Power supply development for microwave systems poses some of the same challenges imposed by other, non-RF/microwave applications. Microwave system designers often need the power supplies to be efficient, cool running and small. They may also need isolation, fault protection and clean, well regulated outputs--all common requirements in non-RF areas.

However, microwave systems often have additional, unique requirements that may stem from special bias requirements of RF circuitry or from the high-reliability demands of aerospace or military applications. The high-rel requirements ratchet up the challenge of power supply design by several notches. Besides needing to derate their components, power supply designers may be restricted in their choice of components to those that are hermetically sealed or radiation hardened.

All of these requirements will continue to shape the design of power supplies for microwave systems in the coming year and beyond. Although the power supply is often the last piece in the system design puzzle, the many challenges associated with powering microwave systems dictates that power supply requirements be considered in the early stages of system development. As microwave systems become more advanced, the performance demands on the power supply will continue to grow. Consequently, it will become more imperative that microwave system designers understand the impact of their power specifications on the design of the power supplies for their systems.

One way to illustrate the interplay of various microwave power requirements is with an example from satellite applications. In satellites that used phased array antennas, numerous solid-state power amplifiers (SSPAs) (perhaps 100 to 150) may be required to drive the antennas. International Rectifier is one of the companies that developed the electronic power conditioners (EPCs) that power these SSPAs through its high rel group in Copenhagen, Denmark.

According to Bjarne Soderberg, program manager of IR's high-rel operations, the electronic power conditioner is a DC-DC converter that typically generates three supply voltages--a main output for the drain supply on the SSPA's GaAs MESFETs and two auxiliary outputs for powering preamps and gate supplies. Typically, the main output is an adjustable 6 V to 9 V, while the auxiliary outputs are +/-5 V. Input voltages vary. In the past, a 28-V nominal supply voltage was common, while in newer satellites, higher voltages such as 50, 70, or 100 V may be used. Transformer-based isolation between input and output is typical.

During flight, a thermistor array measures ambient temperature in the vicinity of the amplifier. The thermistor signal is then used to adjust the drain supply to optimize the amplifier's efficiency. The supply voltage is lowered at cold temperatures and raised at high temperatures. The power delivered to the amplifier remains constant, so the supply current varies accordingly. However, this means that components used to generate the main output of the EPC must be rated to deliver the maximum current seen at low voltage and the maximum voltage seen at low current.

Another requirement for powering the SSPA is the sequencing of the positive and negative supplies. At turn on, the negative supply must ramp up before the positive supply to protect the GaAs MESFETs. The order is reversed on power down.

Because of the large number of SSPAs, the EPCs must handle a large fraction of the overall system power, so power supply efficiency is critical. Typically, overall efficiency for the power converter must be greater than 90 percent for a supply that's rated somewhere in the 50 to 250 W range. The efficiency requirement usually demands use of synchronous rectification.

Component selection is further limited by space requirements for radiation-hardened components. The power supply designer has many fewer parts to choose from than when designing for commercial applications. In addition to hermeticity and radiation hardening, components in the EPC must typically be derated by 50 percent.

All of these requirements add to the challenge of building a supply that meets the high-efficiency requirements of the satellite applications. What's more, the power supply designer is constrained by the desire to minimize the weight of the design. Generally speaking, power supply designers can trade weight for efficiency, using larger conductors and bigger transformers to achieve higher efficiency. However, the cost of launching a satellite, which rises with every gram added to the payload, dictates a lighter design.

Meeting these goals requires a complex DC-DC converter design. However, the system designer can make the design of the EPC easier by following a few guidelines in defining system power requirements. Although these requirements were developed with the EPC and the satellite's SSPAs in mind, they also should apply to other microwave systems now being developed.