Susceptibility of offline SMPS to Common Mode transients

The conventional offline switched mode power supply (SMPS) and near unity power factor converters (NUPF) are exposed to differential mode (DM) and common mode (CM) LT power line transients. The front-end rectifier system converts the CM transients to DM transients because of the variation of the diode capacitance in the duration of transients. The NUPF are more vulnerable to such CM transients than conventional SMPS because of the smaller value of hold up capacitor. It has been found experimentally that the magnitude of DM converted transients varies with the time of occurrence with respect to AC mains voltage wave and is maximum when it occurs at zero crossing. Its magnitude is higher for lesser rise times and higher coupling capacitance to ground. Simulation and experimental studies have verified this. The results obtained are in close agreement. Three types of protective elements i.e. MOV, Line Filter, and RC Snubbers were used to mitigate this transient. It has been found that RC Snubbers were most effective. RC Snubbers are conventionally used to suppress over voltage DM transients occuring on LT power lines and are effective only to DM transients. There can be four such snubbers one across each of the diode in the bridge or even one snubber across the bridge. But, for reducing the CM converted DM transients it is essential to connect snubbers across all the four bridge rectifiers. If not the imbalance of two circuits, line to ground and Neutral to ground will persist and Mode converted transients will not be reduced. A method to calculate the RC snubber values for this purpose has been proposed. The simulations were carried out in Pspice and are supported by experimental results.