Back to Basics: Powering Dynamic Loads

August 21, 2014
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Modern electronic loads can demand rapid changes in the supply current. An RF power amplifier, for example, may present a steady-state load of 10% of the rated input power, but this can increase to nearly 100% almost instantly during a transient. Other loads capable of imposing similarly large transients include RF transmitters, test systems, displays, designs with low voltage ASICs and laser diode transmitters.

A DC-DC converter can take a few hundred microseconds to respond to a large load step, while a power amplifier may require a recovery time below 100 microseconds to avoid unwanted modulation of the output. The result when the load step changes, however, is typically a significant droop in converter output voltage, followed by an overshoot at the end of the transient event. Fig.1 shows a typical converter’s response to a 7.7 A transient load current with no external filtering or capacitance.

Fig.1: Converter response to transient current pulse

Fig.1: Converter response to transient current pulse

One solution would be to fit a large capacitor across the converter’s output, to provide an energy store that could be drawn down by the load during a transient. There are, however, a number of drawbacks to this approach. The capacitor presents a short-circuit output to the converter at turn-on, and changes the characteristics of the converter’s output control loop, leading to possible instability. It also slows the control loop’s response, which is the exact opposite of the requirement for a faster response. Another problem is that suitable capacitors can be large in terms of volume, footprint and cost – deeply unattractive features as designers endeavour to reduce size, cost and weight to remain competitive.

Much better results can be obtained by using an active solution rather than passive components. Vicor’s QuietPower (QPO) and MicroRAM Output Ripple Attenuator Modules are active filter products that are simple to drop into most applications. As Fig.2 shows, they sit between the converter output and the load, and offer considerable improvements on the capacitors they replace. They respond to transients much faster than converters by decreasing the internal resistance path between their positive input and output pins.

 

Fig.2: Showing QPO active filter installed between converter output and load

Fig.2: Showing QPO active filter installed between converter output and load

If the converter was originally delivering 5 V to the load, the active output filter can be chosen to supply 5 V in its place, but while it does so, it uses the Trim input to force the converter output to a higher level: 5.3 V would be a typical value. The 0.3 V differential across the active output filter is known as its headroom voltage, and is dissipated during steady-state operation. However, in the event of a transient, this same 0.3 V becomes the energy source from which the output filter can uphold its output while the converter output droops. Fig.3 shows the filter’s input and output waveforms during a transient.

Fig.3: QPO input and output waveforms during transient

Fig.3: QPO input and output waveforms during transient

 

In addition to providing an improved transient response, these active filters offer significant size and weight reductions compared with passive component solutions. Savings of about 60% in area and 80% in volume are possible, while the QuietPower products weigh just 3.1 grams, approximately the same as a single capacitor. These savings are particularly beneficial for portable or mobile equipment applications.

For further information on this topic, see Vicor’s White Paper ‘Reduce Load Capacitance in Noise-Sensitive, High-Transient Applications, through Implementation of Active Filtering’.

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