I had no problems to imagine the purpose of the moved Rg. But I didn't see the undershoot protection in this "extreme" way yet. Thanks for clarifying. I would probably provide both resistor options (Rg and Rs) and check for the actual switching node undershoot in your circuit.
You didn't yet comment my first statement about possible boostrap failure.
There's nothing wrong, it's a regular DCM discontinuos mode buck converter waveform, as observed at low output loads.could anyone explain why the Vs waveform has a single hump before it floats correctly to 30V.
I wonder how you are measuring Vgs? You'll need a differential probe which apparently isn't used in your measurement. If you ground the switching node by the oscilloscope probe, other unwanted effects may occur.
I see it when I know it.you'll see the maths option being lit up, instead of CH1 or 2.
I see it when I know it.
Differential is basically O.K., but the proble compensation may be slightly wrong. You would want to check it first by connecting both probes to the same test point. If you can verify correct differential measurement, than the Vgs value should be expedcted correct. It's however easy to get parasitic effects in the breadboard setup. It may be necessary to connect a z-diode across the bootstrap capacitor.
Connecting the z-diode directly to gate-source has advantages and disadvantages. The disadvantage is adding capacitannce and slow down switching speed. But it can be compensated by reducing Rg, if necessary. O.K. considering the breadboard setup, it's probably safer to place a 15 to 18V z-diode near the FET.
Reviewing your initial post, I see that you intend a 100 to 28 V converter. In this case, high duty cycle won't be an issue, if the PWM controller implements a duty cycle limiting means.
The other possible problem is however low or even no load, resulting in DCM (discontinuous mode) operation. Diode conduction time will be very low, at no load, the diode possibly doesn't switch on at all. Imagine a load current step to zero, the output voltage does a small overshoot, compensated by a long duty cycle 0 period. If the bootstrap capacitor discharges in this phase, you need to completely shut down the output voltage to restart the converter.
My coclusion is, that reliable operation can't be achieved without either a synchronous low-side switch or an auxilary charge pump to supply the bootstrap circuit.
Typical HCPL-3180 supply current is about 3 mA (< 6 mA), you should add several mA for MOSFET gate charge multiply 100 kHz.
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