PCB News - Switching power supply PCB design and electrical safety specifications

In any switching power supply design, the physical design of the PCB board is the last link. If the design method is improper, the PCB may radiate too much electromagnetic interference and cause the power supply to work unstable. The following are the matters needing attention in each step analyze:

one,
Design flow from schematic to PCB

Establish component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verify design -> review -> CAM output.

two,
parameter settings

The distance between adjacent wires must be able to meet electrical safety requirements, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum spacing must be at least suitable for the voltage tolerated. When the wiring density is low, the spacing of the signal lines can be appropriately increased. For signal lines with a large gap between high and low levels, the spacing should be as short as possible and the spacing should be increased. Generally, Set the trace spacing to 8mil. The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, which can avoid the defects of the pad during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed into a drop shape. The advantage of this is that the pads are not easy to peel, but the traces and the pads are not easily disconnected.

three,
Component layout

Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, the signal waveform will be delayed and reflected noise will be formed at the terminal of the transmission line. The performance drops, so when designing the printed circuit board, you should pay attention to adopting the correct method. Each switching power supply has four current loops:

(1). Power switch AC circuit

(2). Output rectifier AC circuit

(3). Input signal source current loop

(4). Output load current loop input loop

The input capacitor is charged by an approximate DC current, and the filter capacitor mainly acts as a broadband energy storage; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier, and at the same time eliminate the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should only be connected to the power supply from the terminals of the filter capacitor respectively; if the connection between the input/output loop and the power switch/rectifier loop cannot be connected to the capacitor The terminal is directly connected, and 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 of these currents are very high. The frequency is much greater 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. The transition time is usually About 50ns. These two loops are the most prone to electromagnetic interference, so these AC loops must be laid out before the other printed lines in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, inductors or transformers. Place them next to each other and adjust the position of the components to make the current path between them as short as possible.

The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

1. Place the transformer

2. Design the power switch current loop

3. Design the output rectifier current loop

4. Control circuit connected to AC power circuit

Design input current source loop and input filter
When designing the output load loop and output filter according to the functional unit of the circuit, when laying out all the components of the circuit, the following principles must be met:

(1) First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. The best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. The components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.

(2) When placing the device, consider the subsequent soldering, not too dense.

(3) Take the core component of each functional circuit as the center and lay out around it. Components should be uniform,
Arrange neatly and compactly on the PCB, minimize and shorten the leads and connections between the components, and the decoupling capacitors should be as close as possible to the VCC of the device

(4) For circuits operating at high frequencies, the distributed parameters between components should be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass produce.

(5) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

(6) The first principle of the layout is to ensure the wiring rate, pay attention to the connection of the flying leads when moving the device, and put the devices with the connection relationship together.

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

Four,
wiring

The switching power supply contains high-frequency signals. Any printed line on the PCB can function as an antenna. The length and width of the printed line will affect its impedance and inductance, thereby affecting the frequency response. Even printed lines that pass DC signals can couple to radio frequency signals from adjacent printed lines and cause circuit problems (and even radiate interfering signals again). Therefore, all printed lines that pass AC current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and other power lines must be placed very close.

The length of the printed line is proportional to its inductance and impedance, and the width is inversely proportional to the inductance and impedance of the printed line. The length reflects the wavelength of the printed line's response. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, and it can radiate more radio frequency energy. According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance.
At the same time, make the direction of the power line and the ground line consistent with the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply. It plays a very important role as a common reference point for the circuit. It is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout. Mixing various groundings will cause unstable power supply operation.
The following points should be paid attention to in the ground wire design
:

1. Correctly choose single-point grounding. Generally, the common end of the filter capacitor should be the only connection point for other grounding points to couple to the AC ground of high current. It should be connected to the grounding point of this level. The main consideration is that the current returning to the ground in each part of the circuit is changed. The impedance of the actual flowing line will cause the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and the inductance between the devices have little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so one point grounding is used, that is, the current loop of the power switch
(The ground wires of several devices are connected to the ground pin, and the ground wires of several devices that output the current loop of the rectifier are also connected to the ground pin of the corresponding filter capacitor, so that the power supply works more stable and is not easy to self-excite. When it is not possible to do a single point, connect two diodes or a small resistor at the common ground. In fact, it is enough to connect it to a relatively concentrated piece of copper foil.

2. Make the ground wire as thick as possible
If the ground wire is very thin, the ground potential will change with the current, which will make the timing signal level of the electronic equipment unstable, and the anti-noise performance will deteriorate. Therefore, it is necessary to ensure that each large current ground terminal is as short and wide as possible. To make the wire, widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, if possible, the width of the ground wire should be greater than 3mm, it can also be used The large area copper layer is used as ground wire, and the unused places on the printed circuit board are connected to the ground as the ground wire. When performing global wiring, the following principles must also be followed

(1).
Wiring direction: From the perspective of the soldering surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction is best to be consistent with the wiring direction of the circuit diagram. Because various parameters are usually tested on the soldering surface during the production process, so This is convenient for inspection, debugging and maintenance during production (Note: It refers to the premise that the circuit performance and the installation and panel layout requirements of the whole machine are met).
Wiring direction: From the perspective of the soldering surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction is best to be consistent with the wiring direction of the circuit diagram. Because various parameters are usually tested on the soldering surface during the production process, so This is convenient for inspection, debugging and maintenance during production (Note: It refers to the premise that the circuit performance and the installation and panel layout requirements of the whole machine are met).

(2). When designing the wiring diagram, the wiring should not bend as much as possible, and the line width on the printed arc should not change suddenly. The corner of the wire should be ≥90 degrees, and the line should be simple and clear.

(3). Cross circuits are not allowed in the printed circuit. For the lines that may cross, you can use "drilling" and "winding" to solve the problem. That is, let a lead "drill" through the gap under other resistors, capacitors, and triode pins, or "wind" from one end of a lead that may cross. In special circumstances, how complex the circuit is, it is also allowed to simplify the design. Use wires to bridge to solve the cross circuit problem. Due to the single-sided board, the in-line components are located on the top surface and the surface-mount devices are located on the bottom surface, so the in-line devices can overlap with the surface-mount devices during layout, but overlap of the pads should be avoided.

3. Input ground and output ground This switching power supply is a low-voltage DC-DC. If you want to feedback the output voltage back 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, you must Connect together to form a common ground

Five, check

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and at the same time, it is necessary to confirm whether the established rules meet the requirements of the printed board production process. Generally, check the lines and lines, the line and the component pads, and the lines. Whether the distance from the through hole, the component pad and the through hole, the through hole and the through hole is reasonable, and whether it meets the production requirements.
Whether the width of the power line and the ground line are appropriate, and whether there is a place to widen the ground line in the PCB. Notice:
Some errors can be ignored. For example, a part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; in addition, each time the traces and vias are modified, the copper must be re-plated.

The review is based on the "PCB checklist", which includes design rules, layer definitions, line widths, spacing, pads, and via settings. It should also focus on reviewing the rationality of the device layout, the routing of power and ground networks, and high-speed clock networks. The wiring and shielding, the placement and connection of decoupling capacitors, etc.

six,
Design output

Precautions for exporting Gerber files:

a. The layer that needs to be output has a wiring layer (bottom layer)
, Silk screen layer (including top screen printing, bottom screen printing), solder mask (bottom solder mask), drilling layer (bottom layer), and also generate drilling files (NC Drill)

b. When setting the Layer of the silk screen layer, do not select Part Type, select the top layer (bottom layer) and the Outline, Text, and Line of the silk screen layer.
c. When setting the Layer of each layer, select Board Outline. When setting the Layer of the silk screen layer, do not select Part Type, select the Outline, Text, and Line of the top layer (bottom layer) and the silk screen layer.
d. When generating drilling files, use the default settings of PowerPCB and do not make any changes.

The above is the introduction of switching power supply PCB design and electrical safety specifications. Ipcb also provides PCB manufacturers and PCB manufacturing technology.