Electronic – SMPS: What is current mode instability (aka “sub-harmonic oscillation”)

dc/dc converterpower electronicsswitch-mode-power-supply

In peak current controlled switching power supplies, there is a phenomenon called “current mode instability” aka “sub-harmonic oscillation”. What is that? Can't seem to get a good explanation of this….

Bonus:
(To mitigate this side effect, they recommend using something called "slope compensation".)

Best Answer

Rather than going into the mathematics of this, it's quite easy to see this graphically. Consider a peak current mode controller operating at <50% duty cycle. Then you can see below that perturbing the system results in the perturbation decaying and the system returning to steady-state operation.

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But at >50% duty-cycle the system does not return to steady state operation. Instead, it enters a "sub-cycle oscillation" mode.

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Simple as that.

See source for a more detailed explanation.