Traditional switching power supplies generally operate at frequencies of about 1 MHz or less. But new designs are pushing frequencies higher in the quest to cram power supply circuits into smaller and smaller volumes.
An indication of the trend can be seen at the Power Electronics Research Group within the Massachusetts Institute of Technology. There researchers are working on power supplies switching at speeds between 30 MHz and 300 MHz. To do so, the supplies rely on a special design that goes beyond such traditional topologies of boost converters, buck converters, and buck-boost converters.
The VHF prototype LED driver designed by MIT’s Power Electronics Research group.
For example, a group organized by Power Electronics Research Group head David Perreault is working on a prototype LED lamp driver that works at frequencies high enough to use ceramic capacitors for energy storage instead of electrolytic devices. Perreault says the prototype LED driver hits a power factor better than 0.7, the level recommended for LED drivers by the EPA Energy Star program. The driver also provides a power density five to 10 times that of current commercial systems, Perreault says.
The power conversion techniques developed at MIT have also been commercialized in what is billed as the world’s smallest laptop power adapter supply. The device comes from MIT spinoff company FINsix. Said to be shipping this fall, FINsix’s 65-W Dart adapter weighs a mere 2 oz and looks a little like an over-sized electrical plug.
The techniques MIT researchers use to devise power supplies operating at VHF frequencies become clearer from patents recently granted for the group’s work. One such circuit includes a reconfigurable switched capacitor transformation stage coupled to a magnetic converter (or regulation) stage. The dc-dc converter described in the patent can be used to power logic devices in portable battery operated applications, for example, or to power digital circuits with dynamic voltage scaling, or where wide output voltage ranges are commonly required.
The FINsix 65-W laptop power supply adapter.
The patent points out that conventional power converters must typically employ semiconductor switches rated for voltages roughly that of the input voltage. These relatively high-voltage blocking devices are inherently slower than lower-voltage devices and suffer from a higher on-state resistance or large gate capacitance. Both reduce overall efficiency. By working at higher frequencies, the group’s design avoids these difficulties.
The first stage of their power converter circuit includes a reconfigurable switched capacitor network that produces an intermediate voltage smaller than the input voltage and which varies over a much smaller range (ratio) than the input voltage. Basically, the switches allow capacitors in the network to charge up from the input, then switch the capacitors so they discharge into the next stage, called a regulation stage. The regulation stage sees a voltage substantially lower than the switched-capacitor circuit input voltage, so it can use fast, low-breakdown semiconductor switches. Making the apparent input resistance of the auxiliary converter higher than the ESR of the switched-capacitor and the switch resistances lets the circuit recover a majority of the capacitor-charging energy.
The first stage of their power converter circuit includes a reconfigurable switched capacitor network that produces an intermediate voltage smaller than the input voltage and which varies over a much smaller range (ratio) than the input voltage. The regulation stage sees a voltage substantially lower than the switched-capacitor circuit input voltage, so it can use fast, low-breakdown semiconductor switches.
The transformation stage and regulation stage each include two or more switches. The switches in the regulation stage are selected to operate at a switching frequency which is higher than the switching frequency of the switches in the transformation stage. This idea augments the efficiency, power density and control bandwidth of the converter, the designers say. For a given switching frequency at small size scales, the power density of this sort of switched capacitor converter can be much higher than that of a magnetic converter.
The whole circuit can be configured with CMOS switches on a CMOS chip with discrete or integrated storage elements.
