Detection and Control of DC Bias Flux in Dual Active Bridge Medium Frequency Transformers

This paper introduces a novel method of DC bias flux detection in the medium frequency transformer (MFT) of high power dual active bridge (DAB) converters. First, the sources of DC bias flux in DAB MFTs are discussed. Previously proposed approaches to measure bias flux are shown to be invasive to the MFT design and/or add an excessive amount of volume/mass to the converter. Experimental measurements of nanocrystalline magnetic cores show that the non-linearity of the core produces non-linear mixing of the odd harmonics in the excitation voltages to produce measurable even current harmonics. These even harmonics can be related directly to the magnetic flux bias in the core and therefore measurement of even current harmonics offers a path to higher-power density, less invasive measurement of bias flux. Sensor requirements are discussed and switch-averaged equations for PWM duty cycle to DC bias are provided to facilitate the design of closed loop bias controllers. Finally, hardware verification of closed-loop DC flux control is performed using a 20kW DAB converter.