Cambridge-based power chip start-up CamSemi has revealed its first product, a controller for off-line power supplies between six and 40W. It does not use the firm’s flagship ‘PowerBrane’ MEMS transistor.
“When we went to potential customers, they were as interested in the control technology as they were the PowerBrane,” CEO David Baillie told Electronics Weekly. “As a start-up it was in our interest to get a product to market as soon as possible, so we delivered the same technology with a very cheap external bipolar junction transistor.”
Unusually for low-power off-line switching supplies, which are generally flyback converters, CamSemi’s topology is flyback, and resonant which inherently cuts high-frequency EMI.
“Resonance has not been popular recently, certainly with low power,” said v-p of business development John Miller. “Our sophisticated control and the topology allows us to use a bipolar transistor that is a third of the cost of an equivalent Mosfet and keep it within its safe operating area when operating at high voltage.”
Resonant converters have inherently low switching losses, even with bipolar switch which is slower than a Mosfet. The firm is claiming over 80 per cent efficiency between 2W and 7W for an example design. No load dissipation is a creditable 100mW.
Being a forward converter with no feedback, the output voltage is not regulated, but a fixed ratio of the mains voltage, much like a conventional transformer. “The characteristics of a transformer are what our customers are looking for,” said CEO Baillie. “At output power ratings above 6W it is cheaper than a transformer supply.”
The chip, dubbed C2470, is made in a conventional 3.3V double-poly CMOS process and the SO23-6 part is $0.45. It is in full production having been designed-in by development partners, said Baillie.
PowerBrane is a novel 700V insulated gate transistor made on a 30V silicon-on-insulator wafer by micromachining away the substrate under the device.