Optimization of bi-directional DC to DC converters for battery applications Florian Krismer Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory ETH Zentrum / ETL I16 Physikstr. 3, CH-8092 Zurich/Switzerland krismer@lem.ee.ethz.ch

Outline Applications Dual Active Bridge (DAB) Efficiency Optimization Fuel-cell powered and hybrid vehicles Photovoltaic systems Uninterruptible power supplies Dual Active Bridge (DAB) Operation principles Challenges Efficiency Optimization Improved modulation methods Hardware improvement Hardware Results

Unidirectional DC/DC converter, e.g. buck or boost converter Bi-directional DC to DC Converter Unidirectional DC/DC converter, e.g. buck or boost converter Bi-directional DC/DC converter Discussed topologies: current direction changes, voltage sign remains unchanged

Environmental impact caused by traffic must be reduced Applications Conventional Car vs. Fuel-cell Powered Car Environmental impact caused by traffic must be reduced Pollution: CO2, ozone, particles Noise Efficiency improvement: up to 10% with combined fuel-cell and reformer drive system compared to a modern diesel propulsion („Emissionsproblematik von Strassenfahrzeugen“, Dr. St. Hausberger)

Fuel-Cell Powered Car Super-Cap in Series to Fuel Cell Max power: 80kW Applications Fuel-Cell Powered Car Super-Cap in Series to Fuel Cell Max power: 80kW Top speed: 150km/h Vehicle range: 395km Tank capacity: 156.6l (max. 3.75 kg)

Applications Hybrid Car Series Parallel

Hybrid Car Power System Architecture 300…500V High Voltage DC Bus Applications Hybrid Car Power System Architecture 300…500V High Voltage DC Bus 14V Battery

Photovoltaic Systems Energy production is hard to predict Applications Photovoltaic Systems Energy production is hard to predict  Battery assisted power supply PBattery = 2kW … 4kW UBattery = 10V…16V, 20V…32V, or 40V … 64V

Uninterruptible Power Supplies Applications Uninterruptible Power Supplies Conventional systems need high voltage battery  Disadvantageous with respect to space, cost, reliability, and safety Conventional UPS Extended UPS Bi-directional DC/DC converter Low voltage battery

Typical Requirements P max = 2 kW V1 = 11…16 V V2 = 220…447 V Applications Typical Requirements P max = 2 kW V1 = 11…16 V V2 = 220…447 V I1,max ≈ 200 A Galvanic isolation High efficiency,  > 90% Low converter volume Low number of components V1= 11…16V V2= 220…447V

Bi-directional DC/DC Converter Topologies with Galvanic Isolation Dual Active Bridge Bi-directional DC/DC Converter Topologies with Galvanic Isolation Single stage topologies Multiple stage topologies Current-fed converter topologies Voltage to voltage converters without choke Dual Active Bridge Series Resonant Converter Dual Active Bridge

Dual Active Bridge Converter Advantages Low number of components No resonant topology Comparably small converter inductor L Simple control Disadvantage Poor switch utilization may occur when operated within wide voltage and power ranges

Dual Active Bridge Hard Switching Turn-On of T1 Turn-Off of T2

Dual Active Bridge Soft Switching Turn-On of T1 Turn-Off of T2

Low Voltage Side Switching Losses Dual Active Bridge Low Voltage Side Switching Losses → lower switching losses for hard switched operation

Low Voltage Side: No Soft Switching Dual Active Bridge Low Voltage Side: No Soft Switching Turn-On of T1 → Turn-off losses due to lead inductance Turn-Off of T2

High Voltage Side Switching Losses Dual Active Bridge High Voltage Side Switching Losses Full current range Soft switching range

Conventional Operation: Phase-shift Modulation Dual Active Bridge Conventional Operation: Phase-shift Modulation 12V → 336V, P = 1kW

Phase-shift Modulation: Power Flow Dual Active Bridge Phase-shift Modulation: Power Flow d = V2 / (n V1) Φ … phase-shift Po … output power

Operation at Low Power Conditions Dual Active Bridge Operation at Low Power Conditions High Switching Current Hard Switching High Transformer Current 16V → 220V, P = 500W

Improving the Dual Active Bridge Conventional Dual Active Bridge Phase-shift operation is simple Low number of components Bad converter utilization when a wide operation range is required Performance improvements Improved modulation algorithms: triangular/trapezoidal modulation Two stage topology Phaseshift: 12V → 336V, P=1kW Input voltage (blue) 5V/Div Output voltage (red) 100V/Div Transformer current (black) 5A/Div

Triangular Current Mode Modulation Efficiency Optimization Triangular Current Mode Modulation Advantages Low switching losses Utilization of parasitic inductors on the low voltage converter side Disadvantages Limited power transfer compared to phase shift modulation Inefficient utilization of the converter at high transfer ratio 12V → 400V, P=1kW Input voltage (blue) 5V/Div Output voltage (red) 100V/Div Transformer current (black) 5A/Div

Trapezoidal Current Mode Modulation Efficiency Optimization Trapezoidal Current Mode Modulation Advantages Converter operated with voltages which are close to the transformer turns ratio Efficient utilization of low voltage side as well as high voltage side of the converter Disadvantage Increased switching losses 12V → 250V, P=1kW Input voltage (blue) 5V/Div Output voltage (red) 100V/Div Transformer current (black) 5A/Div

Transition between the Modulation Methods Efficiency Optimization Transition between the Modulation Methods Triangular, V2 > n  V1 Trapezoidal, V2 > n  V1 Trapezoidal, V2 < n  V1 Triangular, V2 < n  V1 Calculation time: 3-4µs (16 Bit DSP @ 160Mhz)

Two Stage Converter V1 = V2 / n Efficiency Optimization Two Stage Converter Idea: galvanic isolated converter is most efficient when operated close to V1 = V2 / n Solution: the given specifications suggest a second converter stage to achieve better utilization of the galvanic isolated converter

Two Stage Converter: Voltage Conversion Ratios Efficiency Optimization Two Stage Converter: Voltage Conversion Ratios Voltage Gain of the Galvanic Isolated Converter Voltage Gain of the Non Isolated Converter

Single Stage Converter Efficiency Optimization Two Stage Converter: Calculated Efficiencies Single Stage Converter Two Stage Converter

Hardware setup of the new dual active bridge Hardware Results Hardware setup of the new dual active bridge Low Voltage Side Switches: eight IRF2804 in parallel CDC,1: 72 x 10µF/25V in parallel High Voltage Side Switches: SPW47N60CFD CDC,2: 6 x 470nF / 650VDC in parallel Transformer Core: planar core ELP 64 Low voltage side turns: 1 High voltage side turns: 24 Digital control HF transformer High voltage side Heatsink Low voltage side

Low Voltage Side Conduction Losses Hardware Results Converter Loss Model Low Voltage Side PCB and contact losses MOSFET conduction losses MOSFET switching losses Transformer Copper losses Core losses High Voltage Side Low Voltage Side Conduction Losses

Phaseshift Modulation, Measured vs. Calculated Efficiencies Hardware Results Phaseshift Modulation, Measured vs. Calculated Efficiencies

Measured Efficiency: Conventional and Alternative Modulation Hardware Results Measured Efficiency: Conventional and Alternative Modulation

Future Tasks Thorough experimental verification Verification within full operating range Implementation of an optimal modulation Investigation of converter variants Two stage topologies Series resonant converter Solving technical details Reduction of switching losses Avoiding transformer saturation

Summary Successful hardware implementation Improved Loss Model Successful implementation of modulation and control Efficiency of more than 90% at 12V / 2kW achieved Improved Loss Model Switching loss measurements Improved model of the low side conduction losses Efficiency optimization Improved modulation Hardware improvement: two stage topology