igbt-low inductive design.pdf

04.12.2007 1

Dealing with IGBT ModulesDealing with IGBT ModulesDealing with IGBT Modules

04.12.2007 2

Table of Contents

Dealing with IGBT ModulesMotivationLow inductive DC-link designChoice of right SnubberGate Clamping Thermal managementParalleling Application of driver circuitParalleling Low inductive AC-Terminal connectionUsage of single switch GA type modulesConclusion

04.12.2007 3

vCE(t)iC(t) VCC

IO

0t

0

t1

0t

pv(t)iC vCE

t2

Dependence of VCE, IC, Pv, Eswitch

04.12.2007 4

vCE(t)iC(t) VCC

IO

0t

t1

0t

pv(t)

t2

Eswitch

vCE(t)iC(t) VCC

IO

0t

t1

0t

pv(t)

t2

Eswitch

di/dt

Influence of switching speeds

Increased switching speed, decreases the switching losses EswitchBut, leads to increased di/dt and therewith to higher over voltages

04.12.2007 5

Porsche 911 - 2004

Porsche Diesel - 1960Would you use these different vehicles with

the same driver and in the same environment?

Motivation

04.12.2007 6

Table of Contents


04.12.2007 7

Motivation

Why low inductive DC-link design?

Due to stray inductances in the DC link, voltage overshoots occur during switch off of the IGBT:

These voltage overshoots may destroy the IGBT module because they are added to the DC-link voltage and may lead to VCE > VCEmax

With low inductive DC-Link design (small Lstray) these voltage overshoots can be reduced significantly.

dtdiLv strayovershoot =

linkDCovershootCE vvv +=

04.12.2007 8

Lstray = 100 nH

Low Inductance DC-link Design

The comparison of stray inductances showInside the module SEMIKRON reduced the inductances significantly

Outside the module the reduction of stray inductances is necessary, too

Lstray = 20 nH

04.12.2007 9


The mechanical design has a significant influence on the stray inductance of the DC-link

The conductors must be paralleled

Lstray = 100 %

Lstray < 20 %

loop

1 cm 10 nH

04.12.2007 10



The connections must be in line with the main current flow

Lstray = 100 %

Lstray = 30 %

remaining loop

04.12.2007 11



Also the orientation must be taken into regard

Lstray = 100 %

Lstray = 80 %

+-

+-

04.12.2007 12

+ bus bar - bus bar


Simulation of current distribution for the case of Lstray = 80 %

04.12.2007 13

Lstray = 100 %

Lstray = 50 %



A paralleling of the capacitors reduces the inductance further

04.12.2007 14


For paralleling standard modules a minimum requirement is DC-link design with two paralleled bars

04.12.2007 15


04.12.2007 16

++

--~


Paralleled half bridge IGBT modules

04.12.2007 17

DC-link

SnubberCapacitor

3 x 2 x IGBT parallel

Heat Sink

Fan

Driver

Apple

SEMIKRON 3 Phase and Low Inductance Inverter

04.12.2007 18

-+

-+

-+

-+

-+-+

2 IGBT Modules

Capacitor

Low inductive solution

-+

-+-+

-+

-+-+

2 IGBT Modules

Capacitor

+

+-

-

+ ++

Typical solution

loop


Comparison of different designsTwo capacitors in seriesTwo serial capacitors in parallel

parallel current paths

04.12.2007 19

Low Inductance DC-link Capacitors

Also the capacitors have to be decidedCapacitors with different internal stray inductance are availableChoose a capacitor with very low stray inductance!Further: low ESR Equivalent Series ResistanceHigh IR Ripple Current Capability

Lstray = ?

Ask your supplier!

04.12.2007 20

Table of Contents


04.12.2007 21

Motivation

Why use a snubber?

Due to stray inductances in the DC link, voltage overshoots occur during switch off of the IGBT:

These voltage overshoots may destroy the IGBT module because they are added to the DC-link voltage and may lead to VCE > VCEmax

The snubber works as a low pass filter and takes over the voltage overshoot (caused by the energy which is stored in the stray inductances)

dtdiLv strayovershoot =

linkDCovershootCE vvv +=

04.12.2007 22

Snubber Networks

Different snubber networks are in use

a) b) c) d)

04.12.2007 23

Snubber Networks

SEMIKRON recommends for IGBT applications:Fast and high voltage film capacitor (MKP / MFP) as snubberparallel to the DC terminals

Not to increase Lstray, the snubber must be located directly at terminals of the IGBT module

DC-link Snubber

04.12.2007 24

Not Sufficient Snubber Capacitors

But still: the snubber networks need to be optimisedThe wrong snubber does not reduce the voltage overshootsTogether with the stray inductance of the DC-link oscillations can occur

IGBT switch off (raise of VCE )before optimisation

Voltage overshoot

Oscillation

04.12.2007 25

00

IGBT-switch-o ff.xls

VCE

t

V1VDC

V2

iC = operating currentdiC/dt = turn off

Determination of a snubber capacitor

Influence of DC-link stray inductance and snubber capacitor stray inductance

=

=

=

=

04.12.2007 26

Not Sufficient Snubber Capacitors

These capacitors did not work satisfactory as snubber:

04.12.2007 27

Available Snubber Capacitors

From different suppliers different snubber capacitors are available.In a trial and error process the optimum can be find, based onmeasurements.The different snubber capacitors have different stray inductance values. Again it is necessary to find one with lowest inductance.

good better

04.12.2007 28

Optimal Snubber Capacitor

After introduction of optimised snubber capacitor:Significantly reduced voltage overshootsNo oscillations

IGBT switch off (raise of VCE )after optimisation

Voltage overshoot

No oscillation

04.12.2007 29

Table of Contents


04.12.2007 30

Gate Clamping

Over voltages at the gate VGE > +/- 20 V can occur due toInduction at stray inductancesBurst impulses by EMC

The introduction of an additional gate clamping is necessaryClose to the gate terminals, what means 5 cmUse twisted pair wiring

-20 V VGE +20 V

04.12.2007 31

Gate Clamping

Gate clamping with RGE from gate to emitter potential Keeps gate potential always on defined level also when supply voltage of the driver dropsPrevents charging of the gate, for highly resistive driver outputsOnly RGE is not sufficient for gate clamping. (See the following charts.)

VGE

04.12.2007 32

Gate Clamping

Gate clamping with Schottky Diode from gate to supply voltage of driver

On driver board (distance to module 5 cm, twisted pair wires)Additional RGE is recommended

VGEV+ supply

04.12.2007 33

Gate Clamping

Gate clamping with Zener Diode or Avalanche Diode from gate to emitter potential

On driver board (distance to module 5 cm, twisted pair wires)Or on auxiliary PCBParallel RGE is recommended

VGE

04.12.2007 34

Gate Clamping

Auxiliary PCB directly at the IGBT moduleThe additional RGE ensures off-state of IGBT in case of failed wiring

RGoffRGon

RGEZ-diode

RGoffRGon

RGEZ-diode

04.12.2007 35

Table of Contents


04.12.2007 36

Thermal Management

Taking thermal management into regardNo space between the paralleled modules lead to low stray inductances and minimum spaceBut the thermal stacking makes a current derating necessary

04.12.2007 37

Thermal Management

20 30 mm space between the modules increase the inductances but reduces the thermal resistance to the heat sink significantly

Optimised thermal management leads to maximum possible current ratings

04.12.2007 38

Table of Contents


04.12.2007 39

Worst Case: All Contacts Shorted

Different IGBT modules with differentSwitching speeds ton and toffGate thershold voltages VGE(th)Gate charge characteristic VGE = f(QG) and Miller Capacity CresTransfer characteristic IC = f(VGE)

C

AEG

EVGE VGE VGE

Due to hard connected gates, all IGBTs must have the same VGEThis means: all IGBTs do not switch independently from each other

04.12.2007 40

Hard Connected Gate with Common Resistor

Hard connected GatesAll IGBTs have different gate threshold voltages VGE(th)IGBT1, with the lowest VGE(th) turns on first.The gate voltage is clamped to the Miller-Plateau. Therefore IGBTswith higher VGE(th) can not turn on. They turn on only after t1.The IGBT1 with low VGE(th) takes all the current and switching losses during turn on.On going process by negative thermal coefficient of VGE(th)

VGE

t

t1

VGE(th)

VGE

t

t1

VGE(th)

VGE

t

t1 1

VGE(th)

t1 n

04.12.2007 41

Introduction of Gate Resistors

Separated by gate resistorsThe gate voltage of each IGBT can rise independent from the other one.Note: The gate resistors must be tolerated < 1 %

With individual gate resistors all IGBTs are independent from each other

C

AE

G

EVGE 1 VGE 2 VGE n

04.12.2007 42

Introduction of Gate Resistors

Separated by gate resistorsAll IGBTs still have different gate threshold voltages VGE(th)But: The gate voltage of each IGBT can rise independently from the other ones.The higher Miller-Plateau will be reached after a short time t1. Only small differences in current sharing and switching losses between paralleled IGBTs.

VGE

t

t2t1

VGE(th)

04.12.2007 43

Worst Case: All Contacts Shorted

Taking stray inductances into regardDue to hard connected gates and varying transfer characteristics, all IGBTshave different switching times and speeds; dix/dt varies in each legThe circuit also has different stray inductances; LxTherewith vx = Lx x dix/dt varies in each leg (e.g.: 1000 A/s x 10 nH = 10 V)Nearly unlimited equalising currents i flow also via the thin connecting wiresOscillations between parasitic capacitances (semiconductors) and-inductances are not damped.

V1 V2 Vni =

C

AEG

E

04.12.2007 44

Introduction of Auxiliary Emitter Resistors

The introduction of REx ( 10 % of RGx but min. 0,5 ) leads toLimitation of equalising currents i 10 ADamping of oscillations

C

AE

E

G

V1 V2 Vni 10 A

RE1RE2 REn

04.12.2007 45


The introduction of REx leads also to a negative feedback:The equalising current i leads to a voltage drop VREx at the Emitter resistors REx

C

AE

E

G

i

VRE1 VRE2

fast IGBT slow IGBT

04.12.2007 46


The introduction of REx leads also to a negative feedback:The voltage drop VRE1 reduces the gate voltage of the fast IGBT and decreases therewith its switching speed.The voltage drop VRE2 increases the gate voltage of the slow IGBT and makes it faster.During switch off: vice versa.

C

AE

E

G

i

fast IGBT slow IGBT

VRE1 VRE2

04.12.2007 47

Additional Proposals

The introduction of Shottky-Diodes parallel to RExhelps to balance the emitter voltage during short circuit case.Dimensioning 100V, 1A.

04.12.2007 48

Additional Proposals

The introduction of clamping diodes prevents over voltages at the gate contacts.Therefore these clamping diodes must be placed very close to themodule connectors

C

AE

E

G

04.12.2007 49

Conclusion Driving Paralleled IGBTs

Balanced switching behaviourIndependent switching due to introduction of RGxBalanced switching speeds due to negative feedback by introduction of REx

Limitation of equalising currentsDamping of oscillationsPrevention of gate over voltagesRefer also to SEMIKRON Application Manual - Power Modules

GermanEnglishChineseKoreanJapaneseRussian (on internet only)

04.12.2007 50

Additional Parallel Board

PCB for paralleling IGBT close to the module connectorsSame track length on the boardShort, twisted pair wires from the board to the modules ( 5 cm)

RGonRGoff

RERGonRGoff

RE RGoffRGon

RE RGoffRGon

RE

04.12.2007 51

Additional Parallel Board

Top Bot

IGBT Driver

04.12.2007 52

Auxiliary Printed Circuit Board

Auxiliary PCB directly at the IGBT moduleThe additional RGE ensures off-state of IGBT in case of failed wiring Same track length on the boardShort, twisted pair wires from the main driver to the auxiliary PCB at the IGBT module

RGoffRGon

RGEZ-diode

RERGoffRGon

RGEZ-diode

RE

04.12.2007 53

Table of Contents


04.12.2007 54

Motivation

Why symmetrical AC terminal connection for paralleled IGBTs?When the connection between the AC terminals have high inductance and different inductances, the current sharing of IC(output current) will be inhomogeneous and oscillations may occur.This would make a current derating necessary.

Simulation of 4 paralleled IGBT modules with

inhomogeneous current sharing

leads to oscillations

IC

t

04.12.2007 55

Symmetrical AC Connection

Why symmetrical AC terminal connection for paralleled IGBTs?The sketch shows that Lstray,DC and Lstray,AC are connected in seriesThis makes clear why both have to be reduced and both have to besymmetric in each legto ensure even current distribution to avoid oscillations

C

G

E

04.12.2007 56

Symmetrical AC Connection

AC link designShort connections with identical current path length for each moduleWide and thick barsFlexible interconnections for large systems might be necessary to compensate differences in thermal expansionLong hole drillings' can compensate mechanical tolerances

Isolated supporting poles take over vibrations and forces from heavy AC cables

Look for a symmetric AC-connection so that the current sharing will be even over all modules

04.12.2007 57

Table of Contents


04.12.2007 58-

~

+1

2

3

1

2

3

-

+

~

1

1

2

2

?

Motivation

Optimisation problemIn order to optimise the thermal management it seems to be useful splitting the current of one half bridge topology into two modules.

The question is: what is better use two paralleled half bridges, or two single switches in series connection?

04.12.2007 59

-

~

+1

2

3

1

2

3 -

+

~1 1

2 2

3 3

Paralleling GB Modules

How to parallel half bridge IGBT modules

04.12.2007 60

-+

~

1

2

2

1

-

+

~

1

1

2

2

Paralleling of GA Modules

How to use single switch IGBT modules as half bridge

04.12.2007 61

Increased switching speed, decreases the switching losses EswitchBut, leads to increased di/dt and therewith to higher over voltages

vCE(t)iC(t) VCC

IO

0t

t1

0t

pv(t)

t2

Eswitch

vCE(t)iC(t) VCC

IO

0t

t1

0t

pv(t)

t2

Eswitch

di/dt

Influence of Switching Speeds

04.12.2007 62

-+

~

1

2

2

1

-

~

+1

2

3

1

2

3

GA or GB?

Comparison For GB modules the diodes for commutation are placed in the samemodule. Therewith the stray inductance is as low as possible.Paralleled GB modules allow higher switching speeds

04.12.2007 63

GA or GB?

Comparison In half bridge modules the snubber capacitors can be placed closed to the terminals with short - and therewith low inductive connections. So that the snubbers work very efficient.Paralleled GB modules allow higher switching speeds

04.12.2007 64

Conclusion

Advantages of paralleled half bridges

The current per module is only 50 % of the maximum currentThe di/dt is much reduced, therewith the voltage overshoot is small (v = - L x di/dt)The half bridge module has much lower stray inductances, what reduces the voltage overshoot againSnubber capacitors can be placed very close to the terminals, so that they work very efficientThe switching speed can be increased and therewith the switchinglosses are reduced

SEMIKRON recommends the use of paralleled half bridge modules instead of single switch modules

04.12.2007 65

SEMIKRONs Recommended Solution

04.12.2007 66

Table of Contents


04.12.2007 67

Conclusion

When using latest generations of IGBT modules it is recommended and advantageous to

Do a low inductive (sandwich) DC-link designDecide for low inductive DC-link capacitorsOptimise the snubber capacitors Optimise thermal management which leads to maximum possible current ratings

04.12.2007 68

Conclusion

For paralleled modulesThe driver must be powerful enough

Some additional components are necessary (e.g. REx) and must be located close to every single module

The DC- and AC connection must be symmetric and low inductive

04.12.2007 69

Thank you very much for your attention

Refer also to SEMIKRON Application Manual - Power Modules

04.12.2007 70

Document status: preliminary

Date of publication: 2006-06-08

Revision: 1.3

Prepared by: Christian DaucherWith assistance fromDr. Arendt WintrichNorbert Pluschke

Information furnished in this document is believed to be accurate and reliable. However, no representation or warranty is given and no liability is assumed with respect to the accuracy or use of such information. Furthermore, this technical information specifies semiconductor devices but promises no characteristics. No warranty or guarantee expressed or implied is made regarding delivery, performance or suitability. Specifications mentioned in this document are subject to change without notice. This document supersedes and replaces all information previously supplied and may be supersede by updates.

Dealing with IGBT ModulesTable of ContentsDependence of VCE, IC, Pv, EswitchInfluence of switching speedsMotivationTable of ContentsMotivationLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignSEMIKRON 3 Phase and Low Inductance InverterLow Inductance DC-link DesignLow Inductance DC-link CapacitorsTable of ContentsMotivationSnubber NetworksSnubber NetworksNot Sufficient Snubber CapacitorsDetermination of a snubber capacitorNot Sufficient Snubber CapacitorsAvailable Snubber CapacitorsOptimal Snubber CapacitorTable of ContentsGate ClampingGate ClampingGate ClampingGate ClampingGate ClampingTable of ContentsThermal ManagementThermal ManagementTable of ContentsWorst Case: All Contacts ShortedHard Connected Gate with Common ResistorIntroduction of Gate ResistorsIntroduction of Gate ResistorsWorst Case: All Contacts ShortedIntroduction of Auxiliary Emitter ResistorsIntroduction of Auxiliary Emitter ResistorsIntroduction of Auxiliary Emitter ResistorsAdditional ProposalsAdditional ProposalsConclusion Driving Paralleled IGBTsAdditional Parallel BoardAdditional Parallel BoardAuxiliary Printed Circuit BoardTable of ContentsMotivationSymmetrical AC ConnectionSymmetrical AC ConnectionTable of ContentsMotivationParalleling GB ModulesParalleling of GA ModulesInfluence of Switching SpeedsGA or GB?GA or GB?ConclusionSEMIKRONs Recommended SolutionTable of ContentsConclusionConclusionDealing with IGBT ModulesTable of ContentsDependence of VCE, IC, Pv, EswitchInfluence of switching speedsMotivationTable of ContentsMotivationLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignLow Inductance DC-link DesignSEMIKRON 3 Phase and Low Inductance InverterLow Inductance DC-link DesignLow Inductance DC-link CapacitorsTable of ContentsMotivationSnubber NetworksSnubber NetworksNot Sufficient Snubber CapacitorsDetermination of a snubber capacitorNot Sufficient Snubber CapacitorsAvailable Snubber CapacitorsOptimal Snubber CapacitorTable of ContentsGate ClampingGate ClampingGate ClampingGate ClampingGate ClampingTable of ContentsThermal ManagementThermal ManagementTable of ContentsWorst Case: All Contacts ShortedHard Connected Gate with Common ResistorIntroduction of Gate ResistorsIntroduction of Gate ResistorsWorst Case: All Contacts ShortedIntroduction of Auxiliary Emitter ResistorsIntroduction of Auxiliary Emitter ResistorsIntroduction of Auxiliary Emitter ResistorsAdditional ProposalsAdditional ProposalsConclusion Driving Paralleled IGBTsAdditional Parallel BoardAdditional Parallel BoardAuxiliary Printed Circuit BoardTable of ContentsMotivationSymmetrical AC ConnectionSymmetrical AC ConnectionTable of ContentsMotivationParalleling GB ModulesParalleling of GA ModulesInfluence of Switching SpeedsGA or GB?GA or GB?ConclusionSEMIKRONs Recommended SolutionTable of ContentsConclusionConclusion

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