What Is Charging Mechanism of Chargers?
Charging Mechanism of Chargers
This section illustrates the charging principle of chargers by separately presenting examples of the topological structures of unidirectional and bidirectional charger circuits.
Unidirectional Charging Topology
The charger realizes the conversion between AC and DC through power electronic devices. Inevitably, power electronic devices introduce reactive power, and excessive reactive power can lead to power grid voltage fluctuations, reduced power supply quality, and increased line losses. The ratio of active power to apparent power in a circuit is defined as the power factor. To suppress excessive reactive power supplied by the end-user to the power grid, strict restrictions on the power factor are imposed for both residential and industrial electricity consumption, usually no less than 0.8~0.9. One of the methods adopted is PFC (Power Factor Correction) technology, which can eliminate harmonic pollution from power electronic devices and improve the input power factor.
The single-stage full-bridge PFC technology offers advantages such as simple structure, high efficiency, and high-frequency transformer with double-end excitation, making it suitable for high-power applications. A single-stage full-bridge PFC converter based on the full-bridge structure is shown in Figure 11-21. It operates in two states: upper and lower arm conduction and opposite arm conduction. During upper and lower arm conduction, the current in the input inductor rises. During opposite arm conduction, the current in the input inductor falls. The control system adjusts the ratio (duty cycle) of the upper and lower arm conduction time within the charging and discharging cycle of the input inductor to adjust the magnitude of the current in the input inductor, making the input current a sine wave in phase with the input voltage. This ultimately eliminates high-order current harmonics and achieves power factor correction.
Analyzing the energy flow process, it can be seen that during upper and lower arm conduction, the voltage across the high-frequency transformer is 0, and the output filter capacitor supplies energy to the load; during opposite arm conduction, the high-frequency transformer transfers the energy stored in the input inductor and supplies it
The energy from the input cable is transferred to the secondary side of the transformer. After high-frequency rectification and filtering, it supplies energy to the load. By regulating the duty cycle of the system, the output voltage can be changed, keeping the output voltage at the rated value. Within one operating cycle, the input inductor completes two charge and discharge cycles, and the high-frequency transformer is excited twice, with the two excitation directions being opposite. This utilizes the magnetic core in a push-pull manner, improving the magnetic core utilization rate of the transformer.
Bidirectional Charging Topology
Figure 11-22 shows the topological structure of the main circuit for charging and discharging of a bidirectional charger, which includes a three-phase half-bridge voltage-source PWM rectifier and a bidirectional DC/DC converter.
Three-phase AC power sources are commonly used in industrial high-voltage and high-power applications. Bidirectional means that the energy flow can be from the grid side to the vehicle battery, or from the battery side to the grid side. The three-phase half-bridge voltage-source PWM rectifier in the figure is a type of bidirectional PWM rectifier, which has advantages such as achieving bidirectional energy flow, fast dynamic response, and good steady-state performance. When it is in the rectification state, energy flows out from the grid side, the current is sinusoidal, and its phase is the same as the grid voltage; when it operates in the active inversion state, the energy stored in the electric vehicle battery is fed back to the power grid, and the grid-side current and current waveform are both sinusoidal, with a phase difference of 180°.
The bidirectional DC/DC converter has advantages such as fast dynamic response, high energy conversion efficiency, and fewer power devices. As shown in Figure 11-22, when the charger is charging the electric vehicle battery, switch S1 is conducting, while switch S2 is always off. Therefore, the bidirectional PWM rectifier operates in the rectification state, and the bidirectional DC/DC converter is in the step-down buck state, and energy flows from the grid side to the battery side; when the battery is discharging, switch S2 is off, switch S1 is conducting, the bidirectional DC/DC converter is in the step-up boost state, and the bidirectional PWM rectifier operates in the active inversion state, and the energy stored in the battery is fed back to the power grid through the rectifier.
