square feet. The formula for determining the CFM (volume) rating of the fan when the required LFM (velocity) is known is as follows:

CFM = LFM x Area (in square feet)

Therefore, in this example:

CFM = 295 LFM x 0.0312 ft² = 9.2 CFM (min. fan rating)

Fans are rated in CFM based upon the expected free-flow of the air coming from them, without obstructions, which cause back-pressure. Of course, real world applications always include some obstructions. To ensure the least amount of back-pressure, it is best to have the exit ports in the enclosure about 1.5 times the area of the minimum entry port. In most applications there are other heat loads and components that can obstruct the path or free flow of the cooling air. It is therefore wise to select a fan with a higher rating than is calculated. Perhaps a 10 CFM or larger fan should be used in this application.

Tip: The use of a larger fan running at a slower speed can deliver the same airflow as a smaller fan running at a higher speed, but the larger fan will be much quieter.

Since most fans have round air outlets and square mounting patterns, the air-flow from the fan may require ducting within the end-product’s enclosure to direct the cooling air to the high power devices including the power supply.

The same process would be used to determine the correct fan rating for AC-DC power modules or DC-DC converters, with or without heatsinks that require forced-air-cooling. When heatsinks are used (see photo below), always direct the air flow in the same direction as the slots between the fins of the heatsink.
In all situations, the system must be tested with the selected fan and all other devices in-place to confirm that the power supply or converter and the load it drives do not exceed their maximum operating temperature, under worst case conditions (maximum ambient inlet air temperature, 100% power load, etc.). If problems are observed, a higher CFM rated fan or dual fans may be required.

In the metric world, fans are sometimes rated in “m³/hr” (Cubic Meters per Hour) and the air velocity is rated in “m/s” (Meters per Second). The following Metric to English conversion factors may be useful.

1 m³/hr = 36 ft³/hr ÷ 60 min. = 0.60 CFM (cubic feet per minute)
1 m/s = 3.28 ft/sec x 60 sec = 196.85 LFM (linear feet per minute)

Some fans and power supplies have dimensions in mm (millimeters).
Just remember that 1 inch = 25.4 mm, and 1 mm = 0.04”

There are a number of very good online calculators to assist you in determining the fan size and ratings required for various forced-air-cooling applications. Here are a few of those websites:

http://www.airperformancetech.com/conversion-tools.htm
http://www.aavidthermalloy.com/technical/airflow.shtml
http://www.calculatoredge.com/optical%20engg/air%20flow.htm

What is the SEMI F47 line sag spec all about?

2008-12-23T10:27:00.000-08:00

We have all seen lights dim at home or at work and this is an indication that the AC line voltage has been reduced or sagged. Although an occasional dimming of lights can be tolerated, when it comes to factory automation equipment, line sag can be the source of a production shutdown, resulting in significant revenue losses. Since the production of semiconductors, including microprocessors, is a very precise and expensive process, back in 1999 the Semiconductor Equipment and Materials Institute (SEMI), established standards relative to AC line sag immunity. This specification is called the SEMI F47 Voltage Sag Immunity Standard and has been revised periodically. Because many other factory automation processes are equally critical, some of these production products need to comply with the SEMI F47 standard as well.

Basically, this standard requires that the AC-DC power supply that is used in semiconductor production, or in other factory automation equipment, continue to provide the required output voltage and current, even if the input voltage dips below its specified limits. As can be seen in the chart below, in the blue area, the basic specs require that the power supply perform normally even if the input voltage sags down to 50% of its nominal voltage for up to 200 ms, or sags to 70% for up to 500 ms, and sags to 80% for up to one second. Since this sag percentage refers to the nominal line voltage, this means for example that with a nominal 220VAC input, the AC voltage can sag down to 50% or 110VAC for up to 200 ms, down to 70% or 154VAC for 500 ms, and down to 80% or 176VAC for up to one second.

There are additional sag ride-through “recommendations” within the latest version of the standard, which is the SEMI F47-0706 (these recommendations are not requirements) that includes operation of the power supply with 0% input power (no power) for up to 20 ms. This recommendation can be accommodated by insuring the selected power supply has a “hold-up time” specification of 20 ms or longer. Other newly recommended thresholds within the SEMI F47-0706 include sags of 80% for 10 seconds, and continuous sags of 90%. Most power supplies that meet the previous versions of this standard, the SEMI F47-0200, and have a hold-up spec of 20 ms or greater, should be able to meet the new recommendations (not requirements) as well.

The simplest and lowest cost method of complying with the SEMI F47 standard is to use a power supply with a universal input, such as 90 to 264VAC, and operate it from a 220VAC or higher line input. In this way you automatically meet and exceed the standard since this type of power supply can operate down to 90VAC (even lower than the 50% line sag spec of 110VAC). Note that this method does apply to auto-strap power supplies.

Another way of meeting the SEMI F47 is to draw less power than the supply can normally provide (de-rate the supply). If you do this, always check with the manufacturer to confirm that your reduced load will allow the supply to fully meet the SEMI F47 standard. This may require extra testing to confirm compliance, either by the power supply manufacturer or the end-product OEM. Alternatively, the power supply manufacturer may be able to modify the supply to meet the SEMI F47 standard.

Some factory automation equipment require the use of SEMI F47 “certified” power supplies, which means the supplies were tested by an outside agency or laboratory and found to fully comply with the standard (similar to UL certification). If this is a requirement, always look for supplies that have existing certifications from a reliable manufacture, because the cost of getting this type of certification can amount to $2,000 or more. There is a grandfather clause in the updated standards that provides for equipment that was tested or certified under the previous versions of the standard to not require re-testing or re-certification.

Many industrial-type power supplies are designed and/or certified to meet the SEMI F47. These supplies may be a bit more expensive, but it will be the lowest cost solution, especially if you compare it to the cost of adding an external constant voltage transformer or UPS to the input of the power supply.

Power supply manufacturers such as TDK-Lambda offer supplies that are SEMI F47 certified and supplies that operate with a wide universal input of 90 to 264VAC. In addition, modified supplies can be provided that meet this and other prevailing power supply specifications.

Ripple & Noise Specs and Measurements

2008-11-21T10:28:00.000-08:00

AC-DC power supply and DC-DC converter datasheets should always include output “Ripple & Noise” specifications. The Ripple & Noise spec is sometimes referred to as Periodic And Random Disturbances or PARD. The following drawing shows how ripple and noise may look when viewed on an oscilloscope that is attached to the output of a typical switchmode power supply.

The output “Ripple” frequency is primarily determined by the switching frequency of the power supply. The higher frequency “Noise” spikes are generated by the fast rise and fall times of the pulses associated with the switching and rectification components of the power supply. Typical ripple and noise specs are defined as peak-to-peak measurements in mV units.

Ripple & Noise Measurements
Unfortunately, there is no universally accepted method for measuring ripple and noise. It seems that each manufacture, and sometimes different products from the same manufacturer, may have varying methods for these measurements. In some cases the bandwidth of the test oscilloscope is defined as 20MHz or 100MHz. In addition, added components such as capacitors, resistors, twisted wires, and/or coax are sometimes required in the test set-ups that are defined by the manufacturer. In order to meet the power product’s specified ripple and noise specs, care must be taken to follow the manufacture’s defined test set-up. There are a few standardized methods for ripple and noise measurements; one of which is the JEITA-RC9131A standard.

Fig 1: JEITA-RC9131A Ripple & Noise Test Set-Up

The above drawing (Fig 1) shows the test set-up per JEITA-RC9131A. This standard defines a custom oscilloscope connection comprised of a length of 50 ohm coax that is connected to the output of the power supply with the other end terminated at the scope with a 50 ohm resistor in series with a 4700pF capacitor. Notice that the coax is attached to the output of the power supply within 150mm or 6 inches of the output terminals and has two added capacitors (22uF electrolytic and 0.47uF film type) soldered across those points. The 50 ohm coax should not exceed 1.5M or 5 feet in length. All coax pigtails and added component’s lead lengths should be kept to a minimum to prevent pick-up of radiated noise.

Other Measurement Precautions
Some ripple and noise measurements can be made with the use of a standard oscilloscope scope probe that has been modified by removing the plastic tip cover and ground clip wire and replacing the ground connection with a short length of bare copper wire that is wound around the probe’s ground ring. In this way the probe’s tip and ground connections are kept to a minimum length, thereby reducing the chance of the ground lead acting as an antenna and picking up radiated noise signals, which can result in out-of-spec measurements.

Figures (a), (b), and (c) below show incorrect set-ups for ripple and noise measurements.

When making ripple and noise measurements a standard load should be used. This precaution is to prevent any noise from the power supply’s normal system load, which may contain noisy digital or RF circuits, from feeding noise back to the output of the supply, which again can result in out-of-spec test measurements. In some cases, to reduce ground loops, it may be necessary to isolate or float the oscilloscope from the AC source by plugging it into an isolation transformer.

Unless otherwise stated, the ripple and noise specifications are usually based on measurements taken while operating the power supply with its nominal input voltage, at the rated output voltage and current load, and at or near room temperature (typically 72°F to 77°F).

Over Current Protection in Power Supplies & Converters

2008-09-19T10:45:00.000-07:00

Most AC-DC power supplies and DC-DC converters have internal current-limiting circuits to protect the power device, and to some degree its load. The majority of over-current-protections include an automatic recovery feature. In practice, the current limit feature typically starts operating when the output current excedes it maximum rating by 10 to 20%.

In many cases, should an overload (e.g., short circuit) be allowed to exist for a prolonged period, it can reduce the product’s field life by temperature stressing the electrolytic capacitors, and in extreme cases, it can damage the user’s printed circuit traces. Therefore, always check the power supply’s “Instruction Manual” to be sure you understand the precautions associated with the power product’s over-current-protection feature. Also, if the power product has an Output Good signal, this can be used as an indication that the power supply is either faulty or could be in an over-current mode.

There are a number of ways to implement over-current-protection (OCP), and below are descriptions of the most common methods.

Fold-Back Current Limiting: When this method is employed if an overload condition exists, the output voltage and current reduce to safe levels. As can be seen from the following curve, should an overload occur the supply will provide current up its current limit point (aka ‘knee’), and then the output current will fold-back to a lower value as the output voltage reduces towards zero.

This technique is employed in linear power supplies because it reduces the strain on the supply’s internal power devices to minimum. One drawback of fold-back current limiting is that if the supply turns on into a heavy capacitive load, it could latch-up at a reduced current before reaching its full output voltage. Depending upon the design, recovery from a fold-back current limit condition can be automatic, or after a built-in time delay when the overload condition is removed.

Fold-Back Current Limiting

Fold-Forward Current Limiting: In this method, when an overload is sensed the output voltage reduces towards zero, but the current increases. When driving motors, pumps, or highly capacitive loads, employing a fold-forward current feature can help overcome the electrical inertia of these loads. Recovery from a fold-forward current limiting situation is usually automatic when the overload is removed.

Fold-Forward Current Limit

Constant Current Limiting: In this method, should an overload occur, the output current stays at its limit point and the output voltage reduces towards zero in a somewhat linear fashion. This technique is used in many switchmode power supply designs. Typically, the supply will automatically return to its normal output voltage when the overload condition is no longer present.

Constant Current Limiting

Current Limit Shutdown: In some power supply designs, when an overload occurs the power supply will begin to go into a constant-current limit mode, but when the output reaches a preset reduced voltage, the supply will shutdown. Recovery from this condition can be automatic or require recycling of the input power.

Hiccup Mode Current Limiting: Some low power supplies have what is termed a hiccupcurrent-limit feature. As the name implies, if a current limit is sensed, the supply will reduce its output voltage to zero and then, after a short time, it will attempt to provide its normal voltage. These On-Off attempts at operation are referred to as a hiccup-mode. Should the overload condition be removed, the supply will again operate normally.

Peak-Current Power Supplies
It should be mentioned that some power supplies are designed specifically to provide large peak-currents, which can range from 200 to 300% of the maximum current rating for a short duration, without going into a current-limit condition. These are especially useful when powering loads that include electric motors such as computer hard drives, fans, actuators, pumps, etc. When using this type of power supply it is important to limit the “average power” that is delivered to load. More information about peak-current-rated supplies will be provided in a separate article.

Power Supplies with Wide Range Adjustable Outputs

2008-08-05T09:25:00.000-07:00

For some power supply applications it is desirable to change the output voltage over a wide range. There are a number of ways to control the output voltage of power supplies that are designed to provide wide adjustment ranges. Remotely adjustable output voltages can be implemented by using one of the following methods.

Variable Voltage Control
In this case an external variable control voltage (e.g., 1-6V) is connected to the designated input of the power supply, sometimes called the PV input. As the input control voltage is varied it will cause the output voltage to change in a fairly linear fashion over a wide range (e.g., 20% to 120% of the nominal output voltage). For some applications this is a low cost method of providing a programmable power supply. Below are diagrams showing an example of this type of remote voltage adjustment for Lambda’s HWS/PV and SWS-L series of power supplies.

External Variable Voltage Control (1-6V)

Output Voltage Change (20-120%) with Ext. Variable Voltage Control (1-6V)

Variable Resistive Control
Some power supplies can be remotely adjusted via a variable resistive control (external potentiometer). This method has the advantage that an external voltage is not required since an internal Ref. voltage is provided by the supply. As the resistance changes, it will cause the output voltage to change in a non-linear fashion over a wide range (e.g., 20% to 120% of the nominal output voltage) as shown in the diagrams below (Lambda’s HWS/PV series). For some applications this is a low cost method of providing a programmable power supply.

External Variable Resistive Control (50k ohm pot.)

Output Voltage Change (20-120%) with Ext. Variable Resistive Control (50k ohm pot.)

Serial Digital Control
Programmable Power Supplies can be remotely controlled via a serial digital port such as RS232 or RS485. Both the output voltage and current can be controlled from zero to the maximum output ratings. In addition, alarm signals from the supplies can be sent back to the remote computer or controller via the same digital link. Programmable Power Supplies are more expensive than wide adjustable supplies mentioned above, but they have a large array of local and remote control features that are not found elsewhere. Lambda’s ZUP series is a good example of a feature-rich Programmable Power Supply.

Up to 31 ZUP Series Programmable Supplies can be Remotely Controlled via RS485 Interfaces

What is Remote Sensing?

2008-07-03T11:21:00.000-07:00

Most medium to high power AC-DC power supplies and DC-DC converters have “Remote Sense” connection points (+/- Sense) that are used to regulate the supply’s output voltage at the load. Since the cables that connect a power supply’s output to its load have some resistance, as current flows it will cause a voltage drop in the cables. Since it is best to regulate the voltage at the load site, the use of the two Remote Sense wires connected from the supply to the load will compensate for these voltage drops.

Typical remote sensing circuits are capable of correcting from 0.3V to 1.0V of voltage-drop in the output cables. However, to be sure, always check your power supply’s instruction manual to determine the maximum remote sense compensating range. If the voltage drop across the cables exceed the range of the remote sense circuits, this can be remedied by either reducing the length of the cables or increasing the size of the cable’s conductors. The remote sense leads carry very little current, so light gauge wires can be used. Steps should be taken to ensure the remote sense wires do not pick up noise by either twisting the +/- Sense wires together and/or shielding the wires from noise. It is important to observe the correct polarities, i.e., the +Sense wire should connect at the load to the +V output cable and the –Sense wire should connect at the load to the –V output cable. Refer to Figure 1.

Fig. 1: Power Supply with Remote Sense Wires Connected at the Load

When not using the remote sense feature, Local Sense (LS) connections should be used. In this case the +/-Sense points should be connected to their corresponding output or local sense terminals at the power supply (+Sense to +V output or +LS and, –Sense to –V output or -LS). Most power supplies are shipped from the factory with these “Local Sense” connections in place.
Refer to Figure 2.

Fig. 2: Power Supply with Local Sense Jumpers Installed

Harvesting More Energy without Building More Power Plants

2008-06-13T13:38:00.000-07:00

An AC-DC switchmode power supply without Power Factor Correction (PFC) can draw approximately 950 Watts from a typical 115VAC wall socket protected by a 15A circuit breaker before exceeding the UL mandated limit of 12A. A simple load like a toaster can draw almost 1400 Watts. The difference between the two is due to the higher Power Factor (PF) of the toaster, which presents a resistive load to the power line. If we correct the Power Factor of the switchmode supply it can then draw about as much power as the toaster, allowing it to provide more output power to its load from the same 115VAC/15A wall socket.

What is Power Factor Correction (PFC)?
Power Factor (PF) is technically the ratio of real power consumed to the apparent power (Volts-RMS x Amps-RMS), and is expressed as a decimal fraction between 0 and 1. PF is traditionally known as the phase difference between sinusoidal voltage and current waveforms. When the AC load is partly capacitive or inductive, the current waveform is out of phase with the voltage (Fig. 1, the dotted line is the current waveform). This requires additional AC current to be generated that isn't consumed by the load, creating wasted I2R (wattage) losses in the power lines.

An electric motor is inductive, especially when it is starting. The current waveform lags behind the voltage waveform, dropping the PF to well below 1 (similar to Fig. 1). This is why many motors have “starting” capacitors installed to counteract the inductance, and therefore correct the PF during motor startup.

Figure 1. Voltage and current waveforms are sinusoidal but out-of-phase; PF <1.

A simple resistive load has the highest PF of 1. An AC voltage across the resistor causes an AC current which is identical to and in-phase with the voltage waveform (Fig. 2).

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase; PF=1.

A switchmode power supply when viewed as an AC load is neither capacitive nor inductive, but non-linear. A switchmode supply conducts current in short pulses or spikes that are in-phase with the line voltage (Fig. 3). The product of “Volts-RMS x Amps-RMS” is considerable higher than the real power consumed, and thus the PF is much less than 1, typically around 0.65 or less.

Figure 3. Voltage waveform is sinusoidal, current waveform is non-sinusoidal but in-phase; PF<1.

Improving the Power Factor
Low Power Factors can be improved via Power Factor Correction (PFC) circuits. The types used for switchmode power supplies “smooth out” the pulsating AC current, lowering its RMS value, improving the PF and reducing the chances of a circuit breaker tripping. There are two basic types of PFC: Active and Passive. Active PFC is more effective, a bit more expensive, generally integrated into the switchmode power supply, and can achieve a PF of about 0.98 or better. Passive PFC is less expensive and typically corrects the PF to about 0.85.

Harvesting Additional Output Power
To determine just how much more power is available from the AC line and a power supply with PFC, the user needs to understand the following equation, which defines the amount of output power (Pout) available from a switchmode supply:

Pout = VL-RMS x IL-RMS x PF x Eff

For example, UL limits a system's line current to 80% of the circuit breaker's rating. For a typical 15A breaker, 12A is the maximum allowed, and the best-case power available is therefore 120VAC x 12A = 1440 Watts. Referring to the above equation, here are two examples of supplies with different power factors:

A switch-mode supply with 0.65 PF and 85% efficiency can only deliver (120 x 12 x 0.65 x 0.85) = 796 Watts (Pout).
However, if the power factor is corrected to 0.98, the same power supply can now deliver (120 x 12 x 0.98 x 0.85) = 1200 Watts (Pout), a 51% increase.

From the examples above, it can be seen that by employing power supply’s with PFC, more output power can be delivered to the OEM’s end-product, without the need to increase the AC power wire sizes, increase the circuit breaker’s rating or draw more current from the power plants. Thus, PFC has a significant effect on our environment relative to reducing the pollutants coming from electric power plants.

Meeting International Regulations
Since switchmode power supplies without PFC tend to draw the AC line current in a non-linear fashion, many unwanted harmonic currents are generated and reflected back on the AC power lines. These reflected harmonic currents are “pollutants” to the power grid that have a negative affect on other devices connected to the same power lines. These unwanted harmonic currents can range in frequency from the 100 Hz on up to over 2,000 Hz, and have a direct relationship to the Power Factor of switchmode power supplies.

An important reason to have PFC within your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. These regulations were put in place to maximize the available power generated each day by electric power plants located worldwide. The intent was to make the most of the power we have today without expanding our carbon footprint. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies. Power supplies with PFC circuits that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or better.

Summary
As previously mentioned, "Power Line Harmonics" are created whenever the line current is not a pure sinewave, as is the case with a switchmode power supply's input, which tend to have “pulsed” currents (Fig 3). Measuring power line harmonics is a mathematical means to describe a complex waveform's Power Factor by resolving it into a fundamental frequency and its many harmonics.

The harmonic currents do not contribute to the output load power, but cause unwanted heating in the wall socket, wiring, circuit breaker, and distribution transformers, resulting in wasted energy. When personal computers first hit the mainstream market, their power supplies lacked PFC. As a result, circuit breakers that seemed to be sized correctly for the load were tripping for no apparent reason. After investigation, it was determined that the poor power factor of the PC’s power supplies was the culprit.

Today new “green initiatives” are dictating that personal computers include power supplies which must have Power Factor Correction (PFC) and high efficiencies. PFC significantly reduces harmonics, resulting in almost a pure “fundamental” current frequency that will be in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial attenuation of harmonic currents. The vast majorities of AC-DC power supplies manufactured by Lambda Americas employ active PFC, are in accordance with EN61000-3-2 and provide typical power factors in the range of 0.97 to 0.99.

Lambda’s RTW Series of switchmode power supplies range in output power from 50 to 300-Watts. All units include PFC and have typical Power Factors of 0.98 (meets EN61000-3-2). Efficiencies reach up to 89%.

Choosing an Input EMI Filter for a Power Supply

2008-05-19T15:42:00.000-07:00

There are two primary functions that an input EMI filter can perform:

Minimize outgoing electrical noise to avoid interfering with neighboring equipment
Attenuate (reduce) incoming electrical noise that could damage the system

Regarding outgoing noise, although most power supplies meet the governmental regulations for EMI, noise is additive and if there are multiple power supplies or high speed processor boards, it can result in a failing grade.

If the noise is only slightly out of specification, then a (lower cost) single stage filter may suffice. If the noise is considerably out of specification then a higher performance two stage filter will be required.

An example of these would be Lambda’s MA (single stage) and MX (two stage) filters. Look for the terms “wideband” or “low frequency attenuation” in the features.

Incoming electrical noise is usually in the form of a spike or burst of energy. It can be generated from natural causes such as a lightning storm or man made by a large piece of industrial equipment.

This type of filter may have “high pulse attenuation” listed as a feature and will have internal values optimized to reduce these potentially harmful spikes from reaching the power supply. The filter will also have some outgoing noise attenuation, but may not be as effective. An example would be Lambda’s MZ series of filters.

Benefits of Environmentally Friendly Power Supplies

2008-04-01T16:17:00.000-07:00

Increasingly, Lambda’s customers are asking for and receiving Environmentally Friendly (EF) or “Green” power supplies. What constitutes an EF/Green power supply? There are a number of factors which contribute to a power supply being considered EF/Green. These include: RoHS compliance, electrical noise (EMI) suppression, high efficiency and power factor correction (PFC).

RoHS Compliance: Lead, mercury and other “hazardous” elements and chemicals used in the production of electronic devices have been identified, and by-law (since July 2006), have been banned or severely limited to their content in these devices, by the European Union (EU). These limitations are spelled out in what is called the RoHS Directives (Restriction of Hazardous Substances). Currently, all products sold to the countries within the EU must comply with the RoHS Directives. Many of the States here in America have adopted similar restrictions. Lambda has been a leader in providing RoHS-compliant power supplies even to their US customers who may not currently require them. This represents Lambda’s dedication to making the earth a greener and cleaner place to live. Over 99% of Lambda’s power products are RoHS-compliant with a few exceptions being for those industries that are RoHS-exempt such as specific military applications.

EMI Suppression: All electrical devices contribute in some way to the electrical noise that “pollutes” our power lines and airways in the form of unwanted noise spikes and radio interference. These unwanted noise generators are restricted to the amount of EMI (Electro-Magnetic Interference) they are allowed to emit by strict standards that are produced and maintained by the FCC and international organizations (e.g., IEC, EN). Power supplies are among the electrical devices that have an inherent electrical noise problem (both conducted and radiated) which must be addressed by the power supply designer and manufacturer. Without properly designed internal EMI filters and RF shielding, power supplies would become huge polluters of our electrical and electronic environment. Lambda power supplies comply with the most rigorous EMI suppression standards. For example, the very restrictive FCC Class-B EMI compliance is available, as standard, on many Lambda power supplies.

High Efficiency: The efficiency rating of a power supply is measured by comparing the AC power going into the power supply divided by the DC power coming out of the supply. For example, if 100 watts of AC power is used by a power supply for it to provide 90 watt of DC output power, the efficiency of the supply is calculated by dividing 90W by 100W, which equals 0.90 or 90% efficiency. When comparing a 75% to a 90% efficient power supply, the savings in electricity usage and wasted energy (in the form of heat) is quite significant.

Power Factor Correction (PFC): Modern switchmode power supplies should include active Power Factor Correction circuits in order to maximize the AC power that is used by the supply. The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power (Volts-rms x Amps-rms) and is a decimal number between 0 and 1.0. If left uncorrected the Power Factor (PF) of switchmode supplies will generally be around 0.65 or less. With active PFC, switchmode power supplies can achieve power factors from 0.96 to 0.99. Without PFC, switchmode supplies would draw their AC line current in the form of spikes or pulses, instead as a clean sinewave; the net result being that the AC power wires, circuit breakers and power generating plants need to be sized larger than if they are driving products that contain PFC power supplies.

So what does High Efficiency and PFC mean relative to being Environmentally Friendly?
Most electricity used within the USA is generated by burning fossil fuels such as oil, natural gas or coal. It has been shown that there is a green-house-effect that is increasing at an alarming rate due to the large amount of carbon dioxide that power generation plants produce. So, whenever we can reduce the amount of electricity used, we are contributing to a cleaner environment, a reduction of CO2 in the atmosphere and our dependence on, and the transportation costs of foreign oil from the OPEC countries (the oil tankers burn diesel fuel, which further adds to the pollution).

In addition, there can be significant money savings to the power supply user. Basically the savings of just 1-watt dissipated in the power supply = 365 days x 24 hrs x 1W = 8.76kW/hrs per year. If electricity costs $0.30 per kW/hr, that would amount to $2.63/year per each watt saved. If we boost the efficiency of a power supply by only 5% on a 150W unit from 85% to 90%, that saves 7.5W [(150W x 0.15 = 22.5W) - (150 x 0.10 = 15W) = 7.5W]. That translates to $19.73 per year savings (7.5W x $2.63/year = $19.73/year), which is more than the purchase price of the power supply over its typical lifespan (4 to 5 years), plus, we have reduced the amount of carbon dioxide in the air. If we consider an OEM who uses a 90% efficient power supply (instead of 85%) in their end product and produces 100,000 units/year, the “total electric power savings” would amount to 100,000 units x 7.5W = 750,000 watts per hour, a significant power savings.

Furthermore, the higher the efficiency of the power supply, the less power is wasted in the supply and the less energy is needed to remove that wasted energy (in the form of heat), by cooling the supply or the room in which the equipment is used (via use of electromechanical fans, blowers, air conditioning, etc). The combination of high efficiency and PFC in power supplies allows the use of smaller gauge power distribution wires, lower-rated circuit breakers and fewer power generating plants (air polluters).

Lambda’s HWS Series, 1000-Watt Switchmode Power Supply is fully RoHS-Compliant, Meets FCC Class B EMI, has integral active PFC (Power Factor =0.98) and has a typical operating Efficiency of 88%.

In Summary: As mentioned above, there are many factors that comprise an “environmentally friendly/green” power supply. And, the benefits to the end-user are significant. Lambda is a leader in the provision of these advanced power devices and our international presence insures that we will continue to be at the forefront of these “Green & Clean” power products.

Droop Mode Current Share

2008-03-05T11:15:00.000-08:00

If two power supplies are to be connected together to produce more power or share the load, then a parallel capable model should be selected. Lambda’s DPP100, 120, 240 and 480 models are all parallel capable. On the front of each power supply is a small black switch. For parallel operation this switch should be set to “parallel” (Fig. 1).

In single mode the load regulation (the amount the output voltages chances with load) is minimal, the difference being less than 0.24V from zero load to full load for a 24V output power supply.

In parallel mode that load regulation is artificially increased to 1.2V using internal circuitry (Fig. 2).

The extra voltage drop or “droop” is proportional to the load drawn, so that when two or more power supplies are connected in parallel the output load is shared between the power supplies. If on of the paralleled power supplies tries to provide more current, its output will droop slightly and the other supplies will balance.

For optimal performance, all power supplies should have their outputs set to the same voltage.

What is PFC and why do I need it?

2008-02-08T09:07:00.000-08:00

Switchmode power supplies without Power Factor Correction (PFC) tend to draw the AC input current in short bursts or spikes relative to the line voltage, as shown in Fig. 1. The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power (Voltsrms x Ampsrms) and is a decimal between 0 and 1.0. If left uncorrected the Power Factor (PF) of switchmode supplies will generally be around 0.65 or less.

Figure 1. Input of switchmode power supplies without PFC. The voltage waveform is a sinewave and the current waveform is a pulse or spike. PF<1

The Power Factor can be improved by using PFC circuits. These circuits “smooth out” the pulsating AC current, improving the PF, and reducing the chances of a circuit breaker tripping prematurely. There are two basic types of PFC, passive and active. Passive PFC circuits are less expensive and typically can correct the PF to about 0.85. Active PFC circuits are the most popular, are built into the switchmode power supply and can increase the PF to 0.98 or higher. The closer the PF comes to being 1.0, the better the performance of the power supply. Ideally, we want to end up with the input voltage and current waveforms being sinusoidal and in phase with each other as shown in Fig. 2.

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase. PF=1.

PFC is Required by International Regulations
An important reason to have PFC in your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies.

For example, the first harmonic is the primary input frequency, typically 50 Hz for the EU countries. The third harmonic is 150 Hz, and the 39th harmonic is 1,950 Hz. These unwanted harmonic currents have a direct relationship to the Power Factor of switchmode power supplies. Therefore, power supplies that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or higher.

PFC Increases the Supply’s Output Power Capability
The PF, much like the supply’s efficiency rating, determines the amount of useful power a switchmode power supply can draw from the AC line and then deliver to its output load. Specifically, the formula that determines this is:

VLrms x ILrms x PF x Eff = Pout

As an example, if a power supply is operating off of 120VAC line, which is protected by 15A circuit breaker, UL guidelines say you should not draw more than 12A. So, using the formula above, we can compare two power supply examples with different Power Factors, as follows:

Example A: No PFC, PF = 0.65, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.65 x 0.85 = 796 Watts Output Power

Example B: PFC used, PF=0.98, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.98 x 0.85 = 1200 Watts Output Power

As can be seen above, the power supply in Example B (with PFC) can deliver 404 Watts or 51% more power to its output load than the non-PFC supply, a significant increase.

Why do I need PFC?
A power supply with PFC can supply higher output load currents than those without PFC. PFC significantly reduces the AC current harmonics, leaving mainly the “fundamental” current frequency that is in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial reduction of harmonic currents. The vast majority of AC-DC power supplies manufactured by Lambda Americas has active PFC, is in accordance with EN61000-3-2 and provides typical power factors in the range of 0.97 to 0.99.

What does 1U, 2U or 3U mean?

2008-01-03T16:16:00.000-08:00

Many rack-mounted power systems are specified as being 1U, 2U, 3U, etc. What does this mean? For electronic equipment racks (e.g., 19 or 23 inches wide), the term 1U is used to define one rack unit of height.

1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75”, which equals 3.5-inches high. A 3U height would be 3 x 1.75” = 5.25-inches.

It should be noted that the 1U, 2U, 3U, etc., heights are maximum dimensions. In order to allow for mechanical tolerances and to provide some space between panels, typically, for each 1U of height manufacturers may deduct about 0.03” (see Photo #1). For example, a 2U panel, which has a nominal height of 3.50” may be only 3.44” high [3.50” – (2 x 0.03”) = 3.44”].

Individual power supplies are sometimes mounted within rack-mounted enclosures that require integral power. In these cases, the power supply needs to be a bit shorter than the equipment’s overall height to allow for the top and bottom covers. So a 1U high enclosure-mountable power supply needs to be shorter than 1.75-inches; a 2U enclosure-mountable supply needs to be shorter than 3.5-inches, and so forth (see Photo #2).

Examples

Photo #1: This 19” rack-mountable power system can hold up to 3 plug-in, hot-swap and redundant power supplies. The enclosure with mounting ears is 1.72” high (= 1.75” minus 0.03”) and is therefore considered 1U high.

Photo #2: This 1000-watt switch-mode power supply is 3.25” high and, therefore, can be mounted in a 2U rack-mountable enclosure, which can vary between 3.44” to 3.50” high.

Since we still live in an English and Metric measurement world, here are a couple of handy conversion factors: 1 inch = 25.4 millimeters (mm), 1 mm = 0.03937 inch

As a side note, Lambda ran a clever ad campaign that those who understand what “1U” or “2U” really means would appreciate. Here is a copy of that ad, which hopefully you will find humorous.