ZVS converter - snubber helpful at turn-off?

The only problem you get with a cap directly across the device is that at a few tens of nS switching time for the fet, it becomes a series tuned circuit with it's associated lead and track lengths. You could get ringing that exceeds the drain source voltage limit for the device and also have to consider the losses through the cap itself, which will need to be dv/dt rated. The usual snubber design uses either a series cr across the device, or a fast diode coupled to a cr circuit between hv+ and ground.

Have done a little work on hv bridge inverters and the key to stop ringing is to keep all tracks as short as pos (vhf radio techniques) and use polypropylene type caps for hv+ decoupling as close as possible to the device. Also, optimise the switching time of the device by gate drive control, usually via a series resistor, which also damps the tuned circuit formed by the Cg-s and track lengths. The gate drive power can be quite significant as the Cgs is typically 100's of pF. The losses through the device itself shouldn't be significant if you get the rest of it right, though I found it all a bit of a of a black art to start with.

Both IR and Fairchild had loads of app notes on this sort of stuff last time I looked...

Regards,

Chris

That's basically it, though you have two problems; one is the other parasitics you get, as ChrisQ mentioned, and the other is turn-on transients.

I was having a problem with dV/dt causing ringing in an early gate drive circuit:

Inverter output, basically feeding an inductor (square voltage, triangular current). This is above the LC resonant frequency, but essentially identical to your situation. Anyway, notice the drool on the falling edge. That doesn't belong there. Let's take a closer look at it: Eww! dV/dt is coupling through the gate drive coupling transformer, causing it to oscillate slightly. The period corresponds to the propagation delay of the drive circuit, about 300ns.

So I straightaway just put a 0.1uF MKP on the output terminal. Yes, awful short circuit current, but if it starts, it'll stay started and keep running okay without problems. The energy is small (0.1uF charged to full supply voltage is only 5mJ), so if transient voltages don't explode it, it won't die thermally at least. No, not ideal, but unfortunately I don't have a 0.1 ohm 10W resistor to put in series. Anyway, this is the result,

You can see a little chunk at turn-off, as current commutates from transistor to capacitor. The voltage slope kind of wiggles by 10V, resonating with Lstray (ESL of the snubber cap) and Cout (junction capacitance), while the current waveform splats as it always does (eh, it's an inductor, it picks up trash fairly well). At higher current, of course, the slope steepens, and commutation time is reasonable. It's a bit slow at this current, but well illustrates the effect.

Ultimately, I went without the snubber, because I changed the output network from LLC to transformer coupled series resonant, which has far better characteristics, and which is ZVS for most of its operating range anyway (except at resonance, where power factor approaches 1). I'm also building more noise-tolerant gate drives (or so I hope).

Speaking of which, here's a shot of the more recent output. You can see the phase shift is exactly where it should be. Approaching resonance (with more load applied!), phase gets completely in phase. It even goes capacitive below resonance.

Tim