Design of a Zero voltage switching Flyback synchronous rectification controller for high voltage applications

Abstract

The recent launch of high voltage switching devices of new semiconductor materials such as Silicon Carbide, have shown better performance. This makes it possible to use the single stage Flyback converter for low to moderate power, high input voltage applications, which is attractive due to its inherent advantages of low cost and simplicity. However, the losses in the Metal-Oxide-Semiconductor-Field-Effect Transistor (MOSFET) are aggravated due to the high input voltage, which decreases the efficiency. Additionally, the losses in the secondary diode can be considerable, thus deteriorating the efficiency further. With the current trend towards high power density converters, the replacement of through hole component with surface mounted technology (SMT) components for the active devices is often the solution, under the condition that they require minimal thermal management i.e. using SMT components with minimal footprint without heat-sinks. In the light of thermal management it is important to reduce the losses in the active components, thereby effectively decreasing the generation of heat. Therefore, the objective of this thesis is to design a high power density 40W flyback converter by reducing the losses in the active components. First the nature of the losses are identified theoretically and afterwards the results are verified experimentally. It was found that the switching losses in the primary MOSFET and the conduction losses in the secondary diode are the major loss contributors. To reduce both these losses a synchronous rectification controller was built which ensures zero voltage switching (ZVS) by making the converter bidirectional. The experimental results show that for loads greater than 0.5 Inominal,, the losses in the active components are reduced effectively to levels which allow the use of SMT component with minimal footprint. For light load conditions the converter showed erratic behavior due to the limited blanking time of the primary controller. To address this issue, a solution was proposed which has proven to work successfully for the voltage range of 400V-600V. Under a few special condition for voltages higher than 600V, the proposed solution has shown some erratic behavior, which is expected to be a result of the control loop stability.