MIC2584_05 Datasheet, PDF (24/28 Pages) Micrel Semiconductor – Dual-Channel Hot Swap Controller/Sequencer

MIC2584/2585
MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum continuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
• The value of I_{LOAD(CONT, MAX.)} for the output in
question (see "Sense Resistor Selection").
• 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?).
The data sheet will almost always give a value of on resis-
tance given for the MOSFET at a gate-source voltage of 4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, add the two values together and divide by two
to get the on-resistance of the part with 8V of enhancement.
Call this value R_ON. Since a heavily enhanced MOSFET acts
as an ohmic (resistive) device, almost all that’s required to
determine steady-state power dissipation is to calculate I²R.
The one addendum to this is that MOSFETs have a slight
increase in R_ON with increasing die temperature. A good
approximation for this value is 0.5% increase in R_ON per °C
rise in junction temperature above the point at which R_ON was
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R_ON of 10mΩ at a
T_J = 25°C, and the actual junction temperature ends up
at 110°C, a good first cut at the operating value for R_ON
would be:
Micrel
R_ON ≅ 10mΩ[1 + (110 - 25)(0.005)] ≅ 14.3mΩ
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:
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.
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.
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.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I²R power
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
V_IN
12V
D1
(18V)
R_SENSE1
0.006Ω
1 ^5% 2
3
4
C1
1µF
Q1
IRF7822
(SO-8)
*D2
1N5240B
10V
R1
33kΩ
16
VCC1
15
SENSE1
GATE1 ¹⁴
R3
10Ω
R4
100kΩ
1%
6 _ON
C2
0.01µF
MIC2584
FB1 ¹²
R2
33kΩ
CPOR
7
C3
0.05µF
GND
9
11
/POR
DOWNSTREAM
SIGNAL
R5
13.3kΩ
1%
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 VGS(MAX) = 12V)
Channel 2 and additional pins omitted for clarity.
Figure 12. Zener Clamped MOSFET Gate
V_OUT
12V@6A
C_LOAD1
220µF
MIC2584/2585
24
March 2005