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2004 Microchip Technology Inc. DS00799B-page 1
AN799
INTRODUCTION
There are many MOSFET technologies and silicon
processes in existence today, with new advances being
made every day. To make a generalized statement
about matching a MOSFET driver to a MOSFET based
on voltage/current ratings or die sizes is very difficult, if
not impossible.
As with any design decision, there are multiple vari-
ables involved when selecting the proper MOSFET
driver for the MOSFET being used in your design.
Parameters such as input-to-output propagation delay,
quiescent current, latch-up immunity and driver current
rating must all be taken into account. Power dissipation
of the driver will also effect your packaging decision
and driver selection.
This Application Note discusses the details of MOSFET
driver power dissipation in relation to MOSFET gate
charge and operating frequency. It also discusses how
to match MOSFET driver current drive capability and
MOSFET gate charge based on desired turn-on and
turn-off times of the MOSFET.
Microchip offers many variations of MOSFET drivers in
various packages, which allows the designer to select
the optimal MOSFET driver for the MOSFET(s) being
used in their application.
POWER DISSIPATION IN A MOSFET
DRIVER
Charging and discharging the gate of a MOSFET
requires the same amount of energy, regardless of how
fast or slow (rise and fall of gate voltage) it occurs.
Therefore, the current drive capability of the MOSFET
driver does not effect the power dissipation in the driver
due to the capacitive load of the MOSFET gate.
There are three elements of power dissipation in a
MOSFET driver:
1. Power dissipation due to the charging and
discharging of the gate capacitance of the
MOSFET.
EQUATION 1:
2. Power dissipation due to quiescent current draw
of the MOSFET driver.
EQUATION 2:
3. Power dissipation due to cross-conduction
(shoot-through) current in the MOSFET driver.
EQUATION 3:
As deduced from the equations above, only one of the
three elements of power dissipation is due to the
charging and discharging of the MOSFET gate
capacitance. This portion of the power dissipation is
typically the highest, especially at lower switching
frequencies.
In order to calculate a value for Equation 1, the gate
capacitance of the MOSFET is required. The gate
capacitance of a MOSFET is comprised of two capaci-
tances: the gate-to-source capacitance and the gate-
to-drain capacitance (Miller Capacitance). A common
mistake is to use the Input Capacitance rating of the
MOSFET (C
ISS
) as the total gate capacitance of the
MOSFET. The proper method for determining gate
capacitance is to look at the Total Gate Charge (Q
G
) in
the MOSFET data sheet. This information is typically
shown in the Electrical Characteristics table and as a
typical characteristics curve in any MOSFET data
sheet.
Author: Jamie Dunn
Microchip Technology Inc.
P
C
C
G
V
DD
2
F××=
Where:
C
G
= MOSFET Gate Capacitance
V
DD
= Supply Voltage of MOSFET Driver (V)
F = Switching Frequency
P
Q
I
QH
DI
QL
1D–()×+×()V
DD
×=
Where:
I
QH
= Quiescent current of the driver with
the input in the high state
D = Duty cycle of the switching waveform
I
QL
= Quiescent current of the driver with
the input in the low state
P
S
CC F V
DD
××=
Where:
CC = Crossover constant (A*sec)
Matching MOSFET Drivers to MOSFETs
