【 Weak current College 】 switch power generation and of EMI Suppression 】

Switching power supply's prominent drawback is able to produce strong electromagnetic interference (EMI). EMI signals in a stream output has a wide frequency range, have certain amplitude, conduction and radiation in the electromagnetic environment after pollution, communications equipment and electronic products cause interference. If not handled properly, switch the power supply itself becomes a source of interference. Electromagnetic interference of the switching power supply efficiency and safety and the use of increasingly become the focus of attention. This paper analyzes the switching power supply cause electromagnetic interference and propagation path and introduced effective measures to suppress interference. 1, introduction to electromagnetic compatibility EMC is the abbreviation for the English electromagneticcompatibility. It includes two meanings, one is the equipment in the work of electromagnetic radiation must be limited to a certain level, the second is the device itself to a antijamming ability, it must have three elements: the source of interference, the coupling channel, sensitive body. To the electronic circuit power switching power supply for interference suppression on guaranteed electronic systems normal and stable operation. This article by analyzing switching power supply sources of interference and the coupling channel, introduced effective measures to suppress interference. And switching power supply transformer design and production methods. 2, switching power supply sources of interference and the coupling channel switching power supply first alternating rectifier to DC, and then after switching valve control becomes a high-frequency, finally after rectifier filtering circuit output, getting a stable DC voltage, thus, itself contains a large number of harmonic interference. At the same time, because transformer leakage and output diode reverse recovery of peak current, will produce different degrees of electromagnetic interference.Switching power supply disturbance mainly concentrated in the voltage and current changes (i.e., dv/dt and di/dt is large) components, especially the output transistors, diodes and high-frequency transformer, etc. At the same time, the stray capacitance will power of noise transmission to the electronic system of power and on the work of the electronic circuitry. Here we'll look at several causes of interference and the coupling of the path. 2.1 output rectifier filtering circuit filtering interference of the switching power supply outputs common bridge rectifier, capacitance filtering circuit. Because of the rectifier diode capacitance of Nonlinear and filtering of energy storage function, making the output current of a peak time is very short, very high the cyclical peak current. This distortion of input current, it is in addition to fundamental wave, is also rich in higher harmonic components. 2.2 switch circuit interference switch circuit's core is also one of the main sources of interference, it is mainly composed of switch tube and high-frequency transformer. Switching from dv/dt greater impulse, band wide and rich in harmonics. This pulse interference generated mainly due to: (1) in the switch tube via instant, transformer primary coil produces a large surge in the ends of the primary coil has a high surge voltage spikes; switch disconnects the moment, because of the primary coil flux, making part of the energy from a coil is transferred to the secondary coil, stored in the leakage of this part of the energy will and switch tube itself interelectrode capacitance and resistance of formation with spikes, damped oscillatory superimposed on switching valve's switching voltage on off peak voltage. The noise will be transmitted to the input and output side, the formation of conducted noise. (2) output diode in Wizard-pass, PN was received in charge accumulation, the diode with reverse voltage when the accumulated charge will disappear and reverse current. Because secondary rectification circuit V in switch conversion rate is very high, i.e. by conduction to the closing time is very short, in a short time to let the storage charges disappearing resulting reverse current surge. Because the DC output of distributed capacitance, inductance, the result of a surge of interference into high-frequency attenuation reducing oscillations. (3) high-frequency transformer primary coil, switching valve and filter capacitor consists of high-frequency switching current loop may produce larger space radiation, forming a radiation interference. 2.3 coupling channel due to the interference of the transformer's initial stage there are stray capacitance between the coil, switching circuits of common mode interference through transformer in former Deputy edge mutual communication. Phase contrast, differential mode interference path is simple and easy to handle. This article describes common-mode noise generation and suppression. 3. measures to suppress interference below several disturbing story we make switching power supply, suppression method. 3.1 power input EMI filter for power line ends are usually shown in Figure 3. The circuit and differential mode for common mode interference have ripple is better. The component in action: (1), L, C1, C2 is used to filter out common mode interference signals. L is the common mode inductance, usually around for 2MH-33mH inductance. C1, C2 as bypass capacitors, also known as Y capacitors. About electric capacity requirements 2200pF. Effect of electric capacity over the Assembly equipment insulation performance. (2), C3, C4 is used to filter out the differential mode interference signals. C3, C4 power crossover capacitors, also known as X capacitors. Common ceramic capacitor or polyester film capacitor. Capacitance from 0.22 μ F-0.47 μ F. 3.2 switch tube and output buffer circuit diode, because switching valve and high speed switching diode output caused by interference, can increase the buffer circuitry to reduce. 3.3 high-frequency transformer design and production of switching power supply transformer is the most critical devices. Transformer not only design reasonable, in the production is also on. A good transformer has to meet with load capacity, should also be able to play a little and the role of the inhibitor interference. First of all should be based on the size of the output load Select transformer type and cores models. Determine the transformer of wire diameter and number of lines. Basis of slot widths and bobbin to current density 6A/mm2 for reference, the comprehensive consideration of current's skin effect, decided transformer wire diameter and number of lines. According to the circuit topology and design elements, calculate the initial level of inductance, if it is a Flyback power supply you should calculate the size of transformer air gap, the gap size determines the transformer with load capacity, but also affect the size of transformer leakage. And interference leakage inductance is an important reason to meet with a load capacity of leakage to smaller as well. The structure design and transformer windings. Transformers there are two common around method: order around the law and the mezzanine round the law. Order around the law generally leakage inductance for original sideOf 5%, but since there is only one primary, secondary contact, former Deputy edge stray capacitance between smaller. Mezzanine round the law generally leakage inductance for original side of 1-3%, but since there is only one primary, secondary contact, former Deputy side large stray capacitance between. Leakage is an important factor in producing interference, former Deputy stray capacitance between edges is interference propagation channel for interference, it is necessary to reduce leakage and to reduce leakage and scattered among former Vice edges. Therefore, the design should be considered both aspects of design, specifically the round method should be based on the actual situation. Transformer shield. Strong in EMI interference, often in the initial stage of transformer into a layer mask between layers, by joining the shielding cut initial stage stray capacitance of the path so that it is the formation of the land, its capacitance shielding effect is very good, you can greatly reduce EMI, at the same time for power system transient series into a degree of interference. But the transformer production process and costs are rising. The shield is copper layers and winding layer two copper layer works best. Output rectifier filtering in order to increase the interference filter effects, can the power of the second output side to join the secondary filter and a common mode inductors. L1, C1, the second-level filtering appliances composed of filtering out differential mode interference; L2 is output common-mode inductance, filtering the output voltage in the common mode interference. 3.5PCBLAYOUT of wiring switching power supply includes a high-frequency signal, PCB printed lines on any can play the role of the antenna, the printed line length and width affect its impedance and inductance, resistance to impact frequency response. It should be all through high-frequency AC current and printing line design as possible short, wide, which means that all connections must be to the printed line and connect to other line components placed close to each other. Length of printed lines and the inductance and resistance, while the width is proportional to the printed line inductance and resistance inversely. According to the current printed wiring, should, as far as possible, bold power line width, reduce the loop resistance. 4. concluding remarks we design and manufacture Flyback power supply, using the above steps, transformer adopts order around method with copper shield, the output side with common mode filter inductance, + 5v output voltage waveform of Burr and ripple on the peak of the smaller 60Mv, using these measures before the burrs and ripple500Mv peak is greater than. By comparison, we find that these measures are to be able to interfere with the very good effect. EMI Suppression switched mode power supply and there are many measures, such as shielding, grounding, etc. In the design of switching power supply should be considered a variety of factors, as far as possible the various switching power supply noise, the electromagnetic compatibility can be improved, so that switching power supply more widely applied.