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A NOVEL HIGH-FREQUENCY MULTIPHASE CROWBARLESS HIGHVOLTAGE DC POWER SUPPLY Mangesh Borage#, Alok Singh, Sunil Tiwari and A. C. Thakurta Raja Ramanna Centre for Advanced Technology, Indore 452013, India Abstract A novel topology based on high frequency switching resonant immittance converters (RICs) is proposed in this paper. The principle of operation, design, simulation and experimental results on a -20 kV, 1 A dc prototype power supply, that uses a three-phase RIC operating with 120o phase shift involving switching at 25 kHz and a dc-dc step down converter with energy recovery snubber in the front-end, is presented. INTRODUCTION High voltage (HV) dc power supplies are required for myriad of applications such as X-ray tubes, electric discharge machining, corona discharge processes, electrostatic precipitators, ozone generation, electrostatic accelerators, electron guns, klystrons, modulators, vacuum pumps, beam diagnostic devices, low temperature plasma generation, lasers, pulsed power supplies, etc. Design of a HV power supplies is complex because of non-negligible effect of circuit parasitic components, e.g. high leakage inductance and winding capacitance associated with the transformer. Besides, partial discharge and arcing can occur frequently in HV loads. HV power supplies required for sensitive and expensive loads like klystrons need to have minimum energy storage in the output filter as stored energy is dissipated in the load in case of arcing. Fast closing switches (spark gap, thyratron, ignitron, light activated thyristors) operating in parallel to the klystron path (known as crowbars) are used for protection. To avoid false firing and reliability issues of crowbars, a crowbar less power supply is preferred, wherein stored energy in the power supply is kept less than the maximum energy handling capability of klystron. In order to reduce stored energy in the power supply, the value of output filter capacitor must be reduced. To achieve this without sacrificing the output voltage ripple, a way to reduce the output ripple amplitude and increase the ripple frequency must be built in to the power converter circuit. This often calls for the use of highfrequency power converter. Use of single power converter is practically not feasible, particularly in the case of highpower applications, because the required switching frequency becomes prohibitively high. Instead, many converters, with their outputs connected either in series [1] or parallel [2] and operating in phase-staggered manner, have been proposed for applications demanding a crowbarless power supply. Alternatively, a technique known as pulse step modulation, in association with many low voltage switch-mode converters connected in series, ___________________________________________ #[email protected] has been successfully implemented [3]. In these techniques, the phase staggered operation is mainly responsible for reduction of the output filter capacitor and associated stored energy. However, the power supply, being a voltage source, is not inherently immune against arcing and short circuits. In this paper a new approach is suggested, wherein it is proposed to use phase staggered current sources in parallel instead of the conventional arrangement of series-connected voltage sources. The principle of operation, design, simulation and experimental results on a -20 kV, 1 A dc prototype power supply are presented. THE CURRENT SOURCE Resonant converters (RCs) have been a potential candidate in many power electronics applications due to soft switching, high frequency operation, high efficiency, small size, etc. A new family of RC, known as resonant immittance converters (RICs) is recently proposed [4] that can transform a voltage source into a current source. Salient merits of RIC are: Easy parallel operation without any complex control for equal current sharing Modularity and redundancy In-phase bridge output voltage and current under all loading conditions – lowest conduction loss and softswitching is guaranteed Inherently short-circuit proof – device current drops to very small value under load short-circuit condition. Constant-frequency operation – simplifies the design of magnetic and filter components. Gainful utilization of transformer parasitic components. In all 24 RIC topologies with three and four reactive element, the circuit shown in Fig. 1 is chosen as the basic building block for the presented power supply, mainly because it gainfully utilizes the transformer leakage inductance and winding capacitance into the resonant network [5]. It is a half bridge converter followed by a network of reactive elements feeding to a transformer, rectifier and filter. Defining, o 1 L1C1 , n o , L2 L1 , C2 C1 and Z n L1 C1 , it has been shown that the converter operates as a current source Io Cdc S1 D1 L2 Vd L1 C1 Dc1 1:n Dr1 + Vo C2 Cdc Dr3 Cf RL Tr S2 D2 Dc2 Dr2 Dr4 Figure 1: Circuit diagram of a half-bridge RIC. - when designed with the condition 1 and when it is operated at ωn=1. The current gain, nI o Z n Vd , under these condition is equal to 8 2 . Addition of two clamp diodes Dc1 and Dc2, as shown in Fig. 1, impart inherent constant-voltage characteristics to the circuit [6]. PHASE-STAGGERED, PARALLELCONNECTED CURRENT SOURCES Phase-staggered operation of power converters connected in series or parallel on the input and output side can reduce the ripple amplitude and increase the ripple frequency, thereby reducing the filter, without increasing the switching frequency. Since a RIC is inherently a current source, it lends itself for easy paralleling without any additional care for equal current sharing, as illustrated in Fig. 2. For m identical modules operating in parallel at the same switching frequency but with phase shift θ, the components of rectified output current are: 4I 2I it t m m m h 2, 4,... 1 cos nt x h 1h 1 x 0,1,2,... m1 (1) The dc components of individual modules directly add independent of θ and the total dc current of the paralleled modules is m times the output current of one module. However, harmonics can selectively get cancelled in the total current depending on θ. CA SA 1:n Dr1 Tr SB Dr2 Table 1: Specifications and component details Parameter Value Output -20 kV, 1 A Ripple and regulation <0.5 % Stored energy <10 J Input 415 V, 3-ph., 50 Hz Dr3 Resonant Immittance Network (RIN) CB In the three-phase RIC, each half-bridge IGBT operates at 25 kHz. However, the operation of each half bridge is phase shifted by 120o. Three identical resonant immittance networks composed of inductors L1A, L1B, L1C as well as L2A, L2B, L2C and external capacitors C1A, C1B, C1C have been used. The capacitors C2A, C2B and C2C (nearly 70 nF) are in fact the parasitic winding capacitors of the respective transformers. Similarly, the leakage inductance (nearly 3 µH) is absorbed as the part of resonant inductors L2A, L2B, L2C. The step-up transformers are Y-Y connected and have turns ratio of 1:50. At the output, three-phase voltage doubler rectifier is used to get the required high voltage. Three sets of clamping diodes, Dc1A Dc2A, Dc1B Dc2B and Dc1C Dc2C are used to clamp the open circuit output voltage passively at the designed value. Semikron make SKM100GB128D IGBTs are used. Ultrafast diodes DSEI 2 X31 - 100 are used as the clamp diodes. In the output rectifier, 40 nos. of MUR4100 diodes are connected in series to realize each of the diode stack used in high voltage rectifier. Eight capacitors rated for 0.22 µF/ 5000 V are connected in series to form the output filter capacitor. This capacitor stores only 5.5 J at 20 kV, enabling the crowbarless operation. The output voltage sensing is done using compensated resistive voltage divider and output current is sensed indirectly by measuring transformer primary current. Dr4 Lf Figure 2: Parallel connected RICs. Db 415V, 3 Ph Mains Cf Cd1 Ds3 Cs1 Rd1 PROTOTYPE POWER SUPPLY A prototype power supply has been developed using three-phase RIC with clamp diodes. Major specifications are summarized in Table 1 and the schematic diagram is shown in Fig. 3. In the front-end, three phase diode rectifier and a damped LC filter generates unregulated dc bus. A dc-dc step-down converter is used for the control and regulation of output high voltage, which is accomplished by changing the output dc voltage using PWM. To reduce the switching loss in the dc-dc converter, an energy recovery snubber, composed of only passive components (Ls, Cs1, Cs2) and diodes (Ds1, Ds2, Ds3) has been implemented [7]. The switching frequency of IGBT in the dc-dc converter is 25 kHz. The output filter capacitor is divided into two parts (Cdc) so that it can also perform as the split capacitors for the subsequent half-bridge converter stages. Ldc Sb Ls Lf Cs2 S1B D1B Rd2 Cdc Ldc Dc1A Dc1B Dc1C S1A D1A Cdc Ds2 Ds1 Vdc(+) Vdc(+) Cd2 Vdc(-) Dr1A Dr1B Dr1C S1C D1C Cf1 L2A L1A C1A L2B L1B C2A TrA L1C C2B TrB C1B L2C C1C C2C TrC S2A D2A Vdc(-) S2B D2B Cf2 S2C D2C Dc1A Dc1B Dc1C Dr2A Dr2B Dr2C Figure 3: Circuit diagram of the power supply. HV Va Va Ia Ia Vb Vb Vc Vc A (b) B C (d) (c) 3.5 3.0 23V Arc Energy (J) @ 15 kV DC 15V @ 10 kV DC voltage current 2.5 2.0 1.5 1.0 0.5 @ 5 kV DC 8.5V 0.0 0 50 100 150 200 time (us) 250 300 (f) (e) (g) (a) Figure 4: (a) Photograph of the power supply. (b) IGBT bridge output voltage and current waveforms under loaded condition. (c) IGBT bridge output voltage and current waveforms under output short circuit condition. (d) Transformer primary current waveforms under output short-circuit condition showing balanced operation and equal current sharing (e) Output ripple voltage waveforms under different settings. (f) Output voltage and current waveforms under arc condition. g Calculated arc energy under arc condition. EXPERIMENTAL RESULTS Figure 4(a) shows the photograph of the power supply. It is housed in a 0.6 m X 0.8 m cabinet of 32U height. Only semiconductors are mounted on water-cooled heatsink. All magnetic components, except for the high voltage transformers are air-cooled. The high voltage transformers, rectifiers and filter capacitor are placed in an oil tank for insulation and cooling purpose. The circuit functionality has been confirmed under various steady-state and transient operating conditions. Fig. 4(b) shows the waveforms of three IGBT halfbridges and the current in one of the half-bridge under the loaded condition. It can be seen that voltage waveform is clean and free from any ringing. This is because of the soft-switching operation of the IGBTs which is evident from the voltage and current waveforms, which are nearly in phase. The similar set of waveforms under output short-circuit condition is shown in Fig. 4(c). Here it is important to note that the soft-switching is still maintained and the bridge output current has dropped to a very small value as compared to the case when the converter was loaded. This demonstrates that the converter can be safely operated under load short-circuit conditions. The transformer primary current waveforms under output short-circuit condition demonstrating balanced operation and equal current sharing are shown in Fig. 4(d). The high frequency output ripple under loaded condition was measured at different output levels as shown in Fig. 4(e), which is seen to be well within the design specifications. The transient performance of the converter was examined under simulated arcing conditions created by using sphere-gap. Fig. 4(f) shows the output voltage and current waveform under the output short-circuit condition created when the converter was operating at -15 kV output. A small inductor has been used in series with the output to limit the peak discharge current of output filter capacitor within its safe limits. The recorded voltage and current waveforms are then used to calculate the energy deposited in the arc. The results shown in Fig. 4(g) confirm that the experimental arc energy of 3.11 J is very close to the calculated value of 3.09 J at -15 kV output. It is possible to make the power supply with positive output voltage polarity just by changing the way output rectifier diodes are connected. The number of phase can be increased with suitable phase shift to further reduce the output ripple. Also, the same power supply can be configured to function as a capacitor charging power supply by making suitable changes in the control circuit. ACKNOWLEDGMENT The authors acknowledge the contribution of Shri P. Renukanath and his team at PSIAD, RRCAT in the development of high voltage transformers used in the prototype, and, Shri Vinod Somkuwar for his assistance during assembly, wiring and testing of the power supply. REFERENCES [1] H. Kwon et al., "Klystron power supply for KOMAC," APAC'98, http://www.JACoW.org [2] H. Kozu et al., J. Synchrotron Rad. 5 (1998) p. 374-375. [3] J. Alex et. al, “A new klystron modulator for XFEL based on PSM technology," PAC’07, http://www.JACoW.org [4] M. Borage, et. al, IEEE Trans. Ind. Electron, 58(3) (2011) p. 971-978. [5] M. Borage, et. al, IEEE Trans. Ind. Electron, 56(5) (2009) p. 1420-1427. [6] M. Borage, et. al, IEEE Trans. Ind. Electron, 54(2) (2007) p. 741-746. [7] A. Singh, et. al, Adv. Power. Electron., 2012 (2012).