The UCC27324-Q1 drivers are capable of delivering 4 A of current to a MOSFET gate for a period of several hundred nanoseconds. High peak current is required to quickly turn on the device. Then, to turn off the device, the driver is required to sink a similar amount of current to ground. This repeats at the operating frequency of the power device. A MOSFET is used in this discussion, because it is the most common type of switching device used in high-frequency power-conversion equipment.

Reference [1] in the Section 10.2.1 section discuss the current required to drive a power MOSFET and other capacitive-input switching devices and includes information on the previous generation of bipolar gate drivers.

When a driver is tested with a discrete capacitive load, calculating the power that is required from the bias supply is fairly simple. Use Equation 2 to calculate the energy that must be transferred from the bias supply to charge the capacitor.

where

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

An equal amount of energy transferred to ground when the capacitor is discharged which leads to power loss. Use Equation 3 to calculate this power loss.

where

• f is the 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 should help clarify this.

Use Equation 4 to calculate the power loss with the following values: V_DD = 12 V, C_LOAD = 10 nF, and f = 300 kHz.

For a 12-V supply, use Equation 5 to calculate the current

The actual current measured from the supply was 0.037 A, which is very close to the predicted value. But, the I_DD current that is due to the internal consumption should be considered. With no load, the current draw is 0.0027 A. Under this condition, the output rise and fall times are faster than with a load. This could lead to an almost insignificant, yet measurable current due to cross-conduction in the output stages of the driver. However, these small current differences are buried in the high-frequency switching spikes and are beyond the measurement capabilities of a basic lab setup. The measured current with 10-nF load is reasonably close to the expected value.

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 needed 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, one can determine the power that must be dissipated when charging a capacitor. Use Equation 6 and the equivalence Q_g = CeffV to calculate this power.

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