Thanks for the info. A little ringing doesn't matter to me and it should be greatly reduced with my new pcb and when I stop using wire wound resistors so I think I'll try skipping the RC across the switch.
That said, you mention resistors don't need to be big since it's just for peak. Are there any rules of thumb. Let's say I have 5A and 20V for 5 us. Do I need a 1/2 watt, 1W, 2W, or 5W resistor?
I'm happy to use surface mount resistors, but again I need to have an idea of what wattage I need to handle for these pluses.
What's pulse rep rate / duty cycle?
5A * 20V * 5us = 500uJ, about a flea fart. An SMT chip resistor might poof, but I don't think the average 1/2W resistor would even mind.
Strictly speaking, you need to shop for pulse-rated resistors, and find where on their power(time) curve 5us and 100W lies.
Informally, resistors usually go about as t^1/3 below the dominant (thermal) time constant. 1/2 exponent would make more sense (thermal diffusion), or proportional (fixed energy), but, eh, for whatever reason, that seems to be the most common pattern. Most power resistors are rated for a 5x or 10x overload for 5 seconds. Combining these two properties, we might expect a 6-decade faster pulse to have 2-decade more power. So a 100W pulse could be handled by a 1W resistor (no overload condition needed) or a 0.2W resistor (5x overload 5s). Resistors quite this small, I would worry that their time constants will be much shorter, so erring on the side of 1W would be better.
This assumes a pulse rep rate much longer than the longest thermal time constant, i.e., so that the resistor temperature returns to ambient before the next hit.
For a one-shot flash, I'm guessing this will be sufficient? If you're instead doing (or "also considering", since it's trivial once you've built the hardware) a stroboscope, then the temp may not return to zero between pulses, and you have to worry about peak loading on top of the baseline temperature.
It seems likely that, for resistors over 1W, you'll have no problem with peak loading, so you can use average power just as well. For example, 2W resistors would get you a duty cycle up to 2% (a pulse rep rate of 5kHz, and more if you reduce the pulse width / intensity).
Tim