Electronic Design - Design specification for PCB circuit board of switching power supply

1.Design flow from schematic diagram to PCB

Establish component parameters - > input principle net list - > design parameter setting - > manual layout - > manual wiring - > verify design - > recheck - > cam output.

2.Parameter setting: the distance between adjacent conductors must meet the electrical safety requirements, and the distance should be as wide as possible in order to facilitate operation and production. The minimum spacing shall at least be suitable for the withstand voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased to meet the high voltage The signal lines with great low-level difference shall be as short as possible and the spacing shall be increased. Generally, the routing spacing shall be set as 8mil. The distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, which can avoid the defect of the pad during processing. When the wiring connected with the pad is thin, the connection between the pad and the wiring should be designed into a water drop shape. This has the advantage that the pad is not easy to peel, but the wiring and the pad are not easy to be disconnected.

3.The practice of component layout has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have an adverse impact on the reliability of electronic equipment.For example,if two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed and the reflected noise will be formed at the terminal of the transmission line;

4.Due to power supply 

The interference caused by the thoughtless consideration of the ground wire will degrade the performance of the product.Therefore,when designing the printed circuit board, 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 loop

(4).Output load current circuit

The input circuit charges the input capacitor through an approximate DC current, and the filter capacitor mainly plays a role of broadband energy storage; Similarly, the output filter capacitor is also used to store the high-frequency energy from the output rectifier and eliminate the DC energy of the output load circuit. Therefore, the terminals of input and output filter capacitors are very important. 

The input and output current circuits should be connected to the power supply only from the terminals of filter capacitors respectively;If the connection between the input / output circuit and the power switch / rectifier circuit cannot be directly connected with the terminal of the capacitor, the AC energy will be radiated into the environment by the input or output filter capacitor. 

The AC circuit of the power switch and the AC circuit of the rectifier contain high amplitude trapezoidal currents. The harmonic components in these currents are very high, and their frequency is much higher than the fundamental frequency of the switch.The peak amplitude can be as high as 5 times the amplitude of the continuous input / output DC current, and the transition time is usually about 50ns. 

These two circuits are most prone to electromagnetic interference, so these AC circuits must be laid before other printed wires in the power supply. The filter capacitors of the three main components of each circuit Power switch or rectifier Inductors or transformers shall be placed adjacent to each other, and the position of elements 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:

*Place transformer

*Design switching current circuit of power supply

*Design output rectifier current loop

*Control circuit connected to AC power circuit

*Design the input current source circuit and input filter

When designing the output load circuit and output filter, the layout of all components of the circuit according to the functional unit of the circuit shall comply with the following principles:

1) First, consider the size of PCB. When the size of PCB circuit board is too large, the printed line is long, the impedance increases, the anti noise ability decreases, and the cost also increases; If it is too small, the heat dissipation is poor, and the adjacent lines are easy to be disturbed. The best shape of the circuit board is rectangular, with an aspect ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.

2) When placing devices, consider future welding and do not be too dense

3) Take the core components of each functional circuit as the center and layout around it. Components shall be uniform Tidy It shall be compactly arranged on the PCB circuit board to minimize and shorten the leads and connections between components, 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 should be considered. For general circuits, components shall be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld and easy to mass produce.

5) Arrange the position of each functional circuit unit according to the circuit flow, make the layout convenient for signal flow, and keep the signal in the same direction as far as possible.

6) The primary principle of the layout is to ensure the routing rate of the wiring. Pay attention to the connection of flying wires when moving devices, and put the devices with connection relationship together.

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

The wiring switching power supply contains high-frequency signals. 

Any printed line on the PCB can act as an antenna. 

The length and width of the printed line will affect its impedance and inductive reactance, thus affecting the frequency response. Even printed lines passing through DC signals will be coupled to RF signals from adjacent printed lines and cause circuit problems (or even radiate interference signals again). Therefore, all printed lines passing through AC current should be designed as short and wide as possible, which means that all components connected to printed lines and other power lines must be placed close to each other. The length of the printed line is directly proportional to its inductance and impedance, while the width is inversely proportional to its inductance and impedance. The length reflects the wavelength of the printed line response. The longer the length, the lower the frequency that the printed line can send and receive electromagnetic waves, and it can radiate more RF energy. According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time Turn off the power cord The direction of the ground wire is consistent with the direction of the current, which helps to enhance the anti noise ability. Grounding is the bottom branch of the four current circuits of switching power supply. As the common reference point of the circuit, it plays a very important role. It is an important method to control interference. Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing various grounding wires will cause instability of power supply.

Pay attention to the following points in the design of ground wire

1).  Select single point grounding correctly. Generally, the common end of the filter capacitor should be the only connection point of other grounding points coupled to the AC ground with large current. The grounding point of the same level circuit should be as close as possible, and the power filter capacitor of the current level circuit should also be connected to the grounding point of this level, mainly considering that the current flowing back to the ground of each part of the circuit is variable, Because the impedance of the line actually flowing through will lead to the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and inductance between devices have little influence, while the circulating current formed by the grounding circuit has a great influence on the interference, so one point grounding is adopted, Power switch current circuit (the ground wires of several devices in the are connected to the grounding pin, and the ground wires of several devices of the output rectifier current circuit are also connected to the grounding pin of the corresponding filter capacitor, so that the power supply works more stably and is not easy to self excite. When a single point cannot be achieved, connect two diodes or a small resistance at the common ground, which can be connected to a relatively concentrated piece of copper foil.

2).  Thicken the grounding wire as much as possible. If the grounding wire is very thin, the grounding potential will change with the change of current, resulting in unstable timing signal level and deterioration of anti noise performance of electronic equipment. Therefore, it is necessary to ensure that the grounding terminal of each high current adopts the printed wire as short and wide as possible and widen the power source as much as possible The width of ground wire is better than that of power supply. Their relationship is: ground wire > power wire > signal wire. If possible, the width of ground wire should be greater than 3mm. Large area copper layer can also be used as ground wire. All unused places on the printed board are connected with the ground as ground wire.

When conducting global wiring,the following principles must also be followed

1). Wiring direction: from the welding surface, the arrangement direction of components shall be consistent with the schematic diagram as far as possible, and the wiring direction shall be consistent with the wiring direction of the circuit diagram. Because various parameters are usually required to be tested on the welding surface in the production process, this is convenient for inspection in production, Commissioning and maintenance (Note: on the premise of meeting the requirements of circuit performance, whole machine installation and panel layout).

2). When designing the wiring diagram, the routing shall be as few turns as possible, the line width on the printed arc shall not be abrupt, and the wire corner shall be ≥ 90 degrees, so as to make the line simple and clear.

3). No cross circuit is allowed in printed circuit. For lines that may cross, use "drill" "Around" two solutions. That is, let a lead from another resistance capacitance. The gap at the foot of the triode is "drilled" or "wound" from one end of a lead that may cross. Under special circumstances, the circuit is very complex. In order to simplify the design, wire bridging is also allowed to solve the problem of cross circuit. Because the single panel is adopted, the in-line component is located on the top surface and the surface mount device is located on the bottom surface, the in-line component can overlap with the surface mount device during layout, but the pad overlap should be avoided.

4). Input ground and output ground the switching power supply is a low-voltage DC-DC. In order to feed back the output voltage to the primary of the transformer, the circuits on both sides should have a common reference ground. Therefore, after laying copper on the ground wires on both sides, they should be connected together to form a common ground

PCB inspection

After the wiring design is completed PCB layout, it is necessary to carefully check whether the wiring design meets the rules formulated by the designer, and also confirm whether the rules formulated meet the requirements of the printed board production process. Generally, check the wires and wires Wire and component pads Line and through hole Element pad and through hole Whether the distance between through holes is reasonable and meets the production requirements. 

Whether the width of the power line and ground wire is appropriate, and whether there is any place in the circuit board where the ground wire can be widened. Note: some errors can be ignored. For example, a part of the outline of some connectors is placed outside the board frame, and an error will occur when checking the spacing; In addition, copper cladding shall be carried out again every time the wiring and vias are modified.

Recheck according to the "PCB board Checklist", including design rules and layer definitions Line width Spacing Pad For via setting, we should also focus on rechecking the rationality of device layout and power supply The routing of ground wire network, the routing and shielding of high-speed clock network, the placement and connection of decoupling capacitors, etc.

Precautions for designing and outputting photo files:

1). The layer that needs to be output is the wiring layer (bottom layer) Screen printing layer (including top screen printing and bottom screen printing) Solder resist layer (bottom layer) Drilling layer (bottom layer), and also generate drilling file (NC drill)

2). When setting the layer of the silk screen layer, do not select part type, select the top layer (bottom layer) and the outline of the silk screen layer Text.  Linec.  When setting the layer of each layer, select the board outline. When setting the layer of silk screen layer, do not select part type, and select the outline of top layer (bottom layer) and silk screen layer Text.  Line. d. When generating drilling files, use the default settings of powerpcb board without making any changes.