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Power transistor with avalanche diode

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CMOS

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Could anyone explain what is the purpose of an avalanche diode connected between base and collector of NPN power transistor like in this transistor array chip **broken link removed**?

What are these transistors called? I was searching for an equivalent in Digikey, but couldn't find one since I am not sure what to search for.

 

I've seen this sort of configuration before in motor drives using IGBTs. The diodes (usually TVS devices) are there to prevent peak switch voltage from exceeding some level, which would cause failure. When the voltage rises higher than the reverse breakdown of the diode, then the transistor will turn on regardless of what the controller is doing. The reasoning behind this is that it's better to have a momentary turn on than to let the switch die, which would likely result in it becoming a permanent short circuit.
 
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Thanks for quick reply. Do you know any transistor with diode built in? I am looking for something with Ic ~3A and Hfe ~500.
 

I doubt you'll find many with that built in. The breakdown voltage is usually very application specific, so they're usually external. Also I've never seen it done on a BJT before.
 

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