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The BM2Pxxx Series High Efficiency AC/DC Converter IC with Internal MOSFET

Original Super-junction MOSFETs Hold the Key

- Part 3-

  • Super-junction MOSFET
  • Efficiency
  • ON resistance
  • Gate charge amount
  • Miniaturization
  • Element size
  • Switching frequency

-We have learned how the BM2Pxxx series has contributed to achieving the four goals of high efficiency, reduced power consumption, low standby power, and miniaturization. However, in places there have been references to "the benefits of super-junction MOSFETs". Next, we would like an explanation of these super-junction MOSFETs (hereafter "SJ MOSFETs").
To begin with, please summarize once again the problems to which SJ MOSFETs can contribute solutions.

The first of these is efficiency improvement; the second is miniaturization.

-Then let's start with efficiency. How can efficiency be improved by using SJ MOSFETs?

First, let's briefly explain SJ MOSFETs. Conventional MOSFETs have what is called a planar structure, but SJ MOSFETs have a structure that is, in essence, different. Of course there are also detailed differences, such as regarding impurity concentrations, but it is the different structures that enable SJ MOSFETs to achieve greatly lowered ON-resistances RDS(on) and gate charge amounts Qg.

SJ MOSFETs themselves are also manufactured by other companies, but ROHM is conducting its own original development. The SJ MOSFETs incorporated in these devices achieve a high withstand voltage of 650 V, while also achieving low ON-resistances and small gate charge amounts, and are also capable of very high-speed switching. This greatly improves both conduction losses and switching losses of the MOSFET as a switch. When both these losses are low, improved efficiency results. Moreover, lower losses mean less heat generation, and this in turn affects the types and sizes of IC packages that can be used.

-In other words, this relates to the second goal, miniaturization.

To continue, in general the on-resistance of a MOSFET depends heavily on the element size. This is true for both planar MOSFETs and for SJ MOSFETs. In other words, if there is a need to lower the on-resistance, the element size can be increased. Even so, larger elements cannot be the method used to address this issue, given the current situation.

We have explained that SJ MOSFETs have low ON-resistances, but to be more precise, the on-resistance per unit area is small, and if the area is the same as for a planar MOSFET, the on-resistance is low, or if the on-resistance is the same, the area is small. In terms of the amount of power that can be handled, for a given element area, more power can be handled, and for a given power level the area, that is the element size, can be made smaller. These features are used selectively according to the requirements imposed and the purpose of the application.

-Previously you explained that depending on the amount of power to be handled, a smaller package can be used.

That's right. ROHM’s SJ MOSFETs have an extremely low on-resistance relative to the device size, and so with a chip size compatible with a compact SOP8 package, we achieved the remarkably low on-resistance of 4Ω. As a result, we were able to attain an industry-first high power density of 8 W for a SOP8 package.

The ICs of other manufacturers ranging up to 8 W use DIP7 or DIP8 packages, which are the same as ROHM devices extending up to 25 W. In general, applications for 8 W or below are often smaller than those for 25 W. It is natural to want to use an IC in a smaller package for applications using less power. In this respect, SJ MOSFETs make a major contribution.

-However, in AC/DC converters, other components are rather large.

Earlier I discussed this a little bit too. The high-speed switching performance of SJ MOSFETs has an effect as well.

In essence, when the switching frequency is raised, inductors/transformers and output capacitors with smaller values can be used. The smaller the values, generally the smaller the physical size of the device. Put simply, if the switching frequency is raised, external components become smaller in roughly inverse proportion. In actual DC/DC converters, there are ICs with high-speed switching at frequencies up to 10 MHz, and extremely small components are used to configure compact, portable power supplies.

-Well then, the BM2Pxxx series could also be used for megahertz-band switching.

Certainly components would be smaller, but there is the drawback that as switching speeds are increased, efficiency falls. In the previous example of DC/DC converters, this is an application in which only small amounts of power are handled, and the highest priority is placed on miniaturization, even at the sacrifice of some efficiency. This is another so-called tradeoff.

The reason for reduced efficiency is switching losses. However, an internal SJ MOSFET is capable of high-speed switching, or rather, switching losses are low and so the device can be used at high frequencies. Hence while handling larger amounts of power, the required efficiency can be maintained, and small external components can be used at appropriate switching speeds. These ICs are optimized for 65 kHz.

-In this photo of an evaluation board, components of a certain size are used....

This is a BM2P094FEVK-001 evaluation board, using a BM2P094F device with a SO-8 package. The output is 5 W at 5 V/1 A. The BM2P094F with the SO-8 package is mounted on the center-left of the board surface. In fact, it has been made quite small for a 5 W output device with a universal input. Of course, external components are required to have withstand voltages in the several hundreds volt range, and so the result is this size. It can't really be compared with a DC/DC converter.

-What you say about miniaturization is clear. Where efficiency is concerned, I'd like a more detailed explanation.

Yes, well, where efficiency is concerned, another feature of the BM2Pxxx series is its support for the Energy Star standard. Let's explain this next, including Energy Star updates.

The BM2Pxxx Series of high efficiency AC/DC converter ICs with an internal proprietary super-junction MOSFET

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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