1. 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

2. 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

3. 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

4. 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)

5. 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: kW
Top speed: 150km/h
Vehicle range: 395km
Tank capacity: 156.6l (max kg)

6. Applications
Hybrid Car
Series
Parallel

7. 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

8. 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

9. 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

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

11. 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

12. 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

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

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

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

16. 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

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

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

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

20. 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

21. 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) V/Div
Output voltage (red) V/Div
Transformer current (black) 5A/Div

22. 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) V/Div
Output voltage (red) V/Div
Transformer current (black) 5A/Div

23. 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) V/Div
Output voltage (red) V/Div
Transformer current (black) 5A/Div

24. 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 160Mhz)

25. 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

26. 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

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

28. 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: x 10µF/25V in parallel
High Voltage Side
Switches: SPW47N60CFD
CDC,2: x 470nF / 650VDC in parallel
Transformer
Core: planar core ELP 64
Low voltage side turns:
High voltage side turns:
Digital control
HF transformer
High voltage side
Heatsink
Low voltage side

29. 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

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

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

32. 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

33. 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