EMI/Transient Filtering Stage

The problem with PSUs is that their switching transistors produce EMI/RFI that could seriously affect other electronic devices in the house. Also we must protect the PSU from incoming noise and voltage spikes coming out of the power grid, so the role of this stage is twofold and serves as protection in both directions.

Noise can be classified into two types, according to the conduction mode: Common Mode Noise (CMN) and Differential Mode Noise (DMN).

1. CMN is electrical interference with reference to the ground or common wire. It consists of high frequency spikes and comes from faulty wires or from EMI/RFI of nearby devices. Common mode choke coils along with Y capacitors are used to suppress CMN.
2. DMN represents the noise that is measured between two lines with respect to a common reference point, excluding common-mode noise. To suppress DMN, X capacitors are placed across the lines.
   

The EMI/Transient filter in PSUs is always placed before the bridge rectifier, because in this position it also suppresses the noise coming from the bridge's diodes (yes, even those produce noise, especially at the moment they are turning off). The necessary parts for a proper EMI/Transient filter are two Y and two X capacitors, two coils, an MOV (Metal Oxide Varistor) and a fuse. Very briefly an MOV is a voltage-dependent resistor that protects the PSU/system from voltage spikes coming from the power grid.

However, especially in low-end PSUs, manufacturers omit some components in order to save money. Usually the first to be left out is the MOV. If your PSU does not have an MOV in the EMI/Transient filter then you should always operate it along with a surge suppressor or a UPS, otherwise a spike could damage permanently not only the PSU but your system too.


After the EMI/Transient filter an NTC (Negative Temperature Coefficient) thermistor is usually used to protect the other components from large inrush currents. A thermistor is simply a resistor that adjusts its resistance according to its operating temperature. The resistance of a “cold” thermistor is usually 6-12 Ohms and after the start up of the PSU the thermistor heats up and lowers its resistance to approximately 0.5 to 1 Ohm.

High efficiency power supplies use a relay that bypasses the thermistor after the PSU starts up, in order for the thermistor to cool down and operate normally in a hot switch restart (off/on) of the PSU. Also by bypassing the thermistor efficiency is improved a little bit since no energy is wasted by heating the resistor.