FAN3226_11 Datasheet, PDF (20/25 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, P_GATE and P_DYNAMIC:
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, F_SW, is determined by:
P_GATE = Q_G • V_GS • F_SW • n
(2)
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 “I_DD (No-Load) vs. Frequency”
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):
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
T_B = board temperature in location defined in Note
1 under Thermal Resistance 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
V_GS = V_DD = 7V. At a switching frequency of 500kHz, the
total power dissipation is:
P_GATE = 60nC • 7V • 500kHz • 2 = 0.42W
(5)
P_DYNAMIC = 3mA • 7V • 2 = 0.042W
(6)
P_TOTAL = 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, T_J would be limited to 120°C. Rearranging
Equation 4 determines the board temperature required
to maintain the junction temperature below 120°C:
T_B = T_J - P_TOTAL • ^_JB
(8)
T_B = 120°C – 0.46W • 43°C/W = 100°C
(9)
For comparison, replace the SOIC-8 used in the
previous example with the 3x3mm MLP package with
_{JB = 3.5°C/W. The 3x3mm MLP package could operate}
at a PCB temperature of 118°C, while maintaining the
junction temperature below 120°C. This illustrates that
the physically smaller MLP package with thermal pad
offers a more conductive path to remove the heat from
the driver. Consider tradeoffs between reducing overall
circuit size with junction temperature reduction for
increased reliability.
© 2007 Fairchild Semiconductor Corporation
FAN3226 / FAN3227 / FAN3228 / FAN3229 • Rev. 1.0.7
20
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