A Zener diode is characterized by its ability to protect an object from transient overvoltage pulses as well as overvoltage pulses close to DC, which are difficult to protect against with ESD protection diodes.
In addition to ESD, there are induced lightning surges and switching surges with long pulse widths (in the order of microseconds to milliseconds) as overvoltage pulses to be considered in circuit design.
In order to protect an object from overvoltage pulses with long pulse widths, it is necessary to select a product according to the assumed surge energy.
In the case of overvoltage pulses such as that shown in the figure above, the grey portion in the figure will be energized to the Zener diode, so it is necessary to check whether the power is acceptable.
Use the above equation to approximate a square wave and calculate the pulse width. The surge power tolerance for each product is set as shown in the graph. Confirm that the Zener surge power tolerance for tW after approximating the square wave is not a problem in designing.