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Re: Yo John

morricab: ""
Are you saying that back EMF can then prevent the transistor from shutting off properly or suffer from secondary breakdown?""

Yes. For inductors, it's the stored energy which manifests as a pseduo constant current, and it will continue to supply that current even if the output goes to the opposite polarity. Perhaps it's easier to envision a large capacitor as load..have the amp ramp slowly to the negative rail, then once the load cap is charged, change the output to the positive rail..the cap doesn't move quickly, so the output stage draws lots of current in an attempt to move the cap. The pass devices trying to pull the cap up are from the positive rail, so the devices are dissipating both rail voltages and whatever current being forced..in the case of an inductive load, it's quite easy for the pass devices to move the out to the other rail, however, since an inductor cannot change it's current as quickly, it continues to have current in the same direction for a while. A more controlled current, but the passes still have to hold it for double rail voltage momentarily. (it's easier for most to envision caps rather than inductors, but they are both reactive storage mechanisms.)

If this brings the outputs close to the secondary breakdown, poof. (bipolar discussion). As devices approach this dangerous region, several mechanisms happen..as you raise the C-B voltage, the neutral base region gets thinner (early effect), increasing the gain of the device. Also, the leakage collector to base increases, this leakage is amplified by the device gain.. And thermally, gain is also going up..these conspire to make the output devices "cramp up"..this is secondary breakdown.

..Consider a darlington config output..if nothing is put in to pull the output bases down when this "cramping" happens, the device will clamp on. As this starts, the only way to stop it is to pull the base down hard. If the only thing the base sees is an emitter, then that driver has to go reverse breakdown base-emitter to get any current into the base. (note, I've been using current nomenclature loosely for ease of discussion). That'll probably toast the driver e-b junction.

Cheers, John

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  • Re: Yo John - jneutron 06:20:46 03/22/07 (4)
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