ieee journal of solid-state circuits volume 48 issue 12 2013 [doi 10.1109%2fjssc.2013.2287592] choi,...

IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 48, NO. 12, DECEMBER 2013 2989

Resonant Regulating Rectifiers (3R) Operating for6.78 MHz Resonant Wireless Power Transfer (RWPT)

Jun-Han Choi, Student Member, IEEE, Sung-Ku Yeo, Seho Park, Jeong-Seok Lee, Member, IEEE, andGyu-Hyeong Cho, Senior Member, IEEE

AbstractThe design of a Resonant Regulating Rectifier (3R)capable of switching-mode operation is presented. The pro-posed 3R is a highly efficient receiver circuit intended for usein Resonant Wireless Power Transfer (RWPT) application witha 6.78 MHz resonant frequency. Owing to the inductance ofresonant coils, the 3R does not require any additional inductorfor the switching-mode regulation. The transmitted power via theRWPT with the 3R ranges from 0 W to 6 W, and its peak effi-ciency reaches 86%. It employs the Continuous Conduction Mode(CCM) and Discontinuous Conduction Mode (DCM) for differentoutput power levels. This helps to increase the output power andto lower the voltage stress on the transistor. Fabricated in 0.35 mBCD technology, the 3R circuit occupies an area of 2.35x2.35 mm.The functionality of the 3R is successfully demonstrated using atransmitter circuit with resonant coils.

Index TermsA4WP, frequency 6.78 MHz, frequency13.56 MHz, inductive power transmission, IPT, medium wirelesspower, phasor transformation, Qi, receiver circuit, resonantcharger, resonant regulating rectifiers, resonant tanks, resonantwireless power transfer, RWPT, secondary pwm control, wirelessbattery charger, wireless charger, wireless power, wireless powertransfer, wireless power transmission, WPT, wireless power 5 W,wireless power 6 W, wireless power control, WPC, wireless SMPS,wireless voltage control.

I. INTRODUCTION

R ECENTLY, Wireless Power Transfer (WPT) technologyhas become a popular research subject again, especiallyin relation to cellular phone-charger market. Since Tesla firstsuggested the concept of WPT, it has slowly evolved for morethan one hundred years. Researchers have tested the possibilityof the WPT for various applications ranging from electrical carsto bio-medical applications, as categorized in Table I. However,a majority of these applications have failed to be commercial-ized or have remained as further research items. What blockedthe WPT from becoming more wide spread in the consumermarket? There may be a number of different reasons dependingon the cases, but seen from a distance, the major issues are re-lated to cost, efficiency and spatial freedom.

Manuscript received April 19, 2013; revised October 03, 2013; acceptedOctober 04, 2013. Date of current version November 20, 2013. This paperwas approved by Guest Editor Piero Malcovati.J.-H. Choi and G.-H. Cho are with the Department of Electrical Engineering,

Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea (e-mail: [email protected]).S.-K. Yeo, S. Park, and J.-S. Lee are with Samsung Electronics Co. Ltd,

Suwon, Gyeonggi-do, Korea.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JSSC.2013.2287592

Given that the WPT system is composed of two separateparts, the transmitter and the receiver, there is a disadvantage interms of cost and efficiency compared with plugged-in system.Therefore, the key issue is convenience before people willbuy WPT applications, and this is closely related to the spatialfreedom. A specific WPT application can be established in theconsumer market only when its spatial freedom outweighs costand efficiency issues. Currently, the wireless phone-chargermarket also faces with the similar problems.WPTs can be classified into two types: inductively coupled

power transfer (IPT) [1] and resonant wireless power transfer(RWPT) systems [2][6]. In this paper, RWPT is defined asany WPT method using resonant coupling. Generally speaking,the IPT has higher efficiency but requires very short distancesand precise alignment between the transmitter and the receiver.On the other hand, the RWPT allows longer distances and lessprecise alignment under a handicap of somewhat lower effi-ciency. In the WPT phone-charger field, IPT products have de-veloped near completion [1], and numerous products have beenreleased in the last few years. Meanwhile, some companies andresearchers have attempted to develop the RWPT systems [2],[3], but few of which are feasible for completed system haveappeared thus far. Table I estimates the maturity of each tech-nology. If a RWPT set is made successfully, it can be one of thesolutions for the feet-stuck situation of the medium power WPTmarket.In the RWPT, transmitter and receiver coils do not use a core

material, whereas the IPT uses cores material in the center ofthe coils. This characteristic comes from the fact that the RWPTuses loosely coupled coils to achieve better spatial freedom.However, it is very difficult to ensure a resonant condition withpalm-sized coils at a low frequency such as several hundredkHz; a resonant frequency of at least several MHz isrequired. Our research focuses on the development of a RWPTsystem that supports cellular phone chargers by transferringmedium power of around 5 W using a fixed of 6.78 MHz.The WPT system is consisted of two basic parts: the trans-

mitter and the receiver. In terms of its systematic design, how-ever, it is better to divide the system into three parts: the trans-mitter part, the receiver part, and the resonant tanks part on bothsides, as shown in Fig. 1. The overall efficiency can be cal-culated by multiplying the efficiency values of the respectiveparts. Among them, the efficiency of the receiver part is mostimportant, as it is specially related to the thermal emission of thehands-on mobile device and, therefore, has to meet strict speci-fications. Furthermore, regarding recent smartphones which al-ready utilize most of their thermal margin on the applicationprocessors, the importance of designing the receiver circuit withhigh efficiency for a WPT application is even greater now.

0018-9200 2013 IEEE

2990 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 48, NO. 12, DECEMBER 2013

TABLE IRESEARCH TRENDS OF RELATED TO WIRELESS POWER TRANSFER (WPT) AND THE MATURITY OF TECHNOLOGY

Fig. 1. Conventional wireless power transfer (WPT) system.

In this paper, we suggest new receiver circuits for the RWPT,with the term, Resonant Regulating Rectifier (3R), introducinga combination between a simple structure and high efficiency.The adjective resonant is added to 3R, because it works underthe presence of the resonant condition. This paper introducestwo different receiver circuits, one-switch 3R and three-switch3R. There is a big difference in regulation topologies, but aremany common features in control blocks in these circuits. Theone-switch 3R is explained in Section II first, including its com-parison to conventional receiver circuits. Solving the problemsin the one-switch 3R, the three-switch 3R circuit is evolved andanalyzed in Section III. In Section IV, several important circuitsand related techniques are discussed. The experimental resultsand performances levels of 3R are provided in Section V, andthe conclusions follow in Section VI.

II. INITIAL CONCEPT OF A RESONANT REGULATINGRECTIFIER (3R)

A. Design Principles of a WPT Receiver Circuit and theStructure of the One-Switch 3RFor most power converter circuits, output voltage regulation

is the main purpose, and this is much the same for the WPTsystems. A regulating technique can be implemented in eitherthe transmitter [4], [5] or the receiver [1], [3], [6]. It can varydepending on the requirements of the target application, suchas the required speed of the response and the output voltagetolerance that application can endure. If the voltage regulationmechanism of the WPT is placed in the transmitter, the overallsystem can take advantage of improved cost and efficiency, butthe error signal between the output voltage and the referencevoltage needs to be transmitted from the receiver wirelessly. Itusually takes several period cycles to process the signal, which

brings about a delayed response and oscillation of the outputvoltage for a transient load change. Because charging the bat-tery of a cellular phone with a level of 5 W requires both a rapidresponse and precise voltage conditioning, the regulating func-tion has to be placed in the receiver circuit.Usually, the conventional receiver circuits adopted in WPT

consumer electronics have nearly identical structures with atwo-step design consisting two separate parts, a rectifier and astep-down converter. The latter part can be an LDO or a buckDC-DC converter, as shown in Fig. 1. The LDO is simplerwith higher loss levels, while the buck DC-DC converter hashigher efficiency but requires an extra inductor, which causes aburdensome increase in cost and interferes with the design ofthe receiver board.A switching topology without an extra inductor can increase

the efficiency and lower the cost, providing simplicity for the de-sign of the board. The design concept of Fig. 2 is a good startingpoint, which meets these requirements. Controlling the rectifiedcurrent with pulse width modulation (PWM) control ofan switch is the underlying concept of the idea. Meanwhile,by shorting in the off-duty state, the voltage stress on iseliminated and the resonant current freewheels. The prin-cipal method of controlling is similar to those appearing inprevious papers [4], [7]; these methods were termed resonantswitching and the integral cycle mode, respectively. Comparedto both papers, in which the transmitter controls , the controlmechanism is in the receiver part in this paper.While simulating the above topology, we soon realized that

shorting in the off-duty state can cause several seriousproblems, such as an excessively rising and an increase inthe conduction loss of the resonant coils and . Furthermore,because flows back and forth, the requirement of a body

CHOI et al.: RESONANT REGULATING RECTIFIERS (3R) OPERATING FOR 6.78 MHz RESONANT WIRELESS POWER TRANSFER (RWPT) 2991

Fig. 2. Initial concepts of a regulating rectifier.

Fig. 3. Resonant Regulating Rectifier (3R) topologies with the estimated waveforms: (a) Initial sub-switch topology (b) improved serial sub-switchand limiting capacitor topology, and (c) simplified limiting capacitor topology, for managing IRS in the off-duty state.

switching technique is inevitable. Putting a limiting capacitorin series with reduces in the off-duty state as

shown in Fig. 3(b), and this also helps reduce the conductionlosses proportional to . Moreover, inserting makesthe voltage stress on manageable during the off-duty state.The circuit with a sole , also shown in Fig. 3(c), can dothe same job accomplished by the circuit in Fig. 3(b). Theoperations of both circuits are identical in on- and off-dutystates. As a result, we chose the circuit shown in Fig. 3(c) as thefirst 3R. Since there is no need for body switching, controllingthe switch is relatively easy. We term this one-switch 3R todifferentiate it from the latter one in Section III.

B. Optimizing the One-Switch 3R and Introducing V-I CurvesThere is a prior condition in which the capacitive reactance

of should be several times larger than the input impedanceof the full bridge rectifier at the given . This is necessary be-cause most of the resonant current should flow into the rectifier.Furthermore, the designers of resonant tanks should considerthe resonant condition, especially for the case of shiftingdue to the parallel connection between and the rectifier, as

shown in Fig. 4(a). Optimizing the impedance and interpretingthe appropriate resonant condition with the rectifier are difficulttasks in a frequency range of several MHz. For the systemat-ical analysis, we develop curves through a simulation andcompared them depending on , as shown in Fig. 4.In the curves shown in Fig. 4(b), the -axis and -axes

represent and or in the resonant coil, respectively.Here, the rectified energy and the buffered energy

are the two key criteria for optimizing the capacitanceof . In the RWPT system, the energy or power travelsthrough the coils, so the area of curve can be consid-ered as transferred energy from the transmitter to the receiverresonant tanks. Similar to that, the curve, which is

, imitates the transferred energy from the resonant tank tothe rectifier output. Next, the area of is the differencebetween and curves in Fig. 4(b), and itdescribes the temporally stored energy in at each cycle.However, since these curves uses the cylindrical coor-dinate, the area does not exactly represent the energy or thepower. Therefore, and are somewhat difficult torepresent, but we named it in convenience.


Fig. 4. To optimize the limiting capacitor , (a) a simple rectifier circuit is introduced with the mark of the current path of and , after which(b) current and voltage waveforms are converted to I-V curves. (c) Simulated curves with varying are compared to attain the optimum value of at amaximum load condition.

If the resonant condition is well maintained under variousvalues, the radius and the area of during the half

phase of a resonant period will not be changed. In particular,the radius of should be viewed in detail. In Fig. 4(c), thecircuit is simulated with various to ratios from 0 to 1with a 1 A load current and a 5 V ; the resonantcondition is changed when is larger than 0.5 . Fromthe simulation result, a range of value can be chosen aslong as the value does not exceed the maximum , which is0.5 .

C. Easing Voltage StressPreviously, we explained briefly the necessity of , but it

is better to understand the role of if the detailed operationis presented. is the stored energy in at the begin-ning of the phase change, in Fig. 4(c); and have aproportional relationship and itself is not dissipative en-ergy. is defined as the differential voltage of and

or the voltage across ; it is closely related to ,which is the rectifier output voltage. In the on-duty state,is connected to and limits so that it is not increasedby more than , while limits lower than

during the off-duty state, where is the diode dropvoltage.The major role of is to block so that it does not rise

too high during the off-duty state. Detailed voltage and currentwaveforms of the one-switch 3R are descried in Fig. 5. Duringthe on-duty state, increases gradually, and most of the cur-rent turns into . During the off-duty state, decreases tozero, but a small amount of charges back and forth.When the state is changed from the on-duty to the off-dutystate, increases instantly, as the only remaining current

Fig. 5. Current control mechanism of the one-switch 3R.

path of in Fig. 4(a) is that with . The amount of in-rush current into at the switching moment determines the

value, which is proportional to the value of the priorphase. Because has a linear relationship with the load cur-rent and the output power andhave an effect on the peak level. In other words, the peak

increases when rises.In the view point of the circuit design, the highest value of the

terminal voltage is very important; is the highest voltagein the proposed one-switch 3R circuit. The explicit and therelationship with the transistor break-down voltageare established as follows:

(1)


Fig. 6. Current control mechanisms in (a) the Discontinuous Conduction Mode (DCM) (b) Continuous Conduction Mode (CCM).

With LDMOS transistors, can endure several tens ofvoltage. However, because there is a trade-off relationshipbetween the break-down voltage and the mobility of the tran-sistor, the MOS transistors with lower break-down voltagesare desirable. Therefore, all of the parameters, including

and , are closely related in theone-switch 3R circuit design. For example, the circuit with

W shows a 60 V peak , avalue that the design with 40 V LDMOS cannot withstand.Although the one-switch 3R circuit met the conceptual re-

quirements of a switching converter without an extra inductor,its performance was not satisfactory due to the low andthe low efficiency. First, could not reach the expectedlevel. The level of the peak value exceeded the voltagethat the switch could hold when increased. With ahigher device, could be increased but the cir-cuit would suffer more from undesirable parasitic effects. Sec-ondly, the efficiency was poorer than that of conventional WPTreceiver ICs. Considering the conduction loss from an extra in-ductor in the buck converter and the drop-out voltage of theLDO, the one-switch 3R did not show an impressive result.

III. PROPOSED 3R

In this section, a different type of resonant regulating rectifier(3R) is introduced. It uses a new type of switching method toregulate the output voltage. Its design is explained in detail bymodeling an equivalent inductor to derive the duty for theequation.

A. The Proposed 3R and the Operation

We confirmed that the one-switch topology worked and thatthe duty of with a receiver circuit was successfully imple-mented. However, we decided to create another version of 3R inorder to improve the and efficiency. To do so, the problemwas at the level of . Specifically, the peak current level wastoo high to reduce the conduction loss, which is calculated by

, where represents the DC resistance of the res-onant coils. If we can change the shape of from that shownin Fig. 6(a) to that shown in Fig. 6(b), the RMS current can bedecreased while keeping the average current at the same level.Moreover, the voltage will not be a big problem becausethe current in the off-duty state release the voltage stress. Forconvenience, we refer to the mechanisms in Figs. 6(a) and (b)

Fig. 7. The proposed three-switch 3R.

as the Discontinuous Conduction Mode (DCM) and the Contin-uous Conduction Mode (CCM), respectively.In order to support the CCM, a new 3R circuit is derived, as

shown in Fig. 7. This 3R consist of three switches, and, along with two capacitors, the output capacitor

and the flying capacitor . The key of switching sequenceof the three-switch 3R is to control up and down via an inte-gral cycle mode, where on or off switching is determined withinhalf of the resonant cycle [7]. Fig. 8 shows the operations in theCCM and DCM, respectively, where the waveform of dif-fers depending on the case. In the on-duty state in the CCM,both and are closed and is open, where and

are connected in parallel, as shown in Fig. 8(a). Whilein the off-duty state in the CCM, both capacitors are stacked ina series with opposite switch statuses. Therefore, the resonantcurrent charges the capacitors in parallel orin series , alternately, while alternating be-tween and for each duty. While in the on-dutystate in the DCM, the switch statuses are identical to that in theCCM, as shown in Fig. 8(b). During the off-duty state in theDCM, however, becomes zero due to opening of all threeswitches, and dramatically increases depending onand the amount of induced. In fact, the operation of theDCM is identical to that in the one-switch 3R, as introducedin Section II.The magnitudes of and follow simple dynamics

under a resonant condition. When is low, the resonant cur-rent increases slowly, whereas when is high, the cur-rent is decreased or is reduced to zero. If we consider the en-velope magnitudes only in CCM, the envelope of andbehaves akin to the current and voltage in a buck-type switching


Fig. 8. Operation principle of 3R, depending on (a) the CCM and (b) the DCM.

Fig. 9. Phasor transformation from (a) a rotational system to (b) a stationary system.

DC-DC converter. Meanwhile in the DCM, is instantly de-creased to zero immediately when entering the off-duty statesuch that is regulated more precisely under a light load.

B. Modeling a WPT System With Phasor Transformation

The proposed 3R uses the inductance of the resonant tanksfor regulating the voltage. For example, the peak cannotchange instantly, which means that there is a type of force sim-ilar to the force an inductor creates. In fact, there is a very usefulmethod for interpreting such a case, called phasor transforma-tion [8]. It is used to analyze the characteristics of a series res-onant converter quantitatively. In Fig. 9, a simple example isshown in which a rotational system is transformed into a sta-tionary system. In this Figure, the peak envelope of at theinput and output node in a rotational system is identical to thecurrent waveforms in a stationary system. Therefore, the fre-quency component at a given can be deleted and we canconcentrate on designing the topology.

With the phasor transformation, the resonant tanks can be ab-stracted equivalently to a single inductor at a value which is fourtimes of the leakage inductance . Because the resonantcoils usually have a very low coupling ratio , the valueof is nearly equal to the value of . As a result, wecan assume the inductance equivalent to .Applying phasor transformation, we can model the entire

system as a simple structure, but it is still an AC-AC converteras shown in Fig. 10. To simplify this even more, the Class-Eamplifier and the rectifier can be transformed into ideal trans-formers, after which the transformers are abstracted in a singleinductor. The final version of the abstracted model is somewhatsimple: an inductor with equivalent DC resistanceand the 3R circuit. Here, the includes every resistiveparasitic.The phasor transformation, however, is not a useful tool for a

conventional two-step receiver circuit whose structure consistsof a rectifier and a step-down converter, but is only useful whenthe peak envelope of changes. Therefore, only a one-step


Fig. 10. Class-E and rectifier are modeled as ideal transformers. In this case, the circuit can be created in an abstracted form with a single inductor. EquivalentDC resistance was introduced to model the parasitic effects that cause conduction losses.

circuit using resonant switching or an integral cycle mode canhave an advantage when using this method [4], [7].Here, one-step circuit means the circuit that is impossible to

be separated into two different parts, in an analytical manner.Structurally, both one-switch 3R and three-switch 3R consist ofa full-bridge rectifier andMOSFET switches, so it could be seenas another modification of the conventional receiver circuit withtwo parts. However, MOSFET switches alone cannot work bythemselves and operations of full-bridge rectifier of both 3Rs aredifferent from that of the two-step circuit. Therefore, in order toanalyze both 3Rs, we need to deal with them as one-step AC-DCconverters.

C. 3Rs Duty- RelationshipMerging the abstracted model with the 3R circuit, a topology

of the DC-DC converter results, as shown in Fig. 11(a). Ignoring, we can calculate the to relationship ac-

cording to the duty, by using the voltage-second balance rela-tionship. The result is as follows:

(2)

when on-duty, , when off-duty, ,

(3)

In Fig. 11(b), a duty vs. graph is featured by the dottedline. The distinctive feature is that is regulated between

and half of . In fact, 3R in the CCM cannot secure aregulating duty when the load current is very small. Further-more, if is applied, the duty vs. graph becomessomewhat degraded, which is the solid line in Fig. 11(b). Com-paring the dotted line and the solid line, we can anticipate that anunwanted nonlinear effect can arise due to . In the ex-periments, we actually observed the slanted envelope waveform(dotted line) of , as shown in Fig. 11(c). The same happenswhen the inductance is small and is large in a conven-tional SMPS.In conclusion, it was demonstrated that the 3Rwith the RWPT

system is a switching power converter to which PWM con-trol can be applied. Moreover, by analyzing the duty vs.graph, we found that a higher duty is more efficient. There is an-other possibility that should be adjustable in order to keepthe duty high.

D. Optimizing Between CCM and DCMAs described in Fig. 8, the on-duty states of the CCM and

DCM are identical, but the off-duty states of each mode are

Fig. 11. (a) The abstracted model of 3R with the WPT system, (b) theduty- relationship, and (c) the slanted waveform.

different. State flow diagram is in shown in Fig. 12(a) and amode changing event has to be arranged.As touched upon above, each the CCM and DCM has its own

range of , which is advantageous for specific case. TheCCM has higher efficiency than the DCM, but the regulation of

fails when is very small, as shown in Fig. 12(b). Onthe other hand, DCM has a good regulation characteristic for alow . Actually, when is low, the efficiency does notbecome a problem because the static power loss of the WPT isrelatively high. However, because rises depending on thelevel of , as in the one-switch 3R in Section II, the DCM


Fig. 12. (a) State flow diagram for the CCM and DCM mode change and (b) agraph to define the appropriated reference of the output power, .

cannot be used at high due to avoiding the break-downof the MOS transistor.The maximum depends on how much power the trans-

mitter can push to the receiver. When the transmitter is set tobe capable of 6 W for the WPT, the fail position of thevoltage regulation with the CCM is where is slightlyless than 1 W. Therefore, we posed a mode changing point

at 1 W. In this way, we can guarantee the regula-tion ability of 3R for entire range of , and take advantageof the improved efficiency as much as possible by spanning theCCM region.

E. Increased and Reduced

With a continuously flowing in the CCM, increased per-formance not only for better efficiency but also pertaining to thecapability are obtained. The increased efficiency can beeasily anticipated because the RMS current is decreased with thesame output current, as noted earlier. In fact, the impressive re-sult comes from capability. is fixed to in theCCM such that we do not have to worry about rising witha high . As a result, is not limited by the voltagestress on , which is why the one-switch 3R cannot lead to ahigh . In other words, and do not have corre-lation in the CCM. Furthermore, this leads to the possibility of afurther improvement when using a lower break-down transistorwith greater mobility in the future.In the DCM, the operation of the proposed 3R in Section III

is identical to that of the one-switch 3R in Section II; theand relationship is also identical in the DCM. However,

the DCM only works when is lower than 1 W. There-fore, we only need to consider the peak of when is

.It was noted that in Fig. 4 is not dissipated energy inin Section II. This is mostly true and the efficiency andwith value of 0 to 0.5 do not change greatly.

However, precisely, does have an effect on the efficiencybecause increases in keeping with , creating con-duction loss more in the coils. Therefore, if there is no otherproblem such as MOS break-down and reliability or noise is-sues, it would be better to reduce . As a result, to determine

, both the reliability of and the efficiency have to beconsidered together.

IV. KEY BLOCKS AND ITS DESIGN TECHNIQUESAll of the system blocks are displayed in Fig. 13. The basic

requirements of the 3R circuits were to support 5 W andoperation. Among the frequencies of 6.78 or 13.56 MHz, at

which most RWPTs operate [2][6], the 3R uses 6.78 MHz forthe . There is another important frequency figure, which isthe PWM frequency . The was set to 848 kHz,which is in sync with and is one eighth of . In otherwords, eight resonant cycles are used for one duty period.Originally, our goal was to implement a complete receiver

IC. However, we chose to fabricate the switching converterblock first, owing to the complexity of the 3R receiver board.As a result, the rectifier was excluded from IC fabricationprocess. Instead, it was assembled outside with Schottky diodes

. On the other hand, IC components, including threemain switches and some other peripherals, were successfullyimplemented, marked with the gray background in Fig. 13.In the figure, the trail of the feedback loop of the 3R follows

the dotted arrows, passing through a resistive voltage divider, anOTA with a compensation filter, a comparator with a saw-toothwaveform, a latch for quantizing the duty, the gate drivers, andlastly, the switches. Given that is around 1 MHz, mostof the above circuits do not have to work around , exceptfor the circuits related to synchronization. The synchronizingtechnique is explained in the following chapters.The mode change between the CCM and DCM is accom-

plished with an external signal, as shown in Fig. 13, and thereare three-bit duty information ports coming out from a simplethree-bit counter. These three-bit ports can be used as a dutymonitor optionally. As stated in Section III, a high duty ispreferable for better efficiency. Therefore, we added a digitalfeedback, which is optional. We tested the 3R design withor without the digital feedback component, which will beexplained in detail.

A. ZCS and the Sync BlockZero Current Switching (ZCS) improves the efficiency as

well as the switching noise. Therefore, it is crucial for the on/offswitching time to synchronize with the zero-crossing time,as shown in Fig. 14(a). There are two means of synchroniza-tion, as shown in the circuit in Fig. 14(b): one is a differen-tial comparator sync and the other is an inverter sync. While inthe on-duty state in both the CCM and DCM, a simple inverterconsisting of a high voltage MOS is enough to ensure the ZCS


Fig. 13. The complete system block diagram is displayed. IC parts are shown with a grey background, and the feedback loop path is indicated by a dotted arrow.

Fig. 14. (a) For zero current switching (ZCS), (b) sync control is needed; (c) a specific delay has to be applied in order to match the half resonant cycle and.

sync. While in the off-duty state in the DCM, however,is charged by in one direction, which keeps the and

nodes floating as shown in the waveforms of Fig. 14(c).Therefore, the comparator sync with a forced offset ofhas to be used for faultless operation. These inverter sync andcomparator sync are merged into one pulse by a SEL switch. Themerged pulse is intentionally delayed by to formulate CLK.Then, CLK is used for the D-FF clock in Fig. 13 to accomplishZCS. In this case, the additional delay is controlled outsideby a four-bit digital input in order to compensate for the delay

. That is, the sum of and is set to half the period ofto secure a zero-crossing time of by creating 180-degreephase lag.

B. Boost Bias GeneratorThe Boost Bias Generator (BBG) in Fig. 15(a) is designed to

drive the and gates in Fig. 13. In the CCM, the peakvoltage of is fixed at , though it can be much higherin the DCM. Therefore, limiting the gate bias voltage to a propervalue is important to prevent a break-down of the gate oxide


Fig. 15. (a) Boost Bias Generator (BBG) Circuit and (b) to fromBBG relationship.

as can be seen in Fig. 15(b). The voltage formed by a seriesof diodes with the diode-connected of Fig. 15(a) acts asa reference voltage, and with an edge-enhancement filter(EEF) stimulates to charge . On the other hand,limits by forming a current path to five series diodes.

C. Digital Feedback

For only the efficiency, a fully turned on-duty state will be thebest. However, in such way, voltage regulation is impossible. Asa result, around 80% of the duty will be the point of compromisefor both the efficiency and the regulation, as shown in Fig. 11(b).A digital feedback loop can ensure that the duty is located atthe optimum point by trimming the drain voltage of the Class-Eamplifier in Fig. 13. This feedback does not have to be fast. Infact, this digital feedback has to be much slower than the mainfeedback loop, as indicated by the dotted arrows. Hence, theloops do not interfere with each other in the frequency domain.The three-bit duty information for the digital feedback ap-

pears during every duty cycle. Then, 1000 samples of informa-tion are averaged in the MCU to operate the duty precisely. Byprocessing the average duty, is controlled five times persecond.

V. FABRICATION AND EXPERIMENTAL RESULT

Both the one-switch 3R in Section II, and the proposed 3Rin Section III were fabricated in a 0.35 m BCD process. Be-cause the proposed 3R is capable of superior performance thanthe one-switch 3R, most of the figures in this Section V are fromthe proposed three-switch 3R. Fig. 16 shows a chip micrographwith the receiver board. The IC has three switches, M1, M2,and M3; it also has a sync control block (TCON). On the PCB,full-bridge rectifiers consisting of four diodes, capacitors: 20 Fand 10 F capacitors are used for the and , respec-tively, and several sliding switches are soldered; these switchescontrol the four-bit delay and the operation modes. The duty in-formation pins can be connected, or not connected, to the MCU.The experimental setting is shown in Fig. 17 with a transmitterand a transmitting coil at the bottom, and the 3R board and re-ceiving coil on top. The spatial freedom is measured with a gridplate; the WPT works while the receiver coil moves within anarea of 7 2.5 cm .

Fig. 16. IC micrographs of the three-switch 3R and the receiver board.

Fig. 17. Experimental setting and measured spatial freedom of the 3R RWPTsystem.

Fig. 18 shows the measured waveform of andthe rectifier current for both the CCM and DCM withvalues of 2.5 W and 0.5 W, respectively, at 6.78 MHz. In thiscase, a class-E PA operates in the transmitter at a fixed .In the CCM, is switched between 5 V and 10 V, while

is increased to 12.5 V in the DCM with a value of200 pF. The peak envelope from (a) and (b) shows slanted risingand falling waveform. Meanwhile, the small fluctuations in theenvelope waveform are seems to be came from the resonanttanks inherent characteristic.The load regulation characteristic is measured, as shown in

Fig. 19. deviation of 3.3% appeared during a changein the load current from 70 mA to 700 mA. The load changefrom 50 mA to 1.2 A is also experimented, as shown in Fig. 20,with the variable in our transmitter controlled by theMCU using three-bit duty signals, as illustrated in Fig. 13. Atthe points marked by the arrows at which switching occurs inFig. 21, noise with ZCS is eliminated compared tonoise when is disabled, as indicated by the dotted circles.Related to efficiency, from and and

from and in Fig. 13 are measured; is the powerwhich flows from the rectifier output to the IC input, which is thepart of the 3R circuit. Using the measured values, the efficiencyof the individual IC is calculated, and the efficiency of the 3Rreceiver board is estimated while subtracting the power loss dueto the external Schottky diodes, as shown in Fig. 22. Here, theRMS voltage of is higher than . This


Fig. 18. Measured waveform of and in (a) the CCM and (b) the DCM.

Fig. 19. Measured waveform of and with a load transientbetween 70 mA and 700 mA is shown. The duty change is highlighted in theenlarged window.

Fig. 20. The proposed 3R operates while the transmitter is controlled usingduty information. The Measured waveform of and with avariable load between 50 mA and 1.2 A is displayed.

means that a diode drop voltage occupy less portion whenthe voltage of is high, and also means thatis less than . This fact is applied when the loss of diodesis calculated.The level of when dividing the DCM and CCM is set

to 1 W and the efficiency is measured by varying from300 mW to 6 W when MHz and V.

Fig. 21. With or without ZCS, the waveforms of the vary drastically.

Fig. 22. The efficiency plots for the individual IC (red dotted line) and theoverall 3R receiver board (black solid line).


TABLE IICOMPARISON CHART FOR 3R SYSTEMS WITH RECENTLY REPORTED RWPT SYSTEMS

When is around 0 to 60 mW, the efficiency is below 10%.Since the focus of this research is on 5 W-level WPT, consider-ation on small leakage power is not carefully taken into. The ef-ficiency reaches 85% to 86% when 3.5 W W, thenit slightly reduces as is further increased. This is a nat-ural result, because switches size is optimized for 5 W .However, curiously, the efficiency increases whenW. This is because of the unintentional pulse skipping phenom-enon due to the discrete PWM control. It possibly benefits 3Rin extending the maximum , but may worsen regulationcapability.Table II shows the evaluated performance of the 3R, among

recently reported RWPTs fabricated in IC [2][6]. 3R has thelargest and supports a 5 W wireless charger. It can regu-late the output voltage, with and without the help of transmittercircuit.

VI. CONCLUSION

A Resonant Wireless Power Transfer (RWPT) system witha power transfer capability of more than 5 W and an indepen-dently regulating ability is introduced in this paper. For the pro-posed system, receiver circuits, termed 3R, are fabricated in0.35 m BCD technology. Among the two, one that uses threeswitches and a synchronizing technique achieves superior per-formance; the maximum reaches 6 W and the peak effi-ciency of the receiver board is 86%. Impressively, these char-acteristics were used in the frequency of 6.78 MHz. In thisfrequency, the parasitic effects degrade the performance muchmore than when in several hundred kHz, where most of the

IPT systems usually operate. The switching operation with thedistinguished topology of the three-switch 3R solves numerousproblems, including the high voltage from the inrushing, the low due to the break-down of the transistor, and

the low efficiency with a high RMS current despite the sameaverage current. Most importantly, this 3R does not require anextra inductor, as it uses phasor-transformed inductance in theresonant tanks, and does regulate the voltage, using a switchingmechanism.

REFERENCES

[1] Qi Specification, Version 1.1.2, Jun. 2013 [Online]. Available: http://www.wirelesspowerconsortium.com/developers/specification.html

[2] Datasheet WiT-2000 M, 2013 [Online]. Available: http://www.witricity.com/pdfs/WiT-2000M-developers-kit-for-mobile-data-sheet-v14.pdf

[3] J. H. Choi et al., A resonant regulating rectifier (3R) operating at 6.78MHz for a 6 W wireless charger with 86% efficiency, in IEEE ISSCCDig. Tech. Papers, 2013, pp. 6465.

[4] R. Shinoda et al., Voltage-boosting wireless power delivery systemwith fast load tracker by -modulated sub-harmonic resonantswitching, in IEEE ISSCC Dig. Tech. Papers, 2012, pp. 288290.

[5] K. Tomita et al., 1-W 3.316.3-V boosting wireless power transfercircuits with vector summing power controller, IEEE J. Solid-StateCircuits, vol. 47, no. 11, pp. 25762585, Nov. 2012.

[6] H. M. Lee and M. Ghovanloo, An adaptive reconfigurable activevoltage doubler/rectifier for extended-range inductive power transmis-sion, IEEE Trans. Circuits Syst. II, vol. 59, no. 8, pp. 481485, Aug.2012.

[7] G. B. Joung et al., Integral cycle mode control of the series resonantconverter, IEEE Trans. Power Electr., vol. 4, no. 1, pp. 8391, Jan.1989.

[8] C. T. Rim and G. H. Cho, Phasor transformation and its application tothe DC/AC analyses of frequency phase-controlled series resonant con-verters (SRC), IEEE Trans. Power Electr., vol. 5, no. 2, pp. 201211,Apr. 1990.


Jun-Han Choi (S13) received the B.S. degree fromYonsei University, Seoul, Korea, in 2006, and theM.S. degree from the Korea Advanced Institute ofScience and Technology (KAIST), Daejeon, Korea,in 2011, both in electrical engineering. Since 2011,he has been working toward THE Ph.D. degree inelectrical engineering at KAIST, Daejeon.From 2006 to 2008, he worked in the research

center of Samsung Thales, Gyeonggi-do, Korea,where he is working on the development of IR cam-eras. His research interests are in the field of analog

circuit designs for power electronics and application processor applications,including IC-based wireless power transfer systems, envelope modulators forenvelop tracking power amplifiers, very high frequency switch-mode buckconverters, multiphase buck converters and voltage-tolerant power converters.

Sung-Ku Yeo received the B.S., M.S., andPh.D. degree in electrical engineering from theKorea Advanced Institute of Science and Tech-nology (KAIST), Korea, in 2003, 2005, and 2009,respectively.In 2009, he joined Samsung Electronics, Suwon,

Korea, where his research interests are wirelesspower transfer system.

Seho Park received the B.S., M.S., and Ph.D. de-grees in photonics from the Materials Science De-partment, Pohang University of Science and Tech-nology, Korea, in 1995, 1997, and 2002, respectively.In 2001, he joined Samsung Electronics Co. Ltd.,

Suwon, Korea, and was involved in the opticalfiber amplifier, Gbit multimode fiber and wide-bandNZDSF project in photonics business. Since 2007, hehas lead the chip embedded PCB, nano-ink patternedflexible PCB and the wireless power transfer projectin DMC R&D center and mobile communications

business. In 2011, he became a Principal Engineer and was honored with theSamsung Electronics annual award of the best patent. He holds more than 43patents in the wireless power transfer technology.

Jeong-Seok Lee (M13) received the M.S. and Ph.D.degrees in solid-state physics from the Department ofPhysics, Seoul National University, Seoul, Korea, in1993 and 1997, respectively.In 1997, he joined Samsung Advanced Institute

Technology. In 2002, he has researched many topicsrelated to wireless devices in Samsung ElectronicsCo. Ltd., Suwon, Korea, for example, haptics, opticalwireless pen, wireless charger systems, and more.

Gyu-Hyeong Cho (S76M80SM11) receivedthe B.S. degree from Hanyang University, Korea,and the M.S. and Ph.D. degrees, all in electricalengineering, from the Korea Advanced Institute ofScience and Technology (KAIST), Daejeon, Korea,in 1975, 1977, and 1981, respectively.During 19821983, he was with the Westinghouse

R&D Center in Pittsburgh, PA, USA. In 1984, hejoined the Department of Electrical Engineering,KAIST, where he has been a full Professor since1991. His early research was in the area of power

electronics until the late 1990s and worked on soft switching converters andhigh power converters. Later, he shifted to analog integrated circuit design,and now he is interested in several areas including power management ICs,Class-D amplifiers, touch sensors and drivers for AMOLED and LCD flat paneldisplays, biosensors and wireless power transfer systems. He has authored onebook on advanced electronic circuits and authored or coauthored over 200technical papers and 80 patents.Dr. Cho received the Outstanding Teaching Award from KAIST. He served

as a member of the ISSCC international technical program committee, and isnow an associate editor of IEEE JOURNAL OF SOLID-STATE CIRCUITS. At theISSCC 60th Anniversary in 2013, he received the ISSCC Author RecognitionAward as one of the top 16 contributors of the conference.

ieee journal of solid-state circuits volume 48 issue 12 2013 [doi 10.1109%2fjssc.2013.2287592] choi,...

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