Design of Power Electronics Converter: beyond...

ISP3D - March 16, 2017

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Design of Power Electronics Converter: beyond topology

---On the use of optimization in Power Electronics

JL.SchanenUniversity Grenoble Alps – CNRS - G-INP

Introduction: Converter Design

What is a « good converter »?Low cost

Good efficiency

Low Volume, weight

Safe operation, quite commutation, no external disturbance

… in some cases accepting wideinput/output voltage

The definition may vary depending on the industrial application (aerospace, trains, grid converters, consumer electronic equipments, …)

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IEEE Xplore: « Design Power Electronics » - 147,049 hits !

It is our every day work

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We are designing converters !


SMPS design:Wide space of solutions

• Several topologies• Several possible design• Several candidate technologies

Multidisciplinary (Power Electronics & control, thermal, electromagnetics, mechanics, …)

Evoluting requirements, especially during the preliminarysteps

A task for experienced engineers, but …A lot of time spent in converter design (iterative design)

Often solutions are inherited by history

Are we using the emerging technologies at the best ? What should be the next technological breakthrough ?

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• Is it the good one ?


Topology & Control

• Switchingfrequency

• Component design



Components choice

CoolingSystem

Layout

Mechanics

• Meeting Standards

• Filter design


• Filter design

EMC

What we usually do…

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Local optimization

ExperimentalValidation





Local optimizationLocal optimizationLocal optimization

Local optimization



Topology & Control





Components choice

CoolingSystem

Layout

Mechanics


• Filter design


• Filter design

EMC


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Not easy to meet all criteria …


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Specifications

No

YesInputs OutputsModel OK?

…in a reasonable amount of time!


Illustration: DC-DC conversionDC-DC conversion example: Aircraft application: Vin=540V, Vout=28V, P=5kW, FS=50kHz

Minimum Weight

Respecting all aircraft criterion, especially the EMI limits from DO160

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Illustration: DC-DC conversionDC-DC conversion example

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Dual Active Bridge

Current Fed (half or full Bridge)Hard switching Bridge

Series resonant converter (unidirectional)

Illustration: DC-DC conversion

All previous converters exhibit 8 switches, 1 output capacitor, 1 transformer, 1 Inductor

Design results from converter requirements + components constraints

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Topology Control parameter Transformer Main component

Hard Switching Bridge d = 88% m = 0.06 Lout = 10µH2% current Ripple

Current Fed (Full Bridge) d = 80% m = 0.041SC Max Voltage

Lin = 200µH60% Current Ripple

Dual Active Bridge δ = π/2d = 50% for both bridges

m = 0.075 L = 100µHfixed by the output current at max Power

Series Resonant Converter FS/FR = 1.084 m = 0.055Max voltage on CR

LR = 238µHCR = 50nF

* sizing constraint


Input current is really different… and input EMC filter will bepart of the final weight

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HBC

DAB SRC

CFC


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Input currents are really different… and input EMC filter will be part of the final weight to fulfill the D0160 EMC standard

Current Fed converter’s input inductance is very big, but helps in reducing EMC spectrum

Comparison is not straightforward

Outline

1. Introduction

2. Design by optimization methodology1. Principle

2. Designing in a virtual world: order 1 method

3. Examples of application

3. From component choice to converter realizationImpact of Layout - Optimization again

1. Keep semiconductors in the SOA

2. Mitigate ground current

3. Accounting for stray couplings

4. Conclusion

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2.Design by optimizationGoal

Explore a wide space of solutions and find the « best » one regardingone or several objectives, and satisfying several constraints

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Final solution

2.1. DbO PrinciplePrinciple

Identifying coupled phenomena and solving them together

Not a step by step approach

• Example switching frequency

RequirementsAnalysis of all interactions

Defining the constraints

Defining one objective function

• Multiobjective optimization: Pareto fronts

Providing models

Defining an optimization strategy

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Increasesswitching

losses

Decreasespassives’ weight

2.1. DbO PrincipleSome design concepts

Design is not finding the solution to a well specified problem, but findingthe best way to formulate it, so as at least one possible solution can befound

… and if several solutions exist, finding the best trade-off

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2.1. DbO Principle


Opimizationsupervisor

Circuit

Simulator

FunctionalComponent level

Advanced Component level 1

EM simulation

Materialmodels

Thermal simulation

Mechanicaldescription

The most straightforward idea: supervision of accurateand detailed modeling tools

ComponentDatabase

MaterialWiresCore

Database

MaterialDatabase

A unified framework for computationally efficient power converter design optimisation [UnivBristol-Manchester]

An Optimization Approach for Designing Multilevel Converters [University of British Columbia Vancouver]

An advanced tool for optimized design of power electronic circuits [Aalborg University]

…

2.1. DbO PrincipleDiscussion

Practical issues:

• Optimization time, number of variables• time simulation in the loop + convergence…• Coherence of different descriptions: 3D geometries in different physics, electrical values, …

More complexity, less accuracy

• Finding the best level of description• Keep all phenomenon at equivalent

level of accuracy

Formulation of constraints

• Design constraints are huge: limits ofparameters, physical limits –saturation,max temperature, voltage, …

• Stochastics algorithms have limited ability to handle constrained problems (penalty in the objective function)

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%error

Nb parameters describing the phenomenon

2.2. Designing in a Virtual World Order 1 Method

Some optimization algorithms are more suited to handlelarge number of variables and constraints

SQP, relaxation…

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Forbidden AreaThey need the derivative of functionsand constraints: Order 1

All models have to be derivable

The difficulty is now to elaborate the model

Order 0: usual models, sophisticated algorithms

Derivable models = no discrete functionsTurn number, core reference, …

We optimise in a virtual world where all is possible

If no solution exists in this world, no solution will exist in the real Power Electronics World


Derivable model example: Inductor1 base core, homotetic variation

Turn number not integer

L = N²/Reluctance

Material property extrapolation: µr(Idc,F)

Copper and Core losses analytic evaluation

• skin effect, proximity effect• Steinmetz, GSE, IGSE, Loss Surface, …

Heat exchange coefficient evaluation

Weight, Volume

Cost (BoM)

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Derivable model example: CapacitorInterpolation of adequate quantities function of capacitance

• Esr, esl• RMS current• Weight, volume

Interpolation has to be

performed for all

technologies

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Derivable model example: SemiconductorAnalytic expression of voltage & current waveforms

Temperature dependence

Semiconductor parameters (transconductance, voltage drop, threshold voltage, capacitors) extracted from datasheets and extrapolated for voltage and current capability

• Ciss(Vb, I), Coss(Vb, I), Crss(Vb, I)• The extrapolation is physic based (I ↔ S, V ↔ doping thickness)

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0

50

100

150

200

250

300

350

0 20 40 60 80 100

Rd

s_

on

(mΩ

)Id (A)

Rds_on = f(Id)

Id à 25°C

Puissance (Id à25°C)

Sw

itch

ing

En

erg

y (

mJ)



Derivable model example: Ripples, EMC

Solved in the frequency domainusing equivalent sources

High frequency effects (beyond1MHz) are not impacting so muchthe weight – Layout effect, seepart 3

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Heatsink model

Heatsink geometry(area, fins height, …)

Fan characteristics

Heatsink weight

Rth

Derivable model example: HeatsinkPure analytic model from [Kolar]



Example of a SiC switching cellSC Losses

Heatsink (Rth, Weight)

Thermal (SC temperature evaluation)

Inductors/transformers

(weight, saturation, leakage,

Losses)

Capacitors

(weight, max rms current)

Ripple

(at switching frequency)

EMC

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Example of a SiC switching cell

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Power ConverterPower 1kW

DC Input Voltage 400V

DC Output Current 5 A

Duty Cycle 0.5

Input/output Voltage Ripples ∆Vin/out = 1%

EMC requirements DO160F

TechnologyMOSFET Device Cree C2M0080120D

Diode Device Cree C4D20120D

HeatsinkFinned heatsink - Forced air cooled – 2 possiblefans

InductorsWound on torroidal core material µr = 5000 (CMfilter) or 160 (DM/output inductor)

Cx, Cy Ceramic

Cin Ceramic

Cout Electrolytic

LayoutLayout stray capacitance (generates CMcurrent) = 100pF Effect of EMC standard:

sudden need of EMC Filter

2.2. Designing in a Virtual World Order 1 Method: Summary

Example « Stratobus Project »Halfway between a drone and a satellite

• Autonomous airship, operating at an altitude of about 20 kilometers (stratosphere)

• Electrical airship: solar panels, fuel cell, electrolyze unit

Interleaved buck converter

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Order 1 method necessitates efforts for developping dedicatedmodels but is very powerful

Risk of local optimum – Hybrid Algorithm


Stratobus3 month PHD student to develop and implement the models in a dedicated framework, CADES

Optimization problem summary:

• Several set of specifications often evolving (≈10 versions)• 1 objective function• 15 design parameters• 62 design constraints

≈10s each optimization

Ongoing project,

some improvements needed

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MatériauMode de

conductionDensité

Fréquencede

découpage

Masse self de

puissance

Pourcentage sur la masse

totaleLimites atteintes par l’optimisation

MPP 26 Discontinue7,2

kW/kg155 kHz 146 gr 80 % Température self, rendement, THD

MPP 60 Discontinue7,5

kW/kg125 kHz 91,1 gr 76 % Température self, rendement, THD

MPP 125 Continue7,6

kW/kg230 kHz 223 gr 85 %

Température jonction MOSFET,conduction Pmin, η, Température self

KoolMu 60 Discontinue7

kW/kg115 kHz 105 gr 78 % Température self, rendement

KoolMu 125 Continue7,4


HighFlux 147 Continue6,4



Other example: UPS design – NPC vs Half Bridge

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Other example: UPS design – NPC vs Half BridgeHalf bridge, 1 phase, 10kVA, Filter, _40 Material

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Other example: UPS design – NPC vs Half BridgeNPC – 100kVA – Filter – Different materials

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Cos

t


Other example: UPS design – NPC vs Half Bridge10kVA, _40 Material, Filter

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150 200 250 300 350 400 45020

30

40

50

60

Coû

t

_40, NPC _40, Half Bridge

Cos

t

Losses

2. Discussion & ConclusionA « good » converter is a good adequation between

Well defined requirements

An adequation between the topology and technology

Optimization can help the designer inFinding a solution

Choosing the best compromise

Order 1 methodtakes some time to be implemented

but very powerful especially in the preliminary design phase

Optimization will never replace the designerHelp in formulating the problem

Shows possible designs compromises (Pareto analysis: e.g. Weight vs Cost, Weightvs Losses, … )

Helpfull with new emerging technologies where no experience has been accumulated

Key method for challenging the impact of requirements (EMC standards, …)

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4. Conclusion

Converter design is not only choosing an appropriate topologyand control strategy

Design by Optimization methodology may help engineers in better formulating the design problem, and explore widespace of solutions, especially with the new devices which maychallenge past solutions

Order one method applied to power electronics seems veryperformant, even if derivable models are quite challenging to develop

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4. Conclusion

New high speed devices necessitate improvements in semiconductor packaging, both on power and gate part

Common mode generation can be mitigated at source

Stray interactions between components and layout trackshave to be managed, and even used in a positive way

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Thank you !

M.Delhommais, PHD studentA.Baraston, PHD StudentP.Pelletier, MScC.Martin, PHDT.De Oliveira, PHD

JP FerrieuxF.WurtzPO.JeanninJM.Guichon

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