Reverse Vbe BJT (BD202)?

[boiss]

I have been trying to find out what is the max reverse Vbe of the the BD202. The data sheets I have found don't quote this.
This is a medium power PNP jobbie which I have pressed into service for my wireless charger etc. If this device was used in a complementary PP output stage then I would have thought that this could reach quite high values ie >20V.
Mike

[Keithstv]

Typically the reverse Vbe of a silicon transistor is 5v.

Keith

[Refugee]

I would test it using an HT power supply and a 220K resistor. A DMM will then reveal it for you.

[boiss]

That sounds a bit destructive to me, I remember reading that even seconds at the reverse breakdown point can cause relevant damage. At the moment I've got a pulse of about 10 (limited by a zener), reckon I'd better replace it with a simple diode.
Damage done ?????
Mike

[AlanBeckett]

I remember reading that even seconds at the reverse breakdown point can cause relevant damage.

Reverse biased B-E Junctions of Small Signal Transistors are/were often used as Zeners. It will be OK provided you limit the Current.
The Data Sheet I looked at said 5V Absolute Maximum.

Alan

[Refugee]

Reverse bias is not destructive as long as the power dissipation is limited to a low level. Limiting the currant to 1ma will suffice for almost everything except those special diodes found in expensive cable testers and such like.

[Kalee20]

If this device was used in a complementary PP output stage then I would have thought that this could reach quite high values ie >20V

I've puzzled about this - in a normal complementary P-P stage the reverse Vbe doesn't reach more than one diode drop backwards - and is almost always less. When the bases are taken in such a direction to turn one transistor off, the other transistor turns on all the harder and takes the turned-off transistor's emitter with it.

As to can reverse Vbe do any harm? I have read that noise performance in small-signal devices (and on input stages of IC op-amps) can be degraded, but I have no experience of this. On the other hand, power switching stages are fairly often operated with Vbe in reverse breakdown to get the best possible turn-off speed. Current flow and duration are limited thus excessive energy dissipation in the wrong part of the device doesn't cause any thermal problems.

[boiss]

Hi Kalee,
I hadn't thought about the other emitter going down......
The next bit is what interests me;-
On the other hand, power switching stages are fairly often operated with Vbe in reverse breakdown to get the best possible turn-off speed. Current flow and duration are limited thus excessive energy dissipation in the wrong part of the device doesn't cause any thermal problems.
The best efficiency I have achieved was driving Rev Vbe to about 10V but the Zener used for limiting at this voltage was getting a tad too hot (1W) device. It has now been replaced by 16 1N4007s. Quiescent input 2.5W.
Using a single 1N4007 reduces the efficiency and Quiescent input rises to around 7W with the device becoming hot on its' small heatsink.
The best Output was achieved at around 8V but the heating effect on the device was quite noticeable, Quiescent input around 4W.
Thanks
Mike