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boost converter in light load

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grizedale

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Hello,

Please can you confirm that if a boost converter is correctly compensated for max load and it is in ccm at max load.

Then it will automatically be stable when it goes into DCM due to being lightly loaded?
 

In general yes, I'd expect it to remain stable. The RHP zero frequency should increase at light load, and as your controller starts operating in DCM, its gain should decrease. Those two things should only increase stability (unless the system was only conditionally stable). The only thing it could worsen is the damping of your LC, which might cause some extra phase lag, but it's unlikely to be enough to overcome the loss of gain and cause instability.

Is this still the same issue as in your other thread? Based on what you explained over there, it sounded like it must be a relaxation oscillation, not a small signal instability. Did you notice that the amplitude or frequency of the ripple changes depending on the load impedance?
 

yes its the same issue as the TPS40210 problem.

I thought in current mode the resonance in the LC never occurred because the inductor is being controlled, and its pole is thus removed from the transfer function......?

.....or is it that when slope sompensation is involved, one is not fully in current mode, and so therfore the inductor's pole raises its ugly head, and resonates with the output capacitor to give instability?
 

Right, I forgot that it was a current mode controller. Yes, it theoretically eliminates the pole caused by the inductor. In practice, it just moves it to a much higher frequency (I'm not clear on the math). But that can't be your problem, since your oscillation frequency is ~5KHz, which is far below the poles in your converter. There should be negligible small signal phase shift at such low frequencies.
 

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