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Power tip: Implications of envelope tracking

Posted: 06 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:RF? power system? battery? base stations? amplifiers?

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 controlwhich 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 over-current 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 20kHz control bandwidth ramps the current negative in less than 50?S and linearly discharges the output capacitor from 30 to 20V in 500?S.

Figure 2: Current reverses to discharge output capacitor.

To summarise, 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.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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