How Do You Model and Control a Bidirectional Dual Active Bridge (DAB) Converter for EV Charging?
How Do You Model and Control a Bidirectional Dual Active Bridge (DAB) Converter for EV Charging?
Electric vehicle charging systems require high efficiency, high power density, galvanic isolation, bidirectional power flow, and reliable battery control. One of the most suitable converter topologies for this purpose is the Bidirectional Dual Active Bridge (DAB) DC-DC Converter.
A Dual Active Bridge converter is widely used in EV fast chargers, onboard chargers, battery energy storage systems, DC microgrids, solid-state transformers, renewable energy systems, and vehicle-to-grid (V2G) applications. Its main advantage is that it can transfer power in both directions while providing high-frequency transformer isolation.
In this article, we will understand how to model and control a bidirectional DAB converter for EV charging from beginner to advanced level.
What is a Dual Active Bridge Converter?
A Dual Active Bridge converter is an isolated bidirectional DC-DC converter consisting of two full-bridge converters connected through a high-frequency transformer.
The basic structure includes:
- Primary-side full bridge
- High-frequency transformer
- Transformer leakage inductance or external series inductor
- Secondary-side full bridge
- DC-link capacitors
- Battery-side filter
- Digital controller
The word dual active bridge means that both sides of the transformer use actively controlled switching bridges.
Basic DAB Converter Structure
DC Bus / Grid Side
↓
Primary Full Bridge
↓
High-Frequency Transformer
↓
Secondary Full Bridge
↓
EV Battery Pack
In an EV charger, the primary side is usually connected to a regulated DC bus from an AC-DC PFC stage, while the secondary side is connected to the EV battery.
Why DAB Converter is Used in EV Charging
The DAB converter is highly suitable for EV charging because it provides:
- Bidirectional power flow
- Galvanic isolation
- High power density
- High efficiency
- Soft-switching capability
- Wide voltage conversion range
- Simple phase-shift power control
- Support for V2G and G2V operation
This makes it attractive for both grid-to-vehicle (G2V) charging and vehicle-to-grid (V2G) energy transfer.
Power Flow Direction in DAB Converter
Power flow is controlled by the phase shift between the primary bridge voltage and secondary bridge voltage.
| Phase Relation | Power Flow Direction |
|---|---|
| Primary voltage leads secondary voltage | Grid/DC bus to battery |
| Secondary voltage leads primary voltage | Battery to DC bus/grid |
| Zero phase shift | Ideally zero transferred power |
Single Phase-Shift Control
The simplest and most widely used DAB control method is Single Phase-Shift (SPS) control.
In SPS control:
- Both bridges generate square-wave voltages.
- The duty ratio is usually fixed at 50%.
- Power is controlled by changing the phase shift angle.
The phase shift is usually represented by:
φ = Phase shift angle between primary and secondary bridge voltages
A higher phase shift produces higher power transfer.
Power Transfer Equation of DAB Converter
For a basic DAB converter under single phase-shift control, the transferred power can be approximated as:
P = (n V1 V2 / ω L) × φ × (1 - φ/π)
Where:
- P = transferred power
- V1 = primary-side DC voltage
- V2 = secondary-side DC voltage
- n = transformer turns ratio
- ω = angular switching frequency
- L = leakage or series inductance
- φ = phase shift angle
This equation shows that DAB power depends strongly on DC voltages, switching frequency, transformer ratio, leakage inductance, and phase shift.
Role of Leakage Inductance
The leakage inductance of the transformer or an external series inductor is not just a parasitic element in a DAB converter. It is the main energy transfer element.
It controls:
- Power transfer
- Current ripple
- Soft-switching range
- Peak current stress
- Efficiency
If inductance is too small, current stress becomes high. If inductance is too large, transferred power becomes limited.
Transformer Turns Ratio Selection
The transformer turns ratio is selected according to the voltage range of the DC bus and battery.
A practical design goal is to match the reflected voltages:
V1 ≈ nV2
This condition reduces circulating current and improves efficiency.
For example, if the primary DC bus is 800 V and the battery nominal voltage is 400 V, a transformer ratio near 2:1 may be selected depending on the exact design requirement.
Modeling of DAB Converter
DAB converter modeling can be done at different levels depending on design stage and accuracy requirement.
1. Switching Model
The switching model uses actual semiconductor switches, gate signals, transformer, leakage inductance, and capacitors.
Used for:
- Detailed waveform analysis
- Switching loss calculation
- Device stress estimation
- Soft-switching verification
- Gate signal validation
Tools: LTspice, PLECS, PSIM, MATLAB/Simulink, Typhoon HIL.
2. Averaged Model
The averaged model removes high-frequency switching details and represents the converter using average power transfer behavior.
Used for:
- Control loop design
- Battery charging profile analysis
- System-level simulation
- Stability study
This model is faster than a full switching model.
3. Small-Signal Model
A small-signal model is used to design controllers and analyze stability.
The control input is usually the phase shift angle, and the output may be battery voltage or charging current.
Input: Phase shift φ Output: Battery current or battery voltage
This model helps design PI controllers, compensators, and closed-loop control systems.
Basic EV Charging Control Modes
EV battery charging usually follows two main stages:
| Charging Stage | Control Objective |
|---|---|
| Constant Current Mode | Regulate battery charging current |
| Constant Voltage Mode | Regulate battery terminal voltage |
Constant Current Mode
In constant current mode, the charger controls the DAB converter to deliver a fixed charging current to the battery.
This mode is used when the battery state of charge is low or medium.
Measured battery current
↓
Compare with reference current
↓
PI controller
↓
Generate phase shift command
↓
DAB converter transfers power
Constant Voltage Mode
When battery voltage reaches its upper limit, the charger enters constant voltage mode.
In this mode, the output voltage is regulated and the charging current gradually decreases.
Measured battery voltage
↓
Compare with reference voltage
↓
PI controller
↓
Generate phase shift command
↓
DAB converter controls battery voltage
Closed-Loop Control Structure
A practical DAB EV charger control system usually uses cascaded loops:
- Outer voltage loop
- Inner current loop
- Phase-shift generation block
- Protection logic
The inner current loop improves dynamic response, while the outer voltage loop ensures proper battery charging.
Battery Voltage Reference
↓
Voltage PI Controller
↓
Current Reference
↓
Current PI Controller
↓
Phase Shift Command
↓
PWM Generator
↓
DAB Converter
Bidirectional Control for V2G Operation
In vehicle-to-grid operation, the EV battery sends energy back to the grid or DC bus.
The controller reverses the phase shift direction.
| Mode | Power Direction | Phase Shift |
|---|---|---|
| G2V Charging | DC Bus to Battery | Primary bridge leads |
| V2G Discharging | Battery to DC Bus | Secondary bridge leads |
Advanced Modulation Techniques
Although SPS control is simple, it may produce high circulating current under wide voltage variation.
Advanced modulation methods are used to improve efficiency:
- Dual Phase-Shift Control
- Triple Phase-Shift Control
- Extended Phase-Shift Control
- Variable Frequency Control
- Optimal Current Stress Control
- Model Predictive Control
These methods reduce RMS current, expand soft-switching range, and improve light-load efficiency.
Soft Switching in DAB Converter
One major advantage of the DAB converter is its ability to achieve Zero Voltage Switching (ZVS).
ZVS reduces switching losses and improves efficiency.
However, ZVS depends on:
- Load current
- Phase shift
- Leakage inductance
- Device output capacitance
- Voltage conversion ratio
At light load or large voltage mismatch, ZVS may be lost, increasing switching losses.
Design Parameters for EV Charger DAB Converter
| Parameter | Typical Consideration |
|---|---|
| Input DC Bus | 400 V or 800 V |
| Battery Voltage | 200 V to 900 V depending on EV platform |
| Power Rating | 3.3 kW to 22 kW onboard, higher for fast chargers |
| Switching Frequency | 20 kHz to several hundred kHz |
| Transformer | High-frequency isolated transformer |
| Semiconductors | SiC MOSFETs preferred for high voltage |
Simulation Steps in MATLAB, Simulink, PLECS, or LTspice
- Define input DC voltage and battery voltage range.
- Select power rating and switching frequency.
- Choose transformer turns ratio.
- Calculate leakage or series inductance.
- Build primary and secondary full bridges.
- Add high-frequency transformer model.
- Generate 50% duty square-wave gate signals.
- Apply phase shift between primary and secondary bridges.
- Measure battery current and voltage.
- Design current and voltage PI controllers.
- Validate G2V and V2G operation.
- Check transformer current, RMS current, ZVS, and device stress.
Common Mistakes in DAB Converter Design
- Selecting incorrect transformer turns ratio
- Ignoring leakage inductance design
- Using SPS control for very wide voltage range without optimization
- Not checking circulating current
- Ignoring light-load efficiency
- Not verifying ZVS range
- Poor transformer thermal design
- Improper battery current control
- No protection for overcurrent and overvoltage
Protection Requirements
A practical EV charger DAB converter must include:
- Battery overvoltage protection
- Battery overcurrent protection
- DC-link overvoltage protection
- Transformer saturation protection
- Overtemperature protection
- Short-circuit protection
- Isolation monitoring
- Emergency shutdown logic
Applications of Bidirectional DAB Converter
- EV Onboard Chargers
- EV Fast Charging Stations
- Vehicle-to-Grid Systems
- Battery Energy Storage Systems
- DC Microgrids
- Solid-State Transformers
- Renewable Energy Interfaces
- Data Center DC Power Systems
- Aircraft Electrical Power Systems
Modern Research Trends
- SiC-Based High-Frequency DAB Converters
- GaN DAB Converters for High Power Density
- Triple Phase-Shift Optimization
- AI-Based Charging Control
- Model Predictive Control for DAB
- Multiport DAB Converters
- Integrated High-Frequency Transformers
- Solid-State Transformer Based EV Charging
- Digital Twin Based Battery Charging Control
- V2G and Grid Support Services
Frequently Asked Questions (FAQs)
Why is DAB converter used in EV charging?
DAB converters are used because they provide bidirectional power flow, galvanic isolation, high efficiency, high power density, and simple phase-shift control.
How is power controlled in a DAB converter?
Power is controlled by changing the phase shift between the primary and secondary full-bridge voltages.
What is the role of transformer leakage inductance?
Leakage inductance acts as the main energy transfer element and controls current ripple, power transfer, and soft-switching behavior.
Can a DAB converter support V2G?
Yes. By reversing the phase shift direction, power can flow from the EV battery back to the grid or DC bus.
Which control method is simplest for DAB?
Single Phase-Shift control is the simplest method, but advanced phase-shift methods improve efficiency over wide voltage ranges.
Key Takeaways
- DAB is one of the most suitable bidirectional isolated DC-DC converters for EV charging.
- Power transfer is controlled by phase shift between two full bridges.
- The transformer provides galvanic isolation and voltage matching.
- Leakage inductance is essential for power transfer.
- Constant current and constant voltage modes are required for battery charging.
- V2G operation is achieved by reversing the power flow direction.
- Advanced modulation improves efficiency under wide battery voltage variation.
- SiC and GaN devices improve DAB converter power density and efficiency.
Conclusion
A Bidirectional Dual Active Bridge converter is a powerful and flexible topology for modern EV charging systems. It provides isolated bidirectional power transfer, high efficiency, high power density, and compatibility with both grid-to-vehicle and vehicle-to-grid operation.
Modeling a DAB converter requires understanding its switching behavior, averaged power transfer equation, transformer leakage inductance, and small-signal control dynamics. Controlling the converter mainly involves regulating the phase shift between the two bridges to achieve constant current charging, constant voltage charging, or reverse power flow.
As EV charging infrastructure moves toward higher voltage, faster charging, and bidirectional energy exchange, SiC and GaN based DAB converters will continue to play a major role in next-generation power electronics systems.
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