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Design a buck converter

BENFEI WANG
Prodigy 190 points

I try to build a buck converter with some basic components and controlled by a DSP 28335.

Actually, the inductor current is not directly measured. It is a voltage signal from a current transducer.

When I set a switching frequency as 8.8Khz, the waveform of the inductor current is OK as shown in the figure below:

However, when I set the switch frequency as 88Khz, the waveform of the inductor current has a high resonant part when the switch is ON and OFF. It is shown below:

I do not know whether this distortion is caused by the circuit design or the transducer.  The capacitor and inductor I use are 100uF and 500uH. Is it because of my large cap? Any one can give some advice for trouble shooting?

  • 0
    Prodigy 60 points
    You may be getting it in the upper trace as well, but you are not seeing it due to the slower sweep speed. See the little vertical lines you get on the top blue trace (Especially the one above the "100us" legend)? Try looking at that at the same 2us speed you use on the lower trace, just look at the edges, not the "full" waveform.
    Study your layout, especially the main current loops: from the input cap, through the top FET, through the inductor, through the output cap, and back to the input cap; and from the output cap ground, up through the bottom FET, through the inductor, and back into the output cap. Keep both of those loops as short as you can. Study the layout recommendations for almost any higher power buck controller. You may also need some high-frequency (smaller case size) caps in parallel with your 100uF output cap.