This is the schematic in its basic form. It's an nMOSFET that gets driven by a pin of an Atmega328p, this nMOSFET on his turn, drives a big pMOSFET. The source is connected to a voltage regulator (9V6), the drain is connected to the backlight of an RGB screen. The Atmega generates a PWM-signal (100kHz, duty-cycle is variable) so one can change the brightness of the screen.
However, when looking at the drain, I noticed that the OFF-part doens't go to ground and there's lots of ringing.
The ringing I could damp by adding a snubber network to the drain of the pMOSFET. But the OFF-part still doesn't go to ground and there is still a sudden peak in voltage that I could not remove. It seems something is pulling the voltage back high.
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How comes that there is still a DC component in the OFF-part of the PWM?
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How can I remove that part so it goes to ground?
The only other question I found, is this one: High side mosfet source voltage does not switch back to ground
So I added a parallel RC network at the output.
Measuring now, it goes down to ground, but there is still a peak present in the signal.
Can I clean this up further?
Best Answer
When the P-MOSFET is off, it runs no more current into VLED. It is possible that the input of the LED does not provide a pull-down path, and the drain voltage just keeps floating above ground. You "fixed" this with the pull-down array that discharges C62 and C63, but such does not seem necessary to operate VLED.
The negative peak might be due to the inductor L3. When the current through L3 collapses, the inductor reacts with a voltage spike of opposite polarity. Can you try adding a flyback diode across L3 (me not knowing its purpose to begin with)?