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Old 10th January 2004, 09:07 AM   #1
mikelm is offline mikelm  England
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Default transistor melt down ?

If a tranistor has absolute max ratings of:

Collector current 20mA and Vce 35V

What will happen if I run it at Vce 7V and collector current 35mA ( 0.25W )

Will this damage the device ?

If so can anyone explain to me why ?

The absolute max power is not stated

is 0.25w too much for the average i/p tr ?

I realise that the beta may fall off but that is easy to check.
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Old 10th January 2004, 10:04 AM   #2
mikelm is offline mikelm  England
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.....or to put it another way

I understand that too much voltage or too much heat could cause damage.

Is there anything else I have to watch out for ?
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Old 10th January 2004, 10:19 AM   #3
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Quote:
Originally posted by mikelm
.....or to put it another way

I understand that too much voltage or too much heat could cause damage.

Is there anything else I have to watch out for ?
Yes, all the maximum ratings specified in the data sheet,
including max. Ic, which you asked about first.

As for the reason for the current limit, I am not sure. I seem
to remember some data sheets for power transistors
mentioning the bonding wires as the limiting factor. However,
I doubt that is the case for small-signal devices, as in your
case. High injection certainly comes into play somehow and
it limits the beta at high currents, but I don't think that per
se gives you any maximum rating. After a quick glance in my
semiconductor physics book it seems that the high injection
current is limited by the emitter series resistance. A guess then
would be that exceeding the max rating might cause too much
heat developing due to this resistance, but that's just a guess.
Somebody will probably provide an answer to this.
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Old 10th January 2004, 11:39 AM   #4
mikelm is offline mikelm  England
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Hi Christer

Thanks for the reply

Yes I would agree that you would think that absolute 20mA was there for a reason. I still don't understand what that reason is. Could it be in this case the absolute max's are 20mA and 35V together ?

The datasheet (ssm-2220) quotes a 'a very low bulk resistance' of 0.3ohms.

For my proposed 7V and 35mA I will be asking the transistor to be 200ohms c.e.

In worst possible case ( using a socket ) the device rises 100 deg C / watt.

So with 20 deg C ambient + my .25watt the junction should be about 45 deg C.

both these factors seem reasonable to me.

are there any other ideas as to how this could do damage ?
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Old 10th January 2004, 12:39 PM   #5
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Well, remember that the high voltage is always over collector-
base junction, the base-emitter junction stays around 0.6 to
0.7 V. Hence, most of the power is disspiated in the cb junction.
If you look at the physical design of a transistor you will see
that the collector is very large compared to the emitter and
the collector is the one in contact with the metal tab/case if
there is one. That means the collector can more easily dissipate
power. So maybe it doesn't even have to do with the resistance
as I previously guessed,
perhaps it is just that the max. rated current times the be voltage
drop is about as much power as the tiny emitter can dissipate.
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Old 11th January 2004, 09:13 AM   #6
mikelm is offline mikelm  England
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I think that I will have to do some tests to see what happens.

I'll try running them in spec for a while and then turn up the current and see what happens. If they don't self destruct I will listen for any subtle sound degradation. The alternative is two devices in paralell which I can use as a reference for comparison.

It might be a while but I will post my results when I have finnished.

I would be interested if anyone else has some input on this.
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Old 11th January 2004, 09:27 AM   #7
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I don't think they will go up in smoke if you exceed the current
rating at Vce where you stay within the power handling capacity.
They may stop working, but more likely you will just degrade
reliability and life time of the devices.

If you attempt going beyond the power rating, on the other hand,
then don't forget to wear protective glasses. These things can
explode!
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Old 11th January 2004, 11:24 AM   #8
mikelm is offline mikelm  England
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Quote:
Originally posted by Christer
If you attempt going beyond the power rating, on the other hand, then don't forget to wear protective glasses. These things can explode!
Yes - It only happened once to me but it's not something that I will forget in a hurry - 50V straight across a power transistor, full on - it was like a gun going off... ... ...
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Old 11th January 2004, 12:55 PM   #9
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in this particular case, the maximum Ic rating is probably restricted by the junction area (or die size as a proxy).

I don't think it will explode if you ran 35ma through a 20ma small power transistor. It will probably just fail or work (I have ran both 2n5401 and 5551 over their rated Ic capacities) but not as reliably.

most, if not all, datasheets usually warn you of staying within the absolute electric ratings AND SOA. unfortunately, they usually don't state SOA for small signal transistors.
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Old 11th January 2004, 01:16 PM   #10
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Quote:
Originally posted by millwood

I don't think it will explode if you ran 35ma through a 20ma small power transistor. It will probably just fail or work (I have ran both 2n5401 and 5551 over their rated Ic capacities) but not as reliably.
the 5401 and 5551 are my workhorse small signal transistors. While ON-SEMI doesn't have an SOA chart for these, Zetex has SOA charts for some of their "z"mall signal transistors.
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