2. Background
Switching regulators have long been the workhorse of efficient conversion of
DC power rails to lower or higher voltages, either directly for the load or as part of a distributed power architecture. The first designs from the 1950s used vacuum tubes and showed dramatic improvements in conversion efficiency compared with the alternative ‘linear regulator’ approach and also opened up the possibility of boosting DC voltages, only practical previously with unwieldy mechanical ‘vibrators’. It was only in the 1970s that the first switched-mode power supply IC controller appeared, the Silicon General SG1524, using ‘voltage mode’ control.
The success of this device opened the floodgates to alternatives using different control and conversion techniques. Today, there are many alternative topologies and control methodologies; from synchronous vs asynchronous, minimum on time vs minimum off time, averaged vs cycle-by-cycle current control or fixed frequency vs variable frequency designs with pulse-skipping for light loads, to name just a few examples. Although each topology has its own advantages and disadvantages, the selection and specification of the main power stage components; the switching transistor, the inductor and the output capacitor are still often key to achieving the optimum performance and the lowest possible losses under all operating conditions.
A measure of the development of the switching regulator is its conversion efficiency - over the years, the figures have been steadily climbing from around 70-80% to 97% and higher in the latest designs. Higher efficiency allows a
higher power density, measured in watts/volume, representing how much power can be delivered from a given size without the converter overheating. Much of this improvement in power density is due to increasing integration; the low RDS(ON) switching transistor(s) are commonly integrated alongside the controller die in the same package and increasingly the inductor is also on-board. Peripheral functions such as fault monitoring, current sharing, synchronisation and sequencing have also been increasingly swept into the controller IC design. In addition, the switching frequencies are moving upwards from a few hundred kilohertz up to 1MHz or higher. Increasing the switching frequency allows smaller components to be used, but at the cost of increased power dissipation and higher EMI levels. The challenge is to find the optimum balance between physical size, thermal performance and EMC filter costs.