rlcZONE Products for the week of October 29, 2007

Cree Says...

Zero Recovery Rectifier Boosts Efficiency in Computer Power Supplies

Cree, Inc., a market leader in silicon carbide (SiC) power semiconductors, announces release of a new 8-amp, Zero Recovery rectifier that significantly increases power-supply efficiency in computer servers. The new CSD08060 Schottky diode extends Cree's leadership in rectifiers that save energy while boosting power-supply performance.

The CSD08060 8-amp SiC rectifier also expands Cree's Zero Recovery product line, which includes 600-V diodes with 1A, 2A, 4A, 6A, 10A, and 20A ratings. Compared with traditional silicon-based diodes, Cree SiC-based rectifiers can:

• Simplify PFC Boost design
• Eliminate the need for snubbers
• Reduce power losses, leading to cooler operating temperatures
• Produce significantly less electromagnetic interference (EMI)
• Offer faster switching speeds without reverse recovery currents
• Enable streamlined circuit design resulting in smaller board size and component counts
• Allow higher power density designs for compact power supplies for high performance applications

"Cree is taking a leadership role in solving a significant global energy problem -- the increasing power demands from large server farms. Our new Schottky diode can significantly improve the efficiency of the power factor correction circuits used in switch mode power supplies," said John Palmour, Cree executive vice president of advanced devices. "By replacing silicon-based rectifiers with the new 8-amp SiC Zero Recovery rectifier, power supply manufacturers can cut power losses by at least 10 percent in a typical 1kW server power supply."

The Cree CSD08060 is available in production quantities. Additional information about Cree Silicon Carbide Zero Recovery rectifiers may be obtained by calling Cree at 919-287-7836 or by visiting www.cree.com.

EN-Genius Says...

The advantages of silicon-carbide (SiC) semiconductors are well known, but the barriers to making defect-free SiC wafers have prevented manufacturing and dissemination by vendors. Obstacles include the high temperatures at which SiC must be formed, and its resistance to chemical treatment and cutting. The material also tends to crystallize in a variety of atomic arrangements -- all different! It’s a tough beast to tame.

Regardless of these drawbacks, Cree apparently has perfected a route to low defect rates, and reasonable costs, making SiC material commercially achievable. In fact, Cree claims it has fabbed SiC transistors that operate at temperatures as great as 650°C, and it has a SiC-based CMOS op amp that can work at a fiery 350°C. Cree has also demonstrated a SiC-based vertical power MOSFET.

These devices can operate at extremely high temperatures, without intrinsic conduction effects. SiC can also withstand voltage gradients (electric field) over eight times higher than that of ordinary silicon, or fast gallium-arsenide (GaAs) -- without avalanching. That high breakdown rating is key to Cree's fast high-voltage high-power devices.

SiC is also a great conductor of heat. At room temperature, SiC even shows higher thermal conductivity than metal. That's a unique property that lets Cree's SiC rectifiers operate at high power levels and still dissipate excess heat.

An Expanding Family
As its press release above notes, with this 8-A addition the company expands its family of Zero Recovery rectifiers. Not stated is that all of these devices exhibit essentially no reverse recovery, even at 300-V, 600-V, and 1.2-kV breakdown levels. No reverse recovery time specs are indicated on their factory data sheets, nor at any distributor’s Web site spec tables.

The 300-V parts are available to handle 10 A and 20 A. Cree's 600-V rectifiers are available in type numbers to handle currents from 1 A to as high as 10 A. The 600-V devices have parts rated for use from 1 A to 20 A, and the 1200-V devices can rectify 10 A and 20 A.

Temperature-Independent Switching
Regardless of voltage and current ratings, the latest TO-220-packaged CSD08060 8-A devices (peak forward-current is rated at 17.5 A) are fast switching under all conditions. Significantly, they exhibit temperature-independent switching, with a positive forward-voltage tempco. Not only do these so-called Zero Recovery rectifiers let you reduce the size and proportions of heatsinks, but also they ensure operation without thermal runaway. The 8-A devices themselves can dissipate over 100 W at room temperature. Forward voltage drop at that temperature is about 1.6 V.

Unfortunately, I couldn’t get Cree’s product manager Ken Allen to return my telephone calls, so I couldn’t nail down a price for these devices. However, Cree’s CSD04060 devices, rated at 4 A, go for a bit less than $4 a pop in small quantities at Digikey, and Cree’s CSD06060 devices, rated at 6-A, sell for about $5.50 each. That gives you a reasonable point of comparison with conventional silicon rectifiers carrying similar ratings.

Data Sheet