PNP BJT as a 'switchable' flyback diode?

Question

I have an automotive ignition coil which I'm using as an ignition source for a project I'm working on.

During normal operation, the primary coil current will be interrupted periodically to induce large counter-EMF spikes on the primary (just as it does in an automobile). However, I would like to prevent this spike from occurring when powering-off.

The ignition circuit uses an IGBT on the 'low side' of the primary coil, so my original thought was to simply 'ramp off' the IGBT gate voltage using an RC circuit. Unfortunately the transfer characteristics were just too steep and this did little to prevent the inductive spike (without resorting to prohibitively large values).

My next thought is to simply short the primary during power-off. While other devices are perhaps better suited for this purpose, design constraints have narrowed my options to a PNP BJT.

I've created a (simplified) schematic to better illustrate my intentions:

Here, S1 (used in place of the aforementioned IGBT) will cycle the current through the coil on and off at anywhere from 2 to 200 Hz during normal operation . Upon powering off, S2 (or, more likely, the IGBT and an NFET) will interrupt current through the primary coil while simultaneously pulling the base of the BJT low, allowing current to flow between the primary's terminals, 'bleeding off' the counter-EMF.

Is this a viable and robust approach? I'm concerned about the BJT's ability to withstand the repetitive and profound dV/dt. I'm looking for thousands of hours of continuous normal operation, so a robust design is a must.

Any advice or insight is, as always, very appreciated.

Answer 1

Based on points made below, it seems S2 would be able to handle both primary voltage spike voltages and primary current so simply using an S2 contact and a diode across the coil primary would seem to achieve the desired results. See discussion below.

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Can you instead leave the IGBT off after an intended spark and then turn it on for long enough and then charge when a spark is required. As it must be able to charge when running at a 200 Hz rate, the maximum delay introduced by this method will be <= 5 ms.

If using the method shown I would probably bias and connect the transistor slightly differently to the circuit shown, but the method is tolerable provided the transistor is able to withstand the repeated reverse polarity primary spikes. How large these are will depend on your system design, and especially on Vspark and turns ratio. In a spark coil design like this the turns ratio is usually << Vspark/Vdc_in so Vprimary_spike is > to >> Vdc_in x turns_ration.
eg if turns ratio = 10 and Vspark is 5kV the Vprimary_spike ~= Vspark/TR = 5000/10 = 500V. Transistors capable of repetitively withstanding this voltage repetitively are available, but a gentler method is probably desirable. (A BJT with a voltage rating at least equal to that of the IGBT at S1 is required. Depending on the size and cost of S1, whatever is used there would work for the clamping transistor.

The bottom contact of S2 is exposed to the primary spike voltage, and the top contact of S2 carries the primary coil current. So, using S2 to instead switch in the transistor and diode only when required would mean the transistor was never exposed to primary spikes.

Alternatively, given the above abilities of S2, simply using an S2 contact and a diode across the coil primary would seem to achieve the desired results.

Answer 2

The primary and secondary are magnetically coupled. If you want a "Kick" out of the secondary expect a similar (through scaled by the turns ration) "Kick" on the primary. By "Kick" I mean L di/dt.

You want a fast turn off on the primary least you dump energy that you would rather have transferred to the secondary. You will simply need to clamp the primary voltage to a safe level and live with the resulting clamped output on the secondary.