What’s all this SOA
stuff anyway?
The safe operating area of a transistor describes all maximum values of combinations of Ic (or Id) and Vce (or Vds) that will not destroy the transistor or FET. (There is also a 3rd dimension, time. Some combinations of Vce and Ic are allowed only for very small periods, like milliseconds).
Especially bipolar power transistors are very sensitive to a destructive overload phenomenon called Secondary Breakdown. Since the die (the active chip) on the transistor is never perfectly homogenous, some small spots conduct more current than other spots. Those spots get hotter than others, which means they conduct even better, etc. The result is a very hot small spot that 'hogs' the current which can destroy the transistor. The effect is stronger with higher Vce so especially at high Vce the allowed Ic gets quite small. This is reflected in the SOA curve.
You can see that at say, 60V Vce, the allowed Ic is only 150mA or so, which represents only 9 Watts of dissipation. And this a a transistor with a nominal Pc of 50 Watts! On the other hand, you CAN dissipate 50 Watts in this transistor at Vce = 60V, as long as you limit it to 1uS…
Not all loads are created equal. The situation is getting worse when we use a real loudspeaker load with crossover filter for an amp. Such a load causes a phase shift between the amp output voltage and the amp output current (the load current) The phase shift may cause the output voltage, for example, to become negative, while the current is still positive, coming from the top half of the amp. That means that the top transistor has even more Vce while still having to source current, and Secondary Breakdown becomes a real danger.
There are several
remedies:
- Use the transistors with lower supply voltages;
- Use several parallel transistors so they can divide the load
current between them;
- Built-in a protection circuit that limits the output load
current when the device comes in the danger zone.
Most amplifiers incorporate some form of SOA-limiting.
To help with the calculations for such a protection circuit, I
have developed a spreadsheet.
