New-Tech Europe Magazine | August 2016 | Digital edition

Switching hundreds of volts and amps without loss Reinhard Zimmermann, RECOM Power GmbH

Efficient use of electrical power is a goal shared in every area of electrical engineering and electronics. Wind and solar have come to provide a significant part of our daily energy needs, placing the focus on power electronics components able to switch hundreds of volts and amps at a high degree of efficiency before this green power can be fed into the grid. We refer to state-of-the-art power switches, IGBTs and SiC- FETs, and how to supply the respective gate drivers with voltage in the most effective possible way. These semiconductor technologies are to be found today not only in wind and solar power inverters, but

across the board in power electronics, ranging from motor control to welding equipment, induction furnaces and vacuum generators to e-mobility and more. Developers only had bipolar power transistors available to them in the early stages of the power electronics; these were suitable for reverse voltages of up to 500V and could switch currents of 100 amps and more; this might be a perfectly respectable level of performance, but it comes at the cost of very high control currents - usually ten to twenty percent of collector current - to keep switching losses from relatively sharp edges in limits. The first power MOSFETs (metal oxide field effect transistors) to be developed solved this problem. A MOSFET gate is insulated by design, only requiring negligibly small currents to charge

and discharge the gate capacitor. This development significantly reduced power losses from gate control. Obviously, there were still two sources of loss as known from bipolar semiconductors: 1. Switching losses from limited edge steepness during switching cycles 2. ON losses while the semiconductor is in its ON state ON losses are intrinsic to the technical specification of MOSFETs and are therefore largely fixed; switching losses on the other hand depend on control quality, which may be be improved in the development phase by suitable circuit designs. IGBTs combine the advantages of MOSFETs and bipolar transistors The disadvantage of higher ON losses on the drain-source path

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