Page 78 - Troubleshooting Analog Circuits

6. Understanding Diodes and

Their Problems

After the first five chapters about troubleshooting passive parts, this chapter shifts
over to troubleshooting active components. We begin with the simple stuff-diodes
and rectifiers, optically coupled devices, solar cells, and batteries.
Even the simplest active devices harbor the potential for causing baffling trouble-
shooting problems. Consider the lowly diode. The task of a diode sounds simple:
To conduct current when forward biassed, and to block current when reverse biassed,
while allowing negligible leakage. That task sounds easy, but no diode is perfect. and
diodes’ imperfections are fascinating. Even these two-terminal devices are quite
complex!
All diodes start conducting current exponentially at low levels, nanoamperes and
up. An ideal diode may have an exponential characteristic with a slope AVIAI of
g = (38.6 mS/mA) X IF,
where mS = millisiemens = millimhos, and 1, = forward current. And indeed transis-
tors do have this slope of 38.6 mS/mA at their emitters, at room temperature. This
corresponds to 60 mV per decade of current. But the slopes of the exponential curves
of different real (two-terminal) diodes vary considerably. Some, like a 1N645, have a
slope as good as 70 or 75 mV per decade. Others like 1N9 14s have a slope as poor as
1 13 mV per decade. Others have intermediate values such as 90 rnvldecade. When
you go shopping for a diode, the data sheets never tell you about this. To tell the
truth, I didn’t even really recognize this. When I wrote the first version of this, as
published in EDN, I assumed that the slopes started out from 60 mV per decade and
then got worse-shifted over to 120 mV per decade at higher current levels, and I
said so. But I was wrong. And nobody ever contradicted me. Such a strange world!
Please refer to Figure 6.1, which shows just a few of the different curves you may
get when you buy a diode. None of these slopes are characterized or guaranteed; if
you change vendors, all bets are off. So, qualify each vendor of diodes carefully for
each application. (Note: For a detailed explanation of this graph and a table detailing
exactly which diode is which, see Appendix E.)
As the current level continues to increase, the conductance per milliampere gets
even worse due to series resistance and high-level injection and other nonlinear fac-
tors. Therefore, at a large forward current, a diode’s forward voltage, VF, will be
considerably larger than predicted by simple theory-and larger than desired. Of
course, some rectifiersaepending on their ratings-cun handle large currents from
amperes to kiloamperes; but the VFs of all diodes, no matter what their ratings, err
from the theoretical at high current levels.
These days, Schottky diodes have smaller VFS than ordinary pn diodes. However,
even germanium diodes and rectifiers still have their following because their low VFs
are similar to the Schottky’s. Just the other day I read about some new Germanium
Schottky diodes that have even lower VFs.
High-speed and ultra-high-speed (sometimes also called high-efficiency) silicon

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