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A FREQUENCY CONTROLLED LCL - T RESONANT CONVERTER FOR H- ION SOURCE V.K. Gauttam, A. Kasliwal, R. Banwari, T.G. Pandit, A.C. Thakurta Power Supplies and Industrial Accelerator Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India L1 Abstract - An H ion source is being developed at RRCAT, Indore. An LCL-T resonant power converter with variable 400 V AC frequency control is proposed which is utilized to develop 50Hz a -20 kV/ 100 mA high voltage (HV) power supply for extraction of H ̄ ions. The LCL-T resonant topology offers many advantages like gainful utilization of the transformer parasitics as a part of resonant network and low circulating current. The power converter is operated with variable frequency control and above resonance to get well known zero-voltage switching (ZVS) advantages for full bridge semiconductor switches in full load range. The converter energizes the symmetrical CockcroftWalton (CW) based HV generator to achieve required high voltage. The CW circuit is an attractive solution for HV generation since it has features like low stored energy and low output ripple. The HV power supply is operated in constant current (CC) mode with closed loop control and soft start of the power supply is achieved by sweeping the switching frequency from 40 kHz to defined operating point. Design parameters, simulation results and experimental results of the power converter are presented in this paper. INTRODUCTION High frequency resonant converters are widely used in many applications like high voltage (HV) power supplies, induction heating, power factor correction etc. owing to their features of small size, zero voltage switching (ZVS) at above resonant frequency (operating in lagging power factor mode), lower switching losses, high efficiency, low electromagnetic interference . Recently high frequency resonant converter with three or more resonant elements are more attractive solution because of their distinct advantage of having load independent operation. An Hion source is being developed at RRCAT, Indore and it requires a high voltage power supply (HVPS) for extraction of H- ions. The power supply consists of two stages. First one is high frequency resonant power converter and second one is a balanced symmetrical Cockroft-Walton (CW) scheme based HV generator. L2 generator should be energized by a high frequency sine wave source to achieve required +load regulation, +ripple Lo G and low stored Genergy. The LCL – T resonant tank circuit D D excited by an H bridge voltage S S V source inverter V(VSI) Co C with variable frequency control is chosen as power circuit G topology. The G LCL-T resonant tank circuit has the D D advantage that itS can absorb the - leakage inductance of S2 transformer as a part of resonant network and output can be controlled even at no load by sweeping the switching frequency. The basic schematic of LCL-T resonant circuit is shown in Fig. 1. 3 1 1 3 1 3 i 4 2 4 L1 D1 Vdc - C2 Pri 2 4 + C1 L2 D3 S1 S3 C Cdc D4 RL L O A D D2 S4 S2 Figure 1: VSI with LCL-T resonant topology The resonant frequencies f01 and f02 of the LCL-T resonant tank can be defined by complex input impedance (Z). These frequencies can be found by calculating those frequencies which result in either infinite or zero input impedance . Z→∞ f01 = Z →0 f02 1 2π√C. L2 = (1) 1 2π√C.L1 L2 ⁄(L1 +L2 ) 𝑍𝑛 L 𝑍𝑛 = √ , Q = C 𝑅𝐿 (2) (3) ANALYSIS AND DESIGN PARAMETERS The voltage gain vs. frequency plot of LCL-T with resistive load at different Q is shown in Fig. 2 which FREQUENCY CONTROLLED LCL- T RESONANT CONVERTER The high frequency power converter energizes the symmetrical CW based HV generator which consists of multiplier circuits connected in cascade. The HV ____________________________________________ [email protected] Figure 2: Plot of voltage gain vs. frequency. C'1 L1 L2 + ~ Vi + C V0 (-20 kV) VPri VSense - - L O A D ISense Figure 3: Circuit diagram of HV power supply with LCL-T resonant topology. shows two resonant frequencies at 18.38 kHz (f01) and 26 kHz (f02) from light load (Q=9) to rated full load (Q=0.5) respectively. The operating region is shown in Fig. 2 which is from operating point B (10 % of rated load) to A (full load) and switching frequency varies from 36 kHz to 26 kHz respectively. The depicted operation region shows above resonance operation of the converter from light load to full load. This ensures the soft switching (ZVS) for all four IGBT switches of H - bridge VSI because of lagging power factor in all loading conditions. The designed parameters are shown below in table 1. input over current and arc protection are implemented and tested successfully. Table 1: Design Parameters Sr. No. Design Parameter Value 1 Power (Po) 2 kW 2 Voltage (Vo) - 20 kV DC 3 Current (Io) 100 mA 4 L1=L2=L 250 µH 5 C 0.3 µF 6 f01 18.38 kHz 7 f02 26.0 kHz The power circuit diagram of HV power supply with LCL-T is shown in Fig. 3 in which VSI is operated with fixed 50 % duty cycle. The diagonal devices are switched simultaneously to generate square wave voltage which excites the LCL tank circuit. The sinusoidal output of the resonant tank energizes the centre tapped step up HV transformer (1:20:20). The HV transformer then feeds the HV generator to generate required high voltage. EXPERIMENTAL RESULTS - A 100 mA -20 kV H ion extraction power supply has been designed, developed and commissioned in Ion Source Laboratory (ISL) at RRCAT, Indore as per designed specifications shown in table 1. The soft start of the power supply is implemented by sweeping the switching frequency from 40 kHz (point C in Figure 2) to operating point defined by closed loop control. The power supply is controlled in constant current mode and various protections like output over current, output over voltage, Figure 3: Experimental waveform of voltage across and current through the IGBT of VSI Experimental waveform of voltage across and current through the IGBT switch is shown in Fig. 3 which shows the soft switching (ZVS) at turn on. The parasitic output capacitance of the IGBT is used as a lossless snubber capacitor to reduce the turn off losses. The HV power supply which consists of the power converter and HV generator is operated in closed loop control while taking the output current as a control parameter. The voltage gain of the resonant network is controlled by varying the switching frequency which in turn controls the output voltage and hence the output current. The HV generator which is basically a symmetrical CW circuit consists of three doubler multiplier stacks. The measured value of voltage build up ratio (first stack to top) is 3.2 at no load. CONCLUSION The frequency controlled LCL –T resonant converter is well suited for HV power supplies with variable load and the power supply can be operated either in constant current mode (CC) or constant voltage mode (CV) by sweeping the switching frequency in the preferred operating region. Furthermore, the transformer parasitics can be integrated as part of resonant network in LCL –T resonant topology and ZVS at turn on for IGBT switches can be achieved for full load range. ACKNOWLEDGMENT Authors are sincerely thankful to Mr. Deepchand, Mr. Rajesh Nagdeve, Mr. Nathan Singh, Mr. G. H. Ansari and Mr. S. K. Sonawane for their help in fabrication, assembly, testing and commissioning of the HV power supply. REFERENCES  Mangesh Borage, K. V. Nagesh, M. S. Bhatia and Sunil Tiwari, “Design of LCL – T resonant converter including the effect of transformer winding capacitance,” IEEE Trans. Ind. Electron., vol. 56, no. 5, May 2009, pp. 1420– 1427.  S. Dieckerhoff, M. J. Ryan and R. W. De Doncker, “Design of an IGBT – based LCL-resonant inverter for high frequency induction heating,” in Proc. IEEE Ind. Application Conference, vol. 3, 1999, pp. 2039–2045.  R. P. Severns, “Topologies for three element resonant converters,” IEEE Trans. Power Electron., vol. PE-7, Jan. 1992, pp. 89-97.  M. Borage, S. Tiwari, and S. Kotaiah, “LCL-T resonant converter with clamp diodes: A novel constant-current power supply with inherent constant-voltage limit,” IEEE Trans. Ind. Electron., vol. 54, no. 2, Apr. 2007, pp. 741– 746.  A. K. S. Bhat, “Analysis and design of LCL-type series resonant converter,” In Proc. of IEEE International Telecommunications Energy Conference (INTELEC), 1990, pp. 172 - 178.