Igbt reliability

IGBT reliability in converter
design
Zhou Yizheng

Temperature
• Tj >150°C
• Tcase< -40°C
How to destroy an IGBT module ?
Current
•IC short circuit
• surge current
•RBSOA / SOA
Mechanical stress
• shock & vibration
• forces on terminals
• heat sink bending
Voltage
• VCE Over-voltage
• VGE Over-voltage
Faulty control
• shoot through(dead time)
• short pulse
Wrong handling
• ESD
• wrong mounting proc.
Thermal stress
• thermal cycling
• power cycling
by
Other components
• driver
• bus bar

Converter reliability
 Component qualification
 Correct assembling
 Proper design
Hardware
control
Lifetime & reliability estimation
 Sufficient protection
Over voltage
Over current
Over temperature
Set date Copyright © Infineon Technologies 2010. All rights reserved. Page 3

Assembling
 Mounting torque
 DCB crack
 terminal broken
 Mounting sequence
 thermal grease distribution

Assembling
 Applying thermal grease
 thermal grease thickness  high Rthch
 thermal grease distribution  DCB crack
TIM(thermal interface material)
Screen printer

Assembling
 ESD
IGBT is ESD sensitive component

Proper design(hardware)
 Vce overvoltage  must not exceed blocking voltage
Low commutation loop stray inductance
Proper Rgoff
Suitable protection in abnormal condition
 Vge overvoltage  can not exceed 20V
 influence SC capability
Proper driving voltage level
Short gate cable length
Efficient clamping

 RBSOA
Maximum turn off two times of nominal current  over
current protection point
 SOA (Diode)
Peak power limitation
¬ IGBT turn on speed
¬ Stray inductance
3000
2000
1000
0
1000
2000
2
2000
1000
1
time [400ns/div]
VR [500V/div] IR [500A/div]
1
3
!
0
0 1000 2000 3000
0
VR(t) [V]
IR(t) [A]
locus iR(t)*vR(t)
2
3
!
0

 Maximum junction temperature
Maximum operation temperature within limitation (including
overload condition and temperature ripple)
 accurate losses calculation
- Switching losses
 accurate thermal impedance value
- Rthch
- Rthha
 certain design margin
- Considering aging issue
- Considering lifetime

 Temperature sensing
Detect junction temperature
almost impossible for real products, but in lab…
1.Gate Resistor of IGBT chip as a sensor (RGINT)
2.Infrared Camera (IR-Camera)
3.Thermocouple
4.Infrared sensor

Protection(temperature)
C
E
F1 S1 S2 F2
G
RGINT RG = f(T)
IGBT-VRG
VGINT Chip
I0
Gate bond ±V0
Ri
 RGINT method
¬ can detect chip junction temperature ripple
¬ synchronization and sophisticated data acquisition are needed
¬ measurements at high voltage are possible

 IR- camera
¬ Temperature ripple detection is possible
¬ requires an open module
¬ Limited by high voltage

 Thermocoupler
¬ Special module need to be prepared
¬ Not suitable for junction temperature ripple
Customer made sample
 Infrared sensor
Assembly IR-Sensor
fixture
¬ Not suitable for junction temperature ripple
¬ Limited by high voltage

 Mechanical stress (vibration)
¬ Fixing block (force direction)
¬ Soft copper bus bar
¬ Fastness of capacitor

Proper design(control)
 Dead time (avoid short through)
driver delay may shrink dead time
Worst case is at small current condition
Software dead time VS. hardware dead time
tDT=[((tdoff(max)+tf(max))-tdon(min))+(tPHLmax-tPLHmin))]×1.5
 Minimum pulse width
Short pulse will speed up switching
¬ IGBT switching voltage spick
¬ Didoe reverse recovery
Care about hardware dead time

Proper design(Lifetime & reliability estimation)
 Power cycling
Bonding wire reliability
¬ Junction temperature ripple
¬ Junction temperature
¬ Cycling time
 Thermal cycling
Soldering reliability
¬ case temperature ripple
¬ case temperature
¬ Cycling time

 Comparing to old generation chip, IGBT4 have around 4 times
improvement with same max. junction temperature.
By improvement of bonding technology and chip metallization

By improvement of material, soldering process, DCB shape…

How to estimate lifetime of IGBT module
 What’s needed: Basic system parameters
¬ Output current
¬ Output frequency
¬ Power factor
¬ Modulation index
¬ Switching frequency

 Calculate the losses and further more get temperature ripple.
losses Temperature ripple
Thermal model of system

 Compare PC/TC curve with estimated number off temperature
ripple

 Cosmic radiation
DC link voltage
Altitude
 FIT

 High altitude effect
FIT rate(due to cosmic radiation)
Cooling
Clearance
1,E+07
1,E+06
1,E+05
1,E+04
FF450R17ME4
Cosmic Radiation Induced Failure Rate
per Device
RT, 4000m
FIT
1,E+03
1,E+02
1,E+01
1,E+00
Voltage [V] RT, sea level
125°C, 4000m
1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500

Protection (voltage)
 DC link voltage overvoltage
 IGBT blocking voltage(active clamping voltage) limitation
 IGBT turn off snappy
 Vce overvoltage
 more severer at overload and short circuit condition
 soft turn off, two level turn off
 active clamping
 Vge overvoltage
 zener diode, TVS
 clamp to 15V

Protection (current)
 Over current
¬ Two times of nominal current
¬ Transient junction temperature within limitation
 Short circuit
¬ Short circuit time within 10us
¬ Short circuit gate voltage limitation (SC energy, current)
¬ Short circuit turn off after IGBT goes into desaturation

 Over temperature
Hundreds of ms
Several s
Tens of s

Sensing case temperature
 time delay is around several seconds
 Require prior estimation delt Tjc max.
Sensing heatsink temperature
 time delay is around tens of seconds
 Require prior estimation delt Tjh max.
 suffer from Rthch changing due to thermal grease aging

 Over temperature
How to realize fast and accurate temperature protection
¬ Real time transient losses calculation, and
¬ Temperature detection point(as close as to chip), and
¬ Thermal impedance model
 Real time calculation of the junction temperature

Igbt reliability

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