New designs for switch-mode power supplies are pushing frequencies higher as a way of cramming circuitry into smaller and smaller volumes. An indication of the trend can be seen from the recently developed Dart power supply from Finsix, a start-up with roots in the Massachusetts Institute of Technology. The Finsix device fits in the palm of your hand, but puts out 65 W, as much as a conventional brick power supply for laptops. It includes a USB connection so it can power phones as well as a computer.
The key to its small size is its switching power supply operating at VHF switching frequencies—around 100 MHz according to company literature. In comparison, ordinary switching supplies usually work in the range of tens of megahertz. The use of higher frequencies reduces the size of the circuit components involved.
VHF-range switching supplies are a hot research topic, but usually involve wide bandgap SiC or GaN power devices. However, company officials said the Dart uses only silicon power transistors.
Finsix said it uses a fully-resonant Class-Φ2 converter topology. A resonant converter is basically a switching supply that incorporates a resonant L-C circuit as a part of the power conversion process. These converters operate by putting energy into the resonant circuit and then transferring some or all of it into the load.
The Class-Φ2 converter topology, which does not originate with Finsix, was inspired by RF power amplifiers, which use resonant harmonic peaking of their input or output network to reduce the peak voltage on the semiconductor switch. But RF amp topologies can’t be borrowed unchanged to make power supply circuits; their switching devices are typically never 100% on or off, so they tend to make inefficient power supplies. Class-Φ converters address the efficiency problem by putting a high-order lumped network in the supply input to shape the waveform in a way that reduces the stress on the switching device.
A problem is that these inverters use high-order resonant structures with many energy storage components that make the circuitry complicated. The Class-Φ2 is a simplified version of a Class-Φ inverter, which, among other things, absorbs switching device capacitance into the wave-shaping network. This lets the supply put out the same amount of power regardless of changes in the device output capacitance. The components of the inverter are tuned to get a low peak voltage across the semiconductor switch and to allow near zero voltage at switch turn-on and turn-off. Judicious choice of energy storage components can yield a zero dv/dt across the switch at turn on, desirable for operating at VHF-UHF frequencies.
A point to note is that resonant converters are characterized by a storage of energy in the device parasitic capacitance. The energy moves to a complementary reactance over each cycle so that most of the energy gets recovered. This results in only a small energy loss per switch transition, so the resulting power supplies are energy efficient.
The Finsix founders came out of the Power Electronics Research Group (PERG) within MIT. Work on high-frequency supplies continues there as well. For example, a group organized by PERG head David Perreault is developing a prototype LED lamp driver that operates at frequencies high enough to use ceramic capacitors for energy storage instead of electrolytic devices. Perreault said 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, he said.
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