High Step-up Flying Capacitor Multilevel Converters

Zitao LiaoECE590

March 27, 2017

Outlines

▪Motivation

▪ Comparison between boost and FCML▪ Switches▪ Inductors

▪ Loss calculation and reduction

▪ Conclusion and future works

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Motivation – Compact High Voltage DC Generation

▪ Satellite Propulsion System▪ Ion Thruster Unit

▪ Pulse Electric Field (PEF)▪ Food and beverage

preservation

▪ Research Goals▪ 100’s V to 1 kV Output, 1 kW

power converter▪ High power density▪ High efficiency

3

Source: elea-technology.eu

Source: NASA

Boost converters- switches

4

IL∆𝐼𝐿

𝐷𝑇𝑠𝑤 𝑇𝑠𝑤

VL

𝐼𝑖𝑛

𝑉𝑖𝑛

𝑉𝑖𝑛 − 𝑉𝑜𝑢𝑡

IL →

+ -

𝐷𝑇𝑠𝑤 𝑇𝑠𝑤

Vout

▪ 1 kV, 10 A

▪ High blocking voltage:▪ Large Rds_on -> conduction loss▪ Large Qg, Qoss-> switching loss

▪ Thermal ▪ Hard to cool a single hot spot

▪ Availability?

FCML Boost Converters- switches

5

1

6𝑉𝑜𝑢𝑡

2

6𝑉𝑜𝑢𝑡

3

6𝑉𝑜𝑢𝑡

4

6𝑉𝑜𝑢𝑡

5

6𝑉𝑜𝑢𝑡

▪ Natural balancing of flying capacitors

▪ S1 open: 𝑉𝑠𝑤 = 𝑉𝑐1 =1

6𝑉𝑜𝑢𝑡

▪ S2 open: 𝑉𝑠𝑤 = 𝑉𝑐2 − 𝑉𝑐1

=2

6𝑉𝑜𝑢𝑡 −

1

6𝑉𝑜𝑢𝑡

▪ Switch Rating: 1

6𝑉𝑜𝑢𝑡, 10 A

▪ 7-level Flying Capacitor Multilevel Converter

▪ 166 V, 10 A

▪ Lower voltage rating:▪ Lower Rds_on▪ Lower Qg

▪ Thermal▪ Heat is distributed to more switches

▪ Availability▪ More likely to find in stock…

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FCML Boost Converters- switches

FCML Boost Converters- switches

▪ EPC 2034, GaN

▪ 200 V, 48 A

▪ Rds_on =7 mOhm

▪ Qg = 8.8 nC

▪ Size: 0.18 X 0.1 inch2

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▪ IXYS, IXFR26N100P, MOSFET

▪ 1 kV, 15 A

▪ Rds_on = 430 mOhm

▪ Qg = 197 nC

▪ Size: 0.62 X 0.54 inch2

Boost Converters- sizing the inductor

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IL∆𝐼𝐿

𝐷𝑇𝑠𝑤 𝑇𝑠𝑤

𝐼𝑚𝑎𝑥

𝐼𝑚𝑖𝑛

• 𝐿 =𝐷𝑉𝑖𝑛

∆𝐼𝐿𝑓𝑠𝑤

• 𝐼𝑚𝑎𝑥 = 𝐼𝑖𝑛 +∆𝐼𝐿

2

• 𝐸𝑃𝑒𝑎𝑘 =1

2𝐿𝐼𝑚𝑎𝑥

2

• 𝐸𝑃𝑒𝑎𝑘 ≤ 𝐸𝐿, 𝑚𝑎𝑥 = 𝜌𝐿𝑉𝑜𝑙𝐿

Higher switching loss, gate driving loss etc.

Higher RMS conduction loss, core loss.

𝑓𝑠𝑤↑

→ →

𝜂 ↓

• 𝐸𝑝𝑒𝑎𝑘 =𝐷𝑃𝑖𝑛

2𝑓𝑠𝑤𝑓(𝛼)

• Where 𝑓 𝛼 =1

𝛼+

𝛼

4+ 1, (0 < 𝛼 ≤ 2)

Inductor energy density: 𝜌𝐿=𝐸𝐿, 𝑚𝑎𝑥

𝑉𝑜𝑙𝐿=

𝐵𝑠𝑎𝑡2

2𝜇

Current ripple ratio: 𝛼 =∆𝐼

𝐼𝑖𝑛

∆𝐼 ↑ (𝑓 𝛼 ↓)

▪ 𝐸𝑃𝑒𝑎𝑘 ↓ → 𝑉𝑜𝑙𝐿 ↓→𝜂 ↓

▪ For a given 𝑓𝑠𝑤, 𝐸𝑃𝑒𝑎𝑘, 𝑚𝑖𝑛 =𝐷𝑃𝑖𝑛

𝑓𝑠𝑤(𝛼 = 2, largest current ripple).

Thinking process:

Conclusions:

FCML Boost Converter- sizing the inductor

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▪ 7-level Flying Capacitor Multilevel Converter

▪ 𝐸𝑝𝑒𝑎𝑘, 𝑏𝑜𝑜𝑠𝑡 =𝐷𝑃𝑖𝑛

2𝑓𝑠𝑤𝑓 𝛼

0

0.2

0.4

0.6

0.8

1

2 3 4 5 6 7 8 9 10

Norm

aliz

ed I

nduct

or

Volu

me

FCML Level

Normalized Inductor Size v.s. FCML Level

(D=0.9)

13.5x reduction

Total Passive Component Volume

1

6𝑉𝑜𝑢𝑡

2

6𝑉𝑜𝑢𝑡

3

6𝑉𝑜𝑢𝑡

4

6𝑉𝑜𝑢𝑡

5

6𝑉𝑜𝑢𝑡

→ 𝐸𝑝𝑒𝑎𝑘, 𝑓𝑐𝑚𝑙 =1 − 1 − 𝐷 𝑁 − 1 𝑃𝑖𝑛

2𝑓𝑠𝑤 𝑁 − 1𝑓(𝛼)

Y. Lei, W-C Liu, R.C.N. Pilawa-Podgurski, “An Analytical Method to Evaluate and Design Hybrid Switched-Capacitor and Multilevel Converters,” IEEE Transactions on Power Electronics, in press

▪ Effective switching frequency ▪ D to (1-(1-D)(N-1))

▪ Effective duty ratio▪ fsw to (N-1)fsw

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▪ 𝐸𝑝𝑒𝑎𝑘, 𝑏𝑜𝑜𝑠𝑡 =𝐷𝑃𝑖𝑛

2𝑓𝑠𝑤𝑓 𝛼 → 𝐸𝑝𝑒𝑎𝑘, 𝑓𝑐𝑚𝑙 =

1 − 1 − 𝐷 𝑁 − 1 𝑃𝑖𝑛2𝑓𝑠𝑤 𝑁 − 1

𝑓(𝛼)

FCML Boost Converter- sizing the inductor

Energy transfer and delivery (D=0.8)

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10 J 8 J

2 J

10 J

4 J

1 J

10 J

3 J

4 J

1 J

2 J

3 J

2 J

Source Load

Conventional Boost Converter

Conventional Boost Converter:Switch twice as fast

inductor source

3-level FCML

Hardware Prototype

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DiodesGaNSwitches

Isolated DC-DC for gate drivers

Flying capacitorsInductor Digital Isolators

Input

Input

Z. Liao, Y. Lei and R.C.N. Pilawa-Podgurski “A GaN-based Flying-Capacitor Multilevel Boost Converter for High Step-up Conversion,” IEEE Energy Conversion Congress and Exposition, Milwaukee, WI, 2016

Loss distribution

▪ Conduction loss▪ Rds_on ,diode Ron and inductor DCR

▪ Switching loss▪ Overlap loss and reverse recovery loss

▪ Inductor core loss▪ Current ripple and switching frequency

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p-n junction reverse recovery ▪ Parameters we care about:▪ Peak reverse current : Irrm

▪ Total reverse recovery charge : Qrr

▪ Total reverse recovery time : trr

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Diodes in hard-switched boost converters

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Vsw

Ifet

t0 t2 t4

𝑊𝑟𝑟 = 𝑉𝑠𝑤 𝑡𝑟𝑟𝐼𝑖𝑛 + 𝑄𝑟𝑟 𝑓𝑠𝑤

𝐸𝑟𝑟 = 𝑉𝑠𝑤𝐼𝑖𝑛𝑡𝑟𝑟 + 𝑉𝑠𝑤𝑄𝑟𝑟

▪ Reverse recovery increases switching loss because:▪ Longer switching transition: trr

▪ Extra charge: Qrr

Iin

Qrr

Loss distribution

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Reduction of reverse recovery

▪ Parallel diodes

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▪ Both Qrr and trr increase with temperature

▪ Temperature increases with input current

▪ Wrr∝ IinN

, (N>1). Paralleling diode should reduce Wrr .

▪ I assumed linear relationship between 𝑄𝑟𝑟, 𝑡𝑟𝑟 and Iin, so N = 2

𝑊𝑟𝑟 = 𝑉𝑜𝑢𝑡 𝑡𝑟𝑟𝐼𝑖𝑛 + 𝑄𝑟𝑟 𝑓𝑠𝑤𝑄𝑟𝑟, 𝑡𝑟𝑟 ∝ 𝐼𝑖𝑛

Test result

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Comparison

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[1] [2] [3] FCML Boost

Rated power 450 W 250 W 2 kW 820 W

Input voltage 25- 30 V 28- 38 V 275 V 100 V

Max outputvoltage

400 V300 –980 V

2 kV 1 kV

Switching Frequency

100 kHz 100 kHz 13.56 MHz 72 kHz

Peak efficiency

96% 97% 84% 94.1%

Overall powerdensity 38 W/in3 19 W/in3 250 W/in3

337 W/in3

19Power Density

Efficiency

[1] L. Müller and J. W. Kimball, “High gain dcdc converter based on the cockcroftwalton multiplier,” IEEE Transactions on Power Electronics,vol. 31, pp. 6405–6415, Sept 2016.[2] M. Kim, D. Yang, and S. Choi, “A fully soft-switched single switch isolated dc-dc converter,” in 2014 IEEE Applied Power ElectronicsConference and Exposition - APEC 2014, pp. 1106–1111, March 2014.[3] L. Raymond, W. Liang, K. Surakitbovorn, and J. Davila, “27.12 mhz isolated high voltage gain multi-level resonant dc-dc converter,” inEnergy Conversion Congress and Exposition (ECCE), 2015 IEEE, pp. 5074–5080, Sept 2015.

Improvements- QSW-ZVS

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t1 t2 t4

▪ ZVS turn-on of FET

▪ Qrr and Qoss of FET are both discharged back to the input source

▪ Trade-offs:▪ High RMS conduction loss and peak current

▪ High core loss

▪ Variable frequency control

▪ Operating at shallow DCM

▪ For FCML Boost

𝐿𝑓𝐹𝐶𝑀𝐿 ≤ 0.5 1 − 𝐷 2 𝐷 −𝑁 − 2

𝑁 − 1𝑅𝑜𝑢𝑡 …(𝐷 >

𝑁 − 2

𝑁 − 1)

QSW-ZVS

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Hard-Switching QSW-ZVS

Pin= 108.68 WPout= 101.99 WSwitching Frequency: 69 kHzInductor: 4 uH

Pin = 106 WPout = 100 WSwitching Frequency: 72 kHzInductor: 22 uH

▪ Switches are cooler: switching loss ↓ > conduction loss ↑

▪ Need better inductor design for lower core loss

Conclusions

▪ FCML:▪ Lower rating device▪ Smaller inductor

▪ Improvements:▪ Soft-switching techniques▪ Better inductor design

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Thank you!