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Five Engineers Talk About New Medium-Power Device Products

Part 3
Super junction MOSFETs Achieve Low On-Resistance, Fast Switching with High 650 V Voltage Rating

Hello! I am Onodera of ROHM. I'm in charge of the development of new high-voltage MOSFET products. In this article, I'd like to talk about our newly developed high-voltage super junction MOSFETs.

First, a few words about high-voltage MOSFETs. At ROHM, MOSFETs having a drain-source voltage rating between 500 V and 900 V are classified as high-voltage MOSFETs. High-voltage MOSFETs are mainly used as switching elements in the power supply circuits of various electrical products such as home appliances and personal computers, and in inverter circuits for refrigerators, washing machines, air conditioners, and the like. They are a key item in the switching-type power conversion circuits that can be said to be essential for the trend toward power saving of recent years.

The elements we have just developed are a lineup of 650 V super junction MOSFETs that we call the "R65xxKNX3 series". As the package, they use the TO-220AB, which is the standard package type for power MOSFETs. The TO-220AB package affords superior heat dissipation because the metal pad (heat-dissipating fins) on which the element sits is exposed on the rear side of the package, and is well-suited to power supply applications handling large amounts of power such as server power supplies with their considerable heat generation.

Fig. 1. A TO-220AB package

Features of the R65xxKNX3 series include the achievement of a low on-resistance and fast switching, even with a 650 V voltage rating. This is due to the adoption of a super junction structure. Discrete components such as MOSFETs are individual elements, and so performance depends directly on the processes and structures used. Here the discussion turns to element processes and structures; we will use diagrams to explain the most important points.

On the left in Fig. 2 is the planar structure used in conventional elements; shown on the right is the super junction structure used in these new elements.

Fig. 2. Planar structure (left) and super junction structure (right)

Next, Fig. 3 shows the difference in the widening of the depletion layer when a voltage is applied in each case. The depletion layer is closely related to the rated voltage.

Fig. 3. Difference in the widening of the depletion layer for a planar structure (left) and a super junction structure (right)

In a planar-structure device, the depletion layer expands in the depth direction when a voltage is applied, and so if the impurity concentration is raised in order to lower the on-resistance, the depletion layer width is decreased, and a high rated voltage cannot be maintained. In contrast, in an element with a super junction structure, column-shape P layers are arranged in the drift layer, which is an N layer, so that when a voltage is applied the depletion layer widens laterally, and a depletion layer can be formed over the depth of the column-shape P layers (called P columns). As a result, a high rated voltage can be maintained even when the impurity concentration in the drift layer is raised in order to lower the on-resistance, so that a high rated voltage and a low on-resistance can both be attained.

Further, a super junction structure is less capacitive than a planar structure, so that switching losses can also be reduced.

Well then, let's introduce the actual product lineup. Five models are provided, with on-resistances of from 98 mΩ to 280 mΩ (Typ.), enabling use in a wide range of applications. (Click on part numbers in the table to access pages with details for the product in question.)

Finally, a few words about future plans. ROHM has just released a new series of products, but we are already working to develop next-generation products. We are aiming at next-generation product with even lower on-resistances, and have set a target of an on-resistance that is 40% lower per unit chip area. In order to protect our global environment, greater energy conservation and enhanced efficiency are needed; and as one step in this direction, I think that, as mentioned at the outset, improved performance for switching elements is extremely important.

I hope you will find time for the next article in this series.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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