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The figure to the right shows one of the ESD protection means known
to be employed for protecting laser diodes. In this scheme, a resistor is
connected directly across the laser diode terminals, within the head, and
with the resistance being typically around 100 ohms.
Using the
human body model shown in the figure as a guide,
it is easy to see that this protection means will not be effective. As
discussed above, for a 15,000 volt ESD event, the resistance would need to
be less than 44 milliohms, in order to prevent the laser diode voltage
from exceeding the maximum reverse bias voltage of 2.0 volts
and exceeding a similar forward-bias voltage. If a 100-ohm resistor is
used, it would allow (15,000 volts / (330 ohms + 100 ohms)) / 100 ohms =
3488 volts to surge into the laser diode. Since this is far in excess of
the typical 2.2-volt lasing threshold or 2.0-volt maximum reverse bias
voltage, this would almost surely destroy the laser diode.
Although it might seem that the 100-ohm resistor
could simply be replaced with a 44-milliohm resistor, this is not
practical, because it would mean that, during operation, far more power
would be expended in the operation of the protection resistor than the
laser diode itself.
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ESD polarity terminology used on this
web site
The term “positive-ESD” is used to
mean electrostatic discharge (ESD) whose voltage polarity would tend
to forward-bias a laser diode. “Negative-ESD,” means ESD whose
voltage polarity would tend to reverse-bias a laser diode. |
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