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Development of Optical Fibre based High Voltage compatible IGBT driver with status acknowledge and protection Y.Kelkar#, Y.P. singh, A.C.Thakurta Power Supply and Industrial Accelerator Division, Centre for Advanced Technology, P.O. CAT, Indore -452 013 Abstract IGBT is one of the preferred switch in many power application. With the availability of wide range of HV IGBTs the need to drive them reliably with protection is a challenging task. Some of the important requirements to drive the IGBT is to have galvanic isolation, appropriate output power, detection of fault condition and subsequent protection. An IGBT driver was developed for driving two IGBT’s in series via Optical fibre interface and with the capability of status acknowledge and protection. INTRODUCTION IGBT is a minority carrier device with high input impedance and large current carrying capability. It can be viewed as a functional integration of MOSFET and BJT. The main advantages of IGBT are- low on state voltage drop, low drive power, excellent forward and reverse blocking capabilities. The drawbacks are slow turn off speed due to current tailing and possibility of current latch up. Typical features of the assembled OFC IGBT driver are- a. Providing requisite On and Off gate drive voltages that are independent of Collector emitter voltage b. Capable of isolating system high voltages and switching noise c. Providing requisite drive power d. Short circuit detection and protection f. Over Voltage protection g. Status acknowledge (health of driver and also the acknowledge path) h. Providing isolated DC power supply to floating circuit . The details of above mentioned features is discussed furtherA. Providing requisite On and Off gate drive voltages that are independent of Collector emitter voltage- The driver circuit must be capable of providing stable On and Off gate drive voltages which are independent of Collector emitter voltage and system duty cycle. Secondly enough power must be provided by the drive circuit so as to charge and discharge IGBT gate capacitances and device stays fully saturated and maintaining short circuit capability. In the assembled circuit IGBT driver Powerex make M57962CL-01 in conjunction with Powerex make DC to Dc converter VLA 106-15242 was used which provides +15V and ~ – 8V . Negative bias provides additional noise immunity B. Capable of isolating system high voltages and switching noise- As the IGBT switch is floating in many configurations, the gate drive control interface must be designed for high common mode voltage and high transient noise immunity. A pulse transformer with low inter winding capacitance or a optocoupler / optical fibre interface with high common mode transient noise immunity may be used for this purpose. Width of the gate drive pulse and required isolation voltage will limit the use of pulse transformer. Minimising gate drive length will reduce noise coupling as the developed driver can be placed very near to driven IGBT c. Providing requisite power level in the driveDuring each turn On and Turn Off the driver must charge/ discharge the effective gate capacitance. The driving power required is P(W)= Ceff X (ΔV gate)2 X Fswitching The Ceff is more than the input capacitance value due to miller effect. D. Short circuit detection and protection-The short circuit protection was provided by desaturation detection, i.e. Collector emitter voltage sense. The desaturation detection feature and subsequent withdrawal of gate drive is the inherent feature of Powerex make M57962CL-01 IGBT gate driver. As the desaturation is detected the IGBT driver will start soft shut down of IGBT which helps to combat large transient voltages which may be generated due to large fault current interruption. f. Over Voltage protection- In the series operation of devices, one of the devices may be burdened by over voltage due of a. failure of one of the series devices b. unequal off stage voltages c. delay in turn On of any device will lead to unequal voltage sharing d. failure of any device to turn ON will lead to over voltage across that device. In the assembled system a voltage sense from resistive divider across each series connected is compared with reference voltage and decision is taken. The output of the comparator is gated with other faults to generate healthy/ fault condition g. Status acknowledge (health of driver and also the acknowledge path)- The rising and falling edge of the gate drive gives as indication of health of IGBT. This signal is gated with over voltage comparator signal and overall information is transmitted back to control card via OFC. Power failure at the driver section or break in OFC transmission path is also taken care while generating trip signal in control card. h. Providing isolated DC power supply to floating circuit – The floating power supply was needed for driver section which has voltage comparator, pulse width generator monostable IC’s, DC to DC converter, OFC transmitter and receiver and gating. This supply was provided by multi winding 5kV isolation transformer. OPERATION The driver system was designed to trigger two series connected IGBT’s(IXGR32N170AH1,1700V) driving a resistive load to generate a voltage pulse of ~11us duration across the load at a repetition rate of 1 Hz. The block diagram of power circuit is shown in figure 1 and voltage pulse across load is shown in figure 6. The input DC is obtained from single phase HV step up transformer, rectification and subsequent filtration. The number of series connected switched can be increased if needed and also the output pulse width can be adjusted according to need. standard plastic OFC with 1mm diameter and attenuation 0.22 db/m was to link the control and trigger cards. The function of trigger card is to receive trigger pulse width information via Agilent make OFC receiver HFBR 1521. The block diagram of the trigger card is shown in figure 3. High Voltage Rectifier High Voltage AC mains OFC receiver Driver Card#1 Step up Driver Card#2 OFC IGBT Driver OFC OFC Control Card Transformer HFBR 2521 VLA 106-15242 . Figure 1 Isolated DC Power suply The assembled driver system has two cards- namely Control card and Trigger card. The block diagram of control card is shown in figure 2. The function of the control card is to generate 1Hz master clock using timer IC NE555 in astable configuration. The falling edge of the master clock acts as reference for events to follow. The requisite Pulse width from master clock falling edge is obtained from monostable multivibrator CD 4538. The pulse repetition rate and High Voltage output pulse width can be adjusted according to load requirement. The healthy/fault condition from the series connected IGBT is received using Agilent make OFC receiver HFBR 2521. The received pulses during normal and desaturation detection are shown in figure 4 and Figure 5. Astable Multivibrator 1 Hz Source NE555 OFC receiver OFC Monostable Multivibrator Pusle width generator CD 4538 M57962CL-01 DC-Dc converter To other Driver Hardware Monostable Multivibrator Rising and falling edge pulse generator CD 4538 - OFC + Trip Set Point Figure 3 The output of the receiver is used to drive Powerex make IGBT driver M57962CL-01. The requirement of +Vcc and -VEE of IGBT driver is catered by Powerex make DC to Dc converter VLA 106-15242. This card also contains floating DC power supply for monostable multivibrator , comparator and gating IC’s. The monostable multivibrator will generate two pulses, one from the rising edge and another from falling edge of the IGBT gate drive. OFC Transmitter Monostable Output during fault OFC OFC Transmitter CD 4538 S S/R Flip Flop OFC receiver OFC HFBR 2521 CD4044 OFC break detect OFC HFBR 1521 R . Figure 2 The received pulses from respective IGBT driver cards and the master width are gated together to identify healthy/ desaturation detect/ OFC break/ power supply unavailability at the trigger card. The fault condition will trigger a set/reset flip flop and will generate a trip signal and will also inhibit further transmission of pulses to trigger card. A reset is also provided to again resume the operation. The output of the set/reset flip flop can be used to trip the incomer of High voltage transformer so as to inhibit generation of High voltage. The pulse width generated is optically transmitted after amplification using Agilent make OFC transmitter HFBR 1521 if the healthy condition prevails. In the assembled system Figure 4 The voltage across the IGBT device also compared by a reference value based on attenuation ratio of resistive attenuators and safe limit voltage the IGBT can handle. The output of multivibrator mentioned earlier and output of voltage comparator were gated together and the overall bundled information transmitted by Agilent make OFC transmitter HFBR 1521. Each series connected IGBT will have their own trigger card. The information received from all the trigger cards will be processed on a single control card. Figure 8 Figure 5 EXPERIMENTAL RESULTS The voltage across the load measured by Tektronix make P5100, 100X probe is shown in figure 6. The delay between the falling edge of the 1 Hz generator which acts as the master edge, and the IGBT gate voltage is 980ns as shown in figure 7. All the measurements were taken ob Tektronix make TPS2024B DSO. Across load Acknowledgement We are grateful to Shri U.Karandikar, Shri R.Barothiya and Shri Y. Raikwar who helped us in the development of charger. Authors wish to thank Shri Yunus Khan ,Shri S.D. Yadav and M.D.Jha for technical assistance for wiring and fabricating various parts of assembly. Conclusion A setup to drive two series connected high voltage IGBT’s were developed using OFC interconnect with acknowledge and protection. The developed scheme can be extended to drive more numbers of IGBT keeping their drive requirement in consideration. References [1] Heinz Ruedi, Peter Kohli,” Driver Solutions for High Voltage IGBT’s”, Power Electronics Magazine, April 2002 [2] ICT power application notes, NTAN-06 [3] IXYS application notes, AN-401 [4] Datasheet of Powerex make DC-DC converter M 57145L-01 and IGBT driver M57962CL-01 [5] Datasheet of Agilent make Fibre Optic Versatile link series 555 o/p Figure 6 Figure 7 The jitter between the falling edge of the 1 Hz generator and the voltage across the load was measured to be ± 1.8ns as shown in figure 8.