How to limit the flyback voltage?

[mschmahl]

Hi all. I´m building the crossbow with some guy´s. It is a variant of the hammerhead. Some of us stumble over the problem with a to high (over 500V) flyback-voltage from the coil. Is there a way to limit this voltage? I have attached a part of the schematic of the crossbow. The problem is first R24 gets really hot, then the IRF gets hot also. So we have changed to an IRF 840. It is not so fast hot, but gets also hot. So the IRF would not last for a long time. Next change is R24 to 7W rating (before it was 3W). So only R24 gets really warm. If you now use a bit more power than 12 V, e.g. 14.4 V so R24 and the IRF get´s hot again.
This high flyback is no problem of the uH of coils. I have try different coils from 236 uH TX(military-coil with separate RX), 297uH (homebuilt mono-coil for mirage), 302 uH (homebuilt basket coil), 506 uH (35cm mono-coil).
Reduce the pulse width of the coil pulse is no option, because we need a longer pulse width for relic-hunting. Actually it is around 110 uS.

[eclipse]

On the place of IRF740 on Delta Pulse (modified) there is diode STTH1210D so maybe it will do better job than the resistor for you, if you can obtain it you can try that?

[mschmahl]

Sorry it doesn´t help.

[chemelec]

I Don't know what your Supply Voltage is?

But Check the Gate Voltage on the Mosfet with a Scope.
If it is less than 10 Volts, your Mosfet will not fully Turn on, and than it Generates Heat.

Either Try to increase this, Or you could put a Heat sink on the Mosfet to help reduce this heat.

[mschmahl]

Thank´s a lot. The gate voltage is around 11V. So only the heat sink can help. Is it better to change to a higher rated FET?

[Davor]

You'll surely get better off if you ensure faster discharging of a MOSFET gate. It discharges through 220ohm resistor. If Q8 and Q9 are arranged opposite, the gate would discharge through an emitter instead of a resistor, also Q9 would react faster, and MOSFET would cut of faster, and get less heated.

[Davor]

Upon checking the schematic more thoroughly, and running a simulation of the circuit, I conclude that the majority of heat is generated by current flowing through body diode at avalanche voltage. Faster switching the MOSFET gate would help only a little.
If you are not going for a higher voltage MOSFET, you may consider a snubber circuit to keep flyback voltage below avalanche limit.
Or simply put a bigger heat sink.