JAJSCB0E June 2016 – December 2021 UCC21520
The total loss, PG, in the gate driver subsystem includes the power losses of the UCC21520 (PGD) and the power losses in the peripheral circuitry, such as the external gate drive resistor. Bootstrap diode loss is not included in PG and not discussed in this section.
PGD is the key power loss which determines the thermal safety-related limits of the UCC21520, and it can be estimated by calculating losses from several components.
The first component is the static power loss, PGDQ, which includes quiescent power loss on the driver as well as driver self-power consumption when operating with a certain switching frequency. PGDQ 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 7-4 shows the per output channel current consumption vs operating frequency with no load. In this example, VVCCI = 5 V and VVDD = 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 IVCCI = 2.5 mA, and IVDDA = IVDDB = 1.5 mA. Therefore, the PGDQ can be calculated with
The second component is switching operation loss, PGDO, with a given load capacitance which the driver charges and discharges the load during each switching cycle. Total dynamic loss due to load switching, PGSW, can be estimated with
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:
QG 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 UCC21520 gate driver loss on the output stage, PGDO, is part of PGSW. PGDO will be equal to PGSW if the external gate driver resistances are zero, and all the gate driver loss is dissipated inside the UCC21520. 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, PGDO is different in these two scenarios.
Case 1 - Linear Pull-Up/Down Resistor:
In this design example, all the predicted source/sink currents are less than 4 A/6 A, therefore, the UCC21520 gate driver loss can be estimated with:
Case 2 - Nonlinear Pull-Up/Down Resistor:
For some scenarios, if only one of the pull-up or pull-down circuits is saturated and another one is not, the PGDO 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 UCC21520, PGD, is:
which is equal to 102 mW in the design example.