Electronic devices and equipment that operate in the aerospace and defense (A&D) environments must be rugged and operate reliably without human attention over long intervals. Their power requirements are very specific and demanding. To ensure product performance and reliability, extensive testing to ensure specifications are met is essential.
Power Design Requirements
A&D system designs must meet many stringent requirements including operating within a wide temperature range and withstanding shock and vibration Adherence to high mean-time-between failure (MTBF), electromagnetic interference (EMI), and Size, Weight and Power (SWaP) requirements is also a must, affecting the overall design cost.
Power designs based on commercial off-the-shelf (COTS) components must meet military standard specifications including MIL-STD-704. Therefore these components must be accurate, reliable, and easy to implement.
Unlike industrial applications, which operate in the 50 Hz/60 Hz range, A&D electronic device and equipment frequencies typically fall in the 400 Hz AC and 28 VDC range. The newer requirements include operation of 800 Hz AC and 270 VDC.
To ensure equipment can perform reliably according to the specification even when, say, the supply voltage has dropped by 10%, the equipment has to be tested accordingly. Therefore, the input frequency (F), voltage (V) or current (I) from the test power supply sources must be able to vary for testing purposes. One way to convert an electrical energy source to various input frequencies, voltages, or currents has been to use motor generators. The generators convert the frequency and variable transformers to change the voltage. More recently, solid state technology has emerged, impacting the A&D test industry.
Why Is Standard Compliance Important?
Equipment used in the A&D applications installed inside an airplane, on the ground, or on the battlefield must perform reliably. It is critical that the power sources or power supplies used for testing are accurate. Otherwise the test results will be inaccurate, directly impacting mission-critical equipment. The power sources used to test the A&D devices need to comply with the two relevant standards, IEC 61000-4-11 (released by the International Electrotechnical Commission) and the military MIL-STD-704, to ensure product quality. (Note that there is another standard with a similar name to the IEC 61000-4-1. EN 61000-4-11 was released by a different standard body called CENELEC, the European Committee for Electrotechnical Standardization body, which is recognized by the European Commission. Even though these two standards have different names, the technical contents are identical.)
IEC 61000-4-11 is an international standard. It covers test parameters of 50 Hz/60 Hz AC equipment including AC/DC power supplies with input current less than 16A per phase. Testing as it relates to 800 Hz will be covered in a future IEC edition. Equipment that complies with the IEC 61000-4-11 standard will be able to withstand predefined voltage dips, interruptions, variations, and inrush currents up to 500A for 220V/240V and 250A for 110 V/120V accordingly.
The Department of Defense published the MIL-STD-704 military standard to ensure airborne military equipment performs reliably. As with the IEC specification, MIL-STD-704 states that the power output behaviors, including steady state voltage, voltage unbalance, modulation, phase difference and distortion, steady state frequency, and frequency modulation, must perform according to specification.
Another recommendation is compliance with Restriction of Hazardous Substances (RoHS) directive. Set up by the European Union (EU), RoHS requires EU member countries to avoid using hazardous materials including cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants. The purpose is to increase environmental safety. RoHS compliant equipment is manufactured without the above hazardous materials.
Solid State Versus Traditional Rotary Solutions
Aerospace and defense devices and equipment, whether they are used inside an airplane or on the battlefield, go through compulsory extensive tests before release. The unit under test (UUT) (Figure 1) refers to the device being tested. Depending on the specification, the test power sources need to provide clean (without harmonic distortion) power with variable voltages and frequencies to test the UUT.
Traditionally, a motor-driven variable transformer is used to change voltage. The process is straightforward. Power comes directly from the power grid, and the transformer changes it to the desired voltage. If the input AC sine waves are distorted, an event commonly known as harmonic distortion, the UUT will also experience these distortions.
To change the frequency, a motor and a generator connected to a common shaft, commonly known as motor-generator sets (M-G sets) are used. Electromagnetic induction in the motor converts electrical energy to mechanical energy. A generator, on the other hand, converts mechanical energy to electrical energy. In the process, variable frequencies are generated. This type of conversion is called rotary conversion.
In 1980 solid state insulated gate bipolar transistors (IGBTs) technology was introduced. Solid state components do not use any motors or generators and therefore have no moving parts. This type of voltage and frequency conversion is called static conversion.
How Does IGBT Technology Work?
IGBT is a solid state device that uses a common MOSFET power device. By constructing an N-channel power MOSFET with a p-type substrate, IGBT-based digital programmable power supplies have been developed. With minimum conduction loss, the voltage and frequency conversion is very efficient and is suitable for high current and voltage applications.
Compared with traditional rotary solutions, the static solid state approach has many advantages. Solid state designs have no moving parts. So, routine maintenance such as inspection and replacement of bearings used in motors is not required. Eliminating such maintenance lowers overall costs, including capital investment, size, and weight. What’s more, designs with no moving parts cut down on unwanted noise.
Solid state programmable power supplies have a number of advantages over power supplies using traditional rotary conversion and can be summarized as follows:
Lower harmonic electrical noise
Higher output frequency stability and system efficiency
Response time in milliseconds rather than seconds in voltage control. The rotary M-G set takes longer to adjust to a new setting.
Reduction of maintenance costs: The rotary M-G set requires regular replacement of bearings and belts, while static frequency conversion only requires cleaning fans and exhausts.
As an illustration, the AFV+ series, a solid state IGBT-based digital programmable power supply from Preen, is able to achieve results not possible with the traditional rotary approach (Figure 2). The AFV+ is able to deliver the power range of 10 kVA to 2000 kVA with total harmonic distortion (THD) less than 0.5%. The best the rotary method can achieve is less than 5%. Better device test results can be achieved with lower THD which enable cleaner power. (Figure 3).
The unit is programmable to output a frequency range from 45 Hz to 800 Hz and voltage range from 0 to 600 VAC. Compared with the digital conversion, the rotary method requires a combination of the M-G set and variable transformers to do the job. Overall, the solidstate approach is more accurate and efficient. Because the design is digital, the unit can be monitored and controlled remotely making it more convenient in equipment testing. It is expected that over time, solid state solutions will replace traditional rotary solutions.
A&D systems and devices need to perform reliably in rugged environments including operation in a wide temperature range and the ability to withstand shock and vibration. Additionally, they need to comply with IEC 61000-4-11, MIL-STD-704, and RoHS. To ensure these systems and devices perform according to the specifications, it is critical to provide accurate power sources with adjustable voltages and frequencies during the test cycle.
Traditionally, the rotary method has been used to vary voltages and frequencies, but it has some limitations. The solid-state technology innovation using the static conversion method has advantages over the traditional approach. They include cleaner AC power, higher output frequency stability and system efficiency, faster response time, and lower maintenance cost overall.
In the future, it is expected that solidstate solutions will play a more important role, including reduction in size and weight, replacing most of the traditional rotary solutions over time.
This article was written by Brian Hsu, Product Marketing Manager, Preen (AC Power Corp.) (Taipei City, Taiwan). For more information, visit here .