Auxiliary Subsystems of a General Purpose IGBTStack for High Performance Laboratory Power

Converters

Anil Kumar Adapa, Venkatramanan D, Vinod JohnDepartment of Electrical Engineering

Indian Institute of ScienceBangalore, Karnataka, India

Email: venkat86ram@ee.iisc.ernet.in, aniladapa@ee.iisc.ernet.in

Abstract—A PWM converter is the prime component inpower electronic applications such as static UPS, electric motordrives, power quality conditioners, renewable energy based powergeneration systems etc. While there are a number of computersimulation tools available today for studying power electronicsystems, value added by the experience of building a powerconverter hardware and controlling it to function as desired isunparalleled. A student, in the process, not only understandspower electronic concepts better, but also gains insights into otheressential engineering aspects of auxiliary subsystems such as sens-ing, protection, circuit layout design, mechanical arrangementand system integration. This paper presents a laboratory builtGeneral-Purpose IGBT Stack (GPIS) which facilitates students topractically realize different power converter topologies. Essentialsubsystems for a complete power converter system is presentedcovering details of semiconductor device driving, sensing circuit,protection mechanism, system start-up, relaying and critical PCBlayout design, followed by a brief comparison with commerciallyavailable IGBT stacks. The results show the high performancethat can be obtained by the GPIS converter.

Keywords—IGBT stack, Auxiliary subsystems, pre-charge cir-cuit, forced-air cooling, system start-up , DC-link capacitor, protec-tion mechanism, IGBT de-saturation, dead-time, gate-drive.

I. INTRODUCTION

IGBT based power electronic converters are widely em-ployed today in a variety of the power conversion applicationssuch as adjustable speed motor drives, off-grid or grid-tiedrenewable energy based power generation systems, powerquality conditioning systems, consumer electronics and light-ing system power supplies etc. [1]–[5]. Admittedly, a PWMpower converter is the chief subsystem in any power electronicsystem. Owing to efficiency considerations, for low voltage(up to 200 V) high current systems, typically MOSFET baseddesigns are preferred and for systems with voltages above 400V and high current, IGBTs are invariably employed. This isdue to the fact that for a given die size, an IGBT can handlethree times the current density than that of a MOSFET, byvirtue of conductivity modulation [6]. A variety of IGBT basedpower stacks are available today commercially and althoughthey house a number of desirable system level features, theyhowever are typically available only for 50 kVA power leveland above [7]–[9]. Also, numerous computer simulation toolsare available today and are widely used to perform simulationstudies on power electronic systems by various academicinstitutions.

TABLE I: Power converter ratings

Item Value

Power 25 kVA

DC bus voltage Vdc 800 V

Output voltage (l-l) 400 V

Output current (RMS) 36 A

Nominal power factor UPF

Nominal modulation index ma 0.9

Switching frequency fsw 7.5 kHz

While there is a great deal of published literature on designmethods specific to various subsystems of a power electronicsystem such as power converter topologies, control architec-ture, filters, PWM techniques etc. [5], [10]–[14], publicationsthat actually deal with design aspects of essential auxiliarycircuits which would aid a graduate level student to builda PWM power converter practically in laboratory are ratherlimited.

The goal of this work therefore is to enable laboratory pro-totyping of a power converter system in the form of General-Purpose IGBT Stack (GPIS), where the focus is particularlylaid on the balance of system, which includes gate drive designfor semiconductors, protection mechanisms, start-up sequenceand self-test for fault diagnosis. As an example, the designdetails of a discrete-IGBT based GPIS developed in PowerElectronics Group, Department of Electrical Engineering, IISc,is presented. Such a generic design is helpful in realizing avariety of power circuit topologies as elucidated in [15], andthe exercise of building a GPIS would greatly enhance hard-ware design and troubleshooting skills of a student working onthe power electronics technology. The results show the highperformance that can be obtained by the GPIS converter.

II. POWER CONVERTER AUXILIARY SUBSYSTEMS

The presented GPIS is a two-level three-phase four legpower converter, that uses state-of-the-art discrete IGBTs andother components. The circuit schematic and system speci-fications are shown in Fig. 1 and Table-I respectively. Thevarious subsystems of GPIS include on-board gate-driver, gatedrive power supply, sensor-card, protection, dead-time andannunciation (PDA) card etc., in the form of auxiliary cards

TABLE II: Component details of GPIS

Item Value

IGBT IKW40N120H3

DC-link Capacitor 382LX102M45

High-frequency Capacitor B32654A0474J000

Current Sensor HLSR-32P

Output Relays 832HAWP-1C-F-S

Pre-charge relay RTE24024F

Pre-charge diode S5M (from Vishay)

Pre-charge PTC B59412C1130B070

Bleeder resistors 20J50KE

DC fan 9G0824H101

that are explained in detail below. The component details areshown in Table.II.

A. Gate-drive circuitry

Gate-driver is an integral part of any power convertersystem. It converts logic or signal level commands fromexternal digital signal controller (DSC) or analog controllerto appropriate power level signals capable of reliably turningon and off the power semiconductor device. Most often, oneor more power supplies isolated from control circuitry arerequired for driving the devices. Another desirable featureof gate-driver is to protect the device against short-circuitfaults by sensing VCEsat and detecting switch de-saturation.Commercially available gate drive cards, in addition to theabove, offer few other attractive features such as soft deviceturn-off after fault detection, under-voltage lockout and fault-detection feedback signal to DSC [16], [17].

A simple method to obtain isolation and other desirable fea-tures using off-the-shelf driver ICs is reported in literature [16].In this work, similar method is adopted with ACPL-339J gate-driver IC that is capable of driving MOSFET based currentbooster. Also, the design in this work is modular, compactand suited for individual discrete devices. A half-bridge basedconverter is designed for powering the gate-drive cards whichdrive 8 discrete devices of GPIS.

Fig. 2(a) shows the circuit schematic of the gate-driverand Fig. 2(b) shows the corresponding assembled PCB card.Fig. 3 shows the IGBT drive circuitry on power-board thatuses the mentioned gate-drive card. In this design, separateturn-on and turn-off resistances are used for better control ofswitching times, along with a TVS diode (SMBJ13CA), whichis present between gate and source to limit voltage excursionsto ±15 V. Also, provision to place ferrite-bead and additionalgate-source capacitance is provided to appropriately dampenexcessive ringing, if any. A suggested 4-layer layout design fordiscrete IGBT is presented in Fig. 4. A tight physical layoutof on-board drive circuit components is highly recommendedas this plays a decisive role in determining the parasiticsand hence the nature of switching transitions. The parasiticinductance between the switching device and the gate-driver ismuch smaller in the suggested layout than what is encounteredin typical IGBT module gate-drivers that employ wire leadsand extension connectors.

B. Protection Mechanism

A generalized protection card that releases all PWM signalsfrom DSC to the power converter and corrects inadequatedead-time, if any, to the preset value is presented in [18].Also, it protects the power converter by shutting off thePWM pulses in the event of a fault. Such an interface cardreduces the burden on the DSC by relieving it of the dutyof providing protection and thus creates additional room forcontrol algorithm implementation.

In this work, an advanced Protection, Dead-time and An-nunciation (PDA) card is employed, as shown in Fig. 5. Ituses Lattice FPGA MACHXO2-640UHC as the controller andis programmed with VHDL for digital logic implementation.This card monitors key sensor signals and reports fault in caseany quantity exceeds the preset limits. Various features of thiscard are enunciated below.

1. De-saturation fault detection for 8-IGBTs and faultreporting.

2. Programmable dead-time lockout for 4 legs whilefacilitating independent and/or complementary controlof the two devices of each leg.

3. Programmable minimum pulse width suppression.

4. Over-temperature detection of the heat-sink and faultreporting.

5. Serial communication with the DSC and with otherPDA cards, if any.

6. Over-current (OC) sense for 4 current signals, over-voltage (OV) and under-voltage (UV) sense for 2 dc-link voltages.

7. A push-button or toggle switch feature to manuallypower on or shut off the power converter.

8. DSC based enable-signal to allow or inhibit all PWMsignals as required.

9. A manual and a software based reset signal to resumeconverter operation after fault clearance.

10. Fault indication to user through LEDs.

11. Fan fault indication to DSC.

A state-machine implemented in FPGA representing the sameis shown in Fig. 6. The above comprehensive protectionscheme ensures that any potential damage to the powerconverter is avoided even when used by a novice withoutsignificant prior hardware experience.

C. Sensing Circuit

A non-isolated sensor-card capable of sensing up to fivevoltages, four currents, and six temperature quantities is em-ployed in this work. The card is designed to measure ac or dcquantities rendering a measuring range of ±660 V for threevoltage channels, ±1200 V for the remaining two, ±80 Afor current using HLSR-32P open-loop Hall sensors and 0-150◦C for temperature using thermistors. Adequate filteringis provided on-board and sensed signals are level-shiftedsuitably, using resistive dividers and a reference voltage, to

Fig. 1: Power circuit configuration of two-level three-phase four leg converter

8/6/2015 8:33 PM f=1.25 C:\Users\ANIL ADAPA\Dropbox\Projects\My_CAD_Files\GDR_MOS_BUF\GDR_MOS_BUF.sch (Sheet: 1/1)

DGND

+5V

DGND

B64290L0044

VCC VCC

VEE

VEE

GN

DA

GN

DA

GN

DA

GN

DA

GN

DA

VEE

VCC

SI7414DN GN

DA

DGND

+5V

+5V

74AHC1G14DBV

R9

C11

C12R8

R12

NC1

K2

K12

A3

DESAT15

VCC213

VE16

VGMOS14

VOUT_P12

VOUT_N11

NC10

GND15

VCC16

FAULT7

GND18

VEE9

C10

R13

S2

S1P1

P2

T2

T1

TF1

VCONTROL5

IN7

IN8

SET4

NC6

EP9

OUT1

OUT2

OUT3

R5

R2

R4

R3

R14

R7

R6 U1

135

246

7 8

135

246

7 8

R15

R10

R11

IC12 4

GN

DVCC

35

IC1P

+5V

+5V

+5V

VEE

VEE

VEE

GNDA

GNDA

VCC

VCC

PWM

FAULT

FAULT

(a) (b)

Fig. 2: Gate driver for the GPIS IGBTs (a) circuit schematic and (b) assembled PCB

Fig. 3: IGBT gate drive configuration on power board

form unipolar output signals falling in the range of 0 -3V. The 3 V level-shifting reference voltage is derived fromTL431 voltage regulator. The sensor-card thus can directly beinterfaced with modern external DSC that work with 3.3 V.A differential amplifier based non-isolated input stage, usingTL074BC op-amp is employed for voltage sensing, followedby a level-shifting circuit as shown in Fig. 7. Since current andtemperature sensors employed in the present design already

4 cm

Fig. 4: Recommended PCB layout (top view) for discrete-IGBT drive circuit on the power board

Fig. 5: Assembled PCB of PDA card

PWR On Idle Ready Normal

PWR ON=0

PWR ON=1

CONV ON=0

PWR ON=0

RST=0,CONV ON=1

PWR ON=0

CONV ON=0 ORFAULT=0

EN DSC=0

EN DSC=1

PWR ON=0

CONV ON=0OR FAULT=0

EN DSC=0

EN DSC=1

Fig. 6: State machine implementation of digital logic in PDAcard

generate unipolar outputs, only buffers and filters on-board areused for the same. The use of differential voltage sensors andisolated current sensors ensures that the DSC can be safelyoperated with its own ground reference while ensuring signalintegrity.

D. Relaying

The poles of four legs of the power converter are takenthrough PCB mounted SPDT power relays (832HAWP-1C-F-S) that are independently controllable through DSC. Once self-test routine check is complete, which is explained in section-III, appropriate relays depending on the desired power circuittopology are closed. A separate DPDT relay is present forDC-link pre-charge as explained in section-III.

III. START-UP SEQUENCE AND SELF-TEST

In order to pre-charge the DC-link, a single-phase fullbridge diode rectifier configuration is used together with Pos-itive Temperature Co-efficient (PTC) resistors and a DPDTrelay, as shown in Fig. 1. The relay when closed connects theoutput terminals to Normally Open (NO) input terminals, andthus connects the diode bridge to the grid voltage. The gridmay either be in line-line configuration for three-phase systemsor line-neutral configuration for single phase systems. Ratingsof diode bridge rectifier, as illustrated by Table-III, is muchsmaller than that of the inverter.

A self-test may be performed to diagnose faults eitherin DC-link capacitors or semiconductor devices before theclosure of the output relays. The DC-link charging time-constant may be used as an indicator of defects in the DC-linkcapacitors. Time-constant is determined with known values ofPTC resistance and chosen capacitance. By measuring the timefor the DC-link to charge to a preset value, it is possibleto diagnose a fault, if any, in the DC-link. PTC resistorsinherently provide fault tolerance by limiting the current atsteady-state to safe values in case of short-circuit condition onthe DC-link.

IGBT faults may render the device either short or open.A short-circuit fault in a particular IGBT can be diagnosedby applying a turn-on pulse to the complementary device andchecking its VCEsat fault report. The turn-on pulse must beadequately long for VCEsat protection to function, and shortenough to keep the device junction temperature below absolutemaximum specified, 175◦C in this case. An asserted VCEsatfault signal in the complementary device indicates a short-circuit fault in the IGBT under test. Similarly, an open-circuitfault in a particular IGBT may be detected by applying aturn-on pulse of sufficient duration to devices and checkingthe corresponding line to line inverter output voltages. Forexample, Vry measured will be equal to VDC when topdevice of R-leg and bottom device of Y -leg are turned on,and a fault may be reported if the measurement reads adifferent value. This process may be repeated sequentially forall the semiconductor devices. Such a self-test is useful inverifying the intactness of power components before startingthe intended system operation. Details of this procedure arediscussed in [19]. The pre-charge and start-up diagnosticesthat is incorporated in the GPIS convereter helps to improvethe system reliability.

IV. COMPARISON WITH COMMERCIAL IGBT STACKS

A comparison drawn between GPIS and commercial IGBTstacks in terms of key features is shown in Table-III. Com-mercial stacks are typically available only for 50 kVA powerlevel and above [7]–[9], while the typical requirement inacademia is most often around 10kVA at graduate level. AGPIS however can be appropriately designed to fit the requiredratings. With the chosen state-of-the-art IGBTs and gate-drivecard, the GPIS is operable up to 70 kHZ switching frequency,three folds better than what is offered commercially. However,operating power level needs to be suitably de-rated to maintaindevice junction temperature to safe values. Also, the flexibilityin the GPIS makes it feasible to fine-tune, add or disableselected features as desired, and this makes it suitable fora research laboratory environment. It may be noted that thefeatures offered by GPIS in terms of modularity, protection,power density, compactness and performance supersedes thatoffered by typical commercial stacks. All the circuit boardsand auxiliary subsystem designs can be downloaded from thePEG, Department of EE, IISc[ref].

V. CONCLUSION

A general-purpose IGBT stack or GPISwww.semikron.comdesign is discussed with emphasis on various auxiliary circuitfunctionalities that are essential for building a practical lab-oratory power converter. A modular design of compact PCB

Fig. 7: Voltage-sense and current-sense circuitry

Feature PEG-EE, IISc(GPIS)

SemikronPE teaching

system

MethodeElectronics

(SPS022B3DA120E)

Rated power level 25 kVA 50 kVA 50 kVA

Switching frequency ∗ 5-70 kHz 20 kHz(max) 10 kHz (max)

De-saturation faultprotection Yes Yes Yes

Over-current (OC)protection

Yes(adjustable) No Yes (370 A)

Over-voltage (OV)protection

Yes(adjustable) No Yes (900 V)

Under-voltage (UV)protection

Yes(adjustable) No No

Over-temperature cut-off Yes Yes Yes

Temperature sensing Yes (upto 6) No Yes

Shoot-through protection Yes(adjustable) Yes Yes (2µs)

Converter statuscommunication Yes No Yes

Cooling mechanism Forced air Forced air Forced air

Fan fault detection Yes No No

Diode-bridge front-end Only forpre-charge

For ratedpower For rated power

TABLE III: Comparison of GPIS with commercial IGBTstacks

mountable gate-driver card housing MOSFET based currentbooster stage and VCEsat protection is provided. Also, anIGBT drive circuitry best suited for discrete-IGBT based powerconverter designs is explained along with a suggested PCBlayout routing scheme. A tight component layout minimizescircuit parasitics and lends itself to high switching-frequencyoperation.

A protection scheme against excessive voltage, current ortemperature excursions is discussed in the form of a state-machine. A PDA-card with Lattice FPGA that provides pro-tection, dead-time and annunciation functionalities is presentedalong with a generic non-isolated sensor-card design. Mea-suring range of the sensor is adequate for voltage, currentand temperature up to a power level of 30kVA and since thecard outputs unipolar signals, it may be directly interfacedwith external digital controller. Also, a start-up scheme andself-test procedure is employed that is capable of diagnosing

faults in DC-link and IGBT devices with adequate coverage.A comparison with commercial IGBT stack reveals that theGPIS offers features that supersede that offered by typicalcommercial converter stacks. Such a GPIS built in laboratoryis helpful in realizing a variety of power circuit topologiesneeded for research and also adds valuable hardware designand troubleshooting experience to a student working on powerelectronics technology.

REFERENCES

[1] Z. Chen, J. M. Guerrero, and F. Blaabjerg, “A review of the state of theart of power electronics for wind turbines,” Power Electronics, IEEETransactions on, vol. 24, no. 8, pp. 1859–1875, 2009.

[2] B. A. Karuppaswamy, S. Gulur, and V. John, “A grid simulator toevaluate control performance of grid-connected inverters,” in PowerElectronics, Drives and Energy Systems (PEDES), 2014 IEEE Inter-national Conference on. IEEE, 2014, pp. 1–6.

[3] F. Brucchi and F. Zheng, “Design considerations to increase powerdensity in welding machines converters using TRENCHSTOP 5 IGBT,”in Proceedings of International Exhibition and Conference for PowerElectronics, Intelligent Motion, Renewable Energy and Energy Man-agement, PCIM Europe, May 2014, pp. 1–8.

[4] C.-C. Yeh and M. D. Manjrekar, “A reconfigurable uninterruptiblepower supply system for multiple power quality applications,” PowerElectronics, IEEE Transactions on, vol. 22, no. 4, pp. 1361–1372, 2007.

[5] D. Venkatramanan and V. John, “Integrated higher-order pulse-widthmodulation filter–transformer structure for single-phase static compen-sator,” IET Power Electronics, vol. 6, no. 1, pp. 67–77, 2013.

[6] “IGBT applications handbook,” ON Semiconductor, HBD871/D, Rev.2,2012.

[7] “Datasheet of 6PS03012E33G34160, IGBT Stack,” Available at:www.infineon.com, last accessed on July 2015.

[8] “Datasheet of SPS022B3DA120E, SmartPower Stack,” Available at:www.methode.com, last accessed on July 2015.

[9] “Semikron IGBT stack,” Available at: http://www.semikron.com/products/product−classes/stacks.html, last accessed on July 2015.

[10] A. Ghoshal and V. John, “Active damping of LCL filter at low switchingto resonance frequency ratio,” IET Power Electronics, vol. 8, no. 4, pp.574–582, 2015.

[11] D. Venkatramanan and V. John, “A resonant integrator based PLLand AC current controller for single phase grid connected PWM-VSI,”National Power System Conference (NPSC), 2010.

[12] V. M. Iyer and V. John, “Low-frequency dc bus ripple cancellation insingle phase pulse-width modulation inverters,” IET Power Electronics,vol. 8, no. 4, pp. 497–506, 2015.

[13] A. Kulkarni and V. John, “Mitigation of lower order harmonics ina grid-connected single-phase PV inverter,” Power Electronics, IEEETransactions on, vol. 28, no. 11, pp. 5024–5037, 2013.

[14] J. W. Kolar and S. D. Round, “Analytical calculation of the rms currentstress on the dc-link capacitor of voltage-pwm converter systems,” IEEProceedings-Electric Power Applications, vol. 153, no. 4, pp. 535–543,2006.

[15] S. Anand, R. Singh, and F. B. Fernandes, “Unique power electronics anddrives experimental bench (PEDEB) to facilitate learning and research,”Education, IEEE Transactions on, vol. 55, no. 4, pp. 573–579, 2012.

[16] A. K. Adapa and V. John, “Gate drive card for high power three phasepwm converters,” in 5th National Power Electronics Conference 2011,2011.

[17] A. K. Jain and V. Ranganathan, “Sensing for igbt protection in npc threelevel converterscauses for spurious trippings and their elimination,”Power Electronics, IEEE Transactions on, vol. 26, no. 1, pp. 298–307,2011.

[18] A. K. Adapa and V. John, “Digital dead time logic and protectioncircuitry for pwm voltage source converters,” in 5th National PowerElectronics Conference 2011, 2011.

[19] N. Agrawal, “Control and start-up diagnostics of three phase inverters,”Master of Engineering (ME) thesis, Department of Electrical Engineer-ing, Indian Instite of Science (IISc), Bangalore, 2011. Available at:http://www.ee.iisc.ernet.in/new/people/faculty/vjohn/stud.html.