Research on combined hybrid charging power supply and its power sharing strategy

In order to meet the needs of the development of large-capacity and modular charging power supplies, a charging power supply based on a series-parallel hybrid structure is proposed, which consists of 4 improved phase-shifted full-bridge circuit modules. The charging power supply adopts a two-stage charging method of constant current first and constant voltage later. The controller adopts a structure of parallel switching of current loop and voltage loop. At the same time, a power sharing strategy of outer loop control plus average current is introduced, and the current and voltage sharing controllers are designed according to the frequency domain analysis method. Finally, a set of 2-parallel and 2-series experimental prototypes are designed. The experimental results show that with this charging circuit topology and control strategy, the imbalance of output current and voltage is less than 5%, which well verifies the correctness of the analysis and design.


1 Introduction

The switching power supply system adopts a series-parallel hybrid structure, which has the characteristics of high reliability, redundant configuration, good module characteristics, etc., and is easy to manage and maintain the system. However, current and voltage equalization measures must be taken between the power modules of the hybrid structure to ensure that the output current and voltage are evenly distributed among the modules. The current equalization technology is divided into the droop method and the active current equalization method. The presence or absence of a current equalization bus is the fundamental difference between the two. The droop method is only suitable for low-power applications. The active current equalization strategy actually includes the control method and the method for forming the current equalization bus. The control methods include outer loop control (OLR), inner loop control and dual loop control; the methods for forming the current equalization bus include the average current (BAP) method and the master-slave (MS) method. Among them, the MS method includes the designated master module method and the automatic master selection method. The charging power supply system designed here adopts the improved phase-shifted full-bridge converter and the power sharing strategy of the OLR+BAP method, which effectively solves the power sharing problem of the power module.

2 Series-parallel hybrid charging power supply topology

The block diagram of the four-module series-parallel hybrid structure is shown in Figure 1. First, modules 1 to 4 are connected in series, and then two series branches are connected in parallel to form a four-module hybrid structure.

The single module circuit is shown in Figure 2. The main circuit adds two clamping diodes VD7 and VD8 to the primary of the transformer of the traditional phase-shifted full-bridge ZVS converter, which can effectively suppress the high-frequency oscillation of the secondary rectifier diode of the transformer and reduce the voltage reverse recovery spike.

3. Outer loop control plus average current power sharing strategy

3.1 Current Sharing Strategy

The power sharing strategy of OLR+BAP method is adopted, and its control circuit is shown in Figure 3. When UI=Ub, the voltage across R Uab=0, then Uc=0, and current sharing is achieved. When there is a current sharing error, U1≠Ub.Uab≠0, then the current sharing regulator outputs Uc≠0, which controls the output current of the power stage by controlling the voltage error amplifier, and finally achieves current sharing.

3.2 Controller Design

The main circuit parameters are: input voltage Uin = 520 V, output voltage Uo = 80 V, transformer primary and secondary turns ratio n = 7:7:24, resonant inductor Lr = 20μH, output filter inductor Lf = 100μH output filter capacitor G = 30μF, switching frequency fs = 50 kHz, voltage sampling coefficient Fv = 0.037 5, current sampling coefficient Fi = 0.087 5. 3.2.1 Design of voltage loop When designing the voltage loop, the influence of the current balancing loop on the voltage loop is not considered. As long as the cut-off frequency of the current balancing loop is far away from the cut-off frequency of the voltage loop, the influence of the current balancing loop on the voltage loop is very small and its influence can be ignored. The voltage closed-loop control block diagram is shown in Figure 4.

The duty cycle of the phase-shifted full-bridge circuit is transferred to the output voltage:

Figure 5 shows the Bode plots before and after the voltage loop is corrected. The Bode plot of the system open-loop transfer function is shown by the dotted line in Figure 5. It can be seen that there is a large steady-state error in the system, and a compensation network needs to be introduced. Here, a PI controller is selected.

It can be seen from formula (2) that the controlled object is a second-order system. The filter's corner frequency. When designing the parameters of the PI controller, the zero point of the PI controller is set at the filter's corner frequency, and fz=Ki1/(2πKp1)=fn=2.91 kHz, Kp1, Ki1 are the proportional and integral coefficients of the PI controller respectively. When determining the crossover frequency fc after compensation, a compromise should be made between system stability and system dynamic response. Here, fc=fn/10=291 Hz is selected. The open-loop transfer function of the voltage loop after compensation is:

According to the designed PI controller, the Bode plot of the system after correction can be obtained as shown in the solid line in Figure 5. It can be seen from the figure that the phase margin of the corrected system is 90.3°, fc=292Hz, and the steady-state performance of the system is significantly improved.


3.2.2 Design of the current sharing loop

According to Figure 4, the control block diagram using the OLR+BAP method shown in Figure 6 can be constructed. From the figure, the open-loop transfer function of the current sharing loop can be obtained as follows:

Figure 7 is the Bode diagram before and after the current balancing loop is corrected. The Bode diagram of the controlled object of the system is shown by the dotted line in Figure 7. As can be seen from the figure, the system has a large steady-state error and a slow response speed, and a correction link needs to be designed. In order to minimize the impact of the current balancing loop on the previously designed voltage loop, and because the general dynamic response requirements for the current balancing loop are not high, the current loop crossing frequency ωc'=12.6 rad.s-1 is taken. In order to meet the suppression of the low-frequency interference of the current loop on the DC bus, the turning frequency ωm'=126 rad.s-1 of the PI correction link is:

Substituting the known parameters, we get Kp2=0.0857, Ki2=37.28, and we have:

From the solid line in Figure 7, we can see that the crossover frequency of the current sharing loop is 43.7 rad.-1, and the two loops do not affect each other. The phase margin is 94.5°, which means that the current sharing loop is stable.

The dual problem of series connection to parallel connection has the same voltage equalization strategy as the parallel current equalization strategy. The voltage equalization controller can be obtained by using the same design method as the current equalization controller, and its current and voltage equalization compensators are:

4 Experimental results

The system uses a state bus to synchronize the operating status of each series-parallel hybrid module. The charging power supply uses a voltage slow-start. When the output voltage reaches the initial terminal voltage of the battery, the system starts the charging current of the battery slowly. After the current rises to the set value, constant current charging is performed. When the system detects that the output voltage reaches the set charging cut-off voltage, the state bus forces each module to switch to constant voltage charging synchronously. During the constant current charging process, the voltage equalizing controller controls the voltage of the two groups of series modules; during the constant voltage charging process, the current equalizing controller controls the current of each parallel branch, thereby ensuring that the power of each charging power module is evenly distributed during the charging process.

Figure 8 shows the output current and output voltage steady-state waveforms of the four-module series-parallel hybrid charging power supply with battery load. As shown in Figure 8a, the output currents of the two parallel branches are 16.1 A and 15.5 A respectively, and the current imbalance is 3.8%; as shown in Figure 8b, the output voltages of the series modules are 43.6 V and 42 V respectively, and the voltage imbalance is 3.7%.

5 Conclusion

A charging power supply based on a series-parallel hybrid structure is proposed. The system adopts an improved phase-shifted full-bridge circuit and a power sharing strategy of outer loop control plus average current method, thereby ensuring that the output power is evenly distributed among the modules. The experimental results prove the correctness of the analysis and design, and the current and voltage imbalance are both less than 5%, meeting the relevant national standards.