A Novel Control Scheme to Reduce Storage Capacitor of Flyback PFC Converter

Transcription

1 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 A Novel Control Schee to Reduce Storage Capacitor of Flyback PFC Converter Boyang Chen and Lei Li College of Autoation, NUS, Nanjing, Jiangsu, 94, China Eail: 46557@qq.co, lileinjust@njust.edu.cn Xinran Zhang City Power Developent Corporation, Beijing,, China Eail: zxr658@6.co Abstract DCM Flyback PFC converter is ainly used in ediu and low power applications, having such advantages as zero-current turning on of switch, no reverse recovery of diode and high PF. At the tie power electronics widely used today, passive coponents volue liits the iproveent of power density. his paper derives the expressions of the switching turn-on tie and the input current of DCM Flyback PFC converter, and based on which, a variable duty control is proposed so as to ake the energy storage capacitor reduce to the original 65.6% at the sae voltage ripple level while PF is not less than.9. he siulation results fro a W universal input prototype are given to verify the effectiveness of the analysis. DCM Flyback PFC converter which has the advantage of siple control, unity power factor correction, no reverse recovery current, feedback loop stability and fast response, etc., is generally suitable for sall and ediu occasions. [], [4] II. Fig. is the ain circuit of Flyback PFC Converter. In order to facilitate analysis, ake the following assuptions: ) All devices are ideal coponents; ) he output voltage ripple is sall copared to its direct flow; ) he switching frequency is uch higher than the input voltage frequency. Index ers power factor correction, energy storage capacitor, variable duty control, power density I. ANAYSIS OF DCM FLYBACK PFC INRODUCION Power Factor Correction (PFC) converters have been widely used in ac-dc power conversions to achieve high power factor (PF) and low haronic distortion. With the developent of the power electronics, high frequency, iniaturization is the trend. In the process of power electronic devices running, it brings a lot of haronic injection public grid, seriously affected the noral operation of the grid power quality and other devices. In order to reduce the haronic pollution power electronic devices on the grid to eet the haronic standard IEC6-- constitute by international organizations, we need a power factor correction (PFC) converter to suppress it. [], [] Flyback PFC Converter copared with the sae type of PFC converter has the advantage of isolated input and output devices, siple structure, low cost, is one of the ost coon active power factor correction (PFC) converter. According to the current of the inductor at the priary side of the converter is continuous or not, the operating ode can be divided into Continuous Conduction Mode (CCM), Critical Conduction Mode (CRM), Discontinuous Conduction Mode (DCM). Figure. Main circuit of flyback PFC Converter Fig. is the wavefor change of the inductor current and the flux of the DCM Flyback PFC Converter at one period. Figure. he inductor current and the flux in one period When Qb is on, the secondary side is not conductive because of the diode D. he voltage between the priary sides of the transforer is vg, and its current ip rise fro Manuscript received February 7, 5; revised July, 5. Project 5777 supported by National Natural Science Foundation of China. 6 Int. J. Electron. Electr. Eng. doi:.878/ijeee.4..-5

2 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 zero in the slope of v g /L, the flux increases linearly, and the energy stored in the transforer. When Q b is off, the voltage of the secondary side increases linearly, until its voltage is bigger than the voltage of the storage capacitor, the secondary side is conductive. he current of the secondary I s down to zero and the energy is transferred to the secondary side at the sae tie. In the frequency of positive half cycle, assue that the input voltage is v ( t) V sin( t), then the input peak g current in a period can be expressed as: i V sin( t) D () in _ pk s Lp And the input average current can be expressed as: V D t sin( ) iin _ av iin _ pk D L f he input power in half frequency period can be expressed as: p s () D V Pin v ( ) ( ) in t iin t dt 4 Lp fs () he Power Factor in ideal can be expressed as: PF Pin V I _ rs So the conclusion we can get is that the PF of Flyback PFC is in ideal and it s very suitable to sacrifice soe PF in exchange of reducing the storage capacitor. III. ANAYSIS OF RELAIONSHIP BEWEEN HE SORAGE CAPACIOR AND PF Because of the input power of PFC converter is pulsed. In one period, the average input and output power is balanced with the unbalanced instantaneous power. So it is necessary to balance the pulsation power by using the power storage capacitor. [5] On the other hand, if the instantaneous power of the input and output are balanced, there is no need of power storage capacitor, and the input current can be expressed as: Po iin () t (5) V sin( t) Fig. shows the input current in one period. Figure. he current and voltage without the storage capacitor By Fourier decoposition of the input current, it can be seen in the Fig., we can conclude that when the input and output instantaneous power balance, input current (4) contains a large nuber of odd haronics and PF is alost zero. In order to achieve the effect to reduce the storage capacitor, we can inject appropriate odd haronics to the input current. IV. HE IMPAC OF INJEC HE HIRD HARMONIC O HE INPU POWER When the input voltage is V sin( t), the input current fundaental can be expressed as: i sin( ) in I t (6) he instantaneous input power of fundaental current can be expressed as: p V I t (7) sin ( ) in If the input current have the sae initial phase as the third haronic, i sin( ) in I t (8) p V I sin( t)sin( t) in Injecting a certain aount of third haronic current which has the sae phase as the fundaental, controlled by a certain way, the input current is expressed as: iin( t) I sin( t) I sin( t) I (sin( t) I sin( t)) () I is the input current per unit on the third haronic of the fundaental. In this case the input power factor of the converter: PF I (9) () According to Energy Star standards of coercial lighting, the power factor of the powered devices are not less than.9. In this case, the aplitude of third haronic of the fundaental is 48.4%. Assuing the efficiency of the converter is %, then: l in o in in l P ' P ' v ( t) i ( t) dt VI ( V sin( ) t I(sin( t) Isin( t))) dt () Instantaneous power per unit value (reference the value of output power) is: v ( t) i ( t) V sin( t) I (sin( t) I sin( t)) in in in Po' VI / sin( t) (sin( t) I sin( t)) p () When using fixed duty cycle control, the instantaneous input power per unit of DCM FLYBACK PFC converter can be expressed as, v ( t) i ( t) p ( t) sin ( t) (4) in in in Po ' As it is shown in the Fig. 4, the storage capacitor C o is charging when p in( t). he storage capacitor C o is discharging when p in( t). Assuing start fro t, 6 Int. J. Electron. Electr. Eng.

3 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 under the control of fix duty cycle and variable duty cycle, the wavefor of pin (t ) intersection with corresponding the axis at t and t. he energy storage by Co in frequency of half period can be expressed as, Do (4) V hen: D D I ( 4sin ( t )) Pin (5) o achieve the duty is difficult because of the coplex forula, so it s necessary to siplify the forula. For convenient, set y sin( t ), then:.5 Pin.5 Dy D I ( 4 y ) ωt ωt.5 t [ pin (t )]dt l t E [ pin (t )]dt l E According to the aylor series expansion forula, expand the function at y y : (5) Co (Vo o ) Co (Vo o ) E Pol / (6) Po CoVo Po CoVo t (7) t (9) [ pin (t )]dt () I ( 4 y ) Next we discuss the suitable point of y to ake sure the PF is not less than.9. he above analysis already has I.484, then (8) lead to: Dy _ fit D y y t ).484sin( t ) sin( t )]d t (9) Lp f s D y y () Assue that the efficiency of the converter is %, then the average input power can be expressed as: Pin '' Po '' ().5 l v (t )i (t )d t l in in t ) V sin ( Lp f s () D ( y y) d t hen: By the ethod of injecting the third haronic, with the sae requireent of the ripple, the storage capacitance can be 65.6% as the original. PF Pin Pin Vin _ rs Iin _ rs (iin (t )) d t V HE CONROL CIRCUI After injecting the third haronic, the input current following the equation: Po (sin( t ) I sin( t )) V V sin( t ) iin ( t )]d t 6 [ sin ( sin ( t ) ( y sin( t )) d t 4 sin ( t ) ( y sin( t )) d t () ().95 PF=.9.9 Lead () into (), D 4I y I ( 4 y ) ( y y ) I ( 4 y ) (8) Aong the D D (8) [ pin (t )]dt 4 [ sin iin (t ) Bring (9) into (), then : V. ( y y )... (7) D ( I 4I y y) With the sae requireent of the ripple, I ( 4 y ) Lead to: Dy D I ( 4 y ) C V o o o Pol o 4I y Ignore higher order ters, take only the forula of the first two to fit, then: C V o o o Pol Co (Vo o ) Co (Vo o ) E Pol / Dy D I ( 4 y ) According to the forula of the energy storage capacitor, the axiu energy per unit and can be expressed as: o (6) Figure 4. he instantaneous input power per unit Co Co Lp f s Po.85 Lp f s Po (sin( t ) I sin( t )) V sin( t ).8.75 ().7 Assue that: 6 Int. J. Electron. Electr. Eng Figure 5. he relationship between the PF and y.9

4 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 he size of PF is associated with the y, so it s easy to find y=.77 and PF=.9 in Fig. 5. Lead to: Aong the the input voltage is divided by R and R VI..75 Lp f s Po V ( sin( t ) ) va k V sin( t ) ; R, R4, D, C constitute of peak sapling circuit, then vz vb k V ; After the op ap, vx kvref k4va ; he output voltage is Dy _ fit D ( sin( t ) ) and () divided by R and R, the voltage through PI adjustent and obtain v y ; hrough the Multiplier the Electric signal can be expressed as DESIGN AND SIMULAION RESUL vea he Fig. 6 shows the control circuit basis of forula (). vy (kvref kk4v sin( t )) k V (4) vea intersects with the sawtooth wave and obtain the PWM signal. Design and siulation by the forula (). In order to verify the validity of the proposed variable duty cycle control, a prototype has been built and siulate in Saber. he paraeters of the prototype are as follows: ) Input voltage: vin 9 ~ 64Vac / 5Hz ; ) Output voltage: vo 5.75Vdc ; ) Output power: Po W ; 4) Switching frequency of the converter: khz he paraeters of the PFC stage are as follows: ) Magnetizing inductance: Lp.6uH ; ) urns ratio: n ; ) Storage capacitor: Co 7.7F (constant duty cycle) Co 4.68F (variable duty cycle) Figure 6. he control circuit 6 Int. J. Electron. Electr. Eng. (a) 9Vac (b) 5Vac

5 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 (c) 65Vac Figure 7. Siulation wavefors of output voltage, input current and voltage with constant duty cycle (a) 9Vac (b) 5Vac (c) 65Vac Figure 8. Siulation wavefors of output voltage, input current and voltage with variable duty cycle Fig. 7 and Fig. 8 show the wavefors of the output voltage, input voltage and input current with constant duty cycle and variable duty cycle at 9Vac, Vac, 64Vac input, respectively, it can be seen that, the input current 6 Int. J. Electron. Electr. Eng. 4

6 International Journal of Electronics and Electrical Engineering Vol. 4, No., April 6 has soe distortion due to DCM operation and it ainly contain rd haronic which has a phase difference of and with fundaental coponent, especially at high input voltage. And because of the peak input current is decreased by injecting the rd haronic, the efficiency is iproved, particularly at low input voltage. VII. CONCLUSIONS DCM Flyback PFC Converter has the advantage of topology siple and efficient, widely used in sall and ediu power applications while the volue of passive coponents constraint its further iprove of the power density. With variable duty cycle control of DCM Flyback PFC Converter the storage capacitance is reduced to the original 65.6% at the sae ripple level. Over a wide input voltage rage ake the topology of the power density further iproved, ore in line with sall, odular trends. REFERENCES [] ENERGY SAR Progra Requireents for Solid State Lighting Luinaries, USA, 7. [] K. Yao, X. Ruan, X. Mao, and Z. Ye, DCM boost PFC converter with high input PF, in Proc. IEEE Applied Power Electronics Conference,, pp [] B. Singh, et al., A review of single-phase iproved power quality AC-DC converters, IEEE ransactions on Industrial Electronics, vol. 5, no. 5, pp ,. [4] K. Yao, et al. Variable-Duty-Cycle control to achieve high input power factor for DCM boost PFC converter, IEEE ransactions on Industrial Electronics, vol. 58, no. 5, pp ,. [5] L. L. Gu, X. B. Ruan, M. Xu, et al., Means of eliinating electrolytic capacitor in AC/DC power supplies for LED lightings, IEEE ransactions on Power Electronics, vol. 4, no. 5, pp , 9. Boyang Chen was born in Beijing, China, in 99, received the B.S. degree fro the Electrical engineering and autoation, Nanjing University of Science and echnology, Nanjing, China in. He is currently working towards the M.S. degree in power electronics and power transission at collage of Autoation, Nanjing University of Science and echnology, Nanjing, China. His research interests include PFC technique. Lei Li (M 9) received the B.S. degree fro the Departent of Electrical Engineering, Shandong University of Science and echnology, Qingdao, China, in 997, and the Ph.D. degree fro the Departent of Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 4. He is currently an Associate Professor with the College of Autoation Engineering, Nanjing University of Science and echnology, Nanjing. He has published ore than 5 technical papers. His research interests include ultilevel technique, high-frequency power conversion, and control technique. Dr. Li was the recipient of one first class reward production of science and technology of Jiangsu Province and is the holder of three China patents. Xinran Zhang was born in Beijing, China, in 967. He received the B.S. degree at work. He is currently a vice general anager of City Power Developent Corporation in Beijing. His research interests include electric power syste and the static var copensator equipent. 6 Int. J. Electron. Electr. Eng. 5