Design Consideration of MHz Active Clamp Flyback Converter with GaN Devices for Low Power Adapter Application

Xiucheng Huang, Junjie Feng, Weijing Du, Fred C. Lee, and Qiang Li Center for Power Electronics Systems, the Bradley Department of Electrical and Computer Engineering

Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061, USA

Abstract— With ever increasing demands of smaller size, ligh-ter weight for all forms of consumer electronics, efficient power conversion with higher operating frequency has always being pursued rigorously. This paper demonstrates high frequency, high efficiency and high power density design of active clamp flyback converter for adapter application. Both the primary and secondary switches are gallium nitride (GaN) devices which can significantly reduce the device related conduction and switching loss. The design procedures, including the selection of active clamping capacitor, optimization of flyback transformer, and EMI filter design, are presented in detail. A 65W (19.5V/3.3A) prototype of active clamp flyback front end converter is devel-oped to verify the feasibility of the system design. The prototype efficiency is 1~2% higher than the state of art product and the power density (exclude case) is more than 40W/in3.

Keywords—Gallium nitride device, active clamp flyback, soft-switching, high frequency, EMI filter

I. INTRODUCTION A growing demand for the size reduction of the power

supplies systems has stimulated substantial development and research effort in high efficiency and high power density power conversion. As the silicon-based semiconductor devices approach their theoretical performance limit, the ability to improve the performance of the next generation of power supplies by optimization of topology, magnetic and power management technique is diminished. The emerging wide-band-gap device, such as gallium nitride (GaN) devices will certainly bring out significant incremental efficiency improvement. GaN devices have a much lower gate charge and lower output capacitance, therefore, are capable of operating at a considerably higher switching frequency than the silicon MOSFET while maintaining high efficiency [1-8].

One of the biggest market of power supplies, in both volume and revenue, is the ac-dc adapter/charger for consumer electronics, including laptop, tablet, smart phone, mobile devices, game console, printer, and stereo sound-bars, etc. The market is projected to surpass $8 billion in 2015 and reach $9 billion by 2018; with much of this growth being driven by smart phones, tablets and a number of emerging applications [9]. Adapters are cost driven and are customized for each mo-bile device for each generation. Disposal of these out-of-date adapters becomes a grief environmental concern. Most of the adapters are operating at relative low frequency (<100 kHz) and with an efficiency below 92% and power density below 10W/in3.

Flyback converters are dominant topology for low power offline application due to its simplicity and low cost. Several literatures [6, 7, 8, 10] have demonstrated flyback converter with GaN devices operating over 1MHz in order to reduce passive components volume. Usually, an RCD clamp circuit is necessary to dissipate the leakage energy and suppress the voltage spike when the switch is off. However, the leakage energy loss is proportional to the switching frequency and it is quite considerable at MHz frequency range. Moreover, the voltage ringing causes high voltage slew rate which has significant impact on EMI noise, especially at 10~30MHz range. The active clamp flyback converter can clamp the voltage without any ringing and recycle the transformer leakage energy [11-13]. Zero-voltage-switching (ZVS) for both main switch and clamping switch can be realized by proper design as well. Different from low frequency operation, lea-kage inductance energy is not sufficient to realize ZVS for the main switch at MHz. The magnetizing current need to reverse direction to help achieve ZVS. It is worthwhile to point out that the clamping capacitor plays an important role on reducing converter conduction loss and switching loss. The design trade-off is analyzed in section II.

On the other hand, the traditional flyback transformer is in hand-made fashion which is intensive labor involved manufacturing process. The cost is a concern and the parameter variation is another circuit design issue. PCB winding based transformer is only feasible when the switching frequency is over several hundreds of kHz due to less turns number and smaller core size. The leakage inductance and parasitic capacitance of transformer can be well controlled by PCB manufacture. Moreover, shielding can be easily integrated in the PCB winding to reduce the CM noise [14]. The optimization of flyback transformer with integrated shielding is presented in section III.

High frequency operation has significant impact on EMI filter design as well. The corner frequency of CM and DM noise shifts to higher frequency which requires smaller CM choke and DM choke. Moreover, one-stage filter can be used to achieve the required attenuation over whole conducted EMI noise testing frequency range (150kHz ~ 30MHz) due to much higher corner frequency. The analysis of EMI performance in active clamp flyback front end converter and the filter design are discussed in section IV. A 65W (19.5V/3.3A) prototype of active clamp flyback front end converter is developed to verify the feasibility of the system design. The switching frequency is 900 kHz to 1.2 MHz over wide input voltage range. ZVS is achieved for both main switch and clamp switch by controlling the negative magnetizing current. The prototype efficiency is

978-1-4673-9550-2/16/$31.00 ©2016 IEEE 2334

1~2% higher than the state of art product and the power density (exclude case) is more than 40W/in3.

II. OPTIMIZATION OF ACTIVE CLAMP CIRCUIT

A. Benefits of flyback converter with active clamp circuit Fig. 1 shows the circuit configuration of the active clamp

flyback converter. The basic operation principle is well-known to power electronics researchers/engineers, which will not be discussed in this paper. However, it is worthwhile to point out that the leakage inductance energy is not sufficient to realize ZVS at high frequency (e.g. MHz). The magnetizing current need to reverse direction to help achieve ZVS for the primary switch. The main benefits of flyback converter with active clamp circuit are summarized as follows.

Fig. 1 Active clamp flyback converter with all parasitic

• Drain-source voltage of main switch is clamped when it is turned off. Low voltage rating device could be used as the primary switches.

• Leakage inductance energy is absorbed by clamping capa-citor, and part of it is utilized to achieve ZVS for primary switch and part of it is transferred to the output.

• ZVS can be achieved for both main switch and clamp switch by magnetizing current. The dv/dt is significantly reduced and therefore the common-mode noise is also re-duced.

• The secondary side current is free of high frequency oscil-lation and SR driving is much easier [6].

In addition to the common knowledge on the active clamp flyback converter, it is interesting to find out that the active clamp circuit modifies the transformer behavior. Fig. 2 shows the ideal transformer winding current comparison of the tradi-tional flyback and active clamp flyback converter. It is well-known that the traditional flyback transformer is actually an inductor with two windings conducting current in different dime period. There is no flux cancellation and therefore the winding loss is typically large. However, there is flux cancella-tion effect in active clamp flyback transformer marked as the shaded zone in Fig. 2. When the primary current goes negative, the magnetic field strength is reduced and the current can be more evenly distributed in the winding. The total winding loss reduction is about 20% based on FEA simulation result.

All of these features mentioned above results in higher effi-ciency and lower noise which makes active clamp flyback to-pology very suitable for high density adapter application.

ipri iLm

isec

ipri iLm

isec

(a) Traditional flyback (b) Active clamp flyback Fig. 2 Comparison of ideal transformer winding current

B. Selection of clamping capacitance The clamping capacitance impacts the resonant frequency

as well as the initial condition of the resonant tank. As a result, it determines the current waveforms of the resonant tank and the secondary side.

Fig. 3 shows the simulated current waveforms considering all the parasitics at 90Vac input and full load output condition. With smaller Cclamp, half of the resonant period formed by Lk and Cclamp is close to the main switch Sw off period. iLk is close to iLm when clamping switch is turn off, which means the turn off current of clamping switch is small. On the other hand, the current ripple of iLk is large due to the complete resonant. The secondary side current iSR is the difference between iLm and iLk, and therefore, iSR also has large ripple. On the contrast, the turn off current of clamping switch increases with larger Cclamp, while the current ripple of both primary and secondary side reduces significantly. Fig. 4 summarize the impact of Cclamp on the related converter power loss. It shows that increasing Cclamp in the range of below 100nF significantly reduces the conduc-tion loss and slightly increases the turn off switching loss. Therefore, the total Cclamp related loss reduces with larger capa-citance. Further increasing Cclamp has diminish return in terms of power loss reduction, but the penalty is larger size and rela-tively slow dynamic performance.

In this design, 100nF is chosen in the converter design from lower loss, smaller size and faster dynamic perspective.

iSR

iLk iLm

Cclamp=35nF, 70nF, 200nF

Cclamp=35nF, 70nF, 200nF

c Fig. 3 Impact of clamping capacitor on primary and secondary current waveforms

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 100 200 300 400 500

Los

s (W

)

CClamp (nF)

PSW

Pcon

Ptotal100nF

Fig. 4 Cclamp impact on converter loss

III. OPTIMIZATION OF ACTIVE CLAMP FLYBACK TRANSFORMER

The traditional low frequency flyback transformer is typi-cally in a hand-made fashion with solid wire. The parasitics are hard to control and the parameters varies from piece by piece. It is also difficult to implement shielding layer in order to re-duce common-mode noise. PCB winding based transformer is practical when the switching frequency is close to MHz. It is easier to control the parasitics and also more standardized for automation manufacture. Shielding layer can be easily added in the PCB layer [14]. On the other hand, transformer loss is the major part of flyback converter total loss. The design target is to minimize the loss within 2W, which is 3% of total power of a 65W adapter. Some key design points are discussed in this section. The optimization process shown below is based on 90Vac input and full load output condition.

A. Core material It is critical to select a proper core material for MHz

operation. The core loss density is the key criteria. Table I lists the core materials that may be suitable for MHz application. All the core materials are Mn-Zn ferrite. The core loss data are derived from datasheet which are measured under sinusoid excitation without DC bias. However, the voltage waveform across the active clamp flyback transformer is rectangular shape which will impact the core loss data significantly. Fig. 5 shows the measured core loss under 1MHz rectangular voltage excitation with Mu’s method [15, 16]. It clearly shows that the core loss data is different from the data listed in Table I. The measured data indicates that ML90S has minimal loss at 1MHz with Bm lower than 100mT.

TABLE I. CORE MATERIAL COMPARISON

3F45 N49 P61 ML90S

Manufactory Ferroxcube TDK ACME Hitachi

Initial Permeability 900 1500 900 900

Core Loss (kW/m3) 1MHz, 50mT

300 400 150 200

Core Loss (kW/m3) 1MHz, 100mT

1600 2300 3000 4000

10

100

1000

10000

10 100

Pv(k

W/m

3 )

Bm (mT)

3F45 N49

P61 ML90S

Fig. 5 Measured core loss of 4 materials under 1MHz rectangular voltage excitation.

B. Secondary side turns number: NS The voltage second of the flyback transformer significantly

reduced when the switching frequency increased to MHz range. The flux density of core is determined by voltage second and cross section area as well as the transformer turns number. Since the output voltage is constant, it is used to calculate the voltage second of the active clamp flyback transformer. Accordingly, the relationship between core loss density and turns number is shown in Fig. 6. The cross section area sweeps from 30mm2 to 50mm2 which can be considered as a reasona-ble range from both loss and volume perspective. It clearly shows that the core loss density is too high for NS=1 case even Ae=50mm2. The calculated core loss is over 1W which is unacceptable in 65W adapter application. The core loss density of NS=2 case is in the range of 200~700 kW/m3 with reasonable Ae. This is more practical and chosen for the converter design.

Ae (mm2)30 35 40 45 50

P v(k

W/m

3 )

0

103

2x103

3x103

4x103

5x103NS=1 @1MHzNS=2 @1MHz

Fig. 6 Core loss density vs. NS

C. Core cross section area: Ae In addition to the impact on the core loss density, the cross

section area also impacts the core volume and the length of winding. The relationship between transformer loss and cross section area is shown in Fig. 7 which assumes the window width is constant. The winding structure is shown in Fig. 11(a). The turns ratio is preset as 10:2 and the optimization will be shown in the next sub-section. The winding loss is based on Maxwell FEA 2D simulation. The core loss reduces when Ae increases since the impact of ΔB reduction is overwhelming than the increase of core volume. The winding loss increases with larger Ae due to increased winding length. The optimal

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Ae to achieve minimal total loss is around 50mm2. The standard core shape ER23/3/6/13 is used in this converter design.

Pcore

Pwinding

PtotalAe=50mm2

30 35 40 45 50 55 60

Ae(mm2)

Los

s(W

)

0

0.4

0.8

1.2

1.6

2.0

2.4

Fig. 7 Impact of Ae on transformer loss

D. Turns ratio The transformer turns ratio has several impacts on the

converter design. The voltage stress of primary and secondary switch is determined by turns ratio. The voltage rating of available high voltage GaN switch that used as the primary switch is 600V, and this sets the maximum turns ratio to be 7 with 15% margin. The secondary GaN switch can select 100~200V eGaN according to the turns ratio. The transformer turns ratio also impacts the root mean square (rms) value of primary and secondary current as well as the duty cycle of primary and secondary switch. The rms value of the primary and secondary side current are plotted in Fig. 8. It shows that the primary current decreases with larger turns ratio since the primary side main switch duty cycle increases. On the contrary, the secondary side current increases with larger turns ratio.

1.51.61.71.81.92.0

3 3.5 4 4.5 5 5.5 6 6.5 7

I pri

_rm

s(A

)

Np:Ns

2.1

4.04.55.05.56.06.57.0

I sec_

rms(A

)

Fig. 8 Impact of turns ratio on converter current

The relationship of converter loss and turns ratio is shown in Fig. 9. The core loss reduces with larger turns ratio since the duty cycle of secondary side reduces which results in a reduction of voltage second applied to the transformer. Basically, the total device conduction loss reduces with larger turns ratio since better device could be used in secondary side. The conduction loss including devices and windings increase with larger turns ratio. Overall, the turns ratio between 4 and 5 can achieve minimal loss, and 5 is chosen for the prototype demonstration.

0

0.5

1

1.5

2

2.5

3 4 5 6 7

Los

s(W

)

Turns ratio

Pcore

Pwinding+device conduction

PtotalNp:Ns=10:2

Fig. 9 Impact of turns ratio on converter loss

E. Integrate shielding layer The complete shielding can be easily implemented in the

PCB winding based transformer [14]. Fig. 10 shows the circuit diagram of flyback converter with shielding and Fig. 11 shows the transformer structure with shielding. The shielding is connected to the primary ground. Therefore the CM noise current coming from the primary noise source is circulating within the primary side. Making the shielding layer exactly the same as the secondary side winding generates no voltage difference between secondary side winding and shielding, which means no CM current flowing from either side. Rotating the shielding layer with any angle creates displacement current circulating between secondary side winding and shielding, but it does not create CM current from either side [14].

Vin

VOPri 1-5

Pri 6-10Sec 1

Sec 2

Shield 1

Shield 2

Fig. 10 Flyback transformer with shielding

One of the general concern of adding shielding layer to the flyback transformer is the winding loss. The 3D FEA simulation is carried out to evaluate the impact of the shielding layer on the total transformer winding loss as shown in Fig. 12. It shows that the current distribution in the primary and secondary winding almost remains the same and the current density in shielding layer is also very small. The total winding loss with the shielding layer is 1.35W and it is only 0.05W larger than the case without shielding layer, which is negligible loss increase.

To summarize the flyback transformer design, the ML90S from Hitachi is chosen for the core material; ER23/3.6/13 core shape is selected with optimized cross section area; the primary to secondary winding turns ratio is 10:2; the shielding layer is integrated in the PCB winding based transformer with negligible increase of total winding loss.

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Pri 6-10

Sec 2Sec 1

Pri 1-5Shielding 1

Shielding 2

(a) Transformer winding cross section view

Pri 1-5 Sec 1 Shield 1

(b) detailed PCB layout view

Fig. 11 Flyback transformer winding structure with shielding

Pri 1-5 Sec 1w/o shielding

Shield 1Pri 1-5 Sec 1

w/ shielding

Pwinding=1.3W

Pwinding=1.35W

Fig. 12 3D FEA simulation of flyback transformer with and without shielding layer

IV. EMI ANALYSIS AND FILTER DESIGN OF ACTIVE CLAMP FLYBACK CONVERTER

A. Analysis of shielding impact on CM/DM noise The electromagnetic interference (EMI) noise, especially

the common-mode (CM) noise is more severe due to high dv/dt induced by GaN devices. Fig. 13 shows the CM noise propaga-tion path in the active clamp Flyback converter. The primary switches induce high dv/dt which dominates the overall CM noise magnitude, and it is represented as a noise source VS. Compared to the primary to the power earth parasitic capacit-ance CPE, the interwinding capacitance of the flyback transfor-mer CPS and the secondary ground to power earth parasitic capacitance CSE are much larger and it is the major coupling path of the CM noise.

With the complete shielding layer integrated in the PCB winding based transformer, the CM noise path through CPS is cut off. As a result, the total CM noise current reduces signifi-

cantly due to much reduced capacitance. Fig. 14 shows the CM noise comparison between the converter prototypes with and without shielding. The switching frequency is near 1MHz. It clearly shows that CM noise has 27dB reduction with shiel-ding. More importantly, the high frequency noise, in the range of 10~30MHz, is also significantly reduced. It is usually diffi-cult to be attenuated by filters since the characteristic of CM choke turns to be poor due to core material permeability drop and parasitic impact at that frequency range.

Sw

Sa

SR

LISN

Cps

Cse

Cpe

VAC

+-

VS

iCM_L

iCM_N

D1 D2

D3 D4

Fig. 13 CM noise path of active clamp flyback converter

106

107

40

60

80

100

120

140

Frequency(Hz)

Noi

se L

evel

(dB

uV

)

W/O ShieldingWith ShieldingEN55022B

27dB

Fig. 14 CM noise reduction with shielding

It is interested to notice that the shielding also helps to re-duce the differential mode (DM) noise, as shown in Fig. 15. The cause of this phenomenon is the CM/DM noise transfor-mation when the L and N line impedance is unbalanced. The reduction of total CM noise current results in a reduction of DM noise which is transformed from CM noise. The detailed mechanism of this issue is presented in [17].

106

107

40

60

80

100

120

140

Frequency(Hz)

No

ise

Lev

el(d

Bu

V)

W/O ShieldingWith ShieldingEN55022B

21dB

Fig. 15 DM noise reduction with shielding due to CM/DM transfor-mation

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It is essential to understand the impact of shielding on the total EMI noise in order to design compact EMI filter with required attenuation.

B. High frequency EMI filter design The CM and DM filter corner frequency can be calculated

by the desired attenuation at switching frequency according to the conducted EMI standard EN55022B. By increasing the switching frequency to MHz range, the corner frequency of CM and DM increases to around 100kHz which is much higher than current industry practice. The simple one-stage filter can be used to achieve the required attenuation. The parameters of the EMI filter is summarized in Table II.

LCM

CY1

CY2

CX

LDM

(Lk)

Fig. 16 One stage EMI filter

TABLE II. PARAMETERS OF THE EMI FILTER

Parameters LCM LDM CY1=CY2 CX

Value 1.4mH 0.03mH 1nF 130nF

The conventional core materials for CM/DM chokes have high permeability at few hundreds of kHz and drops sharply at frequency above 500kHz. They are good enough for the con-verter operateing below few hundreds of kHz. However, these kinds of materials are not suitable for MHz application. The desired features for high frequency chokes includes relative high initial permeability and relative stable permeability over the frequency range from 1MHz to 5MHz. The ferrite materials 3D3 from Ferroxcube is chosen for the prototype design.

V. EXPERIMENTAL RESULTS AND DISCUSSIONS

In order to verify the feasibility of MHz converter design, a 65W (19.5V/3.3A) prototype is developed as shown in Fig. 17. The authors intend to implement MCU based control since there are no commercial available controllers that are targeted at MHz switching frequency. The close loop control is still under development and the results shown below are all based on open loop test which presets PWM frequency and duty cycle at certain input and output condition.

Side View

10mm

45mm

58mm

Fig. 17 1MHz 65W active clamp flyback converter prototype

GaN devices are used for both primary and secondary switches to reduce conduction and switching losses. The 600V GaN HEMT from Transphorm is used as the primary main switch and active clamp switch. The 150V eGaN from EPC is used as the synchronous rectifier. The PCB board is 6-layer which includes two layers shielding as shown in Fig. 11. The power density excludes the case is over 40W/in3, which is at least two times higher than the state-of-the-art products.

Fig. 18 shows the key experimental waveforms at low line and high line input condition. There is no voltage spike across the main switch which indicates that the leakage energy is well absorbed by clamping circuit. The primary switches can achieve ZVS over wide input range. It eliminates the turn on switching loss and avoid high dv/dt at the turn on instant.

VDS_SR

VDS_SW200ns/div

ZVS100V/div

20V/div

(a) Key waveforms at 110VAC

200ns/div

30V/div

100V/div

VDS_SW

VDS_SR

ZVS

(b) Key waveforms at 230VAC

Fig. 18 Prototype experiment waveforms

Fig. 19 shows the CM/DM noise spectrum with peak mode measurement under 110VAC input full load output condition which is the worst case for the prototype design. The red curves are the results with shielding but without filters. The blue curves are the final results with EMI filter. It clearly shows that the blue curves are already lower than the quasi-peak standard.

The measured full load efficiency over wide input range is shown in Fig. 20. The efficiency of the prototype is 1~2% higher than the state-of-the-art product. It is worthwhile to point out that the power density improvement is accompanied with efficiency improvement due to thermal restriction. Based on the thermal simulation carried on a converter which is en-closed by case, the converter efficiency should be above 92%

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at worst case to achieve 25W/in3 power density without violat-ing the thermal standard IEC60950. The prototype full load efficiency is above 92.8% over wide input range which makes it possible to achieve high density goal. More testing will be done to verify the prototype design can meet the general speci-fications of adapters.

106

107

20

40

60

80

100

120

Frequency(Hz)

Noi

se L

eve

l(dB

uV

)

W/O FilterWith FilterEN55022B

CM Noise

(a) CM noise measurement

106

107

20

40

60

80

100

120

Frequency(Hz)

Noi

se L

eve

l(dB

uV

)

W/O FilterWith FilterEN55022B

DM Noise

(b) DM noise measurement

Fig. 19 Prototype EMI noise measurement with and without filter

90

91

92

93

94

95

80 100 120 140 160 180 200 220 240 260

Effic

ienc

y

VAC(V)

State of art productCPES prototype

Fig. 20 Efficiency measurement of the prototype

VI. CONCLUSION

This paper presents the design consideration of MHz active clamp flyback converter for adapter application. Each design procedure targets at minimal power loss as well as smaller size. The PCB winding based transformer has the advantage of high density, controllable parasitics, and easy integration of shiel-ding. The prototype verifies the feasibility of the system de-sign. With high frequency operation and dedicated design,

much higher density and higher efficiency are achieved com-pared with state of art product. Future work will focus on light load efficiency and thermal condition evaluation.

ACKNOWLEDGMENT The information, data, or work presented herein was funded

in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006521 with North Carolina State University, PowerAmerica Institute.

This work was supported in part by the Power Management Consor-tium in CPES, Virginia Tech.

This work was conducted with the use of GaN device samples by Transphorm and EPC of the CPES Industry Consortium Program.

DISCLAIMER The information, data, or work presented herein was funded

in part by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

REFERENCES [1] B. Hughes, J. Lazar, S. Hulsey, D. Zehnder, D. Matic, and K. Boutros,

"GaN HFET switching characteristics at 350V-20A and synchronous boost converter performance at 1MHz," in proc. IEEE APEC, 2012, pp 2506-2508.

[2] J. Delaine, P. Olivier, D. Frey, and K. Guepratte, "High frequency DC-DC converter using GaN device," in proc. IEEE APEC, 2012, pp 1754-1761.

[3] X. Huang, Z. Liu, Q. Li, and F C. Lee, "Evaluation and application of 600 V GaN HEMT in cascode structure," IEEE Trans. on Power Electron., vol. 29, no. 5, pp.2453-2461, May. 2014.

[4] X. Huang, Z, Liu, F. C. Lee, and Q. Li, "Characterization and enhancement of high-votlage cascode GaN devices," IEEE Trans. on Electron Devices, vol. 62, no. 2, pp.270-277, Feb. 2015.

[5] X. Zhang, C.Yao, X. Lu, E. Davidson, M. Sievers, M. J. Scott, P. Xu, and J. Wang, "A GaN transistor based 90W AC/DC adapter with a buck-PFC stage and an isolated Quasi-switched-capacitor DC/DC stage," in proc. IEEE APEC, 2014, pp 109-116.

[6] X. Huang, W. Du, F. C. Lee, and Q. Li, “A novel driving scheme for synchronous rectifier in MHz CRM flyback converter with GaN devices, ” in proc. IEEE ECCE, 2015, pp 5089-5095.

[7] Z. Zhang, K. D. T. Ngo, and J. L. Nilles, "A 30-W flyback converter operating at 5 MHz," in proc.IEEE APEC, 2014, pp 1415-1421.

[8] T. Labella, B. York, C. Hutchens, and J. S. Lai, "Dead time optimization through loss analysis of an active-clamp flyback converter utilizing GaN devices," in proc.IEEE ECCE, 2012, pp 3882-3889.

[9] External power adapters and chargers report-2015, IHS Technology. [10] H. Jia, O. A. Rahman, K. Padmanabhan, P. Shea, I. Batarseh, and Z. J.

Shen, "MHz-frequency operation of flyback converter with monolithic

2340

self-synchronized rectifier (SSR)," in proc.IEEE INTELEC, 2010, pp 1-6.

[11] C. T. Choi, C. K. Li, and S. K. Kok, "Control of an active clamp discontinuous conduction mode flyback converter," in Proc. IEEE Power Electronics and Drive Systems Conf., 1999, vol. 2, pp. 1120-1123.

[12] R. Watson, F. C. Lee, and G. Hua, "Utilization of an Active-Clamp Circuit to achieve Soft Switching in Flyback Converters," IEEE Trans.on Power Electron., vol.11, pp162-169, January 1996.

[13] J. Zhang, X. Huang, X. Wu, and Z. Qian, "A high efficiency flyback covnerter with new active clamp technique," IEEE Trans.on Power Electron., vol.25, No. 7, pp1775-1785, Jul. 2010.

[14] Y. Yang, D. Huang, F. C. Lee, and Q. Li, "Analysis and reduction of common mode EMI noise for resonant converters," in proc. IEEE APEC, 2014, pp 566-571.

[15] M. Mu, F. C. Lee, "A new core loss model for rectangular AC voltages," in proc. IEEE ECCE, 2014, pp 5214-5220.

[16] M. Mu, F. C. Lee, Q. Li, D. Gillham, and K. Ngo, "A high frequency core loss measurement method for arbitrary excitations," in proc. IEEE APEC, 2011, pp 157-162.

[17] X. Huang, J. Feng, F. C. Lee, and Q. Li, “Conducted EMI analysis and filter design for MHz active clamp flyback front-end converter,” in proc. IEEE APEC, 2016, to be published.

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