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:

E +

1

2

CV

2

(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:

P +

1

2

CV

2

f

(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.

P + 10 nF

12

2

300 kHz + 0.432 W

(3)

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

I +

P

V +

0.432 W

12 V

+ 0.036 A

(4)

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

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

power:

P + C

V

f

(5)

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

specific bias voltage.

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UCD7100

SLUS651E – MARCH 2005 – REVISED NOVEMBER 2021

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