The worst-case test without any devices limiting energy flow into the input and output chips is presented in Figure 5. The direct application of high voltage 750 V via switch S1 into the output chip represents the worst-case condition, when the intermediate high voltage 750 V is directed to the driver chip without an energy limiting gate resistor.
Another possible worst case exhibits the application of excessive supply voltage to the control chip on the primary side of the driver. The maximum recommended input supply voltage is 5.5 V. If the dc-to-dc converter generating input voltage loses regulation, its output voltage could increase. In case of lost regulation, the converter’s output voltage can increase by a factor of two to three for state of the art dc-to-dc converters. Energy applied into the input chip of ADuM4223 is limited and, as always, other devices, like resistors, power switches, and inductors, are in place. Those devices impede the energy flow into the control chip. A supply voltage of 15 V with current limit 1.5 A was chosen for realistic simulation of the dc-to-dc converter malfunction.
Table 1 presents the result of the overstress tests done using the circuits of Figure 3, Figure 4, and Figure 5. In order to determine the impact of the protection circuit, two tests have been done for each MOSFET/IGBT power switch type. For the worst-case tests of 9, 10, and 11, mechanical switches S1 and S2 were used.
In general, the Zener diode helps to protect the driver circuit, as the table states (when comparing test 1 against test 2). However, when the gate resistor value is too small, the driver will be destroyed despite the Zener diode (compare test 3 and test 4).