Design Review : 140 W , Multiple Output High Density DC / DC Converter

This seminar topic reviews the most common solutions used in multiple output power supply designs. In the focus of the detailed design review is the current-fed push-pull converter, one of the most cost effective solution available for the practicing designer. A step-by-step design procedure will guide the reader through the most important decisions and their effect on the ultimate performance of the circuit. Measurement results, in particular cross-regulation data without using any post regulation technique concludes the topic. INTRODUCTION Designing multi-output converters presents a remarkable challenge for the power supply designer. There is no comparable power supply design task where specification, topology selection and the choice of output voltage regulation technique would have a more profound effect on the performance, cost and complexity of the final circuitry . Converters utilizing a single primary power stage and generating more than one isolated output voltage are called multi-output converters. These circuits provide isolation and several output voltages using one high frequency isolation transformer as opposed to individual power modules for each output as it is frequently done in distributed power systems. The decreased system complexity of the multi-output approach is realized primarily on the account of output voltage regulation accuracy. Several approaches have been explored for multi-output converters to address complexity, overall efficiency, output voltage and cross-regulation issues. These circuit solutions can be divided into two radically different groups. The first group of multi-output converters regulate their main output, from which the feedback signal is generated, with very good accuracy. Their auxiliary outputs are regulated with lesser accuracy since they are not part of a closed loop regulation scheme. These solutions tend to be the most economical ones but their application can be limited because of their inadequate output voltage accuracy. The best known topologies of this category are the flyback, forward and current or voltage-fed push-pull converters. The second circuit family provides a remedy for those applications where tight voltage regulation of each output is necessary .These approaches can be based on almost any topology since they are utilizing some sort of post regulation technique to regulate the auxiliary outputs of a multiple output converter. Accordingly, these solutions are differentiated by the post-regulation technique they are employing. Post regulation of the auxiliary outputs can be achieved by linear post-regulators, individual DC!DC converters, magamp circuits or switchmode S.econdary S.ide p-ost Regulators (SSPR). All of these techniques are based on closed loop output voltage regulation of their respective outputs. Before reviewing these power supplies a little bit closer, Figure 1 can be used to position the different solutions on the Cost/Complexity vs. Performance plane. For simplicity and for fair comparison, a three output forward converter is used where it was possible. In each case, it was assumed that the main output is regulated directly by the PWM controller of the power supply. The performance scores of the different solutions were based on expected efficiency and output voltage regulation characteristic. The cost/complexity score was determined by the number of power components (MOSFET and diode switches, inductive components, energy storage capacitors), by the number of integrated circuits required and by the number of control loops employed in that particular solution.