Five Engineers Talk About New Medium-Power Device ProductsPart 2 Fifth-Generation -40 V/-60 V P-channel Power MOSFETs with Greatly Reduced On-Resistance

Good day. My name is Shiraishi, and I’m with ROHM. In Part 2, I would like to introduce a newly developed series of -40 V/-60 V P-channel power MOSFETs that use ROHM’s fifth-generation fine process technology, selected from among the new low-voltage MOSFET products for which I have responsibility.

First I would like to explain the background to the newly developed P-channel power MOSFETs. In general, N-channel MOSFETs are widely used, but P-channel MOSFETs are also demanded owing to their own features, and there is a need for development of P-channel MOSFETs to accommodate changes in market demand.

N-channel MOSFETs are widely used because their on-resistances are generally low compared with P-channel MOSFETs of the same size. This implies reduced losses, which in recent years especially has become an important advantage. Whereas in N-channel MOSFETs the carriers are electrons, which have high mobility, in P-channel MOSFETs the carriers are holes, with mobilities approximately three times greater than those of electrons, so that currents do not readily flow, and so the on-resistance is higher for a given element size.

However, N-channel MOSFETs also have problems requiring examination. When an N-channel MOSFET is used on the high side of a circuit, if the gate voltage is not set higher than the input (drain) voltage, the MOSFET will not turn on. For this reason, a separate step-up circuit must be provided to supply the gate voltage, complicating the circuit layout. A P-channel MOSFET, on the other hand, can be driven with the gate voltage set lower than the input (source) voltage, and so does not require a special driving circuit, and therefore has the advantage of enabling a simplified circuit layout.

Due to such advantages, P-channel MOSFETs are widely used in machine tools, robot motors, fans for industrial equipment, and for switching in power supply circuits. They are also frequently used in H bridge circuits to drive DC motors and stepping motors, and as high-side switches in switching power supply circuits, in combination with N-channel MOSFETs on the low side (see Fig. 1).

Fig. 1: Typical circuits and applications in which P-channel MOSFETs are used

P-channel MOSFETs are used in these markets and applications, but in recent years it is increasingly frequently the case that industrial equipment and consumer products use higher voltages, such as 24 V, as the input voltage of the power supply, and so demand is mounting for P-channel MOSFETs with higher rated drain-source voltages.

Still, there is the tradeoff in which the on-resistance rises when the required higher voltage rating is achieved.

In general, the switching frequencies of power supplies used in industrial equipment and consumer products are relatively low, and so conduction losses dominate among MOSFET-related losses, and MOSFET on-resistances have a direct impact. This is a matter that cannot simply be ignored. As one means of lowering the on-resistance while achieving higher rated voltage, element size can be increased, but this results in larger sizes for the MOSFETs, including their packages.

Hence ROHM has newly developed a series of -40 V/-60 V P-channel MOSFETs using its proprietary fifth-generation fine process technology in order to achieve low on-resistances while accommodating high input voltages.

By using fifth-generation fine process technology, it was possible to reduce gate trench structures by 75% from conventional sizes, thereby successfully raising current densities. As a result, an industry-best A・Ron (on-resistance per unit area) was achieved. ROHM managed to cut the on-resistance by 62% from that of previous products for -40 V MOSFETs, and by 52% for -60 V MOSFETs (see Fig. 2).

Fig. 2: Improvement of A・Ron of new 5th-generation -40 V/-60 V P-channel MOSFETs over previous products

For example, by optimizing the element sizes of the new RD3L07BAT fifth-generation series (TO252/-60 V/-70 V/12.1 mΩ), the drain current (current capacity) was increased fivefold over the previous series, and the on-resistance (VGS = 10 V Max) was reduced* by 85%. *Compared with the previous RD3L140SP product (TO-252/-60 V/-14 A/84 mΩ) with the same rated voltage

Applications are expected to further expand, starting from industrial equipment, and so the lineup of packages has also been expanded. Packages have been prepared to accommodate a broad range of equipment, applications, and mounting conditions, from the TO-252, SOP8, TSMT8 (size 3028) and TSMT6 (size 2928) packages that have been used in the past for industrial equipment to packages that can dissipate heat from the bottom surfaces with superior heat dissipation such as the HSOP8 (size 5060), HSMT8 (size 3333), and HUML2020L8 (size 2020) (see Fig. 3; click on part numbers in the table to open a page with product details).

Fig. 3: Extensive lineup of packages

Finally, a few words about future plans to release new low-voltage MOSFET series.

There are plans to release 40 V/60 V/100 V/150 V N-channel power MOSFET series using next-generation fine process technology, which support 36 V/48 V/54V inputs for fan motors for industrial equipment, DC-DC power supplies for communication base stations and servers, and synchronous rectification/ORing circuits.

Moreover, there are also plans to release a 100 V P-channel dual power MOSFET series for load switches supporting higher input voltages, and a series of ±40 V/± 60 V/± 100 V/Nch+Pch dual power MOSFETs for use in H bridge circuits.

Packages equipped with these devices are slated to include, in addition to conventional packages, large-current packages using wireless structures as well as more compact packages.

This discussion will be continued; please watch this space.