Global Sources
EE Times-Asia
Stay in touch with EE Times Asia
EE Times-Asia > Power/Alternative Energy
Power/Alternative Energy??

Grasp power MOSFET based on superjunction tech

Posted: 18 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Power MOSFETs? superjunction? AC/DC power supplies? inverters? EMI?

Power MOSFETs based on superjunction technology have become the industry norm in high-voltage switching converters. They offer lower RDS(on) simultaneously with reduced gate and output charges, which allows for more efficient switching at any given frequency.

Prior to the availability of superjunction MOSFETs the dominant design platform forhigh-voltage devices was based on planar technology. However, fast switching at high voltages poses its own challenges in AC/DC power supplies and inverters. Designers making the transition from planar to superjunction MOSFETs often have to accommodate EMI, voltage spikes, and noise-related concerns by compromising switching speed. Below we will compare the characteristics of the two platforms so that the benefits of superjunction technology are fully understood and utilised.

In order to understand the differences between the two technologies, we need to start with the basics. Figure 1a shows the simple structure of a conventional planar high-voltage MOSFET. Planar MOSFETs typically have a high drain-to-source resistance per unit of silicon area, and come with relatively higher drain source resistances. Lower RDS(on) values could be achieved with high cell density and large die sizes. However, large cell densities and die sizes also come with high gate and output charges, which increase the switching losses as well as costs. There is also a limit to how low the total silicon resistance can go. The total RDS(on) for the device can be expressed as the sum of three components: the channel, epi, and the substrate.

RDS(on) = Rch + Repi + Rsub

Figure 1a: Conventional Planar MOSFET Structure.

Figure 1b: Resistive Components of a Planar MOSFET.

Figure 1b shows a breakdown of different components that make up the RDS(on) in a planar MOSFET. For low-voltage MOSFETs the three components are comparable. However, as the voltage rating is increased, the epitaxial layer needs to be thicker and more lightly doped to block high voltages. For every doubling of the voltage rating, the area required to maintain the same RDS(on) increases more than five-fold. For 600 V rated MOSFETs, more than 95 % of the resistance comes from the epitaxial layer. It is obvious that for any significant reduction in the RDS(on) value, it is necessary to find a way of heavily doping the drift region and drastically reducing the epi resistance.

Figure 2: Superjunction MOSFET Structure.

Figure 2 shows the physical structure of superjunction MOSFETs based on the idea of charge balancing. The drift region now has multiple P columns, which cancel the charge in the surrounding N regions under reverse bias. As a result, the Nepi can now be thinner and heavily doped since the combined structure offers a much higher resistance to applied reverse voltage. As the N region becomes more heavily doped, its on-resistance per unit area decreases.

1???2???3???4?Next Page?Last Page

Article Comments - Grasp power MOSFET based on superjun...
*? You can enter [0] more charecters.
*Verify code:


Visit Asia Webinars to learn about the latest in technology and get practical design tips.

Back to Top