In addition to the selection and design of the circuit components , a good printed circuit board (PCB) design Electromagnetic Compatibility is a very important factor. Key PCB EMC design is to minimize the area under reflux , so that the return path in the direction of flow of the design . The most common problem of the return current from the reference plane of the crack , changing the reference plane layer and a signal flowing through the connector . Or decoupling capacitor connected across the capacitor may be able to solve some problems, but will need to take into account the overall impedance of the capacitor , vias, pads and wiring . The hierarchical strategy stresses from PCB layout and routing rules three techniques introduced EMC PCB design technology.
PCB layered strategy
circuit board design thickness , vias and circuit board manufacturing process layers is not the key to solving the problem, fine layered stack is to ensure that the power bus bypass and decoupling , the power ground plane layer or transient voltage on the minimum and electromagnetic shielding of the signal and power up the key. From the signal traces , the good stratification strategy should be all the signal traces on one or several layers , these layers next to the power supply or ground floor . For power , a good strategy should be stratified layer adjacent to the power and ground layers , and the distance between the power supply layer and the ground layer as small as possible , and this is what we call a "layered" strategy. Here we will talk about the specific PCB excellent layering strategy.
1 . Projection plane of the wiring layer should be refluxed in a plane layer region . If the wiring layer of the plane layer is not refluxed projected area, there will be a signal wiring lines outside of the projection area , resulting in " edge radiation" problem, and also causes the signal circuit area increases , resulting in differential mode radiation increases.
2 . Avoid wiring layer disposed adjacent . Because the signal traces adjacent parallel wiring layer causes the signal crosstalk , the wiring layer can not be avoided if the adjacent layers should be properly spacing the two pull-up wiring layer , the wiring layer is reduced and its interlayer spacing between the signal circuit .
3 . Adjacent plane layer should avoid overlapping projection plane . Because the overlapping projection , a coupling capacitance between the layers results in the noise coupling between the layers to each other .
Multilayer Design:
When the clock frequency exceeds 5MHz, or less than the signal rise time 5ns, in order to make the loop area of the signal can be well controlled, generally requires the use of multilayer design . In the design of multilayer should note the following principles:
1 . ( Where the layer clock line , bus , interface signal cable, RF cable, reset signal, chip select signal line and a variety of control signal lines , etc. ) should complete critical wiring layer adjacent to the ground plane , preferably between two planes, such as Figure 1. The key signal lines are generally extremely sensitive to strong radiation or signal lines , the ground plane close to the signal wiring circuit area can be reduced , reducing the intensity of radiation or to improve noise immunity .
2 . Power plane should shrink relative to its neighbor (recommended 5H ~ 20H) in the ground plane. Power plane relative to the ground plane shrink reflux can effectively suppress the "edge radiation" problem.
In addition, the work of a single moderators power plane ( the most widely used power plane ) should be close to its ground plane , in order to effectively reduce the supply current loop area .
3 . Veneer TOP, BOTTOM layer is no ≥ 50MHz signal lines. If so, the best walking in the high-frequency signal between two planar layer to suppress the radiation of space .
Single board and double board design :
For single and double panel board design , the main should be noted that the key signal line and power line design . There must be close to the power ground traces its close , parallel lines , to reduce the supply current loop area .
The key signal lines on both sides of the cloth layer board should "Guide Ground Line", shown in Figure 4 . The key should be on the double plate signal line ground floor a large area of the projection plane , or a way to deal with a single-layer board design "Guide Ground Line", shown in Figure 5 . The key signal lines on both sides to " defend the ground" on the one hand can reduce the signal loop area , in addition, can also prevent crosstalk between the signal line and ground the other signal lines.
PCB layout techniques
PCB layout design should fully comply with the placement of a straight line along the signal flow design principles to avoid back and forth surround , as shown in Figure 6 . This avoids direct signal coupling , signal quality. In order to prevent the circuit between the layout of electronic components interfere with each other and the coupling between the circuit and the components placement should follow the following principles:
1 . If the design of the interface to a single board , " clean" , then filtering, isolation devices should be placed in the " clean" and work to isolation between belt. This avoids filtering or isolation device coupled to each other by plane layer , weakening effect. In addition, the " clean" on , in addition to filtering and protection devices , can not place any other devices.
2 . When multiple modules placed on the same circuit PCB, digital circuits and analog circuits , high-speed and low-speed circuit layout should be separated to avoid mutual interference of digital circuits, analog circuits , high-speed circuit and the low-speed circuits. In addition, when the circuit board while the presence of high , medium and low-speed circuit , in order to avoid high frequency circuit noise radiated outward through the interface , you should comply with the principles of the layout in Figure 7 .
3 . PCB power input filter circuit should be placed close to the interface , has been filtered to avoid lines are coupled again.
4 . Filtering , protection and isolation devices interface circuit is placed near the interface , you can effectively achieve protection , filtering and isolation effect. If both interfaces have protective filter circuit , the protective filter should comply with the first principle. Because the protection circuit is used for external overvoltage and overcurrent suppression , if the protection circuit is placed after the filter circuit , filter circuit is overvoltage and overcurrent damage. In addition , it will weaken due to the input of the output circuit when traces mutual coupling filtering , isolation or protective effect , while the layout to ensure that the filter circuit ( filter ) , input and output line isolation and protection circuitry coupled to each other do not .
5. Sensitive circuit or device ( such as a reset circuit, etc. ) away from the edges of the board side edge of the interface in particular at least 1000mil.
6. Nearby ( such as input and output terminals of the power module and the fan relay ) means there is a big change in the current circuit or device should be placed between the storage capacitor and the high frequency filter , to reduce the large area of the loop current loop .
7 . Filter element to be placed side by side , to prevent the circuit is disturbed again filtered .
8. Crystal , crystal, relays, switching power supplies and other devices away from the intense radiation of the interface connector at least 1000mil. Such interference can be radiated outward or directly on the cable out of the current coupling to external radiation .
PCB routing rules
In addition to the selection and design of the circuit components , a good printed circuit board (PCB) wiring Electromagnetic Compatibility is also a very important factor. Since it is an inherent component of the system PCB , PCB layout enhanced electromagnetic compatibility in the final product will not be completed to bring additional charges . Any person should remember that poor PCB layout EMC can cause more problems rather than eliminate these problems , in many cases, even with the filters and components can not solve these problems. In the end, had to rewire the entire board . Thus , at the beginning to develop good habits PCB layout is the most economical way . The following will be some general rules PCB layout and power lines , ground and signal line design strategies are introduced , and finally, according to these rules, the typical printed circuit board circuit air conditioner suggest improvements .
1. Wiring separation
Separation is the role of the wiring within the same layer PCB noise coupling and crosstalk between adjacent lines is minimized. 3W specification indicates that all signals ( clock , video, audio , reset , etc. ) have to be like that shown in Figure 10 , be isolated between online and offline , edge to edge . In order to further reduce the magnetic coupling , the reference ground cloth placed near the key signal to isolate the other signal line coupling noise generated .
2 . Protection and shunt line
Shunt circuit is provided and the protection of critical signals , such as the system clock signal from a noisy environment to isolate and protect a very effective method. Parallel or PCB protection circuit is placed along the critical signal lines cloth. Isolation of the protection circuit is coupled only by the magnetic flux generated by the other signal line , and also isolate the critical signals from the signal lines coupled to the other open. The difference between the line and the protection line shunt shunt line that does not have to be terminated ( connected to ground ) , but both ends of the protected line must be connected to ground. To further reduce the coupling in a multilayer PCB protection circuit can be coupled to the ground at intervals on the path .
3 . Power Line Design
Depending on the size of current PCB , bold power line width as possible to reduce the loop resistance . Meanwhile, the power cord, in the same direction toward the ground , and data transfer , which helps to enhance noise immunity . In a single panel or double panel, if the power cord is very long traces should be added for every 3000mil decoupling capacitor , the capacitor value is 10uF +1000 pF.
4 . Ground Design
Ground design principles are:
( 1 ) separate digital and analog . If there are both linear logic circuit on the circuit board , so that they should be separated as much as possible . Low-frequency circuit ground should be connected in parallel with a single point grounding , the actual difficult part of the series and then wiring parallel to the ground . High-frequency circuits should adopt multi-point series grounding , ground rent should be short and surrounded by high-frequency components as much as possible with a grid-like large area foil.
( 2 ) ground wire should be bold. If the ground wire with a very sewing lines, the ground potential varies with current changes , make anti-noise performance degradation . Therefore, the ground wire should be bold, to make it through three times the allowable current on the PCB . If possible, the ground wire should be 2 ~ 3mm or more.
( 3 ) constitute a closed loop ground . PCB only by a digital circuit , the grounding circuit loop of cloth into the group most able to improve noise immunity .
5. Signal line design
For critical signal lines , if there is an internal signal traces veneer layer , the clock and other key signal line cloth lining , giving priority to the preferred wiring layer . In addition , the key signal lines must not cross partition alignment, including vias, pads gap caused by the reference plane , otherwise it will lead to increased signal circuit area . And the key signal line should be away from the reference plane edge ≥ 3H (H is the height of the line from the reference plane ) to suppress the edge radiation effects.
For the clock line , bus lines, and strong radiation of RF signal line and a reset signal lines , chip select signal line , the system control signals and other sensitive signal lines , signal lines should go away from the interface . Thus avoiding interference intense radiation signal line coupled to the signal line out , external radiation ; also avoid external interference signal line interface to go with incoming signal is coupled to the sensitive line, cause the system to malfunction.
For differential signal lines should be the same layer , and so long , and walking the line , consistent impedance between differential lines no other traces. Because guaranteed common mode impedance differential pairs are equal, can improve its anti-jamming capability .
According to the above wiring rules, the typical printed circuit board circuit air conditioner to improve optimization.
Overall , PCB design improvements for EMC is : Before wiring , the first research design good return path , there is the best chance of success , can reduce EMI radiation to reach the goal . And no hands before the actual wiring , the wiring layer changes so do not spend any money, is the cheapest way to improve EMC .
