Dual Active Bridge Converter with Phase-Shift Control

The problem

Bidirectional isolated converters are needed for vehicle-to-grid capable chargers, solid-state transformers and DC fast-charger modules. The DAB is attractive because it is symmetrical, isolated and soft-switching in part of its range, but when the voltage ratio departs from the turns ratio, as it does across an EV battery's charge range, single-phase-shift control loses soft switching and circulating current rises. Improving this is an active research area.

Objectives

• Derive and verify the DAB power-transfer equation for single-phase-shift control.
• Build a switching model with two full bridges and a high-frequency transformer.
• Map ZVS range and RMS current over power and voltage ratio.
• Implement an extended phase-shift scheme and compare it with single phase shift.
• Close a battery current loop and test bidirectional steps.

What you need

Key concepts

Single-phase-shift power transfer

P = n·V1·V2·φ·(π − |φ|)/(2π²·fs·L), with φ the phase shift in radians between bridges, n the turns ratio (secondary referred to primary) and L the series inductance.

Voltage conversion ratio

d = V2'/V1, with V2' the secondary voltage referred to the primary; when d = 1, single phase shift achieves ZVS over a wide range with low circulating current.

Extended phase shift

Adding an inner phase shift within one bridge creates a three-level voltage, giving an extra degree of freedom to reduce circulating current when d differs from 1.

Circulating (reactive) current

Current that flows back and forth without transferring net power, increasing conduction loss and component stress.

Step-by-step guide

1. 1

   Define the example application

   Choose labelled example values: a 400 V DC bus on one side and a battery of 250–420 V on the other, 10 kW in both directions, 100 kHz switching, transformer turns ratio 1:1. Set targets for ZVS range and efficiency at nominal conditions, and define the operating grid of power and battery voltage to be studied. These values are examples for the study only.

   Checkpoint: A specification and an operating grid ready for scripted sweeps.

2. 2

   Size the series inductance

   From the power equation, choose L so that rated power is reached at a phase shift around 30–45 degrees at the minimum battery voltage, leaving margin. Compute the RMS current and peak current at the corners of the operating grid analytically, and plot the theoretical ZVS boundaries for single phase shift. Note the inductance tolerance you assume and check that rated power is still reached at the low end.

   Checkpoint: Script prints L and a theoretical ZVS map over the operating grid.

3. 3

   Build the switching model

   Build two full bridges with Simscape Electrical MOSFETs including output capacitance, a series inductor and a transformer with magnetising inductance, a DC source on the bus side and a battery or DC source with series resistance on the battery side. Use a fixed-step local solver around 10–20 ns and include dead time. Log inductor current, both bridge voltages and average power on each side.

   Checkpoint: At a fixed phase shift, transferred power matches the equation within a few per cent.

4. 4

   Verify the power equation over the grid

   Script sim() runs over phase shift and battery voltage and compare measured average power with the equation. Plot inductor current waveforms at d < 1, d = 1 and d > 1 to show how the current shape and its value at switching instants change. Any systematic gap between measured and predicted power usually comes from dead time or device drops; quantify it and decide whether the equation needs a correction term for your design.

   Checkpoint: Measured versus predicted power plot lies close to the unity line across the grid.

5. 5

   Map ZVS and RMS current

   For each grid point, check the sign of the inductor current at every switching instant to determine whether each switch turns on with ZVS, and confirm by inspecting drain-source voltage on selected cases. Compute RMS inductor current. Plot ZVS regions and RMS current maps for single phase shift. Save the maps as data files so that the extended phase-shift results in later steps can be plotted as differences against them on identical axes.

   Checkpoint: Simulated ZVS map agrees with the theoretical boundaries from step 2.

6. 6

   Implement extended phase shift

   Add an inner phase shift on the bridge facing the higher reflected voltage. Derive or compute the power equation for this mode, and for each operating point search for the combination of inner and outer shift that delivers the required power with minimum RMS current, using a grid search or fmincon. Store the result as lookup tables. Check that table entries vary smoothly between neighbouring points.

   Checkpoint: Extended phase-shift tables deliver the required power at every grid point with lower RMS current where d differs from 1.

7. 7

   Compare modulation schemes

   Run both schemes across the grid and compare RMS current, ZVS coverage and estimated losses from example device data. Present maps of the difference. Identify where extended phase shift brings most benefit, typically at light load and at battery voltages far from the bus voltage. Use the same device data, dead time and loss expressions for both schemes, so that the comparison reflects modulation only, and quote losses at nominal and extreme battery voltages.

   Checkpoint: Loss and ZVS comparison maps with the main benefit regions identified.

8. 8

   Close the current loop and test

   Implement a PI controller on battery current whose output is the phase-shift command, feeding either the single or extended phase-shift tables. Test steps from full charge to full discharge and back, and at different battery voltages. Check stability across the operating range, since the plant gain varies with phase shift. If the response varies too much, add gain scheduling by phase shift and show the improvement on the same test sequence.

   Checkpoint: Battery current tracks bidirectional steps with consistent response across battery voltage.

9. 9

   Research write-up

   Document the analysis, model, ZVS and RMS maps, the modulation optimisation and the closed-loop tests. Release scripts where possible. State limitations such as linear magnetics and no core loss, and identify how the results would be checked on hardware. Compare your results with published DAB studies of similar ratings, explain any differences, and set out clearly what is new, for example a wider ZVS range over the full battery charge window.

   Checkpoint: A thesis chapter draft with reproducible maps and a clear comparison of modulation schemes.

Testing & validation

• Power-transfer equation verified over the operating grid.
• Simulated ZVS map compared with theory.
• RMS current reduction from extended phase shift quantified.
• Loss comparison using device data.
• Closed-loop bidirectional current steps across battery voltage.

Expected results

A verified DAB model with mapped soft-switching and circulating current under single and extended phase-shift control, a quantified benefit of extended phase shift, and a working bidirectional current controller.

Take it further

• Add dual phase shift or triple phase shift and compare with extended phase shift.
• Include transformer core loss and optimise switching frequency.
• Add a DC bus voltage loop for grid-tied V2G operation.
• Study the effect of dead time on power transfer at light load.

Viva and review questions

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