RF power system efficiency is becoming increasingly important in low-power systems when it comes to wringing out the last minute of operation from a battery, and in high-power base stations where electric bills can be substantial. One obvious way to improve the situation is to emit only the needed RF power. This is relatively easy to do, as the amplifiers operate as AB amplifiers, and the drive signals can be backed off when lower output power is desired.
This can be taken a step further by reducing the supply voltage to the amplifiers when lower output power is desired. Figure 1 illustrates this; it presents amplifier efficiency as a function of output power for two different supply voltages. Lowering the supply voltage improves efficiency but limits how much power the amplifier can deliver.
Lower supply voltage improves efficiency but limits output power
(view full-size image).
Speed of response can be an issue in these types of systems, as the bandwidth of switching power supplies is usually limited to the tens of kilohertz, and the modulation requirements can be in the multiple megahertz range. There have been combinations of linear regulators and switching power supplies developed, such as the LM3290 and LM3291. These devices feature linear regulators with 50+ MHz control loop bandwidths coupled with a high-performance switching regulator.
The switching regulator provides enough headroom to the low dropout regulator (LDO) for envelope tracking while keeping the power loss low. There also have been efforts to improve control loop speeds by pushing the power supply switching well past 1 MHz with advanced switching devices such as GaN.
In addition to a fast control loop, an envelope-tracking power supply needs to source and sink current. That is, the power supply needs to take charge off the output capacitor to quickly reduce the output voltage, rather than letting the load discharge it. Otherwise, there can be a significant energy loss due to the discharge.
Current sinking has several significant implications on the power supply architecture. The power supply must process energy in both directions. The excess output capacitor charge has to go somewhere, and control, current sense, and current limit need to work in both source and sink operation.
One way to efficiently remove charge from the output capacitors is with a synchronous topology. Synchronous topologies replace output diodes in a power supply with semiconductor switches that enable current flow in both directions. The obvious solution is a synchronous buck regulator. This topology has been used for years in DDR memory applications with source and sink requirements. However, synchronous isolated topologies are just as appropriate, as there is nothing inherently limiting reverse current flow. Flybacks, forwards, and phase-shifted bridges all have been operated successfully with reverse energy flow.
The next issue is where does the excess capacitor energy go? With synchronous topologies, it goes back to the power supply input, where it can be dissipated by other power supplies or be stored on input filter capacitors. If there are no other power supply loads, it is prudent to consider what the input voltage may surge with this energy. Also, you should consider possible interactions between this voltage perturbation and other equipment connected to the input.
The final issue with these types of supplies is that the traditional current measurement techniques are unidirectional. This means that, with a reverse current, you can lose your current sense ramp in current-mode control -- which greatly impacts the loop. It also means that you have no current limit in the reverse direction. Couple this with a very wide bandwidth loop, and there is an overcurrent possibility when taking charge off the output capacitor.
An example of an envelope-tracking power supply is the PMP5726. It is a phase-shifted bridge with synchronous rectifiers to enable bidirectional current. It is operated with voltage-mode control and uses a full-wave rectified current sense transformer for over-current protection in both directions.
Figure 2 shows the converter performance taking current off the output capacitor. A 20-kHz control bandwidth ramps the current negative in less than 50 µS and linearly discharges the output capacitor from 30 to 20 volts in 500 µS.
To summarize, envelope tracking requires a high-bandwidth power supply with the ability to source and sink current. Bidirection current flow complicates the design, as it must comprehend synchronous rectification and energy storage, as well as reversal of current sense voltages. Envelope tracking puts no real limit on topologies as long as they are synchronous.
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