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Why might an inductor current waveform overshoot/oscillate?

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state_of_flux:
I am simulating a power supply I have designed in LTSpice, which is a buck converter feeding a half bridge LCC (not LLC) converter. In the buck output inductor, I get a waveform that overshoots (almost oscillates slightly?) then settles at steady state with required output current and ripple I designed for, but I am confused as to why the oscillation occurs. My guesses are that it is resonating with something, maybe the switch capacitance. As this is a current-fed topology I do not have the output capacitor and instead just have the bridge switches following the inductor.

Any suggestions on why this is occuring and how it can be remedied?

soldar:
What's the load?

With a resistive load and without an output capacitor I cannot see how a buck converter could not show ripple.

A Bode plot analysis would be useful. A damping factor of less than 1 would lead to ringing/oscillation.

http://support.motioneng.com/Utilities/bode/bode_16.html

T3sl4co1l:
Check control compensation.

It... is current mode controlled, right?

If you're just simulating a fixed duty cycle, that's not very meaningful in terms of low frequency dynamics.  Just replace the buck stage with a Thevenin equivalent source.

Tim

state_of_flux:

--- Quote from: T3sl4co1l on September 26, 2019, 07:53:29 pm ---Check control compensation.

It... is current mode controlled, right?

If you're just simulating a fixed duty cycle, that's not very meaningful in terms of low frequency dynamics.  Just replace the buck stage with a Thevenin equivalent source.

Tim

--- End quote ---

The idea is that the I will use buck inductor current and output voltage to control the system, so two error amp stages. At the moment I do not have a control system and I am just trying to simulate the main converter electronics, and design a filter before I look at the compensation.
I am (at the moment) simulating with a fixed duty cycle as you describe because I have yet to design the feedback.

state_of_flux:

--- Quote from: soldar on September 26, 2019, 06:32:03 pm ---What's the load?

With a resistive load and without an output capacitor I cannot see how a buck converter could not show ripple.

A Bode plot analysis would be useful. A damping factor of less than 1 would lead to ringing/oscillation.

http://support.motioneng.com/Utilities/bode/bode_16.html

--- End quote ---

At the moment I have not designed the feedback for the system as I am still having a few issues with the actual converter circuit. The ripple on the inductor is fine, but for some reason the actual waveform is fluctuating before reaching a steady-state. Is it possible to do a bode plot without having first designed the feedback?

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