Page 80 - Troubleshooting Analog Circuits

Turn ’Em Off-Turn ’Em On . . . 67

Speed Demons

When a diode is carrying current, how long does it take to turn the current off?
There’s another wide-range phenomenon. Slow diodes can take dozens and hundreds
of microseconds to turn off. For example, the collector-base junction of a 2N930 can
take 30 ~s to recover from 10 mA to less than 1 mA, and even longer to the nanoam-
pere level. This is largely due to the recombination time of the carriers stored in the
collector region of the transistor. Other diodes, especially gold-doped ones, turn off
much faster-down into the nanosecond region. Schottky diodes are even faster,
much faster than 1 ns. However, one of my friends pointed out that when you have a
Schottky diode that turns off pretty fast, it is still in parallel with a p-n junction that
may still turn off slowly at a light current level. If a Schottky turns off from 4 mA in
less than 1 ns, there may still be a few microamperes that do not turn off for a mi-
crosecond. So if you want to use a Schottky as a precision clamp that will turn off
very quickly, as in a settling detector (Ref. 2), don’t be surprised if there is a small
long “tail.”
Switching regulators all have a need for diodes and high-current rectifiers and
transistors to turn off quickly. If the rep rate is high and the current large and the
diode turns off slowly, it can fail due to overheating. You don’t want to try a 1N4002
at 20 or 40 kHz, as it will work very badly, if at all. Sometimes if you need a mod-
erate amount of current at high speed, you can use several 1N914s in parallel. I’ve
done that in an emergency, and it seemed to work well, but I can’t be sure I can rec-
ommend it as the right thing to do for long-term reliability. The right thing is to engi-
neer the right amount of speed for your circuit. High-speed, fast-recovery, and ultra-
fast diodes are available. The Schottky rectifiers are even faster, but not available at
high voltage breakdowns. When you start designing switching regulators at these
speeds, you really must know what you are doing. Or at least, wear safety-goggles
so you don’t get hurt when the circuit blows up.
Turn ’Em Off-Turn ’Em On.. .

“Computer diodes” like the 1N9 14 are popular because they turn OFF quickly-in
just a few nanoseconds-much faster than low-leakage diodes. What isn’t well
known is that these faster diodes not only turn OFF fast, they usually turn ON fast.
For example, when you feed a cumnt of 1 .O mA toward the anode of a 1N914 in
parallel with a 40 pF capacitance (20 pF of stray capacitance plus a scope probe or
something similar), the 1N914 usually turns ON in less than 1 ns. Thus, the VF has
only a few millivolts of overshoot.
But with some diodes-even 1N914s or 1N4148s from some manufacturers-the
forward voltage may continue to ramp up past the expected DC level for 10 to 20 ns
before the diode turns ON; this overshoot of 50 to 200 mV is quite surprising
(Figure 6.2). Even more astonishing, the VF overshoot may get worse at low repeti-
tion rates but can disappear at high repetition rates (Figure 6.2b-d).
I spent several hours once discovering this particular peculiarity when a frequency-
to-voltage converter suddenly developed a puzzling nonlinearity. The trickiest part of
the problem with the circuit’s diodes was that diodes from an earlier batch had not
exhibited any slow-turn-on behavior. Further, some diodes in a batch of 100 from
one manufacturer were as bad as the diodes in Figures 6.2b and 6.2~. Other parts in
that batch and other manufacturers’ parts had substantially no overshoot.
When I confronted the manufacturers of these nasty diodes, they at first tried to
deny any differences, but at length they admitted that they had changed some diffu-