Ensure that the power loss in the driver and the GaN FETs is maintained below the maximum power dissipation limit of the package at the operating temperature. The smaller the power loss in the driver and the GaN FETs, the higher the maximum operating frequency that can be achieved in the application. The total power dissipation of the LMG3100 device is the sum of the gate driver losses, the bootstrap diode power loss and the switching and conduction losses in the FETs.

The gate driver losses are incurred by charge and discharge of the capacitive load. It can be approximated using Equation 3.

where

• Q_G is the gate charge
• VCC is the bias supply
• f_SW is the switching frequency

There are some additional losses in the gate drivers due to the internal CMOS stages used to buffer the outputs.

The bootstrap diode power loss is the sum of the forward bias power loss that occurs while charging the bootstrap capacitor and the reverse bias power loss that occurs during reverse recovery. Because each of these events happens once per cycle, the diode power loss is proportional to the operating frequency. Higher input voltages (V_IN) to the half bridge also result in higher reverse recovery losses.

The power losses due to the GaN FETs can be divided into conduction losses and switching losses. Conduction losses are resistive losses and can be calculated using Equation 4.

Equation 4.

where

• R_DS(on)HS is the high-side GaN FET on-resistance
• R_DS(on)LS is the low-side GaN FET on-resistance
• I_RMS(HS) is the high-side GaN FET RMS current
• I_RMS(LS) and low-side GaN FET RMS current

The switching losses can be computed to a first order using , t_TR can be approximated by dividing V_IN by 25V/ns, which is a conservative estimate of the switched node slew rate. Equation 5.

where

• t_TR is sum of the switch node transition times from ON to OFF and from OFF to ON
• C_OSS(ER) is the output capacitance of each GaN FET

Note that the low-side FET does not suffer from this loss. The third quadrant loss in the low-side device is ignored in this first order loss calculation.

As described previously, switching frequency has a direct effect on device power dissipation. Although the gate driver of the LMG3100 device is capable of driving the GaN FETs at frequencies up to 10MHz, careful consideration must be applied to ensure that the running conditions for the device meet the recommended operating temperature specification. Specifically, hard-switched topologies tend to generate more losses and self-heating than soft-switched applications.

The sum of the driver loss, the bootstrap diode loss, and the switching and conduction losses in the GaN FETs is the total power loss of the device. Careful board layout with an adequate amount of thermal vias close to the power pads (VIN and PGND) allows optimum power dissipation from the package. A top-side mounted heat sink with airflow can also improve the package power dissipation.