epc1 design model squematics egan® fets in

8/10/2019 EPC1 Design model squematics eGaN® FETs in

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WHITE PAPER: WP014 eGaN® FETs in Wireless Power Tran

INTRODUCTION

The popularity of wireless energy transfer has increased over the last few years and in particular for applications targeting portable device charging. In this arthe focus will be on loosely coupled coils, highly-resonant wireless solutions suitable for the A4WP [1] standard operating at either 6.78 MHz or 13.56 MHz unlIndustrial, Scientic and Medical (ISM) [2] bands.

Many of the wireless energy transfer solutions have targeted portable device charging that require features such as low prole, high efficiency, robustness to changoperating conditions and, in some cases, light weight. These requirements translate into designs that need to be efficient and able to operate without a bulky heatsin

Furthermore the design must be able to operate over a wide range of coupling and load variations. There are a few amplier topologies that can be considered suas the voltage mode class-D, current mode class-D and class-E. The class-E has become the choice for many wireless energy solutions as it can operate with veryconversion efficiency.

eGaN® FETs have been previously demonstrated in a wireless energy transfer application using a voltage mode class-D topology [3, 4] and showed superior pmance when compared to a system utilizing equivalent MOSFETs by as much as four percentage points higher peak conversion efficiency. At output power labove 12 W, the design required a heatsink to provide additional cooling to the switching devices and gate driver. In addition, the traditional voltage mode clastopology requires that the resonant coils be operated above resonance to appear inductive to the amplier. This is needed to allow the amplier to operate in the ZVmode and overcome the COSS of the devices that would otherwise lead to high losses in the devices as opposed to being operated in the ZCS mode. Operating the coabove resonance comes at the cost of coil transfer efficiency and high losses associated with the matching inductor due to the presence of reactive energy.

eGaN FETs have also been demonstrated in a class-E topology by Chen et al [5] with upto 25.6 W power delivered to the load while operating at 13.56 MHz. The wireless en-

ergy transfer system was operated with very high load resistance (350 Ω) which ensureda high Q resonance, and the system efficiency was measured at 73.4% including gatepower consumption. The shunt capacitor in that example was completely embeddedin the EPC1010 device used in the experimental setup, thereby keeping the componentcount low.

COIL AND LOAD SIMPLIFICATIONTo facilitate the discussion and design evaluation, the coil set, device side matching, rec-tier and load will be reduced to a single reactive element (Zload) as shown in gure 1.

All subsequent design and discussion will make use of this single element, which allowsfor equal comparison between various topologies under equivalent load conditions.

CLASS E WIRELESS ENERGY TRANSFER

The ideal single ended class-E circuit for a wireless energy transfer system is shown ingure 2. In this setup Cs is used to resonate out the reactive component of Zload to yieldonly the real portion of the coil circuit to the amplier.

The design and operation of the class-E amplier is well documented in [6, 7, and 8]. Thematching network is designed for a specic load impedance to establish the necessaryconditions for zero voltage and current switching.

eGaN® FETs inWireless Power Transfer Systems EFFICIENT POWER CONVERSION

Alex Lidow, Ph.D., CEO and Michael de Rooij, Ph.D., Executive Director of Applications Engineering

Figure 1. Single element simplicationof the entire coil system and DC load.

Figure 2. Class-E amplier with ideal waveforms.

Lsrc

Ldev

Cdevs

RDCload

Cout

Ldevs

Cdevp

Zload

Coil Set

VDD

Ideal Waveforms

VDS

ID

tim

3.56 x VDD

V / I

50%

Q1

+

Csh

CsLeLRFck

Zload

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WHITE PAPER: WP014

EPC – EFFICIENT POWER CONVERSION CORPORATION |WWW.EPC-CO.COM | COPYRIGHT 2014 |

eGaN® FETs in Wireless Power Tran

Figure 5 shows the thermal performance of the eGaN FETs operating in theexperimental circuit delivering 30 W into a 20.2 Ω load.

Both the gate driver and eGaN FET temperature remain well below 50°Cwhen operating in an ambient temperature of 25°C. No heatsink or forced aircooling was used for this test.

There have been many questions about how to compare the performancebetween a class-E amplier utilizing an eGaN FET or MOSFET, given that COSS

is included in the matching network. The rst step to answer that question isto look at the soft switching gure of merit (FoMSS) [12]. Figure 6 shows thesoft switching gure of merit comparison between an EPC2012 [10] eGaNFET and FDMC86248 [13] MOSFET. The comparison is made for two gate volt-age operating conditions for the MOSFET, 6 V and 10 V. This allows a compari-son in performance for a circuit that uses the same gate driver for both theeGaN FET and MOSFET. From gure 6 is can be seen that the gate charge issignicantly lower for the eGaN FET, which is an important consideration forlow power converters as gate power is a signicant portion of the total powerprocessed by the amplier.

Comparing the output charge, the difference is smaller. However, the eGaN

FET is still lower for the same RDS(on) and has a higher voltage rating than theMOSFET. This allows the class-E amplier to be operated at a higher volt-age than the MOSFET, with resulting higher output power. Operating theMOSFET based amplier with a gate voltage of 6 V yields lower performancethan at 10 V gate despite the halving of the gate charge gure of merit. Figure4 shows the performance of the MOSFET based class-E amplier operatingwith a 10 V gate.

CLASS E SENSITIVITY TO LOAD VARIATIONLoosely coupled wireless energy transfer systems operate with large loadvariations as load power demand uctuates and coupling varies between

the source and device units. These variations introduce changes in the coilimpedance (Zload) as seen by the amplier, which when tuned at a specicload condition can shift from inductive to capacitive in addition to introduc-ing large changes in the resistance component of Zload. These changes mustbe understood and accounted for when designing wireless power systems.Most important is the impact these changes have on the power dissipationof the devices.

The class-E system was tested where the DC load resistance (RDCload) was var-ied while maintaining a xed supply voltage to the amplier. Figure 7 showsthe experimental results for the system efficiency, when using the eGaN FET,as function of the DC load resistance. It can be seen that below the designpoint (20.2 Ω for this example), the efficiency decreases rapidly with decreas-ing load resistance. This condition equates to an increase in load currentdemand by the device. Also shown in gure 7 is the corresponding outputpower as function of DC load resistance.

ZVS VOLTAGE MODE CLASS D WIRELESS ENERGY TRANSFERIn this section we introduce a zero voltage switching (ZVS) variation of thetraditional voltage mode class-D. The ideal voltage mode class-D ampliercomprises a half bridge topology that drives the load Zload. Since the loadmust be tuned by Cs, this capacitor also serves to block the average DC

Figure 6. Soft Switching Figure of Merit comparison for the class-E Am

Figure 8. ZVS Class-D amplier with ideal waveforms.

Figure 7. Experimental performance of the Class E system usingthe eGaN FET as function of DC load resistance.

0

200

400

600

800

1000

1200

F O M

( n C · m )

EPC2012BVDSS = 200 VQOSS = 11.6 nCQG = 1.5 nC

RDS(on)= 70

FDMC86248BVDSS = 150 VQOSS = 13.2 nCQG = 6.4 nC

RDS(on)= 69

FDMC86248BVDSS = 150 VQOSS= 13.2 nC

QG= 3.7 nCRDS(on)= 89 mmm

V G S

= 5

V

VDS= 100 V

V G S = 1

0 V

V G S

= 6

V

QOSS·RDS(on)

QG ·RDS(on)

12.0

12.5

13.0

13.5

14.0

14.5

15.0

70

72

74

76

78

80

82

10 15 20 25 30 35 40 45 50

E ffi c i e n c y ( % )

DC Load Resistance ( )

EPC2012

Pout EPC2012

VDCin = 20 V

Coil becomesInductive

Coil becomesCapacitive

+

VDD

CSQ2

Q1 Zload

LZVS

CZVS

V / I

VDS ID

time

VDD

50% Ideal Waveforms

ZVS tank



epc1 design model squematics egan® fets in

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