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FraCa team's technical approach for Little Box Challenge


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This document presents FraCa cost-controlled 2kW inverter as an answer to the specifications of Google and IEEE Little Box Challenge.

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FraCa team's technical approach for Little Box Challenge

  1. 1. LF switches COLD AIR HOT AIR HF + ARCP switches  8W  4W  4W  4W  4W • Restricted height  selection of low profile SMD components using through PCB cooling (with thermal vias) • Dual HF legs  use of symmetry to create a cooling duct acting as a double-side thermal system • Split inductors are preferred to a single coupled inductor to benefit from the cooling concept and increase losses density • All dimensions must be wisely chosen to align components. 1. PCB thickness + baseplate + D²Pak height  max value for Hind ; E32 Planar cores perfectly match the constraints 2. Thermal performances  Hfin ; Choice of an adequate form factor (H/D) for capacitors with high volumetric energy 2) 3D mechanical assembly of the converter 3) Thermal management approach Conclusions Motivated by reaching the target of the Little Box Challenge with a low-cost approach, a technical solution achieving a power density around 4.3kW/l (71W/inch3) with a CEC efficiency higher than 97% and a 98.1% peak efficiency was presented. With a second revision, the converter volume could be reasonably reduced to 370cm3 (5.4kW/l - 88W/in3) without affecting the cost but this value is definitely capped by the DC link capacitors (1,7mF for PF=0,7). CAPS D²PAK D²PAK Output inductor 5mm HIND CAPACITORS HFIN CAPACITORS PCB Insulating foil (TIM)Baseplate Coupling PCB D²PAK D²PAK INDUCTOR INDUCTORD²PAKD²PAK - Power supplies, DSP board and DC capacitors - Fan and LF leg cooling system LF switches HF board n°1 HF board n°2 PCB Insulating foil (TIM)Baseplate Duct equipped with fins (not represented here) COLD AIR HOT AIR Min 20mm 0 10 20 30 0.5 1 1.5 2 Fins H=12mm Fins H=8mm Fins H=4mm Cooling air thermal resistance (°C/W) Intermediate fins 0 10 20 30 0 1 2 3 4 5 Fins H=12mm Fins H=8mm Fins H=4mm Thermal resistance (°C/W) Intermediate fins 0 10 20 30 0 1 2 3 Fins H=12mm Fins H=8mm Fins H=4mm Air flow (m3/h) Intermediate fins Air flow (m3/h) Thermal resistance (°C/W) Air thermal resistance (°C/W) Intermediate fins Intermediate fins Intermediate fins CM filter 115mm 138mm28mm ARCP chokes Distributed DC link Distributed DC link Power Supply LF leg HF board n°1 DM filter DM choke Meas. units Distributed DC link Estimated losses assuming Tj = 125°C : 24W (ARCP chokes, DC capacitors are cooled via the metallic enclosure) Fan Selection (flow rate ensuring Tair_out < 60°C) Optimized heatsink geometry (Depth is set to 50mm according to the PCB area needed for active devices) Selected model : 2 x Alpha-Novatec UB60-7B (0.4mm thick, 4mm high fins) Model H/L/W (mm) Min/max flow (m3/h) BFB03512 HHA 7.7/24/35 2.9/3.6 24mm Nint=11 HLF leg dedicated cooler with lowP (Home made device : copper drains equipped with fins) Cooling of the DM inductors IN OUT IN Experimental results : HS_HF = 3°C/W (per side), HS_LF = 3°C/W, Air = 1,1°C/W and HS_ind=3,7°C/W 4) Prototype performances overview 88 90 92 94 96 98 100 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Efficiency(%) Ouput power (W) Measurements Calculation Objective • Converter volume  425cm3  77W/in3 (460cm3 w/ enclosure  71W/in3) • Efficiency : 98,1% peak - 97,2% CEC. Optimized control laws of the ARCP circuitry are still possible to improve CEC efficiency • Overall cost : 270€ assuming a MOQ=10kunits +0,5% CEC A cost-controlled, 4.3kW/l 1-ϕ Inverter with a 97.2% CEC efficiency Guillaume Lefevre, Nicolas Degrenne , Jeffrey Ewanchuk, Yoan Lefevre FraCa Team, Rennes, FRANCE Specifications Approach 1. Active devices : avoid using the latest generation of SiC or GaN devices due to cost and EMI requirements (high dV/dt) soft-switched SJ MOSFETs 2. DC Capacitors : conventional design using a wise selection of market sampling 3. AC Inductors : low Evol multilevel PWM + interleaving techniques 4. Cooling system : spread losses and optimized combination of fan and heatsink 5. EMI filter : prefer soft-switching and keep the first CM harmonic below 150kHz 6. Layout : Standard techniques with a minimum number of layers to reduce cost Abstract : The motivation of this document is to describe the integration aspects of the independent Franco-Canadian Team FraCa's entry into the Google Little box Challenge, the team being self-funded by persons working outside their current positions. The cost-motivated design approach is presented, and the selected converter topology namely FraCa topology is described in terms of its passive and active component contributions to meet the objective volumetric power density. Experimental results confirm a 97.2% CEC and 98.1% peak efficiency, meeting both the power density and efficiency targets Rated power 2kVA, PF≥0,7, leading-lagging Power Density > 50W/in3 (3kW/l) Output voltage 240Vrms ±12V@60Hz Input voltage 450V + 10Ω in series Input ripple 20% (I) and 3% (V) pk-pk Maximum THD 5% on both Iac and Vac Maximum Temp. 60°C with TAMB=30°C EMC constraints - FCC Part 15 B EMI rules - Ileak<50mA with 120nF of CM capacitances between terminals 1) Overview of the FraCa topology operating under soft-switching conditions Principles • Combination of a LF-T-type leg and HF soft-switched legs • Two interleaved HF legs were implemented  generation of 5-level waveforms @ 2fsw • T-type leg operating at the grid frequency : 1. no switching losses even with reduced dV/dt to comply with the CM constraints 2. LV MOSFETS are implemented for the mid. switch 3. beneficial use of the capacitive mid. point to implement a simple ARCP circuitry 4. easy to scale for higher power levels • Body diodes related issues are tackled  full advantage of the latest generation of the SuperJunction MOSFET Typical waveforms@Pn Voltage across the load and the low-frequency mid point 2ms 0,5 IL1 0,5 (IL1+ IL2) 0,5 Vload VLF VDS VGS IARCP Increasing I 100ns Max ISW TZVS Current flowing in L1 and (L1+L2) Typical turn-on transitions with the ARCP circuitry LF-T-type leg HF leg n°1 HF leg n°2 ARCP circuit L1 L2 Carcp Larcp VDCp VDCn Load Special thanks to all those who contributed and helped us in this contest (Free samples, testing facilities, measurement devices…) : ENS Rennes Ker Lann, G2Elab, CEFEM Industries, ST Microelectronics 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 1 2 3 4 5 6 Energydensity[J/cm3] Form factor H/D 10 100 1000 10000 100000 0,01 0,1 1 10 100 1000 FOM((mm3*€)/J²) Energy (J) Electrolytic Ceramic Film 0 200 400 600 800 1000 1200 1400 1600 1800 0 100 200 300 FOM=Cost*OnResistance(€*mΩ) Rds_On@25°C (mΩ) HEMT GaN MOS_JFET SiC SJ MOSFET SJ MOS w/ ARCP