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A case study of the ASPJ power supplies.

A Case Study of the ASPJ Power Supplies

A joint venture composed of ITT Avionics (ITTAV) and Westinghouse was one of two teams awarded a contract in 1979 to design the Airborne Self-Protection Jammer (ASPJ) system. The first phase of the contract required a number of critical item demonstrations (CID); among its contract activities, ITTAV built a form, fit and function transmitter. After the successful competitive demonstration, the ITTAV and Westinghouse joint venture was awarded a full-scale development (FSD) contract in 1981.

Twelve ASPJ systems were built during the FSD phase. Successful qualification and flight testing during FSD led to the award of the present production verification (PV) contract in 1987.

The mechanical design of the system accommodated installations in several different aircraft: AV-8 pod, F-14, F-16, F-18 and the A-6. In some of these, existing systems will be replaced with the ASPJ (ALQ-165). A basic system consists of a complement of five weapons replaceable assemblies (WRAs), comprising two receivers, two transmitters and a processor. A basic system is augmented with up to two additional transmitters for various aircraft applications. Figure 1 is a photograph of the WRAs used in the F-16/F-14 ASPJ installation. An additional transmitter and receiver is used in these aircraft.

HIGH-VOLTAGE POWER SUPPLY

Each transmitter and processor contains a power supply. The processor low-voltage power supply (LVPS) provides power to all the WRAs. Each of the two types of transmitters (low band and high band) requires a high-voltage power supply (HVPS) to drive two TWTs (CW and pulse). Twelve LVPSs and thirty-nine HVPSs were built for the FSD program. Figure 2 is a detailed photograph of a high-band transmitter; the dimensions of the low-band transmitter are the same. Also visible are the separate heat exchangers that provide TWT and HVPS cooling. The exchangers are separated by an air space to minimize heat transfer from the TWTs to the HVPS, as each dissipates about five times as much power as the HVPS. Twenty production systems will be built during the PV phase.

The transmitter size was largely determined by the two TWTs and the HVPS volume. The TWTs were well-defined, whereas the HVPS was new. The totally sealed ALQ-165 HVPS delivers 1,700 W, supporting all requirements of both the CW and pulse TWTs in a 535-in.sup.3 volume. The output power density of 3.2 W per in.sup.3 is considered to be state-of-the-art design (Table 1). To achieve this density some innovative electronic and mechanical designs were required. Three patents were issued for the HVPS design, one mechanical and two for electronics.

Liquid encapsulation using Fluorinert liquid from 3M was chosen over the more traditional solid encapsulants for the HVPS after a comprehensive trade-off study. One of the most significant disadvantages of the solid encapsulant is that although the dielectric insulation rating is in the range of 400 V per mil, it must be used well below this limit. Joint venture engineers determined from past experience that air entrapment, surface creepage and poor adhesion require a solid encapsulant be derated to 50 V per mil for reliable use. In comparison, the Fluorinert liquid can be used reliably at 100 V per mil.

A comparative disadvantage of the fluid is its considerable change in volume over the military temperature range of -55[deg.] to 125[deg.]C. Bellows were used in the CID HVPS to compensate for the changing volume. Since a bellows design that compensated for the entire volume change would have used an unacceptable portion of the available space, additives such as small glass beads were used to reduce fluid volume; however, this did not prove successful. A unique approach was then developed and patented by ITTAV that solved all of the expansion problems and did not require the use of a bellows.

The implementation of this approach was to partition the HVPS into low- and high-voltage sections separated by a bulk-head, with the high-voltage section filled with fluid and the low-voltage section only partially filled. Whenever pressure increases, due to expansion, the fluid is forced into the low-voltage compartment. The reverse procedure occurs when the pressure decreases in the high-voltage side. A patented bypass configuration is used to interchange the fluid between compartments effectively, independent of aircraft (HVPS) orientation, while maintaining the high-voltage section filled under all conditions.

Figure 3 is a photograph of an open FSD HVPS, with the high-voltage section removed. The empty chamber contains all the components that are referenced to the two TWT voltages, which range from 5kV to 12kV. On the low-voltage side the backplane, EMI filter, flex tape and some of the connectors can be seen. The low-voltage assembly can be extracted by removing four connectors and "hold down" screws. This assembly contains four modules that plug into the backplane. These modules contain all of the power processing components, control and protection circuitry as well as some magnetics.

The electrical design also had to overcome some severe restrictions. While the FSD HVPS was partitioned into two cavities (Figure 3), the circuit for the CID HVPS had been developed for a single-cavity layout. For the single cavity, conventional low-noise layout techniques were used. However, due to the constrained form factor of the two-cavity approach, buffering was added to provide the circuitry for low-impedence signal flow through the bulkhead. In both CID and FSD, it was required that the HVPS be easily repaired. All the high- and low-voltage modules were designed with connectors to permit disassembly without unsoldering.

HYBRIDS

It became apparent from the preliminary layout that the high overall packaging density would require extensive use of hybrids. Signal, medium power and power hybrids would have to be used as well as flex tape to achieve the desired modularity and repairability of the HVPS.

The signal hybrids, each of which contains almost all of the components normally found on a printed wiring board, were packaged at about 50 components per in.sup.2 of substrate area. As a comparison, the density on a printed wiring board designed for automatic component insertion is about five analog components per in.sup.2.. The medium-power hybrid was less dense than the signal hybrids but required about 1,000V of isolation from the input line. The power hybrid contains four power transistors and associated power diodes. It can dissipate 50 W at 125[deg.]C and also is required to withstand 1,000-V isolation.

Even with the use of power hybrids, a conventional power processing design requires six power transistors. A new power processor topology was developed and patented by ITTAV that uses only four transistors and provides the same performance as the conventional scheme. This design proved so efficient that it was also used for the medium power processing of the various housekeeping voltages that were required throughout the HVPS. This increased commonality enabled the multiple use of the same hybrid design. The power hybrid is used three times and the control signal hybrid is used twice.

KEEPING IT SMALL

The high-voltage capacitors consume significant volume in any HVPS that provides regulation for pulsated TWT loads. Their size is totally dependent on the square of the rated voltage and the capacitance value. In the ASPJ design the rating is 60% higher than the worst-case operating capacitor voltage.

To obtain the 60% voltage margin without requiring a prohibitively large capacitor assembly, a new feedback circuit was developed. The new circuit, also patented, speeds up the voltage step response of the power processing by measuring the charging or discharging rate of the h igh-voltage capacitors. The volues of the capacitors are invesely proportional to the speed of the step response. The faster response allowed the use of lower-value capacitors. Instead of reducing the capacitor volume, it was judged more important to maintain the voltage margin at the present 60% to obtain the highest possible reliability without compromising electrical performance.

A similar scenario was followed for the LVPS used in the FSD phase. Some of the HVPS design technology was also used in the LVPS to achieve the required composite 3.m% per in.sup.3 packaging. The LVPS delivers six regulated output voltages to all the WRAs. Approximately 1,000 W of the total 1,470 W of delivered power is provided for the 5-V logic requirement.

DESIGN ADVANCES

The LVPS and HVPS designs of the ASPJ represent onsiderable advances from earlier ECM systems. A comparison between the ALQ-172 (PAVEMINT), which was developed in the 1970s, and the ALQ-165 power supplies is shown in Table 1. The four LVPSs used in the ALQ-172 system range from 0.7 to 2.5 W per in.sup.3.. The PAVEMINT HVPS provides power to one pulse TWT and does nto include a grid modulator. The delivered power is about 800 W at a density of 1.7 W per in.sup.3. The PAVEMINT HVPS design uses all-discrete components and a solid encapsulant. This HVPS was also developed by ITTAV and an alternate vendor. There are two grid modulators in the ALQ-165 HVPS, one for each possible TWT failure, which are separately controlled. In the event of a TWT or any other fault, the defective side is shutdown automatically with full performance still available from the unaffected section.

Judgment about power supply density or complexity is not conclusive when only output power density is compared. An excellent treatment of this subject can be found in the recently released NAVMAT P-4855-1A "Navy Power Supply Reliability" publication.

In the PV phase the joint venture ASPJ team will build 20 systems to demonstrate producibility. Recent advances in high-speed logic ICs, high-voltage MOSFETs and multilayer ceramic capacitors have permitted the PV grid modulator to be redesigned with fewer housekeeping functions and about 60% of the components of the FSD version. Because the volume for the grid modulators has decreased, additional producibility opportunities were addressed in the redesign of the HVPS for the PV phase.

Figure 4 shows an opened PV HVPS with the redesigned and repackaged high-voltage section. In the low-voltage section, the additional volume was used to replace three hybrids with discrete components.

It is an ITTAV policy to undertake a make/buy evaluation at all phases of the program to acquire custom-designed power supplies. Internal costs and schedules are also compared with the industry responses by a make/buy committee. The result of this activity determines who is awarded the development of the custom-designed power supply. For a nonproduction program, e.g., the FSD phase of the ASPJ, a sole source award is usual; alternative sources are developed for the production phase such as the PV phase of the ASPJ. Currently in the PV phase there are two suppliers contracted as possible alternative sources for the production HVPS and one source for the LVPS.

The continuing quest to increase density and improve quality will require systems houses to keep up with the state of the art in power supply design. Whether a systems house needs instant support to a technical proposal, design of a state-of-the-art power supply or vendor liaison, a competent inhouse power systems group will be necessary.
COPYRIGHT 1989 Horizon House Publications, Inc.
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Title Annotation:Airborne Self-Protection Jammer system; includes related article
Author:Schwarz, A.; Herskovitz, Sheldon B.
Publication:Journal of Electronic Defense
Date:Jul 1, 1989
Words:1844
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