OK, I had a go at simulating back EMF from a 4mH (which seems 'normal', 40mH is a bit much) inductor with 5.5 ohm series resistance and a 56V supply. Worst case, driven from a FET being switched by a perfect square wave and 10R gate resistor, peak is 180V. So the TVS would clamp the worst of it, but still more than I'd like to subject a 150V device to.
Good news though, the expensive driver I want to protect is 0.9mH and 5.5 ohm, no crossover, so with a 63V supply it only kicks 70V.
Good news though, the expensive driver I want to protect is 0.9mH and 5.5 ohm, no crossover, so with a 63V supply it only kicks 70V.
Let's say we want belt and suspenders, we could connect two extra clamping diodes between speaker output and rails (as is done in some other protection circuits), which limits the maximum drain-source voltage to twice the rail voltage, even without TVS. Then with a 150V device there would be no risk of exceeding the maximum voltage rating, or am I wrong ?