9.1 Drive Current and Power Requirements

The UCC2742x is capable of delivering 4 A of current to a MOSFET gate for a period of several-hundred nanoseconds. High-peak current is required to turn the device ON quickly. 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. This discussion uses a MOSFET because it is the most common type of switching device used in high-frequency power-conversion equipment.

References ⁽¹⁾ and ⁽²⁾ discuss the current required to drive a power MOSFET and other capacitive-input switching devices. Reference ⁽²⁾ includes information on the previous generation of bipolar IC gate drivers.

When a driver IC 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.

Equation 2.

where

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

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

Equation 3.

where

• ƒ 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 ƒ = 300 kHz, the power loss can be calculated as in Equation 4.

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

Equation 5.

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 IC internal consumption should be considered. With no load, the IC 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 laboratory setup. The measured current with a 10-nF load is reasonably close to that which is predicted.

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 = CeffV, to provide the following equation 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.