Posted on 01 November 2019

Advancements in MOSFET Technology for High Switching Frequency Applications

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By Tom Loder, Vice President of Sales & Marketing, Microsemi Power Products Group


Power metal-oxide semiconductor field-effect transistors (MOSFETs) are known for superior switching speed, and low gate drive power. In these respects, power MOSFETs approach the characteristics of an “ideal switch.” Because of these features, power MOSFETs are the best choice for high voltage, high power, high performance switch mode power supply applications including power factor correction, server and telecom power systems, solar inverters, arc welding, plasma cutting, battery chargers, medical, semiconductor capital equipment and induction heating.

In N-channel MOSFETs, only electrons flow during forward conduction – there are no minority carriers. Switching speed is determined by how fast the internal capacitances can be charged and discharged. MOSFETs with low capacitances are therefore capable of very high switching speeds and correspondingly low switching losses. This is why MOSFETs are the device of choice in high switching frequency, switch mode power supplies and inverters operating in excess of 100 KHz.

The drawback of fast switching is that the MOSFET can interact with parasitic circuit elements, causing oscillations and generation of excessive electromagnetic interference (EMI). This effect can cause problems ranging from increased switching losses to catastrophic system failures. Finding the proper balance between high switching speed and oscillation immunity is the key design tradeoff in the latest generation of MOSFET technology.

Companies like Microsemi, with its recent acquisition of Advanced Power Technology, continue to develop new power management technology, assembly and manufacturing techniques to address the needs of highvoltage devices and applications.

 Tom Loder, Vice President of Sales and Marketing, Microsemi Power Products Group

The acquisition combined two strong high performance analog companies offering both differentiated RF product into niche end markets as well as a strong focus on the highpower, high-speed segment of the power semiconductor market.

The latest MOSFET technology advancement by Microsemi has led to the introduction of devices with low switching losses and improved oscillation immunity. The key device parameters are the value and ratio of input and reverse transfer (Miller) capacitances as well as the intrinsic gate resistance of the MOSFET. This optimization results in the elimination of gate oscillation, reduced EMI (“quiet” switching), simple gate drive circuitry and increased drain-gate noise immunity, resulting in high dv/dt ruggedness.

As with previous generations of products, Microsemi continues to focus on high voltage (>500V) devices that are capable of handling high power at high switching frequencies. This family of devices also feature low on-resistance, which results in low conduction losses. Combined with low switching losses the result is an overall efficiency improvement and increased system power densities. Advanced manufacturing processes have lowered thermal resistance and enabled higher current ratings for each die size and package type compared to earlier generations.

These advancements also apply to FREDFET devices. All MOSFETs have an intrinsic body diode that allows reverse current flow. This body diode has a slow reverse recovery. Slow body diode recovery leads to reliability problems in zero-voltage switching (ZVS) circuits. A FREDFET is a MOSFET with additional processing to speed up the body diode recovery. A FREDFET has identical switching characteristics to a MOSFET, with the fundamental difference being the improved reverse recovery speed of the body diode. The key benefit of FREDFETs is improved reliability in ZVS bridge circuits and higher commutation dv/dt ruggedness.

Companies like Microsemi are paving the way for the next generation of high reliability, high efficiency, power conversion systems at ever decreasing costs.



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