AND8199 Datasheet, PDF (1/4 Pages) ON Semiconductor – THERMAL STABILITY OF MOSFETS

AND8199
Thermal Stability of
MOSFETs
Prepared by: Alan Ball
ON Semiconductor
Application Engineering Manager
http://onsemi.com
A variety of applications use hot−swap controllers, often
to increase the reliability of a system. However, a failure in
the hot−swap circuit would defeat that purpose. When you
use MOSFETs in their active region to control current, such
as you would for a controller that operates in a
constant−current mode of operation, they have an inherent
failure mechanism. In this mode, the MOSFET can get
hot spots and fail, long before the device exceeds its
Safe Operating Area (SOA) ratings.
Engineers have long understood that MOSFETs are
positive temperature coefficient devices. Therefore, as the
temperature of the device increases, the resistance increases.
In other words, higher temperatures result in lower currents.
This fact is important if you want to operate MOSFETs in
parallel. With a good thermal path between devices, the
positive temperature coefficient reduces the current in the
hottest device and forces more of it to flow in the cooler
device, thereby avoiding thermal runaway.
Engineers often think of a MOSFET as a single power
transistor, but it is a collection of thousands of tiny power
FET cells connected in parallel. In terms of sharing current,
24
V_DS ≥ 10 V
20
16
12
8
T_J = 25°C
4
T_J = 100°C
T_J = −55°C
00 _{1 2 3 4 5 6 7 8 9}
V_GS, GATE−TO−SOURCE VOLTAGE (V)
Figure 1. Transfer Characteristics for NTD12N10
the same application of the positive temperature coefficient
applies. In this case, the thermal path between the cells is
better than that of separate packaged devices, because the
cells are all on the same die.
As the current density of a small group of cells increases,
those cells heat up, increasing the resistivity of those cells
and forcing current to flow in neighboring cells, which
minimizes the thermal gradient and avoids hot spots. This
process is an essential physical tenet that allows the parallel
array of cells to function reliably.
If the MOSFET exhibits a negative thermal coefficient,
today’s parallel cell structure would cause serious reliability
issues. In fact, in some modes of operation, the thermal
coefficient goes negative. You can easily understand this
phenomenon by looking at the transconductance curves for
a FET device (refer to Reference 1).
A typical set of transconductance curves clearly
demonstrates this effect as shown by Figure 1. Below are
curves from three typical devices used in
hot swap applications.
100
T_J = 25°C
T_J = 175°C
10
4.0
V_DS = 50 V
20 ms = Pulse Width
5.0
6.0
7.0
8.0
V_GS, GATE−TO−SOURCE VOLTAGE (V)
Figure 2. International Rectifier IRF530
 Semiconductor Components Industries, LLC, 2005
1
January, 2005 − Rev. 0
Publication Order Number:
AND8199/D