Single-phase rectifier with novel passive waveshaping filter.

Kazem, Hussein A.

Introduction

DC power supplies are extensively used inside most of electrical and electronic appliances in the world today, such as in computers, monitors, televisions, audio sets and others. They are commonly known as rectifiers (Fig. 1). The nature of rectifiers either it is conventional or switch mode types, all of them contribute to high [THD.sub.i] and low efficiency to the power system. They have the problems of poor power quality in terms of injected current harmonics, resultant voltage distortion and poor power factor at input ac mains and slowly varying rippled dc output at load end, low efficiency, and large size of ac and dc filters, [1-2].

Due to the presence of the considerable distortion power, the power factor of the conventional topology is very low. It is found that the power factor to deliver 1.0 pu power [P.sub.r], input maximum voltage is 1 pu (12 volt) and fundamental frequency is 1 pu (50 Hz), is only about 0.698. This conventional method has many disadvantages, including:

(1) High input current harmonic component and [THD.sub.i] is 55.16% also 3rd harmonic is 49.3%;

(2) Low input power factor, the maximum value of which to deliver 1.0 pu [P.sub.r] is only about 0.698;

(3) Input AC mains voltage distortion because of the associated peak current;

(4) Low conversion efficiency because of large rms value of the input current.

A growing number of current waveshaping methods applied to single-phase rectifier are now available including active and passive methods and selection of the best-suited method for a particular case can be a complicated decision making process, [3].

Among the proposed passive waveshaping methods, the improved topology (Figure 2) proposed by P.D. Ziogas et al. [3] in 1990 is superior to the others in reducing the input current harmonic components and improving the input power factor. On the bases of the Ziogas topology, Ji Yanchao et al [4-5] proposed an improved method (Figure 3), which can further improve the input current waveform and therefore has a better input power factor.

The object of this paper is to propose and analyze a novel single-phase rectifier (Figure 4). Compared with the improved topologies, the novel topology can further reduce the input current THD under rated output power; therefore it can obtain a higher input power factor.

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Ziogas Method For Single-Phase Rectifier

As shown in Fig. 5, the spectrum of such input current ([I.sub.i]) waveforms of the conventional topology clearly shows the presence of a third harmonic component of considerable amplitude which is the main cause for the low input power factor. In 1990, P.D. Ziogas presented an improved single-phase diode rectifier, as shown in Fig. 6. Compared with the conventional topology, the improved topology places a parallel resonant tank composed of a capacitor and an inductor between the AC source and the diode rectifier. The capacitor and inductor are selected so that the input filter presents an infinite (theoretically) impedance to the third harmonic input current component. Consequently, the third input current harmonic component, which affects the topology's performance most, is removed from the input current. Therefore the input power factor is improved effectively. The advantages of the Ziogas method over the conventional method include:

(1) Lower the input current [THD.sub.i], which is about 30.26% also 3rd harmonic is 11.51%.

(2) Higher input power factor, the maximum value of which to deliver 1.0 pu [P.sub.r] is only about 0.931.

(3) Increase efficiency of the rectifier.

[FIGURE 5a OMITTED]

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Yanchao Method for Single-Phase Rectifier

To further lower the input current [THD.sub.i] of the Ziogas diode rectifier, Ji Yanchoa [4] proposed improved method (Fig. 3) by place a capacitor [C.sub.b] in parallel between the parallel resonant tank and the rectifier bridge. The distortion power yielded by waveform distortion has a similar property to reactive power, so that the capacitor will compensate this reactive power. When [C.sub.r] has a value of 7.93[micro]F or 0.39 pu and [L.sub.r] is 140mH or 0.31 pu, the value of [C.sub.b] is selected such that the input power factor at the rated output power reaches its peak value. The input current and voltage waveforms and variation of the input power factor with the value of [C.sub.b] at the rated output power is shown in Figs. 7 & 8 respectively. It is clear from Fig. 8 that for the rated output power, the value of [C.sub.b] should be selected to be 2.5[micro]F or 0.11 pu. Under this condition, the relevant input power factor approaches its maximum value of 0.967.

The advantages of the Yanchao method over the Ziogas and conventional methods include:

(1) Lower the input current [THD.sub.i], which is about 27.80% also 3rd harmonic is 9.59%.

(2) Higher input power factor, the maximum value of which to deliver 1.0 pu [P.sub.r] is only about 0.935.

(3) Increase efficiency of the rectifier.

[FIGURE 7a OMITTED]

[FIGURE 7b OMITTED]

[FIGURE 8 OMITTED]

Novel Single-Phase Rectifier Topology

To further lower the input current [THD.sub.i] of the Ziogas and Yanchao methods diode rectifier, a novel method is proposed in this paper by place inductance [L.sub.o] in series with the output of the rectifier (Fig. 4). When [C.sub.r] has a value of 7.93[micro]F or 0.39 pu, [L.sub.r] is 140mH or 0.31 pu, and [C.sub.b] 2.5[micro]F or 0.11 pu, [L.sub.o] is selected such that the input power factor at the rated output power reaches its peak value.

The input current & voltage waveforms and harmonic spectrum and variation of the input power factor with the value of [L.sub.o] at the rated output power are shown in Figs. 10 & 11 respectively. It is clear from Fig. 11 that for the rated output power; the value of [L.sub.o] should be selected to be 0.35 mH or 0.76 x [10.sup.-3] pu. Under this condition, the relevant input power factor approaches its maximum value of 0.969.

The advantages of the novel method over Yanchao, Ziogas and conventional methods include:

(1) Lower the input current [THD.sub.i], which is about 25.03% also 3rd harmonic is 8.40%.

(2) Higher input power factor, the maximum value of which to deliver 1.0 pu [P.sub.r] is only about 0.969.

(3) Increase efficiency of the rectifier.

[FIGURE 10a OMITTED]

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Table I illustrates a comparison between the four cases. It is clear seen that the novel method have better power factor and less [THD.sub.i].

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System Analysis

The analysis of the Yanchao and Ziogas topology shown in Fig. 2 and 3 is based on the following assumption:

(i) The filter capacitance [C.sub.O] is assumed to be sufficiently large so that the output voltage [V.sub.L] is ripple free constant DC voltage.

(ii) The AC source is considered ideal.

(iii) The losses in inductors [L.sub.r], capacitor [C.sub.O] and the bridge rectifier are neglected.

(iv) The load is modeled as a variable resistance since the effect of high frequency ripple is negligible as per assumption (i).

According to the diode's conduction and turn-off, the novel topology has two operation modes. When two diodes conduct, the operation of the novel topology is similar to Yanchao topology except that the output inductor [L.sub.o] added to the Ziogas inductor [L.sub.r]. Instead of using complicated equation with many assumptions to find the input and output current, which is the disadvantage of Yanchao method due to increasing of the complexity of operation and analysis, simple Kirchhoff's laws used as follows Figure12:

Diodes Turn-on

[v.sub.s] = [Z.sub.r] * [i.sub.1] + [Z.sub.b]([i.sub.1] - [i.sub.2]) (1)

0 = [Z.sub.o] + [Z.sub.L] * [i.sub.2] + [Z.sub.b] * ([i.sub.2] - [i.sub.1]

Diodes Turn-off

[v.sub.s] = [Z.sub.r] + [Z.sub.b] * [i.sub.1] (2)

0 = [i.sub.2]

Where the diode represented by [V.sub.f] and [R.sub.f],. Also, let [R.sub.f] = R[L.sub.r] = [R.sub.Cr] = [R.sub.o] = [R.sub.Cdc] = 0.1 [OMEGA]. Solving simultaneously the above equations lead to find [I.sub.i] (=[I.sub.1]) and [I.sub.o] (=[I.sub.2]). The input current and input voltage solved to find the current waveforms, which is illustrated in Figure 13.

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Design Example and Experimental Results Design example

To illustrate the validly of the simulation analysis in the previous sections, the following design example is presented. The rectifier has the following specifications:

[V.sub.s] = 8.5 rms = 1.0 pu; [P.sub.r] = 500 mW = 1.0 pu; Output voltage [V.sub.L] ripple = 5%. From these values 1 pu angular frequency = 2 [pi] f = 314 rad/sec; 1 pu current = 0.5/8.5 = 59 mA 1 pu impedance = 8.5/0.059 = 143.8[OMEGA] 1 pu inductance = 143.8/314 =457.8mH 1 pu capacitance = 1/(143.8 x 314) = 22.13[micro]F

The value of DC Filter Capacitor [C.sub.o]

The value of [C.sub.o] for 5% harmonic on the capacitor voltage at the optimum operatimg from [3] is given by

[C.sub.o] = 100 x [I.sub.o,2]/2[omega][V.sub.L,o] x 5% (3)

where

[V.sub.L,o] : The dc average value of the output voltage.

[I.sub.o,2] : The rms value of the 2nd harmonic output current.

The value of [C.sub.o] (assuming 5%) can be calculated by using (3). Its value is 102.8[micro]F or 4.61 pu.

The value of ac Compensation Capacitor [C.sub.b] From section-IV: [C.sub.b] = 0.11 x 22.13=2.5[micro]F. The value of dc Filter Inductor [L.sub.o] From the previews section-V: [L.sub.o] = 0.00076 x 457.8= 0.35 mH.

Experimental Results

To verify the predicted results obtained in the previews sections, a 500 mW experimental diode rectifier was implemented with the following circuit parameters: [C.sub.r]= 8[micro]F, [L.sub.r] = 150mH, [C.sub.b] = 2.2[micro]F, [L.sub.o] = 0.3mH, [C.sub.o] = 100[micro]F, [R.sub.L] = 150[OMEGA]. The experimental waveforms of the input voltage and current are shown in Figs. 12, which are obtained under the condition that the output power is 1.0 pu, the input rms voltage is 8.5V and its frequency is 50Hz. Evaluation of Figure14 and 10a shows that the simulation results are in close agreement with the experimental results.

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Conclusion

In this paper, a novel passive input current waveshaping method for single-phase rectifiers has been proposed. The operation of the circuit has been analyzed under steady state, and the relevant waveforms of the input current and voltage obtained from computer simulation and the spectrum of the input current obtained from Fourier analysis have been illustrated. For further reduction in the input current [THD.sub.i] and increase power factor achieved by install a series inductor with the output of the rectifier while installing a parallel capacitor [C.sub.b] between the parallel resonant tank and the rectifier bridge. The validity of the simulation results and the feasibility of the improved method have been verified on a 500 mW laboratory prototype unit.

References

[1] Hussein A. Kazem, Abdulhakeem A. Alblushi, Ali. S. Aljabri & Khmais H. Alsaidi, "Simple and Advanced Models for Calculating Single-Phase Diode Rectifier Line-Side Harmonics", Transactions on Engineering, Computing and Technology, Vol.9, November 2005, pp. 179-183, ISBN 975-98458-8-1.

[2] Hussein A. Kazem, "Input Current Waveshaping Methods Applied to Single-Phase Rectifier", Proceedings of IEEE ICEMS 2007- October 8-11, 2007, Seoul, Korea, pp. 54-57.

[3] Atluri Rama Prasad, Phoivos D. Ziogas & Stefanous Manias, "A Novel Passive Waveshaping Method for Single-Phase Diode Rectifiers", IEEE Transactions on Industrial Electronics, Vol. 37, No. 6, December 1990, pp 521-530.

[4] Ji Yanchao, Liang Xiaobing, Liu Zhuo, Jin Jisheng & Liu Xinhua, "An improved Passive Input Current Waveshaping Method for Single-Phase Rectifier", Industrial Electronics, Control and Instrumentation, IEEE IECON, Vol. 2, 1996, pp 695-699.

[5] Ji Yanchao, and Fei Wang, "Single-Phase Diode Rectifier with Novel Passive Filter", IEE Proc.-Circuits Devices Systems, Vol. 145, No. 4, August 1998, pp 254-259.

Hussein A. Kazem

IEEE Member, Faculty of Engineering, Sohar University,

PO Box 44, PC 311, Sohar, Oman

E-mail: h.a.kazem@soharuni.edu.om

Table 1: PF and THDI for the four Cases.

       Conventional   Ziogas   Yanchao    Novel

PF            0.698    0.931     0.935    0.969
THDi         55.16%   30.26%    27.80%   25.03%
3rd           49.3%   11.51%     9.59%    8.40%