I'm working on a small coilgun, and am running into an extremely annoying problem with my circuit.
- Flyback diode: BYC30X-600PSQ
- Flyback resistor: Ohmite 45 W 1 ohm
- Coil: 95 uH
- Switching MOSFET (Q1): ISC027N10NM6
- MOSFET driver (U2): MCP1406T-E/SN
I've chosen everything to run right at it's limits (probably a bad idea in hindsight).
The coil is 85 mohm, and my battery's internal resistance is 15 mohm. Fully charged it's 8.4 V, so I should be getting around 84 A of current at most. My flyback resistor is 1 ohm, so the flyback voltage peaks out at 92 V. ISC027N10NM6 has a breakdown of 100V. I've tested shorting the resistor though, and it still fails the same way so I don't think this is the cause.
When testing my circuit in a single isolated stage, it works fine. I've repeat fired it 9-10 times and nothing ever got hot and everything behaved as expected.
But then when I got to assembling the stages together, and started testing again in that scenario, the gate driver kept blowing up on the firing test! I tested slight variations in each one, but they all resulted in the gate driver breaking (in the same way), and I'm now out of boards :(
- shorting the flyback resistor (to rule out breakdown voltage issues)
- adding a gate discharge resistor from gate to ground (10k)
But none of these affected the circuit. The driver still broke in the same way (described further down).
My gate driver (MCP1406T-E) and circuit worked perfectly fine when using a test LED, which is just a small LED and a resistor that I plug in instead of the coil's leads. This is how I adjust the comparator before plugging it into the coil.
The driver is inverting, so it provides no signal when it has an input voltage, and only turns on when the input is low. I've verified this functions correctly with the test LED and my oscilloscope.
Since everything looked good on the test LED, I moved onto testing with the coil plugged in, but it kills the gate driver when I insert the projectile! As soon as it turns on, the driver gets stuck on, which leaves the MOSFET on. I've unplugged it and tested with the LED again, and can see that regardless of the voltage comparator signal (which is still outputting correct values), the gate driver is stuck outputting 12 V.
I've also verified that the MOSFET itself is still fine by removing the driver, and poking the gate with the 12 V wire (and adding a discharge resistor).
Some things to note:
- In the future, I'll probably go for a Zener diode instead of a flyback resistor to cap the voltage to a predictably lower value than the MOSFET breakdown. But considering I ran 2 tests with the flyback shorted, I don't think the flyback voltage is the culprit.
- I've done some research and realized I should have added decoupling capacitors at all the ICs (comparator + driver), but that still doesn't really explain why it worked in the old setup but broke in the new one.
- Some people have mentioned the EM pulse could induce voltages in the photo diode leads, which might trick the comparator/driver to turn on. Unfortunately I didn't think to hook up the voltage comparator to my scope while firing, but again it worked in the old setup!
I'm completely at a loss as to what to do to fix this. If anyone has any ideas I will graciously accept anything. It literally feels like I've lost everything right at the finish line and I'm tearing my hair out at trying to figure out the issue.