Page 95 - Troubleshooting Analog Circuits

82 7. Identifying and Avoiding Transistor Problems

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Figure 7.2. When you hit a component or circuit with a pulse of real ESD, you can never be sure what
kind of trouble you’ll get-unless you’ve already tested it with an ESD simulator. (Photo
copyright Peggi Willis.)

cause enough I X R drop to force the entire emitter and its periphery to share the
current. Now, let’s halve the current and double the voltage: The amount of dissipa-
tion is the same, but the I X R drop is cut in half. Now continue to halve the current
and double the voltage. Soon you’ll reach a point where the ballasting (Figure 7.3)
won’t be sufficient, and a hot spot will develop at a high-power point along the
emitter. The inherent decrease of VBE will cause an increase of current in one small
area. Unless this current is turned OFF promptly, it will continue to increase
unchecked. This “current hogging” will cause local overheating, and may cause the
area to melt or crater-this is what happens in “secondary breakdown.” By definition
you have exceeded the secondary breakdown of the device. The designers of linear
ICs use ballasting, cellular layouts, and thermal-limiting techniques, all of which can
prevent harm in these cases (Ref. 3). Some discrete transistors are beginning to in-
clude these features.
Fortunately, many manufacturers’ data sheets include permitted safe-area curves at
various voltages and for various effective pulse-widths. So, it’s possible to design
reliable power circuits with ordinary power transistors. The probability of an unreli-
able design or trouble increases as the power level increases, as the voltage increases,
as the adequacy of the heat sink decreases, and as the safety margins shrink. For
example, if the bolts on a heat sink aren’t tightened enough, the thermal path
degrades and the part can run excessively hot.
High temperature per se does not cause a power transistor to fail. But, if the drive
circuitry was designed to turn a transistor ON and only a base-emitter resistor is
available to turn it OFF, then at a very high temperature, the transistor will turn itself
ON and there will be no adequate way to turn it OFF. Then it may go into secondary
breakdown and overheat and fail. However, overheating does not by itself cause
failure. I once applied a soldering iron to a 3-terminal voltage regulator-I hung it
from the tip of the soldering iron-and then ran off to answer the phone. When I