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I'm working on a small coilgun, and am running into an extremely annoying problem with my circuit.

Circuit schematic:

enter image description here

PCB Layout: enter image description here enter image description here

Key components:

  • 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.

Testing single stage with direct wiring (Works) enter image description here

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 :(

I tried:

  • 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).

Multiple coil assembly with single stage wired up (Fails!) enter image description here

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.

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    \$\begingroup\$ Bypass capacitors are critical, especially for MOSFET drivers that source and sink high currents into high capacitance gate loads. Your schematics don't show ANY capacitors anywhere. The MCP1406 data sheet ww1.microchip.com/downloads/en/DeviceDoc/20002019C.pdf explains this requirement. Fix that and report back. Also show what you use for the flyback diode - it needs to be fast recovery. \$\endgroup\$
    – PStechPaul
    Jul 24, 2022 at 0:39
  • \$\begingroup\$ Thanks @PStechPaul, I've noted the need for a bypass capacitor but I'm just now grasping the importance I think. I will try again once a new shipment of drivers come in to see if that resolves the issue. I do wish I understood why they worked in the previous test setup though without them :( As for the flyback diode -- I will update the post -- but it is a BYC30X-600PSQ, which I think is pretty fast (45ns). \$\endgroup\$
    – saguapang
    Jul 24, 2022 at 2:37
  • \$\begingroup\$ That should be OK. Another thing to consider is inductance in the flyback resistor if it is wirewound. And I don't think it needs to be 45 watt. \$\endgroup\$
    – PStechPaul
    Jul 24, 2022 at 4:22
  • \$\begingroup\$ What @PStechPaul said, and please post your layout. ”voltage peaks out at 92 V. ISC027N10NM6 has a breakdown of 100V” Pretty much bound to fail. You need margins. \$\endgroup\$
    – winny
    Jul 24, 2022 at 6:40
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    \$\begingroup\$ @PStechPaul I made sure to avoid wire-wound flyback resistors, and the Ohmite one is a thick film resistor. I'm not ruling out the avalanche situation completely, but I did test shorting the resistor (so peak of 8V on breakdown), and it still fails. Also I am seeing the driver itself causing the most issues, as it remains stuck on regardless of its input after restarting the circuit (with and without a load). I've got parts coming over next-day delivery, so I will have a chance to try adding the bypass capacitors and other possible solves soon! \$\endgroup\$
    – saguapang
    Jul 25, 2022 at 3:12

2 Answers 2

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So after much trial and error (and about 25 drivers blown up for testing), I've come to find a number of problems and their solutions. Thank you everyone for the comments, and I hope this helps someone who runs into the same issue:

  1. (Biggest problem) I did not account for the flyback RLC voltage oscillations! While the flyback diode + resistor was chosen to clamp the voltage @ ~80-90V, the RLC oscillations on top of that was making it peak out past the 100V MOSFET limit. While the MOSFET was able to function fine afterwards, parasitic capacitance in the circuit was causing the voltage transients to ripple throughout the circuit, which at the MOSFET driver was occasionally hitting 24V (well above the 12V input). This is outside of the driver's 20V max supply range, destroying the driver. An RCD snubber network across the MOSFET removed the RLC oscillations, and I never saw it peak above the expected 80-90V range afterwards (even after 100+ firings!).

  2. Decoupling capacitors around the driver were useful, not just for providing a local burst of current, but because it further damped the voltage transients that rippled through the system. This dropped the transients even lower, to 17V at most.

  3. I should have picked a higher R1 resistor value (sensor resistor). I didn't realize how little current the diode would allow even with full light exposure, so I was only getting about 1/2 the voltage range to work with for sensing. Using a 1M ohm resistor made it better to tune and also minimized noisy switching coupled with the turn on/off transients, which caused erratic switching oscillations.

All 3 of these problems catalyzed each other. Now that they're all addressed, the system is very stable, with almost 0 unwanted switching oscillation! I was able to do a multi-stage test over 100 times, and not a single part got warm to the touch :)

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If I understood correctly, it works with one stage with the projectile. The only major difference between one stage, and multiple stages is that the projectile is travelling faster.

It should be noted that the voltage induced by a magnetized piece of metal traveling through a coil is proportional to the speed. It's possible that with more stages you created enough kickback voltage to break down the isolation between the MOSFET gate and drain, which in turn damaged the driver.

To confirm this, with the power disconnected, I would measure between the MOSFET gate and drain, and also measure between the MOSFET drain and source. If either are short, then that's the likely cause. Also, measure the resistance between the driver output and the positive power supply (between pins 1 and 7) with the power off to confirm a short.

It may also be worthwhile to either use a higher voltage MOSFET or put a TVS across the MOSFET drain/source.

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  • \$\begingroup\$ Sorry, to clarify the setup, the coil is failing with a single stage (but multiple coils next to each other). So the only difference between the previous success and current failure is that there are coils next to each other now. Also I guess the 12V power line is going over the coil instead of around it (red wire with yellow heat-shrink). But I can't imagine that causing any issues. \$\endgroup\$
    – saguapang
    Jul 23, 2022 at 22:52
  • \$\begingroup\$ @saguapang I wouldn't completely rule out problems caused by the 12V wire position. The wire loop created by 12V wire and GND wire could pick up induced voltage from the coil when it is turned on. Ideally you would run the 12V wire right next to the GND wire and stay away from the coils \$\endgroup\$ Jul 24, 2022 at 7:24
  • \$\begingroup\$ @LarsHankeln I should probably run some tests on my oscilloscope, but I always thought induced voltages -- even from strong coils -- would be in the mV range. I will double check though! Hopefully some simple placement or EM shielding will be enough if that is the problem. \$\endgroup\$
    – saguapang
    Jul 25, 2022 at 3:16
  • \$\begingroup\$ @saguapang Were you able to measure the MOSFET and does drain-source or drain-gate measure short? If not then the other likely cause is a power supply glitch, which would be solved with decoupling capacitors. \$\endgroup\$
    – user4574
    Jul 25, 2022 at 4:17
  • \$\begingroup\$ Yep, tested the drain-gate + drain-source and neither are shorted. I also was able to recheck the flyback voltage after removing the broken driver and got 64V at drain, which is still well below the safety margin (I see no clipping either). Once my new shipment comes in, I will be trying the decoupling capacitors and report back in! \$\endgroup\$
    – saguapang
    Jul 25, 2022 at 9:01

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