Series-Resonant Single-phase AC-DC Power Supply with Control of Reactive Power

The series-resonant converter is one of the major classes of electronic power converters that are extensively applied in dc-dc conversion. This technology transfers bulk energy efficiently through a series-resonant circuit in the direct path of the energy transfer. This resonant converter forms a high-frequency ac link between the voltage sources connected to the input and output terminals. The high-frequency link is exploited in the application of a modulation process, thus avoiding the use of bulky and expensive low-order harmonic filters. The achievable high-pulse repetition frequency is rooted in the efficient turn-off mechanism of the semiconductor switches and does not compromise their reliability. The classical objective of an ac-dc converter is to control the (average value of the) output waveform (often the output voltage). The energy emanated from the source is adapted by passive filters, often a compromise between volume and effective use. A rectifier-filter network presents a major disadvantage: a power factor less than one caused by distortion of the source current. The resolution of the high-frequency input current of the series-resonant converter introduces the possibility of accurately controlling the waveform of the source current. In combination with the accurate positioning of the high-frequency current pulses, it is possible to influence the exchange of power with the source by the introduced method of active filtering. A power factor equal to one would lead to optimal operation. However the zero crossing of the ac source voltage introduces the necessity to store energy. Bulky low-frequency storage elements increase the specific volume and weight of the converter, while the exchange of energy with storage elements decrease the overall efficiency. The energy stored in the resonant circuit is fundamentally too low to solve this problem. The stored energy in the output capacitor necessary to decrease the highfrequency ripple voltage is available for these purposes. To meet the optimal conditions for the power factor at the source, a bipolar flow of energy is required. ’ I. POWER CIRCUIT CONFIGURATION ACH input and output line is connected to the switchE ing elements. The configuration in Fig. 1 applies to a single-phase input and output converter. As shown in this figure the sinusoidal source voltage e, is connected to the switching matrix SM1, whereas the load with a dc voltage U , is connected to the switching matrix SM2. Both switching matrices are of the half-bridge configuration. The switching matrices are interconnected by a series-resonant circuit constructed with the passive components L, and Manuscript received February 5, 1990; revised April 26, 1991. J . B. Klaassens and M. P. N. van Wesenbeeck are with Delft University H. L. Lauw is with Oregon State University, Corvallis, OR 97222. IEEE Log Number 910501 1 . of Technology, 2600 GA Delft, The Netherlands. C,. It is further to be noted that each switching element is realized by two semiconductors in antiparallel in order to be able to switch the bipolar resonant current i, for each polarity of the voltages connected to the board. This topology facilitates dc and ac input and output and provides the capability of bidirectional power flow, i.e., fourquadrant operation. The resonant circuit elements of the proposed converter in Fig. 1 are formed by the inductor L, and the parallel combination of the capacitors C,I = $, and Cr2 = $Cr. Capacitors C, and the combination of Col = $CO and CO, = $CO are the input and output filter capacitors that are inserted to form a low-impedance path for the high-frequency components of the source and output currents, respectively. Capacitors C, and CO should be large compared to the resonant capacitor C,. This power circuit does not need additional components in order to ensure 1) Natural commutation 2) Unity power factor at the input (no power factor boost circuit is required) 3) An internal frequency as high as necessary by the desire to meet the requirements with respect to the output performance specification, size, and power density without the need to include bulky and expensive low-order harmonic filters 4) Operation of the converter with ac and dc input 5 ) Bidirectional power flow and four-quadrant operation The use of thyristors makes it possible to build a seriesresonant converter with an internal converter frequency of up to a hundred kilohertz limited by the turn-off time of commercially available thyristors. The use of power FET’s makes it possible to increase this frequency to beyond 500 kHz. The use of staggered power modules increases the pulse repetition frequency at the output port of the converter without increasing the internal frequency and, consequently, improves the indicated mode of modulation [lo]. It is noted that size advantage with increasing frequency is not present in this class of power converters, which has to apply a dc link. The thyristor is significantly cheaper and more robust than gate turn-off switches (GTO’s) in addition to facilitating convenient interfacing between the control circuit and the power circuit due to the low gate current required. 0885-8993/92$03.00