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A New Energy Recovery Circuit for the Capacitive Load using the Current-Balance Transformer J. B. Baek, J. H. Park, B.H. Cho Department of Electrical Engineering, Seoul National University San 56-1, Silim-dong, Seoul, 151-742, Korea E-mail: [email protected] Abstract- This paper proposes a new recovery circuit for the capacitive load. Connecting the one side of windings to the load together, the resonant current is shared in both windings, hence the conduction loss is reduced. Simple circuit structure with transformer achieves the reduced the number of devices. Furthermore, the current-balance transformer enables ZVS operation of the switching device. To confirm the validity of the proposed circuit, prototype hardware with 12-inch mercury-free flat fluorescent lamp is experimented. The results are compared with a conventional energy-recovery circuit in perspective of luminance performances. I. VS SX1 SY1 DX1 DY1 CP DX2 L L SX2 SX4 SY4 DY1 SY2 Fig. 1 Conventional Webber’s ERC INTRODUCTION Cells of the PDP are electrically modeled as an equivalent panel capacitance (CP) because the dielectric and MgO layers cover the electrodes in the unit cell . Recently, research interest toward developing a high luminance and high luminous efficiency mercury-free flat light sources has gained momentum -. It is also modeled as an equivalent panel capacitance (CP). When the sustain voltage (VS), which is the AC square wave with a high peak-to-peak value, is applied to the sustaining and scanning electrodes, energy as much as CPVS2 is lost in each transient. To minimize this energy loss and improve the power efficiency of the plasma discharge system, an energy recovery circuit (ERC) is necessary. . Webber’s type energy recovery circuit (ERC) which employs series resonance is one of the most popular schemes in the industry . Fig. 1 shows the basic scheme of the Webber’s ERC. This ERC uses auxiliary capacitor to recover and inject the energy in the capacitance Cp. Through this series LC resonance, the panel voltage changes with three states such as –Vs, 0 and Vs. The auxiliary capacitor has approximately ten times larger capacitance than a panel equivalent capacitance and it operates as constant voltage source. The voltage is maintained as Vs/2 because recovery and injection has same amount in a period. However, it reveals potential areas of improvements such as the energy recovery efficiency and the complexity of the circuit. It has the high conduction loss due to series-connected bidirectional switches and the possibility of in-rush current by failure of ZVS due to the voltage drop from the MOSFET’s parasitic resistance. Also, it needs extra unidirectional diodes to block the bidirectional voltage. In this paper, a new ERC using the current-balance transformer is proposed to improve the energy recovery efficiency and to reduce the part count of the circuit. Each operational mode of the proposed ERC is explained in the following sections. A prototype hardware using 12-inch mercury-free flat fluorescent lamp (MFFL) is implemented to verify the circuit operation. Its luminance performance is compared with the conventional Webber’s ERC. Experimental result indicates that under low luminance, luminance efficiency was increased by 15%. II. ENERGY RECOVERY CIRCUIT WITH CURRENT-BALANCE TRANSFORMER The proposed ERC has the similar operating sequence with other conventional ERC: capacitive load charging, sustain, energy recovery, and erased-holding. Fig. 2 shows the circuit diagram of the proposed ERC. It has symmetric structure that the load is located between twin bridges for bipolar operation. A current-balance transformer is connected between switches and the load. In this case, the turn ratio of the transformer is 1:1 to balance the current. Using this current-balance transformer, the current is split into each winding in every operational mode. Therefore the current always flow two switches, and conduction loss decreases in half. It reduces the possibility of ZVS failure, because the resonant energy loss decreases due to the on-resistance. In-rush current caused by ZVS failure at the beginning of sustain discharge is reduced, which is critical to the EMI and device life-span. Furthermore, in case of the Webber’s ERC, it needs diodes which are necessary for bidirectional voltage blocking. In case of the proposed ERC, it needs no diodes and current-balance transformer has a filtering function thus it provides a protection from EMI noise. Fig. 3 shows the key waveforms of the proposed ERC. As shown in Fig. 3, there are 8 different operational modes in a cycle, and the first 4 modes are symmetric to the following 4 modes. 1 978-1-422-2812-0/09/$25.00 ©2009 IEEE SY3 SX3 1676 I Cp = VS sin ω0 ( t − t0 ) 2Z 0 (1) VCp = VS (1 − cos ω0 ( t − t0 ) ) 2 (2) where, ω0 = 1 LC p Z0 = L Cp VS SX3 SX1 Fig. 2 Proposed ERC with Auto-Transformer SY3 SY1 CP TX A. Operational Mode Analysis SX4 SX2 x Mode 1 [t0 < t ≤ t1]: TY SY4 X-board SY2 Y-board (a) Mode 1 Before t0, all the bottom switches (SX2, SX4, SY2, SY4) are turned on, and the panel voltage (Vcp) is zero. The load current (Icp) is also zero. At the beginning of Mode 1, Sx1 is turned on, the current is shared in Sx1 and Sx4. The current resonates through the leakage inductance of transformer (L) and the panel capacitance (Cp) as equation (1). Due to the current-balance transformer, the current is distributed to the two switchds with reduced equivalent resistance (Rds_on), and the conduction loss decreases. The transformer-tap voltage reaches Vs and SX3 can be turned on under ZVS condition in Mode 2. Both SY2 and SY4 are kept on, which hold transformer-tap voltage to the ground. SX4 can be turned off under ZCS condition. The panel voltage (Vcp) increases from zero to Vs in resonant fashion. Fig. 4(a) shows the path of conducting current in Mode 1. VS SX1 SX3 SY3 SY1 CP TX SX2 TY SX4 SY4 SY2 SY3 SY1 (b) Mode 2 VS SX1 SX3 CP TX VX1 SX2 TY SX4 SY4 SY2 SY3 SY1 VX3 VX2 (c) Mode 3 VX4 VS VY1 VY3 SX1 VY2 SX3 CP VY4 ICp t0 t1 t2 t3 mode 1 t4 t5 mode 3 VCP mode 2 t6 t7 TX mode 7 mode 5 mode 4 mode 6 SX2 SX4 SY4 SY2 mode 8 (d) Mode 4 Fig. 4 Operational mode diagrams of the proposed ERC Fig. 3 Key waveforms 2 978-1-422-2812-0/09/$25.00 ©2009 IEEE TY 1677 x Mode 2 [t1 < t ≤ t2]: TABLE I COMPARISON OF THE NUMBER OF DEVICES In the previous mode, SX4 is turned off under ZCS condition during the anti-parallel diode on. Mode 2 starts when Vcp reaches Vs. At that moment, the anti-parallel diode of SX3 starts to conduct and SX3 is turned on under ZVS condition. The switches of Y board are still grounded and those of X board maintain Vs. Through the current sharing, the conduction loss also decreases significantly. Fig. 4(b) shows the current sharing path in this mode. Magnetics Storage Capacitor Webber’s 8 4 2 Inductor 4 Proposed 8 0 2 transformer* 0** III. EXPERIMENTAL RESULTS To verify the performance of the proposed ERC, a prototype hardware with a capacitor load has been built. The capacitance is 88[nF] to simulated the MFFL load and the switching frequency is 20kHz. Fig. 5 shows the experimental waveforms of the current (Icp) and the voltage (Vcp) of the load. The figure shows the panel current (Icp) and a winding current (ISX3 – ISX4), and the switches share the load current equally through the transformer winding. In case of Webber’s ERC, the panel current is same as the switching current. However, in proposed ERC, the panel current is splitted up by two winding of the current-balance transformer. It means that conduction loss decreases a half. Fig. 6 shows the ZVS operation in the voltage waveforms of gate-source and drain-source. In Mode 1 and Mode 3, the proposed ERC achieves ZVS turn-on. Fig. 7 shows luminance efficiency comparison at different luminance. In most of operating region, the proposed ERC has higher efficiency than a Webber’s ERC. It is note worthy that the luminance efficiency was increased more at lower luminance region. In this mode, the load voltage starts to decrease and the stored energy in the load capacitance is recovered. The operation is quite similar to Mode 1 except turning on SX for load capacitance discharge. Y board is tied on zero voltage. When SX2 is turned on, the stored charges in the load start moving through SX2 and SX3 with the current-balance transformer. Then the energy starts recovering into the power source by the LC resonance. This mode also includes currentsharing operation and obtains reduction of conduction loss. The current path is shown in Fig 4(c). When the panel voltage drops from Vs to zero under resonance as equation (4), ZCS condition of SX3 is achieved and SX4 also satisfies ZVS condition. The equation of the panel voltage and current is as follows: V (3) I Cp = − S sin ω0 ( t − t2 ) 2Z0 VS cos ω0 ( t − t2 ) 2 Diode * use the leakage inductance of the transformer ** be replaced by input voltage source x Mode 3 [t2 < t ≤ t3]: VCp = Switch (4) x Mode 4 [t3 < t ≤ t4]: After the resonant current becomes zero in Mode 3, the ZCS condition of SX3 is achieved and Vcp reach zero voltage. When the load voltage reaches zero voltage, the diode of SX4 is conducted and that the switch is turned on in ZVS condition. Then, the load voltage maintains zero voltage until new operating cycle begins. Fig 4(d) shows the current path in Mode 4. It can be oscillated when the energy is remained. Sx2 gate waveform Vcp Icp B. Device Count Comparison Using the current-balance transformer, the proposed ERC reduces the number of devices. Table 1 shows the comparison of the number of device count with Webber’s ERC. Diodes used to block the bidirectional voltage are removed in the proposed one. The resonant inductor is replaced by the leakage inductance of the transformer. Because the input voltage source acts as the buck storage device, storage capacitors are eliminated. ISX3 - IISX4 SX3 - ISX4 Fig. 5 Experimental Result using Capacitor load (2μs/div, 100V/div, 10A/div) 3 978-1-422-2812-0/09/$25.00 ©2009 IEEE 1678 REFERENCES Vgs of S x4  ZVS of Sx4  Vds of S x4     Vgs of S x3 ZVS of Sx3  Fig. 6 switch voltage waveforms (0.5[㎲/div] ch.1: [20V/div] ch. 2: [100V/div], ch. 3: [20V/div] )  [lm/W]  32 28  24  20 16  Webber’s ERC Proposed ERC 4000 5000 Luminance [cd/m 2] 6000  Fig. 7 Luminance efficiency comparison  IV. CONCLUSION This paper proposes a new energy-recovery circuit for the AC driving with capacitive load using the current-balance transformer. It improves the luminance efficiency through the current sharing which results in the reduction of the conduction loss. It also helps the ZVS condition which not only improves the efficiency but also overcomes the EMI problem caused by ZVS failure. Furthermore, the proposed ERC reduces the device counts, and thus cost. A prototype was built to verify the performance of the proposed scheme, and compared with a Webber’s ERC.      ACKNOWLEDGMENT This study is partly supported by Samsung SDI, and Research Center for Energy Conversion and Storage. 4 978-1-422-2812-0/09/$25.00 ©2009 IEEE 1679 L. F. Weber, K. W. Warren, and G. S. Weikart, "Quantitative all voltage characteristics of AC plasma displays," Electron Devices ,IEEE Transactions on, vol. 33, pp. 1159-1168, 1986. S. Mikoshiba, "Xe discharge Backlights for LCDs", SID 01 Digest, pp. 286-289, 2001. 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