SiC die target low power AC-DC flyback converters

SiC die target low power AC-DC flyback converters

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By Nick Flaherty

The normally-on SiC JFETs range from 650 V to 1700 V and enable simplified start-up implementation with zero standby dissipation and are designed for the large cascode-based flyback AC-DC applications market, including consumer adapters and auxiliary power supplies.

Controller chip makers can use the small die sizes with very low RDS(ON) and capacitances of the JFETs to meet light and no-load dissipation regulations when coupled with a low Qg low voltage MOSFET integrated in the controller. The die are avaiable on wafers to integrate with controllers and simplify the co-packaging process.

SiC cascodes are extremely robust, due the inherent capability of SiC JFETs when it comes to handling repeated avalanche and short circuits. Since the SiC JFET is in series with the low voltage MOSFET in the controller, the source of the normally-on JFET rises to 12 V before JFET turns off and the IC begins switching. This current path through the JFET can be used as a start-up supply for the controller IC. An auxiliary supply from the converter transformer is then gated-in when the converter starts running, with no further dissipation. 

Typical low-power flyback applications include consumer electronics chargers such as laptop and mobile device chargers (from 20 to 65 W). Other applications range from wide-input (up to 1400 V) Flyback auxiliary supplies for industrial applications such as motor drives to high power lighting applications such as long LED chains.  “With the addition of these new SiC JFETs, UnitedSiC now has one of the largest SiC power portfolios in the industry,” said Chris Dries, CEO of UnitedSiC. “We now have high-performance JFET functionality in both die and discrete package form.” 

A total of seven die are currently available in wafer form only with voltage ratings from 650 V to 1700V and RDS(ON) values down to 140 milliohms, in three die sizes down to 0.8 x 0.8mm for ease of co-packaging.

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