MIC2584/2585
Micrel
MOSFET Steady-State Thermal Issues
R
10mΩ[1 + (110 - 25)(0.005)] 14.3mΩ
ON
TheselectionofaMOSFETtomeetthemaximumcontinuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in °C/W) as that with which the MOSFET’s
performance was specified by the manufacturer. Here are a
few practical tips:
• The value of I
for the output in
LOAD(CONT, MAX.)
question (see "Sense Resistor Selection").
1. The heat from a surface-mount device such as
an SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be sol-
dered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
• The manufacturer’s data sheet for the candidate
MOSFET.
• The maximum ambient temperature in which the
device will be required to operate.
• Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
2. Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down sub-
stantially. If you can, position the MOSFET(s)
near the inlet of a power supply’s fan, or the
outlet of a processor’s cooling fan.
The data sheet will almost always give a value of on resis-
tancegivenfortheMOSFETatagate-sourcevoltageof4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, addthetwovaluestogetheranddividebytwo
to get the on-resistance of the part with 8V of enhancement.
3. The best test of a surface-mount MOSFET for
an application (assuming the above tips show it
to be a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of
the expected final circuit, at full operating
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the device’s
junction temperature.
Call this value R . Since a heavily enhanced MOSFET acts
ON
as an ohmic (resistive) device, almost all that’s required to
determine steady-state power dissipation is to calculate I R.
The one addendum to this is that MOSFETs have a slight
2
increase in R
with increasing die temperature. A good
ON
approximation for this value is 0.5% increase in R per °C
ON
riseinjunctiontemperatureabovethepointatwhichR was
ON
MOSFET Transient Thermal Issues
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R
of 10mΩ at a
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I R power
ON
2
T = 25°C, and the actual junction temperature ends up
J
at 110°C, a good first cut at the operating value for R
would be:
dissipation which it will see, it remains only to verify the
MOSFET’s ability to handle short-term overload power dissi-
pation without overheating. A MOSFET can handle a much
ON
RSENSE1
0.006Ω
Q1
IRF7822
(SO-8)
*D2
1N5240B
10V
5%
VIN
12V
1
2
VOUT
12V@6A
3
4
D1
(18V)
CLOAD1
220µF
C1
1µF
R1
33kΩ
R4
100kΩ
1%
R3
10Ω
16
15
VCC1 SENSE1
14
GATE1
C2
0.01µF
6
ON
12
11
MIC2584
FB1
R5
13.3kΩ
1%
R2
33kΩ
DOWNSTREAM
SIGNAL
/POR
CPOR
GND
7
9
C3
0.05µF
Undervoltage (Output) = 11.0V
/POR Delay = 25ms
START-UP Delay = 6ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less for catastrophic output
short circuit protection. (IRF7822 V (MAX) = 12V)
GS
Channel 2 and additional pins omitted for clarity.
Figure 12. Zener Clamped MOSFET Gate
MIC2584/2585
24
March 2005
MICREL [ MICREL SEMICONDUCTOR ]
