DC-bias Cancellation for Phase Shift Controlled Dual Active Bridge

The dual active bridge topology allows bidirectional power flow and galvanic isolation for DC/DC energy conversion. These features have made it the possible backbone of the future smart transformer for distribution. The different voltage drops and commutation dead-times of the semiconductor switches result in DC-voltage at the transformer terminals. Even small DC-voltage components produce large DC-bias currents as they are only limited by the transformer resistances. The DC-bias degrades the transformer performance by increasing the losses. If the core saturates the resulting current pulses can damage the converter. A typical approach to avoid the DC-bias is placing a capacitor in series with the transformer. This capacitor suffers large current variations, reducing its reliability, and complicates the control. The dual active bridge usually handles the power flow by modifying the phase-shift of the converter square waveforms. In this paper the duty-cycle of the converter waveforms is controlled to cancel the current DC-bias in both transformer sides. A formula relating the reference power and phase-shift is provided for the case of varying duty-cycle waveforms. The implementation with constant sampling frequency and its limitations are explained.