Voltage Regulation Characteristics of Transformer-less Uninterruptible Power Supply without Voltage Divide Capacitor

Transcription

1 Voltage Regulation Characteristics of Transformer-less Uninterruptible Power Supply without Voltage Divide Capacitor Atsushi Hirota The Electrical and Computer Engineering Akashi National College of Technology Akashi, Japan Bin Guo Panasonic Kusatsu, Japan Saad Mekhilef University of Malaya Kuala Lumpur, Malaysia Mutsuo Nakaoka The Electric Energy Saving Research Center, University of Malaya / Kyungnam University Kuala Lumpur, Malaysia / Masan, Korea- South Abstract Power supplies prepare for a power failure often apply converters in the input side, and use inverters in the output side. The input side and the output side of power supply are coupled by high frequency transformer. This method requires a transformer. Because the switch devices included in the power flow increase, the power conversion efficiency declines. Though, high frequency transformer-less circuits are proposed, these circuits often use voltage divide capacitors. And these schemes require the voltage balance control of the voltage divide capacitors. This paper mentions about the transformer-less uninterruptible power supply which can omit the voltage divide capacitors. Delta-sigma modulation technique is introduced to the power supply and the proposed scheme has output voltage regulation function Keywords transformer-less; voltage divide capacitor; output voltage regulation; PI controller I. INTRODUCTION Power supplies prepare for a power failure widely use the combinations of converters and inverters. But this method has a necessity of high frequency transformer, and the switch devices contained in the power flow increases. The demand for cost cutting grows and transformer-less circuit is suggested. To avoid unstable voltage of a load side, this method uses a common line between the input side and the output side. In the transformerless method, the capacitors, which divide dc bus voltage, are often used. This paper treats the transformer-less circuit which does not use dc voltage divide capacitors. The circuit uses one energy storage capacitor. Compare to the combination of a converter and an inverter, the scheme mentioned here reduces the switch device number included in the power flow. Therefore, the power conversion efficiency would improve. To produce the output voltage, delta-sigma modulation scheme is introduced and output voltage regulation characteristic is realized without PI controller. Hysteresis band control is applied to the input current control. In comparison with PWM controlled method, it is mentioned that the proposed technique can regulate output voltage without PI controller designing. II. TRANSFORMER-LESS TECHNIQUE To suppress the load voltage variation from the earth, isolation transformer is used for uninterruptible power supplies. Because the low frequency transformer is heavy and large, an interruptible power supply with this tends to large. With the power electronics technology advances, high frequency operation can be achieved and small sized high frequency transformer can be applied. But the demand for small sized and reduced costs grew, therefore, isolation transformer-less type power supply is used. The transformer-less type power supply shares one common line between the input side and the output side, and suppresses the load voltage variation from the earth. Fig.. represents these type power supplies. III. BASIS OF DELTA-SIGMA MODULATION The basic block diagram of delta-sigma modulation circuit is shown as Fig. 2. The modulation circuit consists of a quantizer and a one sample delay. When the quantizer input is larger than the threshold level, the modulator puts +, and the quantizer input is smaller than the threshold level, the modulator puts -. The error signal e i between the quantizer input and the quantizer output is feedbacked to the input side with one sample delay. As a result, the relation between the input signal sequence x i and the output signal sequence y t is as follows

2 TABLE I Rectifier Load LIST OF SYMBOLS USED IN THIS PAPER Isolated High Frequency Converter vc vc2 Load (c) Fig.. Various types of uninterruptible power supply: Low frequency transformer; High frequency transformer; (c) Transformer-less scheme. Quantizer (error ei) Symbol Definition ffnd Fundamental frequency (6Hz) fs Sampling frequency of delta-sigma modulator (Multiple of fundamental frequency) fc Carrier frequency of PWM (Multiple of fundamental frequency) Vin Amplitude of input voltage (V) ein Instantaneous input voltage (V) Vref Amplitude of output reference voltage (V) Vofnd Amplitude of output fundamental component (V) iref input current reference signal di input current hysteresis band L Input side energy storage inductance L2 Load side inductance C Energy storage capacitor C2 Load side capacitance R Load resistance Q~Q6 Switch device Nsw Q, Q2 switching number in one fundamental cycle Nsw3 Q3, Q switching number in one fundamental cycle Nsw5 Q5, Q6 switching number in one fundamental cycle Input (xi) z - - Output (yi) Fig. 2. Basic block diagram of delta-sigma modulation circuit. y i = x t + (e i - e i- ) () According to this equation, the output signal is composed of the input signal and the diversity of the error noise. In case of analog to digital conversion, the quantizer error noise components distribute evenly in general, the error distribution changes by using delta-sigma modulation and Fig. 3. represents the characteristics. If the sampling frequency is settled enough larger value comparing to the reference signal frequency, the quantization noise around the reference signal frequency could be suppressed. Table I lists the symbols used in this paper. IV. PROPOSED CONVERTER Half bridge circuit composition is used for an interruptible Signal Band Amplitude Modulation Noise of Delta-sigma Modulator Quantization Noise of PWM Frequency Fig. 3. Quantization error noise distribution of delta-sigma modulator. power supply without isolation transformer. Though this construction is simple, voltage divide capacitors are necessary and these voltages should be balanced. Also, high voltage rating switch device is required. This paper mentions about the circuit shown in Fig.. This circuit uses one energy storage capacitor connected to dc bus line and does not need to watch voltage unbalance. To produce the output voltage, delta-sigma modula- Q Q2 il iref vref L ein Q3 Q Hysteresis Controller Amplitude z - vc vc2 Ro C2 il2 L2 Mode Dtermine Logic output voltage Fig.. Proposed uninterruptible power supply: Transformer-less circuit without voltage divide capacitors; Control block diagram. Q5 Q6 Q Q6 C vc

3 tion technique is applied. The output voltage is used as the feedback signal of the delta-sigma modulator. Using this scheme, the output voltage is controlled to the reference value. The input current is controlled by hysteresis comparator. Table II shows the converter switch state, and the input side and the output side state. In the input side, the storage mode which storages energy to the input side inductor, and the transfer mode which transfers the energy from the input side inductor to the load, exist. When the input voltage e in is positive, the output voltage is positive value or zero. And when ein is negative, the output voltage is zero or negative value. Table III indicates switching mode transitional conditions. The input current is controlled within the hysteresis band of the reference signal. When e in is positive, and if the input current reaches the reference + di, the input mode changes to the transfer mode, and if the input current reaches to the reference signal - di, the input mode changes to the storage mode. The same operation is practiced when e in is negative. In the output side, when e in is positive and the deltasigma modulation output is positive, the output voltage puts positive value, and if the delta-sigma modulation output is negative, the output voltage puts zero. And when e in is negative and the delta-sigma modulator output is positive, the output voltage puts TABLE II STATES OF THE P ROPOSED CONVERTER SWITCH STATE; INPUT AND OUTPUT S TATE Mode on state switch Q Q3 Q5 7 Q2 Q Q5 Q Q Q6 3 Q Q Q5 8 Q2 Q Q6 2 Q Q3 Q6 5 Q2 Q3 Q5 6 Q2 Q3 Q6 Mode Conditon ein L energy vc2 + storage 7 + storage + + transfer 3 + transfer storage 2 - storage transfer 6 - transfer - ein >= il > iref + di il < iref - di iref - di <= il < iref + di TABLE III OPERATIONS OF THE PROPOSED CONVERTER L transfer mode mode 3 mode L storage mode mode 7 mode mode 7 (L storage) mode 3 (L transfer) mode (L storage) mode (L transfer) zero, and if the delta-sigma modulator output is negative, the output voltage puts negative value. V. ANALYTICAL CONDITIONS AND RESULTS About the proposed method and the conventional PWM scheme, the analyses are implemented under the conditions in Table IV. Fig. 5. illustrates the waveform of the output voltage. The switching operation number of the proposed scheme seems small. Fig. 6. depicts the input current of the two converters. The both currents of the converters are controlled within the reference signal hysteresis bands. The spectrum distributions are studied about the converters. Fig. 7. expresses the output voltage spectrum. Though the fundamental frequency component of the PWM scheme is not controlled to the setting value, the fundamental voltage component is controlled to the reference value in case of the proposed scheme. The switching noise peaks are same in the both converter, the same performance are kept from the TABLE IV ANALYTICAL CONDITIONS Symbol Condition fs 52 fc Vin (V) Vref (V) di.2 (A) L 2 (uh) L2 (uh) C 5 (uf) C2 (uf) R (ohm)

4 x x Fig. 5. Output voltage waveforms: Proposed scheme; PWM scheme. Fig. 7. Spectrum distributions of the output voltage: Proposed scheme; PWM scheme x x Fig. 6. Input current waveforms: Proposed scheme; PWM scheme. Fig. 8. Spectrum distributions of the input current: Proposed scheme; PWM scheme. point of keeping noise regulations. The spectrum of the input current is shown in Fig. 8. Owing to hysteresis control, the input current harmonic components of the both converters are suppressed. Next, FFT analysis is carried out about the output voltage, and the result is indicated in Table V. The fundamental component is controlled to the settled value in case of the proposed scheme, but the fundamental component is not controlled in case

5 TABLE V FUNDAMENTAL COMPONENT OF THE OUTPUT VOLTAGE Proposed PWM Vofnd (V)..7 TABLE VI SWITCHING NUMBERS OF THE CONVERTERS Proposed PWM Nsw 68 Nsw Nsw5 2 5 of the PWM scheme. From this result, the proposed method has output amplitude control feature. Table VI shows the switching numbers of the both converters. The switching numbers of the proposed converter are small, therefore, the switching loss could be reduced in case of the proposed scheme. From these results, the proposed method needs small switching operation number and has output voltage control function. Furthermore, the converter can omit PI controller gain designing. VI. CONCLUSIONS This paper mentioned about the transformer-less uninterruptible power supply. The circuit used one energy storage capacitor and could omit voltage divide capacitors. The circuit could reduce switch device within the power flows and this contributed to power conversion efficiency. In the circuit, to produce the output voltage, delta-sigma modulation technique was applied, and output amplitude control function was achieved without PI controller gain designing. Compare the proposed method to the conventional PWM scheme, it was clarified that the method could control output voltage with small switching operation. REFERENCES [] Atsushi Hirota, Satoshi Nagai, and Mutsuo Nakaoka, Performance Evaluations of Delta-Sigma Modulated Voltage Source, Proc. of IEEE International Conference on Power Electronics and Drive Systems (PEDS'99), pp , [ 2] Atsushi Hirota, Satoshi Nagai, and Mutsuo Nakaoka, A Novel Delta- Sigma Modulated DC-DC Power Converter Utilizing Dither Signal, Proceedings of Power Electronics Specialists Conference (PESC ), pp , 2.6 [3] Satoshi Nagai, Atsushi Hirota, and Mutsuo Nakaoka, Single phase deltasigma modulated PFC power converter introducing auxiliary switch, ' Korea- Japan Joint Symposium on Advanced Industry Applications, pp-, 2. [ ] J. C. Candy and G. C. Temes, Over Sampling Delta-sigma Data Converters -Theory Design and Simulation, IEEE Press, 992. [ 5] S. R. Norsworthy, R. Schreier, and G. C. Temes, Delta-sigma Data Converters -Theory, Design, and Simulation, IEEE Press, 997.