8.2.2.5 Selecting Gate Resistor R_{ON}/R_{OFF}
Resistor R_{ON} and R_{OFF} are sized to achieve the following:
- Limit ringing caused by parasitic inductances and capacitances.
- Limit ringing caused by high voltage/current switching dV/dt, dI/dt, and body diode reverse recovery.
- Fine-tune gate drive strength to optimize switching loss.
- Reduce electromagnetic interference (EMI).
As mentioned in Output Stage, the UCC27712 has a pull up structure with a P-channel MOSFET providing a peak source current of 1.8A.
For this example 3.3-Ω resistors for R_{ON} and 2.2-Ω resistors for R_{OFF} were selected to provide damping for ringing and ample gate drive current.
Equation 6.
Therefore the peak source current can be predicted with:
Equation 7.
Equation 8.
where
- R_{ON}: External turn-on resistance
- R_{GFET_Int}: Power transistor internal gate resistance, found in the power transistor datasheet.
- I_{O+} = Peak source current. The maximum values between 1.8 A, the UCC27712 peak source current, and the calculated value based on the gate drive loop resistance.
In this example:
Equation 9.
Equation 10.
Therefore, the high-side and low side peak source current is 1.6 A. Similarly, the peak sink current can be calculated with:
Equation 11.
Equation 12.
where
- R_{OFF}: External turn-off resistance
- V_{DGATE}: The diode forward voltage drop which is in series with R_{OFF}. The diode in this example is an MBRM130L.
- I_{O-} = Peak sink current. The maximum values between 2.8 A, the UCC27712 peak sink current, and the calculated value based on the gate drive loop resistance.
In this example:
Equation 13.
Equation 14.