Abstract—This paper presents a low voltage AC/DC converter

using a voltage multiplier circuit for energy harvesting applications where a low voltage AC waveform used to charge a battery. The circuit rectified and boosted the AC input voltages in the range of 0.5-4 V and 40-150 Hz to a DC voltage in the range of 0.8-12 V. A maximum power efficiency of 71% achieved for AC amplitude 1.5 V with temperature measurements between -40 to +50 °C. The proposed circuit fabricated using TSMC 130nm CMOS technology with an active area of 0.249 mm2.

Index Terms—Energy Harvesting, Active AC/DC, Voltage Multiplier

I. INTRODUCTION

Energy harvesting systems aim to eliminate the need to replace batteries, by converting the ambient energy (mechanical, optical, thermal and RF) into electrical energy and charging a battery over time. The ambient energy of thermoelectric generators (TEG), silicon-based micro-fuel and single junction photovoltaic (PV) cells produced a DC voltage [1] without the need of the AC/DC converter. The ambient energy of piezoelectric, electromagnet and RF produced the AC voltage; the AC/DC converter is employed to convert the AC voltage to a DC voltage as shown in Figure 1. The energy harvest system included the voltage booster circuit, followed by storage element to eliminate the need for the batteries.

Conventional AC/DC converters are achieved by diodes

and capacitors. The full bridge rectifier and voltage doubler are commonly used as rectifier circuits to convert the AC voltage to a DC voltage [2]. However, the diode forward voltage drop of 0.7 V cannot be accepted in low voltage applications. Schottky diodes with a low forward voltage drop can replace the diodes to improve the efficiency. However, the production cost is a big problem [3]. The diode-connected MOS transistors can replace the diodes [4]. However, the efficiency is still limited by the drop voltage of the diodes.

Recently, an active diode [5] is used instead of the diode-

connected MOS transistors. The active diode works nearly as an ideal diode with current flowing in only one direction and low voltage drop. The negative voltage converter with an active diode, the cross coupled active full bridge, and active bridge voltage doubler commonly used as an active rectifier to convert the AC voltage to a DC voltage [6].

This paper is organized as follows. In Section II, the

principle of the active diode is introduced. In Section III, a voltage multiplying AC/DC converter is presented. Simulation results in Section IV, followed by conclusions in Section V.

II. THE PRINCIPLE OF ACTIVE DIODE

The active diode is used to control the current direction and to work nearly as an ideal diode. As shown in Figure 2, the active diode is a comparator-controlled PMOS switch. The power consumption of the comparator should be very low due to that the comparator supplied from the storage capacitor. Moreover, since this rectifier is mainly for low voltage energy harvesting applications. It should work at the smallest input voltage amplitude.

Figure 3 shows the proposed active diode included a PMOS switch controlled by a comparator. The transistors M4-M9 work as a comparator to control the gate voltage of the PMOS switch M1. The transistors M10-M12 are two current mirrors to supply a current needed to power on the comparator .The transistor M1 should turns ON and turns OFF completely to reduce the voltage drop. The transistors M2-M3 work as a dynamic bulk regulator, connecting substrate of the transistors M1, M4, and M6 to the highest potentials, which prevent the chance of latch up. In addition, the body effect on the transistor M1 reduced and therefore, the voltage drop and power dissipation will reduce.

Fig. 1.Block diagram of energy harvests system.

Fig. 2.Schematic of an active diode using the PMOS switch.

Ehab Belal1, Hassan Mostafa2, Yehea Ismail3 and M. Sameh Said4 1, 2,4Electronics and Communications Engineering Department, Cairo University, Giza, Egypt.

2,3Center for Nanoelectronics and Devices, AUC and Zewail City of Science and Technology, New Cairo, Egypt. 1ehab.belal@gmail.com, 2(hmoustafa@aucegypt.edu, hmostafa@uwaterloo.ca), 3y.ismail@aucegypt.edu ,4msmsab@hotmail.com

A Voltage Multiplying AC/DC Converter for Energy Harvesting Applications

978-1-5090-5721-4/16/$31.00 ©2016 IEEE ICM 2016229

The reversing current appeared from Vout to Vin if the PMOS switch M1 is still ON when Vout >Vin and the reverse current will reduce the power efficiency of the AC/DC converter.

Fig. 3. The proposed active diode

When Vin decreases and becomes smaller than Vout, the gate voltage of the PMOS switch M1 does not change to high voltage immediately and there will be a reverse current. In order to enhance the power efficiency, the reversing current should reduce. Song Guo and Hoi Lee had used an unbalanced biasing scheme to reduce the time delay [7].

In this work, without the need of this scheme, the proposed active diode can work with very little reverse current by adjusting the size of the M11 and M12. The transistor size M12 should be bigger than the transistor size M11, thus the current through the branch of the transistor M12 will be larger than the current through the transistor M11. A larger ID current makes the voltage of the transistor M6 larger than the voltage of the transistor M7, which makes an offset voltage in the comparator. This will compensate the time delay and eliminate reverse current.

III. A VOLTAGE MULTIPLYING AC/DC CONVERTER

There are two types of voltage multiplier often used as voltage booster, Villard and Dickson circuits [8] achieved by diodes and capacitors. However, the diode forward voltage drop of 0.7 V cannot be accepted in low voltage applications. Active bridge voltage doubler [9-11] is used as a voltage multiplier AC/DC converter as shown in Figure 4. During the positive wave of the AC input signal the voltage at the negative input port of the comparators is greater than 0V, Correspondingly the comparator 2 output becomes low which turns the PMOS switch ON and the comparator 1 output becomes low which turns the NMOS switch OFF. Thus , the current flowing through the PMOS switch charges the loading capacitor Cload.

During the negative wave of the AC input signal, the

voltage at the negative input port of the comparators is lower than 0V, correspondingly the comparator 2 output becomes high which turns the NMOS switch ON and the comparator 1 output becomes high which turns the PMOS switch OFF.

Thus, the current flowing through the NMOS switch charges the loading capacitor Cload.

The circuit does not need any discrete components [12] to

boost the voltage, but the dc power supply required by active parts is obtained through a passive AC/DC doubler. In addition, a voltage reference circuit is used to provide the bias voltage for the comparator [13].

Figure 5 shows the proposed voltage multiplier AC/DC

converter using the active diodes for energy harvesting applications. The use of the active diodes can significantly reduce the input voltage limit and improve the efficiency.

During the negative cycle of the AC input signal, the transistor M1 acts as a diode clamp conducts, and charges the capacitor C1 to the value Vc1=Vin-Vth(M1), the voltage at terminal A will be -Vth(M1) . During the positive cycle of the AC input signal, The transistor M1 is OFF the capacitor C1 discharge very little by applying the Kirchhoff's voltage law, the voltage at terminal A will be 2Vin- Vth(M1) .

Fig. 4. An active voltage doubler.

Fig. 5.The proposed voltage multiplier AC/DC converter using an active AC/DC converter.

When the voltage at terminal A is higher than the voltage at terminal B, the comparator 1 output becomes low which turns the transistor M2 ON to allow the charging of the capacitor C3

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to the value Vc3=Vin-Vth(M1)-Vth(M2). When the voltage at terminal A is lower than the voltage at terminal B, the comparator 1 output becomes high which turns the transistor M2 OFF and the forward conduction path is disconnected.

The voltage at terminal C is shifted by a DC value (Vin-

Vth(M1)-Vth(M2)-Vth(M3) ) plus the AC voltage of capacitor C2.

The active diode includes the PMOS switch M4 controlled by the comparator 2 which used as a second stage of the proposed voltage multiplier AC/DC converter. The active diode works as a follow if the voltage at terminal C is higher than the voltage at terminal D, the comparator 2 output becomes low which turns the transistor M4 ON to allow the charging of the capacitor C4. When the voltage at terminal C is lower than the voltage at terminal D, the comparator 2 output becomes high, which turns the transistor M4 OFF, and the capacitor C4 will discharge through the load RL.

IV. SIMULATION RESULTS

The proposed voltage multiplier AC/DC converter using the active diodes for energy harvesting is simulated by using Cadence Spectre and hardware calibrated 130nm CMOS technology provided by UMC. Figure 6 shows the transient behavior of the proposed voltage multiplier AC/DC.

Fig. 6. Transient behavior of the proposed voltage multiplier AC/DC

under 20Hz,1.5V input.

The voltage efficiency versus different input voltage amplitude "unloaded" and with load of 10kΩ for the proposed voltage multiplier AC/DC is shown in Figure 7. The proposed voltage multiplier AC/DC active bridge efficiency is larger than 300% at input voltage 0.7V unloaded, and 270% at input voltage 0.7V with a load of 10kΩ.

Figure 8 shows the power efficiency versus different input

voltage amplitude for the proposed voltage multiplier AC/DC. The maximum power efficiency is higher than 83%.

Table І shows a comparison with other state-of-the-art voltage multipliers AC/DC using an active diode (AD). Compared to the other voltage multipliers, this proposed voltage multiplier AC/DC converter does not need external voltage reference circuit since the comparator supplied from

the storage capacitor. It also eliminates the need of the passive AC/DC doubler. In addition, it provides the higher voltage efficiency 270% with ohmic load 10kΩ is used and its ripples lower than 5%. The minimum input voltage of the proposed circuit is around 0.3V and the maximum power efficiency around 83%. The proposed voltage multiplier AC/DC converter chip layout occupies an active area of 49.020µm*5086.51µm.

Fig. 7. Voltage efficiency versus input voltage amplitude unloaded and with

ohmic load 10KΩ with 20Hz input frequency.

Fig. 8. Power efficiency versus input voltage amplitude under 20Hz.

In order to assure the performance of the proposed voltage

multiplier AC/DC converter using active diodes, corner simulations across PVT variations is carried on the extracted layout, as shown in Table II. Post-layout system simulations using the proposed voltage multiplier AC/DC converter shows an overall power efficiency of 71% with temperature measurements between -40 °C to +50 °C.

V. CONCLUSION

The proposed voltage multiplier AC/DC converter rectified and boosted the AC voltages in the range of 0.5-4 V and 40-150 Hz to a DC voltage in the range of 0.8-12 V. A maximum power efficiency of 71% achieved for the AC amplitude 1.5 V with temperature measurements between -40 to +50 °C.

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TABLE І PERFORMANCE COMPARISON

Year 2011 2012 2013 2014

Reference 12 11 10 9 This work

Technology External +/‐ 1.25 supplies

Voltage reference circuit +TSMC 90nm CMOS

Voltage reference circuit +TSMC 90nm CMOS

Voltage reference circuit +UMC 180 nm CMOS

TSMC 130nm CMOS

"schematic"

Vin 0.1‐1.2 0.1V‐1V 0.1V‐0.8V 0.15V‐1V 0.3V‐4V

Vout N/A ≃ 0.2V‐1.7V ≃ 0.2V‐1.4V ≃ 0.2V‐2V ≃0.4‐13.2 V

Ripples 10% N/A N/A N/A <5%

Frequency 1Hz‐500Hz 1Hz‐10kHz 10H.z 8H.z 20Hz‐1KHz

Max power efficiency

>80% 92% 67% @ 0.61 V,40 μA load

86%@1.1v

83%

Size N/A N/A N/A N/A 0.249mm2.

TABLE II CORNER SIMULATIONS ACROSS PVT

Vin Vout Frequency

Max power

efficiency

Temp range ° C

Process corner

Post-layout

0.5-4V 0.8-12.38V 40Hz-150Hz 71% -40:50 SS,TT,FF

VI. ACKNOWLEDGEMENT

This research was partially funded by Cairo University, ITIDA, NTRA, NSERC, Zewail City of Science and Technol- ogy, AUC, the STDF, Intel, Mentor Graphics, SRC, ASRT, and MCIT.

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