The external gate-driver resistors, R_G(ON) and R_G(OFF) are used to:

1. Limit ringing caused by parasitic inductances and capacitances
2. Limit ringing caused by high voltage or high current switching dv/dt, di/dt, and body-diode reverse recovery
3. Fine-tune gate drive strength, specifically peak sink and source current to optimize the switching loss
4. Reduce electromagnetic interference (EMI)

The output stage of UCC23511-Q1 has a pull up structure consisting of a P-channel MOSFET and an N-channel MOSFET in parallel, as shown in Figure 8-4. The N-channel MOSFET provides the peak current that charges the gate of the IGBT (or power switch) while the P-channel MOSFET ensures that V_OUT will go all the way upto V_CC. Figure 10-1 shows the internal gate resistance, R_{G_int} of the power switch. R_{G_int} is usually specified in the datasheet of the power switch. When turning the power switch "on", the gate driver sees a total gate resistance R_{G_total} = R_GON + R_{G_int} and when turning the power switch "off", the gate driver sees a total gate resistance R_{G_total} = R_GOFF + R_{G_int} in series with the gate capacitance where,

• R_GON is the external resistance on the pcb for turn "on"
• R_GOFF is the external resistance on the pcb for turn "off"
• R_{G_int} is the internal gate resistance of the power switch that can be found in the data sheet of the power switch
• Ideal diodes — zero resistance and zero voltage drop — are assumed on both the turn-on and turn-off paths, for simplicity.

To illustrate the procedure to select R_GON and R_GOFF , let us consider the following example with the assumptions listed below:

1. Power switch internal gate resistance is 2 ohms
2. Desired charging current is 1.5A
3. Desired discharging current is 1.7A
4. Gate driver supply is 15 V

For this example, since V_CC=15V, use the typical curves shown in Figure 10-5. It can be seen that we need a total turn on resistance of 7 ohms (peak charging current of 1.5A) and a total turn off resistance of 8 ohms (peak discharging current of 1.7A). Subtracting the internal gate resistance of the power switch, we get R_GON= 7 ohms - 2 ohms = 5 ohms, and R_GOFF= 8 ohms- 2 ohms = 6 ohms.

Figure 10-5 Peak drive current vs Total gate resistance, R_{G_total} (Gate driver supply =15V)

Figure 10-6 Peak drive current vs Total gate resistance, R_{G_total} (Gate driver supply = 30V)

Use Table 10-3 to find the minimum gate resistance to be used with UCC23511-Q1 when driving a power switch. The values shown includes the power switch internal gate resistance R_{G_int}. Hence, R_{G_int} must be subtracted from the values shown to determine the value of the resistor to populate on the printed circuit board.

The diodes shown in series with each, R_GON and R_GOFF, in Figure 10-1 ensure the gate drive current flows through the intended path, respectively, during turn-on and turn-off. Note that the diode forward drop will reduce the voltage level at the gate of the power switch. To achieve rail-to-rail gate voltage levels, add a resistor from the V_OUT pin to the power switch gate, with a resistance value approximately 20 times higher than R_GON and R_GOFF. For the examples described in this section, a good choice is 100 Ω to 200 Ω.