Coil driver circuit has to many faliures

[Bob Edge]

View attachment lift.pdf

Hi,

I have a circuit (attached) that is used to drive a coil. The coil switches on for upto 2 seconds. The capacitor C24 charges to roughly 150V, when the coil is energised it gets an initial high voltage, then this quickly settles down to 24V, since the current is supplied through the six resistors. My trouble is that the switching device Q2 fails, not very often, but enough for it to be a problem. I have tried MOSFET's and IGBT's (with Q1 removed) but I still get the odd faliure. The coil is 8 ohms, and 26mH @ 1KHz.

Can anyone see what the problem is, or maybe sugest a bullet proof way to do the job?

Regards
Bob.

 

[WimRFP]

Hello,

As you have the driver transistor connected to the collector of the IGBT, the IGBT doesn't switch, it stays in its linear region, so it will dissipate and may put stress on the chip due to uneven current distribution when using in the linear region (that is non-saturated operation).

As you only need to charge the gate, you may create a 12V source (zener diode, resistor, capacitor?) that you use to feed the collector of Q1 and U17. This enables the IGBT to switch and avoids to high gate voltage.

You may need a second PNP transitor to discharge the gate when off-switching. Look on the web for gate driver circuits for how to use the additional PNP transistor to discharge the gate.