Re: BC847 Destryoed

On Sat, 13 Oct 2007 15:32:07 -0700, Marra
<cresswellavenue@xxxxxxxxxxxx> wrote:

On 5 Oct, 20:41, Pieter <dit3_werkt_ook_n...@xxxxxxxxxxx> wrote:
On Mon, 24 Sep 2007 06:04:33 -0700, Rap...@xxxxxxxxx wrote:
Hello
I have a circuit with a BC857B where the transister gets destroyed.
I am a litle out of ideas. When mounted on the board i have seen
arround 150 transistors out of 1800units where destroyed.
The transistor in first place seems shorted between collector and
emitter, the ressistance is between 5 to 250 Ohm.
The funny part is both "diodes" in the transistor can be measured
from
base. And the one with 250Ohm i have tried to activete in the circuit

Notice that the resistance means nothing here, it is just a value your
multimeter comes up with depending on current it uses. Use the 'diode'
measurement of the meter, it will send a constant current through the
diodes and show the resulting voltage. That should be 0.6, and not
working in the opposite direction.

and the transistor seems to react and make lower ressistance between
collector emitter. (so in fact it looks like a normal working
transistor with a build in ressitor of 5-250ohm between collector and
emitter)

So maybe that really is so.





The product has been produced for several years with out problems,
they started in earlier this year..
I have got several transistors decapped, so there seems not to be any
burned inside the transistor and the boundings where ok. I have only
seen the chip from the top, and collector emitter is in top of each
other.
The product is assambled by proofesinel production house and been
both reflow and wave soldered (only wave soldered on the bottom side)
Another funny things are there are 2 of the same transistor on the
board, we are only seeing one of them with error so far.
I have measred the currents in the circuit it seems ok.
There has been tried to change transistor vendor, and there has been
seen the same problem, but only 25 of 1800 with same error.
Does anyone have a auggestion to the error, or seen similar ??
Best Regards
Rapzak

Hi,

I think that your transistor gets partially damaged. As if it starts
conducting at some places in the transistor. There can be many causes,
but here are two plausible ones:

1) A high voltage peak gives a local breakdown in the chip, and makes
a bad conducting channel. This could be caused by for example this
transistor controlling the coil of a relay without damping diode or
snubber network. Did you change the a relay with biult-in diode to one
without? Or changed it to a relay woth other inductance?

2) SOA, Safe Operating Area. The maximum power of a transistor is not
only defined by current, voltage and power, but also by SOA.
Especially at higher voltages, current must be lower than expected.
Even when voltage, current and power are all within specifications,
the transistor may fail. What happens is that locally at higher
voltages, the p-n layer gets warmer, starts condcuting more there,
gets warmer etc etc. This may also cause your problems. Maybe you are
using different supply voltages? Or maybe your design was already
outside specification, but now it is used in another environment where
it is used differently? Notice that as this problems occures at higher
voltages, it may happen when you do NOT load the circuits.

Pieter- Hide quoted text -

- Show quoted text -

If the transistor is being destroyed then:
1/ The voltage across CE is too much.
2/ The BE voltage is getting too high.
3/ Could be a static discharge problem and they are destryoed by
handling but unlikely.
4/ The transistor has been put in the circuit the wrong way around.

http://www.ckp-railways.talktalk.net/pcbcad21.htm

And some more:
Negative BE voltage (non-conducting direction), can already break down
at 5V and actually destroy the transistor depending on the components
around it.
High temperatures, power must be limited at higher temperatures.
Too high collector current.
Too high emitter current.
Too high base current.

So there can be many more causes, Safe Operating Area can even limit
the maximum current of power transistor to half the current you would
expect. And this also applies to low power transistors.

Notice thet exchanging C and E will not destroy the transistor but it
will give you a transistor with low quality, low gain etc.

Pieter
.

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