9 - Dynamic power supplies

Summary

Power supplies are traditionally designed to provide very stable and well-regulated outputs, primarily a fixed voltage that is maintained irrespective of the current that is drawn by its load. When the supply voltage is fixed in this way, there is no need to be concerned with the reactive characteristics of the load. All that matters is the value of the load resistance, which is often time-varying. Power supply regulation control loop dynamics are designed with this restriction in mind.

Here we require that the value of the output voltage from the power supply be dynamic, meaning not only that it can vary, but that it must be capable of varying at speeds matching the variation of its time-varying load. Combine this with a load having a widely varying resistance value as the voltage across it changes, as seen in Section 5.10, and the design of any dynamic power supply (DPS) becomes a formidable task.

This leads to the important concept that in any DPS transmitter there is a new interface of great importance: the interconnection between the DPS and its associated PA. To date, this interface has not received much attention, probably because of the general complacency afforded traditional design when fixed power supply voltages are used.

This chapter examines the issues involved in DPS design, including some circuit details.

Power objectives

All power into the PA flows through the DPS. The DPS must support the combination of: the PA output RF signal power, the energy in efficiencies of the PA due to its own power dissipation, and any reactive currents required by PA circuitry. Very much like the general electric utility, the DPS must source the total current required by the load, not just the resistive component that leads to output RF power and internal power dissipation. The utility concept of the power factor [9-1] is very important to DPS design.