Design and Analysis of an AC/DC Charger with High Power Factor and High Efficiency

In this paper, an AC/DC charger with high power factor and high efficiency is proposed. The topology of the AC/DC charger makes use of an active power factor correction (PFC) circuit and a DC/DC half-bridge inductor-inductance-capacitor (LLC) resonant converter with high efficiency, which has the following advantages: (1) By incorporating a current-sensing circuit, a zero-current detection circuit, and an active PFC circuit in the front, the AC source can decrease harmonic currents. Therefore, the power factor of the proposed AC/DC charger can approach 0.997. (2) By incorporating a DC/DC half-bridge LLC resonant converter with soft-switching functions in the rear, the power losses of the active switches can be decreased and the efficiency can be improved significantly. Finally, a prototype of the AC/DC charger with high power factor and high efficiency is designed and analyzed. The results of the experiments are presented to prove the function and probability of the AC/DC charger with high power factor and high efficiency.


Introduction
With the increasing awareness of air pollution and the importance of healthy leisure activities, electric bicycles (e-bikes) and electric motorcycles are becoming increasingly popular.E-bikes and electric motorcycles have the following advantages: (1) They do not cause air pollution problems.In cities, personal transportation can be replaced by e-bikes and electric motorcycles, so the problems of air pollution and traffic congestion can be significantly addressed.This will also improve the quality of people's life and health.(2) Green energy issues have become topics of concern worldwide.In the future, high-performance batteries, motor technologies, and power management systems incorporated with sensors, Internet of Things, a global positioning system, and safety protection will be applied to e-bikes and electric motorcycles. (1)Therefore, people and e-bikes as well as electric motorcycles can be connected more closely. (2)here are two main types of important equipment in e-bikes and electric motorcycles: DC brushless motors and lithium ferrous phosphate (LFP) batteries.LFP batteries need an AC/DC charger for charging power energy. (3)Currently, AC/DC chargers with high power factor and high efficiency are required in the consumers' market.Therefore, AC/DC chargers with functions of low current harmonics and switching losses to increase the power factor and efficiency are necessary.In this paper, an AC/DC charger with high power factor and high efficiency is studied, and its structure is shown in Fig. 1.The AC/DC charger with high power factor and high efficiency consists of two stages.In the front stage, there is an AC/DC active power factor correction (PFC) circuit, as shown in Fig. 2. To increase the power factor of the AC/ DC charger, the active PFC circuit is connected to a current-sensing circuit.When a zero-current signal of AC sources is sensed via the current-sensing circuit, a driving signal is delivered to turn on the power switch of the active PFC circuit.−7) In the rear stage, there is a DC/DC half-bridge inductor-inductance-capacitor (LLC) resonant converter with high efficiency, as shown in Fig. 3. To achieve the objectives of electrical isolation, high efficiency, high power, and low harmonic current of the AC/DC charger, a half-bridge LLC resonant circuit is selected in the DC/DC converter.−11) Additionally, the power switches (M 1 and M 2 ) of the half-bridge LLC resonant circuit can be operated under soft-switching conditions during turn-on and turn-off transitions.−16) The overall circuit of the AC/DC charger with high power factor and high efficiency is shown in Fig. 4.

Operational Principles of AC/DC Charger with High Power Factor and High Efficiency
In this section, the operational principles of the AC/DC charger with high power factor and high efficiency are analyzed.There are two stages in the proposed AC/DC charger.In the front stage, there is an AC/DC PFC circuit.In the rear stage, there is a DC/DC half-bridge LLC resonant circuit.The operational principles of the AC/DC charger with high power factor and high efficiency can be divided into seven modes described as follows.

Operational Mode 1
While the power switch (M a ) of the AC/DC charger is turned off, the current i La of the inductor (L a ) is linearly decreased.The active switch (M 1 ) is turned on and the power switch (M 2 ) is turned off.The energy stored in the inductor (L a ) is discharged and the magnetic inductance (L m ) of the transformer (T r ) is excited.The current i La is flowing through the primary side of the transformer (T r ) D a →M 1 →C r →L r →L m →V s .In the secondary side of the transformer (T r ), the power diodes (D 3 and D 6 ) are switched on and the power diodes (D 4 and D 5 ) are switched off.The current i Lb is flowing through the path D 1 →LFP battery→D 4 .
During this operational interval, the input current of the AC source equals the current i La of the inductor (L a ).Therefore, the distorted currents of the AC source are corrected and the high power factor of the AC/DC charger is obtained.Figure 5 shows the equivalent circuit of the operational Mode 1.

Operational Mode 2
When the power switch (M a ) is continuously turned off and the power switches (M 1 and M 2 ) are turned off, the energy stored in the resonant inductor (L r ) is discharged.The parasitic capacitance (C 1 ) of the power switch (M 1 ) is charged and the parasitic capacitance (C 2 ) of the power switch (M 2 ) is discharged.The current i La is flowing through the primary side of the transformer (T r ), which can be divided into two paths.One is D a →C 1 →C r →L r →L m →V s and the other is M 2 →C r →L r →L m .In the secondary side of the transformer (T r ), the power diodes (D 3 , D 4 , D 5 , and D 6 ) are turned off and the current i Lb is flowing through the path C o →LFP battery.
During this operational interval, the current i La of the inductor (L a ) is linearly reduced.Figure 6 shows the equivalent circuit of the operational Mode 2.

Operational Mode 3
When the power switch (M a ) of the AC/DC charger is turned on and the current i La of the inductor is linearly increased, the current i La is flowing through the path D a →M a →V s .The parasitic capacitance (C 2 ) voltage of the power switch (M 2 ) drops to zero and the power switch (M 1 ) is turned off.At this time, the resonant current i Lr equals the current i M2 and the parasitic diode (D 2 ) of the power switch (M 2 ) is turned on.The resonant current i M2 is flowing through the path Cr→Lr→L m →D 2 .In the secondary side of the transformer (T r ), the power diodes (D 3 , D 4 , D 5 , and D 6 ) are continuously turned off and the current i Lb is flowing through the path C o →LFP battery.Figure 7 shows the equivalent circuit of the operational Mode 3.

Operational Mode 4
When the power switch (M a ) of the AC/DC charger is continuously turned on, the current i La of the inductor is linearly increased, and the power switch (M 2 ) is turned on with a zero-voltage switching (ZVS) feature.The resonant current i Lr is flowing through the path Lr→Cr→M 2 →L m .In the secondary side of the transformer (T r ), the diodes (D 4 and D 5 ) are switched on and the  power diodes (D 3 and D 6 ) are switched off.The current i Lb is flowing through the path D 5 →LFP battery→D 4 .Figure 8 shows the equivalent circuit of the operational Mode 4.

Operational Mode 5
When the power switch (M a ) of the AC/DC charger is continuously turned on, the current i La of the inductor is linearly increased.When the power switch (M 2 ) is turned off, the parasitic capacitance (C 2 ) of the power switch (M 2 ) is charged and the parasitic capacitance (C 1 ) of the power switch (M 1 ) is discharged.The resonant current i Lr is revised, which divides into two paths.One is L r →C r →M 2 →L m and the other is L r →C r →M 1 →C b →L m .In the secondary side of the transformer (T r ), the power diodes (D 4 and D 5 ) are switched on and the power diodes (D 3 and D 6 ) are switched off.The current i Lb is flowing through the path D 5 →LFP battery→D 4 .Figure 9 shows the equivalent circuit of the operational Mode 5.

Operational Mode 6
When the power switch (M a ) of the AC/DC charger is continuously turned on and the current i La of the inductor is linearly reduced, the parasitic capacitance (C 1 ) voltage of the active switch (M 1 ) drops to zero and the power switch (M 2 ) is turned off.At this time, the resonant current i Lr  equals the current i M1 and the parasitic diode (D 1 ) of the power switch (M 1 ) is turned on.The resonant current i M1 is flowing through the path C r →D 1 →C b →L m →L r .In the secondary side of the transformer (T r ), the power diodes (D 4 and D 5 ) are turned on and the power diodes (D 3 and D 6 ) are turned off.The current i Lb is flowing through the path D 5 →LFP battery→D 4 →T r .Figure 10 shows the equivalent circuit of the operational Mode 6.

Operational Mode 7
When the power switch (M a ) of the AC/DC charger is turned off and the current i La of the inductor is linearly decreased, the power switch (M 1 ) is switched on with a ZVS feature.The current i La is flowing through the path V s →L a →D a →C b →L m and the resonant current i Lr is flowing through the path L r →C r →M 1 →C b →L m .In the secondary side of the transformer (T r ), the power diodes (D 3 , D 4 , D 5 , and D 6 ) are turned off.The current i Lb is flowing through the path C o →LFP battery.The operational mode of the AC/DC charger with high power factor and high efficiency over one switching cycle is completed.Figure 11 shows the equivalent circuit of the operational Mode 7.After the analysis of these operating modes, it can be seen that the AC/DC charger with features of high power factor and high efficiency can be verified.

Experimental Results
To confirm the function and probability of the AC/DC charger, a 240 W prototype of the AC/ DC charger with high power factor and high efficiency is built.The AC/DC charger has the following specifications: Input Figure 12 shows the experimental input voltage and current waveforms of the AC source.As determined using power quality analyzers, the experimental power factors are 0.996 and 0.966 under AC input voltages of 110 and 220 V rms , respectively.Therefore, a high power factor of the proposed AC/DC charger can be obtained.Figure 13 shows the experimental waveforms of the output current change.When output currents change from 5 to 8 A, the output voltages are stable.Therefore, the load dynamic response of the AC/DC charger is achievable.Figures 14 and  15 show the results of efficiency measurements of the AC/DC charger with high power factor and high efficiency under full load conditions, from which the maximum efficiency can be as high as 96% at the AC input voltage of 220 V rms .

Fig. 1 .
Fig. 1. (Color online) Structure of proposed AC/DC charger with high power factor and high efficiency.

Fig. 4 .
Fig. 4. (Color online) Overall circuit of AC/DC charger with high power factor and high efficiency.
AC voltages: V AC = 85-265 V rms , Output DC voltage: V o = 24 V DC , Output current: I o = 10 A, Total output power: P o = 240 W, and Switching frequency of power switches: f = 100 kHz.

Fig. 12 .
Fig. 12. (Color online) Experimental input voltage and current waveforms of AC source: (a) input voltage of 110 V rms .

Fig. 13 .
Fig. 13.(Color online) Experimental waveforms of output current change from 5 to 8 A.

Fig. 14 .
Fig. 14. (Color online) Plot of efficiency versus output current for AC/DC charger at AC input voltage of 110 V rms .