In the EMC design of switching power supply, the electromagnetic interference problem that engineers need to solve comes from many aspects. So, what should be done in the circuit design process of the switching power supply to make it meet the EMC audit standards? To solve the electromagnetic interference problem of the switching power supply, we can start from the following aspects.

Engineers can reduce the electromagnetic interference generated by the switching power supply itself through the reasonable selection and use of some technical means. At present, soft switching technology, switching frequency modulation technology, active suppression technology of common mode interference, etc. are commonly used. First of all, let’s look at the soft switching technology. Inductance and capacitance elements are added to the original hard switching circuit, and the resonance of the inductance and capacitance is used to reduce du/dt and di/dt during the switching process, so that the voltage drop when the switching device is turned on is earlier than The rise in current, or the fall in current at turn-off, precedes the rise in voltage to eliminate the overlap of voltage and current.

The switching frequency modulation technology is also an advanced technology that engineers often use in the EMC design process. The rational use of this technology can effectively reduce the noise spectrum peak of the switching power supply. Using this technology, by modulating the switching frequency fc, the energy concentrated on fc and its harmonics 2re, 3fc... can be dispersed to the frequency bands around them, so as to reduce the EMI amplitude at each frequency point. This method cannot reduce the total amount of interference, but the energy is dispersed to the baseband of the frequency points, so that each frequency point does not exceed the specified limit of EMI. In order to achieve the purpose of reducing the peak value of noise spectrum, there are usually two processing methods, namely random frequency method and modulation frequency method.

Next, let's take a look at the third commonly used EMC optimization technology, the active suppression technology of common mode interference. The application principle of this technology is to try to take out a compensated EMI noise voltage from the main circuit that is completely opposite to the main switching voltage waveform that causes electromagnetic interference, and use it to balance the original switching voltage.

EMI filter is another common noise reduction component, which can be seen in many large switching power supplies. In the actual use process, one of the main purposes of the EMI filter is to obtain high insertion loss in the frequency range of 150kHz to 30MHz, but it does not attenuate the power frequency signal with a frequency of 50Hz, so that the rated voltage and current can pass smoothly. At the same time, certain size requirements must be met. Conducted interference signals on any power line can be represented by differential mode and common mode signals. And power line interference can be filtered out using a power line filter. A reasonable and effective switching power supply EMI filter should have a strong inhibitory effect on the differential mode and common mode interference on the power line.