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I am working on below schematic:

Sch

+42VA is 42 V DC and +42VAO is also 42V DC when PNP Transistor Q1 is ON. When Q1 is ON, load on +42VAO is 10 mA max. OPTO used is P181.

But it is observed that Q1 (CXT3906) is getting damaged frequently.

I can see in datasheet that VCEO and VBEO max is 40V and VEBO is 5V. Is this main reason of Q1 failure?

Base is also connected to +42VA through R76(100K) and R77(100k), so for base maximum voltage which is the voltage I can check in datasheet?

Is there any reason for Q1 failure other than this OR how can I Improve it? Thanks in advance.

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    \$\begingroup\$ Using a transistor with a VCE max of 40 V in a 42 V application is asking for trouble. I would not even consider using such a transistor here. I'd use one with a VCE max of 50 V or more. \$\endgroup\$ May 27 '16 at 11:08
  • \$\begingroup\$ I am planning to use BC856 with high VCE max voltage (80V), Is it good? Also, I am not getting what is VEBO given in datasheet (5V)? \$\endgroup\$ May 27 '16 at 11:11
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    \$\begingroup\$ If VCE max was the problem then yes, the BC856 is OK. But you might have other problems as Olin mentioned. What will be the collector current when Q1 is on ? VEBO is the max. Emitter-Base voltage when the collector is open. This is a reverse voltage meaning the BE diode is in reverse. This will not happen in your circuit because the voltage on the base cannot be higher than the voltage on the emitter. So no worries about VEBO ! \$\endgroup\$ May 27 '16 at 11:23
  • \$\begingroup\$ when Q1 is on, current is max 10 -15 mA. \$\endgroup\$ May 27 '16 at 11:27
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    \$\begingroup\$ OK, Ic = 15mA, from datasheet: Hfe = 125 (I take the minimum) so Ib will be 15mA/125 = 120 uA There will be approx. 40 V across R77 when U15 is "on" 40V/100k = 400 uA, that is much more than Ib which is good. The PNP will be in saturation and that is what you want. Looks OK to me ! :-) \$\endgroup\$ May 27 '16 at 11:34
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The maximum collector emitter voltage for Q1 is 40V - read the data sheet - you are applying 42 volts.

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  • \$\begingroup\$ Thanks for reply. But base is also connected to +42V through R76 and R77 (100K). Is this a problem? what max voltage base can handle as per datasheet ( i mean, where to check)? \$\endgroup\$ May 27 '16 at 10:49
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    \$\begingroup\$ The base will have about 1V max on it due to current limiting by the 100k resistor in series with it. This is not a problem but the CE voltage IS! Remember the base-emitter junction is just a forward biased diode so more than 1V will be exceptional. \$\endgroup\$
    – Andy aka
    May 27 '16 at 10:51
  • \$\begingroup\$ I mean when Q1 transistor is OFF, +42V is coming on base pin directly. Is this a problem? Is there any parameter to check this or I am wrong in my consideration? \$\endgroup\$ May 27 '16 at 11:01
  • \$\begingroup\$ The base-emitter junction when driven with forward current (about 0.4 mA via the 100kohm resistor) will be about 1V due to the fact that the base-emitter region is a forward biased diode. So, the emitter will be at 42 volts and the base will be at 41 volts approximately. \$\endgroup\$
    – Andy aka
    May 27 '16 at 11:13
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As Andy pointed out, you are already abusing the transistor just when it's sitting there trying to be off.

However, there are other issues. If the collector is connected to a load with inductive component, then the collector voltage can go considerably below ground when the transistor is switched off. You should put a Schottky diode from ground to the collector. That won't conduct in normal operation, but will provide a path for inductive kickback current that doesn't include frying the transistor.

Another possibility is that the transistor is getting destroyed due to heat. You are giving it very little base current, just over 400 µA. I didn't check what the guaranteed minimum gain is at that base current, but even it was 100 (your job to check), that would only support a bit over 40 mA. If the load is trying to draw more, the collector voltage will drop, and the dissipation of the transistor will go up. It doesn't take much to fry a transistor in a SOT-23 package.

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  • \$\begingroup\$ Thanks Olin for kind reply. But, when Q1 transistor is OFF, +42V is coming on base pin directly. Is this a problem? Is there any parameter to check this or I am wrong in my consideration? \$\endgroup\$ May 27 '16 at 11:11
  • \$\begingroup\$ And load is not inductive, pure DC. \$\endgroup\$ May 27 '16 at 11:13
  • \$\begingroup\$ @Ele: I don't know what point you are trying to make with your first comment. Yes, the base and emitter will be at 42 V when the transistor is off. The collector can presumably be as low as 0 V. That puts 42 V across a transistor rated for 40 V. \$\endgroup\$ May 27 '16 at 11:15

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