High power factor pre-regulator with high efficiency.

Transcription

1 High power factor pre-regulator with high efficiency. Introduction. Traditionally, the ac/dc conversion is made using two dc/dc converters in order to obtain a fast regulation of the output voltage and power factor correction ( Fig 1 ); the drawbacks of this scheme are the high cost and size, and low efficiency. Searching to obtain a good efficiency, as well as a fast regulation and PFC, many power topologies have been studied [1-5]; in these schemes the power is processed one and a half times [5]. Also different efforts were made in order to just meet the harmonics standard specifications, with fast regulation of the output voltage and a good efficiency [6-8]; in these schemes the power is processed only once. In [9-10] were proposed other converters that process the power less than once, but these topologies need to use an special control strategy to obtain the desired output voltage. In this paper a new pre-regulator with unity power factor and good efficiency is presented. The proposed scheme do not process all the power delivered to the load and no necessarily needs to use a sophisticated control strategy. The proposed pre-regulator can be complemented with a regulator in order to obtain a fast regulation of the output voltage: the first option as regulator is the traditional that process the power once (e.g. flyback converter), resulting that the total power processed is approximately 1.5 times; the second option is to use a regulator that process the power less than once (e.g. two inputs buck [11]), resulting that the total power is processed less than once, approximately 0.80 times. In the next section the proposed converter is studied; in section three the power processing of the converter is addressed; in section four the simulation results are presented; and finally the conclusions are presented. Proposed Scheme. The block diagram of the proposed converter is shown in Fig. A converter is connected with the rectified ac mains, and also a diode with a bulky capacitor, but the output voltage of the converter is connected in series with the bulky capacitor voltage to produce the output voltage of the converter (). The proposed scheme operates as follows: The bulky capacitor demands a current like the shown in figure 3.a, the main converter must produce the compensating current () in order to demand a rectified ac current (Ir) to the rectified ac mains, this is illustrated in Fig. 3. It is important to notice that the capacitor not necessarily must be bulky. Is Preregulator PFC nverter 1 Regulator nverter Is Ir Ib Main converter Vc Fig 1. Typical ac/dc converter with two stages. Fig. Block diagram of the proposed pre-regulator.

2 Ib Ir Fig 3. Currents of the converter, top to bottom: a) current due to the bulky capacitor, b) the compensating current of the pre-regulator, c) the rectified ac current Main converter Vc Fig. Proposed pre-regulator with TI buck as regulator V The output voltage of the pre-regulator is a contribution of two voltage sources: the main converter output voltage and the bulky capacitor. As the diode that feed to the bulky capacitor switches at low frequency, the efficiency almost only depends on the main converter. Power factor correction A voltage loop is used to generate the current reference, that is where: = Rectified ac mains Ir = * ki ( V V ) dt (1) cb ki = constant of integration ref To shape the rectified current and to regulate the voltage of the capacitor the sliding mode control is used; therefore two loops are used: voltage and current loops. This control strategy is used to introduce a fast response at the input current in load variations. The control law is: u = sgn( s ex s ex ) () where: s 1 and s are controllers parameters. ex 1 and ex are the current and voltage errors respectively. It is important to notice that a traditional control strategy can be used, not necessarily the sliding mode control. Obtaining a regulated output voltage The proposed idea can be mixed with a regulator in order to produce an output voltage with the desired fast response. Two alternatives of regulators can be used: a regulator that process the power once like the flyback converter if isolation is required, or a regulator that process the power less than once like the two inputs buck converter if isolation is not required. In Fig. Is shown the proposed scheme using the two inputs buck as regulator.

3 Po Power processed by the main converter 1 Input Load Fig 5. Power processed by the main converter t Energy storage Fig 6. Power flow representation Power processing The output voltage of the proposed pre-regulator is generated by two series connected voltage sources. One of them is the bulky capacitor voltage, and the other is the output of the main converter. The total power delivered to the load is a contribution of both voltage sources, therefore the power processed by the main converter is less than once; this is shown in Fig 5 (shadowed area). The percentage of the power processed by the main converter can be calculated with: Vb k = 1 (3) where: Vb = Bulky capacitor voltage, = Output voltage. The percentage of the power delivered to the bulky capacitor is just processed for a diode switching at low frequency. nsidering those losses negligible the power flow representation of the proposed preregulator is as shown in Fig 6. The efficiency of the proposed scheme can be calculated approximated to: η = η ( 1 k )( 1 η ) () 1 1 where: η 1 = Efficiency of the main converter.. k = Percentage of the power processed for the converters. As can be observed in (), the efficiency of this scheme is higher than the main converter efficiency. As the percentage k is around 0.36 for the scheme implemented, and considering an efficiency of 90% for the main converter, the efficiency of the proposed scheme is 96.%. If the proposed preregulator is complemented with the TI buck converter with an approximately η=97% [11], the final efficiency of the complete converter including preregulator and regulator is around 93.5%. It is important to note that the main converter is designed with the 36% of the power delivered to the load. The Cúk converter is used as main converter to verify the proposed scheme, in Fig 7 is shown the circuit using the Cúk converter. The isolated converter version may be used if a grounded load is needed.

4 Is C L C L - u =1 u = -1 - a) System modelling Fig 7. Proposed pre-regulator. Fig 8. Simplified circuit of the proposed converter The system was modeled considering for this purpose a simplified circuit (Fig 8). The resultant system in matrix form considering the position of the switch u (1 and 1) is: x *(1 u) L x& x1 x3 vr 1 *(1 u) * u C C L () x& x 0 x x& = * u 3 L 0 L x & x3 ( x x5 ) 0 x& 5 C * R C 0 x5 x x5 * d rd * C3 R * C3 where: x 1 = I, x = V, C1 x 3 = I, x L = V, C x = V, 1 if vr > x 5 5 d = 0 if vr x 5 The sliding mode existence and stability analysis of the converter with the sliding mode control technique is not presented in this digest for abbreviation, but a similar analysis is presented in [10]. Simulation results The converter was designed with the following parameters: Po= 300W, Vin=10Vac, =50Vdc. Some simulation results of the converter are shown in Figs 9 through 1; the ac mains voltage and current are shown in Fig 9, in Fig 10 the rectified ac mains voltage, the bulky capacitor voltage and output voltage are Fig 9. Top to down: AC mains voltage, ac mains current. Fig 10. Top to down: Output voltage, bulky capacitor voltage and Rectified ac mains voltage.

5 Fig 11. Top to down: current due to the bulky capacitor, the compensating current of the pre-regulator, the rectified ac current Fig 1. Top to down: Output voltage, bulky capacitor voltage, rectified ac mains, bulky capacitor current and rectified ac mains current. shown. It is important to notice that a regulator must be added if a constant voltage is required. In fig 11 the output current, the compensating current and the rectified current are shown. In fig 1 the performance of the converter with a not bulky capacitor is shown. It is important to notice that a better performance is obtained with a no bulky capacitor, the output voltage is lower distorted, and also the rectified ac mains current. The input current THD obtained is 3.86%, and the power factor FP=0.99. nclusions In this paper a high power factor pre-regulator with high efficiency is presented. In the proposed scheme one converter is used, but it does not process all the power delivered to the load. The power is processed lees than once, resulting in an efficient converter. The proposed pre-regulator can be complemented with a traditional regulator or a regulator with improved efficiency, resulting that the power is processed once and half times or less than once respectively. The converter operation, analysis and simulations results of the converter are presented. In the final version a complete analysis including to the two inputs buck regulator will be presented, as well as simulation and experimental results. References. [1] R.Redl, L. Balogh, N. Sokal. A new family of single-stage isolated power-factor correctors, IEEE Power Electronics Specialists nference 199 pp [] J. Sebastian, P. Villegas, F. Nuño, M.M. Hernando, Very efficient two-input dc-to-dc switching post-regulators, IEEE Power Electronics Specialists nference 1996 pp [3] E. Rodriguez, F. Canales, P. Najera, J. Arau. A Novel isolated quality rectifier with fast dynamic output response. IEEE Power Electronics Specialists nference 1997 pp [] O. García, J. bos, P. Alou, R. Prieto, J. Uceda, S. Ollero. New family of single stage ac/dc power factor correction converters with fast output voltage regulation, IEEE Power Electronics Specialists nference 1997 pp [5] M. Chow, C. Tse, Y.Lee. An efficient PFC voltage regulator with reduced redundant power processing, IEEE Power Electronics Specialists nference 1999 pp [6] L. Huber, M. Jovanovic. Single-stage, single-switch, Isolated Power supply technique with input current shaping and fast output-voltage regulation for universal input voltage-range applications IEEE Applied Power Electronics nference pp [7] J.Sebastian, M.Hernando, P.Villegas, J.Diaz, A.Fontan. A new input current shaping technique using converters operating in continuos conduction mode IEEE Power Electronics Specialists nference 1998 pp [8] O. García, J. bos, P. Alou, R. Prieto, J. Uceda. A simple single-switch single-stage ac/dc converter with fast output voltage regulation IEEE Power Electronics Specialists nference pp [9] N. Vázquez, C. Hernández, R. Cano, J. Antonio, E. Rodriguez, J. Arau. An efficient single-switch voltage regulator. IEEE Power Electronics Specialists nference 000 pp [10] C. Hernández, N. Vázquez, E. Rodriguez, R. Osorio, J. Arau. " ltage regulator with unity power factor and high efficiency". IEEE Power Electronics Specialists nference 001, pp