Evaluating ESD using a Human Body Model

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 of nanoseconds.
 

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.