UCC27323, UCC27324, UCC27325

UCC37323, UCC37324, UCC37325

SLUS492B – JUNE 2001 – REVISED SEPTEMBER 2002

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APPLICATION INFORMATION

drive current and power requirements

The UCC37323/4/5 family of 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 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.

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:

E

+ 1

2

CV2, 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:

P

+ 2

1

2

CV2f, 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 a 12-V supply, this would equate to a current of:

I

+ P

V

+ 0.432 W

12 V

+ 0.036 A

The actual current measured from the supply was 0.037 A, and is very close to the predicted value. But, the

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

that expected.