A tutorial paper is presented on the influence of stray inductance of the commutation circuit (Lo) in switch mode converters. After a short explenation of turn-on and turn-off transients the discussion is focused on the dissipation losses of the switch (or clamping circuit) caused by stray inductance (Lo). A simple measurement of Lo is introduced which is based on switching transients. With this method Lo can be calculated with a standard deviation of 4%. Then dissipation losses due to Lo are estimated for a 50 kW buck converter and for a 50 W flyback converter. Due to low inductive circuit layout in non-isolated high power converters the additional losses are less then 5% of the switching losses. However, isolated switch mode power supplies have a stray inductance which is approximately two orders of magnitude higher than in non-isolated converters. This requires clamping-circuits which reduce overvoltage and absorb or feed back energy to the intermediate capacitor. In that case, losses caused by Lo amount 10 ... 20 % of the switching losses.
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