This paper presents the hardware topology of the on-board charger and analyzes its working principle. On this basis, a PR digital control strategy with voltage and current closed loops is proposed. To verify the effectiveness of the proposed control method, a digital control system based on voltage and current closed loops is constructed. A simulation model of the system is built using PSIM software, and finally, the effectiveness and reliability of the control method are verified through experiments.
The hardware topology of the on-board charger based on the interleaved BUCK three-level structure is shown in Figure 1. The input side is AC 380 ± 10% V, which comes from the output of the ACM (Auxiliary Converter Module) on the vehicle. Since the output voltage of the ACM contains harmonic components and high-frequency interference, an EMI filter is added on the input side to filter out harmonics and eliminate high-frequency interference. D1-D6 form a three-phase full-bridge rectifier circuit. To make the voltage on the DC bus capacitors C1, C2, C3, and C4 rise slowly, avoiding the impact of high voltage on the capacitors which may affect their service life, a pre-charging circuit is incorporated into the circuit. S1 is the charging contactor, R1 is the charging resistor, and S2 is the separating contactor. This largely protects the capacitors and prevents damage.
Figure 1. Topology Structure of the On-Board Charger
In the figure, C1, C2, C3, and C4 are not only DC bus voltage-stabilizing capacitors but also voltage-dividing capacitors of the subsequent three-level DC step-down circuit. They have large and equal capacities, and the voltage across each of C1, C2, C3, and C4 is half of the DC bus voltage. D7 & D8 are freewheeling diodes, L2 is the filter inductor, and C5 is the filter capacitor. The IGBTs VT1 and VT2 in Figure 1 work interleaved, meaning their drive signals are phase-shifted by 180°. The operating modes of the switching tubes within one working cycle are not analyzed in detail here, and reference can be made to [2].
When the catenary is normal, the pantograph of the metro vehicle rises. After the pantograph rises and the ACM works normally, the input side of the on-board charger receives AC 380V ± 10% voltage. This voltage is converted into a DC voltage signal of 450V-564V after rectification, then converted to an output voltage range of 78V-130V DC through the DC step-down three-level converter, and finally, the output voltage and current are stably output after passing through the filter circuit.
According to the characteristics of the on-board charger, a dual closed-loop control system based on the TMS320F28335 DSP (Digital Signal Processor) is designed, as shown in Figure 2.
In the figure, PT is the voltage sensor and CT is the current sensor. It can be seen from Figure 2 that according to different charging stages of the battery, the control system operates with either dual voltage PR controllers or an outer current PR controller combined with an inner bus voltage PR controller. These controllers interact to control the turn-on and turn-off of the switching tubes VT1 and VT2. The output voltage or output current, after being processed by the PR regulator, serves as the feedback signal for the flying capacitor voltage loop. It is modulated with the carrier signal to generate PWM signals for driving the lower tube VT2. The flying capacitor voltage loop, after being processed by the PR regulator, acts as the feedback signal for the output voltage loop or output current loop. It is modulated with the carrier signal to generate PWM signals for driving the upper tube VT1 (the drive signals of VT1 and VT2 are interleaved and phase-shifted by 180°).
Figure 2. Digital Control System of the On-Board Charger
When the converter is in a stable state, if the voltage across the flying capacitors (i.e., C1 & C3 or C2 & C4) rises due to disturbance, as shown in Figure 2 where Vc > 1/2 Vbus, the output of the capacitor voltage PI regulator becomes negative (i.e., Veb < 0). Consequently, the result of Vea – Veb increases, leading to an increase in the duty cycle of VT1 and a decrease in Vc. The decrease in Vc causes Veb + Vea to decrease, which in turn reduces the duty cycle of Q2, thereby stabilizing the capacitor voltage at 1/2 Vbus [3] [4]. The output voltage and output current are stably output according to the charging curve of the battery.
To verify the rationality of the digital control strategy, a system simulation model is established using the dual voltage closed-loop PR controller as an example, as shown in Figure 3:
In Figure 3, the switching frequency is set to 15kHz, the DC input voltage (i.e., the voltage after uncontrolled rectification by the three-phase full bridge in Figure 1) is set to 520V DC, C2 = C3 = 100 μF, L1 = 0.5 mH, C1 = 500 μF, and the load R is set to 500 Ω.
It can be seen from Figure 4 that the switching tubes VT1 & VT2 are turned on interleaved with a phase shift of 180°, which is completely consistent with the theoretical analysis.
Figure 5 shows the output voltage waveform of the on-board charger. It can be observed that the output voltage waveform has no oscillation or excessive ripple, and the output voltage is stable and reliable.
Figure 3. Simulation Model of the On-Board Charger Control System
Figure 4. Drive PWM Waveforms
Figure 5. Output Voltage Waveform
To verify the control strategy of the on-board charger based on the interleaved BUCK three-level structure, experimental research is conducted on a prototype. The voltage at the AC input terminal is adjusted to make the DC bus voltage stably output 520V DC. Figure 6 shows the drive signals of VT1 and VT2. It can be seen that the two drive PWM signals have the same amplitude and a phase difference of 180°. Dead zones are set at the rising and falling edges of the drive waveforms to prevent the two tubes from being turned on or off simultaneously.
Figure 6. Drive Signals of Switching Tubes VT1 & VT2
In Figure 7, Channel 3 shows the charging current waveform of the charger. It can be seen that there is a soft-start process in the initial stage of charging, lasting for 12 seconds. After the 12-second soft-start, the output current stabilizes at 30 A without oscillation, and the ripple is within ±5%, meeting the requirements.
