Discrete power MOSFETs are necessary for building switching AC-DC converters that convert the AC mains voltage to a DC voltage and DC-DC converters that convert a (DC) voltage from one level to another.
Toshiba is working to further improve MOSFET performance, driven by its resolve to reduce MOSFET losses as much as possible. Reducing conduction and switching losses helps improve the efficiencies of power supply circuits. Toshiba believes this will eventually contribute to the realization of a more energy-conscious and sustainable society.
Toshiba has decades of experience in the development and manufacturing of discrete MOSFETs. Its main products include the mid- to high-voltage DTMOSIV Series with a VDSS of 600 V or so and the low-voltage U-MOS Ⅷ-H Series with a VDSS of 30 to 250 V. This article presents an interview with the engineers who were in charge of the development of these MOSFETs.
Onodera: For mid- to high-voltage applications, Toshiba offers the DTMOSⅣ (d-t-mos-4) Series with a VDSS of 600 V using a superjunction structure. As the suffix "IV" implies, DTMOSIV is our fourth-generation superjunction process.
Generally, a superjunction process increases breakdown voltage by creating vertically oriented P-type pillar layers in the N-type layer to form a depletion layer with uniform electric field distribution across P-N junctions. A superjunction structure also enables a reduction in on-resistance compared to the value typically achieved with conventional mid- to high-voltage MOSFETs.
Besides, DTMOSⅣ is uses a single-epitaxial (single-epi) technology to form P-type pillar layers. While the most commonly used multi-epi process is sensitive to changes in dopant concentration at the P-N junction due to thermal diffusion, a single-epi process leads to P-type pillar layers with an even sidewall. Consequently, a single-epi process provides higher performance and allows for easier geometry shrinking (Fig. 1).
Additionally, a single-epi process shortens production lead time since the P-type pillar layers are formed by performing epitaxial growth only once. I think Toshiba is the first company to use a single-epi process for mass production. Toshiba will continue to strive to keep ahead of our competitors in single-epi technology.
Onodera: RONA, or on-resistance per area, is used as a figure of merit to compare conduction losses. DTMOSIV provides an approximately 30% reduction in RONA over DTMOSⅢ, the third-generation DTMOS process. In addition, DTMOSIV exhibits a lower increase in RONA in the high-temperature region thanks to the single-epi structure. For example, at 125°C, DTMOSIV has an RONA value 10% lower than that of DTMOSⅢ and thus maintains high efficiency (Fig. 2). The switching loss, Eoss, of DTMOSIV is more than 12% lower than that of DTMOSⅢ.
The high-VDSS and low-RDS(ON) DTMOSIV Series is suitable for high-current applications such as power supply units for large IT systems, communications equipment and computer servers. It is also ideal for inverter and converter applications for photovoltaic power generation that requires a series cell connection to handle several hundreds of volts. Toshiba's mid- to high-voltage planar MOSFETs have commanded a large market share, especially in applications requiring high reliability. The addition of the high-current superjunction DTMOSⅣ Series to our MOSFET portfolio will expand the application areas of our MOSFETs.
Yokota: U-MOS Ⅷ-H (u-Mos-8-h) is a low-VDSS discrete MOSFET series suitable for AC-DC and DC-DC converter applications in a wide range of electrical and electronic equipment such as mobile device adapters, game consoles, IT equipment, communication systems, audiovisual equipment, white goods and small industrial equipment.
U-MOS Ⅷ-H has a trench structure. As the name implies, it is our eighth-generation trench process.
The U-MOS Ⅷ-H Series is available with a VDSS from 30 V to 250 V, covering all industrial application needs. It is a competitive product line with industry-leading performance.
Yokota: Let me talk about a typical N-channel MOSFET, for example. RONA, or on-resistance per area, is used as a figure of merit to compare conduction losses. The RONA value of an N-channel U-MOS Ⅷ-H device with a VDSS of 100 V is approximately half that of a U-MOS Ⅳ device with the same VDSS. Also, RONCiss is used as a figure of merit for drive losses that have a significant impact on power supply circuits at light loads. U-MOS Ⅷ-H delivers a 60% reduction in RONCiss over U-MOS IV.
For comparison, we measured the power conversion efficiencies of Toshiba's and competitors' MOSFETs in an AC-DC converter with a VOUT of 19.5 V. The assumption was that the AC-DC converter would be used in AC adapters for notebook PCs. Compared to the MOSFET from Company A, Toshiba's new MOSFET has a higher efficiency at light loads and about the same efficiency at heavy loads.
Both the U-MOS Ⅷ-H and DTMOSIV Series offer the industry's highest performance. Toshiba's MOSFETs are now available with a VDSS from 30 V to 650 V. Take a communication system, for example, that has an AC-DC converter that generates 48 Vdc, a DC-DC converter that generates either 12 Vdc or 5 Vdc and point-of-load (POL) power supplies that generate low voltages required by a CPU and a memory subsystem (Fig. 4). You can now use Toshiba's latest MOSFETs for all these power supply circuits.
Previously, some MOSFETs were only available with old processes. With the release of the U-MOS Ⅷ-H and DTMOSIV Series, a wide range of MOSFETs has become available with the industry's highest performance.
It is true that discrete MOSFETs may contribute only marginally to improving the overall efficiency of a power supply circuit, but continual improvement is necessary to make a difference. Thus, further R&D efforts are required. Toshiba will continue to strive to develop new processes in order to contribute to the realization of a more energy-conscious society.