Semiconductor Galvanic Isolation Based Onboard Vehicle Battery Chargers
Chengcheng Yao · OhioLink ETD Center (Ohio Library and Information Network) · 2018
Power converters with galvanic isolation are widely used in various applications, which is also required by the industry safety standards (e.g., IEC60950 and UL2202).Traditionally, there are mainly three solutions to achieve the galvanic isolation in power converters, including the magnetic field-based isolation, electric field-based isolation, and optics-based isolation.Semiconductor-based galvanic isolation (SGI) is a paradigm shift in realizing galvanic isolation.It uses semiconductor switches' output capacitance to isolate two sides of the circuit when the switches are off.When the switches are on, differential-mode (DM) power can still be delivered from the primary side to the secondary side circuit.It means both the common-mode (CM) current blocking and DM power delivery are handled by the semiconductor switch.This dissertation conducts a comprehensive study of the principle, safety requirements, suitable circuit topologies, the touch current issue and design guidelines of SGI based power converters.The goal is to achieve high power density, high efficiency, and valid galvanic isolation performance that meet safety standards.Onboard vehicle battery charger is selected for the target application.The discussion starts with a review of safety standards on galvanic isolation, which can be evaluated with two tests: the withstand voltage test and the touch current test.Then, principles, benefits, and challenges of the SGI technology are discussed.iii A family of SGI circuit topologies is explored.Basically, the SGI concept can be applied to most traditional inductor-based and capacitor-based circuit topologies.The developed topologies include (a) SGI based buck cell, (b) SGI based buck-boost cell, (c) SGI based boost cell, (d) SGI based switched capacitor converters, and (e) SGI based bridgeless power factor correction converters.Using onboard battery charger as an example, the selection of SGI topologies for different ac/dc power levels is discussed.The touch current issue is one of the key challenges of SGI power converters.Systematic analysis and modeling methods are proposed to predict the touch current.The analysis shows that for a given SGI topology, it is preferred to have smaller Coss and fsw to reduce the touch current.However, these constraints greatly limits the power density and efficiency of SGI power converters.TC compensation is an approach to mitigate the tradeoff between satisfying safety standards and achieving high power density and high efficiency.Several passive and active TC compensation approaches have been investigated.The passive filtering techniques are not very effective due to the low-frequency nature of the touch current.Instead, active compensation methods are far more effective.A comprehensive discussion is presented on various aspects of active compensation methods, including system architecture, TC detection, compensation current injection methods, channel sharing and the timing control.A buck-boost based distribution method with local control is presented to illustrate the design process.Several local detection and control methods are also explored to realize a local control of the TC compensation circuit so that the main controller does not need to involve in the control of the TC compensation.