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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.
To give a numerical example, assume for a 12 V V DD
(V BIAS ) system, the synchronous rectifier switches of
Figure 54 have a total gate charge of 60 nC at
V GS = 7 V. Therefore, two devices in parallel would have
120 nC gate charge. At a switching frequency of
300 kHz, the total power dissipation is:
The total power dissipation in a gate driver is the sum of
two components, P GATE and P DYNAMIC :
P GATE = 120 nC ? 7 V ? 300 kHz ? 2 = 0.504 W
(5)
P TOTAL = P GATE + P DYNAMIC
(1)
P DYNAMIC = 3.0 mA ? 12 V ? 1 = 0.036 W
(6)
P GATE (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, f SW , is determined by:
P TOTAL = 0.540 W (7)
The SOIC-8 has a junction-to-board thermal
characterization parameter of ? JB = 42°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
P GATE = Q G ? V GS ? f SW ? n
(2)
temperature of the device must be prevented from
exceeding the maximum rating of 150°C; with 80%
where n is the number of driver channels in use (1 or 2).
P DYNAMIC (Dynamic Pre-Drive / Shoot-through
Current): A power loss resulting from internal current
derating, T J would be limited to 120°C. Rearranging
Equation 4 determines the board temperature required
to maintain the junction temperature below 120°C:
consumption under dynamic operating conditions,
including pin pull-up / pull-down resistors. The internal
current consumption (I DYNAMIC ) can be estimated using
the graphs in Figure 15 and Figure 16 of the 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)
where n is the number of driver ICs in use. Note that n is
usually be one IC even if the IC has two channels,
unless two or more.driver ICs are in parallel to drive a
large load.
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):
T J = P TOTAL ? ? JB + T B
(4)
where:
T J
= driver junction temperature;
? JB
= (psi) thermal characterization parameter
relating temperature rise to total power
dissipation; and
T B
= board temperature in location as defined in
the Thermal Characteristics table.
T B,MAX = T J - P TOTAL ? ? JB
T B,MAX = 120°C – 0.54 W ? 42°C/W = 97°C
(8)
(9)
? 2007 Fairchild Semiconductor Corporation
FAN3223 / FAN3224 / FAN3225 ? Rev. 1.1.4
22
www.fairchildsemi.com
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