The allowed repetitive SOA for the LMG3425R050 (Figure 5-12) is defined by the peak drain current (I_DS) and the drain to source voltage (V_DS) of the device during turn on. The peak drain current during switching is the sum of several currents going into drain terminal: the inductor current (I_ind); the current required to charge the C_OSS of the other GaN device in the totem pole; and the current required to charge the parasitic capacitance (C_par) on the switching node. 145pF is used as an average C_OSS of the device during switching. The parasitic capacitance on the switch node may be estimated by using the overlap capacitance of the PCB. A boost topology is used for the SOA testing. The circuit shown in Figure 6-3 is used to generate the SOA curve in Figure 5-12. For reliable operation, the junction temperature of the device must also be limited to 125°C. The I_DS of Figure 5-12 can be calculated by:

where drain slew rate at the peak current is estimated between 70% and 30% of the bus voltage, and C_par is the parasitic board capacitance at the switched node.

Figure 6-3 Circuit Used for SOA Curve

Refer to Achieving GaN Products With Lifetime Reliability for more details.