Current Sharing Method for DC-DC Transformers

An ever present trend in the power conversion industry is to get higher performance at a lower cost. In a computer server system, the front-end converter, supplying the load subsystems, is typically a multiple output power supply. The power supply unit is custom designed and its output voltages are fully regulated, so it is not very efficient or cost effective. Most of the load systems in this application are supplied by point-of-load converters (POLs). By leaving the output voltage regulation aspect to POLs, the front-end converter does not need to be a fully regulated, multiple output converter. It can be replaced by a dc-dc transformer (DCX), which is a semi-regulated or unregulated, single output dc-dc converter. A DCX can be made using a modular design to simplify expansion of the system capacity. To realize this concept, the DCX block must have a current sharing feature.

The current sharing method for a resonant DCX is discussed in this work. To simplify the system architecture, the current sharing method is based on the droop method, which requires no communication between paralleled units. With this method, the current sharing error is inversely proportional to the droop voltage. In traditional DCX implementations, the droop voltage depends on the resistive voltage drops in the power stage, which is not sufficient to achieve the desired current sharing error. The resonant converter has the inherent characteristic that its conversion gain depends on the load current, so the virtual droop resistance can realized by the resonant tank and the droop voltage can be obtained without incurring conduction loss. An LLC resonant converter is investigated for its droop characteristic. The study shows the required droop voltage is achievable at very high switching frequency. To lower the switching frequency, a notch filter is introduced into the LLC resonant tank to increase the sensitivity of the conversion gain versus the operating frequency. The design of the multi-element resonant tank is discussed. Depending soly on the resonant tank, the droop characteristic is largely varied with the component tolerance in the resonant tank. The current sharing error becomes unacceptable. The active droop control is imposed to make the output regulation characteristic insensitive to the component tolerance. The proposed resonant DCX has simpler circuit structure than the fully regulated resonant converter. Finally simulation and experimental results are presented to verify this concept.