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Digital control challenges old ideas about power supply design
Find out how digital control is forcing a rethinking about using voltage-mode or current-mode control.
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By
Brendan Daly, Analog Devices
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Page 1 of 4
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Power Management DesignLine
(01/17/2007 2:15 PM EST)
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One of the fundamental decisions required at the beginning of a power supply design is whether to implement voltage-mode control (VMC) or current-mode control (CMC). As with any design, trade-offs exist between both implementations. These tried and trusted trade-offs are being challenged, however, by the introduction of digital control. This article will investigate where the line has been drawn, and how digital control is forcing a rethink in the debate of whether to implement VMC or CMC.
The methods
A wide body of material adequately describes current-mode control and voltage-mode control, so only a brief review is presented here. VMC consists of a single control loop, with the output voltage used as the feedback signal. The loop reacts after the output voltage changes, producing a transient response that takes place after-the-fact. The slow response is an obvious drawback, as the goal is to maintain a constant output voltage. CMC adds an extra feedback loop. In addition to the voltage control loop, it also senses the current flow, and turns off the power switch when the peak current is reached. The current signal is used as the main control loop. One might think that VMC will give a slower response than CMC, but this is not correct, as the loop response is ultimately determined by the system bandwidth. Both methods can be designed to have the same loop bandwidth, so VMC is not necessarily slower than a CMC.
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