March, 2004
Application Note 9034
Power MOSFET Avalanche Guideline
Sungmo Young, Application Engineer
Introduction
The Power MOSFET is a very popular switching device used in switching power supplies and
DC-DC converters. Their operation frequency is being continuously increased to reduce size
and increase power density. This causes high di/dt, intensifies the negative effect from para-
sitic inductances, and results in high voltage spike between the Power MOSFET drain and
source during device turn off. The spike is worst at power on due to empty bulk capacitors and
small inductance because the transformer primary side inductance almost reaches the level of
leakage inductance. Fortunately, the Power MOSFET is equipped to withstand a certain level
of stress, unnecessitating expensive protection circuits. This note presents an effective way to
determine the applicability of a Power MOSFET in an application. The designers can balance
between cost and reliability.
1. A Rating System: Single Pulse UIS SOA
The Fairchild Power Discrete Group has introduced a rating system that specifies the Power
MOSFET capability for single pulse Unclamped Inductive Switching (UIS).
[1]
This system
enables easy determination and/or estimation of device feasibility in any application with sim-
ple parameters : the peak current through the Power MOSFET during avalanche (I
AS
), the
junction temperature at the start of the UIS pulse (Tj), and the time the Power MOSFET
remains in avalanche (t
AV
). By plotting I
AS
and t
AV
on a graph the user can check the UIS
capability of the device. The application specific part of Fairchild UltraFET
and Power-
Trench
provide such rating chart, and a part of QFET
TM
datasheets will soon be updated.
I
AS
V
DS
t
AV
Figure 1. UIS Waveforms
©2004 Fairchild Semiconductor Corporation
1
Rev. A, March 2004
2. Over-Voltage Conditions
The over-voltage conditions in actual applications can be classified into two different groups.
One is when the drain-source voltage of Power MOSFET exceeds the specified absolute max-
imum rating but is still short of the breakdown voltage of the device. This is not an avalanche
situation and the device feasibility can be determined through junction temperature analysis.
Another is when the device breaks down and goes into avalanche mode. The Rating System
is a great tool for avalanche mode analysis.
3. Avalanche Mode Analysis
When the Power MOSFET avalanches, the drain-source voltage is clamped to its effective
breakdown voltage and the current is commutated through a parasitic antiparallel diode. Fig-
ure 2 shows typical avalanche waveforms in switching power supplies. The drain-source volt-
age is over 1kV and a commutating current is observed.
Figure 2. Device Breakdown, 800V Rated MOSFET
The UIS Rating System is very useful in dealing with avalanche situations. There are three
areas in the UIS SOA graph as indicated in Figure 3 : (1) above and right of the 25°C line, (2)
below and left of the maximum junction temperature line, (3) in between the two lines. (1) and
(2) are easy to determine: the device is within the UIS rating ((2)), or beyond the rating ((1)).
But the junction temperature of the Power MOSFET at the start of the UIS pulse is required to
determine (3). The junction temperature analysis methods will be discussed later in detail.
This UIS Rating System can also be applied to repetitive pulses through superposition tech-
nique. Each UIS pulse is evaluated separately just as in single pulse. Usually, the last pulse
in a series of power pulses occurs at the highest junction temperature and is therefore the
worst stress. If the Power MOSFET is within the specified UIS rating for the last pulse, it is
certainly within the UIS ratings for previous pulses which occurred at a lower junction temper-
ature.
[2]
(1)
STARTING T
= 25 C
o
I
AS
, AVALANCHE CURRENT (A)
100
(2)
10
(3)
J
STARTING T
J
= 150 C
o
1
0.01
0.1
1
10
100
t
AV
, TIME IN AVALANCHE (ms)
Figure 3. UIS Capability, FDP050AN06A0
©2004 Fairchild Semiconductor Corporation
2
Rev. A, March 2004
4. Junction Temperature Analysis
Generally, breakdown of the Power MOSFET seldom occurs even if the drain-source voltage
exceeds the absolute maximum rating. The BV
DSS
of the Power MOSFET has a positive tem-
perature coefficient as shown in Figure 4. It reaches about 990V at 120°C in this example.
Therefore, a greater voltage is required to cause device breakdown at higher temperature. In
many cases, the ambient temperature during the Power MOSFET operation is over 25°C and
the power loss causes the junction temperature of the Power MOSFET to rise above the ambi-
ent temperature.
1.2
BV
DSS
, (Normalized)
Drain-Source Breakdown Voltage
1.1
1.0
0.9
※
Notes :
1. V
GS
= 0 V
2. I
D
= 250 µ A
0.8
-100
-50
0
50
100
o
150
200
T
J
, Junction Temperature [ C]
Figure 4. Normalized BV
DSS
vs Tj, FQA11N90C
Also, note that the BV
DSS
in Figure 4 is measured at 250µA of the drain current. In a real
breakdown, the drain current reaches a much higher level and the breakdown voltage is even
higher than the above value.
Figure 5. Waveforms from Switching Power Supply,
600V rated MOSFET
©2004 Fairchild Semiconductor Corporation
3
Rev. A, March 2004
For practical purposes, an actual breakdown voltage in applications is chosen as 1.3 times the
rated low current breakdown voltage
[1]
. Figure 5 is an example of this non-breakdown but
over the absolute maximum rating. The peak drain-source voltage is 668V but there is no
breakdown yet.
Even though the abnormal voltage spike did not cause a device breakdown, the junction tem-
perature of the Power MOSFET should be kept below the specified maximum junction temper-
ature to ensure reliability. The steady state junction temperature can be expressed as
T
J
=
P
D
R
ΘJC
+
T
C
(
1
)
where
T
J
: junction temperature
T
C
: case temperature
P
D
: power dissipated in the junction
R
ΘJC
: steady state thermal resistance from junction to case
In many applications, however, the power dissipated in the Power MOSFET is pulsed rather
than DC. When a power pulse is applied to the device, the peak junction temperature varies
depending on peak power and pulse width. Thermal resistance at a given time is called tran-
sient thermal resistance and is expressed as
Z
ΘJC
(
t
)
=
r
(
t
) ×
R
ΘJC
(
2
)
where r(t) is a time dependent factor regarding thermal capacity. For very short pulses, r(t) is
quite small, but for long pulses it is nearly 1 and transient thermal resistance approaches the
steady state thermal resistance. Most Fairchild Power MOSFET datasheets have a graph
similar to that of Figure 6.
Z
θ
JC
(t), Thermal Response
D=0.5
10
-1
0.2
0.1
0.05
0.02
※
Notes :
1. Z
θ
JC
(t) = 0.42
℃
/W Max.
2. Duty Factor, D=t
1
/t
2
3. T
JM
- T
C
= P
DM
* Z
θ
JC
(t)
10
-2
0.01
single pulse
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
t
1
, Square Wave Pulse Duration [sec]
Figure 6. Transient Thermal Response, FQA11N90C
From this curve, the junction temperature can be obtained as follows:
T
J
=
P
D
Z
ΘJC
(
t
)
+
T
C
(
3
)
©2004 Fairchild Semiconductor Corporation
4
Rev. A, March 2004
For example, the calculation of the temperature rise resulting from single 2kW power pulse
applied to FQA11N90C during 1µs can be expressed as follows:
T
=
P
D
Z
ΘJC
(
1µs
)
=
2000
×
1.49
×
10
–
3
≈
3°C
The applied power is substantial but the temperature rise is only 3 degrees. Note that a power
dissipation rating specified in the datasheet is a steady state power rating, and in a relatively
short time the Power MOSFET can handle even greater power pulse.
In the above example, however, transient thermal resistance of 1µs is not available in Figure
6. In cases where the given time is too short and out of the graph range, the single pulse tran-
sient thermal resistance is known to be proportionate to the square root of time. So Z
ΘJC
(1µs)
becomes
1µs
-
Z
ΘJC
(
1µs
)
=
Z
ΘJC
(
10µs
) ×
-------------
10µs
=
4.72
×
10
–
3
×
0.1
=
1.49
×
10
–
3
where
Z
ΘJC
(10µs): taken from Figure 6
The above thermal response is based on a rectangular power pulse. It is possible to obtain a
response for arbitrary shapes. However, since the mathematical solution would be very com-
plex, it would be easiest to convert it to an equivalent rectangular pulse. Some examples for
triangular and sine wave power pulses are shown in Figure 7.
P
0.7P
0.71t
t
0.91t
t
P
0.7P
Figure 7. Conversion of Power Pulses
The equation (3) can also be applied to applications that have repetitive pulses. The transient
thermal resistance for repetitive pulses can be approximated as follows
[3]
t
1
t
1
Z
ΘJC
(
t
)
= ---- +
1
– ----
r
(
t
1
+
t
2
)
+
r
(
t
1
)
–
r
(
t
2
)
R
ΘJC
-
-
t
2
t
2
t
1
t
1
-
= ----
R
ΘJC
+
1
– ----
Z
ΘJC
(
t
1
+
t
2
)
+
Z
ΘJC
(
t
1
)
–
Z
ΘJC
(
t
2
)
-
t
2
t
2
(
4
)
where
t
1
: pulse width of the power pulse
t
2
: period of the power pulse
©2004 Fairchild Semiconductor Corporation
5
Rev. A, March 2004
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
