Touch current analysis for power supplies designed for energy efficient regulations

Hundreds of thousands of SMPS (switching mode power supplies) are manufactured each year. Most of them are certified to either IEC 60950-1 or IEC 60065, and their derivatives. In most tests, operating conditions of the equipment under test (the term “EuT” will be used throughout this paper) such as environmental, input and output (loading) conditions are specified in these standards. However, loading condition of touch current measurement is not yet specified and hence different practices may arise among different test houses. During a set of experiments conducted couple months ago, it had been observed that loading conditions do influence touch current measurement, but unfortunately the manner and the extent are not predictable. Furthermore, we see more and more techniques spurred by energy efficiency standards are introduced into present SMPS, for instance, to reduce power consumption of stand-by mode by latching off some portion of circuits or changing operating frequencies. This paper intends to provide an overview of touch current measurement and summarize how touch current measurement is affected by loading conditions and modern design techniques.