Design of Peak Efficiency of 85.3% WPC/PMA Wireless Power Receiver Using Synchronous Active Rectifier and Multi Feedback Low-Dropout Regulator

Abstract

An efficient synchronous active rectifier and Multi Feedback low drop out (LDO) Regulator coupled with a wireless power receiver (WPR) is proposed in this study. An active rectifier with maximum power conversion efficiency (PCE) of 94.2% is proposed to mitigate the reverse leakage current using zero current sensing. Output voltage and current are regulated by multi-feedback LDO regulator, sharing the single path transistor. The proposed chip is fabricated in the 0.18 μm BCD technology having die area of 16.0 mm². A 94.2% power conversion efficiency with the load current of 800 mA is measured for the proposed active rectifier.

1. Introduction

Wireless power transfer (WPT) technology is getting significant attention in recent research, especially with mobile phone chargers. Its applications vary from medical components to automobiles [1]. The inductive coupling method is one of the popular WPT methods applicable for a distance below 0.5 cm with a transfer frequency in the range of 87 kHz to 375 kHz. This method is standardized by two consortiums: Wireless Power Consortium (WPC) and the Power Matters Alliance (PMA). Whatever we use the charging technique for, maintaining a high efficiency is necessary which is important because low efficiency will produce heat from the receiver which creates several problems. Under normal conditions, a WPT system has more than 5 W of power at its input, low efficiency of the receiver causes heat which reduces the receiver efficiency [2,3,4,5,6]. Usually, the whole efficiency of wireless power receiver (WPR) is controlled by the rectifier [7]. As low-dropout (LDO) regulators get their DC supply from the rectifier, rectifier efficiency is crucial. The output voltage of rectifier determines the LDO regulator’s efficiency. Protection functions like over current protection (OCP), over voltage protection (OVP), and adaptive communication limit (ACL) are unified with the LDO regulator.

This study proposes an efficient active rectifier and multi-feedback LDO (MF-LDO) regulator coupled with a wireless power receiver. Section 2 describes the architecture and building blocks of inductive coupling WPR. The simulation results are presented in Section 3 and Section 4 summarizes the paper.

2. Proposed Wireless Power Receiver Design and Its Implementation

2.1. Architecture

The simplified block diagram of the wireless power receiver is depicted in Figure 1 where the power is transmitted to the receiver through the coil. The impedance matching network maximizes the power transfer from the receiving coil to active rectifier. The active rectifier converts the input AC signals (AC₁ and AC₂) to DC voltage. The proposed active rectifier uses synchronous control by tracking input frequency by ZCS (zero current sensing) with a monostable circuit to eliminate the double pulse problem. The battery needs regulated DC voltage which is generated by the MF-LDO regulator. Protection functions are integrated to the proposed MF-LDO in this work. A 10-bit ADC converts the internal analog signals from several blocks into digital signals. The digital control block collects them and arranges the packets based on them. To perform the load modulation, it serializes the parallel data into serial data and finally delivers to the modulator. In this work, a complete wireless power receiver (WPR) is designed with proposed active rectifier and MF-LDO.

2.2. Active Rectifier

A design of active rectifier is proposed in this work which receives AC input voltage, the polarity of this input voltage decides which metal oxide semiconductor (MOS) transistor will turn on and off actively in the active rectifier, as exhibited in Figure 2. High power conversion efficiency is achieved because, at MOS transistors, less voltage drop can be made as compared to diode-based passive rectifier [8].

$${\mathsf{\eta }}_{\mathrm{rectifier}}=\frac{{\mathrm{V}}_{\mathrm{out}}}{{|\mathrm{V}}_{\mathrm{in}}|}\times \frac{{\mathrm{I}}_{\mathrm{out}}}{{\mathrm{I}}_{\mathrm{in}}}\approx \frac{{\mathrm{V}}_{\mathrm{out}}}{{\mathrm{V}}_{\mathrm{do}}+{\mathrm{V}}_{\mathrm{out}}}\times \frac{{\mathrm{I}}_{\mathrm{out}}}{{\mathrm{I}}_{\mathrm{loss}}+{\mathrm{I}}_{\mathrm{out}}}$$

(1)

The rectifier efficiency is calculated by Equation (1).

In this equation;

V_do = Voltage drop in conducting path

I_loss = Current loss, defined by the reverse current leakage in power stage

As the power transfer begins, the active rectifier operates in passive mode and operates in active mode when V_RECT gets voltage of the required power level.

The received AC power input rectification power conversion efficiency will be low with high output power level, therefore, the power efficiency of the rectifier is maximized by minimizing V_do [9]. Passive diodes have some forward voltage drop which can limit the efficiency of a rectifier [8,10,11].

On the other hand, MOS transistors have a bidirectional current flow where current flow will be from DC output to AC input. Power conversion efficiency is extremely reduced by this leakage current [12,13,14,15,16].

In Figure 3, the ZCS circuit senses the current of the active rectifier to prevent the reverse leakage current. To generate the gate signals (LI₁, HI₁) that turn on and off M_LS1 and M_HS1 respectively, the ZCS circuit senses source voltages (V_SEN1) of the sensing MOSFET (M_SEN1). Also, gate control signals of LI₂ and HI₂ are generated by M_SEN2 in the same way. The gate control signals (LI₁, HI₁) are turned and off based on Equations (2) and (3).

$${\mathrm{Turn}-\mathrm{on}:\mathrm{V}}_{\mathrm{REF}1}\le \frac{{(\mathrm{VDD}\text{\_}5\mathrm{V}-\mathrm{V}}_{\mathrm{SEN}1})\times {\mathrm{R}}_0}{{\mathrm{R}}_1+{\mathrm{R}}_0}$$

(2)

$${\mathrm{Turn}-\mathrm{off}:\mathrm{V}}_{\mathrm{REF}1}>\frac{{(\mathrm{VDD}\text{\_}5\mathrm{V}-\mathrm{V}}_{\mathrm{SEN}1})\times {\mathrm{R}}_0}{{\mathrm{R}}_1+{\mathrm{R}}_0}$$

(3)

The efficiency of the active rectifier is improved by the ZCS circuits because reverse currents of the active rectifier are prevented. The resistors of R₀, R₁, R₂, and R₃ with a low-temperature variation are used in ZCS circuit. The V_REF1 and V_CS voltages are generated by resistive ratio. Therefore, the ZCS circuit is designed strongly against the change in PVT variation. The monostable circuit and SR latch in the ZCS circuits are used to prevent the double pulse problem by glitches in the gate signals (LI_1,2 and HI_1,2).

The timing diagram of the ZCS circuit is shown in Figure 4a. At zero crossing point of ZCS_SET is generated. In Figure 2, power transistors (M_HS1, M_HS2, M_LS1, and M_LS2) are turned on by ZCS_SET and turned off by the reset signal. To turn on the high side transistors (M_HS1 and M_HS2) with the minimum conduction losses, the bootstrap circuit shown in Figure 2 generates boost voltages (V_BST1 and V_BST2) with the amplitude levels of AC signals (AC₁ and AC₂) plus 5 V since the maximum gate-source voltages of high side transistors (M_HS1 and M_HS2) are 5 V in this process.

The simulation results of the Active rectifier are shown Figure 4b. When I_AC is 20 mA, the LG₁ and HG₁ are turned on. On the other hand, when the I_AC is less than 5 mA, LG₁, and HG₁ are turned off and the reverse leakage current is blocked.

2.3. Multi Feedback LDO (MF-LDO) Regulator

A regulated DC output is provided to the charger IC before the battery and this is provided by the LDO regulator. In the WPR system, the receiver needs various protection functions. In a conventional LDO regulator, the voltage feedback loop is implemented. A MF-LDO regulator is proposed in Figure 5, in which the protection functions are incorporated to low-dropout regulator. The MF-LDO regulator shares the power transistor M_P1, to save die area.

Figure 6 shows simplified functional diagram of multi feedback LDO. The load current, I_OUT, is defined by V_G, V_RECT, and V_OUT voltages. V_OUT and V_RECT voltages are defined by the specification and the active rectifier respectively. Therefore, only V_G controls the I_OUT, and is derived from Equation (4).

$${\mathrm{V}}_{\mathrm{G}}=\frac{\mathrm{t}}{{\mathrm{C}}_{\mathrm{G}}}\times {(\mathrm{I}}_{\mathrm{FB}}+{\mathrm{I}}_{\mathrm{OVP}}+{\mathrm{I}}_{\mathrm{OCL}}+{\mathrm{I}}_{\mathrm{ACL}}+{\mathrm{I}}_{\mathrm{SIN}\mathrm{K}})$$

(4)

In the normal operation mode of MF-LDO, the I_SINK current discharges the V_G node constantly. Also, the I_FB current is generated by voltage feedback loop. Therefore, V_OUT voltage is regulated constantly, and the V_G voltage is changed depending on I_OUT currents. In the protection modes of MF-LDO—such as OCP, OVP, or ACL modes—I_OCP, I_OVP, and I_ACL are not zero current sources. When the I_OCP, I_OVP, and I_ACL are not zero current sources, V_G voltage is increased and the I_OUT current is blocked or limited since the I_SINK current is constant.

Figure 7 shows the adaptive communication limit (ACL) circuit. If the load current (I_OUT) increases rapidly during the WPC communication period, communication errors may occur. In order to prevent it, I_OUT is limited by the ACL circuit. The input signals of control circuits are the V_COMM signal, the output signal (V_IACL) of the current sensor, and references (REF_1,2,3). The ACL is enabled by the V_COMM signal and the current limit level is determined depending on the voltage level of V_IACL signal. When the V_ON is high, the parasitic gate capacitor (C_G) of M_P1 is charged by the limit level block through the diode, D4. Therefore, the voltage level of V_G is increased, and the output current (I_OUT) is limited.

Simulated results of a MF-LDO regulator are presented in Figure 8. The MF-LDO regulator regulates the output voltage to 5 V under the load current of 200 mA. The ACL is enabled at this load current and regulated up to 400 mA.

3. Experimental Results

The proposed WPR chip is fabricated in 0.18 µm 1P4M with MIM capacitors and high sheet resistance poly resistors. Figure 9 shows the chip layout pattern of the WPR. The die area in the WPR is 16.0 mm².

The measurement environment of wireless power receiver is displayed in Figure 10. Inductive wireless power is generated by power transmitter. Below the receiver coil, a transmitter coil is placed.

The measured waveform of the active rectifier is revealed in Figure 11. The ZCS circuit operates the active rectifier. M_HS1 and M_LS1 start to be turned on at 10 mA current of IAC and are active during the interval time T1. On the other hand, M_HS2 and M_LS2 start to be turned on at −15 mA current of I_AC and are active during the interval time T2.

The measured waveform of MF-LDO is shown in Figure 12. The value of load current (I_OUT) varies from 200 mA to 600 mA to check the performance of MF-LDO. When the MF-LDO is in the normal operation mode, VOUT is regulated to 5.0 V. On the other hand, when the ACL is enabled, I_OUT is limited to 450 mA.

In Figure 13, the rectifier output voltage (V_RECT) can change from 6 V to 8 V, whereas the variation of MF-LDO output voltage (V_OUT) is less than 89 mV/A. For this measurement, V_RECT is provided from the power supply.

When the value of load current is 800 mA in Figure 14, the maximum measured PCE of the active rectifier and wireless power receiver are 94.2% and 85.3%, respectively.

The performance comparison with prior works is shown in

Table 1. Performance comparison with prior works.

Parameters	[8]	[17]	[18]	This Work
Technology	0.18 μm BCD	0.18 μm CMOS	0.18 μm BCD	0.18 μm BCD
Supported standards	A4WP	A4WP	WPC and PMA A4WP	WPC and PMA
Input frequency	6.78 MHz	6.78 MHz	85 kHz~500 kHz 6.78 MHz	87 kHz~375 kHz
Input Voltage Range (V)	7–20	7–20	3–20	3–20
Efficiency of rectifier (%)	91.5	94.2 (rectifier only)	91.7 @ 6.78 MHz 92.7 @ 150 kHz	94.2 @ 150 kHz
Post-regulator	DC–DC converter	N/A	DC–DC converter	Low-dropout regulator
System efficiency (%)	80.86	N/A	84.5 @ 6.78 MHz 85.5 @ 150 kHz	85.3
Max. output power (W)	6	8	9	5
Die area (mm2)	12.25	3.45	17.5	16.0

[8,17,18]. The examples from [8,17] are active rectifiers for A4WP standard operating at 6.78 MHz and their efficiencies are 91.5% and 94.2% respectively. The maximum efficiency of [18] is 92.7% when the input frequency is 150 kHz. Therefore, this work achieves an efficiency of 92.4% and has the best performance when the input frequency is 150 kHz. This work shows the highest overall efficiency of a rectifier compared with references.

4. Conclusions

This work describes an inductive coupling (WPC/PMA) WPR having high-efficiency Active rectifier and MF-LDO Regulator. The synchronous Active rectifier with ZCS is proposed to get high efficiency in order to reduce the reverse leakage current. MF-LDO Regulator is proposed to implement the output voltage regulation, OVP, over current limit (OCL), and ACL sharing the single power transistor.

This chip is implemented in the 0.18 μm BCD technology having die area of 16.0 mm². The maximum PCE of the Active rectifier is 94.2% at 800 mA load current.