# Onboard Charging DC/DC Converter of Electric Vehicle Based on Synchronous Rectification and Characteristic Analysis

Authors: Haijun Tao; Guopeng Zhang; Zheng Zheng
Journal: Journal of Advanced Transportation
 (2019)

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## Abstract

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.


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## Body

## 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 Figure1 [13], whereVcc is open-circuit voltage,Re is battery internal resistance,Rp is polarization resistance, andCp is polarization capacitance. The equivalent impedanceZo of the batteries is(1)Zo=Re+Vccs+Rp1+RpCpsFigure 1
Thevenin equivalent circuit.


### 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.Figure 2
Two-stage vehicle charger.


### 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.Figure 3
CC-CV Charging Curve of EV Battery.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:(1) wide range adjustment of the output voltage; (2) soft switching operation with wide load range; (3) 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 Figure4. In the constant voltage stage, the output voltage Vo is compared with the reference voltage Vref. The error is obtained by the PI controller and the modulation signal WCV is obtained. Similarly, the modulation signal WCV can be obtained.Figure 4
CC-CV charge control strategy.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 currentIo is large. At this time, WCC < WCV, the battery is charged in constant current mode. When the voltage rises to the electric reference value Vref, WCV < WCC, the battery is charged in constant voltage mode, and the charging current decreases continuously until the end of charging.


## 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 Figure1 [13], whereVcc is open-circuit voltage,Re is battery internal resistance,Rp is polarization resistance, andCp is polarization capacitance. The equivalent impedanceZo of the batteries is(1)Zo=Re+Vccs+Rp1+RpCpsFigure 1
Thevenin equivalent circuit.


## 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.Figure 2
Two-stage vehicle charger.


## 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.Figure 3
CC-CV Charging Curve of EV Battery.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:(1) wide range adjustment of the output voltage; (2) soft switching operation with wide load range; (3) 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 Figure4. In the constant voltage stage, the output voltage Vo is compared with the reference voltage Vref. The error is obtained by the PI controller and the modulation signal WCV is obtained. Similarly, the modulation signal WCV can be obtained.Figure 4
CC-CV charge control strategy.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 currentIo is large. At this time, WCC < WCV, the battery is charged in constant current mode. When the voltage rises to the electric reference value Vref, 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 Figure5.Vin is the input DC power supply. Switching devices Q1~Q4 (including D1~D4 and C1~C4) form the inverter bridge, Lr is the resonant inductor, T is a high frequency transformer,D1 andD2 form a high frequency rectifier bridge, andLf andCf 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.Figure 5
Traditional ZVS FB main circuit topology.The improved ZVS phase-shifted full-bridge main circuit topology with clamping diodes and operating waveforms are shown in Figure6. 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.Figure 6
The improved ZVS full-bridge DC/DC converter with clamping diodes and synchronous rectification: (a) main circuit; (b) operating waveforms.
(a) (b)


### 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.Figure 7
Equivalent circuit diagram with clamping diodes during [t7,t9]. (a) During [t7,t8]; (b) during [t8,t9].
(a) (b)During [t7,t8], the equivalent circuit is shown in Figure 7(a). At t7, the load current Io all flows throughQ6 and the current passing throughQ5 drops to zero and charges the junction capacitorC5 ofQ5 at the same time. After t7, the resonant inductance Lr andC5 resonance, the voltages of the SR deviceQ5, and the secondary side of the transformer are, respectively, as follows:(2)VQ5t=2Vinn1-cos⁡ωrt-t7(3)Vrt=Vinn1-cos⁡ωrt-t7Expression(1) shows that the voltage on the output rectifier device oscillates equally when no clamping diodes are added. Its peak value is 4Vin/n. Because of the damping effect in the resonance process, the voltage on the output rectifier device will gradually decrease to 2Vin/n, and the secondary voltage of the transformer will gradually decrease to Vin/n.During [t8,t9], the equivalent circuit is shown in Figure 7(b). At t8, because the reverse charging current ofQ5 junction capacitor disappears, the current ip is larger than the current Io converted to primary, so ip decreases rapidly until the two are equal. Because the inductance current iLr cannot mutate, iLr>ip. The neutral point voltage of two clamping diodes is zero,D6 is turned on, and the voltage ofC5 is clamped to2Vin/n; that is, VQ5=2Vin/n. Compared with expression (1), the voltage oscillation of theQ5 is suppressed. At this point, the rising slope of ip is(4)dipdt=Vin/n-VonLfAtt9, ip=iLr, the current flowing through the clamping diodeD6 drops to zero, andD6 is turned off naturally. Then ip and iLr 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 ofQ1~Q4, the current waveforms and driving waveforms ofQ5~Q6 are shown in Figure 8. The driving signals ofQ5 andQ6 can be obtained in the following three ways.(1)
The same driving logic as lagging legQ3(Q2)(2)
And logic: the driving signals ofQ1(Q4) andQ3 (Q2) are carried out and logic(3)
Or logic: the driving signals ofQ1(Q4) andQ3 (Q2) are carried out or logicFigure 8
Driving logic diagram of SR devices.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.


## 3.1. Main Circuit Topology

Traditional ZVS full bridge main circuit topology is shown in Figure5.Vin is the input DC power supply. Switching devices Q1~Q4 (including D1~D4 and C1~C4) form the inverter bridge, Lr is the resonant inductor, T is a high frequency transformer,D1 andD2 form a high frequency rectifier bridge, andLf andCf 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.Figure 5
Traditional ZVS FB main circuit topology.The improved ZVS phase-shifted full-bridge main circuit topology with clamping diodes and operating waveforms are shown in Figure6. 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.Figure 6
The improved ZVS full-bridge DC/DC converter with clamping diodes and synchronous rectification: (a) main circuit; (b) operating waveforms.
(a) (b)


## 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.Figure 7
Equivalent circuit diagram with clamping diodes during [t7,t9]. (a) During [t7,t8]; (b) during [t8,t9].
(a) (b)During [t7,t8], the equivalent circuit is shown in Figure 7(a). At t7, the load current Io all flows throughQ6 and the current passing throughQ5 drops to zero and charges the junction capacitorC5 ofQ5 at the same time. After t7, the resonant inductance Lr andC5 resonance, the voltages of the SR deviceQ5, and the secondary side of the transformer are, respectively, as follows:(2)VQ5t=2Vinn1-cos⁡ωrt-t7(3)Vrt=Vinn1-cos⁡ωrt-t7Expression(1) shows that the voltage on the output rectifier device oscillates equally when no clamping diodes are added. Its peak value is 4Vin/n. Because of the damping effect in the resonance process, the voltage on the output rectifier device will gradually decrease to 2Vin/n, and the secondary voltage of the transformer will gradually decrease to Vin/n.During [t8,t9], the equivalent circuit is shown in Figure 7(b). At t8, because the reverse charging current ofQ5 junction capacitor disappears, the current ip is larger than the current Io converted to primary, so ip decreases rapidly until the two are equal. Because the inductance current iLr cannot mutate, iLr>ip. The neutral point voltage of two clamping diodes is zero,D6 is turned on, and the voltage ofC5 is clamped to2Vin/n; that is, VQ5=2Vin/n. Compared with expression (1), the voltage oscillation of theQ5 is suppressed. At this point, the rising slope of ip is(4)dipdt=Vin/n-VonLfAtt9, ip=iLr, the current flowing through the clamping diodeD6 drops to zero, andD6 is turned off naturally. Then ip and iLr 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 ofQ1~Q4, the current waveforms and driving waveforms ofQ5~Q6 are shown in Figure 8. The driving signals ofQ5 andQ6 can be obtained in the following three ways.(1)
The same driving logic as lagging legQ3(Q2)(2)
And logic: the driving signals ofQ1(Q4) andQ3 (Q2) are carried out and logic(3)
Or logic: the driving signals ofQ1(Q4) andQ3 (Q2) are carried out or logicFigure 8
Driving logic diagram of SR devices.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 Table1.Table 1
Device parameters and models.
Device Model parameter Switch device SPP20N60CFD 325A/1.7kW Drive circuit 56PR3362 100kHz Clamp diode MURS360T3G 3A/600V SR device FDP032N08 235A/75V Drive circuit 2SC0435T2A0-17 100kHz


### 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 Figure9, where Rg is gate resistances, Cgd, Cgs, and Cds, 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.Figure 9
Equivalent model of MOSFET.Since the turn-off process is opposite to the start-up process, only the start-up process is discussed, as shown in Figure10. At t0, the gate voltage Vg is added to the MOS transistor. During t0~t1, Cgs is charged, Vgs rises, and Vgs reaches the turn-on threshold voltage Vth of the MOS transistor at t1. During t1~t2, the MOS transistor is turned on and the drain current id starts to rise. At t2, the drain current Id rises to the rated currentId, while Vgs rises to the Miller platform voltage Vgd. During t2~t3, Cgd starts charging, the drain voltage decreases, and the gate voltage Vgs remains unchanged. During t3~t4, the gate voltage begins to charge Cgs and Cgd at the same time, and the gate voltage Vgs continues to rise. At t4, Vgs=Vg, the MOS transistor is fully turned on.Figure 10
Analysis of MOSFET on-off process.


### 4.2. Loss Calculation

#### 4.2.1. Prestage Power Loss Calculation

( 1 ) The loss of MOSFET during the start-up process is as follows:(5)Pson=12fsVdsIdt12+t23Similarly, the turn-off process is similar to the start-up process, so the switching losses are as follows:(6)Psw=2Pson( 2 ) The conduction loss of MOSFET is(7)PRds=fs∫t4t5idsRdsdt≈DId2RdsonwhereRds(on) is the on-state resistance of MOSFET.( 3 ) Reverse recovery loss of MOSFET is(8)Pbdrr=QrrVdsfswhereQrr is the reverse recovery charge of the body diode.( 4 ) The driving loss of the MOSFET is(9)Pdrive=QgVgsfswhereQg 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 is1/2 of the single transistor:(10)PRds=12Id2RdsonDThe conduction loss of the current flowing through the body diode is as follows:(11)Pdio=VFId1-D+12I2Rdsdiowhere VF and Rds(dio) are an on-state voltage drop and on-state resistance of body diode, respectively.Then the total conduction loss of each MOSFET is(12)Pon=PRds+PdioAccording to the selection parameters in Table1 and the above analysis, the losses of the prestage and poststage can be calculated, as shown in Table 2.Table 2
The losses of the prestage and poststage.
Circuit configuration Loss Value The traditional DC/DC converter Pre-stage power loss 24W Diode rectifier loss 36W  The improved DC/DC converter Pre-stage power loss 24W Synchronous rectification loss 9WAccording to Table2, 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.Figure 11
Efficiency values of two converters.


## 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 Figure9, where Rg is gate resistances, Cgd, Cgs, and Cds, 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.Figure 9
Equivalent model of MOSFET.Since the turn-off process is opposite to the start-up process, only the start-up process is discussed, as shown in Figure10. At t0, the gate voltage Vg is added to the MOS transistor. During t0~t1, Cgs is charged, Vgs rises, and Vgs reaches the turn-on threshold voltage Vth of the MOS transistor at t1. During t1~t2, the MOS transistor is turned on and the drain current id starts to rise. At t2, the drain current Id rises to the rated currentId, while Vgs rises to the Miller platform voltage Vgd. During t2~t3, Cgd starts charging, the drain voltage decreases, and the gate voltage Vgs remains unchanged. During t3~t4, the gate voltage begins to charge Cgs and Cgd at the same time, and the gate voltage Vgs continues to rise. At t4, Vgs=Vg, the MOS transistor is fully turned on.Figure 10
Analysis of MOSFET on-off process.


## 4.2. Loss Calculation

### 4.2.1. Prestage Power Loss Calculation

( 1 ) The loss of MOSFET during the start-up process is as follows:(5)Pson=12fsVdsIdt12+t23Similarly, the turn-off process is similar to the start-up process, so the switching losses are as follows:(6)Psw=2Pson( 2 ) The conduction loss of MOSFET is(7)PRds=fs∫t4t5idsRdsdt≈DId2RdsonwhereRds(on) is the on-state resistance of MOSFET.( 3 ) Reverse recovery loss of MOSFET is(8)Pbdrr=QrrVdsfswhereQrr is the reverse recovery charge of the body diode.( 4 ) The driving loss of the MOSFET is(9)Pdrive=QgVgsfswhereQg 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 is1/2 of the single transistor:(10)PRds=12Id2RdsonDThe conduction loss of the current flowing through the body diode is as follows:(11)Pdio=VFId1-D+12I2Rdsdiowhere VF and Rds(dio) are an on-state voltage drop and on-state resistance of body diode, respectively.Then the total conduction loss of each MOSFET is(12)Pon=PRds+PdioAccording to the selection parameters in Table1 and the above analysis, the losses of the prestage and poststage can be calculated, as shown in Table 2.Table 2
The losses of the prestage and poststage.
Circuit configuration Loss Value The traditional DC/DC converter Pre-stage power loss 24W Diode rectifier loss 36W  The improved DC/DC converter Pre-stage power loss 24W Synchronous rectification loss 9WAccording to Table2, 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.Figure 11
Efficiency values of two converters.


## 4.2.1. Prestage Power Loss Calculation

( 1 ) The loss of MOSFET during the start-up process is as follows:(5)Pson=12fsVdsIdt12+t23Similarly, the turn-off process is similar to the start-up process, so the switching losses are as follows:(6)Psw=2Pson( 2 ) The conduction loss of MOSFET is(7)PRds=fs∫t4t5idsRdsdt≈DId2RdsonwhereRds(on) is the on-state resistance of MOSFET.( 3 ) Reverse recovery loss of MOSFET is(8)Pbdrr=QrrVdsfswhereQrr is the reverse recovery charge of the body diode.( 4 ) The driving loss of the MOSFET is(9)Pdrive=QgVgsfswhereQg 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 is1/2 of the single transistor:(10)PRds=12Id2RdsonDThe conduction loss of the current flowing through the body diode is as follows:(11)Pdio=VFId1-D+12I2Rdsdiowhere VF and Rds(dio) are an on-state voltage drop and on-state resistance of body diode, respectively.Then the total conduction loss of each MOSFET is(12)Pon=PRds+PdioAccording to the selection parameters in Table1 and the above analysis, the losses of the prestage and poststage can be calculated, as shown in Table 2.Table 2
The losses of the prestage and poststage.
Circuit configuration Loss Value The traditional DC/DC converter Pre-stage power loss 24W Diode rectifier loss 36W  The improved DC/DC converter Pre-stage power loss 24W Synchronous rectification loss 9WAccording to Table2, 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.Figure 11
Efficiency values of two converters.


## 5. Simulation and Experiment

The main parameters of phase-shifted full-bridge DC/DC converter for OBC are shown in Table3.Table 3
Main parameters of DC/DC converter.
parameters value Rated input voltageVin 400 V Output maximum voltageVo 12 V Output maximum currentIo 50 A Transformer Ration 21 Resonant inductorLr 26μH Resonant capacitorCf 7.5 nF Output filter inductorLf 2μH switching frequencyfs 100 kHz Input maximum currentIin 2 A


### 5.1. Simulation

The main waveforms of DC/DC converter are shown in Figure12, which is in turn transformer primary current ip, resonant inductance current iLr, two clamping diode currentsiD5 andiD6, transformer primary voltage VAB, and transformer secondary voltage Vr. It can be seen that there is basically no voltage spike in the secondary voltage Vr 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.Figure 12
Main simulation waveforms of DC/DC converter with clamping diodes.Partially amplified waveforms with and without clamping diodes are shown in Figure13. Compared with Figures 13(a) and 13(b), the voltage oscillation at both ends of the transformer secondary voltage Vr and SR tubeQ5 is suppressed by adding a clamp diode.Figure 13
Partially amplified waveforms: (a) without clamping diodes; (b) with clamping diodes.
(a) (b)The drain-source voltageVDS, driving voltage VGS and drain current iD 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 ofQ3, VDS has been reduced to zero before VGS appears, and zero voltage soft switching can be realized.Figure 14
Main waveforms of lagging legQ3 (a) Under rated load; (b) Under 1/5 rated load.
(a) (b)


### 5.2. Experiment

According to the parameters of Table3, 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 signalVc is connected to the phase shift control terminal of the phase shift control chip UCC3895, and the phase shift PWM wave controls main circuit.Figure 15
Experimental test bench.The primary voltageVAB, the primary currentip, and the PWM waveforms ofQ1 andQ3 are shown in Figure 16. These waveforms are consistent with theoretical analysis and simulation, which verifies the rationality of parameter design.Figure 16
Main operating waveforms of ZVS PWM DC/DC converter.The driving voltageVGS and drain voltageVDS 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 thatQ3 can achieve soft switch under rated load and 1/5 load.Figure 17
ZVS of lagging-legQ3 under different loads. (a) Rated load; (b) 1/5 rated load.
(a) (b)The waveforms of output voltageVo and output currentIo are shown in Figure 18(a) when the load current changes from 0 A to 40 A, and the waveforms of the output voltageVo and output currentIo 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.Figure 18
Output voltage and current waveforms under abrupt load change.
(a)
0%~80% load mutation (b)
80%~0% load mutationThe relationship between efficiency and load under different input voltages is shown in Figure19. 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.Figure 19
The relation between efficiency and load under different input voltage conditions.


## 5.1. Simulation

The main waveforms of DC/DC converter are shown in Figure12, which is in turn transformer primary current ip, resonant inductance current iLr, two clamping diode currentsiD5 andiD6, transformer primary voltage VAB, and transformer secondary voltage Vr. It can be seen that there is basically no voltage spike in the secondary voltage Vr 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.Figure 12
Main simulation waveforms of DC/DC converter with clamping diodes.Partially amplified waveforms with and without clamping diodes are shown in Figure13. Compared with Figures 13(a) and 13(b), the voltage oscillation at both ends of the transformer secondary voltage Vr and SR tubeQ5 is suppressed by adding a clamp diode.Figure 13
Partially amplified waveforms: (a) without clamping diodes; (b) with clamping diodes.
(a) (b)The drain-source voltageVDS, driving voltage VGS and drain current iD 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 ofQ3, VDS has been reduced to zero before VGS appears, and zero voltage soft switching can be realized.Figure 14
Main waveforms of lagging legQ3 (a) Under rated load; (b) Under 1/5 rated load.
(a) (b)


## 5.2. Experiment

According to the parameters of Table3, 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 signalVc is connected to the phase shift control terminal of the phase shift control chip UCC3895, and the phase shift PWM wave controls main circuit.Figure 15
Experimental test bench.The primary voltageVAB, the primary currentip, and the PWM waveforms ofQ1 andQ3 are shown in Figure 16. These waveforms are consistent with theoretical analysis and simulation, which verifies the rationality of parameter design.Figure 16
Main operating waveforms of ZVS PWM DC/DC converter.The driving voltageVGS and drain voltageVDS 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 thatQ3 can achieve soft switch under rated load and 1/5 load.Figure 17
ZVS of lagging-legQ3 under different loads. (a) Rated load; (b) 1/5 rated load.
(a) (b)The waveforms of output voltageVo and output currentIo are shown in Figure 18(a) when the load current changes from 0 A to 40 A, and the waveforms of the output voltageVo and output currentIo 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.Figure 18
Output voltage and current waveforms under abrupt load change.
(a)
0%~80% load mutation (b)
80%~0% load mutationThe relationship between efficiency and load under different input voltages is shown in Figure19. 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.Figure 19
The relation between efficiency and load under different input voltage conditions.


## 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.



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*Source: 2613893-2019-06-02.xml*