Wen Cai

Stability analysis and voltage control method based on virtual resistor and proportional voltage feedback loop for cascaded DC-DC converters

This paper proposes a voltage control method based on virtual resistor and proportional voltage feedback loop for cascaded DC-DC converters. The closed-loop DC-DC converter would act as negative impedance load for the front-end converter and might lead to system instability. Based on the analysis of DC-DC converter with distributed parameters, a modified voltage control strategy based on virtual resistor is proposed which is to only add a proportional feedback of inductor current in the control block. Thus, no extra power loss would be generated. However, under some conditions, this method cannot achieve system stabilization. Then, an additional output voltage feedback loop is introduced which can not only stabilize the system under different cases but also improve system performance. Experimental results verified theoretical analysis and feasibility of the proposed control methods.

Stability Optimization Method Based on Virtual Resistor and Nonunity Voltage Feedback Loop for Cascaded DC–DC Converters

This paper proposes a stability optimization method based on virtual resistor and nonunity voltage feedback loop for cascaded dc-dc converters. Oscillating phenomenon or instability would occur occasionally with two or more closed-loop dc-dc converters in series. The virtual resistor and nonunity voltage feedback are used to modify the feedback loop instead of only a direct voltage feedback to improve stability and get rid of oscillating behavior. Based on the stability analysis of dc-dc converters with distributed parameters, several cases have been derived. After that, relative to different cases, two modified methods based on virtual resistor and nonunity voltage feedback loop are proposed to stabilize the overall system. With these methods, no extra power loss would be generated, and it is easy to embed them into any conventional control system. Experimental results verified the theoretical analysis and feasibility of the proposed control methods.

A single-stage single-phase isolated AC-DC converter based on LLC resonant unit and T-type three-level unit for battery charging applications

This paper proposes a four-switch single-phase single-stage isolated AC-DC converter with high efficiency for battery charging applications. By combining one LLC resonant unit and one T-type three-level unit with switch multiplexing technique, a single-stage single-phase isolated LLC resonant (SSPLLC) converter is proposed to achieve AC-DC conversion. In comparison with conventional two-stage isolated AC-DC topologies, the proposed topology works in discontinuous conduction mode (DCM) with small inductor requirement. It can improve both efficiency and power density because of soft-switching capability and wide operating range. The mode analysis, steady-state operation performance and control method are discussed sequentially. Experimental results using a 250W prototype are presented to verify the feasibility and superior performance of the proposed single-stage AC-DC converter.

An integrated multi-port power converter with small capacitance requirement for switched reluctance machine

This paper presents an integrated multi-port converter(IMPC) for Switched Reluctance Machine (SRM). The conventional drive for SRM is based on asymmetrical H-bridge which needs a large capacitor on the dc bus to absorb the transient energy caused by turn-off process. The proposed topology is a multi-port converter which employs the capacitor as an extra port to the dc bus and is integrated based on switch multiplexing. With such converter, the capacitance can be reduced significantly by increasing the capacitor voltage ripple. In addition, the current commutation process can be accelerated with high voltage of capacitor based on such topology. The requirement of capacitance in asymmetrical H-bridge and the proposed topology is analyzed in detail and the corresponding control method for the proposed converter is introduced briefly as well. Simulation results in Matlab/Simulink verified the feasibility of the proposed topology and its superior performance compared with asymmetrical H-bridge structure.

Control method to balance capacitor voltages in split-AC switched reluctance motor drives during startup and at low speeds

This paper proposes a control method to balance capacitor voltages for a four-switch single-stage two phase switched reluctance motor drive with integrated power factor correction. Initial alignment process is necessary for SRM before operation. However, the phase current is unbalanced during that time, which would cause over-voltage issue on DC capacitors as split-AC structure is used. This problem also exists when the motor speed is low, as in startup of electric vehicle. Based on the unified model, an advanced control method is proposed to eliminate the voltage difference between the capacitors. This method does not require any other circuits or components. It also does not affect the performance during normal conditions. Experiments are performed to verify feasibility of the control method.

A power decoupling method with small capacitance requirement based on single-phase quasi-Z-source inverter for DC microgrid applications

This paper proposes a power decoupling method based on single-phase quasi-Z-source inverter (SPQZSI) for DC microgrid applications. Single-phase inverter would cause harmonic issues especially voltage ripple in DC microgrid because of low-frequency (100/120Hz) power ripple. Non-linear load or source would generate other harmonic on the common DC link as well. In order to achieve power decoupling, large electrolytic capacitor is required and connected to DC link in H-bridge inverters. However, electrolytic capacitor would affect the power density, reliability and lifetime of DC microgrid. Quasi-Z-source inverter is utilized instead for capacitance reduction. In terms of low-frequency ripple control with SPQZSI, an effective control method is necessary to regulate the current/voltage of both AC port and DC port. The theoretical analysis about capacitance requirement with power decoupling in SPQZSI is done first. Next, an advanced control method based on generalised predictive control is proposed to decouple low-frequency power ripple. The corresponding control law and predictor are designed as well. Experimental results with 500W prototype verify the stability, feasibility and superior performance of the proposed power decoupling method based on SPQZSI.

Control Algorithm for Soft Start of Split-AC-Switched-Reluctance Motor Drives

This paper proposes a control method to balance capacitor voltages in a four-switch single-stage two-phase-switched-reluctance motor drive with integrated power factor correction during startup and at low speeds. Initial alignment process is incorporated for switched-reluctance machine before startup. However, the phase currents are unbalanced during that time. This will cause overvoltage on dc capacitors in split-ac configuration leadings to their failure. This problem also exists when the motor speed is very low, as observed in the startup process of compressors. Based on a unified model, an effective control method is proposed to eliminate the voltage difference between the capacitors. This method does not require any additional circuits or components. It also does not affect the performance during normal mode of operation. Experiments are performed to verify the feasibility of the control method.

Seamless transition control between motoring and generating modes of a bidirectional multi-port power converter used in automotive SRM drive

In this paper, a control strategy providing seamless transition between motoring and generating modes within a bidirectional integrated multi-port power converter (IMPC) used in switched reluctance machine (SRM) drive is presented. The IMPC offers capacitance reduction in the dc bus while maintaining a very small ripple on the input current from or output current to the dc source under both modes of operation. This facilitates a direct connection to the battery pack in an electric propulsion system. In addition, the braking torque can be controlled even under high speed thanks to the adjustable dc bus voltage. Model of the SRM drive system incorporating the converter and SRM is established for both motoring and generating modes. Accordingly, the seamless transition strategy is developed based on IMPC. Finally, the effectiveness of the proposed system is validated by simulation and experimental results.

A quasi-Z-source integrated multi-port power converter with reduced capacitance for switched reluctance motor drives

This paper presents a quasi Z-source integrated multiport converter (ZIMPC) for switched reluctance motor (SRM) drives to reduce the dc link capacitance. In conventional SRM drives, employing multi-phase asymmetrical H-bridge (ASHB) topology, large capacitors are necessary to absorb the transient energy during phase current commutation. However, electrolytic capacitors would affect the lifetime, cost and power density of the drive system. With switch multiplexing technique, a Z-source integrated multiport power converter is derived to achieve power ripple reduction using relatively small capacitance. Corresponding control method is designed and developed for the proposed SRM drive. Also, the ZIMPC can boost the equivalent phase exciting voltage and widen the constant power speed range (CPSR). At last, simulation and experimental results verify the feasibility of the proposed ZIMPC and its superior performance with smaller capacitance as compared with ASHB.

Low-power LLC resonant AC-DC converter for phone charging applications

A single-stage LLC resonant AC-DC converter is presented to replace conventional flyback type converters for phone charging application. Single-stage flyback AC-DC converter is widely used in low-power AC-DC converters, especially phone charging applications. However, for flyback AC-DC converter, the switching frequency is limited due to the hard-switching and switching losses. Its output current ripple is also higher than other topologies. On the other hand, LLC resonant AC-DC converter can achieve higher switching frequency thanks to zero voltage switching (ZVS). Higher switching frequency can reduce the output current ripple, and it allows to reduce the size of the passive components and the cost. For better quality and reliability with long lifespan of a charger, a compact 135 kHz, 10W (5V/2A) single-stage LLC resonant AC-DC converter is implemented and analyzed to verify its feasibility.