A dual active bridge high frequency transformer transfers bidirectional power between two switched bridges while also contributing leakage inductance to the power-transfer path. Turns ratio, leakage, phase shift, dead time, switch capacitance, flux balance, insulation, and AC winding loss must be coordinated with the control strategy.
Map both power-flow directions
Provide both DC bus ranges, rated and overload power in each direction, switching frequency, modulation method, phase-shift range, dead time, startup sequence, light-load mode, and fault states. The highest RMS current, highest peak current, and worst volt-seconds may occur at different operating points. A design checked only in the preferred direction can miss thermal or saturation limits during reverse power flow.
Select turns ratio from the real gain window
The nominal ratio should place the converter near an efficient operating region across both bus ranges. Semiconductor drops, dead time, leakage-related commutation, and modulation limits reduce the ideal voltage transfer. A ratio that is attractive at nominal voltage may require excessive phase shift or circulating current at an extreme.
Control leakage inductance intentionally
Leakage inductance helps set transferred power and current slew. It may be integrated into the transformer, supplemented with an external inductor, or split between both. Too little can create high peak current for a required phase shift; too much can narrow the operating range and raise conduction loss. Integrated leakage depends on winding placement, margins, interleaving, and production tolerance, so its measurement connection and test frequency belong on the drawing.
Prevent DC flux bias
Unequal bridge timing, device voltage drops, gate delays, or control offsets can create net volt-seconds. Repeated imbalance walks the core toward saturation. Verify primary and secondary current symmetry, blocking or protection strategy, and behavior during startup, mode changes, current limit, and a lost gate pulse.
Balance AC loss and winding capacitance
Square-wave excitation and high RMS current make skin and proximity effects important. Interleaving reduces leakage but raises primary-secondary capacitance and common-mode current. Foil, litz, PCB, and sectional windings each have different loss, capacitance, insulation, and manufacturability tradeoffs. Thermal testing should include both power directions and light load, where circulating current may remain significant.
Insulation sees fast bipolar edges
Define working voltage, reinforced or basic insulation, creepage, clearance, partial-discharge needs, common-mode voltage, altitude, and grounding. The transformer shield strategy must match the system reference points on both bridges.
Prototype verification checklist
- Turns ratio, polarity, magnetizing inductance, and leakage inductance
- Power transfer and RMS current in both directions
- ZVS range, dead-time sensitivity, and circulating current
- Primary and secondary flux symmetry
- Core, winding, gap, and terminal temperature
- Primary-secondary capacitance and common-mode EMI
- Dielectric strength and insulation distances
BaoHui Tech can review a high frequency transformer using bus ranges, measured waveforms, leakage target, insulation, thermal limits, and mechanical envelope. Early coordination lets the transformer manufacturer align magnetic tolerances with the DAB control model.
Frequently asked questions
Is transformer leakage always enough for a DAB?
No. An external inductor may provide tighter and more independent control when transformer leakage tolerance is too sensitive.
Why check light-load operation?
Soft-switching energy and circulating current change, potentially increasing switching loss or losing ZVS.
Does bidirectional operation require two transformer ratings?
Use one complete rating envelope that states power, voltage, current, and thermal requirements for both directions.