FAN3224 Fairchild Semiconductor, FAN3224 Datasheet - Page 19

• Manufacturer Part Number: FAN3224
• Description: Dual 4A High-speed, Low-side Gate Drivers
• Manufacturer: Fairchild Semiconductor

© 2007 Fairchild Semiconductor Corporation
FAN3223 / FAN3224 / FAN3225 • Rev. 1.0.5
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
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 ψ
design (heat sinking and air flow):
T
where:
T
ψ
T
P
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
switching frequency, F
P
n is the number of driver channels in use (1 or 2).
Dynamic Pre-drive / Shoot-through Current: A
power
consumption under dynamic operating conditions,
including pin pull-up / pull-down resistors, can be
obtained using the “I
graphs in Typical Performance Characteristics to
determine the current I
under actual operating conditions:
P
J
J
B
JB
TOTAL
GATE
DYNAMIC
= P
= driver junction temperature
= (psi) thermal characterization parameter relating
= board temperature in location as defined in
= Q
temperature rise to total power dissipation
the Thermal Characteristics table.
= P
TOTAL
JB
loss
= I
G
GATE
was determined for a similar thermal
DYNAMIC
• V
• ψ
GS
GATE
+ P
resulting
JB
• F
DYNAMIC
+ T
• V
and P
GS
SW
, with gate charge, Q
DD
DD
B
SW
• n
• n
, is determined by:
(No-Load) vs. Frequency”
DYNAMIC
DYNAMIC
from
:
drawn from V
internal
current
G
(1)
(2)
(3)
(4)
, at
DD
19
To give a numerical example, if the synchronous
rectifier switches in the forward converter of Figure 52
are FDMS8660S, the datasheet gives a total gate
charge of 60nC at V
would have 120nC gate charge. At a switching
frequency of 300kHz, the total power dissipation is:
The
characterization parameter of ψ
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
Equation 4 determines the board temperature required
to maintain the junction temperature below 120°C:
For comparison, replace the SOIC-8 used in the
previous example with the 3x3mm MLP package with
ψ
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.
JB
P
P
P
T
T
= 2.8°C/W. The 3x3mm MLP package could
B,MAX
B,MAX
GATE
DYNAMIC
TOTAL
SOIC-8
= 120nC • 7V • 300kHz • 2 = 0.5W
= T
= 120°C – 0.52W • 42°C/W = 98°C
= 0.52W
J
= 1.5mA • 7V • 2 = 0.021W
would be limited to 120°C. Rearranging
J
- P
has
TOTAL
GS
= 7V, so two devices in parallel
• ψ
a
JB
junction-to-board
JB
= 42°C/W. In a
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thermal
(5)
(6)
(7)
(8)
(9)