Why Struggle with Loop Compensation?
by Michael O'Loughlin, Texas Instruments, Dallas, TX, How2Power Today, Apr 30 2010
Focus:
Power supply designers sometimes try to get the voltage loop to cross over at a very high
switching frequency to improve large-signal transient response, but then struggle with
stability issues. Even though peak current-mode control is said to be easier to compensate
than
voltage-mode control, power supply designers using this method still struggle with
compensating
the voltage loop because they are unaware of the double pole that occurs (at roughly half
the
switching frequency) in the control-to-output transfer function. This article describes a
compensation method and some design rules that ensure stability. The article starts by
presenting simplified diagrams to explain the control-to-output transfer function of a
peak-
current-mode controlled supply, where its poles and zeros come from, and the
unpredictability
of the double pole. The use of slope compensation to prevent subharmonic oscillations is
explained. General rules are given for stability, including how to set gain and phase
margins,
and crossover frequency for the voltage loop. A technique for measuring the control-to-
output
transfer function using a network analyzer is described, selection of the components
required
for Type 2 compensation is explained, and equations for determining the output capacitance
are
given.
What you’ll learn:
- How to compensate a peak-current- mode-controlled power supply using a network analyzer
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