Page 99 - Troubleshooting Analog Circuits

86 7. identifying and Avoiding Transistor Problems

Figure 7.6. In the old single-diffused structure, n-type dopants were diffused simultaneously into the front
and back of a thin p-type wafer. This structure produced rugged transistors with wider Safe
Operating Areas than the more modern epitaxial-base transistor types, in terms of Forward-
Biassed SOA. However, this fabrication has been obsokted.

Power-Circuit Design Requires Expertise

For many power circuits, your transistor choice may not be as clear-cut as in the
previous examples. So, be careful. Design in this area is not for the hotshot just out of
school-there are many tricky problems that can challenge even the most experi-
enced designers. For example, if you try to add small ballasting resistors to ensure
current sharing between several transistors, you may still have to do some transistor
matching. This matching isn’t easy. You’ll need to consider your operating condi-
tions; decide what parameters, such as beta and VBE, you’ll match; and figure out
how to avoid the mix-and-match of different manufacturers’ devices. Such design
questions are not trivial. When the performance or reliability of a power circuit is
poor, it’s probably not the fault of a bad transistor. Instead, it’s quite possibly the
fault of a bad or marginal driver circuit or an inadequate heat sink. Perhaps a device
with different characteristics was inadvertently substituted in place of the intended
device. Or perhaps you chose the wrong transistor for the application.
A possible scenario goes something like this. You build 10 prototypes, and they
seem to work okay. You build 100 more, and half of them don’t work. You ask me
for advice. I ask, “Did they ever work right?” And you reply, “Yes.” But wait a
minute. There were 10 prototypes that worked, but the circuit design may have been
a marginal one. Maybe the prototypes didn’t really work all that well. If they’re still
around, it would be useful to go back and see if they had any margin to spare. If the
10 prototypes had a gain of 22,000, but the current crop of circuits has gains of
18,OOO and fails the minimum spec of 20,000, your new units should not be called
“failures.” It’s not that the circuit isn’t working at all, it’s just that your expectations
were unrealistic.
After all, every engineer has seen circuits that had no right to work, but they did
work-for a while. And then when they began to fail, it was obviously just a hopeless
case. So, which will bum you quickest, a marginal design or marginal components?
That’s impossible to say. If you build in some safety margin, you may survive some
of each. But you can’t design with big margins to cover every possibility, or your