THE flyback converter represents a widespread topology,

632 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 3, JUNE 2004 Active Voltage Clamp in Flyback Converters Operating in CCM Mode Under Wide Load Variation Nikolaos P. Papanikolaou and Emmanuel C. Tatakis Abstract Active clamp topologies of low power dissipation have become a very attractive solution in order to limit overvoltages in flyback converters. Although many suitable topologies have been introduced for the case of discontinuous conduction mode (DCM), where the duty cycle value depends on the load level, in continuous conduction mode (CCM) it is more difficult to appropriately design such topologies so as to sense load changes due to the small duty cycle divergence under wide load variation. Taking for granted that in order to achieve high power-factor correction in these converters, CCM is a more attractive mode of operation, a drastic solution for this case that will manage to eliminate voltage stresses under wide load changes has become very essential. For this purpose, this paper presents an active clamp topology with small power dissipation, suitable for flyback converters operating in CCM mode. Its main idea is the use of a load-dependent current source, consisting of an auxiliary converter operating in DCM mode. Experimental results highlight the effectiveness of the proposed topology under wide load changes, establishing it as an appropriate solution in order to develop flyback converters, even at the power range of 500 W. Index Terms Active voltage clamp, flyback converter, snubbers. n d NOMENCLATURE Voltage clamp value. Transformer primary to secondary winding turns ratio of the main converter. Snubber power value (dissipative case). Snubber power value (nondissipative case). Snubber resistor. Maximum current value on inductor during ontime [discontinuous conduction mode (DCM)]. Constant switching frequency of the main converter. On time of the main converter. Duty cycle of the main converter. Discharging snubber inductance. Maximum current value on the primary winding of the main converter during on time. Power of the main primary inductance. Main converter primary inductance. Manuscript received June 28, 2002; revised October 30, 2003. Abstract published on the Internet January 14, 2004. The authors are with the Laboratory of Electromechanical Energy Conversion, Department of Electrical and Computer Engineering, University of Patras, 26500 Rion-Patras, Greece (e-mail: Papanik@ee.upatras.gr; E.C.Tatakis@ ee.upatras.gr). Digital Object Identifier 10.1109/TIE.2004.825342 Main converter primary leakage inductance. Auxiliary converter primary leakage inductance. Constant switching frequency of the auxiliary converter. Duty cycle of the auxiliary converter. Transformer primary to secondary winding turns ratio of the auxiliary converter. Zener voltage. Maximum permitted drain to source voltage value of the auxiliary switch. Lower current value of the leakage inductance (normally zero). I. INTRODUCTION THE flyback converter represents a widespread topology, suitable for the development of switch mode power supplies with multiple output voltages, due to the simplicity of its structure. Traditionally, the DCM of operation has been in favor, mainly due to its better dynamic behavior. Nowadays, however, where a lot of discussion has concerned the electromagnetic compatibility of electric and electronic devices [1] and many low-voltage applications that are supplied from the power network have emerged, continuous conduction mode (CCM) of operation has become more attractive, since it leads to higher harmonic regulation and lower power losses. As is well known, the main drawback of this converter is the existence of the transformer leakage inductance, whose value is at the range of 2% 10% of the main inductance; the resonance between this inductance and the parasitic capacitances of the semiconductive switches produces large voltage stresses as well as power losses, decreasing the converter efficiency. Furthermore, in the case of multiple outputs from a single flyback converter only, the cross regulation between the multiple transformer secondary windings is seriously affected by these high voltage spikes [7], complicating the construction of dc power supplies with multiple outputs. Thus, these facts prevent the use of this converter at higher power levels, where the forward and full-bridge alternatives are more preferable. Beyond that, in order to limit the voltage spikes on the controlled switch, an appropriate snubber is necessary. Many snubber topologies have been introduced for the flyback structure and for various power levels. The simplest of them (suitable for lower power levels) are the dissipative snubbers [2], where the leakage energy is led to a resistor. 0278-0046/04$20.00 2004 IEEE

PAPANIKOLAOU AND TATAKIS: ACTIVE VOLTAGE CLAMP IN FLYBACK CONVERTERS 633 Fig. 1. Basic snubber topologies for flyback converter. However, several studies have highlighted the so-called nondissipative LC snubbers [3], [4]; the main idea of these kinds of snubbers is by using an auxiliary LC network either to force the leakage energy to oscillate among the input stage and the leakage inductance (and treat it as reactive power), or to transfer it to the load (and treat it as active power). In both cases the leakage energy is not dissipated to a resistor and so the power losses are decreased. Thus, this solution maximizes the efficiency and permits the use of the flyback converter at higher power levels. Nevertheless, the main drawback of snubbers, in general, is that their effectiveness depends strongly on line and load variations; they are usually sufficient for a narrow load range area and so their design in cases of large load variations becomes difficult. As it concerns the DCM operation, a nondissipative solution has been already presented in the literature [7], which sufficiently deals with these issues. However, in CCM mode the design difficulty increases due to the almost constant duty cycle under wide load changes (if the output voltage is constant) as well as the higher power level. For this purpose, an alternative low-loss active snubber topology will be studied in this paper, suitable for any flyback application including power-factor correction (PFC) and low-voltage supplying. The main idea of this solution is the use of an appropriate auxiliary converter as a controllable current source, so as to regulate the value of its processed power according to load variations. To this direction, a short discussion concerning the main drawbacks of the most popular snubber topologies will take place, highlighting the benefits of the active solution. Analytical presentation of the design procedure as well as experimental results will take place. II. STUDY OF BASIC SNUBBER TOPOLOGIES FOR FLYBACK CONVERTER The block diagrams of the basic snubber topologies for the case of flyback converter are shown in Fig. 1. The snubber in Fig. 1(a) is the least expensive solution; it introduces a damping factor in order to limit the peak voltage value during oscillation. Although simple, its operation depends strongly on load variation, while it is rather difficult to determine the most appropriate values of its elements, limiting its use at small power levels. On the other hand, the topologies of Fig. 1(b) and (c) are more suitable for higher power levels. Using these schemes, the maximum voltage value on the switch is clamped due to the leakage energy storage into voltage source. The snubber of Fig. 1(b) is a dissipative one, while the snubber of Fig. 1(c) is a nondissipative one. In the dissipative version, the voltage source can be consisted either of a parallel combination of a large electrolytic capacitor and a resistor, or it could be a single zener diode [5]. As it concerns the nondissipative case, can be implemented by a large electrolytic capacitor, leading to a constant voltage clamp, which is partially discharged through inductor during on time. This inductor operates in DCM mode and it can be constructed either by using a separate magnetic core (reactive power treatment), or it could be a third winding on the main transformer core transferring so its energy directly to the load (active power treatment). Moreover, an alternative operation mode has been presented, where the resonance frequency of the LC circuit is much higher than the switching frequency. However, this method leads to higher voltages across the semiconductive switch, whether the converter operates in CCM [6] or in DCM mode [7]. For this reason, this mode of operation will not be included in the following analysis. During the design of voltage clamp snubbers, it must be taken into account that the voltage source value should be as close as possible to the theoretical expected value across the primary transformer winding during off time [2] In this way, the voltage across the main switch becomes as small as possible, avoiding so the need for excessively high-voltage MOSFET devices. Of course, these two values cannot become equal, because this it would be a short circuit for the main flyback transformer inductance. Thus, the snubber design should reassure that the voltage source transfers the leakage energy to the resistor or to the inductance, under the desired voltage value. For the dissipative case, the power that is transferred to the resistor is while for the nondissipative case the power can be described as Due to snubber DCM operation, becomes Combining (3) and (4), the power in the nondissipative case can be rewritten as Obviously, the processed power in both cases should be equal to the leakage power. (1) (2) (3) (4) (5)

634 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 3, JUNE 2004 If we assume that the converter is in DCM mode, then the leakage power can be easily described as follows: (6) Similarly, the power of the main primary inductance is Finally, assuming unitary efficiency, the primary inductance power is transferred during off time to the load and, so, Using (6) (8), the leakage power finally becomes (7) (8) Since and should be equal under any load case, the dissipative version becomes less attractive: (2) shows that in order to achieve under any load case, changes according to the load. Thus, for wide load range the voltage value across the main switch becomes prohibitively high making this snubber less attractive. A good choice for wide load range though, seems to be the Zener alternative [5]: in this case the voltage clamp remains constant (due to the constant Zener voltage) under any load condition, making it a rather attractive solution in DCM operation. Similarly, the nondissipative solution is a good choice in DCM mode too: since depends on duty cycle value, which even for constant output voltage depends on the load level in DCM mode, this topology can be appropriately designed so as to meet wide load changes. In this way, both limitation of voltage stress and low power dissipation are achieved, leading to high efficiency. Of course, the cost increase makes it more suitable for power levels above 50 W. In CCM mode however, the snubber design has to deal with the additional fact that the duty cycle value changes slightly under wide load variations theoretically it does not depend on load change. According to (2) and (5), which stand for the dissipative and the nondissipative voltage clamp case, correspondingly, it is obvious that the voltage clamp value depends on the leakage power value as well as the duty cycle value. In DCM mode of operation, the duty cycle value changes as the output power changes, even if the output voltage remains constant. Thus, the change of the voltage clamp value can be kept sufficiently small in this mode of operation under constant output voltage value even if the load presents large variation (e.g. 10% 100% of its nominal value), due to the similar variation of the duty cycle value. For this mode of operation so, we are able to use solutions like those proposed in [3], [4] and in [6], [7], leading to excellent operational characteristics for the main converter. On the other hand, CCM mode of operation is much more complicated, concerning the design of such snubbers; in applications where the output voltage remains constant while the output power presents large variation, the duty cycle value remains almost constant. Thus, the clamp voltage value presents (9) Fig. 2. Equivalent circuit for the case of the load-dependent active clamp. large variation during these large output power changes, in order to absorb the leakage power the value of which changes with the load, for any snubber case like those mentioned in [3], [4] and in [6], [7], leading to high voltage values across the main switch. Even the use of ZVS-active voltage clamp techniques for this specific situation, also leads to high voltage values across the main switch, since the voltage clamp value that they develop changes linearly to the load level. Furthermore, the Zener diode solution becomes less attractive in CCM mode, due its high power level. The above analysis shows that an appropriate snubber for the CCM mode has to take into account load changes, by adjusting properly the energy that is transferred from voltage source to a dissipative resistor or to the input/output converter stage. This can be accomplished by developing a load-dependent current source, as it is shown in Fig. 2. In the following paragraph the appropriate modulation of this current source is discussed. III. CONSTRUCTION OF THE LOAD-DEPENDENT SNUBBER FOR FLYBACK CONVERTER OPERATING IN CCM As has been already mentioned, CCM operation is a rather convenient case for the development of EMC compatible flyback converters. This can be accomplished by using a PFC control technique for CCM operation [10]. Although in PFC converters the duty cycle value is a time function during a line half cycle [8], its average value depends only on the input (peak value) and the output voltages as well as on the transformer turns ratio value. Thus, similar to the dc/dc operation, the average duty cycle value is almost (due to the non-unitary efficiency) constant under load variations and so these two cases can be treated in the same way as it concerns the appropriate snubber design. The load-dependent snubber could be either dissipative or nondissipative. In the dissipative case, its structure has to include a variable load current-dependent resistor, which can be implemented by using a transistor, as shown in Fig. 3. In this way, the power dissipation of is adjusted during the converter operation, minimizing so the voltage stress on the main switch. Although this is a simple approach, the whole circuit

PAPANIKOLAOU AND TATAKIS: ACTIVE VOLTAGE CLAMP IN FLYBACK CONVERTERS 635 inductance are the same. Of course, this condition cannot be fulfilled, because this would be a short circuit for the main inductance. Nevertheless, in order to conclude to a simple expression, we may assume that those two current values are close enough, a fact that is true if the converter operates relatively close to the boundary between CCM and DCM modes. Thus, the power consumption of the auxiliary flyback converter can be expressed as (12) Fig. 3. Block diagram of the dissipative load-dependent snubber. becomes more complicated, while the leakage energy dissipation increases the total losses. Thus, this solution becomes less attractive, mainly due to the fact that the leakage energy is dissipated, even though an additional transistor is used. Thus, the most appropriate solution seems to be the leakage energy transfer to the input or to the output converter stage. This task calls for the use of an auxiliary converter with galvanic isolation, operating in DCM mode so as to behave as a controllable current source. Obviously, the topologies that are most appropriate are the forward and flyback schemes, due to the single controllable switch of their structure. However, the forward converter calls for two magnetic cores as well as for duty cycle values lower than 50%. This last limitation encumbers the efficient converter design under any load case. On the other hand, the flyback converter topology in DCM operation has a simple structure, while the duty cycle value can well exceed 50%. Fig. 4 shows the proposed load-dependent snubber topology, consisted of an auxiliary flyback converter in DCM mode. Of course, its main drawback is the presence of voltage spikes due to the leakage inductance of this converter; thus, its usability depends on the power losses increase due to the leakage energy reset. Obviously, this amount of power can be described by a relation similar to (9), where in this case stands for the power that is transferred from the voltage source to the load-dependent snubber (according to Fig. 2). Since the main flyback converter operates in CCM, its input current waveform becomes as it is shown in Fig. 5. According to this figure, the power that is transferred to the output stage via the primary inductance becomes (10) Additionally, for the leakage power we can modify (6) according to Fig. 6 as (11) Equations (10) and (11) show that (9) can be used in CCM as well, if the lower current values of the primary and the leakage Taking for granted that the leakage inductance in practical flyback converter applications is lower than 10% of the main inductance, we can assume from (12) that the total dissipation due to leakage presence becomes less than 1% of the output power and so a Zener diode dissipative snubber can be introduced. Thus, the wasted energy amount becomes almost insignificant, proving that the use of an auxiliary flyback converter in DCM is rather effective. Furthermore, since the leakage power of the main converter can be kept lower than 10% of the output power, it is feasible to use the flyback structure in CCM mode for power levels even higher than 500 W, where forward and bridge topologies are in favor. Thus, the use of the proposed load-dependent snubber implementation enables the realization of simple switch mode power supplies for a wide power range. At this point, it is worth mentioning that a similar flyback topology has been already used for the case of a forward converter [9], in order to reset the main transformer inductance. However, in this case the voltage across the main switch of the forward converter is a time function whose maximum value depends on the load current. Thus, the whole philosophy of this snubber topology is completely different from the philosophy of the proposed one in the present work, where the voltage across the main switch is kept constant under any load condition. Finally, it is worth mentioning that since the auxiliary converter operates in DCM mode, its dynamic behavior is very good and so it does not have any practical impact on the main converter response under transient conditions. Furthermore, as it is obvious from Fig. 4, in case of short circuit at the output stage the duty cycle of the auxiliary converter becomes zero (since becomes zero) and so it does not contribute to the short circuit current. IV. DESIGN SCHEME OF THE PROPOSED LOAD-DEPENDENT SNUBBER As it is obvious from Fig. 4, an appropriate selection of the proposed snubber basic elements has to be done in order to cover any load condition. As it has been already mentioned, the main converter leakage power can be transferred to the input or output stages or even to the control circuit, depending on its level. However, according to Fig. 4, for the present approach it is assumed that this energy is transferred to the output stage and so it is treated as active power. Beyond that, the output voltage value is assumed constant; although this snubber can be used for variable output voltage as well, the most critical case is that of the constant output (constant duty cycle) voltage value. Thus, if the snubber is appropriately designed for the

636 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 3, JUNE 2004 Fig. 4. Proposed load-dependent snubber topology, consisting of an auxiliary flyback converter in DCM. Fig. 5. Flyback converter input current waveform under CCM operation. Fig. 6. Flyback converter primary leakage current waveform under CCM operation, assuming the use of the load-dependent active snubber. maximum output voltage value, its performance is satisfactory for lower voltage values (due to lower duty cycle values of the main converter). Considering now the snubber design, the first assumption that has to be done is that is close to. The auxiliary converter maximum power level can be estimated from (9), if we first measure the primary leakage and main inductance of the main converter. Furthermore, the power dissipation of the auxiliary converter can be expressed by using of (5) as follows: (13)

PAPANIKOLAOU AND TATAKIS: ACTIVE VOLTAGE CLAMP IN FLYBACK CONVERTERS 637 Fig. 7. Block diagram of the PFC control technique that has been implemented. Equation (13) shows that by using a duty cycle value for the maximum load case beyond 50%, the danger of duty cycle value less than 10% for the lower load case is overcome. Combining (9) and (13), we conclude to the following relation for : (14) Moreover, the auxiliary transformer turns ratio has to be selected in such a way, so as to reassure discontinuous conduction. This can be fulfilled if the following relation is true [2]: Taking into account that, (15) becomes (15) (16) Furthermore, the maximum voltage across the auxiliary transistor is (17) Thus, during the selection of both and it has also to be taken into account the desired voltage level across the auxiliary transistor. Finally, after the final transformer construction and by using (12), we are able to select the appropriate power level of the zener diode, by measuring the leakage inductance of the second transformer too, although in general it can be determined as 1% 2% of the maximum output power. As it concerns its Zener voltage, this has to be selected higher than the expected value from (17) and lower than the maximum permitted drain source voltage value of the auxiliary MOSFET [5] (18) Equations (14) and (16) (18) comprise the design tool of the proposed snubber for any load case. In particular, when the main flyback converter is used as a power-factor preregulator, the use of this active voltage suppressor becomes very essential, due to the high peak input voltage. To this specific application, the design equations can be used considering the average dc/dc converter model, where the input voltage is constant and equal to the average rectified mains voltage. Furthermore, for low-voltage application cases, the auxiliary converter free whiling diode can be replaced by a Schottky diode in order to limit the voltage drop. The introduction of synchronous rectification is not necessary, due to the low power level of the auxiliary converter stage. Nevertheless, for practical low voltage applications the optimum solution seems to be the use of the leakage energy in order to supply the control circuit, minimizing so the converter components and improving the total efficiency. V. EXPERIMENTAL RESULTS The proposed active clamp was implemented for the case of an experimental PFC flyback converter in CCM mode, with 3.0 V low output voltage. The PFC technique that has been implemented (its block diagram is exhibited in Fig. 7) is analytically presented in [8]. Due to the low output voltage value, synchronous rectification has been introduced in its structure. Although the leakage energy could be led to the control circuit, due to the power level of the specific application, the alternative of transferring it to the load has been selected so as to validate the above design strategy. The main characteristics of this converter, as well as the selected components of the auxiliary fly-

638 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 3, JUNE 2004 back converter according to the design procedure of the previous paragraph have been evaluated as follows: Parameter Value Main converter operational 220-Vrms 50-Hz mains characteristics supply, 3.0-V/1.0 10-A dc output, 200-kHz constant Maximum theoretical expected voltage on the main converter primary switch Maximum selected voltage on the auxiliary switch switching frequency. V., the maximum mains voltage value,, V. 39.3 mh. 100 V. By selecting the maximum voltage value at 500 V, we are able to use MOSFET devices of lower cost (600-V breakdown voltage). 60, according to (17), where V,, V. Maximum permitted value 0.65, according to (16). Selected value 0.5, in order to be able to deal with large load variations (10% 100% of the maximum load). Selected value although the main converter frequency can be used, we select a much lower one (25 khz), in order to highlight the fact that the auxiliary converter design does not affect the main converter operation. 21.8 mh, according to (14). 7.6% (measured). 10% (measured). Zener power dissipation 0.21 W at maximum load, according to (12). Zener selected voltage 300 V, according to (18). 400 V, according to (17). In order to verify the effectiveness of the designed active snubber, the whole system was initially tested with low input dc voltage. Fig. 8 shows the voltage across the main converter switch with and without using the proposed active clamp, for 50-V/300-mV input/output voltage value, 1.0-A output current value, and 20-kHz switching frequency. Obviously, the use of the auxiliary converter leads to drastic overvoltage suppression, reassuring the safe converter operation at high voltages. Moreover, the converter was additionally tested with 300-V dc input voltage at maximum load (3.0 V 10 A), in order to verify that during its operation as a power factor preregulator the voltage across the main converter will be well below 500 V. Fig. 9 shows the voltage and current waveforms at the primary switch, for 300-V/3.0-V dc input/output voltage value, at 200 khz constant switching frequency and at maximum load. This experimental result proves that the maximum voltage value at the primary switch remains slightly above 400 V, reassuring the converter safe operation as a power-factor preregulator. Fig. 8. Primary switch voltage and output voltage waveforms of the experimental flyback converter (a) without using a snubber and (b) with the use of the proposed active snubber, for 50-V constant input voltage and 300-mV 1.0-A output characteristics (20-kHz main converter switching frequency). Fig. 9. Primary switch voltage (upper) and current (lower) waveforms, under 300-V constant input voltage and at maximum load (3.0 V 10 A), f =200kHz.

PAPANIKOLAOU AND TATAKIS: ACTIVE VOLTAGE CLAMP IN FLYBACK CONVERTERS 639 Fig. 10. Main converter input current and voltage waveforms (without using an RF input filter) at maximum load. Fig. 12. Auxiliary converter primary transformer winding voltage (upper) and current (lower) waveforms at maximum load. Fig. 11. Main converter input current waveform (lower) and voltage waveform (upper) across the snubber capacitor (V ) at maximum load. Fig. 13. Auxiliary converter secondary transformer winding voltage (upper) and current (lower) waveforms at maximum load. Since the above experimental procedure had shown the effectiveness of the proposed snubber, the converter was finally tested as a PFC topology with low output voltage. Fig. 10 shows the input current and voltage waveform (without using an input RF filter) for the maximum load case, while Fig. 11 shows the input current and the voltage across the snubber capacitor. These results highlight the fact that the proposed active clamp topology does not affect the main converter operation, while it manages to keep the primary switch voltage sufficiently close to its theoretical expected value. Thus, the whole structure produces less electromagnetic interference and so it can be easier integrated into the same board with its control circuit. Furthermore, Fig. 12 shows the input current and the primary winding voltage waveforms, while Fig. 13 shows the secondary winding voltage and current waveforms, for the case of the auxiliary converter and for maximum load. The results that are exhibited in these two figures verify the design procedure of the auxiliary flyback. Beyond that, the overvoltage suppression due to Zener diode reverse conduct is obvious in Fig. 13. Finally, Fig. 14 shows the converter efficiency as a function of the load ( is the maximum output current value), compared to the efficiency of an experimental conventional dc power supply, consisted of two converter stages a forward converter that steps down the rectified mains voltage and a buck dc/dc converter with synchronous rectification that produces 3.0-V output dc voltage. The conventional dc power supply operates at the same power level and with the same switching frequency with the experimental single-stage flyback converter. The reason of this comparison is to examine whether the flyback converter efficiency is at an acceptable level. Therefore, by studying Fig. 14, we can conclude that the flyback converter efficiency is at a very high level indeed, since the conventional converter exhibits lower efficiency under any dc-link voltage value, leading to an efficiency improvement of at least 15% at maximum load. Although many double-stage alternatives could be used instead of the series combination of a forward and a buck converter, the efficiency improvement by using the single stage flyback converter would also be noticeable. Thus, the proposed snubber is

640 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 3, JUNE 2004 Fig. 14. Flyback converter efficiency compared to the two converter stages total efficiency, as a function of the load current and the dc voltage link of the two stages. proved to be almost nondissipative, while the large overvoltage suppression contributes to a high total efficiency level. Furthermore, the proposed snubber, as shown in Fig. 10, does not affect the PFC operation since it does not use the main flyback switch for its discharge and so it does not lead to excessive losses and current stresses. Another important advantage is that the voltage spikes elimination contributes to good cross regulation and so it eases the development of multiple dc outputs from a single transformer core, maximizing so the power density. Finally, the overvoltage suppression enables the use of lower voltage transistors reducing the cost. VI. CONCLUSION An alternative active clamptopologyforflyback converters operating in CCM mode has been presented. The main idea of this structure is the introduction of a load-dependent current source that transfers the leakage energy to the input/output stage or even to the control circuit, depending on the converter power level. Experimental results concerning the performance of the proposed topology have been exhibited, verifying its low power consumption and the large overvoltage suppression that it achieves, leading the main flyback converter to a high efficiency level even for the cases of power-factor preregulation and low-voltage supplying. REFERENCES [1] A. Zuccato and L. Rossetto, Understanding and complying with CISPR and IEC 1000 standards on EMC, presented at the EPE 97, Trondheim, Norway, Sept. 1997. [2] K. H. Billings, Handbook of Switchmode Power Supplies. New York: McGraw-Hill, 1989. [3] T. Ninomiya, T. Tanaka, and K. Harada, Analysis and optimization of a nondissipative LC turn-off snubber, IEEE Trans. Power Electron., vol. 3, pp. 147 156, Apr. 1988. [4] S. B. Yaakov and G. Ivensky, Passive lossless snubbers for high frequency PWM converters, presented at the IEEE APEC 99, Dallas, TX, Mar. 1999. [5] ZenBlock: Zener with integrated blocking diode, Philips Semiconductors, Eindhoven, The Netherlands, Applicat. Note, 2000. [6] G. Spiazzi, S. Buso, and D. Tagliavia, A low-loss high-power-factor flyback rectifier for smart power integration, in Proc. IEEE PESC 00, Galway, Ireland, June 2000, pp. 805 810. [7] C.Chuanwen Ji, K. M.K. Mark Smith, and K. M.Keyue M. Smedley, Cross regulation in flyback converters: Solutions, in Proc. IEEE IECON 99, vol. 1, San Jose, CA, Nov. 1999, pp. 174 179. [8] N. P. Papanikolaou, E. J. Rikos, and E. C. Tatakis, A novel technique for high power factor correction in flyback converters: Theoretical analysis and design guidelines, Proc. IEE Elect. Power Applicat., vol. 148, no. 2, pp. 177 186, Mar. 2001. [9] G. A. Karvelis, M. D. Manolarou, P. Malatestas, and S. N. Manias, Analysis and design of nondissipative active clamp for forward converters, Proc. IEE Elect. Power Applicat., vol. 148, no. 5, pp. 419 424, Sept. 2001. [10] L. Rossetto, G. Spiazzi, and P. Tenti, Control techniques for power factor correction converters, in Proc. PEMC 94, Warsaw, Poland, Sept. 20 22, 1994, pp. 1310 1318. Nikolaos P. Papanikolaou received the Dipl. Eng. and Ph.D. degrees in electrical engineering from the University of Patras, Rion-Patras, Greece, in 1998 and 2002, respectively. He is currently a Post-Doctoral Researcher in the Department of Electrical and Computer Engineering, University of Patras. His research interests are focused on power quality improvement issues. Emmanuel C. Tatakis was born in Alexandria, Egypt, in 1957. He received the Electrical Engineering Dipl. degree from the University of Patras, Rion-Patras, Greece, in 1981, and the Ph.D. degree in applied sciences from the University of Brussels, Brussels, Belgium, in 1989. He is currently an Assistant Professor in the Department of Electrical and Computer Engineering, University of Patras. His teaching activities include power electronics and electrical machines. His research interests include switch-mode power supplies, resonant converters, power-factor correction, electrical drive systems, photovoltaic systems, educational methods on electrical machines, and power electronics. Dr. Tatakis is Member of the European Electronics Association, Société Royale Belge des Electriciens, and Technical Chamber of Greece.