A study on low-power high-voltage step down AC – DC converters

One of the main topics of nowadays research is the efficiency improvement of var- ious electronic systems, powered with a rechargeable battery or directly connected to the main power grid. This interest should be supported by the will to abandon energy generation methods typical of the past two centuries, like non-renewable sources, first among everything coal and liquid or gaseous fossil fuels. These sources have reached their life cycle end and damaged the environment. For these reasons the world is en- couraging the use of energy derived form renewable sources, moreover reducing losses is also a main concern. Following this trend, more stringent requirements have been developed like the U.S. Energy Star or the European Code of Conduct (ECoC), which apply also to power supplies. Due to the increasing number of consumer electronics, the aforementioned standards have been starting to consider not only the efficiency of a device in maximum load conditions, but also in no load or light-load ones. These requirements involve more effort, for hardware engineers, in the design steps. The scope of this work is to design and simulate a circuit able to convert power from the main power grid (AC domain) to a much lower constant voltage (3.3 V ) and which could provide up to one hundred milliamps in full load condition, at the same time a comparison between different integrated circuits is required. The most recent of them have been searched on different product selector guides of various manufacturers, in order to obtain a proper comparison in terms of availability and marketability. Among all the topologies used, the most simple and suitable for the required specifications has been chosen: a non-isolated buck converter. Its working principle has been described in detail, as well as its control system. A device, in order to be sold on the market, should comply important standards; one of the most characteristic, for a switching mode power supply (SMPS) like the chosen buck, is the electromagnetic compatibility (EMC). The device is required to not generate too much electromagnetic noise, both radiated and conducted. The first one is mainly solved by proper placement of components and suitable enclosure, whereas the second one is dealt with an electromagnetic interference (EMI) filter. In the end, also this filter has been discussed, designed and simulated in LTSpiceXVII. Among all the possible integrated circuits founded, the first choice has been the VIPer01 from STMicroelectronics. Its internal structure has been discussed, a buck converter designed with it, simulated in different working condition and then the effi- ciency has been computed at various loads. In order to compare different circuits and find better results in terms of efficiency, an additional research has been conducted focusing attention on buck input voltage reduction methods. The previous research result led to the use of a capacitive voltage divider. This approach not only provides a reduction of stress over components and switching losses, but increases also the number of possible integrated circuits that can be used. In the end, various devices have been simulated under similar working conditions, their efficiencies compared as like as their overall cost and number of components. As expected the best result has been obtained with a synchronous non isolated buck converter. It has been shown how a good research and proper selection of components leads to good results and efficiency improvements.