Micrel, Inc. MIC2593
September 2008
22
M9999-092208
MOSFET Voltage Requirements
The first voltage requirement for each MOSFET is easily
stated: the drain-source breakdown voltage of the
MOSFET must be greater than V
IN(MAX)
for the slot in
question. For instance, the 5V input may reasonably be
expected to see high-frequency transients as high as
6.5V. Therefore, the drain-source breakdown voltage of
the MOSFET must be at least 7V.
The second breakdown voltage criteria which must be
met is a bit subtler than simple drain-source breakdown
voltage, but is not hard to meet. Low-voltage MOSFETs
generally have low breakdown voltage ratings from gate
to source as well. In MIC2593 applications, the gates of
the external MOSFETs are driven from the +12V input to
the MIC2593 controller. That supply may well be at 12V +
(5% x 12V) = 12.6V. At the same time, if the output of the
MOSFET (its source) is suddenly shorted to ground, the
gate-source voltage will go to (12.6V – 0V) = 12.6V. This
means that the external MOSFETs must be chosen to
have a gate-source breakdown voltage in excess of 13V;
after 12V absolute maximum, the next commonly
available voltage class has a 20V maximum gate-source
voltage. At the present time, most power MOSFETs with a
20V gate-source voltage rating have a 30V drain-source
breakdown rating or higher. As a general tip, look to
surface mount devices with a drain-source rating of 30V
as a starting point.
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 7V to 8V of enhancement (11.5V nominal
V
GATE
minus the 3.5V to 4.5V gate threshold of the
MOSFET). 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 I
2
R. 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 10m at 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:
]
14.3m25)(0.05)(110110mR
ON
−
Next, approximate the steady-state power dissipation
(I
2
R) using I
LOAD(CONT,max)
and the approximated R
ON
.
]
1.14W14.3m(8.93A)RIR
2
ON
2
MAX)LOAD(CONT,ON
≅×≅×≅
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 soldered
down to one square inch or more, the copper
trace will act as the heat sink for the part. This
copper trace 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
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, a) withstand both the
applied voltage stresses, and b) handle the worst-case
continuous I
2
R power dissipation that it will endure;
verifying the MOSFET’s ability to handle short-term
overload power dissipation without overheating is the lone
item to be determined. A MOSFET can handle a much
higher pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is that
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. ”Almost all power MOSFET data sheets give
a Transient Thermal Impedance Curve. For example, take
the case where t
FLT
for the 5V supply has been set to
