Bidirectional Information Transmission in SWIPT System with Single Controlled Chopper Receiver

: The wireless power transfer (WPT) technology has the advantages of convenience, safety and reliability due to its non-metal contact power supply and has a broad application prospect in many occasions. In practical applications, the information communication between the primary and secondary side is necessary for output voltage control, load detection, condition monitoring and other functions, which makes the WPT system more intelligent and convenient. A simultaneous wireless information and power transfer (SWIPT) system with controlled chopper circuit receiver is proposed in this paper. The load voltage remains constant by adjusting the pulse width of the secondary controlled device through feedback control. The information bidirectional transmission methods and two modes are proposed, considering di ﬀ erent application scenarios. Simulation and experiment results validate the proposed topology and the method of information bidirectional transmission.


Introduction
Traditional power supply generally means wire transfer, which is not safe because of metal contact failure, electric spark, short circuit, and so on. Most of these failures result from metal contact. The wireless power transfer (WPT) technology employs the form of contactlessness, and supplies power to a load at a certain distance, so that the power supply mode in WPT is more flexible and safer, and the power safety problems existing in the conventional power supply mode can be avoided. Due to its advantages, the WPT technology has been applied in broad fields, such as in electronics, medical equipment, electric vehicles and other fields [1][2][3][4]. According to the mechanism of power transmission, there are two types of WPT, i.e. the far-field wireless transfer and the near-field wireless transfer [5,6].
The information, such as output voltage control, load detection, and status monitoring, can be communicated between the primary and secondary sides in the WPT system [7][8][9][10][11]. The WPT system with simultaneously information transfer is called SWIPT (simultaneous wireless information and power transfer) system. In practical applications, information transmission in the WPT system makes the devices more intelligent and convenient. For example, the wireless charger transfers signals to control the status of the battery, then the charger could be equipped with constant voltage or constant current characteristics, which makes the charger efficient and gives the battery a long lifetime. Another case is an implantable medical device, such as self-contained intracardiac pacemaker. As the implantable medical device is implanted inside the body, the WPT technology makes the device more convenient to use. At the same time, the implantable medical device is able to record human health data and export data during wireless charging, and thus the monitoring of human health is more accurate and convenient.
A direct way for SWIPT is to establish an independent link by using extra coils or antennas to transmit information and power simultaneously [12]. Obviously, this method is not suitable for implantable self-contained medical devices, because the extra components lead to large implementation costs, while the microelectronics or implantable medical devices have strict limits on size and cost. Another way for SWIPT is to transfer information and power by means of modulation schemes [13]. There are currently two main methods of modulation schemes to realize SWIPT function. The first one is referred to as information carrier-based modulation. It is to build an independent information transfer channel on the basis of WPT system, and to transmit information by modulating and demodulating the information carrier [14][15][16][17][18]. The second one employs the power channel as a carrier for information transmission and transmits information by regulating the power carrier [19][20][21][22][23][24][25][26][27][28][29], so this method is referred to as power carrier-based modulation.
Wu et al. [14] developed a SWIPT system, in which the power and data transfer share the same inductive link, they are transmitted by employing different frequency carriers and controlled independently. The ferrite-core coupled inductors are used for data-carrier coupling, and they are magnetically coupled with power resonant tank too. Lee et al. [15] considered a scenario of information exchange between two IoT devices. Simple energy storage devices such as supercapacitors are necessary in this system, and the conventional SWIPT cannot be directly applied to this system. Sun et al. [17] employed four magnetically coupled inductors to transfer data bidirectional. Furthermore, two high-frequency trappers are employed to separate the power and data transfer. Therefore, it is a high order resonant system. Yu et al. [18] designed one inner circuit to realize energy and data reception, the data transmission is realized by controlling several inner switches on and off. Overall, in the information carrier-based method, the influence of information on power is small, but the information modulation and demodulation circuit greatly increases the system cost and size, and the power carrier is much larger than the information carrier, so it has a great influence on the information transmission stability.
As for the power carrier-based modulation, there are many information modulation methods used in this method, such as amplitude shift keying (ASK) [19][20][21], frequency shift keying (FSK) [22][23][24], phase shift keying (PSK) [25][26][27], load shift keying (LSK) [28][29][30][31], cyclic on-off keying (COOK) [32,33], and so on. The ASK is a type of amplitude modulation which represents the binary data in the form of variations in the amplitude of signal. The binary signal, when ASK is modulated, gives a small value for low input while it gives a high value for high input. The FSK is a scheme of frequency modulation in which the frequency of the carrier signal varies according to the digital signal changes. The output of an FSK modulated wave is high in frequency for a binary high input and is low in frequency for a binary low input. The binary 1s and 0s are called Mark and Space frequencies. The PSK is a digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. The LSK modulate the load at the secondary side by adjusting the output impedance. The COOK transmits data by shorting and opening the secondary resonant tank. The LSK and COOK are only applied in backward information transmission.
Wang et al. [19] proposed a method of SWIPT technology, considering four coils magnetically coupled resonant WPT system, which combines ASK modulation and demodulation circuit. Yilmaz et al. [20] use the power transmission circuit as the channel for information transmission; the information forward transmission is realized by FSK, but these forward information modulation methods have a great influence on power transmission. It is the common inherent feature of power carrier-based modulation, because the information transmission depends on power transfer. Mandal and Sarpeshkar [28] proposed wireless data telemetry links for implanted biomedical system. It minimizes power consumption in the implanted system by using impedance modulation to transmit high-bandwidth information from the implanted to the external system. The information backward modulation methods require additional switching devices and driving circuits for signal modulation, which inevitably increases system cost and size. Due to the continuous switching of load, the power of the system will fluctuate and affect the power quality.
In summary, there is no perfect solution, people must carefully seek the balance of information, power, volume, cost and other factors. Due to the independent information channel, the cost and size of the system based on the information carrier modulation is much higher than that of power carrier-based modulation. The magnetic interference of information carrier-based method between power and information transfer carriers reduces the communication quality. Therefore, the power carrier-based method is more suitable for microelectronics or implantable medical devices due to its small size and low cost. This paper designs the SWIPT system using the power carrier-based method.
A SWIPT system with controlled chopper circuit receiver is proposed in this paper. There is only one extra controlled switch on the secondary side. The constant load voltage is achieved by adjusting the voltage angle of the controlled switch through feedback control. The information forward transmission is realized by changing the power transmission frequency and designing forward demodulator. The information backward transmission is realized by changing the switching frequency of the controlled chopper circuit at the secondary side and designing backward demodulator. The predefined instruction mode and continuous data transmission mode are proposed for different working conditions, which makes the information transmission more flexible and convenient.
This paper is organized as follows. Section 2 shows the model of proposed structure, analyzes the principle of power transfer and constant voltage output control strategy. Section 3 elaborates the principles of information forward and backward transmission. Section 4 presents the modulation of bidirectional information transmission and control algorithm of simultaneous power and information transfer. In Section 5, simulation and experiments validate the proposed topology and bidirectional information transmission methods. Finally, Section 6 draws the conclusion.

Model of the Proposed Topology
The schematic diagram of the proposed SWIPT system is shown in Figure 1, V in is the input voltage source, and the Q 1~Q4 constitute the active bridge at the primary side. The controllable switch Q 5 and diode D 5 are connected between the bridge rectifiers and the load at the secondary side, the switch Q 5 implements chopper control at the secondary side. The L 1 , L 2 , C 1 , C 2 , R 1 , and R 2 are the coil inductance, compensation capacitance and resistance, respectively; the subscript 1 and 2 denote the primary and secondary side. R L is the load resistance, V o is the load voltage. The capacitance C o keeps load voltage stable. R e is the equivalent load of the secondary side. system. It minimizes power consumption in the implanted system by using impedance modulation to transmit high-bandwidth information from the implanted to the external system. The information backward modulation methods require additional switching devices and driving circuits for signal modulation, which inevitably increases system cost and size. Due to the continuous switching of load, the power of the system will fluctuate and affect the power quality. In summary, there is no perfect solution, people must carefully seek the balance of information, power, volume, cost and other factors. Due to the independent information channel, the cost and size of the system based on the information carrier modulation is much higher than that of power carrier-based modulation. The magnetic interference of information carrier-based method between power and information transfer carriers reduces the communication quality. Therefore, the power carrier-based method is more suitable for microelectronics or implantable medical devices due to its small size and low cost. This paper designs the SWIPT system using the power carrier-based method.
A SWIPT system with controlled chopper circuit receiver is proposed in this paper. There is only one extra controlled switch on the secondary side. The constant load voltage is achieved by adjusting the voltage angle of the controlled switch through feedback control. The information forward transmission is realized by changing the power transmission frequency and designing forward demodulator. The information backward transmission is realized by changing the switching frequency of the controlled chopper circuit at the secondary side and designing backward demodulator. The predefined instruction mode and continuous data transmission mode are proposed for different working conditions, which makes the information transmission more flexible and convenient.
This paper is organized as follows. Section 2 shows the model of proposed structure, analyzes the principle of power transfer and constant voltage output control strategy. Section 3 elaborates the principles of information forward and backward transmission. Section 4 presents the modulation of bidirectional information transmission and control algorithm of simultaneous power and information transfer. In section 5, simulation and experiments validate the proposed topology and bidirectional information transmission methods. Finally, section 6 draws the conclusion.

Model of the Proposed Topology
The schematic diagram of the proposed SWIPT system is shown in Figure 1, Vin is the input voltage source, and the Q1~Q4 constitute the active bridge at the primary side. The controllable switch Q5 and diode D5 are connected between the bridge rectifiers and the load at the secondary side, the switch Q5 implements chopper control at the secondary side. The L1, L2, C1, C2, R1, and R2 are the coil inductance, compensation capacitance and resistance, respectively; the subscript 1 and 2 denote the primary and secondary side. RL is the load resistance, Vo is the load voltage. The capacitance Co keeps load voltage stable. R e is the equivalent load of the secondary side.  The WPT system is powered by a DC voltage source, and high frequency AC voltage is generated from the primary active bridge [5,6], which converts the power to a time-varying electromagnetic field. The primary and the secondary coils transmit power through electromagnetic induction. The secondary coil picks up the power and converts it back to DC by the bridge rectifier, then the DC current flows through the load. Finally, the power is transferred wirelessly from the primary to the secondary side. The L 1 , C 1 and L 2 , C 2 couple in series and form resonant tank to counteract the reactive power and to improve the power factor.
The power frequency is the same as the resonant frequency of compensation circuits at the primary and secondary sides, so the resonant compensation circuits of both sides are resistive.
According to Kirchhoff's voltage law, the circuit is expressed as

Analysis of Wireless Power Transfer
The typical waveforms of transmitter and receiver are shown in Figure 2, where the blue line indicates voltage, the carmine dotted line is current, 2α and 2β present the voltage angle of V 1 and V 2 , respectively. The frequency of Q 5 is two times the power frequency, and the voltage angle of V 2 is regulated by controlling the pulse width γ. Due to resonance, the primary and secondary circuits are resistive, i.e. V 1 and I 1 , V 2 and I 2 are in phase, and V 2 leads V 1 90 • degrees. The relationship between α, γ and the load voltage V o is obtained through the following analysis. The WPT system is powered by a DC voltage source, and high frequency AC voltage is generated from the primary active bridge [5,6], which converts the power to a time-varying electromagnetic field. The primary and the secondary coils transmit power through electromagnetic induction. The secondary coil picks up the power and converts it back to DC by the bridge rectifier, then the DC current flows through the load. Finally, the power is transferred wirelessly from the primary to the secondary side. The L1, C1 and L2, C2 couple in series and form resonant tank to counteract the reactive power and to improve the power factor.
The power frequency is the same as the resonant frequency of compensation circuits at the primary and secondary sides, so the resonant compensation circuits of both sides are resistive.
According to Kirchhoff's voltage law, the circuit is expressed as

Analysis of Wireless Power Transfer
The typical waveforms of transmitter and receiver are shown in Figure 2, where the blue line indicates voltage, the carmine dotted line is current, 2α and 2β present the voltage angle of V1 and V2, respectively. The frequency of Q5 is two times the power frequency, and the voltage angle of V2 is regulated by controlling the pulse width γ. Due to resonance, the primary and secondary circuits are resistive, i.e. V1 and I1, V2 and I2 are in phase, and V2 leads V1 90° degrees. The relationship between α, γ and the load voltage Vo is obtained through the following analysis. According to the Fourier series and harmonic analysis, the fundamental RMS value of the secondary voltage V2 is denoted as Neglecting the losses of the switches, there be formula as follow According to the Fourier series and harmonic analysis, the fundamental RMS value of the secondary voltage V 2 is denoted as Neglecting the losses of the switches, there be formula as follow Combining the formulas (4) and (5), the relationship between the secondary equivalent resistance R e and the voltage angle of Q 5 is as follows The reflected impedance of the secondary to the primary side can be expressed as The currents of the primary and secondary sides, i.e. I 1 and I 2 are expressed as Equivalent resistance voltage of the secondary side is V 2 = R e I 2 . Combining with formula (9), the following formula holds, Substituting (4) and (6) into (10), the load voltage V o is obtained.
Obviously, the load voltage V o is related to the pulse width γ. The load voltage can be regulated by controlling the angle γ with the specific system parameters. The control structure for constant voltage output with single switch receiver is shown in Figure 3. The output load voltage is sampled and compared with the reference voltage U*. The PID control is adopted to keep load voltage stable during power and information transmission.

Information Forward Transmission
The structure of information forward transmission is shown in Figure 4. The DSP controller of primary side generates PWM signals to drive the converter, then produces high frequency AC power that is received by the secondary side. The frequency of the resonant capacitor voltage at the secondary side is consistent with the AC power frequency, that is, the resonant capacitor C2 and the primary PWM signals are provided with the same frequency. The voltage waveforms of different power frequencies are shown in Figure 5.
The forward information is demodulated from the resonant capacitor C2, as shown in Figure 4. The voltage signal is reduced by the divider resistances R3 and R4; D0 is the transient voltage

Information Forward Transmission
The structure of information forward transmission is shown in Figure 4. The DSP controller of primary side generates PWM signals to drive the converter, then produces high frequency AC power that is received by the secondary side. The frequency of the resonant capacitor voltage at the secondary side is consistent with the AC power frequency, that is, the resonant capacitor C 2 and the primary PWM  Figure 5.
The forward information is demodulated from the resonant capacitor C 2 , as shown in Figure 4. The voltage signal is reduced by the divider resistances R 3 and R 4 ; D 0 is the transient voltage suppressor used to limit the voltage. The rectangular wave signals with the same frequency as the PWM signals of inverter are obtained by a comparator A 1 .

Information Forward Transmission
The structure of information forward transmission is shown in Figure 4. The DSP controller of primary side generates PWM signals to drive the converter, then produces high frequency AC power that is received by the secondary side. The frequency of the resonant capacitor voltage at the secondary side is consistent with the AC power frequency, that is, the resonant capacitor C2 and the primary PWM signals are provided with the same frequency. The voltage waveforms of different power frequencies are shown in Figure 5.
The forward information is demodulated from the resonant capacitor C2, as shown in Figure 4. The voltage signal is reduced by the divider resistances R3 and R4; D0 is the transient voltage suppressor used to limit the voltage. The rectangular wave signals with the same frequency as the PWM signals of inverter are obtained by a comparator A1. In this paper, 85 kHz is selected as the primary side resonance frequency, when the information is forward transmitted; thus, 85 kHz and another frequency are required to represent the 1 and 0 signals, respectively. The second frequency cannot differ greatly from the resonant frequency 85kHz, otherwise the resonant compensation circuit generates a large amount of reactive

Information Forward Transmission
The structure of information forward transmission is shown in Figure 4. The DSP controller of primary side generates PWM signals to drive the converter, then produces high frequency AC power that is received by the secondary side. The frequency of the resonant capacitor voltage at the secondary side is consistent with the AC power frequency, that is, the resonant capacitor C2 and the primary PWM signals are provided with the same frequency. The voltage waveforms of different power frequencies are shown in Figure 5.
The forward information is demodulated from the resonant capacitor C2, as shown in Figure 4. The voltage signal is reduced by the divider resistances R3 and R4; D0 is the transient voltage suppressor used to limit the voltage. The rectangular wave signals with the same frequency as the PWM signals of inverter are obtained by a comparator A1. In this paper, 85 kHz is selected as the primary side resonance frequency, when the information is forward transmitted; thus, 85 kHz and another frequency are required to represent the 1 and 0 signals, respectively. The second frequency cannot differ greatly from the resonant frequency 85kHz, otherwise the resonant compensation circuit generates a large amount of reactive In this paper, 85 kHz is selected as the primary side resonance frequency, when the information is forward transmitted; thus, 85 kHz and another frequency are required to represent the 1 and 0 signals, respectively. The second frequency cannot differ greatly from the resonant frequency 85kHz, otherwise the resonant compensation circuit generates a large amount of reactive power, and the efficiency decreases. Moreover, if the difference between the second frequency and the resonant frequency 85 kHz is too small, the DSP cannot distinguish them, then the signal demodulation could be failed. Therefore, the second frequency needs careful consideration.

Information Backward Transmission
When the frequency of Q 5 is lower than the power frequency, the feedback control of the secondary side is adopted to keep the load voltage stable. If the Q 5 is turned off, the system load is R L , the transmission power is at a high level; if the Q 5 is turned on, the load is shorted, the transmission power is reduced. Due to the power change of system caused by Q 5 turn-on and turn-off in the low frequency state, the input current I in also changes at the same frequency as that of Q 5 . Therefore, the backward information could be demodulated from the input current I in . The structure diagram of information backward transmission is shown in Figure 6. secondary side is adopted to keep the load voltage stable. If the Q5 is turned off, the system load is RL, the transmission power is at a high level; if the Q5 is turned on, the load is shorted, the transmission power is reduced. Due to the power change of system caused by Q5 turn-on and turn-off in the low frequency state, the input current Iin also changes at the same frequency as that of Q5. Therefore, the backward information could be demodulated from the input current Iin. The structure diagram of information backward transmission is shown in Figure 6. As shown in Figure 6 and Figure 7, the sampling resistance Ri and DC voltage source are connected in series in order to sample the input current. Since Ri is small, an amplifier consisting of R5, R6 and A2 is used to amplify the signal. R7 and C3 are pre-low pass filter, R8 and C4 are post-low pass filter. The low-frequency signal is obtained through the pre-low pass filter circuit, which is consistent with the Q5 frequency. The average value of the low-frequency signal is obtained through the post-low pass filter. The cut-off frequencies of two filter circuits are different because their functions are different. The cut-off frequency of the pre-filter circuit is close to the Q5 frequency, while the cut-off frequency of the post-filter circuit is far less than that of the pre-filter circuit. The rectangular wave is obtained by comparing the low-frequency signal with the average value. Considering the demodulation frequency range of the primary side, the rectangular wave with stable frequency could be demodulated at the primary side if the Q5 frequency is set appropriately. Therefore, the Q5 frequency needs careful consideration. As shown in Figures 6 and 7, the sampling resistance R i and DC voltage source are connected in series in order to sample the input current. Since R i is small, an amplifier consisting of R 5 , R 6 and A 2 is used to amplify the signal. R 7 and C 3 are pre-low pass filter, R 8 and C 4 are post-low pass filter. The low-frequency signal is obtained through the pre-low pass filter circuit, which is consistent with the Q 5 frequency. The average value of the low-frequency signal is obtained through the post-low pass filter. The cut-off frequencies of two filter circuits are different because their functions are different. The cut-off frequency of the pre-filter circuit is close to the Q 5 frequency, while the cut-off frequency of the post-filter circuit is far less than that of the pre-filter circuit. The rectangular wave is obtained by comparing the low-frequency signal with the average value.

Method of Information Transmission
transmission power is reduced. Due to the power change of system caused by Q5 turn-on and turn-off in the low frequency state, the input current Iin also changes at the same frequency as that of Q5. Therefore, the backward information could be demodulated from the input current Iin. The structure diagram of information backward transmission is shown in Figure 6. As shown in Figure 6 and Figure 7, the sampling resistance Ri and DC voltage source are connected in series in order to sample the input current. Since Ri is small, an amplifier consisting of R5, R6 and A2 is used to amplify the signal. R7 and C3 are pre-low pass filter, R8 and C4 are post-low pass filter. The low-frequency signal is obtained through the pre-low pass filter circuit, which is consistent with the Q5 frequency. The average value of the low-frequency signal is obtained through the post-low pass filter. The cut-off frequencies of two filter circuits are different because their functions are different. The cut-off frequency of the pre-filter circuit is close to the Q5 frequency, while the cut-off frequency of the post-filter circuit is far less than that of the pre-filter circuit. The rectangular wave is obtained by comparing the low-frequency signal with the average value. Considering the demodulation frequency range of the primary side, the rectangular wave with stable frequency could be demodulated at the primary side if the Q5 frequency is set appropriately. Therefore, the Q5 frequency needs careful consideration. Considering the demodulation frequency range of the primary side, the rectangular wave with stable frequency could be demodulated at the primary side if the Q 5 frequency is set appropriately. Therefore, the Q 5 frequency needs careful consideration.

Method of Information Transmission
Based on the above information transmission principles, two modes called predefined mode and continuous data mode are proposed for different application scenarios. Predefined mode is achieved by transmitting specific frequencies so that a series of the predefined action is realized at the opposite side. Continuous data mode is achieved by transmitting modulated signals so that the complex, large information transmission is received.

Mode A: Predefined Instruction Transmission
For the simple applications, the forward or backward information transmission could be used as a trigger that executes a series of predefined instructions. Based on the above information transmission principle, for example, if the secondary side attempts to ask for the primary side to execute a series of instructions, then the secondary sends a signal to the primary side by changing the switching frequency of Q 5 . The rectangular wave with the same frequency of Q 5 is obtained by the backward demodulator. The DSP controller of primary side recognizes the rectangular wave as a trigger signal and executes the Similarly, when the primary side asks for the secondary side to perform a certain instruction, it transmits signal to the secondary side by changing the working frequency of the converter. Taking backward information transmission as an example, the modulation waveform of Mode A is shown in Figure 8.
For the simple applications, the forward or backward information transmission could be used as a trigger that executes a series of predefined instructions. Based on the above information transmission principle, for example, if the secondary side attempts to ask for the primary side to execute a series of instructions, then the secondary sends a signal to the primary side by changing the switching frequency of Q5. The rectangular wave with the same frequency of Q5 is obtained by the backward demodulator. The DSP controller of primary side recognizes the rectangular wave as a trigger signal and executes the predefined instructions. When transmitting different predefined instructions, the switching frequency of Q5 is changed to a specific frequency. The primary side receives the trigger signal and executes the corresponding instructions.
Similarly, when the primary side asks for the secondary side to perform a certain instruction, it transmits signal to the secondary side by changing the working frequency of the converter. Taking backward information transmission as an example, the modulation waveform of Mode A is shown in Figure 8.

Mode B: Continuous Data Transmission
For applications where bulk information needs to be transmitted in real time, continuous transmission mode should be used. For the information forward transmission of Mode B, if the working frequency of the converter deviates too much from the resonant frequency, the system generates too much reactive power, which seriously affects the system transmission efficiency. Considering the high efficiency of transmission, the modulation frequency of the forward information transmission should be close to the resonant frequency. For the information backward transmission of Mode B, the demodulation frequency is constrained by the hardware parameters, because the DSP is not able to distinguish if the demodulation frequency deviates too little from the resonant frequency. So, the bandwidth of Mode B is limited.
According to the analysis above, the data information could be transferred by the binary signals. Taking data backward transmission as an example, the modulation waveforms are shown in Figure 9. The switching frequency of Q5 is set to two different frequencies that represent 1 or 0 signal. The input current frequency of the primary DC voltage source changes as the same frequency as Q5. The rectangular wave signals with two different frequencies are demodulated at the primary, and the 1 or 0 signals are identified by the DSP controller of the primary side.

Mode B: Continuous Data Transmission
For applications where bulk information needs to be transmitted in real time, continuous transmission mode should be used. For the information forward transmission of Mode B, if the working frequency of the converter deviates too much from the resonant frequency, the system generates too much reactive power, which seriously affects the system transmission efficiency. Considering the high efficiency of transmission, the modulation frequency of the forward information transmission should be close to the resonant frequency. For the information backward transmission of Mode B, the demodulation frequency is constrained by the hardware parameters, because the DSP is not able to distinguish if the demodulation frequency deviates too little from the resonant frequency. So, the bandwidth of Mode B is limited.
According to the analysis above, the data information could be transferred by the binary signals. Taking data backward transmission as an example, the modulation waveforms are shown in Figure 9. The switching frequency of Q 5 is set to two different frequencies that represent 1 or 0 signal. The input current frequency of the primary DC voltage source changes as the same frequency as Q 5 . The rectangular wave signals with two different frequencies are demodulated at the primary, and the 1 or 0 signals are identified by the DSP controller of the primary side.

Simultaneous Power and Information Transfer Control Algorithm
When the proposed SWIPT system is operating, the converter is controlled to keep the load voltage stable as the information is transferring. The control algorithm of the proposed SWIPT system is shown in Figure 10. In power transmission, the load voltage is sampled and compared with the reference value of output voltage. The voltage angle of the Q5 is adjusted in real time to stabilize the output voltage. When transmitting information, the frequency of the control signal is adjusted according to the transmitted signal, the switching frequency of the converter is adjusted

Simultaneous Power and Information Transfer Control Algorithm
When the proposed SWIPT system is operating, the converter is controlled to keep the load voltage stable as the information is transferring. The control algorithm of the proposed SWIPT system is shown in Figure 10. In power transmission, the load voltage is sampled and compared with the reference value of output voltage. The voltage angle of the Q 5 is adjusted in real time to stabilize the output voltage. When transmitting information, the frequency of the control signal is adjusted according to the transmitted signal, the switching frequency of the converter is adjusted during information forward transmission. The switching frequency of Q 5 is adjusted during information backward transmission. After the information transmission is completed, the system frequency is returned to the original frequency.

Simultaneous Power and Information Transfer Control Algorithm
When the proposed SWIPT system is operating, the converter is controlled to keep the load voltage stable as the information is transferring. The control algorithm of the proposed SWIPT system is shown in Figure 10. In power transmission, the load voltage is sampled and compared with the reference value of output voltage. The voltage angle of the Q5 is adjusted in real time to stabilize the output voltage. When transmitting information, the frequency of the control signal is adjusted according to the transmitted signal, the switching frequency of the converter is adjusted during information forward transmission. The switching frequency of Q5 is adjusted during information backward transmission. After the information transmission is completed, the system frequency is returned to the original frequency.  Figure 10. Flow chart of control algorithm.

Verifications
In this section, the experiments are carried out to transfer power and information at different frequencies, and the information is transferred at different rates. The resonance frequency of the system is 85 kHz, while 83 kHz is close to the resonant frequency and the system generates very little reactive power. So, 85 kHz and 83 kHz are set as power frequencies and modulation signal frequencies for information forward transmission. As the backward demodulator of experimental platform is able to demodulate the frequency from 4 kHz to 12 kHz, hence 5 kHz and 10 kHz Q5 frequencies are set as modulation signal frequencies for information backward transmission.
A simulation model is set up in MATLAB/Simulink and an experimental prototype is built. The simulation and experimental results of mode A and mode B with constant load voltage validate the feasibility and effectiveness of the proposed WPT topology and information bidirectional transmission method.

Verifications
In this section, the experiments are carried out to transfer power and information at different frequencies, and the information is transferred at different rates. The resonance frequency of the system is 85 kHz, while 83 kHz is close to the resonant frequency and the system generates very little reactive power. So, 85 kHz and 83 kHz are set as power frequencies and modulation signal frequencies for information forward transmission. As the backward demodulator of experimental platform is able to demodulate the frequency from 4 kHz to 12 kHz, hence 5 kHz and 10 kHz Q 5 frequencies are set as modulation signal frequencies for information backward transmission.
A simulation model is set up in MATLAB/Simulink and an experimental prototype is built. The simulation and experimental results of mode A and mode B with constant load voltage validate the feasibility and effectiveness of the proposed WPT topology and information bidirectional transmission method.
The experimental platform is shown in Figure 11. The DC voltage source is SW-6000W-300V, the controller is TMS320F28335 DSP, the MOSFET is C2M0080120D, the power diode is C4D20120D, and the waveform is acquired using the oscilloscope Tektronix TPS2024B. The system parameters are shown in Table 1. The experimental platform is shown in Figure 11. The DC voltage source is SW-6000W-300V, the controller is TMS320F28335 DSP, the MOSFET is C2M0080120D, the power diode is C4D20120D, and the waveform is acquired using the oscilloscope Tektronix TPS2024B. The system parameters are shown in Table 1.

DC Voltage Source Oscilloscope
Primary side Secondary side Coils and resonant capacitors Figure 11. Prototype of proposed SWIPT system.

Results and Analysis of Information Forward Transmission
The normal power frequency of the system is set to the resonant frequency 85 kHz, as shown in Figure 12. If the power is transferred at a non-resonant frequency, such as 83 kHz, then the system operates at Mode A, i.e. the predefined instruction transmission mode, as shown in Figure  13. For the Mode B, data transmission is carried out by modulating the binary signal, as shown in Figure 14.
The simulation and experimental waveforms of power transmission at 85 kHz are shown in Figure 12. The simulation and experimental waveforms are the voltage of Cs, output of forward demodulator and load voltage in turn. The power frequency is set to 85 kHz, and the voltage frequency of Cs is also 85 kHz. The rectangular wave signal whose frequency is consistent with the power frequency is obtained by forward demodulator. At this time, the load voltage is maintained at 50 V due to the feedback control of secondary side.

Results and Analysis of Information Forward Transmission
The normal power frequency of the system is set to the resonant frequency 85 kHz, as shown in Figure 12. If the power is transferred at a non-resonant frequency, such as 83 kHz, then the system operates at Mode A, i.e., the predefined instruction transmission mode, as shown in Figure 13. For the Mode B, data transmission is carried out by modulating the binary signal, as shown in Figure 14.
The simulation and experimental waveforms of power transmission at 85 kHz are shown in Figure 12. The simulation and experimental waveforms are the voltage of C s , output of forward demodulator and load voltage in turn. The power frequency is set to 85 kHz, and the voltage frequency of C s is also 85 kHz. The rectangular wave signal whose frequency is consistent with the power frequency is obtained by forward demodulator. At this time, the load voltage is maintained at 50 V due to the feedback control of secondary side. When the power frequency deviates from the resonant frequency, it can be used as the transmission instruction for predefined mode. For example, the power transmission frequency is changed to 83 kHz, then the voltage frequency of the Cs is also changed. The rectangular wave of 83 kHz frequency is obtained by forward demodulator, and the predefined instruction transmission from the primary side to the secondary side is completed. When the power frequency deviates from the resonant frequency, the feedback control of secondary side can keep the load voltage constant at 50 V. The simulation and experimental waveforms of power transmission at 83 kHz are shown in Figure 13. The simulation and experimental waveforms are the voltage of Cs, output of forward demodulator and load voltage in turn. The simulation and experimental waveforms of information forward transmission waveforms at 1 kbit/s rate are shown in Figure 14. The simulation and experimental waveforms are transmission signals, the voltage of Cs and load voltage in turn. The modulation signal "1" corresponds to 83 kHz and modulation signal "0" corresponds to 85 kHz. According to Figure 14, the load voltage keeps a constant of 50 V during data forward transmission. Figure 15 and Figure 16 display the transmission waveforms as the frequency of Q5 is 5 kHz and 10 kHz, respectively. When the predefined instructions can not meet the system requirements, data transmission is carried out by modulating the 1, 0 signal, as shown in Figure 17. When the power frequency deviates from the resonant frequency, it can be used as the transmission instruction for predefined mode. For example, the power transmission frequency is changed to 83 kHz, then the voltage frequency of the C s is also changed. The rectangular wave of 83 kHz frequency is obtained by forward demodulator, and the predefined instruction transmission from the primary side to the secondary side is completed. When the power frequency deviates from the resonant frequency, the feedback control of secondary side can keep the load voltage constant at 50 V. The simulation and experimental waveforms of power transmission at 83 kHz are shown in Figure 13. The simulation and experimental waveforms are the voltage of C s , output of forward demodulator and load voltage in turn. When the power frequency deviates from the resonant frequency, it can be used as the transmission instruction for predefined mode. For example, the power transmission frequency is changed to 83 kHz, then the voltage frequency of the Cs is also changed. The rectangular wave of 83 kHz frequency is obtained by forward demodulator, and the predefined instruction transmission from the primary side to the secondary side is completed. When the power frequency deviates from the resonant frequency, the feedback control of secondary side can keep the load voltage constant at 50 V. The simulation and experimental waveforms of power transmission at 83 kHz are shown in Figure 13. The simulation and experimental waveforms are the voltage of Cs, output of forward demodulator and load voltage in turn. The simulation and experimental waveforms of information forward transmission waveforms at 1 kbit/s rate are shown in Figure 14. The simulation and experimental waveforms are transmission signals, the voltage of Cs and load voltage in turn. The modulation signal "1" corresponds to 83 kHz and modulation signal "0" corresponds to 85 kHz. According to Figure 14, the load voltage keeps a constant of 50 V during data forward transmission. Figure 15 and Figure 16 display the transmission waveforms as the frequency of Q5 is 5 kHz and 10 kHz, respectively. When the predefined instructions can not meet the system requirements, data transmission is carried out by modulating the 1, 0 signal, as shown in Figure 17. The simulation and experimental waveforms of information forward transmission waveforms at 1 kbit/s rate are shown in Figure 14. The simulation and experimental waveforms are transmission signals, the voltage of C s and load voltage in turn. The modulation signal "1" corresponds to 83 kHz and modulation signal "0" corresponds to 85 kHz. According to Figure 14, the load voltage keeps a constant of 50 V during data forward transmission.

Results and Analysis of Information Backward Transmission
Figures 15 and 16 display the transmission waveforms as the frequency of Q 5 is 5 kHz and 10 kHz, respectively. When the predefined instructions can not meet the system requirements, data transmission is carried out by modulating the 1, 0 signal, as shown in Figure 17. The Q5 frequency is set to 5 kHz, the frequency of input current Iin changes as the same frequency as Q5. The rectangular wave of 5 kHz frequency is obtained by backward demodulator, and the predefined instruction transmission from the secondary side to primary side is completed. When the Q5 frequency changes, the feedback control of secondary side keeps the load voltage constant at 50 V. The simulation and experimental waveforms of Q5 frequency at 5 kHz are shown in Figure 15. The waveforms are the control signal of Q5, output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The simulation and experimental waveforms of Q5 frequency at 10 kHz are shown in Figure 16. The waveforms include the control signal of Q5, output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The Q5 frequency is set to 10kHz, the input current Iin changes at the same frequency. By comparing the output of the pre-low pass filter and the post-low pass filter, the rectangular wave with the 10 kHz frequency is obtained. The Q 5 frequency is set to 5 kHz, the frequency of input current I in changes as the same frequency as Q 5 . The rectangular wave of 5 kHz frequency is obtained by backward demodulator, and the predefined instruction transmission from the secondary side to primary side is completed. When the Q 5 frequency changes, the feedback control of secondary side keeps the load voltage constant at 50 V. The simulation and experimental waveforms of Q 5 frequency at 5 kHz are shown in Figure 15. The waveforms are the control signal of Q 5 , output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The Q5 frequency is set to 5 kHz, the frequency of input current Iin changes as the same frequency as Q5. The rectangular wave of 5 kHz frequency is obtained by backward demodulator, and the predefined instruction transmission from the secondary side to primary side is completed. When the Q5 frequency changes, the feedback control of secondary side keeps the load voltage constant at 50 V. The simulation and experimental waveforms of Q5 frequency at 5 kHz are shown in Figure 15. The waveforms are the control signal of Q5, output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The simulation and experimental waveforms of Q5 frequency at 10 kHz are shown in Figure 16. The waveforms include the control signal of Q5, output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The Q5 frequency is set to 10kHz, the input current Iin changes at the same frequency. By comparing the output of the pre-low pass filter and the post-low pass filter, the rectangular wave with the 10 kHz frequency is obtained. The simulation and experimental waveforms of Q 5 frequency at 10 kHz are shown in Figure 16. The waveforms include the control signal of Q 5 , output of pre-low pass filter and post-low pass filter, output of backward demodulator, respectively. The Q 5 frequency is set to 10kHz, the input current I in changes at the same frequency. By comparing the output of the pre-low pass filter and the post-low pass filter, the rectangular wave with the 10 kHz frequency is obtained.  Figure 17 shows the simulation and experimental waveforms of information backward transmission waveforms at 1 kbit/s rate. The waveforms include the transmission signals, the control signal of Q5, output of backward demodulator and load voltage, respectively. The modulation signal "1" corresponds to Q5 frequency of 10 kHz and modulation signal "0" corresponds to 5 kHz. The modulation rate of the transmission signal is 1 kbit/s and the Q5 frequency varies with the same frequency. According to Figure 17, the load voltage keeps constant of 50 V during data backward transmission.

Conclusions
In this paper, a new SWIPT topology is proposed, together with the information bidirectional transmission method. The working principle of the system is analyzed and the relationship between variables is deduced. The constant voltage output feedback control strategy is designed to achieve constant load voltage. The information forward transmission method based on FSK and the information backward transmission method based on chopper control are proposed. Two modes of information transmission are proposed for different working conditions. An experimental platform is built to verify the proposed topology and information bidirectional transmission method. The simulation and experiment verify that the information is able to be transferred in both directions, and the load voltage keeps stable during information transmission.  Figure 17 shows the simulation and experimental waveforms of information backward transmission waveforms at 1 kbit/s rate. The waveforms include the transmission signals, the control signal of Q 5 , output of backward demodulator and load voltage, respectively. The modulation signal "1" corresponds to Q 5 frequency of 10 kHz and modulation signal "0" corresponds to 5 kHz. The modulation rate of the transmission signal is 1 kbit/s and the Q 5 frequency varies with the same frequency. According to Figure 17, the load voltage keeps constant of 50 V during data backward transmission.  Figure 17 shows the simulation and experimental waveforms of information backward transmission waveforms at 1 kbit/s rate. The waveforms include the transmission signals, the control signal of Q5, output of backward demodulator and load voltage, respectively. The modulation signal "1" corresponds to Q5 frequency of 10 kHz and modulation signal "0" corresponds to 5 kHz. The modulation rate of the transmission signal is 1 kbit/s and the Q5 frequency varies with the same frequency. According to Figure 17, the load voltage keeps constant of 50 V during data backward transmission.

Conclusions
In this paper, a new SWIPT topology is proposed, together with the information bidirectional transmission method. The working principle of the system is analyzed and the relationship between variables is deduced. The constant voltage output feedback control strategy is designed to achieve constant load voltage. The information forward transmission method based on FSK and the information backward transmission method based on chopper control are proposed. Two modes of information transmission are proposed for different working conditions. An experimental platform is built to verify the proposed topology and information bidirectional transmission method. The simulation and experiment verify that the information is able to be transferred in both directions, and the load voltage keeps stable during information transmission.

Conclusions
In this paper, a new SWIPT topology is proposed, together with the information bidirectional transmission method. The working principle of the system is analyzed and the relationship between variables is deduced. The constant voltage output feedback control strategy is designed to achieve constant load voltage. The information forward transmission method based on FSK and the information backward transmission method based on chopper control are proposed. Two modes of information transmission are proposed for different working conditions. An experimental platform is built to verify the proposed topology and information bidirectional transmission method. The simulation and experiment verify that the information is able to be transferred in both directions, and the load voltage keeps stable during information transmission.
By the proposed topology, the secondary requires only one extra switch so that the system is equipped with the capability of bidirectional information transmission. This provides a low-cost solution for upgrading a traditional WPT system to a SWIPT system.

Conflicts of Interest:
The authors declare no conflict of interest.