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December 8 2015

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Annual Meeting

Arizona State University Raja Ayyanar

January 17-19-2017

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Company Profile Role in WBG Technology

• One of the largest universities in the nation, and one of the largest Power Engineering programs

• Significant expertise and research in power converters

–  10-100 GWHz converters –  PV inverters and grid interface –  WBG modeling and characterization –  Automotive applications (new NSF IUCRC - EV-STS)

–  Smart inverters and microgrids

•  FREEDM partner university –  Architecture, SST and large system modeling

2 x 500kHz/3.6kW PFC

2.2 MHz automotive power supply

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Project Objectives Transformer-less PV string inverter •  3 kW, SiC based, high frequency •  Doubly-grounded (no ground

currents) •  Electrolytic-capacitor-less solution

with active power decoupling •  Wide range of power factor •  Power density improvement from 1W/

inch3 to 10W/inch3 •  CEC efficiency >96% (BP1)

97-98% (BP2)

PV microinverter •  Transformer-less and isolated versions •  GaN high frequency microinverter •  Doubly-grounded (no ground currents) •  Electrolytic-capacitor-less solution with

active power decoupling •  Wide range of power factor •  Power density target: 8 and 10 W/in3 •  Efficiency target: 95% CEC (BP1)

96% CEC (BP2)

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Accomplishments and Outcomes

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Two major challenges in PV inverters

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•  Two major challenges in single-phase and transformer-less PV inverters

–  Module capacitive-coupled ground currents at switching frequency

–  Large (electrolytic) capacitors needed for decoupling 120 Hz power pulsations

• Efficiency a key metric – so solutions for above challenges should not compromise efficiency

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Ground leakage currents in conventional transformer-less inverters

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2

1

0Cgnd

dc

when S is ONv

V when S is ON⎧

= ⎨⎩

1S

2S

Leads to large, high frequency, capacitive, ground leakage currents

PV Cgndv

Parasitic capacitances

to chassis

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120 Hz power pulsations and need for storage

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_

_&ac avg

PV desired

P

P

This needs to be supported (usually by a large bank of electrolytic capacitors)

DC link voltage

Vpeak

Vnom Vr

sin(2 )link nom rv V V tω θ≈ + +

4g g

link nom r

V IC V V

ω=

•  When DC link is same as PV input, Vnom is fixed, and Vr is highly restricted (~ 3-5% of Vnom)

•  Conventional converters with additional dc-dc stage have Vnom of 450 V and Vr of 10% pk-pk ripple; require large capacitors

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Requirements for a high performance PV inverter

•  Topology innovations that exploit advantages of high voltage dc link and high voltage film capacitors

•  Doubly-grounded dynamic dc link topology •  High voltage MOSFETs with low RDS ON

•  1200 V SiC MOSFETs •  High switching frequency to reduce size of other

filter components •  1200 V SiC; 200 V and 600 V GaN devices •  Soft switching

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Doubly grounded, dynamic dc link PV inverter

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gv

1Q

2Q

3Q

4Q

gi invLbLlinkv linkClinkCi

bL

ini

invinCiinC

bi

DC link voltage High peak

High nominal value

Large ripple

• Only 4 switches • Direct connection of PV negative

to ac neutral eliminates capacitive leakage currents

• DC link has high nominal value & high pk-pk ripple (Vrating>1kV)

• SiC 1200 V MOSFET is an enabler that allows efficient high voltage and high frequency (100 kHz) operation

•  15uF/kW film capacitor (10X reduction) at 1100 V • No electrolytic capacitors • Wide range of power factor possible for grid support

Boost stage Half-bridge dc-ac stage

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Experimental results of 3 kW string inverter

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linkV

inV

loadI

loadV

•  𝑉↓𝑔  THD = 2.5%, 𝑖↓𝑔  THD = 2.7%

•  𝑉↓𝑙𝑖𝑛𝑘  decouples the 120 Hz power, resulting in very low 120 Hz ripple in the input voltage at < 3% pk-pk of nominal dc voltage

At 3kW, PF = 1

linkVinVloadIloadV

At 3kVA, PF = 0.75lead

350V/div

350V/div

10A/div

100V/div

350V/div

350V/div

20A/div

200V/div

dsv

biinvi

200V/div

20A/div

20A/div

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Dynamic response: Power factor step change

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• Fraction of a cycle response to step changes in power factor command

linkv

inv

gi

gv

inlinkv v−

PF  change  from  0.7  lead  to  1

PF  change  from  1  to  0.7  lag

Power  factor  step  change  from  0.7  lead  to  1,  and  from  1  to  0.7  lag  

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Dimensions and power density

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Parameter Value Dimensions 9.15in x 5.27in x 1.73in = 83.4   𝑖𝑛↑3 

Power density (without EMI filter) 36   𝑊/𝑖𝑛↑3 

Volume breakdown

Power density target: 10 W/in3 Power density achieved: 36 W/in3

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Measured efficiency

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0 20 40 60 80 10095

95.5

96

96.5

97Efficiency at different power

Percentage of rated power

Effic

iency

in p

erce

nt

100kHz75kHz

Control/drive/fan power of about 5W not included in the above plots and EMI filters are also not included

CEC 96.4%

CEC 95.8%

Target: CEC 96%

Technology Readiness Level: 4

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BP2: Three-level dynamic dc link topology

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invL

invi

ini

linkClinkvbL

bi

3tQ4tQ

3Q

4Q

1Q

2Q

gi

gv

inC inv

gL

gC

•  All the major advantages of BP1 topology retained •  Switching voltage stress reduced for dc-ac stage •  Effective frequency doubled, with inductor grid

ripple (or inductor) reduced 50% •  Analysis predicts about 25% loss reduction

compared to BP1 topology

Measured efficiency at different power levels

0 20 40 60 80 10090

91

92

93

94

95

96

97Efficiency at different power

Percentage of rated power

Effic

ienc

y in

per

cent Proposed  T-­‐type

Original topology in BP1

Preliminary experimental results

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BP2: Split-phase string inverter with dynamic dc link

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1gV 2gV

4Q

5Q

2Q

3Q

ini

1D

1Q

linki

linkClinkV

bi bL

auxbL 1tQ1tD

inV inC

1gi1invL

1invi1gL

1gC1gi

2invL

2invi2gL

2gC

•  All the major advantages of BP1 topology •  Hybrid of boost dc-dc and split-phase half-bridge dc-ac •  Peak and RMS value of boost inductor current reduced •  Lower voltage allowed GaN switches (650 V GS66516 T) •  Zero voltage transition in dc-dc stage

•  Preliminary prototype fabricated and tested up to 1 kW •  Preliminary efficiency 95.6%, expected to improved at

higher power levels

linkv

1gv 2gv

inv inlink vv −

Preliminary experimental results

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BP1: Transformer-less microinverter

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Extended duty-ratio, high gain DC-DC stage Dynamic DC-link DC-AC stage

•  GaN-based with high gain boost dc-dc, and dynamic link dc-ac stage

•  Transformer-less and retains the main advantages of the string inverter topology

•  Extended D, high gain boost advantages •  Lower voltage stress for some switches •  Interleaving and reduced ripple •  Inherent sharing in zone I •  200 kHz switching frequency •  10 uF dc link film capacitor

3,1

Gain kD

=−

(In Zone I)

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BP1: Microinverter experimental results

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Input side interleaved inductor currents Voltage waveforms at input, output, and intermediate boost stages

Power density: 10.57 W/in3 (Target: 10 W/in3) CEC efficiency: 93.2% (Target: 95%) Frequency: 100&200 kHz (Target: 100 kHz)

3Li

1Li3Cv

2Li2cv

inv1cv

3Cv

_dc linkv

gi

inv

gv

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BP2: Microinverter with ZVT coupled-inductor boost

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Coupled inductor, soft-switching interleaved boost DC-DC stage

Dynamically variable DC link DC-AC stage

Qs1

L1

Qaux1

Laux1

1Li

inv C in

D1

Cdc1

Q1

Q2

Lbb

Q3

Q4

Cdc2Lgvg

1dcv

2dcv

Interleaved DC-DC stage DC-AC stage

1: n1auxi

Daux1

ZVT circuit

•  GaN-based transformer-less microinverter based on the coupled inductor soft-switching interleaved boost converter and the dynamically variable DC-link is developed and currently being tested to prove the performance objectives

•  A passive/active clamp is considered for reducing the switch voltage spike arising due to the leakage inductance of the coupled inductor.

•  Both dc-dc and dc-ac stages are integrated in a single board with the customized DSP-controller

11

oin

V ndV d

+=

−

, 1o

sw ratingVVnd

=+

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Publications from BP1 and BP2 work

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1.  J. Roy; R. Ayyanar, "Sensor-less Current Sharing Over Wide Operating Range for Extended-Duty-Ratio Boost Converter," in IEEE Transactions on Power Electronics , vol.PP, no.99, pp.1-1 (Available at IEEE Early Access)

2.  Y. Xia; J. Roy; R. Ayyanar, "A capacitance-minimized, doubly grounded transformer-less photovoltaic inverter with inherent active-power decoupling," in IEEE Transactions on Power Electronics , vol.PP, no.99, pp.1-1, doi: 10.1109/TPEL.2016.2606344 (Available at IEEE Early Access)

3.  J. Roy; R. Ayyanar, "GaN based high gain non-isolated DC-DC stage of microinverter with extended-duty-ratio boost," in 2016 IEEE Energy Conversion Congress and Exposition (ECCE), presented, Sept 2016.

4.  Y. Xia and R. Ayyanar, " Adaptive Dc Link Voltage Control Scheme for Single Phase Inverters with Dynamic Power Decoupling," 2016 IEEE Energy Conversion Congress and Exposition (ECCE), Milwaukee, WI, 2016, pp. 1-6.

5.  J. Roy; Y. Xia; R. Ayyanar, "A single phase transformerless string inverter with large voltage swing of half bridge capacitors for active power decoupling," in 2016 IEEE Energy Conversion Congress and Exposition (ECCE), presented, Sept 2016.

6.  Y. Xia; J. Roy; R. Ayyanar, " A High Performance T-type Single Phase Double Grounded Transformer-less Photovoltaic Inverter with Active Power Decoupling," 2016 IEEE Energy Conversion Congress and Exposition (ECCE), Milwaukee, WI, 2016, pp. 1-6.

7.  Y. Xia and R. Ayyanar, "High performance ZVT with bus clamping modulation technique for single phase full bridge inverters," 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), Long Beach, CA, 2016, pp. 3364-3369.

8.  J. Roy, “Wide band-gap devices based solar inverters,” presented in Clean Energy Education and Empowerment (C3E) Women in Clean Energy Symposium, 2016, Stanford University, CA, USA.

9.  Y. Xia and R. Ayyanar, "Inductor Feedback ZVT based, Low THD Single Phase Full Bridge Inverter with Hybrid Modulation Technique," 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2017 (accepted and to be presented).

10. Y. Xia; J. Roy; R. Ayyanar, "A GaN based doubly grounded, reduced capacitance transformer-less split phase photovoltaic inverter with active power decoupling," accepted in 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2017 (accepted and to be presented).

11. Y. Xia and R. Ayyanar, "Comprehensive Comparison of THD and Common Mode Leakage Current of Bipolar, Unipolar and Hybrid Modulation Schemes for Single Phase Grid Connected Full Bridge Inverters," 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2017 (accepted and to be presented).

12. J. Roy; Y. Xia and R. Ayyanar, "GaN Based Transformer-less Microinverter with Extended-Duty-Ratio Boost and Doubly Grounded Voltage Swing Inverter," 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2017 (accepted and to be presented).

13. J. Roy; R. Ayyanar, "A single phase transformer-less string inverter with integrated magnetics and active power decoupling," accepted in 2017 IEEE Applied Power Electronics Conference and Exposition (APEC), March 2017 (accepted and to be presented).

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•  Barriers to further maturity •  Challenges with high frequency magnetics especially for

the transformer-isolated microinverter •  Layout and reliable assembly of some special packages

Pathway to Market •  High performance of both the topology and SiC/GaN devices

demonstrated on realistic prototypes •  Impact on system level cost reduction through increased efficiency,

smaller filters, heatsinks and enclosures •  Patent for original dynamic link topology awarded recently, and a

new patent pending for the topological enhancements, and available for licensing opportunities

Patent awarded: R. Ayyanar, “CIRCUITS AND METHODS FOR PHOTOVOLTAIC INVERTERS,” U.S. Patent 9,413,269, Aug. 2016 Patent pending: R. Ayyanar, Y. Xia, and J. Roy, “POWER CONVERTER CIRCUITRY FOR PHOTOVOLTAIC DEVICES,” Patent pending, Application No. PCT/US16/64930, filed on 12/05/2016

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• Validation of commercial WBG devices in real applications with operation near rated voltages

• Validation of WBG-enabled performance enhancements in PV applications and risk mitigation

• New topologies that fully exploit WBG characteristics •  Training of 4 research students and >100 course students,

and dissemination through short videos

Broader Impact on the WBG Community

•  Completion of transformer-less string inverter at 240V and a separate split-phase interface with >30 W/in3 and 97% CEC, and grid support using three-level dynamic link topology and integrated magnetics

•  Completion of high performance microinverter with tapped-inductor, zero voltage transition (ZVT) topology at 200 kHz

Future Direction