A Soft-Switched Square-Wave Half-Bridge DC-DC Converter
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A Soft-Switched Square-Wave Half-Bridge DC-DC Converter C. R. Sullivan S. R. Sanders From IEEE Transactions on Aerospace and Electronic Systems, vol. 33, no. 2, pp. 456–463. °c 1997 IEEE. Personal use of this material is permitted. However, permission to reprint or republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. I. INTRODUCTION This paper introduces a constant-frequency zero-voltage-switched square-wave dc-dc converter Soft-Switched Square-Wave and presents results from experimental half-bridge implementations. The circuit is built with a saturating Half-Bridge DC-DC Converter magnetic element that provides a mechanism to effect both soft switching and control. The square current and voltage waveforms result in low voltage and current stresses on the primary switch devices and on the secondary rectifier diodes, equal to those CHARLES R. SULLIVAN, Member, IEEE in a standard half-bridge pulsewidth modulated SETH R. SANDERS, Member, IEEE (PWM) converter. The primary active switches exhibit University of California zero-voltage switching. The output rectifiers also exhibit zero-voltage switching. (In a center-tapped secondary configuration the rectifier does have small switching losses due to the finite leakage A constant-frequency zero-voltage-switched square-wave inductance between the two secondary windings on dc-dc converter is introduced, and results from experimental the transformer.) The VA rating, and hence physical half-bridge implementations are presented. A nonlinear magnetic size, of the nonlinear reactor is small, a fraction element produces the advantageous square waveforms, and of that of the transformer in an isolated circuit. provides isolated terminals from which the output voltage may The active switches (metal-oxide-semiconductor be controlled. Semiconductor stresses and losses are minimized field-effect transistors (MOSFETs)) are operated both by the square waveforms and by low switching losses. The open loop at a fixed frequency with 50% duty cycle. active switches and the output rectifiers both exhibit zero-voltage Control can be effected by varying the dc current in an auxiliary winding on the nonlinear reactor. switching. The active switches (metal-oxide-semiconductor A plethora of soft-switched and/or resonant field-effect transistors (MOSFETs)) are operated at a fixed converter circuits have emerged over the last ten frequency with 50% duty cycle, which allows the use of a simple years. To the best of the authors’ knowledge, there open-loop gate drive. Experimental implementations verify is no constant-frequency soft-switched half-bridge predicted operation. circuit featuring square current waveforms and simple control. The circuit introduced here is expected to be of use where power levels cannot justify use of a full bridge configuration. In addition, the control and gate drive scheme is considerably simpler than that typically used in soft-switched full-bridge converters. A number of authors [1—4] have reported on soft-switched full-bridge circuits that rely on phase-shifting between the legs of the bridge for control. A potential difficulty with this classofcircuitsisthecomplexityofthegatedrive timing. In particular, in these designs, commutation is achieved via the ringing of leakage inductance (or an auxiliary commutating inductor) and switch capacitance. Careful timing needs to be implemented to allow for zero-voltage transitions. These circuits typically exhibit rectifier switching losses. In any Manuscript received August 9, 1994; revised February 23, 1996. case, phase-shifting is not possible with a half-bridge configuration. IEEE Log No. T-AES/33/2/03152. One class of circuits that has emerged in the The work of C. R. Sullivan was supported by an NSF Graduate literature [5—7] is similar to the circuit introduced Fellowship. here. The class of circuits in [5—7] features square Authors’ addresses: C. R. Sullivan, Thayer School of Engineering, waveforms and zero-voltage switching for the primary 8000 Cummings Hall, Dartmouth College, Hanover, NH 03755; switch devices. This is accomplished through the S. R. Sanders, Dept. of Electrical Engineering and Computer use of nonlinear reactors. The nonlinear reactors are Sciences, 211-73 Cory Hall #1772, University of California, configured differently, but serve similar functions to Berkeley, Berkeley, CA 94720. those in the circuit introduced here. However, in the simple form of the circuits in [5—7], the secondary 0018-9251/97/$10.00 c 1997 IEEE rectifier devices exhibit lossy turn-off transitions. ° 456 IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 33, NO. 2 APRIL 1997 single-ended circuit. This function of a nonlinear reactor is similar to that of the nonlinear reactor in the present work. II. CIRCUIT DESCRIPTION The circuit of Fig. 1 is built from three stages, as are most resonant dc-dc converters. The front end is a half-bridge inverter, while the output stage is a capacitor-loaded center tapped half-bridge rectifier. The salient element of the schematic in Fig. 1 is the Fig. 1. Soft-switched square-wave half-bridge dc-dc converter. nonlinear reactor, shown enclosed in a box. With constant secondary bias current of Ib, the flux-current The circuit introduced here is also similar to characteristic at the primary terminals of this reactor multiple-output PWM circuits that use mag-amp has the form shown in Fig. 2(c). This characteristic is post-regulators for some of the outputs. The circuit the result of combining the flux-current characteristics could be considered an altered version of a half-bridge of the two cores, Fig. 2(a) and (b) (each shifted mag-amp circuit. Half-bridge mag-amp circuits have appropriately to account for the effect of the bias many disadvantages, as discussed in detail in [8]. windings), by summing the two flux values at each Many of these problems result from the use of PWM current point. For this circuit, it is desirable to operate in the primary circuit. Because the circuit introduced the reactor with the flux within the bounds of the here uses a constant 50% duty cycle, it avoids indicated maximum operating range. The reactor has problems with undesired circulating currents during its incremental inductance increase greatly when a the off time of the PWM. There is no requirement current threshold (IbNb) is reached. This clamps the for a freewheeling diode on the output, and in fact current in the primary winding of the reactor near this the absence of a freewheeling diode is beneficial current level (IbNb), and thus gives the converter its in forcing full load current to effect zero-voltage advantageous square current waveforms. switching. The action of the nonlinear reactor can be Another similar use of a saturating reactor is viewed as follows. If the reactor is driven with a suggested in the recent paper of Cuk, et al. [9]. 50% duty cycle square-wave voltage with zero dc This work relies on controlling the reluctance of an component, one would expect to observe a similar auxiliary leg of a transformer core to control the square current waveform that is delayed by a quarter leakage between primary and secondary windings cycle with respect to the drive waveform. The wound on separate legs of this core. The reluctance magnitude of the current waveform is determined of the auxiliary leg, and hence the leakage inductance, entirely by the secondary bias current. When is controlled by a dc bias winding used to saturate this inserted in the half-bridge circuit of Fig. 1, the auxiliary leg. nonlinear reactor faces the sum of the square There have also been a large number of primary voltage waveform and a square voltage publications on soft-switched single-ended circuits. waveform reflected from the secondary. The reflected The single-ended circuits that most closely resemble secondary voltage waveform is approximately the work reported here are in [10, 11]. The paper of in phase with the reactor current since the Erickson, et al. [10] used a nonlinear reactor to clamp capacitor-loaded rectifier is current-controlled from a resonant current waveform in a zero-current-switched the nonlinear reactor. Hence, ideal waveforms for the Fig. 2. Flux-current characteristic of saturating reactor. (a) and (b) are characteristics of individual cores, which are added together to find characteristic of composite reactor, (c). SULLIVAN & SANDERS: SOFT-SWITCHED SQUARE-WAVE HALF-BRIDGE DC-DC CONVERTER 457 Fig. 4. A similar circuit. Since the secondary rectifier is current fed and capacitor loaded, it also exhibits zero-voltage transitions, as long as the two secondary windings are tightly coupled. This is not unlike the rectifier switching behavior in a capacitor-loaded series resonant converter. The rectifier load capacitor may be small because it only handles short-duration transient currents that are generated during the commutation of the nonlinear reactor and rectifier combination. The ratings of the major components are very similar to those of a PWM forward converter. Suppose the circuit is run with an output voltage of VO,an output current of IO, and a conversion ratio of °=(2N), V V °= N N Fig. 3. Ideal circuit waveforms. i.e., O = in (2 ). Here, is the transformer turns ratio, ° accounts for regulation accomplished with the nonlinear reactor, and the factor of two accounts for reactor are as shown in Fig. 3. As such, the reactor the half-bridge configuration. The primary switches current always lags the voltage square wave applied experience a peak voltage-current stress product by the active half-bridge, and zero-voltage switching of 2V =° I . The main transformer primary sees a O ¢ O is readily effected. peak flux—current stress of (VOT=4) IO. A practical The control of the output voltage or current is current source for the bias winding¢ will incorporate effected by controlling the current in a secondary an inductor.