A New
Continuous-Time Model for Current-Mode Control
In 1990,
Dr. Ray Ridley published a PhD dissertation that brought an
end to over ten years of fierce debate about how current-mode
control systems should be analyzed. The resulting model accurately
predicted current loop instability, used average models for
the part of the circuit where it applied, and sampled-data analysis
for the current loop phenomena. A simple approximation then
produced results that were very easy to use.
The definitive
new current-mode model offered several insights that helped
understand the system properly:
1. The best representation
of a second-order converters is given by a third-order transfer
function. A dominant pole represents the current-source effect,
and a double pole at half the switching frequency shows the
subharmonic oscillation. The double pole is damped by compensating
ramp addition.
2. Current-mode
control can go unstable even at duty cycles below 50%, and a
compensating ramp must be added even for some converters that
are limited to 50% duty cycle.
3. The current
feedback loop has two right-half-plane zeros in the transfer
function that lead to instability.
4. The PWM switch
model, developed by Vorpérian, works perfectly well in the current-mode
model. This allows a single model to be used for current-mode,
voltage-mode, and the important case where significant compensating
ramp is used and the resulting system is somewhere in between
the two.
Full details of
the model are now available for free in the current-mode book,
downloadable by chapters below. This 200-page book covers the
history of current-mode control and modeling, detailed analysis,
practical application examples, and PSpice model listings. Complete
unabridged details from the PhD dissertation are included. An
abbreviated summary at the end gives quick results for helping
you with design when you don't have the time to read the full
story.
Download
by Chapter:
Chapter
1 (3.3Mb)
Chapter
2 (3.1Mb)
Chapter
3 (2.0Mb)
Chapter
4 (1.8Mb)
Chapter
5 (1.6Mb)
Chapter
6 (2.9Mb)
Chapter
7 (1.6Mb)
© copyright
Ridley Engineering, Inc. 2007
