SINGLE PHASE POWER FACTOR CORRECTION RECTIFIERGishin Jacob George, G. Vinod Kumar, Jacob James Nedumgatt, Jayakrishnan K.B., Joseph Peter, Rakesh R., Unnikrishnan C.K. SELECT DEPT, VIT UNIVERSITY, VELLORE-6632014, TAMIL NADU, INDIA.

Abstract: This project proposes a simple low-cost modulating duty cycle analogue controller to reduce line frequency harmonics for high power boost rectifier. The proposed method eliminates the need for current sensing, and simultaneously offers the performance results comparable to continuous conduction mode (CCM). This scheme also maintains the simplicity comparable to that of

discontinuous conduction mode (DCM). Only the output voltage and rectified input voltage are monitored to vary the duty cycle of the boost switch within a line cycle so that the third order harmonic which is the lowest order harmonic of the input current is reduced. As a result the total harmonic distortion of the line current and thus the input power factor is improved. Moreover the rectifier shows a good transient performance where the converters output voltage overshoots during input voltage/load transient is reduced. The proposed method is developed for constant switching frequency boost rectifier. Simulation and experimental results are presented to verify the

effectiveness of the proposed control method.

Introduction: The conventional ACDC rectifier consists of a diode bridge rectifier followed by a filter capacitor is cheap and robust, however it operates with heavily distorted line frequency current and poor power factor. To overcome the problem, passive and active power factor correction (PFC) circuits have beendesigned. Active methods are more efficient, lighter in weight and less expensive than passive circuit methods. It is important to limit the harmonic content of the input current drawn by electrical equipment, therefore steps have been taken to develop new active PFC circuits employing switched mode DCDC converters. Single-phase high power factor is most frequently accomplished using boost converter. To have unity power factor, the boost converter must maintain the input current proportional to the input voltage. This proportionality is maintained through closedloop feedback control, similar to the one used in the common continuous conduction mode (CCM).Various techniques are available for CCM active current shaping, most of which are now supported by dedicated integrated circuit. For example using the average current mode control would result in very low current harmonic distortion for a wide range of input line voltages and output load

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currents. However, implementation of the average current-mode control is relatively complex. The inductor current is usually sensed through a resistor at the return path which contains significant noise and creates current regulation problem when several boost converters working in parallel. Additionally, there is considerable power loss to the sensing resistor.The simplest technique for PFC is discontinuous conduction mode (DCM) operation of the converter at constant switching frequency and constant duty cycle throughout the line period. For converters operated in DCM, only single voltage loop control is required and the input current is related to the input voltage by a factor which depends on duty cycle. Rectifiers employing converters of this type are called automatic current shapers. It is not, however, an ideal automatic current shaper as the input current in such case exhibits a non-linear characteristic and results in low-frequency harmonics(third, fifth etc.) because of the slow discharge of the inductor current after the switch is turned off. Another drawback of DCM operation is high current stress and higher noise caused by the pulsating inductor current. A larger electromagnetic interference (EMI) filter is needed for DCM as compared to that of CCM. The purpose of this paper is to propose a simple low-cost controller to reduce the line harmonics for single-phase single-switch boost rectifier. The proposed method eliminates the need for current sensing, and simultaneously offers the performance comparable to CCM scheme. Its simplicity is comparable to that of DCM operation. With this technique a low total harmonic distortion (THD) of the rectifier current can be achieved with a good transient performance which reduces the rectifiers output voltage overshoots during stepup line voltage/load transients. The duty cycle is varied within a fixed switching period such that the low-order harmonics of the input current is attenuated. As a result, the THD and thus input power factor are improved. Review of Boost Converter Operation: The topology of boost converter is shown in figure 1. The diode bridge rectifies the AC line voltage, and the power switch S, the inductor L and the output diode do operate as a boost converter. The ripple in the output voltage is reduced by using the capacitor C. The load is assumed to be purely resistive (R).

Figure 1: Boost converter

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The input line filter reduces the high-frequency components in the input current. The power switch is operated at a constant switching frequency, and the output voltage is varied by varying the duty cycle. It is assumed that the boost converter operates at CCM and the switching frequency is much higher than the line frequency. Hence, the input voltage can be assumed as a constant during one switching cycle.

Figure 2: (a) switch is on (b) Switch is off

Equations: (a)When switch is on L (diL/dt) = vg, 0 t dTs(b) When switch is off

L (diL/dt) = vg Vo, dTs t Ts Design of Boost Converter Vo = Vs / (1-D) D = 0.55 Power Output = 500W Vo2 / R = 500

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R = 100 L = ((Vo Vi + VD) (1 - D)) / (I f ) L = 200H Vs = L (I2 I1) / (t1) t1 = D T t2 = (1 D) T L = 200H Simulink model of Boost Converter

Figure 3: Boost converter

Fig 4: Output Waveform of Boost Converter

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Transfer Function of the Boost Converter Control to output transfer function = Gdo 1 / (1+ s/p1) Gdo = (2Vo / D)( (M -1) / (2M -1) ) p1 = 2M -1 / ((M -1)RCo) M = Vo / Vi D=11/M M = 215 / 95.5 = 2.251 Gdo = 277 p1 = 63.36 Control to output transfer function = (277 / 1 + 0.0157s) Bode plot of the Boost Converter

Fig 5: Bode Plot of Boost Converter Boost Converter with proposed Control The PWM controller detects the Rectified input voltage (Vg) and output voltage (Vo) and calculates the operations on the right-hand side of (6),which is actually the off-time 12 d(t) of the switch. The division by Vo in 6) can be easily accomplished by modulating the amplitude of the carrier waveform with Vo.Therefore it offers an inexpensive method to perform the division operation. It also offers a good transient performance where the converters output voltage overshoots during load transients is reduced. Simulation and experimental results are provided to verify the effectiveness of the proposed control algorithm.

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Transfer Function of the PI Controller

C(s) = kp((s +z)/s(s +p))The parameters were optimised using Matlab sofware such that kp = 20, z = 200 rad/s and p = 3000 rad/s. Bode plot of the PI Controller

Fig 6: Bode Plot of PI Controller MULTISIM Model of PI Controller Circuit

Fig 7: PI Controller circuit

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MULTISIM Model of Carrier Waveform Generator using Integrator

Fig 8: Carrier Waveform Generator

Fig 9: Waveform during a step up input voltage transient

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Simulink Model of Boost Converter with PI Controller

Fig 10: Boost Converter with PI Controller

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Expected Output

Fig 11: Output waveforms

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Prototype Verification: The experimental prototype was built using the following components:MUR1560 for bridge rectifier, boost inductor L = 200 mH, boost power switch (S) IRFP22N50, gates derive circuit HCPL3120, output diode Do = DSEI12- 06A and output capacitor Co = 440 mF. The input LC filter with Lf = 1mH, and Cf = 2 mF mainly reduces the harmonics which are artefacts of the switching. A filter capacitor, Cin = 4.7 mF is placed between the bridge rectifier and the boost inductor L, in order to reduce the high-frequency ripple of the rectifier voltage Vg. Cin can also reduce the peak inductor current as well as the power switch peak current. As for the control circuit the following ICs are used: AD633 for multiplier, LM339 for PWM comparator, XR8038 for carrier signal generator, LM358 for PI controller and LM324 for both the differentiator and subtractor. The carrier generator and the multiplication with Vo could be replaced by integrating Vo using LM356 reset integrator, as shown in. It can be seen in that the carrier signal is generated by integrating a voltage proportional to the output voltage. The output voltage is first sensed and attenuated by voltage divider (Rf1, Rf2) and then inverted before it is brought to the input of the integrator. The integrator is reset at the beginning of each switching cycle by an external fixed frequency (20 kHz) clock signal.

Fig 12: Prototype PFC Rectifier

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Fig 13: Source Voltage and Current

Conclusion: The paper proposes a simple, low-cost harmonic analogue controller to modulate the duty cycle of the boost switch such that the third-order harmonic of the input current is reduced and the overall THD is improved. The proposed method eliminates the need for current sensing and simultaneously offers the performance results comparable to those of CCM scheme . Its simplicity is comparable tothat of DCM operation. The PWM control signal is obtained by monitoring the input and output voltages only. The duty cycle is varied within a fixed switching period such that the loworder harmonics of the input current is attenuated. Moreover, the duty cycle variations are naturally synchronised with the input voltage without using additional expensive phase-detecting, phaselocking circuits. With this technique, a low THD of the rectifier current can be achieved with a good transient performance which reduces the rectifiers output voltage overshoots during step-up line voltage/load transients. It is found that simulated and experimental results match closely. Based on the simulation and experimental results, the line current is almost sinusoidal with nearly unity power factor.

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