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….

(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.