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I recently decided to build a simple single-stage coilgun that is triggered by a SCR. For that, I had to design a low-power boost converter that only draws around 2 W at 9 V so I can power it from a 9 V battery. The booster works as expected and charges the cap in roughly a minute. The booster works in DCM (I believe) at a frequency of 7 kHz and a duty cycle of 75%, I mainly used STP4NK60Z MOSFETs for the converter.

However, when connected as shown below, the MOSFET of the converter always dies (short between drain and source) after the 450 V 1000 uF cap has discharged into the coil, or when the converter charges the capacitor again. I thought about some inrush currents due to the large current intake of the cap at the beginning of its charge, but the converter's alleged peak current (~0.6 A?) doesn't exceed any max. ratings, I also did soft start the cap and had the same problems. Then I thought about MOSFET protection and added a Zener diode between the gate and GND, as well as a snubber, which once again had no impact. I used pretty cheap MOSFETs with on-resistances of 2-4 Ω and max. current ratings of 2-4 A. However, I also tried to use an IRFP460 and it also died after only a few charges. I think that it could have something to do with the MOSFET missing a proper gate driver, some EMF issues, or the inductor going into saturation. What do you think is causing the MOSFET to fail and what do I have to change to prevent the MOSFET from it?

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  • \$\begingroup\$ Is D1 still operational after the MOSFET fails? \$\endgroup\$
    – vir
    Commented Oct 10, 2023 at 22:39
  • \$\begingroup\$ Correction: I mainly used STP4NK60Z mosfets for the converter* \$\endgroup\$
    – kall gakr
    Commented Oct 10, 2023 at 22:39
  • \$\begingroup\$ D1 is still functional after the mosfet failed, as well as the other diode \$\endgroup\$
    – kall gakr
    Commented Oct 10, 2023 at 22:50
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    \$\begingroup\$ @kallgakr Please discuss what you imagine happens to the coil when the capacitor being discharged is finally finished discharging through it and the coil is at its peak current. \$\endgroup\$ Commented Oct 10, 2023 at 23:49
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    \$\begingroup\$ @kallgakr When the 450 V 1000 mF capacitor has no longer any voltage across it (close to it, anyway), the coil must start to have its peak current decline. At that very moment, the voltage across the coil reverses polarity. Then, as the right end of it is close to ground (through the SCR still) the other end goes negative with respect to ground. It must. At this point the cathodes of both D1 and D2 are forward biased (or may be.) Q1's body diode is also forward biased, as well. And you are still also allowing your 555 to run, as well. What do you see there at this moment and moments following? \$\endgroup\$ Commented Oct 11, 2023 at 7:14

2 Answers 2

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If you remove the capacitor in series, you end up with a "standard" pulsing circuit, where D2 is keeps the current flowing through the coil, slowly dissipating the energy.

When you add the series capacitor, you close down this diode path after a slight initial surge, once the capacitor has charged enough.

What then happens, is that you force the freewheeling current instead to go through the SCR (it can cope well) and the Q1 body and D1:

enter image description here

The diode is rather beefy with a generous surge current rating, it looks capable of surviving a few pulses. But with Q1 being a tiny MOSFET, it is not ready for the 100s (?) of Amps.

If you want to achieve a shorter pulse, reduce the inductance and resistance of the coil. This will increase current, but as long as the switch losses remain small, you still transfer most of the energy into the magnetic field.

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  • \$\begingroup\$ So the adding of the 450v 10 uf cap leads to practically the same result as having no reverse current at all? But why is the current able to go through the MOSFET in the blocking direction of D1? Is it because d1 temporarily breaks down? I also fear reducing the size of L2 since it already is rather small and it will worsen the velocity. ( I tried a smaller coil without the reverse cap with a much smaller inductance and had a velocity decrease of like 50%) Are there no other ways to give the reverse current a different, fast way to flow? \$\endgroup\$
    – kall gakr
    Commented Oct 11, 2023 at 8:13
  • \$\begingroup\$ @kallgakr Nono, both D1 and Q1's diode are forward biased. See also the comment by periblepsis. The current flows rightwards in your schematic and the voltage between D1 and L2 will be negative two diode drops during this phase. I have added a sketch of the freewheeling current loop. \$\endgroup\$
    – tobalt
    Commented Oct 11, 2023 at 8:21
  • \$\begingroup\$ Thank you very much for the explanation. Do u think that replacing the 450v 10uf cap with some low ESR film or ceramic capacitors in parallel would help that this current way doesn't get blocked so fast( or is the capacitor in series with d2 just generally a stupid idea?). \$\endgroup\$
    – kall gakr
    Commented Oct 11, 2023 at 8:40
  • \$\begingroup\$ @kallgakr I think it is a bad idea to put the series capacitor at all (as is). I think there are some strategies that use a second capacitor to catch the flyback charge, but these could probably use IGBT instead and implement more switching to exchange the first and second cap (for energy recovery). \$\endgroup\$
    – tobalt
    Commented Oct 11, 2023 at 8:44
  • \$\begingroup\$ What would happen if i put a resistor in series with d2 instead of a cap? One which has a resistance high enough that the magnetic field of the reverse current, even though it exists longer, is too weak to pull the projectile back to the center of the coil and thus decrease the velocity. \$\endgroup\$
    – kall gakr
    Commented Oct 11, 2023 at 11:42
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To confirm guess of @tobalt ... 600 A peak, ~ 1 ms pulse.
Made with microcap v12. https://archive.org/details/mc12cd_202110.

enter image description here

Here is the picture for v(vp) ...

enter image description here

Update with a diode accross M1.

enter image description here

And in fine ... Note that the current in L1 is higher ...

enter image description here

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Added views with \$L2 = 85 uH \$

enter image description here

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  • \$\begingroup\$ Thanks for your effort, ig something like 400A then still flows through the mosfet? \$\endgroup\$
    – kall gakr
    Commented Oct 11, 2023 at 11:47
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    \$\begingroup\$ @tobalt I will add this voltage ... \$\endgroup\$
    – Antonio51
    Commented Oct 11, 2023 at 16:00
  • \$\begingroup\$ @kallgakr perhaps more ... Depending of the "time step". I make a new simulation to confirm. I added also a diode (d3) accross M1 and "some" parasitics components. \$\endgroup\$
    – Antonio51
    Commented Oct 11, 2023 at 17:23
  • \$\begingroup\$ Thank u very much for that simulation \$\endgroup\$
    – kall gakr
    Commented Oct 12, 2023 at 11:17
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    \$\begingroup\$ Added picture fo L2 = 85 uH. Made with microcap v12. archive.org/details/mc12cd_202110 \$\endgroup\$
    – Antonio51
    Commented Oct 18, 2023 at 9:14

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