BC857 Destroyed

You have not said anything about the circuit, or the conditions when it gets destroyed (on test, or in the field, etc).

"John Devereux"

Invariably caused by C-E overvoltage.

Wanna bet his dead BC857s are all relay drivers ?

Whatever, no-one cannot reliably diagnose the issue remotely with a Kraut FOOL like him supplying the only info.

...... Phil

Best to post a schematic and a description of what is connected. Motors, relays, solenoids etc.? Did any of those change? Line lengths? Environment? Often this kind of stuff is caused by spikes that for some reason became larger than they used to be.

Hello

I now the one with circuit, since this is not a private project i can't make the schematic public :(

But yes, it has to do with the relay, but not directly, the driver for the relay is BC847 and is protected by diodes. (across each relay coil) and no one of the bc847 is destroyed...

The transistor is sitting in a voltage doubler for making a higher relay voltage when the relay activates. You charge up a capacitor and add it to the 10V.

The supply voltage is 10V, so it doubles it to 20V. I have measred the current in the C-E to have a peak of arround 40mA. And there is BAV99 across each relay.

The transistor seems to be destryed in the assambly + test, but this is not for sure.

But i want to point out the circuit has been runing in several years and in several thousands of the same units without problem, like also to say the product has passed EMC/Surge/Burst test...

I just wondered if anyone else have seen a transistor with these problems, since the transistor is not comepletly shorted between C - E like a normal burned transistor.

Rapzak

For the schematic i need approvals to do that, but read my other post for 2 min ago

Some of them atleast are destroyed in the production house under powered test or ESD what so ever... (since they are aprroved to ESD things..)

so no wires length or anything has changed, and as i said it is less than 8% there have the error....

Kasper

Yes, I've seen this kind of thing. Probably spot damage which you'd be able to see if the microphotography was done properly.

Show us the circuit, including all the details of the load and the driving circuitry. It's almost certainly a design flaw, barring something strange like the incorrect or counterfeit components. Are we talking about the BC587BS dual? That's not much of a transistor, and is probably quite easy to damage.

Best regards, Spehro Pefhany

You don't need to post the whole thing, only the stuff around that transistor. Just cut it out of a copy, scan and post.

Did they migrate to a RoHS process with higher reflow temperatures?

Does the circuit function within safe limits in SPICE? The description seems like a circuit that can be accurately simulated for diagnosis in SPICE.

D from BC

Schematic:

Hello

I am just trying to get this running...

This is not a design i have made, it is 4-5 years old, så i don't know so much about the details jet.

Rapzak

Hello

No, it is a single in sot23...

I have also seen a report with these spot from dual transistor, but is is quite difficult to see between C and E, while they are laying in top of each other on the chip, i only have a top view

Rapzak

The relay coils are 12V, 576 ohms, right?

So if the doubler is energized simultaneously with the 3 relays the peak collector current is over 100mA with only 2mA of base drive?

It's also possible that there's something wrong with your diodes, since without a diode, the voltage will spike negative at the collector of TR8 (more than enough to break down the transistor) when the doubler is shut off. There are single diodes in the same package with one connection unconnected. If the BAV70s on failed units have other markings than "A4" you should check that out, and check the diodes electrically in any case.

Best regards, Spehro Pefhany

No 18V 1276 Ohm for 1 and the 2 other is 1620 Ohm. Also the software locks for not activating all 3 releays at once. Then we could discuss just when the uC starts up, all 3 + boost is activated... hmm...

The allowed peak current is 200mA.

The biggest problem with that are we have seen problems back to last year, so the problem is made over several production lots. But we have ofcause checked the diodes on several bad units, and they measured correctly.

Rapzak

Do you have a datasheet for that part? *With* an SOA curve?

Under some conditions, the PNP is exposed to as much (or more) stress during switching as the NPN (NPN and PNP switched off simultaneously, for example).

I would *not* use such wimpy parts as those in this application. Put MMBT4403 or MMBT2907 parts in there (same footprint, maybe same pinout (?)) and I'll bet your problems go away, if they are not diode related. There is negligible price difference. Might be a good idea to increase the base current on the PNP a bit too, like up to 5mA or

Best regards, Spehro Pefhany

Please take a look:

Rapzak

Yes, this point also came to my mind after i have used a litle time with spice.

So that i defentlig will try tommorow to measure

(and yes, Kasper is my real name :))

Now, is this your real name?

Be careful with that assumption:

This has a nice thermal graph for pulse loads in there:

Can't you just use a stronger transistor for a while and see if those fail as well?

BTW, what prevents the relays and C10 from ringing? Did you try a zener to ground so at least it can't swing above safe limits for TR8 or below ground? It might be worthwhile to hang a fast digital scope on TR8's collector and trigger that with a relay command. It could yield an ugly surprise.

Okay, here's the data sheet:

The part appears to be a BC856BW.

An MMBT2907A is actually a bit cheaper, BTW, probably because it's more common and will replace the wimpier parts in general purpose switching applications.

Best regards, Spehro Pefhany

There is some stuff SPICE won't show because it doesn't know about it. For example ringing of the relay coil itself. Also, the inrush current of a relay coil can be quite substantial. You can measure that by placing a 10ohm resistor in the emitter to ground node of the switch transistors. If you want to measure current spikes at nodes that aren't at ground level you can make yourself a little current transformer. But never, ever, omit the burden resistors when you do that. It's a very handy addition for your toolbox.

Sounds German ;-)