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Design >> High-Power Design >> Sub-harmonic Oscillations in Current-Mode Control
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Message started by AnilReddy on Dec 4^th, 2013, 11:12pm

Title: Sub-harmonic Oscillations in Current-Mode Control
Post by AnilReddy on Dec 4^th, 2013, 11:12pm

Hi all,

In current programmed control of power-converters, the converter will oscillate when the duty is > 0.5 if not slope-compensated. They say, it will will oscillate at one-half of the switching frequency.

My questions are...
x) what parameter of the converter (namely inductor current, output-voltage..etc..) will start oscillating at that frequency?

y) Normally, the response of any network consists of sum of the natural response and a forced response. Usually, natural response will die-out with time and only forced response remains. But, are there any systems, even for some particular or peculiar combinations of network parameters, where in the natural response will cause the system to oscillate for a small-disturbance in spite there is a forced function?

Thanks,

Bye,
Anil

Title: Re: Sub-harmonic Oscillations in Current-Mode Control
Post by AnilReddy on Dec 16^th, 2013, 5:16am

Hi all,

I have found the answer for the first question.

When the Duty = 0.5 then the initial disturbance neither grows or decreases with time, but, it just stays the same.

The interesting point here is the magnitude of the error after 2 switching cycles, if you see, is exactly equal to the magnitude just before 2 cycles. That means, inductor current is oscillating at (switching frequency/2)...and are called sub-harmonic oscillations.

But, for D>0.5 the converter will saturate finally as there is growth in oscillations.

Bye,
Anil Reddy