FAN3216_12 Datasheet, PDF (14/18 Pages) Fairchild Semiconductor – Dual-2A, High-Speed, Low-Side Gate Drivers

Thermal Guidelines
Gate drivers used to switch MOSFETs and IGBTs at
high frequencies can dissipate significant amounts of
power. It is important to determine the driver power
dissipation and the resulting junction temperature in the
application to ensure that the part is operating within
acceptable temperature limits.
The total power dissipation in a gate driver is the sum of
two components, PGATE and PDYNAMIC:
P_TOTAL = P_GATE + P_DYNAMIC
(1)
Gate Driving Loss: The most significant power loss
results from supplying gate current (charge per unit
time) to switch the load MOSFET on and off at the
switching frequency. The power dissipation that
results from driving a MOSFET at a specified gate-
source voltage, V_GS, with gate charge, Q_G, at
switching frequency, fSW, is determined by:
P_GATE = Q_G • V_GS • f_SW • n
(2)
where n is the number of driver channels in use (1 or 2).
Dynamic Pre-Drive / Shoot-through Current: A power
loss resulting from internal current consumption under
dynamic operating conditions, including pin pull-up /
pull-down resistors, can be obtained using the graphs
in Typical Performance Characteristics to determine
the current I_DYNAMIC drawn from V_DD under actual
operating conditions:
P_DYNAMIC = I_DYNAMIC • V_DD • n
(3)
Once the power dissipated in the driver is determined,
the driver junction rise with respect to circuit board can
be evaluated using the following thermal equation,
assuming ^JB was determined for a similar thermal
design (heat sinking and air flow):
TJ
= PTOTAL • ^JB + TB
(4)
where:
T_J = driver junction temperature;
_{JB = (psi) thermal characterization parameter}
relating temperature rise to total power
dissipation; and
TB = board temperature in location as defined in
the Thermal Characteristics table.
In the forward converter with synchronous rectifier
shown in the typical application diagrams, the
FDMS8660S is a reasonable MOSFET selection. The
gate charge for each SR MOSFET would be 60nC with
VGS = VDD = 7V. At a switching frequency of 500kHz, the
total power dissipation is:
PGATE = 60nC • 7V • 500kHz • 2 = 0.42W
(5)
P_DYNAMIC = 3mA • 7V • 2 = 0.042W
(6)
PTOTAL = 0.46W
(7)
The SOIC-8 has a junction-to-board thermal
characterization parameter of ^JB = 43°C/W. In a
system application, the localized temperature around
the device is a function of the layout and construction of
the PCB along with airflow across the surfaces. To
ensure reliable operation, the maximum junction
temperature of the device must be prevented from
exceeding the maximum rating of 150°C; with 80%
derating, TJ would be limited to 120°C. Rearranging
Equation 4 determines the board temperature required
to maintain the junction temperature below 120°C:
TB = TJ - PTOTAL • ^JB
(8)
T_B = 120°C – 0.46W • 43°C/W = 100°C
(9)
© 2012 Fairchild Semiconductor Corporation
FAN3216 / FAN3217_F085 • Rev. 1.0.0
14
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