SNOSCY4E March 2015 – October 2018 LMG5200
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 LMG5200 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.
There are some additional losses in the gate drivers due to the internal CMOS stages used to buffer the outputs. Figure 1 shows the measured gate driver power dissipation versus frequency and load capacitance. Use this graph to approximate the power losses due to the gate drivers.
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 (VIN) 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.
The switching losses can be computed to a first order using , tTR can be approximated by dividing VIN by 25V/ns, which is a conservative estimate of the switched node slew rate. Equation 5.
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 LMG5200 device is capable of driving the GaN FETs at frequencies up to 10 MHz, 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.