改进具有功率因数校正方案降压型变换器的控制策略.pdf

IMPROVED CONTROL STRATEGY ON BUCK-TYPE CONVERTER WITH POWER FACTOR CORRECTION SCHEME , Katsuya Hirachi“, Touru Iwade*, Takanori Mii*, Hidenobu Yasutsune* and Mutsuo Nakaoka* *Research and Development Division, The 2nd Takatsuki Plant, Yuasa Corporation 2-3-21 Kosobe-cho Takatsuki-City Osaka, 569, Japan *Department of Electrical and Electronics Engineering, The Graduate School of Technology, Yamaguchi University 2557 Tokiwa-dai Ube, Yamaguchi, 755, Japan Buck-Type high-power-factor PWM active converter topology is not only sufficiently capable of the effective elimination of input current harmonics, but also its characteristics include high efficiency, absence of inrush current, the capability to obtain low DC output voltage, ability to protect against short circuit, and so force. The buck-type high-power-factor converter has the inherent capability to become attractive power supplies for telecommunications energy systems. On the other hand, because this type of converter must employ high-inductance reactors, these tend to increase equipment in size and weight, and this has prevented their widespread use. This paper presents a new control strategy for Buck-type high-power-factor PWM converter which can reduce the size and weight of reactor and also eliminate the ripple component included into output voltage. The operating principle and simulation results are described. Introduction High-powerfactor converters are classified into three types: buck-type, boost-type, and buck-boost-type topologies. Fig.1 shows the typical non isolated circuit configurations for these three types. All three operate on the basis of the processing of accumulating energy in reactor L1 while the switching device T1 is ON, and transferring reactor energy to the capacitor C1 when the switching device T1 is OFF. Appropriate control of the reactor current forms input current Iin into a sinewave with the same phase as the utility-grid voltage Vin. In the case of boost-type and buck-boost-type converters, the AC input voltage is directly applied across the reactor U, when the switching device T1 is conducting. But in the case of buck-type converter, the voltage applied to the reactor is a (a) Buck-Type F i g . 1 Typical Non-Isolated dBerence between the absolute value of the AC input voltage and the DC output voltage. Thus, it is possible to accumulate energy at all times in the reactor L1 of the boost-type and buck-boost-type converters, while in the buck-type it is impossible to accumulate energy in the reactor when the absolute value of the input voltage is lower than the output voltage, i.e., around the zero cross point of the input voltage. For this reason, the buck-type makes it necessary to accumulate sufficient energy in the reactor in order to provide the energy needed when the absolute value of the input voltage is low. This means that the buck-type requires much larger inductance than that of boost-type or buck-boost-type, and larger inductance increases physical size and weight of the reactor. This makes it necessary to reduce the reactor inductance as much as possible in buck-type high-power-factor converters, but to reduce the inductance brings an increase in reactor current ripple, which causes a large amount of distortion in the AC input current. In order to solve this problem, a control strategy defmed as pulse area modulation by which hardly any distortion i s arised in the input current even when the reactor current contains a large ripple. Operation of Buck-Type High-Power-Factor Converters Fig.2 shows the circuit configuration of the buck-type high-power-factor converter. Reactor Lout has an inductance of adequate magnitude, and current I(Lout) keeps continuous mode. When T1 is ON, current takes the following path, Vin - + D1 + T1 + Lout - + C1 - D4 + Vin, and input current l(Vin) becomes equal to I(Lout). ) Boost-Type Circuit Conftgurations for Three Types of High (c) BUck-BWSt-Type -Power-Factor Converters 0-7803-2795-0 826 When T1 is OFF, I(Lout) circles through the following path, Lout -+ C1 - Df - Lout, which makes the input current I(Vin) zero. Thus, when the value of Lout is sufficiently large and the ripple of its current is negligible small, the converter employs the control circuit shown in Fig.3, which compares the sine wave in the utility grid to the sawtooth wave carrier. By this procedure, the switching device is controlled with PWM strategy, and the input current is controlled to so as to become a perfect sine wave. i vout Vref Fig.2 Main Circuit Codguration of Buck-Type High-Power-Factor Converter to T1 Fig3 Conventional Control Circuit Codgyration Fig.4 shows the simulated waveform. Waveform V(20), which is a rectified sinewave with the same phase as the input voltage, is compared with the sawtooth wave V(lO), and the drive signal of switching device T1 is generated. This results in the creation of the input current waveform I(%) shown in Fig.4. Fig.5 shows the result of the fourier analysis of the waveform I(vin). All the harmonic components are under 2%. Fig.4 Simulation Analysis with no Ripple Current at Reactor Lout 0 2 3 4 5 6 7 8 91011121314151617181920 Harmonics o r d e r s Fig5 Fourier Analysis of Input Current I(%) with no Rippie Current at Reactor Lout The simulation assumes as an operating frequency of 2 kHz in order to make the PWM control easier to understand. In actual circuits, the operating frequencies are set at high levels of several dozen kHz, and the high frequency components in the input current I(Vm) can be easily removed with a small filter. But when the ripple current of I(Lout) cannot be negligible, employing the control strategy in Fig.3 brings -an input current waveform distortion that corresponds to the magnitude of the ripple current. Fig.6 shows the simulation results when the ripple current of I(Lout) cannot be negligible. In this case, reactor current I(Lout) contains a ripple current of BAP-p. As a consequence, input current I(Vin) has the waveform shown in Fig.6. Fig.7 shows the result of the fourier analysis of the waveform I(Vin). There is a large third harmonic component of about 13.5%. Simulation specifications are shown in Table 1. Fig.6 Simulation Analysis with LaxpERipple Current at Reactor Lout . Pulse area modulation Control circuit implementation and control strategy Even if the reactor current I(Lout) contains a large ripple, shaping the input current into a sinewave requires that the pulse width are appropriately controlled according to the instantaneous value of reactor current. Pulse area modulation which modulates the area of the current pulse that passes 827 The circuit also compares the DC output voltage Vour to the reference voltage Vref, and uses the multiplier to control the amplitude of V(20), t making it possible to control Vout at a constant voltage Fig9 illustrates the principle of the pulse area modulation which is employed in this control circuit. Because the sawtooth wave V(10) used in the modulat the integrating reactor current (Lout), its proportionally to Iwut). When reactor current gradually increases, the current becomes a sawtooth wave whose gradient increases gradually as shown in Fig.9. Assuming that the reference wave V(20) has a constant voltage as shown in Fig.9, the duty ratio of T1 gradually decreases. So, the input current waveform I(%) becomes square wave in which peak value gradually increases gradually decreases as shown in Fig.9. The pulse shown with hatched lin value as that shown with dotted area, pulse width with equal area. If the r constant, the areas of these pulses will reference waveform increases or decreases, the pulse areas increase or decrease proportionally. And pulse area is equal to the instantaneous value of the input current I(Vin). Thus, if the reference waveform i s changed into a sinewave as shown in Fig& the input curr through a switching device i s suitable for this kind of control scheme. 9 2 3 4 5 6 7 8 91011121314151617181920 Harmonics Orders Fig.7 Fourier Analysis of Input Current I(Vin) with Large Ripple Current at Reactor Lout Pulse area modulation being introduced to a buck-boost- type circuit has been proposedl. But it would seem that applying the pulse area modulation to buck-type converter circuits have significant advantage than applying it to the buck -boost-type circuit whose reactor operates at high frequency2. Fig.8 shows the proposed control circuit implementation include the pulse area modulation. Reactor current (Lout) is detected at shunt SH1, whose voltage V(SH1) is amplified and fed into the integrating circuit. This integrating circuit is reset at a constant interval, and its output is the sawtooth wave V(lO), which has a gradient that is proportional to the value of I(Lout). T h i s sawtooth wave is compared to the reference wave V(20), which is specified by the fill-wave rectified input voltage V1, thereby obraining the P W M wave that drives the switching device T1. . / I , . t o T1 Integrating Circuit Drive V(SH1) Amplifier Multiplier VOUt Reference Wave lvinl vfefq-i I F i g . 8 Proposed Control Circuit Configuration F i g . 9 Principle of Pulse Area Modulation Simulation results u s b the DUI se area mod ulatiog he pulse area modulation. Simulation specitications 89 Table 1. It is obvious that the frequency of wave is constant and its gradient is proportional to the reactor current Fig.10 shows some simulation result I(Lout). Fig.11 shows an another simulation pulse area modulation. The peak value of input current I(Vm) is equal to the reactor current I(Lout). And the pulse width of I(%) is controlled so that the area of each pulse can be changed in accordance with the AC input voltage V(2,l). Fig.12 shows the result of a fourier analysis on the input 828 current waveform I(Vin) as shown in Fig.11. Harmonics ExDerimental results of proposed circuit components are suppressed much more than in Fig.7 in which pulse area modulation is not used. Just as in Fig.4 and Fig.6, the operating frequency for simulations in Fig.10 and Fig.11 is set at 2 kHz to make the operation easy to understand. In practical circuit, the operating frequency is set at high frequency of several tens of kHz, and the high-frequency Fig.13 and Fig.14 show the waveforms of small capacity active converter breadboard ivhich is controlled on the bases of the pulse area modulation strategy. Although the reactor current have large ripple component as shown in Fig.13, the input current has almost no distortion as shown in Fig.14. components in the input current I(Vii) are easily removed with a small filter. EL Bv m 9 85ms alm S m S 0 U(1B) * U(2Clo) Tim Fig10 Control Circuit Waveform under Pulse Area Modulation scheme 8oms 84ms 88ms 92m 95ms l00ms 0 -1(Uin) Time Fig.11 M a i n Circuit Waveform under Pulse Area Modulation scheme 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Harmonics O r d e r s FIJ2 Fourier Analysis o f Input Current I(Vin) under Pulse Area Moduration Scheme - 2mseJdiv Input Voltage: sOV/div Reactor Current: 5Ndiv Fig.l.3 Measured Reactor Current and Input Voltage Waveforms - ov - ov Input Voltage: 50Vldiv _* 2mseddiv Input Current: 5Ndiv Fig.14 Measured Input Voltage and Inpu! Current WaveBorms Elimination of ripple voltage Control circuit configuration and control strategv In power supplies for telecommunications energy systems, preventing the noise of communication equipment makes it necessary to suppress the ripple voltage in the DC output voltage to a sufficiently small value. But in single- phase input high-power-factor active converters, the reactor current ordinarily includes a large amount of ripple current whose frequency is twice that of the utility grid AC frequency, and for this reason, a ripple voltage with a frequency twice that of the utility grid AC also appears i n the output voltage. Buck-type high-power-factor converter has also a large amount ,ef ripple voltage in DC output voltage. kig.15 shows 829 the simulation results of the reactor current I(Lout) and the DC output voltage V(7) under the conditions in Table 1. Reactor current I(Lout) includes a ripple current of 28Ap-p with 100 Hz, and DC output voltage V(7) includes 0.74Vp-p ripple voltage. Tim Fig.15 Waveform of Output Voltage Ripple Fig.16 shows the newly proposed circuit configuration of the buck-type high-power-factor converter with auxiliary switching circuit which suppress the output voltage ripple. Auxiliary switch T2 in series with diode D5 are connected in parallel to the reactor Lout. When T2 is ON, the reactor current circulates through T2 and D5, but when TZ is OFF, the reactor current is supplied to C1. T h i s means that the circuit in Fig.16 has the three operational modes listed in Table 2 in accordance with the state of switching devices T1 and T2. The mode i n which both devices are O N should be removed. - Fig.16 New Main Circuit Collrguration of Buck-Type High-Power-Fador Convertex with A u x i l i a r y Circuit Table 2 Three operational modes Number Current N OFF Increase Increase FF OFF Decrease Decrease As mentioned above, the duty ratio of T1 is to be determined by the pulse area modulation, and the input current is formed into a sine wave. The duty ratio of T2 is to be also determined by the pulse area modulation, and the pulse width is determined so that the ripple voltage of DC output voltage will be eliminated. Fig.17 shows the proposed circuit configuration which includes a control circuit with auxiliary switch T2. The gate Drive signal of T1 can be produced in the same way as in Fig.8. The gate drive signal to T2 is to be produced by comparing the output voltage V(10) of the integrating circuit and the control voltage V(30). When V(10) is larger than V(30), the auxiliary switch T2 closes and stops the delivery of power to C1. When V(10) is smaller than V(30), the auxiliary switch is OFF, and the power is supplied to C1. Because the control voltage V(30) i s a constant voltage, the power supplied to C1 has a constant value, and DC output voltage Vout has no ripple. V(S circuit Amplifier Multiplier TT Vout 4 Reference Wave vref - I I to T2 lvin I - % / - + Comparator V30 Drive vcont Circuit Fig.17 Proposed Circuit Con6guration with Control Circuit for AuxZary Switch T2 Simulation results with auxiliary switching circuit Fig.18 shows the *simulated waveforms of the control circuit. Just as i n Fig.10, the gate drive signal to T1 is to be obtained by comparing the sawtooth wave V(10) with the sinusoidal waveform V(20), which has the same phase as the input voltage. The gate drive signal to T2 can be obtained by comparing the sawtooth wave V(10) with the DC voltage V(30). The DC voltage V(30) is set as high as possible without overshooting the peak values of the V(10) voltage. Fig.19 shows the simulation results for T1 current waveform I(Sw and T2 current waveform I(SAUX). Where the current peak value is high, I(SAUx) is controlled to have a large pulse width, on the other hand, where peak value is low, pulse width diminishes. Under this type of control, the energy transferred to the DC output side is kept constant. Fig20 shows the simulation results for the reactor current waveform Iwut) and the DC output voltage 830 waveform V ( 7 ) . Although the reactor current includes a large ripple component of 214-p, the DC output voltage has hardly any low-frequency ripple voltage. Fig.18 Control Circuit Waveforms with Auxiliary Switch Fig.19 Main Switch and Auxiliary Switch Waveforms A 2 kHz ripple in the DC output voltage is included as seen in the simulated waveform in Fig.20, but because practical circuit is operated at several tens of kHz, the ripple voltage of the operating frequency component can be completely eliminated by the capacitor C1. The simulation specification are the same as i n Table 1, and the PSpice circuit iile used in the simulation is shown at the appendix. Application for high frequencv link topology / 831 The circuit configuration illustrated in Fig.16 is the simplest circuit of the buck-type high-power-factor converter, but the full-bridge type circuit configuration shown in Figdl can be selected for the large-capacity power supplies. While power supplies for telecommunications energy systems normally require the isolation between the input and the output sides, if one uses the circuit configurations in Figs. 22 and 23, the input and output isolation can be realized with a high4 eque ncy transform er link. 0 1 Fig.21 New Topology Suitable for Large Capacity Power Supply J p p q Fig.22 Full-Bridge Circuit with High Frequency Link Fig.23 Single-Ended Circuit with High Frequency Link Conclusion A control strategy that yields an distortion-free input current waveform even when there i s a large ripple component in the reactor current of a buck-type high-power- factor converter has been proposed. A circuit configuration with auxiliary switching circuit offering nearly complete control of the ripple voltage i n DC output has been developed newly, which is impossible to realize with the conventional buck-type high-power-factor converter.