Device Application Note AN912

VISHAY SILICONIX
Power MOSFETs
Application Note 912
Power MOSFET in UIS/Avalance Applications
By Kandarp Pandya
Inductive loads require adequate attention in electronic
control circuits, since otherwise they can lead to unclamped
inductive switching (UIS) or avalanche conditions, which
may occur when the device is turning off or it is already
turned off. In fact, inductive loads may cause catastrophic
failure of a MOSFET that appears otherwise to be operating
within its specified current and power ratings. Failure
analysis (FA) has shown that such conditions can result in
electrical over stress (EOS), where the total energy
dissipation exceeds the thermal capabilities of the MOSFET
on the printed circuit board (PCB) assembly. The failure
signature further points to localized dissipation of energy
through a very small, randomly located die area. Insofar as
the effects of UIS or avalanche are heterogeneous with
respect to the die, failure can occur at current and power
levels below the maximums specified by the datasheet.
Avalanche in a MOSFET, like the natural phenomenon for
which it is named, is an uncontrolled behavior, and one that
cannot be characterized due to the non-homogeneity of die
utilization during the event.
Solenoid actuators for two-position on/off control, automotive
engine control units (ECU) with high frequency inductive
switching control for fuel injection, and anti-lock braking
systems (ABS) are among the applications that involve
inductive loads. In the case of solenoid actuators, designers
need to understand the significance of single pulse
avalanche current ratings for different inductor values and
corresponding permissible energy values, as shown on
product datasheets. In the case of ECU and ABS
applications, designers need to interpret the datasheet
information to derive a repetitive avalanche current rating.
Maximum operating temperature, TJ (max.)
Drain-source breakdown voltage, VDS
Rth(j-a) (max.) steady-state value
Normalized thermal transient impedance, junction-toambient characteristics
The datasheet specification for single pulse avalanche
current, IAS is normally derived from a test that involves the
MOSFET’s destruction, so the measured value must be
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Document Number: 64717
Revision: 22-Dec-08
The datasheet value for junction-to-ambient maximum
thermal resistance, Rth(j-a) (max.), and transient thermal
characteristics are derived from empirical measurements.
These are also limited by boundary conditions. The
MOSFET is soldered on a double-sided FR4 PCB with
dimensions of 1" by 1" by 0.062" (25.4 mm by 25.4 mm by
1.5 mm) with 2 oz. (0.076 mm) of 100 % copper on both sides
- ignoring the slit isolation to insulate the MOSFET drain,
source, and gate terminals. The thermal characteristics of
MOSFET in an actual system can be different, of course, so
there is no easy way to relate datasheet information with the
actual repetition rate, duty cycle, and board thermal
capabilities the MOSFET will encounter in an end product.
However, the good news is that most real-world PCB
assemblies have better thermal performance characteristics
than those used for datasheet characterization. This
provides an inherent design margin when using the
datasheet information as outlined below.
There are five parameters related to MOSFET stress:
• Avalanche current, single pulse, IAS or repetitive, IAR
• Time in avalanche, TAV
• Circuit inductance, L
• Avalanche energy, single pulse, EAS or repetitive, EAR
• Starting junction temperature, TJ(Start) - mostly Tamb
The following reference characteristics can be developed
using datasheet values and mathematical formulas which
relate the above parameters and extrapolations:
• IAV vs. TAV - single pulse avalanche current vs. time in
avalanche
• IAS vs. L - single pulse avalanche current vs. inductance
• EAS vs. TJ(Start) - single pulse avalanche energy vs. starting
junction temperature
• IAR vs. TAV at 25 °C ambient - 2 % to 50 % duty cycle repetitive pulse avalanche current vs. time in avalanche at
25 °C ambient
• IAR vs. TAV at 150 °C ambient - 2 % to 50 % duty cycle repetitive pulse avalanche current vs. time in avalanche at
150 °C ambient
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APPLICATION NOTE
The thermal capability of the MOSFET in the system is the
key factor that determines its survivability in the
UIS/Avalanche mode operation. Therefore the key
specifications we need to use from the datasheet are the
following:
derated before turning it into a specification. Typically a 50 %
guard band is applied to the test result value of the drain
current at the failure point. The corresponding single pulse
energy ratings EAS are also derived by applying the same
derating factor. Note that the IAS and EAS ratings are limited
to specific test conditions, i.e. to a certain inductor value, test
current pulse, and thermal capability for the PCB setup.
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
The above characteristics can help designers to minimize
the stress to the MOSFET under UIS/avalanche operation.
We should note that the above characteristics are only for
reference because they are applicable only to the part
mounted on the specific PC board used for datasheet
characterization. Designs following these guidelines will
safeguard MOSFET operation under UIS conditions for most
applications. In fact many real life automotive PC board
assemblies are far superior in thermal performance, but our
approach provides an extra margin of safety to minimize the
MOSFET failure.
In the following example we shall develop the above
characteristics
for
Vishay
Siliconix
MOSFET
SQM110N04-03[1].
Datasheet information:
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Maximum operating temperature, TJ (max.) = 175 °C
Thermal resistance junction-to-ambient, Rth(j-a) = 40 °C/W
Drain-source breakdown voltage, VDS = 40 VDC
Normalized thermal transient impedance, junction-toambient characteristics, figure 1
Transient Thermal Impedance (Junction-to-Ambient)
1
0.5
0.2
10-1
0.1
Zth(j-a) Normalized
0.05
10-2
Duty Cycle = 0.02
10-3
Single Pulse
Tamb = 25 °C
10
-4
10-4
10-3
10-2
10-1
1
10
1000
100
Pulse Time PT (s)
Fig. 1 - Normalized Thermal Transient Impedance, Junction-To-Ambient Characteristics
IAV VS. TAV - SINGLE PULSE AVALANCHE CURRENT VS. TIME IN AVALANCHE
The pulse time in figure 1 can be used as Time in Avalanche, TAV for the square wave pulse. Thus figure 1 represents Zth(j-a),
normalized thermal transient impedance, junction to ambient vs. time in avalanche TAV characteristics for a single pulse:
Zth(j-a) (°C/W) vs. TAV (s)
The corresponding thermal resistance Rth(j-a) values can be derived by using the following equation:
Rth(j-a) (°C/W) = Zth(j-a) normalized (number) x Rth(j-a) (°C/W)
Equation (1)
APPLICATION NOTE
Where Rth(j-a) max. = 40 °C/W
Now extract the value for single pulse avalanche current, IAV (A) corresponding to TAV (s)
Maximum permissible junction temperature, TJ (max.) = 175 °C
Ambient, Tamb = 25 °C
Hence, the junction temperature rise, ΔT can be derived by using the following equation:
ΔT (°C) = TJ (max.) (°C) - Tamb (°C)
ΔT (°C) = 175 °C - 25 °C = 150 °C
Equation (2)
While maintaining the temperature rise, ΔT = 150 °C
ΔT (°C) = Rth(j-a) (°C /W) x PD (W)
Equation (3)
Where PD = Pulse power dissipation in watts
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Document Number: 64717
Revision: 22-Dec-08
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
Rearranging equation (3)
PD (W) = ΔT (°C)/Rth(j-a) (°C/W)
Equation (4)
Also
PD (W) = ½ x VBR (V) x IAV (A)
Equation (5)
Where
VBR = Breakdown voltage in volts
VBR = 1.3 x VDS Maximum Drain-Source Voltage
VBR = 1.3 x 40 V (from datasheet)
Rearranging Equations (4) and (5)
IAV (A) = 2 x (ΔT (°C)/Rth(j-a) (°C/W))/VBR (V)
Extract IAV (A) corresponding to TAV (s) for ΔT = 150 °C
IAV (A) = 2 x (150 (°C)/Rth(j-a) (°C /W))/1.3 x 40 (V) for TJ (Start) 25 °C
Equation (6)
Similarly, extract IAV (A) corresponding to TAV (s) for ΔT = 25 °C
TJ (max.) = 175 °C
Tamb or TJ (Start) = 150 °C
While maintaining the temperature rise, ΔT = 25 °C
IAV (A) = 2 x (25 (°C)/Rth(j-a) ( °C /W))/1.3 x 40 (V) for TJ (Start) 150 °C
Equation (6a)
IAV vs. TAV - Single Pulse Avalanche Current vs. Time in Avalanche characteristics with Tamb = 25 °C and Tamb = 150 °C are
shown in figure 2.
Single Pulse Avalance Current (peak) vs. Time in Avalanche
1
10-1
IAS (peak) (A)
Tamb = 25 °C
10-2
Tamb = 150 °C
10-3
10-3
10-2
10-1
1
10
100
1000
TAV (s)
Fig. 2 - IAV vs. TAV - Single Pulse Avalanche Current vs. Time in Avalanche Characteristics
Document Number: 64717
Revision: Revision: 22-Dec-08
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APPLICATION NOTE
10-4
10-4
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
IAS VS. L - SINGLE PULSE AVALANCHE CURRENT VS. INDUCTANCE
The first step is to extract the permissible value of inductor, L (mH) corresponding to TAV.
Using the basic Ohm’s Law equation for the inductor:
V = L x dI/dt
Equation (7)
Where
V = Voltage across the inductor in volts
L = the inductance in Henry
dI/dt = rate of change of current A/s
Rearranging Equation (7) for the value of inductance in avalanche condition:
L (mH) = [(VBR (V) x TAV (s))/IAV (A)] x 1000
Equation (8)
Single Pulse Avalance Current (peak) vs. Load Inductance
1000
100
IAS (peak) (A)
Tamb = 25 °C
Tamb = 150 °C
10
1
10-1
10
1
100
Inductance (mH)
Fig. 3 - IAV vs. L (mH) - Single Pulse Avalanche Current vs. Inductance, L (mH) Characteristic
Figure 3 is the plot for maximum single pulse avalanche current vs. inductance
IAV (A) vs. L (mH) for ΔT = 150 °C or Tamb = 25 °C
IAV (A) vs. L (mH) for ΔT = 25 °C or Tamb = 150 °C
The values are extracted for an inductor value range 0.1 mH to 100 mH, which covers most of the applications.
APPLICATION NOTE
EAS VS. TJ (Start) - SINGLE PULSE AVALANCHE ENERGY VS. STARTING JUNCTION
TEMPERATURE
Single pulse avalanche energy, EAS vs. starting junction
temperature, TJ (Start) for the value of single pulse avalanche
current, IAV at 90 A, 30 A. and 15 A. These values are chosen
to cover the most practical range of avalanche current;
around 80 %, 30 %, and 15 % of drain current rating
respectively. These characteristics enable user to limit the
total avalanche pulse energy from both the staring junction
temperature and peak avalanche current.
Extract database for EAS (mJ) corresponding to TJ (Start) (°C) for IAV = 90 A
Rearrange equation (2) by substituting TJ (Start) for Tamb and TJ (max.) = 175 °C
ΔT (°C)= 175 (°C) - TJ (Start) (°C)
Equation (9)
While maintaining the temperature rise, ΔT = 150 °C, i.e., TJ (Start) = 25 °C
Extract Single Pulse Avalanche Energy, EAS (mJ) as follows:
EAS (mJ) = PD (W) x TAS (s) x 1000
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Equation (10)
Document Number: 64717
Revision: 22-Dec-08
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
Substitute PD(W) from Equation (3)
EAS (mJ) = [ΔT (°C)/Rth(j-a) (°C /W)] x TAV (s) x 1000
EAS (mJ) = [150 (°C)/Rth(j-a) (°C /W)] x TAV (s) x 1000
Equation (11)
Substitute the values of Rth(j-a) (°C/W) and TAV (s) corresponding to IAS = 90 A from single pulse database developed in
equation (1) above. The equation simplifies to:
EAS (mJ) = [175 (°C) - TJ (Start) (°C)/0.0642 (°C/W)] x 0.0005 (s) x 1000
Equation (12)
Similarly, substitute the values of Rth(j-a) (°C/W) and TAV (s) corresponding to IAS = 30 A from single pulse database developed
in (1) above. The equation simplifies to:
EAS (mJ) = [175 (°C) - TJ (Start) (°C)/0.198 (°C/W)] x 0.005 (s) x 1000
Equation (13)
Similarly, substitute the values of Rth(j-a) (°C/W) and TAV (s) corresponding to IAS = 15 A from single pulse database developed
in (1) above. The equation simplifies to:
EAS (mJ) = [175 (°C) - TJ (Start) (°C)/0.3826 (°C/W)] x 0.032 (s) x 1000
Equation (14)
Figure 4 shows plot of EAS (mJ) vs. TJ (Start) (°C) ranging from 25 °C to 175 °C.
Single Pulse EAS (peak) vs. TJ (Start)
100 000
IAS (peak) = 15 A
EAS (peak) (mJ)
10 000
IAS (peak) = 30 A
IAS (peak) = 90 A
1000
100
25
50
100
75
125
150
TJ (Start) (°C)
Fig. 4 - EAS (mJ) vs. TJ (Start) (°C) - Single Pulse Avalanche Energy vs. Starting Junction Temperature (°C) Characteristic
REPETITIVE, MAXIMUM AVALANCHE PULSE CURRENT, IAR VS. TIME IN AVALANCHE, TAV
FOR SINGLE PULSE AND 0.02 DUTY CYCLE TO 0.5 DUTY CYCLE
Using Equation (1) extract junction to ambient thermal transient impedance characteristics for duty cycles 0.02, 0.05, 0.1, 0.2,
and 0.5 (from datasheet)
Rth(j-a) (°C /W) vs. TAV (s)
0.02 duty cycle
Rth(j-a) (°C /W) vs. TAV (s)
0.05 duty cycle
Rth(j-a) (°C /W) vs. TAV (s)
0.1 duty cycle
Rth(j-a) (°C /W) vs. TAV (s)
0.2 duty cycle
Rth(j-a) (°C /W) vs. TAV (s)
0.5 duty cycle
Using Equation (6) for single pulse for different duty cycles 0.02 to 0.5 extract
IAR (A) corresponding to TAV (s) for ΔT = 150 °C
TJ (max.) = 175 °C
Document Number: 64717
Revision: Revision: 22-Dec-08
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APPLICATION NOTE
This characteristic helps to relate the repetitive peak current to the value of time in avalanche at different duty cycle ratios.
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
Tamb or TJ (Start) = 25 °C
While maintaining the temperature rise, ΔT = 150 °C
IAR (A) = 2 x (150 (°C)/Rth(j-a) (°C /W))/1.3 x 40 (V) for TJ (Start) 25 °C
Equation (15)
See figure 5.
Similarly, extract IAR (A) corresponding to TAV (s) for ΔT = 25 °C
TJ (max.) = 175 °C
Tamb or TJ (Start) = 150 °C
While maintaining the Temperature Rise, ΔT = 25 °C
IAR (A) = 2 x (25 (°C)/Rth(j-a) (°C /W))/1.3 x 40 (V) for TJ (Start) 150 °C
Equation (16)
See figure 6
IAR (peak) vs. TAV
10
Duty Cycle = 0.02
0.05
IAR (peak) (A)
1
0.1
0.2
0.5
10-1
Tamb = 25 °C
-2
10
10-4
10-3
10-2
10-1
1
10
1000
100
TAV (s)
Fig. 5 - Shows plots of IAR (A) vs. TAV (s) for Tamb = 25 °C or ΔT = 150 °C for duty cycles 0.02 to 0.5
IAR (peak) vs. TAV
10
Tamb = 150 °C
Duty Cycle = 0.02
IAR (peak) (A)
APPLICATION NOTE
1
0.05
0.1
0.2
10-1
0.5
10-2
10-4
10-3
10-2
10-1
1
10
1000
100
TAV (s)
Fig. 6 - Shows plots of IAR (A) vs. TAV (s) for Tamb = 150 °C or ΔT = 25 °C for duty cycles 0.02 to 0.5
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Document Number: 64717
Revision: 22-Dec-08
Application Note 912
Vishay Siliconix
Power MOSFET in UIS/Avalance Applications
SUMMARY
Extensive testing is required to determine if a MOSFET can
sustain a given repetitive UIS operating condition. The above
approach, which relies on datasheet information including
maximum and steady-state junction temperature,
junction-to-ambient thermal resistance, and normalized
thermal transient impedance junction to ambient
characteristics Zth(j-a) of the device on a standard PC board
assembly, allows us to estimate device performance in both
single pulse and repetitive avalanche mode operation.
The characteristics shown in figures 2, 3, and 4 enable a
designer to relate the ability of the system to handle peak
single pulse avalanche current, IAS, versus time, and shows
the corresponding useable maximum inductance values and
maximum single pulse energy.
The characteristics shown in figures 5 and 6 show the impact
of repetitive peak avalanche current, IAR versus time in
avalanche, TAV at TJ (Start) 25 °C, and 150 °C.
A robust design should include recirculation components to
quench the energy stored in the inductor and thus minimize
the stress upon and risk of failure for the MOSFET.
Designers should also take care that optimization and
cost-cutting measures do not result in overstressing the
MOSFET.
The power MOSFET’s capability to survive UIS is
determined by the thermal capabilities of the device itself and
the PCB assembly. In practical and sound designs, the
MOSFET needs to operate within its thermal limits as
determined by such parameters as circuit inductance,
avalanche energy, time in avalanche, duty cycle, current and
voltage rating, in addition to the transient thermal properties
of the MOSFET and the PCB assembly.
The author would like to thank Ibrahim Darwish for his
valuable technical inputs to this article.
The superior performance of real-life automotive printed
circuit board designs can indirectly provide additional safe
design headroom when these guidelines are followed.
REFERENCE
[1]
Vishay Siliconix Power MOSFET SQM110N04-03 Datasheet URL: www.vishay.com/doc?68605
[2]
Vishay application note AN601 “Unclamped Inductive Switching Rugged MOSFETs for Rugged Environment” Application
Note URL: www.vishay.com/doc?70572
APPLICATION NOTE
Document Number: 64717
Revision: Revision: 22-Dec-08
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