AND8199 Datasheet, PDF (2/4 Pages) ON Semiconductor – THERMAL STABILITY OF MOSFETS

AND8199
50
Pulse Duration = 80 ms
Duty Cycle = 0.5% Maximum
40 V_DD = 15 V
30
20
T_J = 25°C
10
T_J = 100°C
T_J = −55°C
0
2
3
4
5
6
V_GS, GATE−TO−SOURCE VOLTAGE (V)
Figure 3. Fairchild HUF75631SK8
−40°_25°
10
9
125°
8
7
6
5
Point of Inflection
4
3
2
1
125°
−40°
1 2 3 4 5 6 7 8 9 10
V_GS
V_GS−
V_GS+
Figure 4. Typical Transconductance Curve
All three devices shown have one thing in common:
a point of inflection at which the temperature coefficient is
zero. At greater gate−to−source voltages, the coefficient is
positive, and, at lower gate−to−source voltages it is negative.
Figure 4 illustrates the change from negative to positive.
At a gate−to−source voltage greater than that of the
inflection point (V_GS+), a positive temperature coefficient
exists. At this gate voltage, the drain conducts more than
9.0 A of current. However, at 125°C the drain current
reduces to less than 7.0 A. The arrow at the left of Figure 4,
which shows the current decreasing due to an increase in
temperature, indicates this drop.
At a gate−to−source voltage below the inflection point
(V_GS−) a negative temperature coefficient exists. At −40°C,
the drain current is close to zero. However, at 125°C, the
drain current is more than 1.0 A. A second arrow at the left
of Figure 4 indicates this relationship, and the current rises
due to an increase in temperature.
The implication is that when you are controlling the FET
with a gate−to−source voltage below the inflection point,
thermal runaway can occur. When one cell or a small group
of cells becomes hotter than the surrounding cells, they tend
to conduct more current. This situation, in turn, creates more
heat, which allows more current to flow. These cells can pull
a large amount of current and, if not limited in time, can
cause the device to fail.
This situation is similar to the well−known phenomenon
of secondary breakdown that occurs in bipolar transistors
except that a bipolar junction transistor is a single device,
and you can take steps to avoid its destruction. A power
MOSFET contains thousands of parallel devices that are
internal to the die, and you cannot individually protect them.
If hot spots occur, the SOA characteristics of the heavily
conducting cells differ greatly from those of the marginally
conducting cells.
The thermal−runaway situation occurs when you use large
devices at low current−limit settings. Even though it would
appear to be desirable to use a very large MOSFET for an
application such as a hot swap and limit it to a low current,
it may be an inappropriate approach. Use of a
very−low−on−resistance device offers low losses for
steady−state operation but may cause the device to fail
during a short circuit or an overload.
One way to overcome this problem is to directly sense the
die temperature of the MOSFET by integrating the
MOSFET with the controller using a monolithic approach.
ON Semiconductor takes this approach with its new line of
hot−swap ICs. In this case, the temperature can be sensed
directly on the FET die. The location of the sense element on
the die is critical for ensured protection of the device. If a hot
spot occurs too far from the sense location, the device may
be unable to protect itself.
Discrete hot−swap controllers employ a number of
protection schemes. Thermal instability is an issue only if
the controller can go into a constant−current mode of
operation. Some protection circuits simply shut off the
MOSFET switch if a number of conditions indicate a
dangerous area of operation. Controllers that use a
constant−current method of protection can use timers or
other schemes along with the current−limit circuit to reduce
the risk of failure.
Because system efficiency is an important parameter, it is
tempting to use the largest MOSFET possible to reduce
losses. It is important to keep in mind, however, that this
approach may require you to make a trade−off with the
system reliability if you are not mindful of the possible
thermal instability. You can reliably use a large power device
at a low current limit level if you handle it properly.
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