Buck Converter Snubber Design

[jonathan_nadar]

I am designing a snubber circuit for my buck converter. I am following the procedures mentioned in the attached application note. When I do the procedure mentioned on page 3, step 2 where it says to connect a capacitor, a 1nF, 100V 0603 ceramic cap in my case, the low side mosfet (SIR882BDP-T1-RE3) heated and eventually blew up. The test conditions were as follows: Vin=80V, Vout 12V, Iout=1.7A, fsw=180kHz.

I am attaching an image of Vds waveform of low side FET without snubber, there is an overshoot of about 120V, which is higher than the rated value, but for some reason, it didn't blow up before.

I'll be taking into account the DC bias of the capacitor during calculations and using a ground lead adaptor as mentioned in the note.

Do let me know how I should proceed with the design.

[DavidAlfa]

A buck converter doesn't need snubber, as the bemf is taken by the output capacitor and flows through the diode at the input of the coil, I've never seen any buck converter with that.
Step up converters don't need it either because of the same reason.

The problem comes when using transformers, there're two separate coils, so the primary bemf will kick back when opening the driver, there's no direct current path in the very moment it's switched off, until the secondary rectifier takes the stored power.

Maybe the coil causes a small spike before the diode starts conducting, but they're nowhere as huge spikes as with
 flyback converters.

That's also why you use very fast diodes in such converters.
I'm not a expert here, so I could be wrong, coils are a bit abstract for me.

[Terry Bites]

Obviously you didnt goolge "snubber design". Thousands of documents. eg
 https://www.digikey.co.uk/en/articles/resistor-capacitor-rc-snubber-design-for-power-switches

[T3sl4co1l]

0. Schematic and layout?

1. Use 150V+ transistors.  100 is way too close; what's nominal maximum input voltage?  Transients?  Unless it's supplied by another regulated source, that thing is dead even without the overshoot.

2. Just slow down the gate drive.  It doesn't look like you need to push it all that fast.

Tim

[shakalnokturn]

It should be obvious that adding a snubber capacitor is also adding current through the switches that will at least partially charge/discharge it.
You may have over stressed the MOS by loading it with the test capacitor.
Switching at too high frequencies can make the compromise on snubbering even harder between killing the ringing and killing the switches.
Maybe a light RC snubber and TVS if you don't want to change switching frequency?

[jonathan_nadar]

0. Schematic and layout?

1. Use 150V+ transistors.  100 is way too close; what's nominal maximum input voltage?  Transients?  Unless it's supplied by another regulated source, that thing is dead even without the overshoot.

2. Just slow down the gate drive.  It doesn't look like you need to push it all that fast.

Tim

I have attached the schematic and layout.

The input comes from a 20s Li-ion battery pack, so that's 84V peak and 72V nominal. I have a TVS diode at the input to take care of the transients.

I wanted to keep the converter compact and lightweight hence opted for a higher switching frequency. I have increased the high side gate resistance to 4.7 ohms to increase the turn on time.

[jonathan_nadar]

It should be obvious that adding a snubber capacitor is also adding current through the switches that will at least partially charge/discharge it.
You may have over stressed the MOS by loading it with the test capacitor.
Switching at too high frequencies can make the compromise on snubbering even harder between killing the ringing and killing the switches.
Maybe a light RC snubber and TVS if you don't want to change switching frequency?

I measured the waveforms with and without snubber and there doesn't seem to be any difference. I increased the input voltage to 8V and doubled the output power by adding another bulb and the converter continued to work without any issues. As suggested by DavidAlfa, I concluded that they might not be there present and are due to probe parasitics, which I have tried to minimize.

[T3sl4co1l]

I can't see half the routing or pads, can you turn on transparency and screenshot again?

Suggest this setting (extract the attached zip then load it from Layer Settings):

Also, what's the controller?  Why is current sense at the end, not at the inductor?

Tim

[jonathan_nadar]

Also, what's the controller?  Why is current sense at the end, not at the inductor?
Tim

The converter is controlled by an STM32 microcontroller. The current sense is used for charging a 12V battery, so it's after the inductor.