Page 100 - Troubleshooting Analog Circuits

MOSFETS Avoid Secondary Breakdown 87

design will become a monster. That’s where experience and judgment must be
invoked.. . .
An old friend wrote to me from Japan, “Why do you talk about having to trou-
bleshoot 40% of the units in a batch of switching regulators? In Japan that would be
considered a bad design. , . .” I replied that I agreed that it sounds like a problem, but
until you see what is the cause of the problems, it is unfair to throw any blame
around. What if it was a bad workmanship problem? Then that does not sound like a
bad design-unless the design was so difficult to execute that the assembly instruc-
tions could not be followed. Or maybe a bad part was put in the circuit. Or maybe it
was a marginally bad design and part of the circuit does need to be changed-per-
haps an extra test or screening of some components-before the circuit can run in
production. But you cannot just say that if there is ever trouble, it is the design engi-
neer’s fault. What if the design engineer designed a switching regulator that never
had any problems in production-never ever-but it only puts out 1 W per 8 cubic
inches, and all the parts are very expensive, and then there is a lot of expensive
testing on each component before assembly, to prove that there is a good safety
margin. Is that a good design? I doubt it. Because if you tried to build a plane with
too big a safety factor, it might be bigger than a 747, but able to carry only 10 passen-
gers. Every circuit should be built with an appropriate safety factor. If you only use a
transistor that is always SURE to work well, that may be an uneconomic safety
factor. Judgment is required to get the right safety factor.

MOSFETS Avoid Secondary Breakdown

When it comes to power transistors, MOSFETs have certain advantages. For many
years, MOSFETs have been available that switch faster than bipolar transistors, with
smaller drive requirements. And MOSFETs are inherently stable against secondary
breakdown and current hogging because the temperature coefficient of IDS vs. V,, is
inherently stable at high current densities. If one area of the power device gets too
hot, it tends to carry less current and thus has an inherent mechanism to avoid run-
ning away. This self-ballasting characteristic is a major reason for the popularity of
MOSFETs over bipolar transistors. However, recent criticism points out that when
you run a MOSFET at high-enough voltages and low current, the current density gets
very small, the temperature coefficient of IDS vs. VGs reverses, and the device’s
inherent freedom from current hogging may be lost (Ref. 5). So at high voltages and
low current densities, watch out for this possibility. When the VDS gets high enough.
MOSFETS can exhibit current hogging and “secondary breakdown” similar to that of
bipolars! !
The newer power MOSFETs are considerably more reliable and less expensive
than the older devices. Even though you may need a lot of transient milliamps to turn
the gate ON or OFF quickly, you don’t need a lot of amps to hold it ON like you do
with a bipolar transistor. You can turn the newer devices OFF quicker, too, if you
have enough transient gate drive current available.
However, MOSFETs are not without their problem areas. If you persist in dissi-
pating too many watts into a MOSFET, you can melt it just as you can melt a bipolar
device. If you don’t overheat a MOSFET, the easiest way to cause a problem is to
forget to insert a few dozen or hundred ohms of resistance (or a femte bead) righyat
the gate lead of the device. Otherwise, these devices have such high bandwidths that
they can oscillate at much higher frequencies than bipolar transistors.
For example, the first high-fidelity, all-MOSFET audio amplifier I ever saw blew
up. It worked okay in the lab, but some misguided engineer decided that if a band-