parasitic inductance hindering utilization of power devices · › modul: ff450r12me4 › l s =...
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Parasitic inductance hindering utilization of power devicesDr. Reinhold Bayerer Infineon Technologies AG, Warstein, Germany
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
22015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
32015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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V
I
Overvoltage is not the only consequence of parasitic inductance
Turn-off
Load
LS
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
52015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Effect of parasitic inductance during IGBT Turn-on – diode reverse recovery
V
I
Diode
IGBT
IGBT
Diode
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at high inductance› Peak of Reverse Recovery current is widened, charge is removed earlier › no tail for soft decline of current
Diode with low Tail-Charge, IF=1/10 IN
65nH 200nH
Example of reverse recovery: 1200A/3,3kV-Module
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Diode with high tail charge
› soft Diode recovery leads to higher switching losses
Diode with low tail charge
managing high parasitic inductance by higher tail charge, 1200A/3,3kV-Modul
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XHP: Lower parasitic inductance reduces peak power in diode, di/dt at turn-on can increase
› IGBT turn-on under nominal conditions– VCE = 1800 V; IC = Inom– Tj = 25°C
› Eon - 21%, at same diode stress
102015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
XHPIHM
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V
I
Diode
IGBT
IGBT
Diode
IGBT Turn-off - Diode Turn-on
112015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Turn-off under different parasitic inductance (3,3kV/1200A-Module)
L=95nH L=350nH
› limited change in Eoff, if voltage can develop › Tail gone› oscillation
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Example XHP: IGBT Turn-off
› VCE = 2400 V; high current IC = 2 x Inom› Tj = 25°C
› Early pull off of carriers, no tail current and oscillation› Low parasitic inductance less overvoltage and tail for damping of
oscillations
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Example: Loss minimized 750V IGBT cannot be used because of high parasitic inductance
› Exceeding voltage rating› Early pull off of carriers, no tail current and oscillation› Low parasitic inductance less overvoltage and tail for damping of
oscillations
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Compromise IGBT has excessive tail current for lower peak voltage and softness
› To compromise for high parasitic inductance tail charge has to be added
› It means higher switching losses
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
162015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Procedure for 450A/1200V IGBT4
› Modul: FF450R12ME4
› LS = 25nH
› Measure Eon, Erec, Eoff, at Inom, VDC=600V, RGnom=1Ohm, TJ = 150°C
› Adjust RG to stay below 1150V at TJ=25°C, VDC=800V, I=5*Inom
› Measure Eon, Erec, Eoff again at Inom, VDC=600V, TJ = 150°C and adjusted RG
› Compare losses
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Details for worst case condition
Mess.Nr. T[ー ] Rg_on Rg_off Vce Ic Vce Ic Eoff Vce_max1 25 15 15 887 1800 799.664 1800.624 429.235 11442 150 15 15 622.5 450 599.59 449.576 72.194 8243 150 1 1 622.5 450 599.504 450.512 59.498 7764 25 12 12 837.5 900 800.71 899.36 155.76 11525 150 12 12 623.5 450 600.221 450.36 68.002 832
#1
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Details: Eoff for RG,nom and increased RG
Mess.Nr. T[ー ] Rg_on Rg_off Vce Ic Vce Ic Eoff Vce_max1 25 15 15 887 1800 799.664 1800.624 429.235 11442 150 15 15 622.5 450 599.59 449.576 72.194 8243 150 1 1 622.5 450 599.504 450.512 59.498 7764 25 12 12 837.5 900 800.71 899.36 155.76 11525 150 12 12 623.5 450 600.221 450.36 68.002 832
#3#2
192015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Increasing RG to manage parasitic inductance for worst case, doubles switching losses
› RG_high means increased RG to stay within VDSS at VDC=800V, IC=5xInom, TJ=25°C
› Energies are measured at same conditions but different RGs
202015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
212015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Today’s IGBT increase di/dt when RGincreases until a maximum di/dt is reached
› Turn-off IGBT3: 6.5kV, 25A, RT› Maximum di/dt at RG = 100 Ohm
› To lower di/dt by increase of RG the >350 Ohm are required› dV/dt is slowed down, increasing switching losses
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Low LS allows soft switching at low losses
› di/dt, dV/dt, Eoff over RG
› Crossing the maximum in di/dt and reaching lower di/dt than for low RG = 1…30 Ohm increases Eoff by 60%
› Parasitic inductance should be low enough to be able to stay in the left of the maximum in di/dt
232015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
242015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Even low inductance may deteriorate gate voltage, significantly
› Turn-on of an IGBT
› See di/dt in IC, it is typical
› See impact on gate voltage
› Emitter shifts by approx. 6V
› Influence on current share of paralled devices
250A/200ns=1250A/µs
VL2 = induced voltage at stray inductance
VG = gate to ground
VGE = gate to emitter
IC
VC
VC IC
Representing short conductor
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Paralleling of devices is the way to achieve the required power level – current to be shared
› current flows perpendicular to row of semiconductors› Equal LC’s, equal LE1’s, equal LE2’s from power terminals to each
semiconductor› The asymmetric Gate to Aux. Emitter circuit may be accepted › LCx and LEx are ineffective because of current sharing
LC LC LC
LE1 LE1 LE1
LE2 LE2 LE2
LCx LCx
LEx LEx
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Cross current within paralleled devices causes voltage drop among devices
› current flows along the row of semiconductors› current flows through LCx and LEx causing voltage drop from
semiconductor to semiconductor› Voltage drop at Lex deteriorates gate voltages› Imperfect current sharing
LC
LE1 LE1 LE1
LE2
LCx LCx
LEx LEx
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How big is LEx?
› Shield on back plane assumed
› Approx. 1 nH from terminal to terminal on copper track
thickness 1mm:= width 6mm:= Length 6mm:=
L1Length µ0⋅
2 π⋅width
2⋅
thickness−
2
thickness
2
yatan
width
2
thickness2
y−
atan
width
2
thickness2
y+
+
⌠⌡
d⋅:= L1 1.126 109−
× henry⋅=
L µ 0 Length⋅db⋅:=
Lhenry
1.3 10 9−×=
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Significant impact during turn-on, when considering track inductance, only
Inductance of track on PCB
IC_1
IC_2
VC
VG_1,2
Vext
› An inductance of ≈1nH between 2 IGBT
– causes a difference in gate voltage by approx. 1 V
– A difference of 1 V in gate voltage results in a difference of current per chip by a factor of approx. 2
2 switches out of the previous chain
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Inductance of leads improves balancing of current in asymmetric design
Inductance of leads added
IC_1
IC_2
VG_1,2
› Difference in gate voltage reduced to approx. 0.2 V, resulting in less difference in current
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“Parallel plate waveguide design” – backside shield for DC+, symmetric terminals
› Parallel plates for DC+ and DC-› plate for phase
Single switch Half bridge
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Parallel plate waveguide design for half bridge; also phase should be symmetric
332015-10-30 Copyright © Infineon Technologies AG 2016. All rights reserved.
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Rule for orientation of switches, parallel plates and current
› Row of switches along x-axis› Current perpendicular to x-axis› Low L, Low Z, No breaks in L, Z › Line contact at interfaces, required
paralleled switches
substrate
current
x
y
z
X
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Module sample; Half bridge 1200V/600A+
› Parallel plates from chip level to terminal, approximated › Press-fit interface to external plates › Low resistance at conductors (wide and short)
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With DC-link
› Capacitance C=1,64mH
› Total inductance
– LKK = LDClink + LModul = 4,3nH + 5,3nH = 9,6nH
Pulse cap
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turn off 400A, TJ=25°C, 100ns/div
› Low L low voltage overshoot› No lumped parasitics – no localized resonant circuits
New moduleVDC=900V
FF400R12K
T3
vSense: 1V/div
Standard moduleVDC=700V
vCE: 200V/div iL/iC: 200A/div iC*: 200A/div vGE: links 20V/div, rechts: 10V/div *skaliert auf iL
ICE
VCE
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Current sharing of paralleled IGBTs and diodes: Example IHM
› how to implement into DC bus?
› 3 subsystems to be paralleled outside the module
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Equivalent circuit for IHM containing 3 sub-systems
› Power terminals to be connected outside
› Gate- und auxiliary emitter connected, internally
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Symmetric set up of phase leg - 2 IHMBottom ModuleTop ModuleCapacitor Module
Phase Output+-
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Current measurement in each emitter› 3 Rogowski coils for individual emitters
› Pearson for total current
› DC capacitor behind module 2nd module of phase behind capacitor
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Equivalent circuit for 2 IHMs in a phase leg
› Symmetric design
› no voltage between emitters or collectors same current in each subsystem
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results
› Total current: turquoise; voltage: black
› Partial currents: red, green, black
› Minor differences
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Asymmetric setup
modules oriented with long side along the bus
Phase Output
+-
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Equivalent circuit for asymmetric setup› Voltage drop along the emitter or collector chain during di/dt and › Voltage along the auxiliary emitter› Different voltage at each subsystem
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Results with same basic differences › Differences in conduction phase
› Circulating currents through aux. emitter
› Largest differences in turn-on
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Conduction mode
› Current per sub-system: 800A, 550A, 450A
x3
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Turn-on
› Current per sub-system: 1400A, 580A, 300A
x3
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
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Half bridge Module
(+)
(Phase)
C
Parallelplates
LS
Load
(+)
(-)
(Phase)C
phase leg with capacitor, minimized inductance
› Effective within single phase
(-)
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Inductance from half bridge to half bridge causes current flow from the side
› 3 Phase inverterInductance from half bridge to half bridge
Half bridge Module
(+)
(Phase)
(-)
Half bridge Module
(+)
(Phase)
(-)
Half bridge Module
(+)
(Phase)
(-)
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Example: 3 phase inverter with low inductance within half bridge
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Hal
f br
idge
M
odul
e
Hal
f br
idge
M
odul
e
Hal
f br
idge
M
odul
e
To prevent current flow form the wrong side and overall low inductance and symmetry
› 3 Phase inverter
› Modules side by side along main current flow of half bridge
› Low inductance from capacitor to all half bridges / phases
› Phase output to be symmetric, as well
Main direction of current(+
)(-
)
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Content
Introduction
Effect on switching characteristics and electrical stress
System losses, adjusting RG for worst case operation
Switching speed (di/dt) as a function of RG
Current sharing of paralleled power devices
Symmetry in 3 phase inverter
Conclusion
1
2
3
4
5
6
7
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Conclusion
› Parasitic inductance does not only generate overvoltage
› It changes switching characteristics of bipolar power devices
› Excessive tail charge is required to deal with parasitic inductance
› Simple gate drivers: High parasitic inductance hinders the use of RG values which result in low switching losses.
› For paralleled devices even very small parasitic inductance may result in a large imbalance of current
› For paralleled devices parasitic inductance has usually more impact on current sharing than the spread of the devices’ characteristics
› big potential for better utilization of power devices by strict geometry of conductors
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Acknowledgment
› Many thanks to
– F. Wolter and Th. Geinzer for data on 750V IGBT – Chr. Urban for data from worst case operation– D. Heer for data from 6.5kV turn-off characterization
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