Novel Insights of Linear Power MOSFETs and Their Applications in Switching Converters


Student thesis: Doctoral Thesis

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Award date23 Dec 2020


Power electronics is the technology to manipulate electric power, based on elec-tronic devices. The core part of a modern electric power converter is the semiconductor switch, which is turned on or off in high frequency for the converter to buffer and then transform the electric energy in a regulated manner, fulfilling the requirements of both the source and load side. These switches are usually power MOSFETs, fabricated with a dedicated process to achieve fast and efficient switching.

The author’s work is concerning the power MOSFETs used in power electronics, but linear applications. It is common knowledge that linear mode transistors exhibit very high conduction loss, opposing the will of the power electronics field which pursues uni-ty efficiency. Therefore, unlike the switching behavior, the linear operation is not draw-ing general research interests. However, as power electronics technology evolves, the opinion might be facing changes.

The author appreciates the advantages of linear mode power transistor, including the capabilities of fast response, tight regulation, as well as simple and monolithic im-plementation. These advantages are useful in many aspects of a converter system. Once the excessive loss is avoidable or made acceptable, the application of linear mode tran-sistors is quite reasonable. In this work, the power MOSFETs serve in two external assis-tive circuits at the input of switching converters.

One of the applications is to change a diode bridge rectifier into an active EMI-filtering rectifier. Existing diode bridge rectifiers have considerable but inevitable con-duction loss. Thus, replacing the diodes with MOSFETs would not worsen the loss problem. These MOSFETs must work in the third quadrant (i.e., Synchronous Rectifica-tion mode or SR mode), which in common knowledge is merely the reverse-conducting state. The author, however, reveals a linear property in the third quadrant, where a MOSFET exhibits a simple constant ratio between the gate-source and the drain-source voltages. Thus, fast linear control of the AC small-signal drain voltage is implemented, producing a high-frequency voltage compensation signal to reduce differential-mode (DM) EMI. The proposed rectifier is named an active filtering bridge rectifier (AFBR), which plays the role of AC line rectification and EMI reduction at the same time. The MOSFET in linear third quadrant operation mode is modeled and verified in detail. Tests are carried out on a boost-type PFC with AFBR installed, showing the EMI per-formance improved significantly. The proposed technique facilitates the reduction of passive filter size, as well as the potential of solid-state integration as an active filter module. Moreover, by applying the AFBR, the power loss on the rectifier is reduced.

The other application in this thesis is concerning the power network stability. In DC power networks, power converters with input EMI filters sometimes encounter un-wanted input voltage and current oscillations due to interactions among the bus imped-ance, filters, and converters. A solid-state single-port series damper (“S3 damper”) is proposed that can deal with such oscillations, also based on the linear application of a MOSFET. The MOSFET is considered and operated as an AC damping resistor, con-nected in series with the converter. It lowers the quality factor of the filter and thus in-creases the damping effect on the interaction between the filter and converter. It also increases the input impedance of the filter to avoid voltage and current oscillation be-tween the filter and the bus. The control mechanism is autonomous; therefore, no modi-fication would be necessary for the converter. The FET is maintained at a low dropout voltage to reduce power dissipation. Furthermore, the proposed damper does not require any power capacitor or inductor, favoring high compactness, and reliability. Modeling, design, and comprehensive analysis of the S3 damper is provided. A prototype damper for a commercially available 100W DC-DC converter operated on a 48V bus has been built and evaluated.