Factors in Selecting Programmable AC Power Sources

Many electrical testing applications require a stable power source. Instead of having a custom design for each application, the programmable power supply (PPS) allows users to adjust the output frequency (F), voltage (V) or current (I) to meet the testing requirements.

There are varying design approaches for converting an electrical energy source to one which is programmable for F, V or I. Traditionally, people used motor generators to convert the frequency and variable transformers to change the voltage. But solid-state designs are gaining momentum. Some programmable power supplies are transformer-based while others are transformerless. Which approach is better? And what role does standard compliance play in the PPS design in relation to product quality and performance? This article provides an overview of the PPS design and selection guidelines for various applications.

Definitions of technical terms:

• Clean power source: an electrical power source converted from the electrical grid system to stable, clean power. If there is noise on the grid system or the voltage or frequency fluctuate, the power source will rectify from AC to DC and rebuild to AC or DC form with regulated and low noise output.
• Programmable power supply (PPS): the function of a PPS is to convert the input electrical power to the stable, filtered AC or DC power source to meet the specific requirements of the load or testing units. A PPS can be used to set up steps with voltages, frequency, or duration and run the steps automatically. It can also be connected to a remote PC via interfaces such as RS232, Ethernet, GPIB, USB, or RS485.

Selection Considerations and Guidelines

Aerospace/Defense vs. Industrial Applications

Typically, aerospace and defense applications have more demanding requirements than those of industrial design. Therefore, PPS designs based on commercial-off-the-shelf (COTS) components demand high mean-time-between failure (MTBF) and military standard compliance. Aerospace and defense PPS frequencies typically fall in the 400 Hz range rather than the 50 Hz/60 Hz range found in industrial applications. But the use of 400 Hz AC with 28VDC is also changing. Some aerospace/ defense equipment uses up to 800 Hz AC and 270VDC.

Size, weight and power (SWaP) considerations also affect the design throughout. Especially for aerospace and defense platforms, it is critical to have the PPS be as small and lightweight as possible. However, high-performance use cases usually require a more sophisticated design and/or larger-size components. Therefore, it is a constant challenge to design products which optimize for both SWaP requirements and overall cost considerations.

Space and defense manufacturers have very tight requirements for equipment such as radio, radar, and other communication receivers that may be simultaneously used in the same environment. Therefore, it is important that all the equipment stay within design specifications to meet the electrometric parameters. Ensuring the equipment will not be harmed by electrical noise caused by noise spikes is crucial. At the same time, one must make sure that the PPS will not cause excess electromagnetic interference (EMI) to the equipment which is in the same environment as other equipment being tested.

Defense applications, more so than industrial, require electrical or galvanic isolation. One of the sources of electrical noise is ground loops. To avoid ground loops, the use of an isolation transformer is necessary to stop the ground loop from the input to the output on the equipment being tested. Here is how it works. When two devices are connected, they may have different ground potential or currents (commonly known as stray currents) caused by an AC power source. The stray currents can damage a device at times. By using the isolation transformers, galvanic isolation can be achieved, and the problems can be avoided.

Rotary vs. Static Frequency Conversion

Under the industrial category, applications can be subdivided into research laboratory use, automatic test equipment (ATE), test labs and others. Often, each has a need for AC power output with different but stable frequencies (25, 50, 60, 100, and 400 Hz). With one fixed AC power source, a conversion needs to take place to achieve this.

There are two different design approaches to convert the frequency, namely, rotary frequency conversion and static frequency conversion. Rotary frequency conversion is also known as the motor generator (M-G) set conversion. In this approach, a motor and generator are attached to the same shaft. The M-G set receives electricity from a power source. The motor converts the energy into a mechanical form to turn the generator, which in turn converts the energy to the final electrical energy. After this electrical-mechanical-electrical conversion, the final output will be the desired voltage and frequency. The conversion includes multiple forms:

• AC to AC (to a different frequency)
• AC to AC (fixed to regulated and/or variable voltage output)
• AC to DC (use AC motor and a DC generator)
• DC to AC (use a DC motor and an AC generator)

On the other hand, static frequency conversion uses solid state components without any motors or generators. This type of test equipment is referred to as either static or solid-state frequency conversion (SSFC), but they represent the same type of test equipment. An SSFC-based system first converts AC input to DC, and then reconverts it to AC again using the popular insulated gate bipolar transistors (IGBTs) technology. The results are the adjusted frequencies and voltages required by the device loads.

Traditionally, M-G set was the solution of choice until the arrival of solid-state based solutions. IGBT technology was first introduced in 1980 and is a key part of the solid-state solution. But using IGBT took an expensive initial investment and included technological challenges. As IGBT matured, the static frequency conversion started to gain recognition. Because of the many advantages of static over rotary solutions, more and more applications are being used with power supplies which employ the static design.

How does IGBT work? It is a solid-state device that uses the common power device MOS gate and combines it with minimum conduction loss, and is a perfect fit for high current and voltage applications. It enhances power circuitry design by constructing an N-channel power MOSFET with a p-type substrate. As a result, the IGBT-based PPS has proven to be very efficient. Because the solid-state components are much smaller and more efficient than the motor generator type products, the overall costs, including capital investment, size, and weight have been reduced. Additionally, there are no moving parts in the static design, the noise pollution is much lower, and it does not require routine inspection or replacement of bearings used in motors, further reducing maintenance costs.

Perhaps most important is the environmental impact. Solid-state IGBT PPS do not pollute the environment. In some rural areas where diesel fuel is used to generate the needed frequencies and voltage, it is more economical to run a power cable to the static power supplies. The static solution using IGBT technology overcomes the disadvantages associated with conventional rotary solutions.

To summarize, the benefits of static frequency conversion include:

• Reduction of electrical noise caused by the motor and the generator (M-G set)
• Higher output frequency stability
• More suitable for remote monitoring and control
• Reduction of maintenance costs: the rotary M-G set requires regular replacement of bearings and belts, while static frequency conversion only requires cleaning fans.
• Higher overall system efficiency

The benefits of static voltage conversion include:

• Prevents harmonic noise from entering the electrical load end
• Faster response time for voltage adjustment: The rotary M-G set takes longer to change the revolution.
• Much better output regulation and control
• Reduction of maintenance costs, similar to the above: The rotary M-G set requires regular replacement of bearings and belts, while static only needs cleaning of fans.

Finally, if the electrical loads need simultaneous adjustment of frequency and voltage, using the rotary approach will require the M-G set and an additional variable transformer, increasing costs. The static approach can provide 47~63 Hz for industrial use and 400 Hz /800 Hz for military use with 0~300 V.

PPSs With and Without Output Transformer Design

Designing a PPS involves tradeoffs when considering designs which include an output transformer and those which do not. In the case of a PPS designed without an output transformer, the power supply can provide wider frequency range and more features on power line disturbance testing (e.g., transient test or DC offset test, etc.) Due to having no galvanic isolation on the output, damage can occur to the devices if there is a malfunction on the test unit or if reverse current goes back to the power supplies.

It is important to point out that some applications must have transformers, as isolation is critical when testing equipment such as refrigerators, motors, compressors, and air conditioners. Programmable power supplies that include an output transformer are also good for different output voltage range requirements (ex: 0-690VL-L or 0-400VL-N), as it can regulate the output in the second end of the transformer.

Overall, a PPS design that includes an output transformer is more robust with regard to inductive load, while those without an output transformer more closely match the needs of transient testing, wider frequency applications, or harmonic testing.

Table 1 compares programmable power supplies (PPS) that include an output transformer to those that do not.

Table 1: Comparison of programmable power supplies with and without output transformers

Importance of Standard Compliance

Other than the UL listing in relation to product safety, the other relevant standards are IEC 61000-4-11 (from International Electrotechnical Commission) and MIL-STD-704. Standard compliance assures product quality and performance outlined in the specification. Without such references, it is more difficult to select and evaluate a PPS with good power quality and performance. Therefore, selecting a PPS with such compliance is strongly recommended.

(For clarification, there is another standard EN 61000-4-11 released by CENELEC, the European committee for Electrotechnical Standardization body, which is recognized by the European Commission. The technical portions of the two specifications are identical.)

IEC 61000-4-11 is an international standard which defines a set of testing methods applied to 50 Hz/60 Hz AC equipment including AC/DC power supplies with input current not to exceed 16 A per phase. (Testing as it relates to 400 Hz is expected to be addressed in a future edition of the standard.) These tests cover parameters relating to voltage dips, interruptions and variations. An IEC 61000-4-11-compliant PPS will be able to withstand these voltage dips, interruptions and variations. Other requirements include specification of inrush currents. The PPS needs to provide inrush currents up to 500 A for 220 V/240 V and 250 A for 110 V/120 V accordingly.

MIL-STD-704 is a military standard developed by the Department of Defense to ensure airborne military equipment will perform well with the MIL standard-compliant PPS. While most people refer to the MIL-STD-704 as a standard, it is a reference guide which outlines what the users can expect. However, most PPS manufacturers will follow the specification to deliver what is expected.

Similar to the IEC specification, the 33-page MIL-STD-704 document provides detailed information relating to the power output behavior. In the MIL documentation, recommended values and the AC wave forms of the power output in steady state and transient are presented. Steady state characteristics include steady state voltage, voltage unbalance, modulation, phase difference and distortion, steady state frequency and frequency modulation. The transient characteristics include peak voltage, voltage transient and frequency transient.

The PPS is expected to function normally most of the time. When there are conditions such that the power output is in a transient stage behaving abnormally, whether it is an over or undervoltage, the device on the airborne system should not suffer damage.

Summary and Conclusions

In this article, the guidelines for programmable power supply (PPS) selection have been discussed. They include rotary vs. static frequency conversion, transformer-based vs. transform-less as well as the significance of standard-based compliance. Two types of frequency conversion are available to achieve the same goal. The conventional solution is rotary frequency conversion. The static method based on the insulated gate bipolar transistors (IGBTs) technology has many advantages over the conventional rotary method, such as lower overall costs, including capital investment and maintenance costs. Products are smaller in size and weight. It is “green” without impacting the environment. Even though IGBT has been around for many years, there have been many technical hurdles to overcome to make it viable.

Now that the technology is maturing, static frequency conversion using solid state components will be the technology of choice. The choice of a PPS that includes an output transformer or one that does not will depend on the application. Transformers provide impedance matching and isolation. While it is safer, it is also larger in size and costlier overall. A transformer-less PPS, on the other hand, is smaller and costs less but is limited in current supply. Finally, the two relevant standards including IEC-6100-4-11 and MIL-STD-704 were discussed. PPS complying with those standards will have measurable power quality and performance and are strongly recommended.

Daniel Tseng, Product Manager for Preen AC Power Corporation, is responsible for global strategic and product marketing overseeing the programmable AC and DC power supply, frequency converter, and automatic test solutions. Mr. Tseng’s area of expertise includes programmable power supply applications for R&D, production and automotive validation tests. He can be reached at daniel.tseng@acpower.net

Bill Dowd, Regional Sales Manager forPreen AC Power Corporation, has over 30 years of experience in the power supply industry. He has helped many manufactures with their power supply requirements. He is responsible for the western region sales based in Irvine, CA, and can be reached at bill.dowd@acpower.net