Energies-28497_Wang ZhiHui ChongQing Unv

8/13/2019 Energies-28497_Wang ZhiHui ChongQing Unv

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Energies 2013 , 6 , 1-x manuscripts; doi:10.3390/en60x000x12

energies3ISSN 1996-10734

www.mdpi.com/journal/energies5 Article6

A Common Inductive Power Transfer Platform for Kitchen7Appliances with Different Power Capacity8

Zhi-hui Wang *, Yu-peng Li, Yue Sun, Chun-sen Tang and Xin Dai9

Room 1903, Main Teaching Building, District A, Automation College, Chongqing University,10Chongqing, 400044, China; E-Mails: [email protected](Y.P.L.);11

[email protected](Y.S.); [email protected](C.S.T.); [email protected](X. D.)12

* Author to whom correspondence should be addressed; E-mail: [email protected];13Tel.: +86-23-65112750; Fax: +86-23-65112750.14

Received: / Accepted: / Published:1516

Abstract: This paper aims at studying a high-efficiency and high-stability inductive power17transfer (IPT) common energy launch platform for kitchen appliances with different power18capacity. At first, the fundamental structure and basic circuit topology of IPT system for19kitchen appliances are introduced. Most of all, in order to make the designed IPT system20achieve the target efficiency under all different power levels, main losses of the selected21IPT system have been analyzed, and then, an efficiency calculation model is presented,22with the aid of such model and the designed online capacitor array, the control strategy of23the efficiency optimization as well as the segmented optimal frequency dynamic tracking24method has been proposed. Finally, experimental results show total efficiency of the IPT25system for kitchen appliances using the proposed control strategy is increased significantly26

under all different power capacity loads. Meanwhile, the reliability and stability27arent declined.28

Keywords: capacitor array; dynamic frequency; efficiency optimization; kitchen29appliances; inductive power transfer (IPT) 30

31

1. Introduction32

The convenience and safety of power supply for kitchen appliances has been widely concerned. The33traditional power supply way of kitchen appliances may easily result in electrical safety issues such as34loose-contact, electric-spark and short-circuit. Therefore, a more safe, convenient and reliable35

OPEN ACCESS


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electricity access solution is needed urgently in the field of kitchen appliances. Fortunately, a wireless36 power transmission solution based on the inductive power transfer technology has emerged as the37times require.38

Inductive power transfer (IPT) technology is a practical and flexible technology for delivering39 power efficiently from a stationary power supply to one or more movable loads. Such safe and reliable40technology overcomes disadvantages of the traditional power transfer methods and is developed41rapidly nowadays [1]-[3], moreover, it has been widely used where the electrical isolation is essential42for power supplies [4]-[7]. But in the field of kitchen appliances, there have been only few application43

products so far.44Kitchen appliances are known for their variety of types and differences in the power capacity and45

character of loads. Therefore, the stability of the energy transfer magnetic field and power transfer46capability, the constant current in primary winding and the constant output voltage across loads47

generally must be required in the common energy launch platform of IPT system [8]. Meanwhile, it is48 very important to analyze and optimize the efficiency under all different power capacity kitchen49appliances. In this way, the efficiency can reach to the target value according to the requirements of50consumers.51

A transcutaneous energy transfer (TET) system has been researched in [9]. The quantitative analysis52and comparison of parameter affecting efficiency have been discussed from the aspects of conduction53losses, switching losses and core losses. But there isnt a detailed analysis about how to optimize the54

parameters of IPT system so as to make the efficiency reach to the target value.55There is a comprehensive analysis about four typical resonant network topologies in [10]. A general56

approach to optimize power and efficiency has been given. But it doesnt consider the power and57efficiency optimization method when the IPT system is operated under different power capacity load.58

In this paper, focusing on the high-efficiency and high-stability common energy launch platform of59the IPT system for kitchen appliances with different power capacity, the circuit structure characteristic60and main losses of IPT system have been analyzed, with the help of the efficiency calculation model61and designed capacitance array, the control strategy of efficiency optimization (the IPT system is62allowed to operate at different segmented optimal frequency modes according to the power level of63loads) has been proposed. Finally, a rated power of 1000W experimental IPT system similar to the IPT64system for kitchen appliances has been set up in order to verify the designed efficiency optimization65

control strategy.66

2. Fundamental Analysis67

2.1. Fundamental Structure68

Fig.1 illustrates the fundamental schematic structure of an IPT system that is designed for kitchen69appliances with different power capacity. This system essentially is comprised of one stationary70

primary common energy launch platform and several movable secondary pickups with different power71capacity. At any given time, only one pickup is allowed to operate on this common energy launch72

platform. Kitchen appliances can work properly as long as their pickup winding is within the effective73area of the energy launch platforms energy transmitting winding.74

75


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Fig. 1. Schematic diagram of IPT system designed for kitchen appliances76

7778

As for most of kitchen appliances such as toaster, electric cooker, electric kettle and coffee maker,79their energy consumption is mainly divided into two parts: DC control/driver circuits power supply80and resistor-wire heating. In order to obtain the DC control/driver circuits power supply, a rectifier, a81filter and a switched-mode controller are usually used. However, because the AC high-frequency82voltage with wide range (usually 180V-250V) can be directly added across the resistor-wire in kitchen83appliances, it is not necessary to have any rectifier, filter, voltage regulator or switched-mode84controller in the pure resistor-wire heating part. That is, the high-frequency AC voltage produced from85the secondary resonant network can be directly added across the resistor-wire, as shown in Fig.1.86

2.2. Basic Circuit Topology87

Typologically, IPT systems are either voltage-fed or current-fed. Current-fed IPT systems can be88further classified into PS and PP types based on the compensation style of the primary and secondary89windings [11]-[12]. The present paper focuses on the full-bridge inverter circuit of the PS-type90voltage-fed IPT system and its schematic diagram is shown in Fig.2.91

Fig. 2. Full-bridge inverter circuit of an IPT system designed for kitchen appliances92

1S

2S

3S

4S

dc L

dc E

dci

dc R

p L s L

M

pu

pC

p R s R

pV p I

sC L R

R

C s I

9394

In the primary side of this IPT system, a quasi-current source can be constituted with a DC95voltage source E dc and a DC inductor Ldc. The current source idc flowing in the DC inductor can be96inverted and injected into the resonant tank without creating a conflict with the current between the97resonant inductor L p and compensating capacitor C p. The inverter network comprises four IGBT98


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switches from S 1 to S 4. Two switch pairs (S 1, S 4) and (S 2, S3) operate in the complementary mode to99 produce a square wave current output. The current I p and voltage u p are the current flowing through the100inductor L p and the voltage across the capacitor C p respectively; the resistances Rdc and R p are the101inherent resistances of inductors Ldc and L p. V p is the RMS output voltage of inverter network.102

In the secondary side, the energy pickup winding Ls receives energy from the primary side and103 produces resonant in the secondary pickup network comprising the resonance inductor Ls and104compensating capacitor C s. The current I s is the current flowing through the inductor Ls; Rs is the105inherent resistance of pickup inductor Ls.106

As shown before, the energy in the secondary side mainly flows into two parts: control/driver107circuits power supply and resistor-wire heating. In the control/driver circuits power supply part, a108rectifier, a filter and a switched-mode controller are usually used to obtain the constant DC power109supply. Nevertheless, the high-frequency AC voltage generated from the secondary resonant network110

can be directly added across the resistor-wire R L without any power conversion circuits.111

2.3. Equivalent Model of Secondary Side112

The filter capacitance C may be extremely small due to the low operating voltage and output power113(usually 100 V )114across the resistor-wire R L and its power (usually >50 W ) are generally much larger than the voltage115and power of control/driver circuits. Therefore, only for the application of kitchen appliances, the full116

bridge rectifier paralleled with a small filter capacitor as shown in Fig.2 can be ignored. Then the117circuit in the dashed line frame in Fig.2 is equivalent to a pure resistance Req that can be approximately118expressed as119

L R R (1)120

2.4. Mutual Inductance121

For the proposed IPT system for different power capacity kitchen appliances described in Fig.1,122However, once system parameters and circuit structure design are complete and if the system is123operated at the rated condition, the distance and relative position between the primary and secondary124windings are fixed, that is to say, the mutual inductance M is a constant value. Therefore, in the125

following discussion of IPT system for different power capacity kitchen appliances, the influence of126mutual inductance M is not taken into consideration, the load impedance R is the unique disturbance.127

2.5. Operational Frequency128

As shown in Fig.2, the reflecting impedance Z r of the secondary resonant tank in the primary129resonant network can be written as130

2 2

Re Im1

r r r

ss

M Z Z j Z

j L R j C

(2)131

Where Re( Z r ) and Im( Z r ) are the real and imaginary components of reflecting impedance132respectively. Then, the total impedance of primary resonant network is derived as133


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1 11t t

p p p r

Z Y j C

R j L Z

(3)134

Where Y t is the total admittance of primary resonant network and can be expressed as135

2 2 2 2

Re Im

Re Im Re Im

Re Im

p r p r t p

p r p r p r p r

t t

R Z L Z Y j C

R Z L Z R Z L Z

Y j Y

(4)136

Where Re( Y t ) and Im( Y t ) are the real and imaginary components of total admittance respectively. So137that, the zero-phase-angle (ZPA) frequency of load impedance can be derived from equation138

2 2Im

Im 0Re Im

p r t p

p r p r

L Z Y C

R Z L Z

(5)139

Such ZPA frequency is generally assumed to be the operating frequency of IPT system [11], [13]-140[15]. Hence, in the following discussion, these two different frequencies are considered to be141equivalent. Meanwhile, ZPA operation can be achieved by controlling the inverter current to follow142the voltage across the parallel compensating primary winding.143

2.6.Power Transfer Capability144

There is a parallel compensating primary and a series compensating secondary in the IPT system145shown in Fig.2. In consideration of reflecting impedance Z r , we can get the equivalent circuit model of146

primary and secondary resonant tank with reflecting impedance as described in Fig.3.147

148Fig.3 Equivalent circuit model of primary and secondary resonant tank with reflecting impedance149

pC p L p R r Z

s L sC

s R R

150151

For the parallel compensating primary shown in Fig.3, according to the Kirchhoffs voltage law152(KVL), the primary resonant current I p flowing through the primary inductor L p can be obtained as153

2 2Re Im p p

p p p r

p r p r

V V I R j L Z R Z L Z

(6)154

Where is the operating (ZPA) frequency calculated using (5). Again, for the series compensating155secondary shown in Fig.3, according to the KVL, the load voltage across the resistor R can be derived156as157

0

2 211 ( )

p p L

s ss sss

j MI R MI RU I R

R R L j L R RC j C

(7)158

The primary resonant current I p

can be calculated by (6). As a result, the power transfer capability of159such IPT system can be expressed as160


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2 2 22

02 21( )

p L

s ss

M I RP I R

R R LC

(8)161

2.7. Characteristic162

As expressed in equation (5), (6), (7) and (8), the operating frequency, primary resonant current,163load output voltage and power transfer capability at the steady state can be calculated accurately using164numerical solutions. Then, curves of the operating frequency, primary resonant current, and load165output voltage and power transfer capability varying with loads are illustrated in Fig.4.166

1) Operating Frequency : As shown in Fig.4 (a), it is noted that the inconstant frequency area will167appear under some loads due to the frequency bifurcation phenomena and multiple soft-switching168operating points. Fortunately, a constant frequency area also exists, and from equation (5), the constant169

ZPA frequency can be derived approximately as170 1

p p L C (9)171

2) Primary Resonant Current : As shown in Fig.4 (b), due to the constant frequency area, the172constant primary resonant current area also appears, and the constant primary resonant current value173can be expressed approximately as174

22 2 p p p

p p p p

p p p p

V V C I V

L L R L RC

(10)175

3) Load Output Voltage : As shown in Fig.4 (c), a constant output voltage area will also emerge as176the appearance of the constant frequency and primary resonant current area. If a condition L pC p= LsC s 177can be met, the constant output voltage value can be simplified approximately as178

0 2 2

p p p

s p p p p p

MI R MV M U V

R R L L C R L

(11)179

4) Power Transfer Capability : it can be seen clearly from Fig.4 (d) that a power peak exists, this is180the maximum power transfer capability of designed IPT system. Approximately, before such power181

peak, the IPT system would be extremely unstable due to the inconstant operating frequency, primary182

resonant current and output voltage. On the contrary, after this peak, because of the constant183 frequency, current and voltage area (Fig.4 (a), (b), (c)), the IPT system would be greatly stable.184However, for the constant output voltage, the output power will decrease as increasing the impedance185of load.186

Fig. 4 curves of operating frequency, primary resonant current, output voltage and power187transfer capability varying with loads (in Fig.b, c and d, if the multiple soft-switching188operating points appear, a low operating frequency is only considered).189


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0 50 100 150 200

18

20

22

24

0 50 100 150 20018

19

20

21

22

23

24

25

P r i m a r y r e s o n a n t c u r r e n

t I p

A

190(a) Operating frequency (b) Primary resonant current191

0 50 100 150 2000

50

100

150

200

250

0 50 100 150 2000

400

800

1200

1600

192(c) Output voltage (d) Power transfer capability193

194As shown in Fig.4, there is a constant frequency area, a constant primary area and a constant voltage195

area when the system parameters are designed properly in the IPT system shown in Fig.2 for kitchen196appliances. Loads (kitchen appliances) of such IPT system should be designed to operate within the197constant frequency, primary resonant current and output voltage area as soon as possible. Such design198method can realize the operating frequency, primary resonant current and load output voltage constant199without any traditional switching and closed-loop control circuit. Hence, it reduces the circuit200complexity and electromagnetic interference (EMI) and realizes the low-cost and high-quality wireless201

power supply for different power capacity kitchen appliances.202

3. Efficiency Optimization203

3.1. Losses Analysis204

The loss components of selected IPT system for kitchen appliances shown in Fig.2 can be classified205into the following main categories:206

1) Conduction Losses . Copper losses of primary and secondary windings and losses of reverse207 parallel diodes during conduction.208

2) Switching Losses . Turn-on and turn-off losses of IGBTs and losses of reverse parallel diodes in209the primary inverter network.210

3) Radiation Losses .211


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The switching losses of the primary inverter network are very small under the ZPA condition and212the radiation loss is usually negligible because of the low operating frequency [9]. It is obvious that the213conduction losses in the primary and secondary windings contribute to the largest proportion of the214total losses of whole IPT system shown in Fig.2 [10]. In this paper, with the help of conclusions in [9]215and [10], the copper losses in the primary and secondary windings are only discussed. Hence, in the216

process of efficiency analysis, the following assumptions should be made [16], [17]:2171) The devices in the primary inverter network are ideal devices.2182) The selected IPT system is designed to be operated at the ZPA condition.219

3.2. Efficiency Calculation Model220

According to the characteristics of selected IPT system and losses analysis shown above, the energy221consumption is mainly divided into three parts: losses of primary and secondary windings and the222

output load power. The efficiency of the designed IPT system can be expressed as223

0

0 1 2

P

P P P

(12)224

Where is the efficiency of system, P 0, P 1 and P 2 are the load power, the copper losses of primary225and secondary windings respectively and they are given by226

2 2 22

0 2

p L

s

M I RP I R

R R

(13)227

21 p pP I R (14)228

2 2 22

2 2

p ss s

s

M I RP I R

R R

(15)229

Substituting (13), (14), (15) into (12), the efficiency of the designed IPT system can be rewritten as230

2 2

22 2s p s

M R

M R R R R R

(16)231

According to design guidelines of IPT system proposed in [10], generally, in the PS-type IPT232system, we have R>> Rs, therefore, the equation (16) can be further simplified as233

2 2

1

1 p RR

M

(17)234

As shown in equation (17), it is effective to increase the efficiency of IPT system shown in Fig.2 by235improving the mutual inductance M and reducing the inherent resistance of primary energy236transmitting winding. It is necessary to improve the mutual inductance and reduce the inherent237resistance of primary winding appropriately when the parameters of IPT system are being designed.238However, once the process of system parameter design is complete, these two parameters are fixed.239Therefore, the efficiency is only related to the load and operating frequency.240

Fig. 5 The three-dimensional map between the operating frequency, load and efficiency241


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242243

According to equation (17), the three-dimensional map between the operating frequency, load and244

efficiency can be shown in Fig.5, it can also be seen from the figure that when the efficiency value is245not in the saturation state, it can be increased by improving the operating frequency under the same246load R. While the lighter load, the lower efficiency under the same operating frequency.247

3.3. Efficiency Optimization Strategy248

It is indicated that the efficiency optimization presented in this study is not to make the efficiency249reach a maximum value, but achieve a target value. Once the efficiency of selected IPT system has250achieved the target value, the efficiency optimization design process ends correspondingly.251

If the required efficiency must be higher than the target value , so that2520 (18)253

Substituting (18) to (17), the operating frequency must be satisfied as254

0

20

12 1

p R f R M

(19)255

As shown in (19), the IPT system with a larger load impedance R (a lighter load) is required to256operate at the higher frequency to achieve the target efficiency. The efficiency of IPT system with a257smaller load impedance R (a heavier load) can also be increased by improving the operating frequency258when the efficiency value is not in the saturation state. However, the narrower constant operating259

frequency, primary resonant current and output voltage area of IPT system will appear with the higher260operating frequency, as shown in Fig.6.261

Taking load R=50 for example, when the inherent resonant frequency of IPT system is allowed to262increase to f 1, the practical operating frequency becomes greatly unstable. On the contrary, the263

practical operating frequency remains approximately constant if the inherent resonant frequency is f 5.264Meanwhile, the primary resonant current and output voltage at the inherent frequency f 1 are lower and265more unstable than f 5. Therefore, the power transfer capability of IPT system is minimized at the266inherent frequency f 1.267

Hence, if the inherent frequency of IPT system with a smaller load is increased, the unstable268operating frequency, primary resonant current, output voltage area appears with the disturbance of269load, moreover, the stability and power transfer capability of IPT system would be decreased.270


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Fig. 6 curves of normalized operating frequency, primary resonant current, output voltage271and power transfer capability varying with load under different inherent frequency.272( f 1>f 2>f 3>f 4>f 5, f i (i=1-5) is the inherent frequency, under different inherent frequency, all273resonant parameters of IPT system are consistent apart from primary and secondary274compensating capacitor C p,C s and a necessary criteria L pC p= LsC s must be established.275Meanwhile, the operating frequency of IPT system can be changed with the change of the276inherent frequency. Meanwhile, to facilitate mapping and analyzing, the practical operating277frequencies f in Fig.6(a) are normalized using the inherent resonant frequency f i as u= f / fi278(i=1-5)).279

0 50 100 150 2000.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

f 5 f 4 f 3 f 2 f 1

Load R (

N o r m a l

i z e d

f r e q u e n c y u

f / f i

0 50 100 150 200

8

12

16

20

24

f 5 f 4 f 3 f 2 f 1

280(a) Operating frequency (b) Primary resonant current281

0 50 100 150 2000

50

100

150

200

250

f 5 f 4 f 3 f 2 f 1 O

u t p u

t v o l

t a g e

U 0

V

0 50 100 150 2000

400

800

1200

1600

2000

f 5 f 4 f 3 f 2 f 1

P o w e r

t r a n s f e r

P 0

W

282(c) Output voltage (d) Power transfer capability283

284Though analyzing about characteristic, losses and efficiency in the IPT system for kitchen285

appliances, a segmented optimal frequency dynamic tracking method can be proposed for optimizing286target efficiency using changing the inherent resonant frequency, accordingly, the operating frequency287is also changed. Consequently, the power transfer system is allowed to operate at different frequency288modes dynamically according to the power level of loads, and the lighter load requires a higher289frequency than the heavier load. Moreover, the designed IPT system must be within the constant290frequency and voltage area shown in Fig.4 under different frequency modes.291

The phased-control inductor [18] and the switching-capacitor [19] have been used generally to292change the inherent resonant frequency, but it is difficult to wind and control the phased-control293


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inductor, so an online capacitor array different from switching-capacitor shown in [19] is constructed294to change the inherent resonant frequency in section IV. In this way, the IPT system with a lighter load295is allowed to operate at a higher frequency mode than a heavier load, the target efficiency of IPT296system for kitchen appliances shown in Fig.2 can be achieved effectively.297

298

4. Design Implement of Efficiency Optimization299

4.1. Capacitor Array in the Primary Side300

The online capacitor array can be constructed to change the inherent resonant frequency, it is shown301in Fig.7 and paralleled with the primary resonant inductor of the common energy launch platform, the302capacitance in the primary resonance tank can be changed real-timely by controlling switches S k303

(k=1~m ) to be on and off. And these switches are composed of two reverse series semiconductor304devices such as IGBTs or MOSFETs.305

306Fig.7 The working diagram of capacitor array in the primary side307

1C k C

k S

mC

mS

a

b

1S

308309

According to different combinations of the additional capacitor array shown in Fig.7, the optional310 primary compensating capacitance fuzzy set can be given by311

1 1 1 1, , , , 1, 2i i i i m p p p p p pC C C C C C i (20)312If the required target efficiency of designed IPT system for kitchen appliances shown in Fig.2 must313

be higher than the given value , making use of equation (19), the optimization primary314compensating capacitance must be satisfied as315

2

0

11 p p p

M C L R R

(21)316

The efficiency optimization control subsystem real-timely selects capacitance in the capacitance317fuzzy set described in equation (20) according to inequality (21).318

4.2. Fore-design of Secondary Compensating Capacitor319

In practice, as discussed in Section II- A, before kitchen appliances receive from the power320transmitting part at the rated condition, different from the real-time and online choice of primary321compensating capacitor, appropriate secondary compensating capacitors have been fore-designed and322

installed inside the kitchen appliances permanently according to the power capacity of load, and the323capacitance must be satisfied as324


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2

0

11 p ps

s s p

L C M C

L L R R

(22)325

4.3. Identification326

As shown above, the power capacity of load should be identified to select the primary capacitance.327According to the energy relation of selected IPT system shown in Fig.2, the following equation exists328in the form of329

2 2 2 p

dc pdc

I M i R

E R

(23)330

As shown in (23), the injection current idc includes information of the load condition R. With the331identification result of injection current idc, we can identify the load parameter. The control subsystem332of IPT system needs the real time sampled data about injection current idc. Similar to the primary333

compensating capacitance fuzzy set, the injection current identification fuzzy set can be written as334 1 1 1, , , , 1, 2 1 j n j j mdc dc dc dc dc dci i i i i i j (24)335

4.4. Start-up and Working Frequency336

As shown in Fig.6, the constant operating frequency and output voltage area becomes narrower at337the higher inherent frequency, meanwhile, the power transfer capability is larger at the lower inherent338frequency, and it is easy to control at the lower operating frequency. Therefore, a low frequency is339selected as the start-up frequency of designed IPT system for kitchen appliances.340

Fixed and variable frequency controls are two main control strategies for IPT system [20]. For fixed341frequency control, the frequency is forced with the driving switches at the predetermined value so that342it does not vary with the load and circuit parameters. In the case of variable frequency control, some343extent of frequency variation is allowed to follow the circuit parameters. Moreover, the cost, size of the344IPT system for kitchen appliances may be reduced. Thanks to the constant operating frequency area,345the power capability cannot be reduced. Therefore, the variable frequency control is more feasible and346favorable than the fixed frequency control in the proposed control strategy.347

4.5. Control348

The control design of IPT systems relies strongly on experience and experimental verification349 because of the complexity regarding the interactions of the primary and secondary resonant circuits.350Combing (20) with (24), the control strategy of efficiency optimization can be obtained as351

1

1 1

11 1, 2n n

i j j

p dc dc

m p p dc dc dc

p dc dc

C i i

C C i i i i j

C i i

(25)352

Based on the control strategy shown in equation (25), the control block diagram of variable353frequency control and efficiency optimization control subsystem can be designed in Fig.8.354

355


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Fig. 8 The control block diagram of variable frequency control and efficiency optimization356control subsystem357

dc E dcidc L

pu

358359

Normally, the variable frequency control subsystem needs the real time sampled data about the360

resonant voltage ( u p) across the primary compensating capacitor, such voltage can be divided by a361voltage dividing resistor network. Then, the divided voltage signal should be zero crossing sampled by362the LM311 chip. The FPGA chip (EP2C5T144C8N) sends switching gate signal out according to the363zero crossing sampled signal. The inverter network composed of four IGBTs(FGA25N120ANTD) is364essential to drive the resonant tank and generate the required high-frequency primary current.365

In principle, the efficiency optimization control subsystem acquires the information of injection366current ( idc) pouring into the inverter network with the help of sampling resistor and AD574 chip, then367the sampled digital signal is sent into the FPGA chip, in this chip, the current identification, control368strategy implement of efficiency optimization and capacitance choosing are executed.369

4.6. Operating Procedure370

As described above, in summary, there must be a corresponding primary compensating capacitor C p 371and a secondary compensating capacitor C s for loads of different power capacity when the efficiency372achieves a target value, and L pC p= LsC s. The whole circuit of secondary pickup side including the373secondary compensating capacitor is installed within the electrical equipment (kitchen appliances).374

When the electrical equipment receives energy from energy transmitting winding in the common375energy launch platform normally, the power transfer system starts working at the low inherent376

frequency mode at first. Then, through detecting the injection current, the power capacity of this377equipment is identified. Finally, the corresponding value of inherent frequency mode together with378

primary compensating capacitor is regulated by controlling switches S k .379

5. Experimental Study380

5.1. Experimental Circuit and Parameters381

In order to verify the control strategy of efficiency optimization and realize the high-efficiency and382high-stability wireless power supply for all different power capacity kitchen appliances, a contactless383

power transfer system for kitchen appliances capable of providing power of 1000W has been prepared.384The main parameters of such system are shown in Table I, and the primary side as well as the common385energy launch platform is same with the IPT system shown in Fig.2, but two circuit models prepared386


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for light load R1 and heavy load R2 respectively are applied at the secondary side, as shown in Fig.9.387With the aid of these two loads ( R1 and R2), two kitchen appliances of different power capacity can be388imitated. Meanwhile, different compensating capacitors C s are fore-selected so that the IPT system can389

be operated at the secondary inherent resonant frequency as soon as possible.390

Fig. 9 Circuit model of fore-designed secondary pickup side in the experimental system.391(C si ={C s1 ,C s2}, Ri={ R1, R2})392

s L isC

i R

393394

Table. I Main Parameters of Experimental System395Parameter Values Parameter Values

E dc(V) 310 M( H) 77.51 Ldc(mH) 4.01 Rdc( ) 0.04

C 1( F) 0.30 C 2( F) 0.22 LP( H) 117 R p( ) 0.05 Ls( H) 550 Rs( ) 0.54C S 1( F) 0.11 C S 2( F) 0.063 R1( ) 500 R2( ) 100P 1(W) 100 P 2(W) 500

5.2. Without Efficiency Optimization Control396

The first experiment has been allowed to operate at the lower inherent frequency mode (about 20397kHz) without the optimization control strategy. Therefore, compensating capacitance of two secondary398

pickup circuits can be fore-designed as C s1=C s2= 0.11 F , and the compensating capacitance in the399 primary resonant tank can be selected as C p= 0.52 F . Consequently, steady-state waveforms without400the control strategy under both light load R1 and heavy load R2 are shown in Fig.10.401

Fig. 10 Steady-state waveforms without control strategy under both light load and heavy402 load (Ch1-injection current idc, Ch2-primary resonant current I p, Ch3-output voltage U o)403

404

i d c ( 1 A / d i v ) , I p

( 2 0 A / d i v ) , U

o ( 5 0 V / d i v )

t (50us/div)

Primary resonant current I p

Output voltage U o

Injection current idc

i d c ( 1 A / d i v ) , I p

( 2 0 A / d i v ) , U

o ( 5 0 V / d i v )

t (50us/div)


Output voltage U o



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(a) Light load R1 (b) Heavy load R2 405406

The efficiency of designed IPT system for kitchen appliances is 70.7% at the operating frequency407of 20.6 kHz (lower frequency) with an injection current idc of 0.47 A and output voltage U o of 227 V 408under light load R1. Meanwhile, the efficiency of system is 90.0% with an injection current idc of 1.75 A 409and output voltage U o of 221 V under heavy load R2, while the operating frequency under heavy load R2 410is same with the operating frequency under light load R1. It can be clearly seen that the efficiency of411IPT system for kitchen appliances under light load R1 is lower relatively. Therefore, it is necessary to412optimize the efficiency of IPT system for kitchen appliances under light load R1.413

5.3.With Efficiency Optimization Control414

Anyway, it can be assumed that the target efficiency of IPT system for kitchen appliances is415

80%. Combining (20) with (21), the capacitance array fuzzy set in efficiency optimization control416subsystem can be given by417

1 2, 0.3 ,0.52 p p pC C C F F 418At the same time, according to (22), compensating capacitances of two secondary pickup circuits419

can be fore-designed as C s1= 0.063 F for the light load R1 and C s2= 0.11 F for the heavy load R2 420respectively. Meanwhile, relying on the experience and experimental verification, the injection current421identification fuzzy set can be selected as422

1 1dc dci i A 423Hence, the control strategy of efficiency optimization can also be designed as424

2 1

1 1

p dc dc

p p dc dc

C i iC

C i i 425

Fig. 11 Waveforms with efficiency optimization control strategy under the light load (Ch1-426injection current idc, Ch2-primary resonant current I p, Ch3-output voltage U o)427

428(a) Operating procedure waveforms (b) Steady-state waveforms429

430

Operating procedure waveforms of IPT system with efficiency optimization control algorithm are431illustrated in Fig.11 (a) under light load R1. First of all, the IPT system starts working at the lower432inherent frequency mode f 1= 20.6 kHz at time t=t o, because the practical identification current idc is less433

I p ( 2 0 A / d i v ) , U

o ( 1 0 0 V / d i v )

t (100ms/div)

Output voltage U o

Primary resonantcurrent I p

1 20.6

22.3 p

f kHz

I A

2 26.7

17.1 p

f kHz

I A

Frequencyswitched point

0t 1t i d c

( 1 A / d i v ) , I

p ( 2 0 A / d i v ) , U

o ( 5 0 V / d i v )

t (50us/div)


Output voltage U o



16/19


than 1 A under light load R1, then, at time t=t 1, the primary compensating capacitance is reduced from434C p=C p2= 0.52 F to C p=C p1= 0.3 F through closing the switch S 2, the IPT system under light load R1 is435allowed to operate at the higher inherent frequency mode f 2= 26.7 kHz. In this case, the efficiency of436system is 82.6 % at the operating resonant frequency of 26.7 kHz with an injection current idc of 0.406 A 437and output voltage U o of 228 V , steady-state waveforms are shown in Fig.11(b). It is obvious that the438efficiency of IPT system has been increased by 12 % and achieved the target efficiency under light load439

R1. The results shown above have verified the effectiveness of proposed control method.440However, according to the control strategy, steady-state waveforms with efficiency optimization441

are same with steady-state waveforms without efficiency optimization control strategy under heavy442load R2 shown in Fig.10(b), and they are omitted.443

5.4. Further Discussions444

Furthermore, a new experiment used efficiency optimization control strategy has been done. This445time, the assumed target efficiency is 85%. Fig.12 (a) shows the curves between experimental446measured efficiency and loads under different inherent frequency modes ( f 1


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3 f

2 f

1 f

0 100 200 300 400 500 60070

75

80

85

90

95

100

R

%

50 R

A B

1 R 2 R

0

466(a) Efficiency varying with load (b) Practical frequency varying with load467

468

469(c) Power transfer capability varying with load470

471

6. Conclusion472

To realize the high-efficiency and high-stability wireless power supply for all different capacity473kitchen appliances, in this paper, the structure characteristic, main losses and efficiency calculation474model of designed IPT system have been analyzed. With the aid of such efficiency model and designed475

online capacitance array, the control strategy of efficiency optimization has been proposed. Also, in476order to verify the designed efficiency optimization control strategy, a rated power of 1000W477experimental system similar to the contactless power transfer system for kitchen appliances has been478set up. Experimental results indicate that the total efficiency is increased obviously when the system479under a light load is allowed to operate at a higher inherent frequency mode, but this rule is not480suitable for the heavy load.481

Acknowledgment482

This research work is financially supported by The Research Fund for the Doctoral Program of High483 er Education (No.20100191120024) and China Postdoctoral Science Foundation (No. 20110490799).484485

0 100 200 300 400 500 600

0.95

0.96

0.97

0.98

0.99

1.00

1.01

3 f

2 f

1 f

R Load

Normalized frequency

50 R

0 100 200 300 400 500 6000

200

400

600

800

1000

3 f

2 f 1 f

R Load

0P W Power Transfer

50 R


18/19


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