Analysis of Analogue Current and Flux Balancing for the Dual-Active-Bridge Converter

Inhalt:
The dual-active-bridge converter (DAB) is a popular structure found in the electric vehicle (EV) environment. Built around two full-bridges located on the primary and the secondary sides of the converter, the DAB lends itself well to controlling high-voltage battery charging up to several tens of kilowatts. Galvanic isolation with voltage scaling is obtained using a power transformer and an external inductor which ensures zero-voltage switching (ZVS) under certain conditions. Several modulation strategies exist but the single-phase approach, in voltage-mode control, represents a popular choice owing to its ease of implementation. In this mode, both full-bridges operate with 50% duty ratio, applying symmetrical voltages of equal duration across the transformer windings. Unfortunately, ohmic losses and timing errors affect the transformer magnetic operating point and can cause drift, leading to undesirable consequences: a dc-blocking capacitor is typically installed in the circuit to prevent flux runaway and transformer saturation. While this option ensures safe magnetic operation, the size and the cost of the dc-blocking capacitance can add significant cost to the bill of materials and affect longterm reliability. Considering the high ripple current circulating in this component, designers are often constrained to use many discrete parts connected in parallel. Equal current sharing across these discrete capacitors must be through appropriate PCB layout and reliable assembly. This paper describes a solution to remove the dc-blocking capacitance by monitoring the circulating currents and altering the bridges duty ratio in case of imbalance. Originally proposed as a digital implementation in [1], this document explores a fully analogue method.