The UCD7K family of drivers can deliver high current into a MOSFET gate for a period of several hundred nanoseconds. High peak current is required to turn the 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. 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] discusses the current required to drive a power MOSFET and other capacitive-input switching devices.

When a driver device 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 1.

where C is the load capacitor and V is the 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 the following:

Equation 2.

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.

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

Equation 3.

With a 12-V supply, this would equate to a current of:

Equation 4.

The actual current measured from the supply was 0.037 A, and is very close to the predicted value. But, the I_DD current that is due to the device internal consumption should be considered. With no load the device current drawn 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 close to the value expected.

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. This is done by using the equivalence Q_G = C_EFF x V to provide the following equation for power:

Equation 5.

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