This call started with a real red herring. My caller’s first statement was to the effect that the power op amp he had was oscillating. OK, this happens for a number of reasons. I would normally start by asking about power supply by-pass capacitors, the nature of the load impedance and the circuit function, but first the most important question, “What is the frequency of the oscillation?”
The circuit was oscillating with a period of about two seconds on and two seconds off. With this time interval it sounded very much like a thermal problem. I then asked him to move his finger close to the part, but not touch it and tell me if the part was hot. If my theory was right the junction temperature was in the neighborhood of 140 to 160C and direct contact with the part would be painful. His first comment was to thank me for telling him not to touch the part as it was very hot.
He had built a type of relaxation oscillator. The power op amp was generating more heat than the heat sink could move to the ambient environment. The device would heat up until the thermal limit circuit would shut it down. The part would then start to cool off. At the lower side of the thermal limit circuit it would turn on again and heat up, thus repeating the cycle. Most power op amps have an upper thermal limit of about 160C and a lower switch point of about 140C.
Now that we knew why the amplifier circuit was oscillating it was a simple task to fix it. This design was still on the breadboard and a simple increase in the heat sink would lower the thermal resistance to the ambient which cured the problem. But what if the conditions that caused the amplifier to overheat were a fluke or this event happened under known conditions where the event could be accepted? What happens if the thermal limit is exceeded often? Would it cause any harm?
This is a very interesting question. From a pure logical point of view, continual operation under conditions that exceed the thermal limit means that the device is being operated at a junction temperature averaging 150C. This is usually the absolute maximum junction temperature and continued operation under these conditions will degrade the reliability of the device.
When is it acceptable to run a device into the thermal limit? What difference does it make if it is on a regular basis or random time interval? In general, it is wise to assume that a thermal shutdown circuit is provided to protect the device from unusual conditions. Frequent cycling in this condition could eventually cause degradation and possible failure.
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*Applications guru Bill Klein in host the Analog eLab web seminars sponsored by Texas Instruments and CMP Media.