UCD7100
SLUS651C –MARCH 2005–REVISED MAY 2010
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Drive Current and Power Requirements
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:
1
2
2
E + CV
(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:
1
2
2
P + CV 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.
With VDD = 12 V, CLOAD = 10 nF, and f = 300 kHz, the power loss can be calculated as:
2
P + 10 nF 12 300 kHz + 0.432 W
(3)
With a 12-V supply, this would equate to a current of:
0.432 W
12 V
P
V
I +
+
+ 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 IDD
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 QG, one can determine the power that must be dissipated when
charging a capacitor. This is done by using the equivalence QG = CEFF x V to provide the following equation for
power:
2
P + C V f + Q V f
G
(5)
This equation allows a power designer to calculate the bias power required to drive a specific MOSFET gate at a
specific bias voltage.
10
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