Automotive MOSFETs Current-Handling in Power-Applications

Aut o moti ve MOS FE T s C urr en t -H andli ng in
Po wer -A p plicatio ns
AN 2015-05
by Dr. Nicolae-Cristian Sintamarean
by Nicolae-Cristian Sintamarean and Marco Püerschel
Applic atio n N ote
V1.0 2015-05
Aut o moti ve Hi gh Po wer
Edition 2015-05
Published by Infineon Technologies AG,
81726 Munich, Germany.
© 2015 Infineon Technologies AG
All Rights Reserved.
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NOTE.
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Automotive MOSFETs Current-Handling in Power-Applications
AN 2015-05
Document Change History
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Changed By
Change Description
2015-05
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Application Note
3
V1.0, 2015-05
Application Note
AN 2015-05
Table of Contents
1
Introduction – Background and Motivation .................................................................................... 5
2
2.1
2.2
2.3
2.4
2.5
Device Current Capability ................................................................................................................ 6
The silicon (Si) chip current limitation ................................................................................................. 6
Device Copper-Clip current limitations ................................................................................................ 7
Die-Pad, Lead-Post and Solderable area current limitations .............................................................. 7
Current density and Electro-Migration ................................................................................................ 9
Device current capability definition according to IFX-standards ......................................................... 9
3
Device current limitations in automotive-power applications .................................................... 11
4
Device current capability – Laboratory Measurements .............................................................. 15
5
Conclusions ..................................................................................................................................... 18
Application Note
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Automotive MOSFETs Current-Handling in Power-Applications
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Introduction – Background and Motivation
1
Introduction – Background and Motivation
In order to respond to the increased current requirements in high power applications, a lower package
resistance is of high interest. The total MOSFET resistance is a consequence of the Si-chip
resistance (depending on the technology and total die size) and package resistance (influenced by
the bond-wire or Cu-clip, pins and tab-thickness). Therefore, the transition from bond-wire (Figure 1 a)
to Cu-clip (Figure 1 b) has been proposed in some of the packages. By using Cu-clip a significant
reduction in package resistance (with a factor of 3 times) can be reached compared to the bond-wire
based packages.
As shown to the AN2008-01 “Power Bond Technology for High-Current Automotive Power
MOSFETs”, in the past the device current capability limitation was due to the bond-wire current
ratings.
Due to the fact that modern packaging technologies moved from bond-wires to Cu-clips (in order to
decrease the package-resistance), the device current capability limitation has switched from bondwire limitations to the device interface areas and electromigration. This is especially important for the
new power MOSFET technologies.
Presently, there are many device-producers companies and each of them has their own way on
defining the device current ratings. The definition of the device current-capability is a very sensitive
topic which is mainly depending on the company-standards.
Unfortunately, many companies are intentionally overrating their devices by not considering the whole
current limitation chain when defining the device current capability. Therefore a difference of up to 3.5
times higher can exist in the current ratings when considering definition-standard of some competitors
compared to Infineon approach.
In order to support customers during selection of the proper device (according to their requirements),
it is very important to have a common and realistic approach on defining the device current ratings.
This application note is proposed to understand and to be aware of the details regarding the current
limitation chain from device-level and application-level standpoint.
(a) Bond-Wire based package
(b) Cu-Clip based package
Figure 1: The device internal structure for a package based on (a) bond-wires and (b) Cu-clip
Application Note
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Device Current Capability
Device Current Capability
2
Drain
Si-Chip temp: 175°C
TAB
Si-Chip
TAB temp: 85°C
Die-Pad
Cu-Clip
Cu-Clip temp: 220°C
Lead-Post
Source-Pins Temp: 85°C
Source
Gate
Soldering-Area
Fused-Leads
Figure 2: The internal structure of the power MOSFET device and the main parts responsible for device
current capability
The device-level current capability depends on the following main factors:
 Si-Chip current limitation
 Cu-Clip current limitation
 Die-Pad, Lead-Post and Soldering area current limitation
 Current density and Electro-Migration
The internal structure of the automotive MOSFET (based on leadless-package) is presented in Figure
2 where it can be clearly seen the main components which define the device current limitations.
It is worth mentioning that the fused-leads concept introduced by Infineon improves the device current
ratings.
A short description of the main current limiting factors is presented below.
2.1
The silicon (Si) chip current limitation
The Si-chip current rating (ID_Chip) is defined by the MOSFET dissipated power via the thermal path (1)
which leads to a junction temperature rise from the case temperature of TC=25°C (kept constant) to
the maximum allowed junction temperature of TJ_max=175°C. Therefore, considering the device
maximum allowed power losses dissipation and the on-state resistance, the Si-chip maximum allowed
current is defined in (2).
_ − 
ℎ
(1)
_ − 
_
=√
_175℃
ℎ ∗ _175℃
(2)
_ =
_ℎ = √
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Device Current Capability
Where:
PLoss_tot – device power loss dissipation,
TC – device case temperature,
TJ_max – device junction temperature,
RthJC – junction-to-case thermal resistance,
RDSon_175C – device on-state resistance at an operating temperature of 175°C,
ID_Chip – maximum allowed device Si-chip current.
Comparing the Competitor X and IFX devices, the Si-chip current limitation for a junction temperature
of 175°C is presented in Table 1. In case of the Competitor X it can be clearly seen that the
Continuous Drain Current (ID) stated in the datasheet is equal with the obtained Si-chip current
limitation (ID_Chip175°C). This means that the Competitor X is not considering the main limitations in
terms of Cu-clip and electromigration. In the following sections the impact of these limitations in the
device current capability is explained.
Table 1: The IFX versus Competitor X Si-Chip current limitation
Product name
Parameters
IFX S308
Competitor X
Junction-to-case thermal
resistance RthJC
Device on-resistance
RDSon_175°C
Device power loss
dissipation PLoss_tot
Maximum allowed Si-chip
current ID_Chip175°C
Continuous Drain
Current ID
2.2
2.1 K/W
2.2 K/W
5.1 mΩ
6.48 mΩ
72 W
68 W
120 A
102 A
40 A
102 A
Device Copper-Clip current limitations
The Cu-clip current limitation has been determined by a model which considers the lead-frame
geometry, the Cu-clip geometry (the cross-section area and the distance from the die-pad to the leadpost) and the maximum allowed temperature of the Cu-clip (which is limited by the influence on the
molding compound material decomposition if the Cu-clip temperature exceeds 220°C). The
temperature target (Figure 2) for the Si-chip, Cu-clip, Tab and source pins are also considered in the
model for the calculation. All this defines the current capability of the Cu-clip of the power MOSFET.
As a final result, in order to decrease the package resistance, the Cu-clip current capability is higher
than the Si-chip. Therefore the Cu-clip das not represent a limitation in device current capability.
2.3
Die-Pad, Lead-Post and Solderable area current limitations
According to IFX standards, the current density limitations (in order to avoid electro-migration issues
at high current and high temperature operation) are defined up to be 50 [A/mm2]. Therefore this value
will be considered for further calculations according to the connection areas. The current limitation of
the device is therefore defined as follows:
 Die-Pad – connection area of the Cu-clip to the Si-chip.
 Lead-Post – connection area of the Cu-clip to the Source-Pins.
 Soldarable area – represents the common connection area of the source pins to the PCB pad.
Application Note
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Device Current Capability
The total die-pad area of the device will introduce a first current limitation lower than the Si-chip limit.
Furthermore, due to the fused-leads concept (Figure 2) introduced by Infineon, the IFX-devices offers
a contact area of the Lead-Post (Cu-Clip to the Source-pins) and a Source-Soldarable area (of the
Source-Pins to the PCB-pad Figure 3) of >2 times higher than the Competitor X device, which will
have a positive impact increasing the IFX-device current capability.
By considering the whole chain of limitations, the IFX-device has a current capability of 2 times higher
than the Competitor X device.
Moreover the Competitor X is stating a 2.5 times higher current ratings (Table 2) than the IFX-device
but with one half (50 % lower) Source-Connection area. By considering the stated device current
capability and the Source-Connection area, the Competitor X device (260 [A/mm2]) will end-up with a
current density of 6 times higher than the IFX device (45 [A/mm2]). All this determines a higher
current density, increasing the risk of electro-migration at high current and high temperature operation
for the Competitor X device.
Table 2: Main parameters comparison of the S308 IFX versus Competitor X device
Product name
Continuous Drain
Current ID
VG= 10V
ON-resistance
VG= 4.5V
Thermal resistance
Solderable area
Current Density
IFX S308
Competitor X
Improvement
40 A
102 A
2.5 X
2.8 mΩ
3.8 mΩ
2.1 K/W
2
0.9 mm
2
45 A/mm
3.6 mΩ
5.1 mΩ
2.2 K/W
2
0.4 mm
2
260 A/mm
Infineon S308
30%
2.3 X
6X
Competitor X
1.64mm
Fused-Leads
S
S
S
0.65mm
0.31mm
0.3mm
G
G
S
S
S
0.43mm
0.3mm
0.34mm
Contact/Solder area: 0.43*0.3*3=0.4 mm2
=>260 A/mm2
Contact/Solder area: 0.65*0.34*3
+0.31*0.35*2=0.9 mm2 => 45 A/mm2
(a) Soldering area of IFX device
(b) Soldering area of Competitor X device
Figure 3: Total Source-Soldering area of the IFX versus Competitor X device
Application Note
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Device Current Capability
2.4
Current density and Electro-Migration
Electro-Migration
(b) Zoom with the area affected by ElectroMigration
Figure 4: The Electro-Migration impact in MOSFET source-pins connection
(a) Cross-section area of the device
In order to emphasize the actual effect of electro-migration when having high temperature and high
current operation, a test at high-current (2 times higher current density than the maximum allowed by
the IFX standards) and high-temperature operation has been performed with an IFX device. The main
objective is to emphasize the impact of electro-migration in the source pins to PCB connection
interface due to the high-current operation.
After performing the test, the device was analyzed. A cross-section area of the device is shown in
Figure 4 (a) and a zoom to the source-to-PCB connection interface is presented in Figure 4(b).
According to the obtained results it can clearly be seen that due to 2 times higher-current density and
high temperature operation, the copper-electrons are migrating through the soldering-material (Figure
4). This will have a negative impact on solder joint reliability and could result in soldering-cracking and
voids. With continued exposure to the described conditions, contact loss between the device-pins and
PCB can result leading to failure of the device to PCB interface.
Therefore, special attention has to be paid to the electro-migration impact when defining the device
current capability of the device.
2.5
Device current capability definition according to IFX-standards
According to the obtained results, the IFX way to define the device current capability is to consider the
whole limitation-chain encountered by the current flow through the device as shown in Figure 5 (a):
Si-Chip -> Die-Pad -> Cu-Clip -> Lead-Post -> and Soldering area. The Competitor X way of defining
the device current capability is to consider only the Si-Chip current limitation (Figure 5 (b)), without
considering the main limitation in the chain. In Table 3 the device current capability is presented
according to IFX and Competitor X definition standards.
According to IFX definition standard (considering the whole current limitation chain) the Competitor X
device would have a continuous drain current of 20 A.
Finally, by considering the obtained results it may be concluded that the Competitor X Continuous
Drain Current (ID) stated in the datasheet is equal to the Si-chip current limitation (ID_Chip175°C). Some
competitors are over-rating their device current ratings, requiring special attention in analyzing the
Application Note
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Device Current Capability
MOSFET-device before selecting it in order to avoid electro-migration problems due to high-current
and high-temperature operation.
Table 3: The device current capability according to IFX and Competitor X definition standards
Device Current Capability
S308
Competitor X
Product
Competitor X
definition standard
IFX definition
standard
120 A
102 A
-
40 A
20 A
2X
Competitor X way
The IFX way
Si-Chip I Limit 102 A
120 A Si-Chip I Limit
50 A/mm2
Improvement
Die-Pad I Limit
Die-Pad I Limit
Cu-Clip I Limit
Cu-Clip I Limit
260 A/mm2
Lead-Post I Limit
Lead-Post I Limit
Soldering-Area (0.9 mm2)
Soldering-Area (0.4 mm2)
102 A
40 A
(a) IFX current capability definition
(b) Competitor X current capability definition
Figure 5: Device current capability limitation-chain definition according to IFX (a) and Competitor X (b)
standards
Application Note
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Device current limitations in automotive-power applications
Device current limitations in automotive-power applications
3
MOSFET
Si-Chip
Tj
Source
Tc
TAB
PLoss
Rth_jc
Drain
Rth_cPCB
IMOS
PCB
TPCB
Heatsink
Rth_PCB
Rth_PCBh
Rth_ja
Rth_h
Ta
Figure 6: Power device thermal chain from junction to ambient in automotive application
In normal operation the device current level is limited by the junction temperature which is increasing
with the power losses and thermal resistance as shown in (3). If the junction-to-ambient thermal
resistance has a lower value, higher power losses are allowed to reach the same junction temperature
(the device current may be increased).
 =
 − 
ℎ
(3)
The device current capability in automotive power-applications is depending on the following main
factors:
 Device power losses – the device total power losses are composed of the conduction losses
and switching losses. For Electric Power Steering (EPS) application the typical switching losses
have a contribution of 30 % to the total power losses. Therefore, the main focus in this specific
case is on the device conduction losses. The conduction losses are directly related to the
device on-state resistance RDS_on which is mainly depending on the Si-chip resistance and on
the device-package resistance.
 Junction-to-ambient thermal resistance – as shown in Figure 6, the junction to ambient thermal
resistance is depending on the junction-to-case RthJC and on the case-to-ambient thermal
resistance RthCA. The junction-to-case thermal resistance RthJC (defined by the Si-chip size) has
a low contribution in the total thermal resistance chain (Figure 6) from the junction-to-ambient.
The RthCA has a large variation and mainly depends on the device packaging tab size and the
cooling concept.
In order to understand the impact of the above mentioned parameters, an example on how to calculate
the device current capability in Electric Power Steering (EPS) applications (Figure 7) is further
presented.
Application Note
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Device current limitations in automotive-power applications
Safety Switch
UDD
Inverter
i0
M1
M5
M3
a
Star-point
relay
b
Battery
ib
c
M2
Motor
ia
M4
ic
n
M6
N
Figure 7: Schematic of the Electric Power Steering application
The schematic of the EPS-application is shown in Figure 7 and the main system specifications are
given in Table 4.
Table 4: Electric Power Steering (EPS) system specifications
System parameters
Symbol
Value
Inverter Power
P [kW]
0 to 1.8
DC-link voltage
VDD [V]
14
RMS output current
I [A]
0 to 70
Switching frequency
fSW [KHz]
20
As shown in Table 5, according to the device-packaging it is possible to reach a device ON-state
resistance RDS_on which may vary from 2.8 mΩ in S308 package, to 0.7 mΩ in TOLL package for 40V
IFX portfolio.
Table 5: The IFX best-in-class RDS_on according to device packaging for automotive products
Device Packaging Type
RDSon_min [mΩ]
1
S308 (TSDSON 8)
2.8
Leads-Less
2
SS08 (TDSON 8)
2
3
TOLL (H-PSOF)
0.7
4
DPAK (TO252-3/5)
2
Leads
5
D2PAK (TO263-3/5/7)
0.87
The junction-to-ambient thermal resistance and the package tab-size play a key role on dissipating
the power losses from the device. This value also varies according to the device packaging (Table 6)
from 20 [K/W] in S308 to 3 [K/W] in TOLL package. These are typical values which may be found in
applications which are using devices with these packages.
Table 6: The device junction-to-ambient thermal resistance in automotive-applications according to
device packaging
Thermal resistance
Tab Size of the
Tab Thickness of the
Device Packaging Type
2
2
RthJA [K/W]
Package [mm ]
Package [mm ]
1
S308 (TSDSON 8)
20
4
0.2
Leads-Less
2
SS08 (TDSON 8)
10
18
0.25
3
TOLL (H-PSOF)
3
50
0.5
4
DPAK (TO252-3/5)
5
29
0.9
Leads
5 D2PAK (TO263-3/5/7)
3.5
64
1.3
Application Note
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Device current limitations in automotive-power applications
All the above mentioned parameters were considered in order to calculate the required R DS_on
according to the application power and device packaging.
A short explanation on how to calculate the required RDS_on according to device operating conditions
and device packaging is given in the following.
In order to calculate the device current capability for the EPS application the eq. 4 will be used:
 =
 − 
 − 
⇒  ∙  2 ∙  ∙  =
ℎ
ℎ
 − 
25° =
ℎ_ ∙  2 ∙  ∙ 1.3
Where:
 ( ) = 25° ∙ (1 +
(4)
∝ −25°
∝ −25°
)
⇒  = (1 +
)
100
100
(5)
α=0.4 is the technology related constant for power MOSFETs,
Ct- is the temperature coefficient when converting the RDSon from 25°C to the Tj value.
fPSW – the impact of switching losses in the total power losses
The ambient temperature of 125°C and the junction temperature of 150°C are considered as a worst
case scenario for the calculations.
The power (current) is increased until the required MOSFET RDS_on reaches the minimum allowed
value according to the device-packaging technology. In this way the maximum allowed power
(current) may be determined according to the device packaging and the device RDS_ON.
Figure 8 shows the RDS_on requirements according to device-package and application power ratings by
considering the best in class 40V devices from IFX portfolio. The same idea is presented in Figure 9
according to the device current capability.
Dedicated Power ratings according to device Packaging
S308
SS08
TOLL
DPAK
D2PAK
Device RDSon_25°C [mΩ]
100.00
10.00
1.00
0.10
0
200
400
600
800
1000 1200
EPS Motor Power [W]
1400
1600
1800
Figure 8: The device RDS_on requirements according to package and application power ratings by
considering the best in class 40V IFX portfolio
Application Note
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Device current limitations in automotive-power applications
Dedicated Power ratings according to device Packaging
S308
SS08
TOLL
DPAK
D2PAK
Device RDSon_25°C [mΩ]
100.00
10.00
1.00
0.10
0.0
10.0
20.0
30.0
40.0
50.0
Device RMS Current [A]
60.0
70.0
Figure 9: The device RDS_on requirements according to package and application current ratings by
considering the best in class 40V IFX portfolio
According to the device packaging and the best in class RDS_on, the maximum current capability of the
IFX devices is presented in Table 7 for EPS application.
This approach may be used in selecting the devices according to the application power ratings and
device-packaging.
It is worth to mention that IFX portfolio is based on two type of packages, the leaded (e.g. D2PAK)
and lead-less devices (e.g. TOLL). The new generation of lead-less devices offers a larger portfolio of
optimized-for-power packages. Therefore the customer has many options on selecting the proper
device according to their specifications.
Table 7: Device current ratings according to device packaging for the IFX products in EPS application
Device Packaging Type
1
S308 (TSDSON 8)
Lead-Less 2
SS08 (TDSON 8)
3
TOLL (H-PSOF)
4
DPAK (TO252-3/5)
Lead-Based
5
D2PAK (TO263-3/5/7)
Application Note
14
Device current capability
15 A
28 A
70 A
30 A
60 A
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Device current capability – Laboratory Measurements
4
Device current capability – Laboratory Measurements
UDD
RLoad
iD
PWM
GD
RG
1
M1
2
VG
3
(a) Test-board schematic
(b) Hardware implementation
Figure 10: Test board schematic (a) and hardware implementation (b): 1 – Load resistors, 2 – Test-board
with the MOSFET device, 3 – Gate-Driver.
A test setup has been developed in order to compare the current capability of the IFX versus
Competitor X device. The devices used in this study case are different than the ones presented in the
Section 2 of the application note. The test-board schematic (a) and the hardware implementation (b)
are presented in Figure 10.
The target of the experiment is to measure the length of time it takes for the device to heat-up to a
certain case temperature. Therefore, as shown to Figure 10, the test setup consists of the test-board
and the Infra-Red (IR) thermal camera. The IR-Camera is used to measure and record the device
temperature at different operating conditions.
The laboratory test conditions for the devices are presented in Table 8. There are two main test
conditions which are performed, first one at 20 kHz operation (typical EPS-applications) and the
second one at 100 kHz operation (typical DC/DC converter application).
1
2
Figure 11: Test-setup laboratory implementation: 1 – Thermal camera, 2 – Test-board
Application Note
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Device current capability – Laboratory Measurements
Table 8: Test conditions for IFX versus Competitor X devices
Test No.
Test 1
Test 2
Devices
1.1
1.2
2.1
2.2
IFX
Competitor X
IFX
Competitor X
Test Conditions
Load
Duty Cycle
Current
DCVoltage
VDD=12V
IL=67A
Switching
frequency
App.
20 kHz
EPS
100 kHz
DC/DC
50%
Test 1: 12V/67A/20kHz
100
90
Temperature [°C]
80
70
60
50
Tc-IFX
40
Tc-Competitor X
30
20
10
0
25
50
75
100
125
150
175
200
Time [s]
Figure 12: Thermal-camera measured temperature for IFX versus Competitor X device by considering
the Test 1 operating conditions
The only difference in the test conditions is the switching frequency which is changed from 20 kHz
(Test 1) to 100 kHz (Test 2). The used gate-driver parameters are: gate resistance RG=3 Ω and gate
voltage VG=10V.
According to the obtained results from Test 1 (Figure 13) it can be stated that even though Competitor
X device has a 30 % lower RDS_on (due to a 40% larger chip-area), the switching losses are much
higher than the IFX device. Therefore in the Test 1 operating conditions for 20 kHz, the higher
switching losses are compensating the difference in terms of RDS_on between the devices. The
temperature trend of both devices is similar with a slightly advantage for IFX device (Figure 12).
Based on the obtained results it can be stated that both devices have a similar current rating
capability for the EPS-applications (Test 1).
If the switching frequency is further increased to 100 kHz according to Test 2, the IFX device
temperature rising is much slower than the Competitor X device. In Figure 13 it can be seen a 60%
difference between temperature rising time of IFX and Competitor X device. This it means that the IFX
device has lower power-loss dissipation, thus it can operate at a higher current capability than the
Competitor X device in DC/DC-converter applications (Test 2 - 100 kHz switching frequency). It is
worth to mention that the Competitor X device has a double current density therefore this device is
prone to electromigration issues at high-current and high-temperature operation.
Application Note
16
V1.0, 2015-05
Automotive MOSFETs Current-Handling in Power-Applications
AN 2015-05
Device current capability – Laboratory Measurements
Test 2: 12V/67A/100kHz
130
Temperature [°C]
110
90
70
Tc-IFX
60%
50
Tc-Competitor X
40 s
64 s
30
10
0
15
30
45
60
75
Time [s]
Figure 13: Thermal-camera measured temperature for IFX versus Competitor X device by considering
the Test 2 operating conditions
An example of the thermal IR-camera measured device case temperature is presented in Figure 14.
Figure 14: Thermal IR-camera results with the device-case temperature (Tc) measurements
Application Note
17
V1.0, 2015-05
Automotive MOSFETs Current-Handling in Power-Applications
AN 2015-05
Conclusions
5
Conclusions
This application note is proposed in order to explain the current limitation chain for power MOSFETs
from device-level and application-level standpoint.
Based on the obtained results it may be stated that some competitors are overrating their devices by
not considering the entire current limitation chain (encountered by the current flow through the device)
when defining the device current capability. Therefore a difference is evident of up to 3.5 times higher
current ratings when considering the definition-standard of some competitors compared to Infineon
approach.
In Table 3 the device current capability is presented according to IFX and Competitor X definition
standards. According to IFX definition standard (considering the whole current limitation chain) the
Competitor X device would have a continuous drain current (20 A) equal to half of the IFX device.
Furthermore, a method for calculating the device current capability in automotive applications for IFX
power MOSFETs by considering the device packaging is also introduced. According to the device
packaging and the best in class RDS_on, the maximum current capability of the IFX devices is
presented in Table 7 for EPS application.
In order to determine the Competitor X device current capability, a comparison with the IFX best in
class device has been done. By knowing the IFX-device thermal limitations, a test setup has been
developed in order to compare the IFX device with the Competitor X device.
Two main test conditions were performed according to Table 8. Based on the obtained results, it can
be stated that the Competitor X device has similar power ratings to IFX device for 20 kHz based
applications (eg. EPS). For the applications which requires a higher switching frequency operation
(eg. DC/DC converter), the IFX device has a higher current capability than the Competitor X device.
It is worth mentioning that the Competitor X device has a double current density therefore this device
is prone to electro-migration issues at high-current and high-temperature operations.
Application Note
18
V1.0, 2015-05
w w w . i n f i n e o n . c o m
Published by Infineon Technologies AG
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