MIC2582_09 Datasheet, PDF (20/25 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
MIC2582/MIC2583
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
resistance 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 ºC 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 = 25ºC, and the actual junction
temperature ends up at 110ºC, a good first cut at the
operating value for R_ON would be:
R_ON ≅ 10mΩ [1 + (110 − 25)(0.005)] ≅ 14.3mΩ (13)
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 ºC/W) as that with which
the MOSFETs performance was specified by the
manufacturer. Here are a few practical tips:
1. The heat from a surface-mount device such as
an SOIC-8 MOSFET flows almost entirely out
of the drain leads. If the drain leads can be
soldered 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
substantially.
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 MOSFETs 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 MOSFETs ability to handle
short-term overload power dissipation without
overheating. A MOSFET can handle a much higher
pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is
that, like everything else, thermal devices (silicon die,
lead frames, etc.) have thermal inertia.
In terms related directly to the specification and use of
power MOSFETs, this is known as “transient thermal
impedance,” or Z_θ(JA). Almost all power MOSFET data
sheets give a Transient Thermal Impedance Curve. For
example, take the following case: V_IN = 12V, t_OCSLOW has
been set to 100msec, I_{LOAD(CONT. MAX)} is 2.5A, the slow-
trip threshold is 50mV nominal, and the fast-trip
threshold is 100mV. If the output is accidentally
connected to a 3Ω load, the output current from the
MOSFET will be regulated to 2.5A for 100ms (t_OCSLOW)
before the part trips. During that time, the dissipation in
the MOSFET is given by:
P = E x I; E_MOSFET = [12V-(2.5A)(3Ω)] = 4.5V
P_MOSFET = (4.5V x 2.5A) = 11.25W for 100msec.
At first glance, it would appear that a really hefty
MOSFET is required to withstand this sort of fault
condition. This is where the transient thermal impedance
curves become very useful. Figure 10 shows the curve
for the Vishay (Siliconix) Si4410DY, a commonly used
SOIC-8 power MOSFET.
Taking the simplest case first, we’ll assume that once a
fault event such as the one in question occurs, it will be
a long time– 10 minutes or more– before the fault is
isolated and the channel is reset. In such a case, we can
approximate this as a “single pulse” event, that is to say,
there’s no significant duty cycle. Then, reading up from
the X-axis at the point where “Square Wave Pulse
Duration” is equal to 0.1sec (=100msec), we see that the
Z_θ(JA) of this MOSFET to a highly infrequent event of this
duration is only 8% of its continuous R_θ(JA).
This particular part is specified as having an R_θ(JA) of
50°C/W for intervals of 10 seconds or less.
April 2009
20
M9999-043009-C