Input Line Filter

Input Line Filter: An internally or externally mounted low-pass or band-reject filter at the power supply input which reduces the noise fed into the power supply.

The input line filter consists of an electronic circuit connected between the ac mains and the rectifier input stage of the switching power supply. All the input a.c voltage must pass through the filter before reaching the rectifier. An effective filter should therefore attenuate all the higher frequencies and only let the mains 50Hz or 60 Hz pass through to the next stage.

The input line filters are incorporated in most switched mode power supplies to reduce the interference from the electromagnetic and other electrical noises present in the ac lines. The filters are also used to ensure that the power supplies comply with government regulations and agency standards.

The two primary functions of the input line filter are:

  • Preventing the EMI signals generated within the power supply from reaching the input ac power line and affecting other equipment connected on the same line.
  • Preventing high frequency voltage and EMI on the power line from passing through and reaching the supply’s output.

The design and component selection of the input filter is important in ensuring that it does not unnecessarily increase the volume and cost of the supply or compromise the power supply performance.

Even though there are various filter designs with different characteristics and effects on power supply performance, the passive L-C filter achieves both the filter functions above while still offering the best balance between size, cost and performance. However, passive filters may introduce undesirable effects, it is therefore important to understand the load and use the appropriate filter design.

The L-C passive filters may father be classified according to the design and characteristics. The common types include the undamped LC filter, parallel damped filter and series damped filter.

Multiple Section Filters

Multiple filters may be used to provide better performance and allow the use of smaller sized components such as the capacitors and inductors. A typical filter is as shown below
What is Input Line Filter
Figure 1: A two-section input filter (image: Texas Instruments)

Component Selection

The selection of components is critical in the performance of the filter. Low ESL and ESR capacitors are required for high frequency attenuation and ripple current performance.
Aluminum capacitors commonly used for this purpose may be connected as smaller sized and smaller capacitance in parallel so as to get the large resultant value.

To avoid parasitic capacitance from the inductors, the banked or single layer windings are used while ensuring that their inputs, and output leads, are kept apart.