AN2385
Application note
Power dissipation and its linear derating factor, silicon
Limited Drain Current and pulsed drain current in MOSFETs
Introduction
Datasheets of the modern power MOSFET devices, either of low voltage or of high voltage,
show in the section entitled "Absolute Maximum Rating" the values of some important
parameters that regard the SOA (safe operating area). As it is well kwown in literature, SOA
is the area that includes all the I
D
-V
DS
operating points where the device works in safety
conditions.
These important parameters are studied in this technical article. In particular, attention will
be focused on the Power Dissipation and its Linear Derating Factor, Silicon Limited Drain
Current and Pulsed Drain Current. This technical article will explain what these parameters
are and how they can be calculated. It will recall some basic and simple technical concepts
and can be a useful tool for customers to understand and facilitate reading of a power
MOSFET datasheet.
June 2006
Rev 1
1/14
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Contents
AN2385
Contents
1
2
3
4
5
6
Determination of the SOA limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Silicon limited drain current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Determination of the power dissipation and its derating factor . . . . . . 9
Pulsed drain current (I
DM
) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2/14
AN2385
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Typical FBSOA of a Power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
R
DSON
limit for MOSFET's FBSOA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Typical RBSOA of a Power MOSFET. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Silicon limited drain current vs case temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Power dissipation vs T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
IDM determination considering the output characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Junction to case maximum effective transient thermal impedance for TO-220 package.. . 10
Determination of the maximum rectangular pulse duration . . . . . . . . . . . . . . . . . . . . . . . . 11
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AN2385
Determination of the SOA limits
1
Determination of the SOA limits
SOA is the acronym of Safe Operating Area. It includes all the I
D
-V
DS
operating points
inside where the device works in safety conditions. There are two kinds of SOA. The first
one is named Forward Biased Safe Operating Area (FBSOA), while the second one is
named Reverse Biased Safe Operating Area (RBSOA). FBSOA is the SOA during the
device on state, while RBSOA is the SOA when the MOSFET switches off. Supposing that
the I
D
and V
DS
axis are in log scale, a typical FBSOA can be depicted as in
Figure 1.
Figure 1.
Typical FBSOA of a Power MOSFET
I
Dmax
is the maximum drain current limit of the MOSFET. It is usually fixed by the wires that
connect the drain and source pads to the package pins respectively. BVdss is the maximum
drain-source voltage that the device can sustain (breakdown voltage). P
max(t)
is the
maximum power that the device can dissipate. It depends on the junction temperature and
power pulse interval time and on the package used. In fact, if the junction temperature
overcomes typically 150'C or 175'C, as defined in the automotive devices, the MOSFET
could fail or, however, the device works out of the guaranteed temperature spec.
Furthermore, increasing the case temperature, the P
max(t)
value decreases due to a lower
energy, which is necessary to bring the junction temperature to the maximum guaranteed
value. P
max
is also a function of the power pulse interval time because, fixing the power
pulse, the energy dissipated in the MOSFET, as well as the junction temperature rises,
increasing the power interval time. In particular, when the power pulse interval time
increases, P
max(t)
decreases and the SOA area decreases too.
Another limit for FBSOA is established by R
DSON
of the device. In fact, when the device is in
on state without loads connected to the component, all the voltage feed is applied on the
drain-source terminals and, thus, the maximum drain current that can flow in the transistor is
fixed by V
DD
and R
DSON
(Figure
2.).
5/14
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