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Delft University of Technology

Advantages and Tuning of Zero Voltage Switching in a Wireless Power Transfer System

Grazian, Francesca; Van Duijsen, Peter; Soeiro, Thiago B.; Bauer, Pavol DOI

10.1109/WoW45936.2019.9030626 Publication date

2019

Published in

2019 IEEE PELS Workshop on Emerging Technologies

Citation (APA)

Grazian, F., Van Duijsen, P., Soeiro, T. B., & Bauer, P. (2019). Advantages and Tuning of Zero Voltage Switching in a Wireless Power Transfer System. In 2019 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer, WoW 2019 (pp. 367-372). [9030626] (2019 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer, WoW 2019). Institute of Electrical and Electronics Engineers (IEEE). https://doi.org/10.1109/WoW45936.2019.9030626

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Advantages and Tuning of Zero Voltage Switching

in a Wireless Power Transfer System

Francesca Grazian, Peter van Duijsen, Thiago B. Soeiro, Pavol Bauer

Delft University of Technology The Netherlands

Email: (F.Grazian, P.J.vanDuijsen, T.BatistaSoeiro, P.Bauer)@tudelft.nl

Abstract—In charging applications, wireless power transfer (WPT) is mostly used in the form of inductive power transfer with magnetic resonant coupling. Therefore, both the transmitter and the receiver coils are combined with capacitors, such that only active power is transferred. To evaluate the operation of the WPT charging system, its equivalent circuit can be analyzed in the frequency domain. However, this is limiting since the H-bridge inverter operation is not intrinsically considered. As an example, the operating points of both zero current switching (ZCS) and zero voltage switching (ZVS) operations might be still analyzed, but it is not possible to assess their performance in terms of efficiency. In this paper, the advantage of ZVS over the ZCS is evaluated in terms of the efficiency and the delivered output power. To enable the full potential of ZVS, this is tuned considering the switch capacitance and the dead time.

Index Terms—Efficiency, Inverter control, wireless power transfer (WPT), zero voltage switching (ZVS), zero current switching (ZCS)

I. INTRODUCTION

According to [1], radio-frequency wireless technologies can be divided into three categories: wireless communication of data, wireless sensing and wireless power transfer (WPT). In WPT, a considerable amount of energy is sent from the transmitter to the receiver and the two most common applications are energy harvesting (solar) and battery charging. In wireless charging, inductive power transfer with magnetic resonant coupling is generally used [2]–[4], in which the transmitter and receiver are loosely coupled coils. The inductive power of these coils is compensated by capacitors, such that only active power is transferred. These capacitors with the coils form resonant circuits and, depending on the configuration, the compensation network can be either series-series (S-S), series-parallel (S-P), parallel-series (P-S) or parallel-parallel (P-P) [5]–[7], as shown in Fig. 1. Any of these compensation networks can be analyzed in the frequency domain through their relative phasor equations, which can be computed from the equivalent circuits shown in Fig. 1. This analysis relies on the fundamental harmonic approximation (FHA) named in [8], which considers all voltages and currents to be sinusoids operating at the chosen frequency. The FHA describes well the behavior of the resonant circuits and different operating points can be analyzed in the frequency domain. Using this approach, it is possible to have a first estimation of the voltage and current values in both circuits at different operating frequencies. As an example, the operation at zero current switching (ZCS) can be analyzed by imposing

A B Rac Iab L1 L2 M I1 I2 C2 R2 c) R1 C1 a) A B a b Rac Iab L1 L2 M I1 I2 C2 R2 R1 C1 IAB C2 a b b) A B a b Rac Iab L1 L2 M I1 I2 R2 R1 C1 IAB A B a b Rac Iab L1 L2 M I1 I2 R2 R1 d) C1 VAB IAB IAB VAB

Fig. 1: Compensation networks: a) S-S, b) S-P, c) P-S, d) P-P. the power factor (PF) of the primary circuit to be unity. This means that the ZCS occurs at the frequency that gives a zero phase shift between the primary voltage and current. The zero voltage switching (ZVS) operation can also be analyzed through the equivalent circuit imposing circulating reactive power by making the primary current lagging the fundamental frequency component of the primary voltage. However, evaluating the performance of ZCS and ZVS in terms of efficiency is not possible only by using the equivalent circuits in Fig. 1, because the inverter is not included in this analysis. In reality, the inverter supplies the source (either voltage or current) at a chosen operating frequency and its losses have impact on the total efficiency. Moreover, the inverter’s output is a square wave instead of a sinusoid, that makes the FHA more critical as the PF differs from unity.

This paper analyzes the advantages of ZVS over the ZCS operation in a S-S compensation network. Among all the possible compensation networks, S-S is taken into account because it is the only one in which the compensation capacitance values are independent of both the coupling and the load [5], [6], [9]. The minimum ZVS operating point is tuned considering the dead time and the switch capacitance. Then, the optimum operating point for ZVS is defined in terms of efficiency a nd a mount o f p ower d elivered t o the output. Measurements on an e-bike WPT charging system are executed as a proof of concept. The equivalent circuit of the used WPT charging system is shown in Fig. 2, which consists of an H-bridge inverter, a S-S compensation network, a diode-bridge rectifier and a resistive load.

The analysis in the frequency domain based on the FHA is explained in Section II. Then, the characteristics of both the ZCS and the ZVS operations are discussed in Section III.

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A B RL Iab L1 L2 M I1 I2 C2 R2 IAB c) R1 C1 a) A B a b RL Iab L1 L2 M I1 I2 C2 R2 R1 C1 IAB C2 a b b) A B a b RL Iab L1 L2 M I1 I2 R2 R1 C1 IAB A B a b RL Iab L1 L2 M I1 I2 R2 R1 IAB d) C1 VAB VAB VAB VAB Vin Cin Cout RL L1 L2 M I1 I2 C2 R2 R1 C1 A B a b Iout Iin Q1 Q2 Q3 Q4 D1 D2 D3 D4 + -Vout

Fig. 2: WPT charging system.

In Section IV, the minimum operating point that gives ZVS is tuned at different input voltage and dead time conditions. The measured performance of ZVS in terms of efficiency and output power is compared to ZVC in Section V. The results of this analysis are discussed in Section VI. Finally, Section VII gives some conclusions on the ZVS tuning.

II. FHAANALYSIS

From the equivalent circuit in Fig. 1 a), the equations for the primary and secondary circuit can be computed as in (1) and (2) using the Kirchhoff’s voltage law. The mutual inductance M is computed through the coupling factor k and the coil inductances L1 and L2 as in (3). According to [10]–[12], it

is possible to define an equivalent load resistance Rac as in

(4) for the analysis in the frequency domain, where RL is

the equivalent resistive load after the rectifier stage in Fig. 2. In turn, RL models the charging behavior of the battery at a

specific operating point of voltage and current. VAB “ pR1` jωL1` 1 jωC1 qI1` jωM I2 (1) 0 “ jωM I1` pR2` Rac` jωL2` 1 jωC2 qI2 (2) M “ kaL1L2 (3) Rac“ 8 π2RL (4)

The FHA analysis using (1)´(4) can be used as frequency analysis of the equivalent circuit’s operating points. However, the performance of these operating points in terms of both efficiency and delivered output power cannot be evaluated only by using the FHA model, because the influence of the inverter is not included. Therefore, their performance must be assessed by considering the whole WPT charging system in Fig. 2.

III. ZCSANDZVS

ZCS occurs when there is no current flowing through the switch during the switching transition. In the WPT charging system of Fig. 2, it is possible to achieve the ZCS at both turn-on and turn-off by detecting the zero-crossing of i1

and switching the inverter leg exactly at that moment. The fundamental component of vAB and i1 are in phase which

means that the PF is unity. Therefore, i1 does not have large

harmonic components which is good for electromagnetic compatibility (EMC). On the other hand, the drain-source capacitance Cds of the switch is not completely discharged

and, at turn-on, its charge is dissipated inside the switch. In case of short dead time tdead, it might be difficult to tune

Fig. 3: Picture of the laboratory set-up: e-bike WPT charging system. the switching exactly at the zero-crossing of i1 and the ZCS

could be lost.

ZVS occurs when the voltage across the switch is zero during the switching transition. As explained in [11], it is generally easier to achieve this condition at the turn-off, because during the conduction the voltage across the switch is 1-2 V and, in case of MOSFETs, the current drops to zero fast enough. On the other hand, at turn-on, the switch voltage goes from the blocking voltage Vin(considering an H-bridge)

to its conduction value. The ZVS is realized if the switch starts conducting when the voltage across the switch is already equal to the conduction value. According to [7], it is possible to realize this in the WPT charging system of Fig. 2, by making sure that i1 lags vAB. Therefore, considering the half period

in which Q2 and Q3 are conducting and i1 is negative, these

two switches must be turned off while the current is still negative and the anti-parallel diodes of Q1and Q4would start

conducting. After this, Q1and Q4must be turned on when i1

is still negative such that they would naturally take i1from the

diodes when it becomes positive. The reverse recovery would also not occur [13] and ZVS is achieved. In [14] and [15], the minimum amount of negative current IOF F that assures the

ZVS is defined as in (5). The ZVS operation does not have a completely unity PF, but the overall losses could be reduced especially in case of large Cds.

IOF F ą

2CdsVin,max

tdead

(5) The value of IOF F should be kept low such that the

turn-off is also a soft-switching. However, in case of an under-estimation of IOF F, ZVS at turn-off can be lost.

IV. TUNING OFIOF F

According to (5), it is clear that the value of the negative current IOF F can be tuned by setting an appropriate tdead.

The value of Cdsalso plays a role in this tuning. In MOSFETs,

Cdsis highly dependent on the blocking drain-source voltage

Vds which is equal to Vin in a H-bridge inverter. The

dependence of Cds on Vin is an intrinsic property of each

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TABLE I: Parameters in the laboratory set-up.

k L1pµHq L2pµHq C1pnFq C2pnFq R1pΩq R2pΩq

0.28 67.7 46.3 35.9 52.3 0.11 0.16

conditions of a certain device. In this analysis, the device used is IPP030N10N5 which is a 100V MOSFET with a nominal conduction resistance Rdsponq lower than 3 mΩ. From the device’s datasheet [16], the output and the reverse transfer capacitances Coss and Crss are known depending on

Vds. Therefore, Cds can be computed as Cds“ Coss´ Crss.

Typical values of Cds are shown in Table II. Other parasitic

capacitances from the resonant circuits may add to Cds, but

for this WPT charging system they have been found negligible. In this paper, the analyzed WPT charging system is used to charge e-bikes. Therefore, the target load is typically a low-voltage battery which ranges between 24-48 V. A picture of the laboratory set-up used as a proof of concept is shown in Fig. 3, in which the secondary coil is the double kickstand of the bike and the primary coil is placed under a charging tile which is placed on the ground. The circuit schematic is equal to the one in Fig. 2 and each component’s value has been experimentally resumed and can be found in Table I.

To gain an initial understanding of the ZVS operation, the minimum IOF F values at three conditions of Vin and tdead

have been computed according to (5) and they are shown in Table II. It is clear how the minimum IOF F decreases

when tdead becomes larger. This happens because, at the

same voltage condition, that capacitance has more time to discharge and, consequently, it requires less current. These theoretical values of IOF F are compared with measurements

on a laboratory set-up that have been done at the same Vin

and tdead conditions, using a resistive load RL “ 10 Ω. The

measured values of IOF F are shown in Fig. 4 together with

the theoretical ones of Table II. According to the plot in Fig.4 and as expected, the measured IOF F values are higher than

their respective theoretical ones.

The output power and efficiency have also been measured at the same IOF F, Vinand tdeadas shown in Fig. 4, and these

are plotted in Fig. 5. The efficiency η%is computed as defined

in (6), referring to the DC input and DC output power of Fig. 2. η%“ VoutIout VinIin ¨ 100 “Pout Pin ¨ 100 (6)

V. COMPARISON BETWEENZCSANDZVS

A. Performance at the minimum IOF F value

After measuring the performance at ZVS, both the output power and the efficiency are compared respectively with the ones achieved at ZCS in Fig. 6 and 7 for the same Vin and

tdead conditions. In case of ZCS, the only two differences

are that IOF F is equal to zero and, obviously, that ZVS and

ZCS work at different operating frequencies. It is possible to tune both the frequency and tdeadof the inverter with the two

potentiometer knobs of the controller in Fig. 3.

TABLE II: Theoretical values of IOF F from (5).

Vin(V) Cds(nF) tdead(ns) IOF F(A) 24 2.30 100 1.10 200 0.55 400 0.28 36 1.72 100 1.24 200 0.62 400 0.31 48 1.13 100 1.08 200 0.54 400 0.27

Vin Cds nF tdead ns Ioff c Ioff m Vin Iin Vout  24 2.3 100 1.104 1.3 24.025 1.8622 19.483 24 2.3 200 0.552 1 24.03 1.8733 19.623 24 2.3 400 0.276 0.8 24.031 1.8624 19.55 36 1.72 100 1.2384 1.5 36.075 2.8649 29.855 36 1.72 200 0.6192 1.3 36 2.8367 29.737 36 1.72 400 0.3096 1.15 36.012 2.797 29.542 48 1.13 100 1.0848 @ 2 47.989 3.7488 39.414 48 1.13 200 0.5424 * 1.7 48.022 3.7318 39.443 48 1.13 400 0.2712 ** 1.45 48.032 3.7222 39.437 39.166 38.103 39.222 38.803 39.308 38.002 38.83 0 0.5 1 1.5 2 2.5 0 100 200 300 400 IOF F (A) tdead (ns) T M 36 V 24 V 48 V 0 40 80 120 160 100 200 400 100 200 400 100 200 400 Output Pow er (W ) tdead (ns) ZVS ZCS 24 V 36 V 48 V

Fig. 4: IOF F at Vin“24, 36, 48 V. T= theoretical, M= measured values. B. Performance at higherIOF F values

It is difficult to ensure that the operation is always at an exact point for any possible circuit condition. Therefore, to complete the tuning of the ZVS, it is important to analyze the performance of the WPT charging system when the values of IOF F are higher than the minimum measured values in Fig.

4. For higher values of IOF F, the operation would still be

ZVS. However, if IOF F becomes too high, the turn-off losses

could become considerable and, at some point, they could have a negative impact on the overall efficiency. To evaluate the performance of these operating points, measurements have been executed at Vin “ 48 V and tdead “100, 200, 400 ns

and their measured output power and efficiency are plotted respectively in Fig. 8, 9 and 10. In all these charts, four values of IOF F are shown in which the first value corresponds to ZCS

(IOF F “ 0 A), the second value corresponds to the minimum

value of IOF F such that ZVS is achieved in that condition

(according to Table II) and the other two are higher values that give ZVS. In all measurements, the case of ZVS with the minimum value of IOF F gives the best performance with

respect to both output power and efficiency. VI. DISCUSSION OF THE RESULTS

Based on the results shown in the previous sections, some considerations need to be pointed out.

‚ According to Fig. 4, the measured IOF F minimum values

that ensure ZVS become smaller as tdead enlarges. This

measured trend of IOF F agrees with the theoretical trend

from (5). However, the measured values are greater than the theoretical ones at all Vinand tdead conditions. This result

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Iout Pin Pout eff pic ZCS Vin Iin Vout  1.969 44.73936 38.36203 85.7456 312 24.035 1.616 17.937 1.9741 45.0154 38.73776 86.05447 318/57/58/73‐5 24.023 1.5956 17.853 1.9801 44.75533 38.71096 86.49462 329and330 24.044 1.5252 17.577 3.0305 103.3513 90.47558 87.54182 314 36.093 2.4041 27.159 3.0172 102.1212 89.72248 87.85882 321 36.015 2.3463 26.81 3 100.7256 88.626 87.98759 332 36.033 2.2893 26.618 4.0183 179.9012 158.3773 88.03572 316/48/49 48.006 3.3059 36.866 4.0105 179.2085 158.1862 88.26934 336/7/8/43‐4‐5/59/60 48.038 3.2863 36.863 4.0183 178.7847 158.4697 88.63716 334and335 48.05 3.258 36.835 3.9951 178.0464 156.4721 87.88277 363 83 84 85 86 87 88 89 100 200 400 100 200 400 100 200 400 Ef ficiency ( % ) tdead (ns) ZVS ZCS 24 V 36 V 48 V 83 84 85 86 87 88 89 0 40 80 120 100 200 400 100 200 400 100 200 400 (%) (W) tdead (ns) Pout 24 V 36 V 48 V 160 Efficiency

Fig. 5: Measured output power and efficiency achieved with ZVS, operating at the measured IOF F values of Fig. 4.

Vin Cds nF tdead ns Ioff c Ioff m Vin Iin Vout 

24 2.3 100 1.104 1.3 24.025 1.8622 19.483 24 2.3 200 0.552 1 24.03 1.8733 19.623 24 2.3 400 0.276 0.8 24.031 1.8624 19.55 36 1.72 100 1.2384 1.5 36.075 2.8649 29.855 36 1.72 200 0.6192 1.3 36 2.8367 29.737 36 1.72 400 0.3096 1.15 36.012 2.797 29.542 48 1.13 100 1.0848 @ 2 47.989 3.7488 39.414 48 1.13 200 0.5424 * 1.7 48.022 3.7318 39.443 48 1.13 400 0.2712 ** 1.45 48.032 3.7222 39.437 * 2.5 48.022 3.7076 39.166 3.2 48.029 3.5167 38.103 ** 2.5 48.022 3.7126 39.222 2.9 48.024 3.6405 38.803 @ 2.5 47.995 3.7251 39.308 3.3 48.005 3.4935 38.002 1.2 47.998 3.6464 38.83 0 0.5 1 1.5 2 2.5 0 100 200 300 400 IO FF (A) tdead (ns) T M 36 V 24 V 48 V 0 40 80 120 160 100 200 400 100 200 400 100 200 400 Out put Powe r (W) tdead (ns) ZVS ZCS 24 V 36 V 48 V

Fig. 6: Measured output power values at both ZVS and ZCS operations, at the same Vinand tdeadconditions as in Table II.

Iout Pin Pout eff pic ZCS Vin Iin Vout 

1.969 44.73936 38.36203 85.7456 312 24.035 1.616 17.937 1.9741 45.0154 38.73776 86.05447 318/57/58/73‐5 24.023 1.5956 17.853 1.9801 44.75533 38.71096 86.49462 329and330 24.044 1.5252 17.577 3.0305 103.3513 90.47558 87.54182 314 36.093 2.4041 27.159 3.0172 102.1212 89.72248 87.85882 321 36.015 2.3463 26.81 3 100.7256 88.626 87.98759 332 36.033 2.2893 26.618 4.0183 179.9012 158.3773 88.03572 316/48/49 48.006 3.3059 36.866 4.0105 179.2085 158.1862 88.26934 336/7/8/43‐4‐5/59/60 48.038 3.2863 36.863 4.0183 178.7847 158.4697 88.63716 334and335 48.05 3.258 36.835 3.9951 178.0464 156.4721 87.88277 363 3.8865 168.9036 148.0873 87.67565 364/5/6 4.0005 178.2865 156.9076 88.0087 371/72 3.9571 174.8314 153.5474 87.82597 369/370 3.9995 178.7862 157.2123 87.93317 376‐7 3.8717 167.7055 147.1323 87.73259 378‐9 3.9581 175.0199 153.693 87.8146 380‐1 83 84 85 86 87 88 89 100 200 400 100 200 400 100 200 400 Effic ie ncy (%) tdead (ns) ZVS ZCS 24 V 36 V 48 V 83 84 85 86 87 88 89 0 40 80 120 160 100 200 400 100 200 400 100 200 400 (%) (W ) tdead (ns) Pout Efficiency 24 V 36 V 48 V

Fig. 7: Measured efficiency values at both ZVS and ZCS operations, at the same Vinand tdeadconditions as in Table II.

in (5) assumes that the current is constant during the whole tdead. Nevertheless, the current is actually a high-frequency

sinusoid which makes this assumption weak in a first place. Moreover, the values of the internal capacitances Coss and

Crssspecified in the MOSFET’s datasheet are not measured

at the same gate-source voltage and frequency conditions as the operation of the WPT charging system. This means that the actual value of Cds could be different from the

theoretical one. On top of that, there might be other parasitic capacitances that need to be discharged in that interval, so they could add to Cds. Therefore, the definition of IOF F in

(5) must be used only to have an initial insight and a margin must be considered during the actual operation.

pic tdead 322/41/42/61/62 336/7/8/43‐4‐5/59/60 200 363 364/5/6 333 334and335 400 371/72 369/370 315/46/47 100 316/48/49 376‐7 378‐9 380‐1 87 87.4 87.8 88.2 88.6 89 110 120 130 140 150 160 0 2 2.5 3.3 (%) (W) IOFF (A) 48 V, 100 ns Pout Efficiency

Fig. 8: Measured output power and efficiency with different IOF F, at Vin“

48 V, RL“ 10 Ω and tdead“ 100 ns.

Ioff m Vin Iin Vout  Iout Pin Pout eff 0 48.038 3.2863 36.863 3.7565 157.8673 138.4759 87.71663 * 1.7 48.022 3.7318 39.443 4.0105 179.2085 158.1862 88.26934 * 2.5 48.022 3.7076 39.166 3.9951 178.0464 156.4721 87.88277 3.2 48.029 3.5167 38.103 3.8865 168.9036 148.0873 87.67565 0 48.05 3.258 36.835 3.7494 156.5469 138.1091 88.22222 ** 1.45 48.032 3.7222 39.437 4.0183 178.7847 158.4697 88.63716 ** 2.5 48.022 3.7126 39.222 4.0005 178.2865 156.9076 88.0087 2.9 48.024 3.6405 38.803 3.9571 174.8314 153.5474 87.82597 0 48.006 3.3059 36.866 3.7581 158.703 138.5461 87.29897 @ 2 47.989 3.7488 39.414 4.0183 179.9012 158.3773 88.03572 @ 2.5 47.995 3.7251 39.308 3.9995 178.7862 157.2123 87.93317 3.3 48.005 3.4935 38.002 3.8717 167.7055 147.1323 87.73259 1.2 47.998 3.6464 38.83 3.9581 175.0199 153.693 87.8146 87 87.4 87.8 88.2 88.6 89 110 120 130 140 150 160 0 1.7 2.5 3.2 (%) (W) IOFF (A) 48 V, 200 ns Pout Efficiency 87 87.4 87.8 88.2 88.6 89 110 120 130 140 150 160 0 1.45 2.5 2.9 (%) (W) IOFF(A) 48 V, 400 ns Pout Efficiency Fig. 9: Measured output power and efficiency with different IOF F, at Vin“

48 V, RL“ 10 Ω and tdead“ 200 ns.

Ioff m Vin Iin Vout  Iout Pin Pout eff 0 48.038 3.2863 36.863 3.7565 157.8673 138.4759 87.71663 * 1.7 48.022 3.7318 39.443 4.0105 179.2085 158.1862 88.26934 * 2.5 48.022 3.7076 39.166 3.9951 178.0464 156.4721 87.88277 3.2 48.029 3.5167 38.103 3.8865 168.9036 148.0873 87.67565 0 48.05 3.258 36.835 3.7494 156.5469 138.1091 88.22222 ** 1.45 48.032 3.7222 39.437 4.0183 178.7847 158.4697 88.63716 ** 2.5 48.022 3.7126 39.222 4.0005 178.2865 156.9076 88.0087 2.9 48.024 3.6405 38.803 3.9571 174.8314 153.5474 87.82597 0 48.006 3.3059 36.866 3.7581 158.703 138.5461 87.29897 @ 2 47.989 3.7488 39.414 4.0183 179.9012 158.3773 88.03572 @ 2.5 47.995 3.7251 39.308 3.9995 178.7862 157.2123 87.93317 3.3 48.005 3.4935 38.002 3.8717 167.7055 147.1323 87.73259 1.2 47.998 3.6464 38.83 3.9581 175.0199 153.693 87.8146 87 87.4 87.8 88.2 88.6 89 110 120 130 140 150 160 0 1.7 2.5 3.2 (%) (W) IOFF(A) 48 V, 200 ns Pout Efficiency 87 87.4 87.8 88.2 88.6 89 110 120 130 140 150 160 0 1.45 2.5 2.9 (%) (W) IOFF (A) 48 V, 400 ns Pout Efficiency

Fig. 10: Measured output power and efficiency with different IOF F, at Vin“

48 V, RL“ 10 Ω and tdead“ 400 ns.

‚ According to Fig. 5, it is clear that, for all the values of

Vin, the reached efficiency is lower with for shorter tdead.

Two main reasons have been identified. Firstly, when tdeadis

shorter, there is a small margin to realize the soft-switching. On the other hand, when tdead is longer, it is easier to

make sure that Cds is completely discharged and, as a

consequence, the efficiency is higher. These observations are also confirmed by Fig. 11 and 12 which shows the measured waveforms at both ZCS and ZVS respectively for tdead “ 100 ns, 200 ns. According to Fig. 11 b), for

tdead “ 100 ns the ZVS is not perfectly reached because

there is still some overshoot in Vds turn-off. However, that

overshoot is definitely lower than the one at ZCS operation shown in Fig. 11 a). On the other hand, Fig. 12 b) shows that with tdead“ 200 ns the overshoot of Vdsis not present.

The second reason why the reached efficiency is lower when tdead is short is that, as shown in Fig. 4, the minimum

value of IOF F is greater than in the case with larger tdead.

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Fig. 11: Measured waveforms Vdsand i1at Vin“ 48 V and tdead“ 100 ns:

a) ZCS, b) ZVS.

Fig. 12: Measured waveforms Vdsand i1at Vin“ 48 V and tdead“ 200 ns:

a) ZCS, b) ZVS.

a) b)

Fig. 13: Frequency analysis of I1 at Vin “ 48 V through (1) and (2): a)

absolute value |I1|, b) phase angle φpI1q.

they affect negatively the efficiency. Moreover, according to Fig. 5, while the efficiency is considerably affected by tdead,

it is clear that this is not the case for Pout.

‚ From the efficiency comparison between ZVS and ZCS

operation in Fig. 7, it can be seen that the ZVS operation gives overall higher efficiency than the ZVC one. The gain in efficiency is considerable (up to 2%) especially for shorter values of tdeadand lower values of Vin. On the other hand,

from the output power comparison between ZVS and ZCS operation in Fig. 6, it is clear that changing tdeaddoes not

affect considerably its values. Moreover, for all the values of Vin, the output power is greater in ZVS than in ZCS.

This result can be justified from the fact that the efficiency is also greater at ZVS and this makes its output power higher. However, at Vin“ 48 V the difference in efficiency

between the ZVS and ZCS is not that high to justify the considerable difference in output power. This means that also the input power is higher at ZVS than in ZVC. To get a better understanding of the operation of the circuit, both the

pic tdead 396‐7 400‐1 200 402‐4 405‐6 407‐8 409‐10 400 411‐12 413‐14 382 100 392‐94 386‐8 389‐91 87.4 87.8 88.2 88.6 89 89.4 140 160 180 200 220 240 0 2.6 3.6 4 (%) (W) IOFF (A) 48 V, 100 ns Pout Efficiency

Fig. 14: Measured output power and efficiency with different IOF F, at Vin“

48 V, RL“ 20 Ω and tdead“ 100 ns.

Ioff m Vin Iin Vout  Iout Pin Pout eff 0 48.009 5.557 68 3.4503 266.786 234.6204 87.94329 * 2.3 48.024 5.1556 65.97 3.3479 247.5925 220.861 89.2034 * 3.2 48.041 4.7301 63.15 3.2048 227.2387 202.3831 89.06189 4 48.066 4.1043 58.84 2.9848 197.2773 175.6256 89.02476 0 48.01 5.504 67.83 3.4412 264.247 233.4166 88.33272 ** 1.7 48.017 5.342 67.13 3.4068 256.5068 228.6985 89.15883 ** 3 48.033 4.926 64.44 3.2734 236.6106 210.9379 89.14982 4.1 48.065 4.1064 58.82 2.9876 197.3741 175.7306 89.03428 0 48.01 5.556 67.95 3.4409 266.7436 233.8092 87.65316 @ 2.6 48.034 4.9223 64.37 3.2673 236.4378 210.3161 88.95199 @ 3.6 48.052 4.4559 61.17 3.1057 214.1149 189.9757 88.72604 4 48.076 3.8829 57.15 2.8975 186.6743 165.5921 88.70644 87.4 87.8 88.2 88.6 89 89.4 140 160 180 200 220 240 0 2.3 3.2 4 (%) (W) IOFF (A) 48 V, 200 ns Pout Efficiency 87.4 87.8 88.2 88.6 89 89.4 140 160 180 200 220 240 0 1.7 3 4.1 (%) (W) IOFF (A) 48 V, 400 ns Pout Efficiency

Fig. 15: Measured output power and efficiency with different IOF F, at Vin“

48 V, RL“ 20 Ω and tdead“ 200 ns.

Ioff m Vin Iin Vout  Iout Pin Pout eff 0 48.009 5.557 68 3.4503 266.786 234.6204 87.94329 * 2.3 48.024 5.1556 65.97 3.3479 247.5925 220.861 89.2034 * 3.2 48.041 4.7301 63.15 3.2048 227.2387 202.3831 89.06189 4 48.066 4.1043 58.84 2.9848 197.2773 175.6256 89.02476 0 48.01 5.504 67.83 3.4412 264.247 233.4166 88.33272 ** 1.7 48.017 5.342 67.13 3.4068 256.5068 228.6985 89.15883 ** 3 48.033 4.926 64.44 3.2734 236.6106 210.9379 89.14982 4.1 48.065 4.1064 58.82 2.9876 197.3741 175.7306 89.03428 0 48.01 5.556 67.95 3.4409 266.7436 233.8092 87.65316 @ 2.6 48.034 4.9223 64.37 3.2673 236.4378 210.3161 88.95199 @ 3.6 48.052 4.4559 61.17 3.1057 214.1149 189.9757 88.72604 4 48.076 3.8829 57.15 2.8975 186.6743 165.5921 88.70644 87.4 87.8 88.2 88.6 89 89.4 140 160 180 200 220 240 0 2.3 3.2 4 (%) (W) IOFF (A) 48 V, 200 ns Pout Efficiency 87.4 87.8 88.2 88.6 89 89.4 140 160 180 200 220 240 0 1.7 3 4.1 (%) (W) IOFF (A) 48 V, 400 ns Pout Efficiency

Fig. 16: Measured output power and efficiency with different IOF F, at Vin“ 48 V, RL“ 20 Ω and tdead“ 400 ns.

absolute value |I1| and the phase angle φpI1q of the primary

current I1are plotted in Fig. 13 depending on the operating

frequency f and at Vin“ 48 V. The phasor values of I1has

been derived from both (1) and (2). All the measurements have been executed at RL“ 10 Ω which, according to (4),

is equivalent to a load resistance of Rac “ 8.1 Ω for the

frequency domain analysis based on the equivalent circuit in Fig. 1 a). The analysis in the frequency domain of φpI1q

can be used to identify the operating frequencies at which both ZVS and ZCS occur. In case φpI1q is equal to zero,

the operation is at ZCS. On the other hand, when φpI1q

is negative, I1 lags VAB and ZVS can be achieved. After

detecting those frequencies, it is also possible to evaluate the respective values of |I1| at ZCS and ZVS. According to

Fig. 13, it is clear that |I1| is higher at ZVS than at ZCS

with the chosen resistive load RL “ 10 Ω. Consequently,

the input power would also be higher. From Fig. 13 b), φpI1q is zero for a large range of frequencies which is

(7)

be achieved in several operating points and each of them gives different values of |I1|. This relatively wide range

of frequencies that give zero φpI1q can also be noticed in

the ZCS waveforms of Fig. 11 a) and 12 a). As a result, the analyzed load case RL “ 10 Ω is the boundary of the

bifurcation-free operation, because φpI1q crosses the zero

only once at the nominal resonant frequency for greater values of RL. The bifurcation phenomenon occurs when

multiple resonant frequencies exist that make φpI1q equal

to zero. It was initially noticed by [17], [18] and more literature on that can be found in [6], [19]–[22]. In case the resistive load is doubled (RL “ 20 Ω Ñ Rac“ 16.2 Ω), the

frequency response of I1 is considerably different than in

the previous case as it is shown in Fig. 13. With RL“ 20 Ω,

|I1| is lower at ZVS than in ZCS operation. Therefore, the

input power would also be lower at ZVS.

‚ From the analysis of the ZVS operation at higher values of

IOF F, it is clear that the performance is different at the two

different values of RL. According to Fig. 8, 9 and 10, at

RL “ 10 Ω there is a considerable increase in both output

power and efficiency when changing the operation from ZCS to ZVS. For higher values of IOF F, the reached efficiency

drops again, but the output power does not drop as much as it is in the ZCS operation. Similar measurements have been done also at Vin“ 48 V, RL “ 10 Ω and tdead“100, 200,

400 ns, and their output power and efficiency are plotted respectively in Fig. 14, 15 and 16. Also in this case, there is a considerable increase in efficiency when moving the operation from ZCS to ZVS. On the other hand, the output power drops because of drop in input power as a results of the decrease of |I1| shown in Fig. 13 a). For higher values

of IOF F, the output power drops dramatically while the

efficiency is only slightly affected.

‚ The maximum efficiency measured is 89.2% at Vin“ 48 V

and RL “ 20 Ω which is about 0.6% higher than the

maximum one measured at RL“ 10 Ω.

VII. CONCLUSIONS

In this paper, the advantage of ZVS over ZCS is evaluated in terms of efficiency and delivered output power. The ZVS operation has been tuned at different input voltages and dead time conditions. The best way to tune the ZVS is through an experimental evaluation and using the theoretical model just as a support to gain an initial insight into the WPT charging system operation. Depending on the MOSFET capacitance Cds, the dead time must be sufficiently large such that ZVS can

be completely achieved. According to all the measurements, the best operating point of ZVS is the one with the minimum turn-off current IOF F that ensures ZVS. This operating point

guarantees maximum efficiency and enough delivered output power, even if the output power might be lower than in ZCS at some loading conditions. An under-estimation of IOF F

causes the loss of soft-switching. On the other hand, an over-estimation of IOF F causes either an increase in the turn-off

losses which affects the overall efficiency or a considerable drop in the delivered output power.

REFERENCES

[1] K. Wu, D. Choudhury, and H. Matsumoto, “Wireless power transmis-sion, technology, and applications [scanning the issue],” Proceedings of the IEEE, vol. 101, pp. 1271 – 1275, 2013.

[2] X. Lu, P. Wang, D. Niyato, D. I. Kim, and Z. Han, “Wireless charging technologies: Fundamentals, standards, and network applications,” IEEE Communications Surveys & Tutorials, vol. 18, pp. 1413 – 1452, 2016. [3] L. Sun, D. Ma, and H. Tang, “A review of recent trends in wireless

power transfer technology and its applications in electric vehicle wireless charging,” Renewable and Sustainable Energy Reviews, vol. 91, pp. [4] Y. Shi, Y. Zhang, M. Shen, Y. Fan, CanWang, and M. Wang, “Design

of a novel receiving structure for wireless power transfer with the enhancement of magnetic coupling,” AEU - International Journal of Electronics and Communications, vol. 95, pp. 236–241, 2018. [5] C.-S. Wang, G. A.’Covic, and O. H. Scelau, “General stability criterions

for zero phase angle controlled loosely coupled inductive power transfer systems,” in IECON’O1: The 27th Annual Conference of the IEEE Industrial Electronics Society, 2001.

[6] S. Chopra and P. Bauer, “Analysis and design considerations for a con-tactless power transfer system,” Telecommunications Energy Conference (INTELEC), 2011 IEEE 33rd International, 2011.

[7] S. Li, W. Li, J. Deng, T. D. Nguyen, and C. C. Mi, “A double-sided lcc compensation network and its tuning method for wireless power transfer,” IEEE Transactions on Vehicular Technology, vol. 64, 2015. [8] S.-Y. Cho, I.-O. Lee, S. Moon, G.-W. Moon, B.-C. Kim, and K. Y.

Kim, “Series-series compensated wireless power transfer at two different resonant frequencies,” in 2013 IEEE ECCE Asia Downunder, 2013. [9] S. Li and C. C. Mi, “Wireless power transfer for electric vehicle

applications,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 3, pp. 4–17, 2015.

[10] R. L. Steigerwald, “A comparison of half-bridge resonant converter topologies,” IEEE Transactions on Power Electronics, vol. 3, pp. 174– 182, 1988.

[11] R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd ed. Kluwer Academic Publishers, 2004.

[12] A. J. Moradewicz and M. P. Kazmierkowski, “Contactless energy trans-fer system with fpga-controlled resonant converter,” IEEE Transactions on Industrial Electronics, vol. 57, pp. 3181–3190, 2010.

[13] J. D. V. W. Henry W. Koertzen and J. A. Ferreira, “Comparison of swept frequency and phase shift control for forced commutated series resonant induction heating converters,” in Conference Record of the 1995 IEEE Industry Applications Conference Thirtieth IAS Annual Meeting, 1995. [14] T. Kan, T.-D. Nguyen, J. C. White, R. K. Malhan, and C. C. Mi, “A new

integration method for an electric vehicle wireless charging system using lcc compensation topology: Analysis and design,” IEEE Transactions on Power Electronics, vol. 32, no. 2, pp. 1638–1650, Febraury 2017. [15] B. Lu, W. Liu, Y. Liang, F. Lee, and J. van Wyk, “Optimal design

methodology for llc resonant converter,” in IEEE Applied Power Elec-tronics Conference and Exposition, 2006, pp. 533–538.

[16] Infineon, IPP030N10N5 MOSFET, October 2016. [Online]. Avail-able: https://www.infineon.com/dgdl/Infineon-IPP030N10N5-DS-v02 03-EN.pdf?fileId=5546d4624a75e5f1014ac4e0b47c1f49

[17] R. Laouamer, M. Brunello, J. Ferrieux, O. Normand, and N. Buchheit, “A multi-resonant converter for non-contact charging with electro-magnetic coupling,” in IECON’97 23rd International Conference on Industrial Electronics, Control, and Instrumentation, 1997.

[18] J. Boys, G. Covic, and A. Green, “Stability and control of inductively coupled power transfer systems,” in IEE Proceedings - Electric Power Applications, 2000, pp. 37 – 43.

[19] O. Stielau and G. Covic, “Design of loosely coupled inductive power transfer systems,” in International Conference on Power System Tech-nology, 2000.

[20] C.-S. Wang, G. A. Covic, , and O. H. Stielau, “Power transfer capability and bifurcation phenomena of loosely coupled inductive power transfer systems,” IEEE Transactions On Industrial Electronics, vol. 51, no. 1, pp. 148–157, Feb. 2004.

[21] C.-S. Wang, O. Stielau, and G. Covic, “Design considerations for a contactless electric vehicle battery charger,” IEEE Transactions on Industrial Electronics, vol. 52, pp. 1308 – 1314, 2005.

[22] M. Iordache, L. Mandache, D. Niculae, and L. Iordache, “On exact circuit analysis of frequency splitting and bifurcation phenomena in wireless power transfer systems,” in International Symposium on Sig-nals, Circuits and Systems (ISSCS), 2015.

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