Hybrid DC Converter with Wide ZVS Range Bor-Ren Lin, Senior Member, IEEE, and Kuan-Hao Chen
Department of Electrical Engineering
National Yunlin University of Science and Technology, Yunlin 640, Taiwan
Abstract–This paper presents a new soft switching dc-dc converter including a three-level pulse-width modulation (PMW) converter and a series resonant converter with same active switches to extend zero voltage switching (ZVS) range and to reduce circulating current losses on the primary side during freewheeling intervals. The phase-shift PWM scheme is adopted to generate the properly gating signals for active switches in three-level converter and regulate output voltage at the designed voltage level. Series resonant converter with fixed switching frequency is adopted to extend the ZVS range of active switches at lagging leg. The circulating current losses in three-level converter are reduced by parallel connection of output sides of three-level converter and series resonant converter. In order to reduce the circulating current losses in series resonant converter, the switching frequency is higher than the series resonant frequency. Thus, the circulating losses in three-level converter and series resonant converter are all improved. The circuit efficiency of the proposed converter is improved. Finally, experiments with a 1.44k W prototype circuit are provided to verify the effectiveness of the proposed converter.
Index Terms—Soft switching dc-dc converter, full-bridge converter, series resonant converter, zero voltage switching.
Medium or high power ac-dc converters with high power density and high efficiency have been proposed and used in industry switching mode power supplies and plug-in hybrid electric vehicle battery chargers -. In order to meet the power quality demands, single-phase or three-phase bridge/bridgeless power factor correctors (PFC) with boost voltage type have been proposed. Thus, a stable dc bus voltage with V dc=500V-800V is provided by a three-phase PFC. Three-level dc-dc converters - with low voltage stress of active switches are widely adopted in industry applications. Phase-shift pulse-width modulation (PWM) is normally used to generate the gate singles of three-level converters. However, active switches at lagging leg are more difficult to achieve zero voltage switching (ZVS) at light load due to the limited energy stored in primary leakage inductance. If the primary leakage inductance is increased  or external resonant inductance  is added on the primary side, the ZVS range can be improved but the duty cycle loss in freewheeling intervals is also increased that will decrease the effective duty cycle on the secondary side. The more secondary winding turns are needed and it will increase the copper losses of transformer, voltage rating of rectifier diodes and conduction losses of active switches. Auxiliary circuits added on the primary side have been proposed in - to extend the ZVS range. The switching power loss of active switches is reduced but the power losses of additional auxiliary circuits are increased. In -, LLC converter and full-bridge converter share active switches at lagging leg to extend the ZVS range of lagging leg from light load to full load. The circulating current losses on the primary side during freewheeling intervals are the other main issue in conventional three-level converter. The high circulating current will increase circuit conduction losses on active switches and transformer windings and decrease circuit efficiency. The active and passive clamp circuits  and  are adopted at the secondary side to limit voltage overshoots and oscillations across the output diodes when they are turned off and to improve the circulating current losses on the primary side. This paper presents a hybrid three-level ZVS converter including a phase-shift PWM converter and a resonant converter with fixed switching frequency to achieve the functions of wide range of ZVS turn-on for lagging-switch, low circulating current losses and high circuit efficiency. Half-bridge LLC resonant converter shares the same active switches at lagging leg in the conventional three-level PWM converter. Since the LLC converter is operated at f sw (switching frequency) > f r (series resonant frequency), the active switches can be turned on under ZVS from zero to full load with low circulating current losses. The ZVS range of lagging-leg switches can be extended from zero to full load. The leakage inductance or external inductance in three-level converter can be reduced to decrease duty cycle losses and the circulating current during freewheeling intervals. The output sides of LLC converter and three-level PWM converter are connected in parallel with a clamp diode. When the primary voltage of three-level converter is positive or negative, the output power is delivered from input to output through three-level converter. However, the output power is transferred from input to output through LLC converter when three-level converter is operated at freewheeling interval. Thus, three-level converter and LLC converter transfer energy from input to output within the switching cycle to achieve more effective energy transfer. The circulating current of three-level converter during the freewheeling interval can be reduced due to the clamp diode. Finally, the effectiveness of the proposed converter is verified
W prototype circuit.