The UCC2732x-Q1 family of drivers is capable of delivering 9 A of current to a MOSFET gate for a period of several hundred nanoseconds. High peak current is required to turn an N-channel device ON quickly. Then, to turn the device OFF, the driver is required to sink a similar amount of current to ground. This repeats at the operating frequency of the power device. An N-channel MOSFET is used in this discussion because it is the most common type of switching device used in high-frequency power-conversion equipment.

Design And Application Guide For High Speed MOSFET Gate Drive Circuits and Practical Considerations in High Performance MOSFET, IGBT and MCT Gate Drive Circuits contain detailed discussions of the drive current required to drive a power MOSFET and other capacitive-input switching devices. Much information is provided in tabular form to give a range of the current required for various devices at various frequencies. The information pertinent to calculating gate-drive current requirements is summarized here. See MOSFET and IGBT drivers for additional documentation.

When a driver is tested with a discrete capacitive load, it is a fairly simple matter to calculate the power that is required from the bias supply. The energy that must be transferred from the bias supply to charge the capacitor is given by Equation 2:

where

• C = load capacitor
• V = bias voltage feeding the driver

There is an equal amount of energy transferred to ground when the capacitor is discharged. This leads to a power loss given by Equation 3:

where

• f = switching frequency

This power is dissipated in the resistive elements of the circuit. Thus, with no external resistor between the driver and gate, this power is dissipated inside the driver. Half of the total power is dissipated when the capacitor is charged, and the other half is dissipated when the capacitor is discharged. An actual example using the conditions of the previous gate-drive waveform must help clarify this.

With V_DD = 12 V, C_LOAD = 10 nF, and f = 300 kHz, the power loss can be calculated as in Equation 4:

With a 12-V supply, in Equation 5 this equates to a current of:

The switching load presented by a power MOSFET can be converted to an equivalent capacitance by examining the gate charge required to switch the device. This gate charge includes the effects of the input capacitance plus the added charge required to swing the drain of the device between the on and off states. Most manufacturers provide specifications that provide the typical and maximum gate charge, in nC, to switch the device under specified conditions. Using the gate charge Q_g, the power that must be dissipated when charging a capacitor can be determined. This is done by using the equivalence Q_g = CeffV to provide the Equation 6 for power:

Equation 6 allows a power designer to calculate the bias power required to drive a specific MOSFET gate at a specific bias voltage.