Tag Archives: MOSFETS

SiC MOSFETs Enhance Performance and Efficiency

Power applications across industries demand smaller sizes, greater efficiency, and enhanced performance from the electronic equipment they use. These applications include energy storage systems, battery chargers, DC-DC and AC-DC inverters/converters, industrial motor drivers, and many more. In fact, the performance requirements have become so aggressive that they surpass the capabilities of silicon MOSFETs. Enter new transistor architectures based on silicon carbide or SiC.

Although silicon carbide devices do offer significantly enhanced benefits across most critical performance metrics, the first-generation SiC devices had various application uncertainties and limitations. The second-generation devices came with improved specifications. With pressures for time-to-market increasing, manufacturers improved the performance of SiC MOSFETs, and by the third generation of devices, there were vast improvements across key parameters.

While silicon-based MOSFETs significantly enhanced the design of power electronic equipment, the insulated-gate bipolar transistor or IGBT also helped. The IGBT is a functionally similar semiconductor, its construction is vastly different, and its switching attributes are more optimized. This led to power electronic equipment adopting switched topologies, thus becoming far more efficient and compact.

The main characteristics of switched mode topologies are based on some form of PWM or pulse-width modulation. They use a closed-loop feedback arrangement for maintaining the desired current, voltage, or power value. With the increasing use of silicon MOSFETs, the demand for better performance also increased. Regulatory mandates demanded new efficiency goals.

With a considerable effort in R&D, an alternative emerged. This was the SiC power-switching device, that used silicon carbide as the substrate rather than silicon. Deep-physics changes have allowed these SiC devices three major advantageous electrical characteristics over silicon-alone products. These characteristics offer operational advantages and subtle differences.

The first of these three main characteristics is a higher critical breakdown voltage. While silicon-based products offer 0.3 MV/cm, SiC-based products offer 2.8 MV/cm. This results in products with the same voltage rating now being available in a much thinner layer, effectively reducing the drain to source on-time resistance.

The second main characteristic is higher thermal conductivity. This allows SiC-based devices to handle much higher current densities in the same cross-sectional area, as compared to that silicon-based devices can.

The final characteristic is a wider bandgap. This is the difference in energy measured in electron volts between the bottom of the conduction band and the top of the valence band in many types of insulators and semiconductors. This results in a lower leakage current at higher temperatures. Because of the above reasons, the industry also refers to SiC devices as wide bandgap devices.

In general terms, SiC-based devices can handle voltages that are ten times higher than Si-only devices can. They can also switch about ten times faster, besides offering an on-time drain-to-source resistance of half or lower at 25 °C, even when using the same die area as a Si-only device. Moreover, the switching-related loss at turn-off periods for SiC devices is significantly lower than those for Si-based devices. Additionally, it is easier to handle thermal design and management issues with SiC-based devices, as they can operate at much higher temperatures, such as up to 200 °C, as compared to 125 °C for Si-based devices.

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Double-Sided Cooling for MOSFETs

Emission regulations for the automotive industry are increasingly tightening. To meet these demands, the industry is moving rapidly towards the electrification of vehicles. Primarily, they are making use of batteries and electric motors for the purpose. However, they also must use power electronics for controlling the performance of hybrid and electric vehicles.

In this context, European companies are leading the way with their innovative technologies. This is especially so in the development of power components and modules, and specifically in the compound semiconductor materials field.

ICs used for handling electrical power are now increasingly using gallium nitride (GaN) and silicon carbide (SiC). Most of these devices are wide-bandwidth devices, and work at high temperatures and voltages, but with the high efficiency that is typically demanded of them in automotive applications.

Silicon Carbide is particularly appealing to the automotive industry because of its physical properties. While silicon can withstand an electrical field of 0.3 MV/cm before it breaks down, SiC can withstand 2.8 MV/cm. Additionally, SiC offers an internal resistance 100 times lower than that of silicon. These parameters imply that a smaller chip of SiC can handle the same level of current while operating at a higher voltage level. This allows smaller systems if made of SiC.

Apart from functioning more efficiently at elevated temperatures, a full SiC MOSFET module can reduce switching losses by 64%, when operating at a chip temperature of 125 °C. Power control units for controlling traction motors in hybrid electric vehicles must operate from engine compartments, and this places additional thermal loads on them.

Manufacturers are now exploring various solutions for improving the efficiency, durability, and reliability of SiC MOSFETs under the above operating conditions. One of these is to reduce the amount of wire bonding by using double-sided cooling structures. This cools the power semiconductor chips more effectively. Therefore, overmolded modules with double side cooling are rapidly becoming more popular, especially for mid-power and low-cost applications.

As a result of the research at the North Carolina State University, researchers have developed a prototype inverter using SiC MOSFETs that can transfer 99% of the input energy to the motor. This is about 2% higher than silicon-based inverters under regular conditions.

While an electric vehicle could achieve only 4.1 kW/L in the year 2010, new SiC-based inverters can deliver about 12.1 kW/L of power. This is very close to the goal of 13.4 kW/L that the US Department of Energy has set for inverters to be achieved by 2020.

With the new power component using double-sided cooling, it is capable of dissipating more heat effectively in comparison to earlier versions. These double-sided air-cooled inverters can operate up to 35 kW, easily eliminating the need for heavy and bulky liquid cooling systems.

The power modules use FREEDM Power Chip on Bus MOSFET devices to reduce parasitic inductance. The integrated power interconnect structure helps achieve this. With the power chips attached directly to the busbar, their thermal performance improves further. Air, as dielectric fluid, provides the necessary electrical isolation, while the busbar also doubles as an integrated heatsink. Thermal resistance for the power module can reach about 0.5 °C/w.

What Are Super-Junction MOSFETs?

Switching power-conversion systems such as switching power supplies and power factor controllers increasingly demand higher energy efficiencies. For such energy-conscious designers, super-junction MOSFETs are a favored solution, as the technology allows smaller die sizes when considering key parameters such as on-resistance. This leads to an increase in current density while enabling designers to reduce circuit size. With increasing market adoption of this new technology goes up, other challenges are coming to the fore, mainly the requirement for improved noise performance.

High-end power supplies for equipment such as LED lighting, LCD TVs, notebook power adapters, medical power supplies, and tablet power supplies require reduced electromagnetic noise emission. Designers prefer using resonant switching topologies such as the LLC converter with zero-voltage switching, as these have inherently low electromagnetic emissions. Super-junction transistors in the primary side switching in an LLC circuit helps designers achieve a compact and energy-efficient power supply.

Compared to conventional planar silicon MOSFETs, the super-junction MOSFET has significantly lower conduction loss for a give die size. Additionally, architecture of the latter device allows lower gate charges and capacitances, leading to lower switching losses compared to conventional silicon transistors.

Fabricators used a multi-epitaxial process for structuring the early super-junction devices. They doped the N-region richly allowing a much lower on-resistance compared to conventional planar transistors. They adapted the P-type region bounding the N-channel to achieve the desired breakdown voltage.

The multi-epitaxial processes resulted in the N- and P-type structures being dimensionally larger than ideal and led to an associated impact on overall device size. The nature of the multi-epitaxial fabrication also restricted engineering the N-region to minimize on-resistance. Therefore, fabricators now use single-epitaxial fabrication processes such as deep trench filling to optimize the aspect ratios of N- and P-regions to minimize the on-resistance while also reducing the size of the MOSFET.

For instance, the single epitaxial fabrication process allows DTMOS IV family of Toshiba’s fourth-generation super-junction MOSFETs to achieve a 27% reduction in device pitch, while also reducing the on-resistance by 30% for each die area. Similarly, Toshiba’s DTMOS V, based on deep trench process, has further improvements at the cell structure level.

Thanks to the single-epitaxial process, the super-junction MOSFETs can deliver more stable performance when faced with temperature changes. Power converters with conventional MOSFETs are noted for reduced efficiencies at higher operating temperatures, which the super-junction MOSFETs are able to counter. For instance, super-junction MOSFETs show a12% lower on-resistance at 150°C.

Power converters using the fifth-generation super-junction DTMOS V devices can now deliver low-noise performance along with superior switching performance. A modified gate structure and patterning helps to achieve this, resulting in an increase in the reverse transfer capacitance between the gate and drain of the device.

Using eGaN FETs in Wireless Power Transfer Systems

Highly resonant wireless power transfer systems such as the A4WP use loosely coupled coils operating at the standard 6.68 MHz or 13.56 MHz unlicensed industrial, scientific and medical ISM bands. The popularity of such wireless energy transfer is increasing over the last few years specifically for applications targeting charging of portable devices. Usually, such solutions for wireless energy transfer for portable devices demand features such as lightweight, high efficiency, low profile and robustness to varying operating conditions.

Such features call for efficient designs capable of operating without bulky heat sinks and able to handle a wide range of load variations and couplings. Only a few amplifier topologies can meet such extreme demands and these are the current mode class-D, the voltage mode class-D and class-E. Of these, class-E is the most popular choice for several types of wireless energy solutions, chosen for its ability to operate with very high conversion efficiency.

As compared to regular MOSFETs, eGAN FETs have demonstrated superior performance when using voltage mode class-D topologies in a wireless energy transfer application In fact, eGAN FETs showed higher peak efficiencies of more than four percentage points. At output power levels beyond 12 W, regular MOSFETs required the addition of a heat sink to provide the necessary cooling for the switching devices and their gate drivers.

Moreover, in the traditional class-D topology, the resonant coils needed to be operated above resonance for them to appear inductive to the amplifier. Operating the coils above resonance reduced the coil transfer efficiency resulting in high losses in matching the inductor because of its reactive energy.

Working in class-E topology, eGAN FETs were able to deliver as much as 25.6 W of power to the load while operating at 13.56 MHz. Transferring wireless energy with high load resistance of about 350 ohms made sure the system had a high Q resonance. Measuring the system efficiency gave a figure higher than 73%, which included gate power consumption.

In a single-ended class-E circuit, the series capacitance resonates with the reactive component of the load yielding only the real portion of the coil circuit to the amplifier. The design of the matching network works for a specific load impedance and establishes the necessary conditions of zero voltage and current switching.

In tests comparing the performance of MOSFETS and eGAN FETs, temperatures were kept well below 50C, when operating in an ambient temperature of 25C. No forced-air cooling or heat sinks were used during the tests, which used the same gate driver for driving both the eGAN FET and the MOSFET.

Measurements show the eGAN FET requires significantly lower gate charge for the same operating conditions and this is an important consideration for low power converters. Gate power forms a significant portion of the total power processed by the amplifier. Additionally, as the eGAN FET has a 33% higher voltage rating compared to a MOSFET, it can be operated at higher voltages for higher output power.
Therefore, the simple and efficient class-E topology, coupled with eGAN FETs, is well suited for wireless transfer converters.