The total loss, P_G, in the gate driver subsystem includes the power losses of the UCC21521 (P_GD) and the power losses in the peripheral circuitry, such as the external gate drive resistor. Bootstrap diode loss is not included in P_G and not discussed in this section.

P_GD is the key power loss which determines the thermal safety-related limits of the UCC21521, and it can be estimated by calculating losses from several components.

The first component is the static power loss, P_GDQ, which includes quiescent power loss on the driver as well as driver self-power consumption when operating with a certain switching frequency. P_GDQ is measured on the bench with no load connected to OUTA and OUTB at a given VCCI, VDDA/VDDB, switching frequency and ambient temperature. Figure 6-4 shows the per output channel current consumption vs. operating frequency with no load. In this example, V_VCCI = 5 V and V_VDD = 20 V. The current on each power supply, with INA/INB switching from 0 V to 3.3 V at 100 kHz is measured to be I_VCCI = 2.5 mA, and I_VDDA = I_VDDB = 1.5 mA. Therefore, the P_GDQ can be calculated with

Equation 11.

The second component is switching operation loss, P_GDO, with a given load capacitance which the driver charges and discharges the load during each switching cycle. Total dynamic loss due to load switching, P_GSW, can be estimated with

Equation 12.

where

• Q_G is the gate charge of the power transistor.

If a split rail is used to turn on and turn off, then VDD is going to be equal to difference between the positive rail to the negative rail.

So, for this example application:

Equation 13.

Q_G represents the total gate charge of the power transistor switching 800 V at 20 A, and is subject to change with different testing conditions. The UCC21521 gate driver loss on the output stage, P_GDO, is part of P_GSW. P_GDO will be equal to P_GSW if the external gate driver resistances are zero, and all the gate driver loss is dissipated inside the UCC21521. If there are external turn-on and turn-off resistances, the total loss will be distributed between the gate driver pull-up/down resistances and external gate resistances. Importantly, the pull-up/down resistance is a linear and fixed resistance if the source/sink current is not saturated to 4 A/6 A, however, it will be non-linear if the source/sink current is saturated. Therefore, P_GDO is different in these two scenarios.

Case 1 - Linear Pull-Up/Down Resistor:

Equation 14.

In this design example, all the predicted source/sink currents are less than 4 A/6 A, therefore, the UCC21521 gate driver loss can be estimated with:

Equation 15.

Case 2 - Nonlinear Pull-Up/Down Resistor:

Equation 16.

where

• V_OUTA/B(t) is the gate driver OUTA and OUTB pin voltage during the turn on and off transient, and it can be simplified that a constant current source (4 A at turn-on and 6 A at turn-off) is charging/discharging a load capacitor. Then, the V_OUTA/B(t) waveform will be linear and the T_{R_Sys} and T_{F_Sys} can be easily predicted.

For some scenarios, if only one of the pull-up or pull-down circuits is saturated and another one is not, the P_GDO will be a combination of Case 1 and Case 2, and the equations can be easily identified for the pull-up and pull-down based on the above discussion. Therefore, total gate driver loss dissipated in the gate driver UCC21521, P_GD, is:

Equation 17.

which is equal to 102 mW in the design example.