A few weeks ago I got some great help on this forum in this thread:
https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1416524/#msg1416524. A quick refresher is that I designed a test circuit that was supposed to create a microsecond burst of light from an LED.
I fixed several things in my circuit and had a new PCB with things scaled up to where I think it will create enough light for my needs. I'm now running it at 160V with 4 strings of 6 LEDs that I overdrive (measured 8 amp through each led so total current of the system is 32A).
The problem I'm seeing is that the LED doesn't turn off when I ask it to. The issue seems to be a ringing problem, but the odd thing is the ringing doesn't decrease over time. Instead it happens for a long time and then stops.
The basic induced voltage equation is V=L (di/dt). I estimate my circuit is 20 nH, and measured the rise time and current at 32A in 0.1us. This gives me an induced voltage of 6.4V. I measured 6.8V in the ringing circuit so I think that confirms it. I would like to estimate the resonance frequency in the same way, but I can't get the equations to work out. I thought it would f=1/(2*pi*sqrt(L*C)). With a C of 470uF I calculate a frequency of 117Mhz, but I'm only measuring 10Mhz on the scope so that seems wrong to me.
I have also verified that the ringing isn't contained to the critical cap/led/mosfet loop. If I go all the way back to the pin on the micro that is driving the mosfet driver that pin also shows the ringing. Maybe this is why the ringing continues since the micro is actually driving the mosfet on/off?
I would also like to simulate the ringing issue in Spice LT. I have the boost converter and a trigger event simulated. That works as expected by creating a nice square pulse, but when I added in the 20 uH inductor I still didn't see any ringing like I expected. Did I add it to the wrong place or something?
What I'd like to do is verify/understand the issue is ringing and then figure out a way I can make this circuit work. I don't really care if there is some ringing if it's short or doesn't output much light from the LED. I just want to be able to output around a 1 us burst of light from these LEDs. Right now this ringing is causing the LED to go on/off for a long time (20-100 us) after the original 4 us where it should be on.
I have 2 guesses on what might help with this. One is to add a resistor that consumes a few percent of the current across the LEDs. This would help dampen the ringing. Another is to add a resistor between the mosfet driver and the mosfet to slow how fast I turn on/off the LEDs. Are those my best options or is there something else I should consider?
I've attached my schematic (pdf), pcb layout (svg in zip), spice LT file (in zip), and 2 pictures of the voltage across the 1 ohm resistors that limit current through the LEDs (one shows the 4 us pulse followed by ringing and the other is zoomed in on x axis so you can see the sinusoidal shape of the ringing).