1

Magnetics Design Considerations

for Wireless or Contactless Power

Transfer for EV Charging and Data

Center Servers Directly from MV DC

Grid

Presented by:

Subhashish Bhattacharya

Mar. 13, 2020

2

► Outline

Introduction

Motivation and Challenges

Magnetics Design

Converter Topology Selection

Converter Design

Simulation and Experimental Results

Conclusions

3

► Introduction

Fast growing in size of data centers and cloud computing

Conventional system of power distribution is at 240V, and involves

multiple PDUs

System inefficient and inconvenient due to bulky distribution cables

PDU: power distribution unit

4

► Motivation and challenges

Improving system performance by supplying medium voltage (MV) directly to data-

center racks

Physical galvanic isolation barrier between MV distribution and low voltage application

side

Distribution all the way to point-of-use

Elimination of transformer

Explore the feasibility of wide-band-gap devices

Challenge: Medium voltage at the Rack,

Possible solution: Contactless/ Wireless Power transfer

Metric State of the Art Proposed

Application Voltage Multiple conversion stages to 240V Single conversion to 1kV

Overall Data Center Efficiency 81% to 91% 95%

Safety PPE required PPE not required

5

► System Description

SiC wireless power supply for data center server racks

1kV input

20kW output power

48V output voltage

Contactless Power Transfer

Server Power Concept

6

► Contactless Power Transfer

Planner transformer based WPT

converter

Split transformer based WPT

converterGap

HV

Primary

Winding

LV Secondary Winding

Leakage FluxLeakage Flux

Mutual FluxHV Insulation

Barrier

Secondary

Core

Primary

Core

Primary

Coil

Secondar

y CoilSecondary

Insulation

Primary

Insulation

Core

Gap

Winding

Gap

7

• Secondary current ~460A RMS

• Secondary winding – Copper foil

• Primary current ~ 40A RMS

• Primary winding – Litz Wire

Simplified Diagram of the Plug

Transformer

Low Voltage High voltage

Open PlugClosed Plug

► Contactless Power Transfer Plug

8

Comsol FEA verification of magnetic fields of

Quindent design from parametric search, front 1/4 is

cut away to view internals; magnetizing flux (left),

leakage flux (right)

Quindent HPMFT, mid-section profile (left) and 3D

(right) Gray = cores, Red = primary, Green =

secondary

► FEA Validation

9

Calculated effective AC winding resistances due

to skin effect at 50kHz.

Secondary Winding Performance Analysis

Simulated current distribution in solid

conductor secondary

• Winding resistance reduces with thickness

• Resistance reduction limited by skin effect

• Aluminum has 20.5% larger skin depth than

Copper

10

► Major Constrains to Select Power Converter Topology:

1. High degree of step-down voltage (1.2kV to 48V: 25 times)--if converter topology handles

this partly, then transformer design would be simpler, and probably more efficient

2. Controllability in a wide load variation (20%-100% load)

3. Maintaining required efficiency, where the load variation is very wide

4. Maintaining soft-switching even in wide load variation to meet the efficiency and power

density requirements

11

► Possible Converter Topologies

1. Resonant converters

2. Non-resonant converters

3. Two stage power conversion-[can have higher control flexibility, but

meeting required efficiency may be a challenge]

4. Modular parallel converters, or input-series-output-parallel structure-

[efficient solution, but may hit the volume constrain]

Sub-category for the first two categories would be

1. With passive rectification

2. active rectification (dual active bridge/converter)

Topology Selection Criteria

a) Least device count,

b) Stepping-down voltage level

c) Controllability in wide load variation,

d) Soft-switching in wide load variation

12

► Inverter Topologies

Vin

S1

S2

HF power inverter

Vac+

Vac-

Vin

S1

S2

HF power inverter

(symmetrical)

Vac+

Vac-

Vin1

Vin2

Only two devices

Voltage gain 0.5 at maximum duty cycle

Output voltage is pulsed dc—requires series

capacitor at output

Difficult to maintain soft-switching at lower

load

Only variable frequency control possible

Only two devices

Voltage gain 0.5 at maximum duty cycle

Output voltage-ac

Suitable both for resonant and non-

resonant converters

Both variable duty cycle, and variable

frequency control possible

Require additional dc-link caps

Difficult to maintain soft-switching at lower

load

(Asymmetrical)

13

► Inverter Topology - Comparison

Parameters Values for symmetric Half-

bridge

Values for asymmetric

Half-bridge

Input DC

bus current

Average

RMS

Peak

Same

1x

1x

Same

2 times

2 times

Inverter

switch

current

RMS

Peak

Average

Same Same

Tank

capacitor

voltage

RMS 𝑉𝑐𝑎𝑝

𝑉𝑐𝑎𝑝 2 +𝑉𝑖𝑛2

2

Vin

S1

S2

HF power inverter

Vac+

Vac-

Vin

S1

S2

HF power inverter

(symmetrical)

Vac+

Vac-

Vin1

Vin2

(Asymmetrical)

14

► Rectifier Topologies

Io

RL

Vo

Co

D1

D2

HF passive rectifier

Vo

Co

FB passive rectifier

Vo

Co

Current doubler

Only two devices

Suitable for low voltage and

high current at output

Poor transformer utilization—only

half of the coper volume is utilized

at the secondary side winding

voltage stress is double compare

with full bridge diodes

Suitable for low voltage

and high current at output

Require extra inductors

15

► Selected Topology

Llk1C1

Vinv Lm

S1

S2

HB Power inverter

Split-core

transformer

C2

CrIinv Lk2

S3

S4

S5

S6

V1 V2

I2

Vo

Vin

Ro

Io

Co

Iin

Vin1

Vin2resonant

tank

Fig. 1: Selected series resonant converter topology with a split-core transformer isolation

between medium and low voltage coils

Only 2 switching devices

voltage gain of inverter—0.5

tank capacitors blocks dc current to flow through primary coil

Inverter output is ac—symmetric +ve and –ve voltage waveforms

Extended soft-switching possible with variable frequency control and with lagging power factor at

inverter output

Large amount of reactive power flows from inverter at light load

16

► Steady-state operation

S2t

S1

t3 t5t6t2 t7 t9

iS1=iin

S1

iinv

vinv

vS1iS1

t1

S2

0.5vin

0.5vin

t0Cr Llk1

(a)

vin

iin

C1S1

C2

S2

Lm

n 1

vinv

iinv

Cr Llk1

vin

iin

C1S1

C2

S2

Lm

n 1

vinv

iinv

(b)

Cr Llk1

iin

C1S1

C2

S2

Lm

n 1

vinv

iinv

(c)

Cr Llk1

iin

C1S1

C2

S2

Lm

n 1

vinv

iinv

(d)

Fig. 3 Operating waveforms

Fig. 2 Converter equivalent circuit in each switching intervals

17

► Voltage and Current Stresses on Devices

𝜔𝑟=1

𝐶𝑟𝐿𝑙𝑘

መ𝐼𝐷1−4 =𝜋

4𝐼𝑜, 𝐼𝐷1−4 =

𝜋

2 2𝐼𝑜.

𝐼𝑆1 , 𝐼𝑆2 = 0.5 𝐼𝑖𝑛𝑣

𝑉𝑠1 , 𝑉𝑠2 = 𝑉𝑖𝑛

𝑉𝐷1 , 𝑉𝐷2 … 𝑉D4 = 𝑉𝑜

Considering the magnetizing inductance is significantly higher than the leakage, the resonance frequency of the tank is derived as

The peak voltage and rms current ratings of inverter devices are given as

The peak and rms current rating of the rectifier diodes are given as

The peak voltage rating of rectifier diodes are

(1)

(2)

(3)

(4)

(5)

18

► Simulation Results:

Simulation platform: PSIM 11

Table 1: Simulation parameters

Parameters Values

Input and output voltage 1000V/48V

Power output 20kW

Air gap (FR4) 0.5 mm

Insulation breakdown strength 10 kV

Turns ratio (actual turns) 10.5:1 (21/2)

Switching frequency 31.5 kHz

Magnetizing Inductance w.r.t. primary 300 µH

Primary leakage 26 µH

Secondary leakage 0.5 µH

ZVS of both the inverter devices

Inverter output and rectifier input voltages and

currents at 20kW, 1000V/48V, 31.5kHz

Iinv *10 Vinv

Lagging PF

250

500

750

-750

-500

-250

25

50

75

-75

-50

-25

I2 V2*10

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► Initial Comparison with Non-resonant Converter ~3kW

90

91

92

93

94

95

96

97

500 1000 1500 2000 2500 3000

VG1

VDS1

Vinv

I1

[500v/div]

[400v/div]

[10A/div]

VG2

VDS2

V2

I2

[500v/div]

[40v/div]

[100A/div]

ZVS

turn-on

ZVS

turn-on

Resonant

Non-

Resonant

Power vs. Efficiency

Non-resonant converter

efficiency slightly lower at

lower loads

It reduces sharply at

higher load due reactive

power loss in leakage

VG1

VDS1

Vinv

I1

[500v/div]

[400v/div]

[5A/div]

ZVS

turn-on

ZVS

turn-on

VG2

VDS2

V2

I2

[500v/div]

[40v/div]

[100A/div]

Resonant system @ 3kW power, 1000V/48V

Non-resonant system @ 2.5kW power, 1000V/48V

20

20

20kW TransformerFilter capacitors

(Two in parallel)

To

Load

Resonant Tank Capacitor

InverterGate driver

Major components

Inverter: 1.7kV, 72A SiC Cree,

C2M0045170D

Half-bridge cap: 750V DC, 20uF

Cornell Dubilier UNL7W20K-F

Tank cap: 500V AC, 0.1uF×10

Cornell Dubilier 338-3169-ND

Controller: DSP Texas

Instruments TMS320F28335

Rectifier: 80V, 500A, GeneSiC

MBR50080CT

Rectifier Cap: 400VDC,

380uF×2 Kemet 399-5957-ND

Rectifier

Secondary

Core

Primary

Core

Primary

Coil

Secondar

y CoilSecondary

Insulation

Primary

Insulation

Core

Gap

Winding

Gap

Primary winding Primary assembly

20kW Experimental Set-up

Inverter

21

21

Experimental Results

Vinv

Iinv

At 20kW, 48V output and 1000V input At 15kW, 48V output and 1000V input

ZVS turn-on—100% and 75% of rated load (control with frequency modulation technique)

Single stage power conversion for high voltage ratio

S2 gate

S1 gate

S2 gate

S1 gate

V2

V1

V2

V1

Vinv

Iinv

22

22

Experimental Results

At 10kW, 48V output and 1000V input At 4kW, 48V output and 1000V input

ZVS turn-on—50% and 20% of rated load (control with frequency modulation technique)

Single stage power conversion for high voltage ratio

S2 gate

S1 gate

V2

V1

Vinv

Iinv

S2 gate

S1 gate

V2

V1

Vinv

Iinv

23

Loss distribution @ 20kW

Rectifier Transformer Inverter Cap. ESR

Loss distribution @20kW

Rectifier Transformer Inverter Cap. ESR

► Efficiency

Experimental result with diode: GeneSiC

MBR50080CT200V - 80V, 500A, 880mV@ 250AEstimated result with two parallel Infineon

IPT007N06N — 60V, 371A, 0.75mΩ

90

92

94

96

98

2500 6500 10500 14500 18500

Eff

icie

ncy [

%]

Output Power [W]

efficiency w/o synchronous rectification

92

94

96

98

2000 4500 7000 9500 12000 14500 17000 19500

efficiency with synchronous rectification

24

Plug disconnection ‘video’:

► Arc-flash Test at Rated Voltage and Light Load

Ch1 = Inverter output

Ch2 = Output Voltage

Ch3 = Primary Current

25

► Conclusions

Proposed an efficient method of distributing power to data center racks

SiC based inverter enables single stage power conversion from MV to LV

Safety issues are addressed using contactless power transfer technology

Converter selection, analysis and converter design is reported

Operating inverter output at lagging power-factor, soft switching (ZVS) is achievable for

all the inverter devices

Experimental results obtained from the scaled down prototype verifies the

performances

Possibility of a highly compact converter design using SiC inverter and GaN rectifier

and operating at high frequency (few hundred kHz, Power < 5kW)

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