Back-to-back MOSFET switches to reduce the losses in SCALDO implementation

In today's portable devices, major DC-DC topologies are mixed and matched to provide multiple DC rails to power point of load (POL) rails. The traditional high frequency DC converter approaches to achieve high efficiency have their strengths and draw backs, and none of them could be considered ideal solution to energy efficiency and battery run time issues of modern portable devices. Supercapacitor assisted low dropout regulators (SCALDO) is a novel linear regulator topology developed by combining an array of supercapacitors, an LDO and a very low frequency switching arrangement to achieve high efficiency in linear converter comparable with high frequency switching converters. The technique implemented in commonly used DC-DC converter stages such as 12-5V, 5-3.3 V and 5-1.2 V with efficiency improvement factors of 2, 1.33 and 3 respectively, with the example of increasing the 42% efficiency of a 12-5 V linear converter to 84% theoretically. In these early prototype versions, solid state relays were used as the low frequency switches to eliminate the unnecessary discharging of the supercapacitors (SC), however, high on resistance in these switches increase the switch losses. This paper provides the details of a new design technique to eliminate this SC discharge issue and to further reduce the switch losses by using back-to-back MOSFET switches, together with performance details of 12-5 V practical converter.