2021-09-30
Common circuit design schemes for DC-DC power supply
With the increasing integration of components and miniaturization of equipment, the EMI problem of electronic products is becoming increasingly serious. Reducing module power EMI can reduce the harm of EMI, avoid transmission signal quality issues, cause interference or even damage to circuits or equipment, and prevent equipment from meeting the technical specifications specified in electromagnetic compatibility standards.
Module power supply low EMI design, only for reference:
1. For all applications, there is no flawless EMI strategy, but some basic ideas in advance can make tasks easier. Firstly, ensure the position noise of the components. Decoupling capacitors should be placed as close as possible to the module power supply, especially the X and Y capacitors. Use grounding layers to reduce radiation coupling, reduce the cross-sectional area of sensitive nodes, and reduce the cross-sectional area of high current nodes that may radiate, such as those from common mode capacitors.
2. Keep the current loop small, and the ability of the conductor to couple energy through induction and radiation is reduced through a smaller loop, which acts as an antenna. For paired copper printed circuit boards (PCs), use a wide (low impedance) filter to locate the interference source, basically as close to the module power supply as possible. Filter element values should be selected, taking into account the required attenuation frequency range. If capacitors are self resonant at a certain frequency, beyond which they appear to be inductive, make the bypass capacitor lead as short as possible. When considering the proximity of noise sources to potentially susceptible circuits, locate the components on the PC board.
3. The position of EMI components is crucial to avoid placing the converter near the filter to prevent noise coupling back into the filter. Not only do we need to filter the power supply, but we also need to filter all the circuits that the converter is supplying power to. Most communication cabinets use as much local filtering as possible at the card level, and then use another filter on the module power input.
4. Compared with through-hole modules, surface mount devices (SMD) are better at handling RF energy than lead based devices because they can reduce inductance and place components more tightly. Due to the reduction in physical size of SMD, the latter is possible. This is crucial for double-layer circuit board design as it requires the efficiency of noise control components. Usually, lead capacitors become self resonant (more inductive than capacitors) at around 80MHz. Due to the need to control noise above 80 MHz, serious issues should be raised if only through hole components are used for design execution.
5. The rapid voltage and current changes in the conducted emission (CE) DC/DC converter will cause conducted noise at the input of the module power supply. The rapid rise and fall times of logical loads will generate conductive noise, which will also be reflected back into the input. Conducting noise can generate electric and magnetic fields, and if the circuit is not properly configured, it can generate noise. Usually, good layout and filter design will reduce this impact. In order to better understand the source of CE, emissions are classified as differential (symmetric) or common (asymmetric) mode noise.
6. Whether it is direct interference from circuits or interference between devices, it is the process of electromagnetic disturbance energy coupling from the disturbance source to the sensitive source. There are two ways to transfer energy: one is to transfer it along a conductor in the form of an electric current, and the other is to transfer it through space through electromagnetic induction. The main conductor connecting two circuits (devices) is the power line, so the power line is the main factor causing electromagnetic interference problems. When electromagnetic interference problems occur, whether at the circuit or equipment level, the power supply of the affected circuit (equipment) should be checked first to see if there is any interference voltage, and the ground wire is also an important path for interference propagation.
It is crucial to minimize module power EMI in order to ensure the normal and stable operation of the equipment.
