How to avoid PCB electromagnetic interference in the design of switching power supply

- Jan 09, 2020-

How to avoid PCB electromagnetic interference in the design of switching power supply

 

In any switching power supply design, the physical design of PCB is the last link. If the design method is not appropriate, PCB may radiate too much electromagnetic interference, resulting in unstable operation of power supply. The following is an analysis of the precautions in each step:

 

1. The design process from schematic to PCB establishes component parameters - > input schematic network table - > design parameter setting - > manual layout - > manual wiring - > verification design - > Review - > cam output.

 

2. The distance between adjacent wires must meet the requirements of electrical safety. In order to facilitate operation and production, the distance should also be wider as far as possible. The minimum spacing shall be at least suitable for the voltage to be borne. When the wiring density is low, the spacing of signal lines can be increased appropriately. For the signal lines with high and low level differences, the spacing shall be as short and increased as possible. Generally, the spacing of wiring shall be set as 8mil. The distance from the inner hole edge of the solder pad to the edge of the printed board shall be more than 1mm, so as to avoid the defect of the solder pad during processing. When the wiring connected with the pad is thin, the connection between the pad and the wiring should be designed as a water drop. This advantage is that the pad is not easy to peel, but the wiring and the pad are not easy to disconnect.

 

3. The practice of component layout proves that even if the circuit schematic design is correct and the printed circuit board design is improper, the reliability of electronic equipment will be adversely affected. For example, if two thin parallel lines of PCB are close together, the delay of signal waveform will be formed, and the reflection noise will be formed at the terminal of transmission line; the interference caused by the careless consideration of power supply and ground wire will degrade the performance of the product, so when designing PCB, we should pay attention to the correct method.

 

Each switching power supply has four current circuits:

 

(1) power switch AC circuit

(2) output rectifier AC circuit

(3) input signal source current circuit

 

(4) the input circuit of the output load current circuit charges the input capacitance through a DC current, and the filter capacitance mainly plays a role of wide-band energy storage; similarly, the output filter capacitance is also used to store the high-frequency energy from the output rectifier, while eliminating the DC energy of the output load circuit. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current circuits should only be connected to the power supply from the terminals of the filter capacitors. If the connection between the input / output circuit and the power switch / rectifier circuit cannot be directly connected with the terminals of the capacitors, the AC energy will be radiated from the input or output filter capacitors to the environment. The power switch AC circuit and the rectifier AC circuit contain high amplitude trapezoidal current. The harmonic component of these currents is very high, its frequency is far greater than the switch fundamental frequency, the peak amplitude can be as high as 5 times of the continuous input / output DC current amplitude, and the transition time is usually about 50ns. These two circuits are most likely to generate electromagnetic interference, so these AC circuits must be arranged before wiring other printed wires in the power supply. The three main components of each circuit, filter capacitor, power switch or rectifier, inductance or transformer, shall be placed adjacent to each other, and the position of components shall be adjusted to make the current path between them as short as possible. The best way to establish the layout of switching power supply is similar to its electrical design. The best design process is as follows:

 

placing transformers

Design power switch current circuit

Design output rectifier current circuit

control circuits connected to AC power circuits

 

Design the input current source circuit and input filter design the output load circuit and output filter according to the functional unit of the circuit, when layout all components of the circuit, the following principles shall be followed:

 

(1) the PCB size should be considered first. When the PCB size is too large, the printed line is long, the impedance increases, the anti noise ability decreases, and the cost also increases; when the PCB size is too small, the heat dissipation is not good, and the adjacent lines are easy to be interfered. The best shape of the circuit board is rectangular, with the length width ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm from the edge of the circuit board.

 

(2) when placing the device, the future welding shall be considered, not too dense.

 

(3) layout around the core components of each functional circuit. The components shall be evenly, neatly and compactly arranged on the PCB, the lead and connection between components shall be minimized and shortened as far as possible, and the decoupling capacitance shall be close to the VCC of the component as far as possible.

 

(4) for the circuit working at high frequency, the distribution parameters between components shall be considered. In general, components shall be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass production.

 

(5) arrange the position of each functional circuit unit according to the circuit flow to make the layout convenient for signal circulation and keep the signal in the same direction as much as possible.

 

(6) the primary principle of the layout is to ensure the distribution rate of the wiring. When moving the device, pay attention to the connection of the flying wire, and put the connected devices together.

 

(7) reduce the loop area as much as possible to suppress the radiation interference of switching power supply

4The wiring switch power supply contains high frequency signal. Any printed line on PCB can play the role of antenna. The length and width of printed line will affect its impedance and inductive reactance, thus affecting the frequency response. Even the printed line passing through the DC signal will be coupled to the RF signal from the adjacent printed line and cause circuit problems (even radiated interference signal again). Therefore, all printed wires passing through AC current shall be designed as short and wide as possible, which means that all components connected to printed wires and other power wires must be placed close. The length of the printed wire is directly proportional to its inductance and impedance, while the width is inversely proportional to the inductance and impedance of the printed wire. The length reflects the wavelength of the response of the printed wire. The longer the length is, the lower the frequency at which the printed wire can send and receive electromagnetic waves, and the more RF energy it can radiate. According to the current of PCB, try to rent the width of power line to reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of current, so as to enhance the anti noise ability. Grounding is the bottom branch of the four current circuits of the switching power supply. As a common reference point of the circuit, it plays an important role in controlling interference. Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing various grounding wires will cause unstable operation of the power supply. The following points should be paid attention to in the design of ground wire:

 

1. Correct selection of single point grounding generally, the common terminal of filter capacitor shall be the only connection point for other grounding points to be coupled to the AC ground with large current, the grounding point of the same level circuit shall be as close as possible, and the power filter capacitance of the current level circuit shall also be connected to the grounding point of this level, mainly considering that the current returned to the ground by each part of the circuit is variable due to the resistance of the line actually flowing Anti interference will lead to the change of the ground potential of each part of the circuit. In this switching power supply, its wiring and inductance between devices have little influence, while the circulating current formed by the grounding circuit has great influence on the interference. Therefore, one point of grounding is adopted, that is, the switch current circuit of the power supply (the grounding wires of several devices are connected to the grounding pins, and the grounding wires of several devices of the output rectifier current circuit are also connected to the grounding pins of the corresponding filter capacitance, so The power supply works stably and is not easy to self excitation. When a single point cannot be made, connect two diodes or a small resistance at the common ground, in fact, it can be connected at a relatively concentrated copper foil.

 

2. Thicken the ground wire as much as possible. If the ground wire is very thin, the ground potential will change with the change of current, resulting in the instability of timing signal level of electronic equipment, and the deterioration of anti noise performance. Therefore, it is necessary to ensure that the ground terminal of each large current adopts the printed wire as short and wide as possible, and the width of power supply and ground wire should be widened as much as possible. The best way is that the ground wire is wider than the power line width. Their relationship is: ground wire > electricity Source line > signal line, if possible, the width of the ground wire should be greater than 3mm, or a large area of copper layer can be used as the ground wire, and the unused places on the printed board are connected with the ground as the ground wire. The following principles shall also be followed in the global wiring:

 

(1) wiring direction: from the perspective of welding surface, the arrangement direction of components shall be consistent with the schematic diagram as much as possible, and the wiring direction shall be consistent with the wiring direction of circuit diagram as much as possible. Since it is usually necessary to test various parameters on the welding surface in the production process, this is convenient for inspection, debugging and maintenance in the production process (Note: it refers to meeting the requirements of circuit performance, complete machine installation and panel layout Under the premise).

 

(2) when designing the wiring diagram, the route shall be as few turns as possible, the line width on the printing arc shall not be changed suddenly, the wire corner shall be 90 degrees, and the line shall be simple and clear as far as possible.

 

(3) no cross circuit is allowed in the printed circuit. For the possible cross lines, two methods of "drilling" and "winding" can be used. That is to say, let a lead "drill" through the gap at the foot of other resistors, capacitors and triodes, or "wind" around one end of a lead that may cross. Under special circumstances, how to make the circuit very complex? In order to simplify the design, it is also allowed to use the wire to cross to solve the problem of cross circuit. Because the single panel is adopted, the direct plug-in element is on the top side, and the surface mount element is on the bottom side, so the direct plug-in element can overlap with the surface mount element in the layout, but the pad overlap should be avoided.

 

3. The input ground and output ground are low-voltage DC-DC in the switching power supply. In order to feed the output voltage back to the primary transformer, the circuits on both sides should have a common reference ground. Therefore, after copper is laid on the ground wires on both sides, they should be connected together to form a common ground.

 

V. after the completion of wiring design, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, at the same time, it is also necessary to confirm whether the rules formulated conform to the requirements of PCB production process, generally check whether the distance between wire and wire, wire and component pad, wire and through hole, element pad and through hole, through hole and through hole are reasonable, and whether they meet the production requirements Requirement. Whether the width of power line and ground wire is appropriate, and whether there is any place for ground wire widening in PCB. Note: some errors can be ignored, for example, some connectors' outline parts are placed outside the board and frame, and errors will occur when checking the spacing; in addition, after each modification of the routing and through-hole, copper shall be coated again.

 

Vi. recheck according to the "PCB Checklist", including design rules, layer definition, line width, spacing, pad, through hole settings, as well as the rationality of device layout, wiring of power supply and ground wire network, wiring and screen of high-speed clock network

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