| Electronics > Beginners |
| 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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