Super Junction MOSFETs Tackle Efficiency in the World of High-voltage Power

As the world’s power grid struggles to keep up with demand growth, the power semiconductor industry continues to find innovative ways to do more with less. Rohm, Alpha and Omega Semiconductor, and Toshiba have announced the latest parts in their Super Junction power MOSFET (SJ MOSFET) product lines designed for just that purpose.

If you’ve spent much time with MOSFETs, you well understand the struggles of the R_DS(on) (resistance between drain and source while on). A lower R_DS(on) allows for smaller losses and reduced heat buildup with current. However, R_DS(on) typically increases as voltage, current flow, and switching speed increase. Designers of many classes of equipment, especially in motor and power control, want to use higher voltages to reduce current at the same power rating and improve device noise characteristics. A subsequently higher R_DS(on) works against efficiency goals.

 

Toshiba's new DTMOSVI 600 V SJ MOSFET series. Image used courtesy of Toshiba

 

The other primary contributor to MOSFET power loss is the switching time. When fully off or fully on, the gate current is effectively zero. However, while switching, the devices draw current through the gate. MOSFETs have a small amount of gate capacitance that will affect maximum switching time, drawing current while charging or discharging the gate capacitance. The switching time in the circuit is more or less fixed, so a higher clock speed will result in gate current being drawn for a higher percentage of time.

This, by the way, is one of the reasons you should not float an unused input of a MOSFET device. Noise on a floating gate will lead to switching current losses, even if no current is passing through the part.

 

Three New Super Junction MOSFETs

The SJ MOSFET is one of the ways the industry is addressing R_DS(on) and switching time challenges. In this article, I’ll look at three new 600 V SJ MOSFETs from Rohm, Alpha and Omega, and Toshiba.

 

 

Rohm Claims Fastest Body Diode Reverse Recovery Time

Rohm has added three new models to its PrestoMOS SJ MOSFET product line. The Rohm devices emphasize low noise and fast time for reverse recovery (Trr) of the built-in body diode. At 40 ns, Rohm claims the fastest available Trr for a 600 V MOSFET. Low Trr reduces switching losses by approximately 30% and keeps the noise down. 

 

New SJ MOSFET significantly reduces reverse recovery time. Image used courtesy of Rohm

 

Many devices that have used line-voltage AC motors in the past now use brushless DC (BLDC) motors to lower noise, reduce power consumption, and increase control. This includes appliances such as refrigerators, ventilation fans, air conditioners, and a variety of other medium-duty cycle small motor devices. The newest renditions of these products use MOSFET power inverters to create high-voltage DC power, which uses MOSFET-based power inverters and motor drivers.

By switching from full-on/full-off AC motors, a refrigerator, for example, can use 20–30% less lifetime electricity and will have a much quieter audio noise level. This type of application is what Rohm is targeting with the R60xxRNx series.

Prior generations of MOSFETs required additional complex noise reduction circuitry to operate at high speeds. This generation of SJ MOSFET reduces the need for that circuitry by dropping noise characteristics by 40 dB at 40 MHz switching speed, allowing for a faster design cycle and fewer discrete components.

 

Alpha and Omega Semiconductor Emphasizes Low R_DS(on)

Alpha and Omega Semiconductor introduced the first model in its new 600 V SJ MOSFET line, dubbed the αMOS7. The AOK050V60A7 supports a continuous drain current of 50 A at 25°C and up to 200 A pulsed drain current in a TO-247 package.

 

Solar farms make extensive use of MOSFET power inverters. Image used courtesy of the U.S. Department of Energy

 

While the Rohm devices discussed above are designed for less than 10 A current loads for motors and motor power inverters, the Alpha and Omega SJ MOSFETs are higher-current devices designed for mission-critical power systems such as titanium-rated server power supplies, EV charging, and solar farm inverter equipment.

The Alpha and Omega parts don’t have the switching speed of the Rohm parts but rather are designed to deliver sub-50 mΩ R_DS(on) at higher currents while still offering adequate switching time and reduced Qrr (body diode reverse recovery charge) due to their rugged body diode.

 

Toshiba Adds SJ MOSFETs to Improve Power Supply Efficiency

Toshiba has released its fourth generation of SJ MOSFETs, referred to as DTMOSVI, which focus on process improvements to reduce both R_DS(on) and improve high-speed switching. Similar to those of Alpha and Omega, Toshiba’s newest DTMOSVI part, the TK055U60Z1, is in the 40-A range and targets higher-efficiency data center power supplies, power conditioners for solar power generators, uninterruptible power supplies, and similar applications.

 

Evolution of the DTMOSVI family of SJ MOSFETs. Image used courtesy of Toshiba

 

Toshiba optimized the gate design and process to reduce the per R_DS(on) unit area by 13% and R_DS(on) x gate-drain charge by 25% compared to its current-generation DTMOSIV-H series with the same voltage rating. These improvements make the new parts perform better in both conduction and switching losses.

The TK055U60Z1, the first in the new series of MOSFETs, delivers 40 A continuous drain current at 150°C and 160 A pulsed current in a Toshiba TOLL surface mount package.

 

It's All About Reducing MOSFET Losses

Though the Alpha and Omega and Toshiba parts are designed for power supply and the Rohm product for motor applications, all three are utilizing the characteristics of SJ MOSFET architecture to improve MOSFET efficiency and reduce power losses. With MOSFET-based applications becoming ubiquitous anywhere electricity is used, gains in device efficiency, such as those shown in these components, will have a broad-reaching impact.

Expect to see more offerings in the SJ MOSFET arena from these companies and others as the need to get more out of the world’s electric power grids continues to grow in importance.