Actual Reasons Involving Turn-Off Losses Improvement With Increasing Load and Gate Resistance in MOSFETs Enhanced With Kelvin Source

This article explains the actual reasons behind the improvement in terms of turn-OFF energy losses (Eoff) reduction, obtained using Kelvin pin (4-lead, 4L) MOSFETs instead of 3L ones. Eoff increases by increasing gate resistance and load, but the experimental results reveal that the increment is less in 4L than in 3L MOSFETs. Analyzing the turn-OFF waveforms and adopting circuit analysis, the article first proves that the common argument about the undesired effect of the parasitic source inductance in 3L MOSFETs is misleading. Then, the reasons behind the aforesaid improvement in 4L MOSFETs with increasing gate resistance and load are described by means of the derived analytical expressions. Finally, the analysis of the equations has provided helpful information at the design stage, also accounting for the different device technologies. In particular, the improvement obtained using 4L MOSFETs reduces as the driving loop parasitic inductance increases and evanesces if its value becomes comparable with the power loop parasitic inductance.