5.3 Gate Drive Control

The most crucial time in the gate drive is the turn ON and turn OFF of the MOSFET, and performing this function quickly, with minimal noise and ringing. If the rise/fall time is too fast, it can cause unnecessary ringing, and too slow of a rise/fall time will increase switching losses in the MOSFET.

An example of just the high side gate driver is shown in Figure 5-3 (any selection of gate driver components should be the same for high side and low side drive). Two extra components are shown, R_DH and D_H. With the careful selection of R_GH and R_DH, it is possible to selectively control the rise time and fall time of the gate drive. For turn ON, all current will go from the IC through R_GH and charge the MOSFET gate capacitor, hence increasing or decreasing R_GH will increase or decrease rise time in the application. With the addition of D_H, the fall time can be separately controlled as the turn OFF current flows from the MOSFET gate capacitor, through D_H and R_DH to the driver in the IC to VS. So increasing or decreasing R_DH will increase or decrease the fall time.

Increasing turn ON and turn OFF has the effect of limiting ringing and noise due to parasitic inductances, hence with a noisy environment, it may be necessary to increase the gate resistors. For gate resistor value selection, the exact value depends on the type of application and desired level of noise and ringing. Generally, power supplies switch at a fast speed, and want to squeeze out efficiency from the MOSFETs, so lower values are recommended, for example R_GH = 5Ω - 20Ω. For motors, the switching speed is generally slower, and the application has more inherent noise, so higher values are recommended, for example R_GH = 10Ω - 100Ω.