Second, the control technology of electromagnetic interference of switching power supply
To solve the electromagnetic interference problem of the 30w LED driver, you can start from three aspects: 1) reduce the interference signal generated by the interference source; 2) cut off the propagation path of the interference signal; 3) enhance the resistance of the interfered body Interference ability. Therefore, the main control technologies for electromagnetic interference of switching power supply are: circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit boards.
① Reduce the interference of the switching power supply itself
Soft switching technology: add inductance and capacitance elements to the original hard switching circuit, use the resonance of inductance and capacitance to reduce du / dt and di / dt during the switching process, so that the voltage drop when the switching device is turned on precedes the current rise , Or when the current is turned off, the fall of the current precedes the rise of the voltage to eliminate the overlap between the voltage and the current.
Switching frequency modulation technology: By modulating the switching frequency fc, the energy concentrated on fc and its harmonics 2fc, 3fc ... is distributed to the frequency bands around them 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 limit specified by EMI. In order to achieve the purpose of reducing the peak of the noise spectrum, there are usually two processing methods: random frequency method and modulation frequency method.
Selection of components: Select components that are not easy to generate noise, easy to conduct and radiate noise. Of particular note is the selection of winding components such as diodes and transformers. Fast recovery diodes with small reverse recovery current and short recovery time are ideal devices for high-frequency rectification of switching power supplies.
Active suppression technology for common mode interference: try to take a compensated EMI noise voltage completely out of phase with the main switching voltage waveform causing electromagnetic interference from the main loop, and use it to balance the original switching voltage.
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Filtering: One of the main purposes of the EMI filter is to obtain higher insertion loss in the frequency range of 150kHz to 30MHz, but it does not attenuate the 50Hz power frequency signal, so that the rated voltage and current can pass smoothly. Meet certain size requirements. The conducted interference signals on any power line can be represented by differential mode and common mode signals. In general, the differential mode interference amplitude is small, the frequency is low, and the interference caused is small; the common mode interference amplitude is large, the frequency is high, and radiation can also be generated through the wire, which causes large interference. Therefore, to weaken the conducted interference and control the EMI signal below the limit level specified by the relevant EMC standards, an effective method is to install an electromagnetic interference filter in the input and output circuits of the switching power supply.
PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, and its purpose is to reduce the electromagnetic radiation of PCB and crosstalk between the circuits on the PCB. The method of switching power supply layout is similar to its electrical design. After determining the size and shape of the PCB, determine the location of special components (such as various generators, crystal oscillators, etc.). Then, according to the functional unit of the circuit, all the components of the circuit are laid out.
Buffer circuit to reduce electromagnetic interference: It consists of a linear impedance stable network. Its function is to eliminate potential interference in the power supply line, including power line interference, rapid electrical transients, surges, voltage level changes, and power line harmonics. These interferences are not great for general regulated power supplies, but they have a significant impact on high-frequency switching power supplies.
② Cut off the propagation of interference signals-common mode and differential mode power line filter design
Power line interference can be filtered using a power line filter. A reasonable and effective switching power supply EMI filter should have a strong suppression effect on the differential mode and common mode interference on the power line.
③Enhance the anti-interference ability of sensitive circuits
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