Power Transistor Failure reasons

[aggpankaj2]

Hi guys,

I am using PNP Power transistors (darlington), BDX34C- ST Micro, In my design, which are working in active mode region.. providing or passing 2.5A thorugh each transistor. Those are configured as constant current source for 10A using 4 transistors (2.5A each). This constant current source works during 50% on cycle of an 100Hz square wave frequency i receive as input. During off cycle transistor remain off.

I need to know the maximum possible reasons for transistor failure, whether it be juntion temp, Vce voltage etc.

Kindly suggest what parameters can make my working transistor fail (short circuit).

Pankaj

 

[Nigel Goodwin]

Hi guys,

I am using PNP Power transistors (darlington), BDX34C- ST Micro, In my design, which are working in active mode region.. providing or passing 2.5A thorugh each transistor. Those are configured as constant current source for 10A using 4 transistors (2.5A each). This constant current source works during 50% on cycle of an 100Hz square wave frequency i receive as input. During off cycle transistor remain off.

I need to know the maximum possible reasons for transistor failure, whether it be juntion temp, Vce voltage etc.

Kindly suggest what parameters can make my working transistor fail (short circuit).

Overheating is the almost universal cause of failure in power transistors. How large are your heatsinks, and what voltages are being dropped across the transistors?.

 

[JimB]

Are you sure that the transistors are sharing the current equally?
Do you have resistors to ensure an equal current split, or are the transistors just connected in parallel?

JimB

 

[aggpankaj2]

yes nigel, i have a considerable large heat sink,, on which i can run my circuit work for hours...

 

[aggpankaj2]

jim! I have out one 100ohm resistor as Vbe biasing.. but how can i ensure that each is drawing the same amount of current (but they are designed so.).. Vce drop across transistors is around 10V

 

[JimB]

Show us your schematic.

JimB

 

[aggpankaj2]

Show us your schematic.

JimB

View attachment 63903

 

[JimB]

My first reaction is "what a dogs breakfast!".

The values on the schematic are difficult to read.

The opamp (TLV2471 ??) has a maximum supply voltage of 7volts, you are running it at 12volts.

The transistor configuration does not appear to be an effective contant current generator.

You are measuring the current in four places and then summing the result. Does not sound like a good idea.

Where is this 100ohm bias resistor you mentioned earlier?

Putting the switching transistor (Q5 ??) at that place in the circuit looks a bad idea.

I think that you need to re-think this thing.

JimB

 

[Ratchit]

aggpankaj2,

How do the answers you get here compare with what you receive there?

Ratch

https://www.edaboard.com/thread250806.html

 

[unclejed613]

pay close attention the Secondary Breakdown portion of the SOA curve. passing this barrier is usually instant death to a transistor. this part of the curve usually begins at the top of the vertical line and goes up to the primary thermal power limit line (which is the upper diagonal portion). shown below is an SOA curve, and the secondary breakdown region of the curve is circled in red.

 

[audioguru]

The schematic is impossible to read because the parts are too far apart which makes the schematic huge. Then to fit on a page the entire huge schematic size is reduced which makes the writing too small.
Also the schematic was saved and is posted as a fuzzy JPG file type instead of as a very clear PNG file type.

 

[ronsimpson]

I don't thing current is shared well. The type of transistor you chose has a B-E voltage drop of 1.6 volts( more or less). You are dropping only 0.2 volts across the emitter resistors. I believe the hottest transistor will deliver more of the current.