EMI/Transient Filtering Stage
The problem with PSUs is that their switching FETs produce EMI/RFI that can seriously affect other electronic devices. In addition, PSUs must be protected from the power grid's noise and voltage spikes, so this stage is very important.
Noise can be divided into two types according to conduction mode: common mode noise (CMN) and differential mode noise (DMN). 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. 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 noise coming from the bridge's diodes (yes, even diodes produce noise, especially when they're being turned off). The parts necessary for a proper EMI/transient filter are two Y and two X capacitors, two coils, a metal oxide varistor MOV and a fuse. However, especially in low-end PSUs, manufacturers omit some components in order to save money. Usually, the first component to be left out is the MOV. An MOV is a voltage-dependent resistor that protects the PSU and system from voltage spikes coming from the power grid. If your PSU does not have an MOV in the EMI/transient filter, then you should always operate your system with a surge suppressor or an uninterruptible power supply (UPS). Otherwise, a spike could permanently damage not only the PSU, but also your system.
After the EMI/transient filter, a negative temperature co-efficient (NTC) 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 to 12 ohms, and after the PSU is started up, the thermistor heats up and lowers its resistance to approximately 0.5 to 1 ohm. Typically, in high-efficiency PSUs, there is an electromagnetic relay that bypasses the thermistor after the PSU starts up to let it cool down and operate normally in a hot restart of the PSU. Also, by bypassing the thermistor, we raise the efficiency a bit, as no energy is wasted on it. A relay is used mostly in higher-end PSUs, while in lower-end units, the same task can be done with a diode.