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6.1.1. Conducted noise

Switched-mode power supplies, because of their nature, can be a radio-frequency interference source. There are two basic interferences or noise types: conducted or radiated. Within the conducted noise, two modes can be distinguished: Differential mode which mode is encountered between the line poles, and common mode; this is the one encountered between ground and any of the line poles. Filters are used to reduce noise to acceptable levels. Fig. 36 shows two types of filters, the PI FILTER which is appropriate to reduce noise in differential mode and the COUPLED FILTER which is appropriate to reduce noise in common mode. Noise in common mode is harmful and, therefore, limited by standards. Voltage limits generated by a piece of equipment can be found in standards such as EN50081-1 or EN50081-2, depending on the environment type in which the equipment is supposed to operate. CISPR limits A and B can be seen in the following figure: CISPR A and B graphs.

Figure 36

Figure 36

Figure 37

Figure 37