【正文】 In the zero current quasi resonant transform, through the power switch current shall prevail sine, di/dt small, which can reduce EMI level. Because, the spectrum with narrow, and focused on the near the resonant frequency, easy to the design of filter. To pay special attention to reduce the power switch of di/dt and dv/dt and reduce the noise of rectifier diode buffer circuit design. filter Filter to suppress the interference is one of the effective measures, especially in dealing with conducted interference, has obvious effect. To weaken conducted interference, the EMI level control in the EMC standards of extreme level below. In addition to restrain interfereing source beyond, the most effective way is in the switch power input and output circuit with EMI filters. In filter circuits, choose to wear heart capacitor, three end capacitors, ferrite. They can can improve filtering properties of the circuit. The EMI filter can inhibit mon mode interferences and can restrain the differentialmode interference. It is switching power supply EMI filter of the basic work structures, including L1 and L2 is around the same on two independent graviation coil, the same circle number, have the same direction of the same end, called the monmode 黃河科技學(xué)院畢業(yè)設(shè)計 (文獻(xiàn)翻譯 ) 第 13 頁 inductor winding or mon mode coil. L3 and is independent of the poor BMD at l1l4 mode rejection inductance, C1 C2 and C3, is capacitors. If the filter end access interference sources, the load in again be interference equipment, then L1 and C1, L2 and C2 were two independent port to constitute the between the lowpass filter to restrain the power cord of the existence of mon mode EMI signal, the attenuation, and controlled to a very low level. L3 and formed an independent BMD at l1l4 poor stray inductance and capacitance mode rejection C3 formed a lowpass filter to restrain the power cord of the existence of differential mode EMI signal. The appropriate design or choose appropriate filter, and properly installed filter is an important part of the antiinterference techniques, concrete measures as follows. in ac input the power supply filter, the circuit diagram shown in figure 2. One L3, BMD at l1l4 and C3 to prevent differential mode noise, L1 and L2, C1 and C2 to prevent mon mode noise. All of the power supply filter must be grounded, because of the filter mon mode bypass capacitors must be grounded to take effect. General groundi ng method in addition to filter and metal case connect, but also with a thick wire will filter shell and equipment on the site is linked together, ground impedance small filter the better the results. In addition, should be installed in close to filter the power the entrance, avoid interference signals from the input directly coupled to the output. in power output terminal and output filter. The high frequency capacitor, increase output filter the inductance of the inductance quantity and the capacity of the filter capacitance, can restrain differential mode noise. If the multiple capacitance parallel, the effect will be better. In the use of the device, we must also pay attention to the following points. filter must have good shielding, shields and power supply good lap. input filter should hold in the input, output filter must be packed in output port place, and stay away from the internal electromagic emission strong inductor, switch power, etc. If possible, as far as possible as an independent ponents and power reasonable 黃河科技學(xué)院畢業(yè)設(shè)計 (文獻(xiàn)翻譯 ) 第 14 頁 connection. filter the input and output line can39。具體的作法是在繞制完初級繞組后，包上一層～ 厚的薄銅皮，銅皮的始端和末端必須有 3～ 5mm 長的重疊（重疊部分必須相互絕緣）。 應(yīng)選用高頻特性好的電容器。其中 L3， L4 和 C3 用于抑制差模噪聲， L1， L2， C1 和 C2 用于抑制共模噪聲。 2 電磁兼容的設(shè)計 電磁兼容性設(shè)計包括電路選擇、元器件的選擇、濾波、屏蔽、接地、布局等。但是這個電感不能大，因為在高頻下 di/dt 很大，也會引入一個騷擾，因此必須折中。 電子設(shè)備或系統(tǒng)內(nèi)部的各個部件和子系統(tǒng)、一個系統(tǒng)內(nèi)部的各臺設(shè)備乃至相鄰幾個系統(tǒng)，在它們自己所產(chǎn)生的電磁環(huán)境及在他們所處的外界電磁環(huán)境中，能按原設(shè)計要求正常運行。這些變化迫使人們把電磁兼容作為重要的技術(shù)問題加以關(guān)注。所以，在電路設(shè)計的初期，即進(jìn)行電路方案的選擇時就應(yīng)著手考慮 EMC 問題。 對于電磁場而言，電場分量和磁場分量總是同時存在的。 這種 EMI 濾波器既能抑制共模干擾又能抑制差模干擾。 黃河科技學(xué)院畢業(yè)設(shè)計 (文獻(xiàn)翻譯 ) 第 5頁 輸入濾波器應(yīng)裝在輸入端口處，輸出濾波器應(yīng)裝在輸出端口處，并遠(yuǎn)離內(nèi)部電磁發(fā)射很強的電感器、功率開關(guān)等。這就需要在變壓器的設(shè)計上下功夫，使原邊繞組和副邊繞組盡可能地靠近，同時和磁芯也要盡量靠近，這樣漏磁通就會減到最小。 結(jié)語 隨著電子產(chǎn)品的電磁兼容性日益受到重視，抑制開關(guān)電源的 EMI,提高電子產(chǎn)品的質(zhì)量，使之符合有關(guān)標(biāo)準(zhǔn)，已經(jīng)成為人們越來越關(guān)注的問題。s size. 3 other reduce the method of electromagic interference the layout of the ponents In the circuit design, usually interfereing source and disturbance of circuit of working conditions due to limited and hard to avoid. At this time, should as far as possible will be connected ponents put together, to avoid the device too far away from caused by printing line too long brought about by the interference。因此，功率開關(guān)管在快速開斷時產(chǎn)生的電磁噪聲就會 通過 黃河科技學(xué)院畢業(yè)設(shè)計 (文獻(xiàn)翻譯 ) 第 7頁 Ck 耦合到散熱片上面。使用通用板子時，還必須注意功率電路的地對信號地的干擾。 在使用濾器器的時候 ,我們還必須注意以下幾點。除抑制騷擾源以外，最有效的方法就是在開關(guān)電源輸入和輸出電路中加裝 EMI 濾波器。 和開關(guān)管一樣，電路當(dāng)中的脈沖變壓器也存在高頻率的 di/dt 變化，也會向空間輻射高頻的電磁波，干擾其他的元器件和設(shè)備。這都是由于功率變換電路中的整流二級管和 功率開關(guān)管在工作過程中所產(chǎn)生的電壓和電流的躍變，并通過高頻變壓器、儲能電感線圈以及電路中的元件布局和器件本身自帶的寄生參數(shù)之間相互作用而造成的。隨著電子產(chǎn)品越來越多地采用低功耗、高速度、高集成度的 LSI 電路，而使得這些裝置比以往任何時候更容易受到電