Hi. I'm suffering switch node ringing on my boost converter on lower FET turn on and I'm not sure exactly how bad the problem is and why my snub attempt has failed. Below I have included pics of the schematic, pcb layout, and 'scope traces with the probe in the position in the photo.

You can see the pcb layout is very tight but the snubber (3.3nF-7.5Ohm) was retrofitted above Q8 (lower FET, FDD86540, 1.4nF Cout) so it's less than ideal.

The first trace is when the current drawn is low. The boost converter is in discontinuous mode but I don't understand why the NGT is gradual.

The second trace is when the current drawn is high so the boost converter is in continuous mode. The NGT becomes much faster with considerable ringing which increases in amplitude with increased load (up to 10A). I'm surprised how bad the overshoot is considering the tight layout. Slowing the turn on of Q8 with increased Rgate (R22,R23) made no noticeable difference, nor did the retrofitted snubber. The ringing is ~80MHz which is below the scope bandwidth of 60MHz. Does that mean that the actual ringing is worse or is the scope possibly adding artifacts?

Or is the Rsnub of 7.5ohm too high? The current under load is typically 5-10A. Rsnub will limit the peak current drawn by the snubber to ~3Amp so perhaps Rsnub should be more like 2 ohms so the snub current is similar to the load.

Any help appreciated. I've blown a few FETs at higher voltages so I'm testing with a 25V output voltage while I clean up the operation.

Robin

 

Picture 2 ringing is probably made by the long ground wire on the scope probe.
View attachment 247951

NGT is gradual

NGT = ?

The fall time of the signal will be slow because the current is near zero. There is no energy to drive the signal.

 

Picture 2 ringing is probably made by the long ground wire on the scope probe.
View attachment 247951

NGT = ?

The fall time of the signal will be slow because the current is near zero. There is no energy to drive the signal.

NGT = negative-going transition.
I haven’t seen one of those ground tips before. I’ll try to get hold of one and see if it makes a difference. Thanks for the pic.

 

I haven’t seen one of those ground tips before.

You can make one.
The idea is to have a very short wire from ground to ground. Solder a short not insulated wire onto PCB ground and then push it up to the scope ground at the tip.

 

I bodged together such a ground clip with some wire and the ringing amplitude is about a quarter of what it was so it seems to be a measurement error. Thanks a lot for the tip (pun intended)

 

Thanks a lot for the tip