3, interference suppression measures 

 Following several interference to talk about the suppression of the production of switching power supply. 

 3.1 Power input EMI filter into the line-side power supply usually shown in Figure 3 circuit. The circuit have good common mode and differential mode ripple interference inhibition. Figure in the role of each component: 

 (1), L, C1, C2, is used to filter out common mode interference signals. 

 L is the common mode inductors are inductance for 2MH-33mH about,. 

 C1, C2 is a bypass capacitor, also known as Y capacitor. Capacitance requirements 2200pF. The capacitance excessive insulation performance will affect the equipment. 

 (2), C3, C4, used to filter the signal of the differential mode interference. 

 C3, C4, for the power supply across the capacitor, also known as the X capacitor. Commonly used in ceramic capacitors, polyester film capacitors. Capacitance of 0.22μF-0.47μF. 

 3.2 switch and output buffer circuit of the diode, due to the interference caused by the high-speed switching of the switch and output diode, can increase the buffer circuit to reduce. 

 3.3 high frequency transformer design and production 

 The transformer is one of the most critical devices of the switching power supply. Transformer not only to design a reasonable, is also very particular in the production. A transformer is necessary to meet the load capability, but also can play less and suppress interference. First select the type of transformer core model should be based on the size of the output load. Determine the diameter and number of lines of the transformer. Based on the width of the bobbin and the current density 6A/mm2 for reference, considering the current skin effect, determine the diameter and number of lines of the transformer. Based on circuit topology and design elements to calculate the inductance of the first winding, if the size of the flyback power supply should also calculate the transformer air gap, the size of the air gap with a load capacity of the transformer, but also affect the transformer leakage a sense of size. The leakage inductance is an important reason for interference in the meet with a load capacity of the case, the leakage inductance smaller as well. Allocation of the structural design, and windings of the transformer. 

 There are two common winding transformer: the order around and laminated around. General order winding leakage inductance of about 5% of the primary side inductance, but because of the beginning of stray capacitance between the secondary only a contact surface, the secondary side. General sandwich winding leakage inductance of about 1-3% of the primary side inductance, but because of early secondary only a contact surface, the stray capacitance between the sides of the original. The leakage inductance is an important factor in interference, the stray capacitance between the sides of the original is the propagation channel interference, interference suppression, it is necessary to reduce the leakage inductance but also to reduce the stray capacitance leakage inductance between the sides of the original. 

 Therefore, the design should be taken into account these two aspects of design, specifically what around the law should be based on the actual situation. Shield of the transformer. Strong EMI interference, often by adding a layer of shielding between the initial level of the transformer, cutting off the path of stray capacitance between the initial level by adding shield, allowed to form the capacitor, the shielding effect is very good can be substantially reduced EMI and transient interference grid in series has a certain inhibition. However, the production process and cost of the transformer to rise. Shield are two kinds of copper layer and routing layer, the effect of the copper layer. Output rectifier filter in order to increase the filtering effect of interference in the second output side of the power by adding the two filter and a common mode inductance. L1, C1, composed of two filtering appliances, filter differential mode interference; L2, the output common mode inductors, contained in the filter output voltage common mode interference. 

 3.5PCBLAYOUT should pay attention to the problem 

 Wiring switching power supply contains a high frequency signal, any trace on the PCB can play the role of the antenna, the length and width of the printed line will affect the resistance and inductance, thus affecting the frequency response. Should therefore be all as short as possible by the high frequency AC current and trace design and wide, this means that all components connected to the printed line and connect to other power cord placed very close. Length of printed lines and its showing the amount of inductance and impedance is proportional to the width is inversely proportional to the trace inductance and impedance. According to the size of the printed line by the current, should be bold, the width of the power cord to reduce the loop resistance. 

 4, Conclusion 

 In the design and production of flyback switching power supply, the use of these measures, the transformers are the order of winding with copper shield, the output side and common mode filter inductor, glitches and ripple on the +5 v output voltage waveform peak less than 60Mv, the only glitches and ripple peak in these measures than 500Mv. By comparison, we found that these measures can indeed interfere with play a good role. Measures to inhibit the switching power supply of electromagnetic interference, such as shielding technology, ground, and so on. Consider various factors in the design of switching power supply should be as much as possible to inhibit a variety of switching power supply noise and improve the electromagnetic compatibility of the switching power supply switching power supply to get a wider range of applications. 

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