In order to
evaluate the effectiveness of an ESD protection scheme, it is useful to
employ an electrical circuit model that helps to illustrate and understand
the voltage and current levels that are experienced during an ESD event.
There are several so-called “human body
models” for the evaluation of ESD, and a useful one is shown in the figure
to the right, wherein a 150-picofarad capacitor charged to a predetermined
voltage is shown. A switch is placed in between the capacitor and a
330-ohm resistor, which is then connected to the device under test (DUT).
When the switch is closed, the ESD event
occurs, discharging the capacitor through the resistor, and also through
the device under test. This is also the human body model used by the IEC
61000-4-2 standard. As mentioned above, the voltage level of an ESD event
ranges from around 4,000 to around 32,000 volts DC. However, for the
purpose of modeling ESD events, it is common to use a representative
voltage of 15,000 volts.
In looking at the human body model, with
the capacitor charged up to 15,000 volts, discharging through a laser
diode using the 330-ohm series resistance of the human body model, it can
be seen that the laser diode will experience a current in excess of 45
amps (!) during the discharge. And a simple R/C analysis shows that this
discharge happens over a very short period of time, no greater than tens
ESD protection requirements
Real-world ESD events have been observed in
the sub-nanosecond range. Thus, in order to be effective, an ESD
protection means must also react in the sub-nanosecond range, or at least
the single-digit nanosecond range, and also have an
effective frequency bandwidth that ranges from around 20 MHz to 1 GHz.
Using the human body model shown in the
figure above as a guide, it can be seen that, if the ESD protection scheme is
implemented as a passive means, and placed in parallel with the laser
diode, it must have an effective impedance less than 44 milliohms in order
to protect a laser diode whose absolute maximum reverse-bias voltage is
2.0 volts and whose forward-bias limitations are similar.
ESD polarity terminology used on this
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.