Page 84 - Troubleshooting Analog Circuits

Zener, Zener, Zener.. . 71

two” falls hollow on my ears. Two may be better than seven or eight, but one is not
better than two.

Zener, Zener, Zener.. .

Just about all diodes will break down if you apply too much reverse voltage, but
zener diodes are designed to break down in a predictable and well-behaved way. The
most common way to have problems with a zener is to starve it. If you pass too little
current through a zener, it may get too noisy. Many zeners have a clean and crisp
knee at a small reverse-bias current, but this sharp knee is not guaranteed below the
rated knee current.
Some zeners won’t perform well no matter how carefully you apply them. In con-
trast to high-voltage zeners, low-voltage (3.3 to 4.7 V) zeners are poor performers
and have poor noise and impedance specs and bad temperature coefficients-ven if
you feed them a lot of current to get above the knee, which is very soft. This is be-
cause “zeners” at voltages above 6 V are really avalanche-mode devices and employ
a mechanism quite different from (and superior to) the low-voltage ones, which are
real zener diodes. At low-voltage levels, band-gap references such as LM336s and
LM385s are popular, because their performance is good compared with low-voltage
zeners.
Zener references with low temperature coefficients, such as the 1N825, are only
guaranteed to have low temperature coefficients when operated at their rated current.
such as 7.5 mA. If you adjust the bias current up or down, you can sometimes tweak
the temperature coefficient, but some zeners aren’t happy if operated away from their
specified bias. Also, don’t test your 1N825 to see what its “forward-conduction
voltage” is because in the “forward” direction, the device’s temperature-compen-
sating diode may break down at 70 or 80 V. This break-down damages the device’s
junction, degrades the device’s performance and stability, and increases its noise.
The LM329 is popular as a 6.9-V reference because its TC is invariant of operating
current, as it can run from any current from 1 to 10 mA. The LM399 is even more
popular because of its built-in heater that holds the junction at +85 “C. Consequently
it can hold 1/2 or 1 ppm per OC. The LM329 and LM399 types also have good long-
term stability, such as 5 or 10 ppm per lo00 hours, typically. The buried zeners in the
LM 129/LM 199/LM 169 also have better stability than most discrete references
(1N825 or similar) when the references are turned on and off.
And before you subject a zener to a surge of current, check its derating curves
for current vs. time, which are similar to the rectifiers’ curves mentioned earlier.
These curves will tell you that you can’t bang an ampere into a 10-V. 1-W zener for
very long.
In fact, most rectifiers are rated to be operated strictly within their voltage ratings,
and if you insist on exceeding that reverse voltage rating and breaking them down,
their reliability will be degraded. To avoid unreliability, you can redesign the circuit
to avoid over-voltage, or you might add in an R-C-diode damper to soak up the en-
ergy; or you could shop for a controlled-avalanche rectifier. These rectifiers are rated
to survive (safely and reliably) repetitive excursions into breakdown when you ex-
ceed their rated breakdown voltage. The manufacturers of these devices can give you
a good explanation of how to keep out of trouble.
If you do need a zener to conduct a surge of current, check out the specially de-
signed surge-rated zener devices-also called transient-voltage suppressors-from
General Semiconductor Industries Inc. (Tempe, AZ). You’ll find that their 1-W de-
vices, such as the 1N5629 through 1N5665A, can handle a surge of current better than