TN1156
Technical note
Irradiated HV Power MOSFETs working in linear zone: a
comparison of electro-thermal behavior with standard HV products
Introduction
This paper studies the thermal instability phenomenon of irradiated HV Power MOSFET
devices working in linear zone operating conditions and compares their electro-thermal
behavior with standard products. Experimental results show that irradiated devices have
more thermal instability than standard devices, therefore, they should be used carefully in
these particular operating conditions.
In most of cases, Power MOSFETs are used in switching operating conditions where
R
DS(on)
is the key parameter to evaluate the device’s performance. However, in some
special cases, devices work in linear zone. A Power MOSFET works in linear zone when
high-currents and high voltages are together applied to MOSFET terminals. Power
MOSFETs are used in linear zone in some dedicated applications especially in automotive
segment such as: standard topologies of audio amplifiers, linear DC-DC converters, DC fan
controllers, electronic loads, current mirrors, smart fuses, etc… In many of these
applications, typically, the LV Power MOSFETs are used. Furthermore, Power MOSFETs
work in linear zone for a short period of time when they are used in switching conditions
during the Miller region where high-currents and high voltages are together applied to
MOSFET terminals. This means that the thermal instability must be also evaluated in
applications where linear zone isn't the typical operating condition. Therefore, this
phenomenon needs to be evaluated even during the slow switching process either in LV or,
in particular, in HV Power MOSFETs when high inductive loads are driven. Considering a
theoretical Power MOSFET FBSOA, linear zone is referred to that area delimited by
maximum allowed dissipated power for different power pulse duration.
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Contents
TN1156
Contents
1
2
3
4
5
Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Experiments, results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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Basic concepts
1
Basic concepts
In linear zone, the Power MOSFET can be subjected to a thermal run-away process that
could lead the device to fail. Failures are experimentally explained as a drain current
focusing phenomenon (hot spots). Failures are generally localized in the center of the die,
close to the source wire bonding. Owing to that, standard theoretical FBSOA does not
exactly describe the safe area in forward-biased conditions since the red area portion is lost
(see
Figure 1).
An analytical approach has been performed to describe this phenomenon. In
linear zone (saturation region), I
D
can be written as:
Equation 1
1 W
2
I
D
= --
μ
----
OX
(
V
GS
–
V
TH
)
- -c
2 L
c
OX
is the gate oxide capacitance by unit area, W is the perimeter of the device, L is the
length of the channel,
μ
the mobility of the carriers in the channel and V
TH
is the threshold
voltage. Introducing the term K as:
Equation 2
W
1
K
= --
c
OX
----
-μ
-
L
2
Equation 1 can be rewritten as:
Equation 3
I
D
=
K
(
V
GS
–
V
TH
)
2
Figure 1. Example of Power MOSFET FBSOA
GIPG141120131502FSR
During the linear zone operating condition, when a power pulse is applied, the device
warms, the junction temperature increases and I
D
changes its value because the mobility of
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Basic concepts
TN1156
the carriers in the channel and V
TH
change theirs values. In particular, either
μ
or V
TH
decrease when the temperature increases. From the derivative of (equation
2)
against T, the
thermal coefficient, TC, of the device can be achieved by:
Equation 4
∂
I
D
I
D
------- = ----
∂
K
-
-
∂
T
K
------
∂
T
∂
V
TH
–
2 K I
D
-------------
∂
T
V
DS
=
const
V
D S
=
const
TC is the main thermal-electro parameter used to monitor the thermal instability
phenomenon. In fact, in linear zone, the electro-thermal stability of each device is evaluated
by considering a graph where TC is achieved versus I
D
. In equation
4,
the first term,
depending on the derivative of K, tries to make TC negative while, vice versa, the second
term, depending on the derivative of V
TH
, tries to make the same coefficient positive. If the
first one is higher than the second one, TC becomes negative and no failure occurs, vice
versa, TC becomes positive and a thermal run-away phenomenon could occurs. However,
even if TC is positive, the device could work in safety region. This depends on the capability
of the whole die thermal system to catch the heat per unit area and time developed by the
electrical power pulse. If the heat produced by unit time can be totally extracted from the
device, then the Power MOSFET works in safety conditions. Otherwise, the heat increases
the internal energy of the system causing a die temperature rise until T reaches the
maximum allowable value (localized silicon) leading the device to fail.
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Experiments, results and discussion
2
Experiments, results and discussion
To evaluate the electro-thermal instability of irradiated HV Power MOSFETs working in
linear zone and to compare their performance with standard devices, two different samples
belonging to two different suppliers have been taken into account. Devices under testing
have a breakdown voltage equal to 600 V, a standard threshold voltage with 50 A of nominal
drain current in continuous mode when the ambient temperature is 25 °C. The only
difference between irradiated and not irradiated devices is the irradiation process. To study
the thermal behavior in linear zone of the devices under testing, thermal coefficients have
been measured: 10 V Vds. TC has been achieved against the drain current fixing the Vds
values to 10 V and considering two ambient temperature values: 25 °C and 75 °C. Testing
has been fulfilled considering the worst conditions where the current isn't fixed by external
equipment or controlled by a feedback, but it depends on the same transistor (open loop
case) only. The graph in
Figure 2
and
Figure 3
takes into account TC versus I
D
for Vds = 10
V.
Figure 2. Comparison between standard and irradiated HV Power MOSFETs: TC
curves vs. I
D
(supplier 1)
GIPG141120131455FSR
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