Page 101 - Troubleshooting Analog Circuits

88 7. Identifying and Avoiding Transistor Problems

width of 5 Hz to 50 kHz was good, then 0.5 Hz to 500 kHz was better. Consequently,
when the speaker cables were extended from 10 feet to 20 feet for a demonstration,
the amplifier broke into a megahertz-region scream and promptly went up in smoke
because of the lack of damping at the sources. I was told that after a minor redesign
the amplifier was perfectly reliable. The redesign involved cutting the bandwidth
down to a reasonable level, adding some ballasting in the sources, and tying anti-
snivet resistors directly to the gate pins. (Note: A snivet is a nasty, high-frequency
oscillation originally found in vacuum-tube TV sets-an oscillation similar to the
oscillation of a MOSFET with no resistance in series with its gate.)
As with bipolar transistors, MOSFETs are very reliable if you don’t exceed their
voltage, current, or temperature ratings. Dissatisfaction with a device’s reliability or
performance usually stems from the drivers or the related circuitry. Most MOSFETs
have a maximum VGs rating of just 20 or 25 V. A MOSFET may temporarily survive
operation with 30 or 50 V on the gate, but it’s not safe to run it up there forever. If
you apply excessive gate voltage, gradual gain or threshold degradation may occur.
So-please-don’t. Also, power MOSFETs are not quite as rugged as bipolars when
it comes to surviving ESD transients. A common precaution is to add a little decou-
pling, clamping, or current-limiting circuih-y, so that terminals accessible to the out-
side world can withstand ESD.
DMOS FETs are so easy to apply that we usually forget about the parasitic bipolar
transistor that lurks in parallel with them. If dV/dt is too large at the drain, if the drain
junction is avalanched at too high a current and voltage, or if the transistor gets too
hot, the bipolar device turns ON and dies an instant death due to current hogging or
an excursion from its safe operating area.
But I’m spoiled rotten. I’m accustomed to linear ICs, which have protection tran-
sistors built right in, so the user rarely has a problem. (But most of the transistor
troubles are left to the IC designer!) Discrete designs are appropriate and cost-effec-
tive for many applications, but the availability of linear ICs-especially op amps-
can simplify your design task considerably, at the same time as it improves relia-
bility. Next time, we’ll discuss the ins and outs and innards of op amps.

References
1. Leonard, Charles, “Is reliability prediction methodology for the birds?” Power Conversion and
Intelligent Motion, November 1988, p. 4.
2. Pease, Robert A., “Picoammeter/calibrator system eases low-current measurements,” EDN,
March 31,1982, p. 143.
3. “A 150W IC Op Amp Simplifies Design of Power Circuits,” R. J. Widlar and M. Yamatake,
AN-446, National Semiconductor Corp, Santa Clara, CA.
4. Applications Engineering Staff, PowerTech Inc., “Speed-up inductor increases switching
speed of high current power transistors,” Power Electronics, May 1989, p. 78.
5. Passafiume, Samuel J., and William J. Nicholas, “Determining a MOSFET’s real FBSOA,”
Powertechnics Magazine, June 1989, p. 48.