There are two EMI noise: switching power adapter input of common mode noise and differential mode noise. In the AC mains 100 ~ 240V input EMI noise is called common mode noise exists between the lead and the earth, it can be seen as an interference signal level of AC input potential of the transmission line is equal to the same phase. EMI noise referred to the AC input line between the differential mode noise, it can be seen as interference signals in the 180 degrees of phase difference between the AC input line transmission. Common mode noise interference into the earth from the AC input line current differential mode noise interference current flowing between the AC input line. Conducted EMI noise of any switching power adapter input line, common mode and differential mode noise to that, and these two EMI noise as independent sources of EMI to suppress.
The 220V/50Hz AC grid or 115V/400Hz of alternator are there is a wide range of power supply noise, the power cord is the main way of EMI equipment incoming and outgoing equipment. Through the power line, power line interference can be passed to the switching power supply adapter, interfere with the normal operation of the switching power supply adapter. Similarly, the switching power supply adapter interference through the power cord to the power line to cause interference to other electrical equipment of the power line. In order to prevent the occurrence of these two cases, it must be installed in the switching power supply adapter at the entrance of a low-pass filter, this filter is only to allow the device operating frequency (50Hz, 60Hz, 400Hz) through higher frequency interference big impediment. This filter is used exclusively for switching power adapter products, called switching power supply adapter EMI input filter.
EMI filter switching power supply adapter differential mode interference and common mode interference inhibited, but due to the circuit structure, the inhibitory effect of differential mode interference and common mode interference. Therefore, the differential mode insertion loss and common mode insertion loss of the points in the technical specifications of the filter. In addition to the special filters allow an ungrounded, all power filter must be grounded, because the only ground of common mode EMI filter bypass capacitor have a role.
Switching power supply adapter itself is also an EMI interference sources, in the switching state of the switching power supply adapter for the power conversion, itself is a strong source of EMI noise, the EMI noise generated by both very wide frequency range, a high strength . These EMI noise also by radiation and conduction pollution electromagnetic environment, thus affecting the normal operation of other electronic devices.
For switching power supply adapter, in addition to its internal power conversion circuit, drive circuit, the control circuit, the protection circuit, the input / output level detection circuit of these circuits is constituted mainly by a general purpose or application specific integrated circuits. When EMI influence of noise to the analog circuit, the signal-to-noise ratio of the signal transmission will deteriorate, not normally submerged serious cause signals to be transmitted by the EMI noise. When EMI noise impact to the digital circuit will cause the power to stop working, the electronic equipment does not work. Power adapter power supply noise filter can effectively prevent malfunction due to external electromagnetic noise interference.
Import EMI noise from the input terminal of the switching power supply adapter, a part of the output terminal of the power adapter, which induced voltage is generated in a load circuit of the power adapter, a circuit malfunction or interfere with the reason of the transmission signal in the circuit. These problems are also available noise filter is to be prevented. Power filter:
1. Prevent external electromagnetic noise interference power adapter control circuit.
2 load equipment to prevent external electromagnetic noise power adapter.
Suppress EMI generated by the power adapter itself.
4 Inhibition of the EMI generated by other devices through the power adapter transmission.