The total loss, P_G, in the gate-driver subsystem includes the power losses (P_GD) of the UCC23511 device and the power losses in the peripheral circuitry, such as the external gate-drive resistor.

The P_GD value is the key power loss which determines the thermal safety-related limits of the UCC23511 device, and it can be estimated by calculating losses from several components.

The first component is the static power loss, P_GDQ, which includes power dissipated in the input stage (P_{GDQ_IN}) as well as the quiescent power dissipated in the output stage (P_{GDQ_OUT}) when operating with a certain switching frequency under no load. P_{GDQ_IN} is determined by I_F and V_F and is given by Equation 1. The P_{GDQ_OUT} parameter is measured on the bench with no load connected to V_OUT pin at a given V_CC, switching frequency, and ambient temperature. In this example, V_CC is 15 V. The current on the power supply, with PWM switching at 10 kHz, is measured to be I_CC = 1.33 mA . Therefore, use Equation 2 to calculate P_{GDQ_OUT}.

Equation 1.

Equation 2.

The total quiescent power (without any load capacitance) dissipated in the gate driver is given by the sum of Equation 1 and Equation 2 as shown in Equation 3

Equation 3.

The second component is the switching operation loss, P_GDSW, with a given load capacitance which the driver charges and discharges the load during each switching cycle. Use Equation 4 to calculate the total dynamic loss from load switching, P_GSW.

Equation 4.

where

• Q_G is the gate charge of the power transistor at V_CC.

So, for this example application the total dynamic loss from load switching is approximately 18 mW as calculated in Equation 5.

Equation 5.

Q_G represents the total gate charge of the power transistor switching 520 V at 50 A, and is subject to change with different testing conditions. The UCC23511 gate-driver loss on the output stage, P_GDO, is part of P_GSW. P_GDO is equal to P_GSW if the external gate-driver resistance and power-transistor internal resistance are 0 Ω, and all the gate driver-loss will be dissipated inside the UCC23511. If an external turn-on and turn-off resistance exists, the total loss is distributed between the gate driver pull-up/down resistance, external gate resistance, and power-transistor internal resistance. Importantly, the pull-up/down resistance is a linear and fixed resistance if the source/sink current is not saturated to 1.5A/2.0A, 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 6.

In this design example, all the predicted source and sink currents are less than 1.5 A and 2.0 A, therefore, use Equation 6 to estimate the UCC23511 gate-driver loss.

Equation 7.

Case 2 - Nonlinear Pull-Up/Down Resistor:

Equation 8.

where

• V_OUT(t) is the gate-driver OUT pin voltage during the turnon and turnoff period. In cases where the output is saturated for some time, this value can be simplified as a constant-current source (1.5 A at turnon and 2.0 A at turnoff) charging or discharging a load capacitor. Then, the V_OUT(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 pullup or pulldown circuits is saturated and another one is not, the P_GDO is a combination of case 1 and case 2, and the equations can be easily identified for the pullup and pulldown based on this discussion.

Use Equation 9 to calculate the total gate-driver loss dissipated in the UCC23511 gate driver, P_GD.

Equation 9.