how does this free-wheeling protection circuitry work?

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I'm looking at the datasheet for a high-side switch (TI TPS2H160B)

I understand the Diode D to protect against revers voltage, but I don't understand what the TVS is doing in this context:

 

 

My understanding is that TVS are zeners, so conduct in reverse.  But with the ordinary reverse voltage diode D there, how can the TVS be effective?  It looks to me like D would still be the limiting factor, and it a TVS is going to be used it should be a single uni-directional TVS without D in series with it.  Is this just a case of the datasheet being sloppy or am I not understanding it? 

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The voltage at outx will go negative when the current is interrupted, therefore the voltage will be limited by the TVS.

When the fet is on, without D, the TVS would be forward biased and conduct - not what we want, so we have D to avoid this.

Last Edited: Sun. Jan 29, 2017 - 10:15 PM
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So the purpose of the TVS is, that when the negative voltage event ends, the TVS stop conducting faster than D?

In this case is it as good to instead use a D with very low reverse recovery time?

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The tvs clamps the reverse voltage to a given value compared with just using a normal diode alone that would clamp early and slow down the colapse of the magnetic field. I don't think the reverse recovery time would be significant as the inductance is determining the speed of colapse.

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Ah that makes sense.  So the TVS represents a trade-off: some negative voltage, but faster field collapse (and therefore turn-off) of the controlled inductive load.

Thanks very much.