The DC/DC converter is the core part of the two-stage electric vehicle Onboard Charger. At present, the phase-shifted full-bridge soft-switching DC/DC converter has problems such as difficulty in commutation of the lagging leg, voltage fluctuation on the secondary side of the transformer, and low efficiency. A full-bridge DC/DC converter with two clamp diodes and synchronous rectification is proposed in this paper. Clamping diodes are used to suppress the voltage oscillation of the secondary side of the transformer and provide the commutating energy of the lagging leg. Synchronous rectification reduces the loss of the switching device. The operating principle and control method of the DC/DC converter are analyzed, and the switching device loss is calculated. The simulation and experimental results show that, compared with the traditional DC/DC converter, the voltage impulse of the secondary side of the transformer is smaller, the efficiency is higher, and the soft switch can be realized in a wide load range, which satisfies the requirement of fast charging of vehicle-mounted batteries.

1. Introduction

Electric vehicles (EV) have developed rapidly because of its high efficiency and pollution-free advantages. The increase in the number of electric vehicles increases the technical requirements for onboard chargers [1]. Due to the limitation of the interior space, onboard charger (OBC) needs to meet the requirements of high power density, high charging efficiency, and good heat dissipation effect [2]. Two-stage OBC includes PFC converter and isolated DC-DC converter. The former converts AC to DC; the latter provides a wide range of DC to charge the vehicle-mounted batteries. The research objectives of EV onboard converters are mostly focused on improving operational efficiency and reducing volume [3]. The research of PFC is relatively mature, and the existing research has achieved more than 98% efficiency [4]. Therefore, its overall efficiency and power density depend more on the design and operation of the DC-DC converter. At present, the high-frequency switching converter is widely used in the DC-DC converter. The switching frequency is generally at the tens of kHz level [5]. Although the increase of switching frequency greatly reduces the volume of equipment, it also brings about problems such as increasing switching loss, decreasing efficiency, and increasing electromagnetic interference. In order to solve these problems, soft switching technologies such as ZVS, ZCS, and LLC have emerged [6, 7]. The application of this technology in conventional switching power supply topology can reduce switching loss and noise interference of power switching devices in the high-frequency state of the converter, which can further improve efficiency and power density and reduce volume and weight of converter.

Traditional topologies of DC-DC converters used in OBC include full-bridge PWM circuit and full-bridge resonant circuit (including LLC resonance and series resonance) [8]. LLC converter has the advantages of switchless off voltage spike and small circulating current power. Combining with the charging curve of vehicle batteries, the output voltage range of DC-DC converter is wider, the switching frequency of converter will deviate greatly from the resonant frequency, and the system loss will increase [9]. Full-bridge PWM circuit can adapt to wide output voltage range and fixed switching frequency, but the traditional full-bridge PWM converter has large reactive power circulation and cannot achieve soft switching under a light load [10]. For this reason, a phase-shifted full-bridge converter with the controllable auxiliary current is proposed, which realizes full-load soft switch of switching transistors, but the cost is high and the control is complicated [11]. The reverse recovery loss of rectifier diode can reduce by using phase-shifting control of the secondary side of the transformer, but the efficiency is low at full load [12]. An improved ZVS phase-shifted full-bridge DC/DC converter is proposed in this paper. Two clamping diodes are used to eliminate the voltage oscillation of the secondary rectifier. Synchronous rectifier (SR) is used to reduce system losses. Finally, an experimental prototype is built in the laboratory.

2. Onboard Charging Method for Electric Vehicles

2.1. Battery Model

At present, there are three-component lithium batteries and lithium iron phosphate batteries for electric vehicles. The three-component lithium batteries have a high energy density, but low charge current and fast capacity attenuation, which are mainly used in Tesla electric vehicles. Lithium iron phosphate batteries are widely used in many electric vehicles because of their high charge-discharge current, slow capacity attenuation, and high safety. The Thevenin equivalent circuit model of lithium iron phosphate battery is shown in Figure 1 [13], where Vcc is open-circuit voltage, Re is battery internal resistance, Rp is polarization resistance, and Cp is polarization capacitance. The equivalent impedance Zo of the batteries is

2.2. Circuit Structure

According to the different topology of charger converter, OBC has a single-stage structure and two-stage structure. The single-stage structure has the characteristics of simple structure and low cost, but it only has a one-stage conversion, which limits the range of output voltage, and the effect of power factor, current harmonic suppression, and conversion efficiency is not ideal [14]. Considering the suppression of input current harmonics, improvement of power factor, and power processing ability, AC-DC converter is divided into the AC-DC converter and the DC-DC converter, as shown in Figure 2. The former AC-DC converter usually uses Boost circuit for power factor correction, while the latter DC-DC converter generally uses isolation converter [15]. On the basis of ensuring the safety of the converter, it provides DC current with a small load ripple coefficient. Two-stage vehicle charger is studied in the paper.

2.3. Charging Strategy

Vehicle-mounted batteries are the power source of electric vehicles, so it is of great significance for the promotion of electric vehicles to adopt charging methods that can achieve rapid charging and have less damage to battery life [16]. At present, battery charging methods mainly include constant current charging method, constant voltage charging method, and stage charging method [17]. Constant current charging is simple to use and easy to control, but if the charging current is too small, the charging time will be too long. If the selected charging current is too large, it is easy to overcharge at the later stage of charging, which will have a great impact on the battery plate, thus affecting the battery life [18]. The constant voltage charging method is also easy to operate and can avoid the problem of the battery overcharging in the later stage of charging. However, in the early stage of charging, because of the lower electromotive force at both ends of the battery, the charging current is larger. The current shock will lead to the bending of the battery plate and the rapid rise of the temperature of the battery, which will affect the life of the battery. In addition, if the selected charging voltage is too low, it will lead to insufficient battery charging and shorten battery life [19]. The stage charging method generally includes a two-stage charging method and a three-stage charging method [20]. The two-stage charging method refers to the constant current charging before the battery is charged. When the voltage at both ends of the battery reaches a certain amplitude, it is switched to constant voltage charging. The charging curve is shown in Figure 3. The two-stage charging method combines the advantages of constant current charging method and constant voltage charging method, avoids the problems of excessive charging current in the early stage and easy overcharging in the later stage, and has high charging efficiency, which can meet the charging demand of lithium iron phosphate batteries. The two-stage charging method is adopted in this paper.

When charging in constant current mode, the output voltage of DC/DC converter of vehicle charger varies in a wide range. In constant voltage mode, the output current of the converter decreases from full load to zero. Therefore, the design of onboard charging power supply should meet the following requirements: wide range adjustment of the output voltage; soft switching operation with wide load range; high power density and voltage and current stress requirements.

2.4. Constant Current and Constant Voltage Switching Control Method

Constant current and constant voltage switching charging control method is as shown in Figure 4. In the constant voltage stage, the output voltage is compared with the reference voltage . The error is obtained by the PI controller and the modulation signal WCV is obtained. Similarly, the modulation signal WCV can be obtained.

The switching mode is “take a smaller value”. At the initial stage of charging, the equivalent internal resistance of the battery is small and the charging current is large. At this time, WCC < WCV, the battery is charged in constant current mode. When the voltage rises to the electric reference value , WCV < WCC, the battery is charged in constant voltage mode, and the charging current decreases continuously until the end of charging.

3. Operation Principle of PWM DC/DC Converter

3.1. Main Circuit Topology

Traditional ZVS full bridge main circuit topology is shown in Figure 5. Vin is the input DC power supply. Switching devices (including and ) form the inverter bridge, is the resonant inductor, T is a high frequency transformer, D1 and D2 form a high frequency rectifier bridge, and Lf and Cf form a high frequency filter. The converter has problems such as difficulty in commutation of the lagging leg, voltage fluctuation on the secondary side of the transformer, and low efficiency.

The improved ZVS phase-shifted full-bridge main circuit topology with clamping diodes and operating waveforms are shown in Figure 6. Clamping diodes D5 and D6 are added to suppress the voltage oscillation of rectifier and increase the soft switch range; synchronous rectification is applied to the secondary side of the transformer, the conduction resistance of MOSFET is smaller than that of diode, and it can improve circuit efficiency.

3.2. Analysis of the Operating Process

The operating process of the phase-shifted full-bridge ZVS DC/DC converter can be referred to in [21], which is not discussed here. The principle of suppressing voltage oscillation of the secondary side of the transformer is explained in detail in Figure 7.

During [], the equivalent circuit is shown in Figure 7(a). At , the load current all flows through Q6 and the current passing through Q5 drops to zero and charges the junction capacitor C5 of Q5 at the same time. After , the resonant inductance and C5 resonance, the voltages of the SR device Q5, and the secondary side of the transformer are, respectively, as follows:

Expression shows that the voltage on the output rectifier device oscillates equally when no clamping diodes are added. Its peak value is 4/n. Because of the damping effect in the resonance process, the voltage on the output rectifier device will gradually decrease to 2/n, and the secondary voltage of the transformer will gradually decrease to /n.

During [], the equivalent circuit is shown in Figure 7(b). At , because the reverse charging current of Q5 junction capacitor disappears, the current is larger than the current converted to primary, so decreases rapidly until the two are equal. Because the inductance current cannot mutate, >. The neutral point voltage of two clamping diodes is zero, D6 is turned on, and the voltage of C5 is clamped to 2/n; that is, =2/n. Compared with expression , the voltage oscillation of the Q5 is suppressed. At this point, the rising slope of is

At , =, the current flowing through the clamping diode D6 drops to zero, and D6 is turned off naturally. Then and increase at the same slope. At this time, power is transferred from the primary side to the secondary side.

3.3. Driving Logic Selection of SR Devices

According to the driving waveforms of Q1~Q4, the current waveforms and driving waveforms of Q5~Q6 are shown in Figure 8. The driving signals of Q5 and Q6 can be obtained in the following three ways.(1)The same driving logic as lagging leg Q3(Q2)(2)And logic: the driving signals of Q1(Q4) and Q3 (Q2) are carried out and logic(3)Or logic: the driving signals of Q1(Q4) and Q3 (Q2) are carried out or logic

When SR device has no driving signal, the current flows through its body diode, but the conduction voltage drop of the body diode is larger than that of the conductive channel, which leads to the increase of conduction loss. Therefore, the current should flow through the conductive channel as far as possible instead of the body diode. By comparison, the time ratio of or logic is the highest and the total conduction loss is the lowest, so the logic is used to obtain the synchronous driving signals.

4. Loss Calculation of Phase-Shifted Full-Bridge ZVS DC/DC Converter

According to the application requirement of the onboard charger, the device parameters and models are shown in Table 1.

4.1. Loss Model of MOSFET

The losses of MOS transistors are mainly considered from four aspects: switching losses, conduction losses, reverse recovery losses, and driving losses. The equivalent model of the MOS transistor is shown in Figure 9, where is gate resistances, , , and , are the capacitors between the electrodes of the MOS transistor, respectively. In order to simplify the analysis, the parasitic inductance of MOS transistors is not considered.

Since the turn-off process is opposite to the start-up process, only the start-up process is discussed, as shown in Figure 10. At , the gate voltage is added to the MOS transistor. During ~, is charged, rises, and reaches the turn-on threshold voltage of the MOS transistor at . During , the MOS transistor is turned on and the drain current starts to rise. At , the drain current rises to the rated current Id, while rises to the Miller platform voltage . During , starts charging, the drain voltage decreases, and the gate voltage remains unchanged. During , the gate voltage begins to charge and at the same time, and the gate voltage continues to rise. At , =, the MOS transistor is fully turned on.

4.2. Loss Calculation
4.2.1. Prestage Power Loss Calculation

The loss of MOSFET during the start-up process is as follows:

Similarly, the turn-off process is similar to the start-up process, so the switching losses are as follows:

The conduction loss of MOSFET is

where is the on-state resistance of MOSFET.

Reverse recovery loss of MOSFET is

where is the reverse recovery charge of the body diode.

The driving loss of the MOSFET is

where is the gate charge of MOSFET.

4.2.2. Poststage Loss Calculation

In order to reduce the conduction loss of the poststage synchronous rectifier circuit, two SR transistors are connected in parallel. The total conduction loss of the poststage synchronous rectifier circuit is 1/2 of the single transistor:

The conduction loss of the current flowing through the body diode is as follows:where and are an on-state voltage drop and on-state resistance of body diode, respectively.

Then the total conduction loss of each MOSFET is

According to the selection parameters in Table 1 and the above analysis, the losses of the prestage and poststage can be calculated, as shown in Table 2.

According to Table 2, the efficiency values of the two converters are shown in Figure 11. It can be concluded that the efficiency of the improved DC/DC converter is 5% higher than that of the traditional DC/DC converter.

5. Simulation and Experiment

The main parameters of phase-shifted full-bridge DC/DC converter for OBC are shown in Table 3.

5.1. Simulation

The main waveforms of DC/DC converter are shown in Figure 12, which is in turn transformer primary current , resonant inductance current , two clamping diode currents iD5 and iD6, transformer primary voltage , and transformer secondary voltage . It can be seen that there is basically no voltage spike in the secondary voltage of the transformer. At the same time, the clamp diode can turn on only once in a cycle, which reduces the conduction loss caused by clamp diode, it shows the rationality of simulation parameter design.

Partially amplified waveforms with and without clamping diodes are shown in Figure 13. Compared with Figures 13(a) and 13(b), the voltage oscillation at both ends of the transformer secondary voltage and SR tube Q5 is suppressed by adding a clamp diode.

The drain-source voltage , driving voltage and drain current waveforms of lagging leg Q3 at rated load and 1/5 rated load are shown in Figures 14(a) and 14(b) respectively. It can be seen that, under the rated load and 1/5 rated load output conditions of Q3, has been reduced to zero before appears, and zero voltage soft switching can be realized.

5.2. Experiment

According to the parameters of Table 3, a 600W onboard charger test bench is designed as shown in Figure 15. The semiphysical simulation controller RT-LAB performs constant voltage-constant current control on the signal sampled by the Hall current/voltage sensor, the output analog control signal Vc is connected to the phase shift control terminal of the phase shift control chip UCC3895, and the phase shift PWM wave controls main circuit.

The primary voltage VAB, the primary current ip, and the PWM waveforms of Q1 and Q3 are shown in Figure 16. These waveforms are consistent with theoretical analysis and simulation, which verifies the rationality of parameter design.

The driving voltage VGS and drain voltage VDS waveforms of lagging leg Q3 at rated load and 1/5 rated load are shown in Figures 17(a)17(b), respectively. It can be seen that Q3 can achieve soft switch under rated load and 1/5 load.

The waveforms of output voltage Vo and output current Io are shown in Figure 18(a) when the load current changes from 0 A to 40 A, and the waveforms of the output voltage Vo and output current Io are shown in Figure 18(b) when the load current changes from 40 A to 0 A. The analysis shows that the backlash value of output voltage is less than 0.4V, which has good anti-interference ability and dynamic performance.

The relationship between efficiency and load under different input voltages is shown in Figure 19. It can be seen that the efficiency can reach 95% in the 20%-100% rated load range. The efficiency of the improved OBC is little affected by the change of input voltage.

6. Conclusion

Aiming at the problems existing in the two-stage electric vehicle OBC phase-shifted full-bridge soft-switching DC/DC converter, a full-bridge DC/DC converter with clamp diode and synchronous rectification is proposed. Clamping diodes are added to realize a wide range of soft switch and eliminate the voltage oscillation of the secondary rectifier. Synchronous rectification technology of secondary side of the transformer is used to reduce high frequency rectification losses. The operating principle and control method of DC/DC converter are analyzed, and the switching device loss is calculated. The simulation and experimental results show that the secondary side voltage oscillation of the transformer is doubled and the efficiency of the OBC reaches 95% under the load of 20%-100% rated load, which meets the application requirements of onboard charger.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors confirm that this article content has no conflicts of interest.


Authors wish to acknowledge assistance or encouragement from colleagues and financial support by Henan mine power electronics device and control innovative technology team, Key Research Projects of Henan Higher Education Institutions, Grant no. 18A410001, and Doctoral Foundation of Henan Polytechnic University, Grant no. B2017-19.