This paper presents a new zero-voltage-switching (ZVS) bidirectional dc-dc converter. Compared to the traditional full and half bridge bidirectional dc-dc converters for the similar applications, the new topology has the advantages of simple circuit topology with no total device rating (TDR) penalty, soft-switching implementation without additional devices, high efficiency and simple control. These advantages make the new converter promising for medium and high power applications especially for auxiliary power supply in fuel cell vehicles and power generation where the high power density, low cost, lightweight and high reliability power converters are required. The operating principle, theoretical analysis, and design guidelines are provided in this paper. The simulation and the experimental verifications are also presented.

A new ZVS bidirectional DC-DC converter for fuel cell and battery application

A bidirectional dc-dc converter typically consists of a buck and a boost converters. In order to have high-power density, the converter can be designed to operate in discontinuous conducting mode (DCM) such that the passive inductor can be minimized. The DCM operation associated current ripple can be alleviated by interleaving multiphase currents. However, DCM operation tends to increase turnoff loss because of a high peak current and its associated parasitic ringing due to the oscillation between the inductor and the device output capacitance. Thus, the efficiency is suffered with the conventional DCM operation. Although to reduce the turnoff loss a lossless capacitor snubber can be added across the switch, the energy stored in the capacitor needs to be discharged before device is turned on. This paper adopts a gate signal complimentary control scheme to turn on the nonactive switch and to divert the current into the antiparalleled diode of the active switch so that the main switch can be turned on under zero-voltage condition. This diverted current also eliminates the parasitic ringing in inductor current. For capacitor value selection, there is a tradeoff between turnon and turnoff losses. This paper suggests the optimization of capacitance selection through a series of hardware experiments to ensure the overall power loss minimization under complimentary DCM operating condition. According to the suggested design optimization, a 100-kW hardware prototype is constructed and tested. The experimental results are provided to verify the proposed design approach.

High-Power Density Design of a Soft-Switching High-Power Bidirectional dc–dc Converter

A soft-commutating method and control scheme for an isolated boost full bridge converter is proposed in this paper to implement dual operation of the well-known soft-switching full bridge dc/dc buck converter for bidirectional high power applications. It provides a unique commutation logic to minimize a mismatch between current in the current-fed inductor and current in the leakage inductance of the transformer when commutation takes place, significantly reducing the power rating for a voltage clamping snubber and enabling use of a simple passive clamped snubber. To minimize the mismatch, the method and control scheme utilizes the resonant tank and freewheeling path in the existing full bridge inverter at the voltage-fed side to preset the current in the leakage inductance of the transformer in a resonant manner. Zero-voltage-switching is also achieved for all the switches at the voltage-fed side inverter in boost mode operation. The proposed soft-commutating method is verified through boost mode operation of a 3-kW bidirectional isolated full bridge dc/dc converter developed for fuel cell electric vehicle applications. The tested result verified the isolated boost converter can operate at an input voltage of 8.5–15V and an output voltage of 250–420V with a peak efficiency of 93% and an average efficiency of 88% at 55-kHz switching frequency with 72 $^circ$ C automotive coolant.

A Novel Soft-Commutating Isolated Boost Full-Bridge ZVS-PWM DC&#8211;DC Converter for Bidirectional High Power Applications

A new design approach achieving very high conversion efficiency in low-voltage high-power isolated boost dc-dc converters is presented. The transformer eddy-current and proximity effects are analyzed, demonstrating that an extensive interleaving of primary and secondary windings is needed to avoid high winding losses. The analysis of transformer leakage inductance reveals that extremely low leakage inductance can be achieved, allowing stored energy to be dissipated. Power MOSFETs fully rated for repetitive avalanches allow primary-side voltage clamp circuits to be eliminated. The oversizing of the primary-switch voltage rating can thus be avoided, significantly reducing switch-conduction losses. Finally, silicon carbide rectifying diodes allow fast diode turn-off, further reducing losses. Detailed test results from a 1.5-kW full-bridge boost dc-dc converter verify the theoretical analysis and demonstrate very high conversion efficiency. The efficiency at minimum input voltage and maximum power is 96.8%. The maximum efficiency of the proposed converter is 98%.

High-Efficiency Isolated Boost DC–DC Converter for High-Power Low-Voltage Fuel-Cell Applications

A soft-commutating method and control scheme for an isolated boost full bridge converter is proposed in this paper to implement dual operation of the well-known soft switching full bridge DC/DC (buck) converter for bi-directional high power applications. It provides a unique commutation logic to minimize a mismatch between current in the current-fed inductor and current in the leakage inductance of the transformer when commutation takes place, significantly reducing the power rating for a voltage clamping snubber and enabling use of a simple passive clamped snubber. To minimize the mismatch, the method and control scheme utilizes the resonant tank and freewheeling path in the existing full bridge inverter at the voltage-fed side to preset the current in the leakage inductance of the transformer in a resonant manner. Zero-voltage-switching (ZVS) is also achieved for all the switches at the voltage-fed side inverter in boost mode operation. The proposed soft-commutating method is verified through boost mode operation of a 3 kW bidirectional isolated full bridge DC/DC converter developed for fuel cell electric vehicle (EV) applications.

A novel soft-commutating isolated boost full-bridge ZVS-PWM DC-DC converter for bidirectional high power applications

Two new start-up schemes for isolated full-bridge boost converters are proposed in this paper. The control timing for each scheme, which is compatible with pulse-width modulated (PWM) control timing for normal boost mode operation, are investigated. Design considerations on the relationship between the turns ratios of the boost choke windings and the main transformer windings, and its effects on the operation of the converter, are studied. The two proposed start-up schemes are experimentally verified on a 1.6 kW, 12 V/288 V prototype.

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New start-up schemes for isolated full-bridge boost converters

The PWM control, design and implementation issues of the bi-directional dual full-bridge DC/DC converter with a unified soft-switching scheme and soft-start capability, which was proposed in a companion paper, are presented in this part of the two-paper sequel. Test results on a 5 kW prototype converter, which is connected between a 12 V battery and a high voltage bus, and targeted for alternative energy applications, validate the secure operation, high reliability and superior efficiency of the proposed converter topology.

Design, implementation, and experimental results of bi-directional full-bridge DC/DC converter with unified soft-switching scheme and soft-starting capability

Two new start-up schemes for isolated full-bridge boost converters are proposed in this paper. The control timing for each scheme, which is compatible with the PWM control timing for the normal boost mode operation, is investigated. Design considerations on the relationships between the turns ratios of the boost choke windings and the main transformer windings, and its effects on the operation of the converter, are studied. The two proposed start-up schemes are experimentally verified on a 1.6 kW, 12 V/288 V prototype.

High power boost converter has become the essential part of the distributed power system that enables energy to be fully utilized in fuel cell powered electric vehicles and stationary power systems. This paper presents a DSP-based fully digital control implementation for an interleaved high power DC-DC boost converter. A dual-loop average current mode current control method is employed to achieve the fast transient response. Different anti-wind up schemes for a typical PI-controller are evaluated through simulations and experiments. Simulation and experiment results of the 20-kW boost converter under a start-up condition and load transient condition are also presented. The results show that this typical controller with proper anti-wind up scheme achieves a better transient performance than without the anti-wind up scheme.

A DSP based controller for high-power interleaved boost converters

Planar transformers provide a distinct advantage over the traditional transformer. However, when the planar transformer is integrated into a power circuit, the interconnect parasitic effects arise that are not shown in a traditional wire-wound transformer. For typical soft-switching converters, a specific leakage inductance is generally needed to charge and discharge the device output capacitances to achieve zero-voltage turn on. This designated leakage inductance value needs to be large enough to extend the zero-voltage switching range. However, it was found that this hard-to-come-by leakage inductance in a planar structure may be overshadowed by the circuit interconnect parasitic inductance. This paper presents a design integration that incorporates the planar transformer and a full-bridge dc/dc converter. Through design calculation, finite element analysis, and experimental verification, it was proven that the role of the parasitic inductance is indeed far exceeding the transformer leakage inductance. Thus, for any design optimization, it is necessary to take into account the parasitic inductance in the integrated structure.

Lizhi Zhu

Papers

New start-up schemes for isolated full-bridge boost converters

Design, implementation, and experimental results of bi-directional full-bridge DC/DC converter with unified soft-switching scheme and soft-starting capability

New start-up schemes for isolated full-bridge boost converters

A DSP based controller for high-power interleaved boost converters

The role of parasitic inductance in high-power planar transformer design and converter integration