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2kJ/s 1kV, 25Hz PRR Capacitor Charging Power Supply with twin phase shifted primary windings to achieve high charge transfer rate and stability 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 The capacitor charging power supply (CCPS) was developed to charge bank of 150uF energy storage capacitor (15uf , 10nos in parallel) upto 1kV in 35ms exhibiting a peak charging power of 2kJ/s at a repetition rate of 25pps. A CCPS observes a large change in load variations at the output. Initially the capacitor will act as a short circuit so the topology must be such that it should withstand short circuit condition repetitively. The High Voltage capacitor charging power supply consist of two identical full bridge resonant converters feeding to two primary windings of a transformer with rectified secondary connected to capacitor load. Topology selection is based on the fact that the series resonant converter with switching frequency fs, below 50% of the resonant frequency fr (fs ≤ 0.5 fr) act as a current source. INTRODUCTION The CCPS described in the paper will be used to charge energy storage capacitors of Septum pulse generator circuit .The complete process is divided into three modes: (1) High Power charging mode (2) Low Power Refresh mode (3) Output Pulse delivery and recovery mode. During High power Charging mode the two resonant converters operating at fs < 0.5 fr corresponds to discontinuous mode of operation will feed the two primary windings of HV step up transformer in same phase. In this mode of operation all devices are turned ON and OFF at zero current so switching losses are reduced to minimum. It also eases critical bearing on switch commutation time and diode reverse recovery time. The reflected load capacitor to primary will be n2 CLoad. The series combination of reflected load capacitor and resonating capacitor will decide the resonating frequency. As the reflected load capacitor value is kept high in comparison with value of resonating capacitor the resonating frequency is mainly governed by resonating capacitor value. The equivalent circuit is shown in figure1 Lr Cr 2 n Cload VDC Figure 1 As the output voltage reaches linearly near the targeted voltage, the Phase Shift PWM IC UCC3895 will start the phase shifting between the two resonant converters depending on the gain of the error amplifier. As a result the net mmf tends to cancel in primary circuit. The phase shift in the primary circuit will reduce rate of charging and subsequently will lead to improved stability of output voltage .Practically complete cancellation of mmf doesn’t takes place, therefore another loop consisting of comparator switches off the converter if the output voltage exceeds desired voltage. During the low power refresh mode the two primary windings are driven 180 ̊ out of phase and the output voltage will be maintained at the desired voltage level. During the output delivery mode the CCPS will be disabled until the pulse output is delivered and voltage recovery process is over. A voltage recovery system at the output enables to reduce power loading on CCPS. The main drawback of Series resonant converter is high circulating currents as compared with PWM. As CCPs deploys IGBT’s so that the conduction losses are function of average rather than rms current so high peaks are not so detrimental. The paper describes the prototype capacitor charging power supply that has been designed, assembled and tested upto full power. OPERATION The inverter utilizes two full bridge series resonant converter topology with fixed on time and fixed frequency operation (fs < 0.5 fr) as shown in circuit diagram Figure 2. The DC bus is obtained by rectifying and filtering three phase AC mains followed by LC filter. The bridge consist of MITSUBISHI make CM75 DY24H, VCES equal to 1200 V and current rating of 75 A. The IGBT bridge was driven by Powerex make IGBT driver M57962CL-01. The tank circuit values were Lr = 25uH and Cr = 0.4uf. The transformer was built on 4 nos of U93 cores with 10 turns in primary and 100 turns in secondary wound with Litz wire. The energy storage capacitor bank consists of 15uf , 10nos energy storage capacitors in parallel. The output switch used was two series connected ABB make 5STF08 F2060, 2kV SCR’s. As their will be voltage reversal during the output pulse delivery, two IGBT’s (IXGH 16N 170) connected in series were placed in series with the output rectifier and will be made disabled during this period. The switches are driven by Phase Shift PWM IC UCC3895. The output is rectified with series connected ultrafast diodes each rated for 1800V. Vdc 150uF Control A1 Cr Control A3 Sense N/W Np=10 Control A4 Ns=100 Control A2 . Control B1 Vdc Lr Voltage Ref Cr Control B3 Np=10 Control B4 Control B2 . Gate drive fir IGBT's Control Card with UCC 3895 Figure 2 The advantage of series resonant circuit is that the leakage inductance of circuit including transformer are absorbed into the tank circuit inductance. The reflected load voltage appears in series with DC bus voltage as a voltage source. Initially when the capacitor is in discharged state the tank current in nearly sine wave with both the lobes of equal magnitude as shown in Figure 3. As the capacitor charges the current in the forward direction increases and current in reverse direction decreases due to increased reflected load voltage. Figure 3 The stability of the output voltage is governed by error amplifier followed by voltage comparator (LM311), which compares voltage feed back from HV energy storage capacitor and the reference signal. In single SRC even the tank circuit switches were disabled, the energy stored in the tank circuit components will overcharge the output capacitor during the refresh cycle. In twin resonant scheme initially when bulk charging is required the power is fed by both the converters in phase which has the advantage that power handled by each converter is halved. The error amplifier will shift the phase between ____________________________________________ # Lr [email protected] the two current sources. As the transformer is wound with two primaries and one secondary on the same core, phase shifting phenomenon will change the coupling flux between primary and secondary windings. As the output voltage reaches near the target voltage the error amplifier starts phase shifting and more mmf cancellation starts at the input of transformer, so net output voltage at the output of transformer will reduce which in turn reduces the output charging rate as shown in Figure 4. The reduction in the output charging rate will reduce the overcharging rate. Practically perfect cancellation of mmf doesn’t takes place in the primary, so another voltage comparator loop was added which will disable the charging by withdrawing drive to bridge switches. During Output Pulse delivery mode energy of the capacitor is delivered to load by triggering the switch. All the events are synchronised with trigger. Application of trigger will inhibit the charging completely and simultaneously inhibit the output switch drive. The output circuit switches are required to prevent short circuit condition created due to voltage reversal phenomenon due to inductive load. The next charging cycle starts after 5ms. Figure 4 The resonating components consist of L = 60 μF (which includes the main resonating inductor and leakage inductance of transformer), C= 0.4uF.The LCEff series combination of these will yields a resonating frequency of 32 kHz. Therefore the switching frequency used was less than fr /2 (16 kHz). The 1:10 step up transformer is made up of single 4 nos U93 core core with 10 turns in primary and 100 turns in secondary. The series resonating inductor is made up of E42 core with 10 turns and 2mm air gap. There is no intentional circuit resistance. The expected resistance of circuit is due to resistance of diodes, IGBT’s, ESR of capacitors, resistance of transformer and inductor. EXPERIMENTAL RESULTS The CCPS is linearly charging 150uF Energy storage capacitor shown in Figure 5. The average output current Iav required to charge the capacitor C to requisite voltage 𝛥V in a given time 𝛥T can be given by 𝐶𝑋∆𝑉 𝐼𝑎𝑣 = ∆𝑇 12:25:45.0 12:30:25.0 12:35:05.0 12:39:45.0 12:44:25.0 12:49:05.0 12:53:45.0 12:58:25.0 13:03:05.0 13:07:45.0 13:12:25.0 13:17:05.0 13:21:45.0 The calculated value of Iav comes out to be is 4.28 A, when C=150uF, 𝛥V =1kV, 𝛥T = 35ms. 2.9474 2.9472 2.947 2.9468 2.9466 2.9464 2.9462 2.946 2.9458 Figure 7 Acknowledgement We are grateful to Shri U.Karandikar, Shri R.Barothiya, Shri Renukanath 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 power supply. Figure 5 Figure 3and Figure 4 verifies the series resonance behaviour of tank current during initial part and at end of charging cycle. Figure 6 shows complete charging voltage waveform across the load and charging current waveform. The CCPS is tested upto 1kV at a PRR of 25Hz. The traces were obtained with Agilent make DSO 9064A and Tektronix make TPS2024 digitizing oscilloscope. The voltage measurements were taken with Tektronix make P5100, 100X probe and Pearson CT. The stability of capacitor voltage was recorded with Agilent make 34401A multimeter and was found to be ±0.015% as shown in figure 7. Figure 6 Conclusion A high frequency capacitor charging power supply has been designed assembled tested up to full power in laboratory. A twin full bridge series resonant topology using IGBT switches was used. The bridge was driven with fixed frequency variable phase shift using commercial phase shift IC UCC3895 and Powerex make IGBT driver M57962CL-01.The CCPS has been tested for charging 150uf energy storage capacitor at 1kV at Pulse repetition rate of 25 PPS. References [1] N.Mohan ,T.M.Undeland,W.P.Robbins, Power Electronics Converters, Applications, John Wiley and Sons, 2001, pp249-264 [2] B.E.Strickland, M. Garbi , F. Cathell, S.Eckhouse and M. Nelms, “2kJ/s, 25kV High Frequency Capacitor Charging Power Supply using Mosfet Switches”, IEEE tran 1990, pp 531-534 [3] A.C.Lippincott,"A capacitor charging power supply using a series resonant topology, constant on-Time /variable Frequency Control, and Zero -Current Switching", IEEE trans on Industrial Electronics, Vol.38, No.6 ,Dec1991 [4] J.S.Oh,S.D.Jang,Y.G.Son,M.H.Cho,W.Namkung, "Development OF Capacitor-Charging Power Supply For A Smart Modulator",Proc. Of the second Asian Particle Accel. Conf,Beijing, China,2001 [5] Y.Kelkar, Y.Raikwar,” A Zero Voltage Switching , Variable Frequency Capacitor Charging Power Supply Using Series Resonant Topology” Inpac 2005, pp 457-458