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I have an switching circuit containing a transistor to switch the inductive load. There is a miller effect compensation capacitor (C1) in parallel with a base resistor. (The values of the two components have been deliberately increased to better show the effect.)

What happens is, that the base voltage gets negative if the signal source voltage is 0V. It could get below -5V. The datasheet of this particular transistor states that VEBO maximum rating is 5V.

Is this already exceeding it's maximum ratings for VEBO?

schematic

simulate this circuit – Schematic created using CircuitLab

Simulation in LTspice shows a bit different (worser) results.

Base voltage

Collector voltage

There you can see additional ringing 6 times the frequency of the signal generator in this particulary case. And also presumably because of back emf there are (desired) spikes in collector voltage that gets capped by the 3 zener diodes at 36V.

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Just add a protection diode between emitter and base. It'll limit the negative base voltage to about 0.7V and might even improve the circuit's performance by discharging the base capacitor at the end of each cycle.

schematic

simulate this circuit – Schematic created using CircuitLab

Edit: You might have to play a little with the other components' values when you add the diode, however. I've added some example values in the diagram above. Note that the base capacitor is way smaller than in your circuit. This makes the capacitor only deliver a short current spike when the transistor switches on or off in order to get it into and out of saturation quickly. For most of the on-time of the transistor, R1 delivers its base current, which makes the transistor not saturate as hard as with the 100nF capacitor, resulting in improved switching performance.

You should simulate it and check the performance however, I just estimated the values. They might be off a bit.

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  • \$\begingroup\$ I also had the idea with the diode, or better a zener diode 3.3V or so. But is the VEBO maximum rating exceeded or near maximum rating? \$\endgroup\$ – bkausbk Nov 10 '17 at 14:03
  • \$\begingroup\$ @bkausbk I've had transistors die from base-emitter reverse breakdown (with a circuit like yours), so I'd say you shouldn't take the risk. A zener isn't needed because the transistor limits the forward voltage on its own already, there's a diode between base and emitter after all. Make sure to change the base driver network like I described in my answer when you add the diode because the transistor might get way too much base current otherwise. \$\endgroup\$ – Jonathan S. Nov 10 '17 at 14:09
  • \$\begingroup\$ I built it on a prototype board and I found out that removing R1 or replacing with a really high value gives the best results. The signals is now AC with about 700mv...-700mV. There is also not that ringing like in LTspice simulation which surprised me a bit. \$\endgroup\$ – bkausbk Nov 10 '17 at 21:25
  • \$\begingroup\$ @bkausbk That's also a valid configuration of this circuit, even though it might not give the best possible switching performance. With a large value for C1, R1 isn't needed anymore. You can't use the circuit at DC in that configuration, however - the transistor can only stay on for so long before the capacitor can't supply base current anymore. The ringing is gone because the base voltage is always being clamped by either the diode or the transistor's base-emitter junction. \$\endgroup\$ – Jonathan S. Nov 10 '17 at 21:55

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