Super Junction MOSFETs Up the Power Efficiency and Cut the Size of MOSFET SiblingDecember 09, 2020 by Adrian Gibbons
From EV charging stations to OLEDs in TV screens, Super Junction MOSFETs can be a useful design option when power efficiency is paramount.
Hardware designers operating in the automotive, industrial, and consumer industries are constantly pressed to design more power-efficient systems. One device growing in popularity for such systems is the Super Junction (SJ) MOSFET.
The voltage switching capabilities of SJ MOSFETs mean higher power efficiencies and smaller package sizes when compared to planar MOSFETs. But how exactly do these devices work?
MOSFETs vs. Super Junction MOSFETs
Traditional planar power MOSFET structures are subject to increasing RDS(on) resistance with higher-rated voltage switching leading to higher conduction losses, according to a Vishay application note.
RDS(on) is composed of three resistivity elements: channel, epitaxial layer, and substrate layer. An increase in RDS(on) occurs principally in the epitaxial layer between the body of the MOSFET and the substrate.
Resistivity model of a planar MOSFET as a function of increasing breakdown voltage. Image used courtesy of Vishay
According to Vishay, for every doubling of the rated voltage, the area required to maintain the previous RDS(on) increases five-fold.
Structure of a planar power MOSFET (left) and a Super Junction MOSFET (right). Images used courtesy of Fuji Electric
Compared to an ordinary MOSFET, the structure of a SJ MOSFET is able to successfully block higher voltages with a lower RDS(on) by balancing doped regions. This results in a linear relationship between the breakdown-voltage rating and the on-resistivity.
Designers must also consider the switching losses caused by gate capacitance. Conveniently, the small area of a SJ MOSFET (when compared to a planar MOSFET with higher-rated voltage) means that the gate capacitance is also improved.
Comparison between two device architectures, a planar and SJ MOSFET, with data indicating values for resistivity, capacitive charge, and power capacity. Table used courtesy of Vishay
One disadvantage with smaller package sizes is that they diminish total power handling capability.
A Recent Super Junction MOSFET in Action
As an example of a Super Junction MOSFET in action, MagnaChip recently announced two new series of SJ MOSFETs for applications requiring switching at 700 V and 800 V.
According to MagnaChip, the series of high-voltage switching MOSFETs are being targeted for multiple applications including television manufacturing of OLED displays (700 V). Additionally, the 800 V chipsets have applications in fast-charge stations and indoor LED lighting.
Block diagram of an organic LED television system with an LED driver operating at either 24 V or >100 V. Image used courtesy of MagnaChip
MagnaChip claims a reduction in gate charge of 30%, thereby improving power efficiency by reducing switching losses. The devices contain an embedded Zener diode between gate and source to improve package protection against ESD events.
“There is a high demand for the 700 V and 800 V SJ MOSFETs, and we are pleased to introduce eight new products,” said YJ Kim, CEO of MagnaChip. “These new product families will allow us to broaden our application coverage in consumer and industrial market segments. We are gratified by the customer response and will leverage our high-quality product line and technological leadership to expand our SJ MOSFET portfolio into other areas.”
Applications for the SJ MOSFET
SJ MOSFETs have found their way into a number of power applications, from electric vehicle charging stations to OLED television screens.
For instance, one of the stated applications for the new 80 V series of SJ MOSFETs released by MagnaChip is EV chargers.
Power efficiency in these devices is of the utmost importance, especially as the voltage of EV drive trains rises year over year. In the past, many EV architectures were using a 400 V charge system. Switching at 400 V means either an increase in the physical size of the cabling infrastructure or a decrease in the power handling capability of the system.
Comparison of planar and SJ MOSFETs in terms of blockage voltage and on-resistance. Image used courtesy of Vishay
Last year, Ars Technica reported on the Porsche Taycan, a new-model EV that operates on an 800 V electric drive train. The 800 V system on the Porsche can draw up to a maximum of 270 kW.
Beyond fast charging, applications in industrial lighting for LEDs are widespread. Last year, EE Power reported on the release of an 800 V LED driver (HVLED001B current mode controller) from STMicroelectronics.
MagnaChip 700 V-series will be finding its way into newly manufactured OLED television screens as the heart of the LED driver technology for the displays.
Do you have hands-on experience with SJ MOSFETs? How do they compare with your experience with traditional MOSFETs? Let us know in the comments below.