UnitedSiC Announces SiC JFET Family for Low Power AC-DC Flyback Converters

March 28, 2019 by Gary Elinoff

These normally-ON devices are well-suited to form the nucleus of cascode-based 20-100 Watt flyback products.

These normally-ON devices are especially well-suited for co-packaging with controller ICs and low voltage MOSFETs to form the nucleus of cascode-based 20-100 Watt flyback products.

The new silicon carbon (SiC) JFETs consist of a series of seven wafer form dies with voltage ratings from 650 V to 1700V and RDS(ON) values down to 140 milliohms. To make co-packaging easier, they are available in three die sizes down to 0.8 x 0.8mm. The devices, members of the UF3N series, have sister products available in TL-347-TL packages.


Image courtesy UnitedSiC.


UnitedSiC CEO Chris Dries notes that the addition of these new SiC JFETs grows their already-large SiC power portfolio. He states, “We now have high-performance JFET functionality in both die and discrete package form.”

The Advantages of Silicon Carbon Cascodes

Because of the inherent capability of SiC JFETs when it comes to handling repeated avalanche and short circuits, SiC cascode pairs are extremely robust. Since the SiC JFET is in series with the LV MOSFET in the control IC, the source of the normally-ON JFET rises to 12 V before JFET turns off and the IC begins switching.

The salient points are illustrated in the diagram below, employing one of the new members of the U3FN series available in wafer form, the UJ3N1704K3S:


SiC JFET enables simple and efficient start-up. Image courtesy UnitedSIC.


The current path through the JFET (Q2) 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.

Flyback Converter Applications

Flyback converter 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.

Circuit Protection Applications

JFETs which are normally-ON, meaning they operate with zero gate-source voltage, can be very useful in circuit protection applications. The normally-ON characteristic is a benefit in protection circuits such as current limiters and solid-state circuit breakers where a default to the device conducting is preferred.

Silicon Carbide (SiC) JFETs are particularly useful in these applications with their extreme tolerance to high peak junction temperatures and low channel resistance for a given die area. With well-defined saturation current and very fast switching, they can be used in applications such as lightning protection, inrush current limiting and as a replacement for slower mechanical circuit breakers. 

What do you think of the use of silicon carbide in low power applications? Let us know your thoughts in the comments.