Thermal runaway in BJT: Can the transistor still work once returned to normal temperature?

Question

During a test of a BJT totem pole driver I noticed that smoke began to come from my circuit. I made the assumption that it was from the BJT as the input current began to slowly climb - indicating thermal runaway. I reduced the frequency and it seems that the BJT again seems to work fine. However, the input power to the circuit seems to be somewhat higher than what is usual.

My question is, as a BJT begins to enter thermal runaway, is it basically dead and worth replacing? The power dissipation is still constant at low frequency, but as frequency climbs, the thermal runaway starts again.

Is there anything in particular experiences during a thermal runaway that could permanently damage the device in regards to the power dissipation it consumes, while it seemingly maintaining good performance at certain conditions?

I plan to replace the parts on Monday and run the tests again to see it this is the case, but I wanted a bit of a sanity check first.

Best, Jven

Answer 1

Power transistors under secondary breakdown focus heat on the amplifying channel which gets more conductive under additional heat. The area in question loses its properties and/or outright melts leaving a hole. The main result is a loss of hFE. If the thermal runaway does not result in a total loss of conductivity, it still can burn out the active area to a degree where only two independent diodes are left.

You should measure hFE if you can. If it is below spec, your power transistor is likely hosed to a degree where it will offload more of the workload to the driver transistor than it is good for longterm. If it blows its driver transistor, this will likely take out significant parts of the control circuitry as well. A power transistor alone should not be that expensive to replace.

Answer 2

Just like any other electronic component, there is a Data sheet that has absolute maximums on it. Temperature is usually in that list of specs.

As with everything on the absolute maximum list, exceeding any value on it means under that condition, the manufacturer can no longer guarantee the part will perform to spec.