How to design the PCB board of the LED switch power supply

　　Author ：Iflowpower – Fournisseur de centrales électriques portables

The physical design of the PCB board in the switching power supply design is the last link. If the design method is not properly, the PCB may radiate too much electromagnetic interference, resulting in unstable power work, and the following is decomposed for matters needed in each step: I. From the principle map to PCB design process establishment component parameters - Enter the principle network table - Design parameter settings - Manual layout - Handmade Wiring - Verification Design - Review -CAM Output.

Second, the parameter setting adjacent wire spacing is not to meet electrical safety requirements, and the spacing should also be as wide as facilitating operation and processing. The minimum spacing is at least suitable for the voltage, when the wiring density is low, the spacing of the signal line can be appropriately increased, the signal line of high and low levels should be as short as possible and increase the spacing, in general Set the line spacing to 8 mils. The distance from the inner hole edge to the printed plate is greater than 1 mm, which prevents the pad defect during production.

When the trace is connected to the pad, the connection between the pads and the traces should be designed to a water droplet, which is that the pads are not easy to lift, but the traces are not easy to disconnect. Third, the components layout practice proves that even if the schematic design is correct, the printing circuit board is not properly designed, and it will also adversely affect the reliability of electronic equipment. For example, if the two fine lines of printed plates are close to the line, the delay of the signal waveform is formed, and the reflection noise is formed at the terminal of the transmission line; due to the power supply, the ground is considered, the product will make the product Performance declines, so, when designing printed circuit boards, pay attention to adopting the right way.

Each switching power supply has four current loops: (1). Power switch AC circuit (2). Output rectification AC circuit (3).

Input signal source current loop (4). Output load current loop input loop through an approximate DC The current is charged to the input capacitor. The filter capacitor is important to a broadband energy storage purpose; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier while eliminating the DC energy of the output load loop.

Therefore, the terminals of the input and output filter capacitors are very tightly, and the input and output current circuits should only be connected to the power supply from the terminals of the filter capacitor; if the connection between the input / output loop and the power switch / rectifier circuit cannot be unable to compete The terminal pen is connected, and the AC energy will be input or output filter capacitance and radiate into the environment. The power switch AC circuit and the rectifier AC loop contain high trapezoidal current. These harmonic components are high.

The frequency is much larger than the switch based frequency, and the peak magnitude can be high to continue the input / output DC current amplitude. 5 times, transition time is usually About 50ns. These two circuits are most likely to generate electromagnetic interference, so these exchange circuits are fabricated before other printed lines in the power supply, and three important component filter capacitors, power switches or rectifiers, inductors, and transformers should be each other.

Place in adjacent, adjust the position of the component to make the current path between them as short as possible. The best way to establish a switching power supply is similar to its electrical design, the best design process is as follows: Place the transformer Design Power Switch current loop design Output rectifier Current circuit Connection to AC power circuit control circuit design input current source loop and input filter design output The load loop and output filter are in accordance with the functional unit of the circuit, and the following principles are in line with the following principles: (1) First, consider the size of the PCB size. When the size of the PCB is too large, the printed line is long, the impedance is added, the anti-noise ability is lowered, and the cost is also increased.

It is too small, and the neighboring line is susceptible to interference. The optimum shape rectangle of the circuit board, aspect ratio of 3: 2 or 4: 3, is located on the edge of the circuit board, which is generally not less than 2mm from the edge of the circuit board. (2) When placing the device, consider the subsequent welding, do not be too intensive.

(3) Taking the core components of each function circuit around it to lay out. The components should be average, neat, compactly arranged on the PCB, minimize lead and connection between the components, and the decoupling capacitance is as close as possible to the VCC of the device. (4) Circuits operating at high frequencies to consider distribution parameters between components.

General circuitry should be arranged in parallel as much as possible. This, not only beautiful, and it is easy to use, easy to batch processing. (5) Arrange the position of each functional circuit unit according to the circuit of the circuit, so that the layout is easy to flow, and the signal is as consistent as possible.

(6) The primary principle of the layout is to ensure the barring rate of the wiring, pay attention to the connection of the flying line, put the device with connected relationship together. (7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply. Fourth, the wiring switching power supply contains high frequency signals, and the print line on the PCB can play an antenna, the length and width of the printing line affect its impedance and induction, thereby affecting the frequency.

Even the printed lines of the DC signal are also coupled from adjacent printed lines to RF signals and cause circuit problems (even radiation signals again). It should therefore be designed to be as short as possible by the printing line through the AC current, which means that all components connected to the printed wire and connected to other power cords are very close. The length of the printing line is proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the printed line.

The length reflects the wavelength of the printing line, the longer the length, the lower the frequency of the printing line can transmit and receive electromagnetic waves, and it can radiate more radio frequency energy. According to the size of the printed circuit board, try to rent the power line width, reduce loop resistance. At the same time, the direction of the power cord, the ground wire is consistent, which helps to strengthen the anti-noise ability.

The ground is the underlying branch of the four current circuits of the switching power supply. It plays a very important purpose as the public reference point of the circuit. It is awertime method for controlling interference.

Therefore, the placement of the ground wire should be carefully considered in the layout, and the various ground mixing will be unstable. The following points should be paid to the following: 1. Correctly selecting a single point grounding usually, the filter capacitor common end should be the unique connection point of other attached places to the exchange of large currents, the same level of the same level should be as best Near, and the power filter capacitance of the level of the circuit should also be connected to the grounding point, it is important to consider the current of each part of the circuit to the ground is changed, and the impedance of the actually flowing will lead to part of the circuit.

Changes to introduce interference. In this switching power supply, its inductance between the wirings and the device has a smaller, and the circulation formed by the ground circuit has a large impact on the interference, and thus uses a little ground, so that the power switch current loop (several devices in several devices in several devices On the foot of the ground, the ground wire of several devices that output the rectifier current circuit also receive the corresponding filter capacitor, so that the power supply is more stable, it is not easy to be self-excited. When you don't do a single point, you can connect two ordered tubes or a small resistance in a common place.

2. Try to the ground wire, if the grounding wire is fine, the grounding potential changes with the current change, causing the on-time signal level on time, the anti-noise performance is deteriorated, so ensure that the ground end of each large current Using the short-wider print line, try to broaden the power supply, the width of the ground, preferably the ground wire is wide than the power line, and their relationship is: ground line> power cord> signal line, if possible, ground line The width should be greater than 3mm, or the large-scale copper layer is used for the ground, and the place where it is not used on the printing plate is connected as a ground. When conducting global wirings, the following principles must also be followed: (1).

Wiring direction: From the weld surface, the arrangement of the components should be consistent with the principle chart as much as possible, and the wiring direction is best consistent with the direction of the circuit diagram. During the machining process, the test is often tested in the welding surface, so this is easy to process inspections, commissioning and overhaul (Note: refer to the premise of meeting circuit performance and machine installation and panel layout requirements). (2).

When designing the wiring diagram, the trace should be turned to the curve. The line width on the printing arc should not be mutated, the corner of the wire should be ≥90 degrees, and the line is simple and clear. (3).

No interwoven circuit is performed in the printed circuit, and the possible lines can be dealt with "drill", "wind around". That is, a lead is "drilled" from the voids of other resistors, capacitors, and triode, or from one end of a lead wire that may be interleaved, how is the circuit in special cases is complicated, and is a simplified design. Also permitted with wire bridge, handling interleaving circuit issues.

Because of the single-sided panel, the direct-in element is located in the TOP surface, the table stickers are located in the bottom surface, so the interpolation device can overlap the table stickers when the layout, but to prevent pad overlay. 3. Input and output grounded DC-DC in the power supply of the output ground, want to feed back the output voltage back to the transformer, and the circuits on both sides should have a common reference ground, so after paving the ground on both sides, Connect together to form a common place.

V. After the check wiring design is completed, it is necessary to carefully check the wiring design is not the rules set by the designer. At the same time, it is also necessary to confirm that the rules set are not in line with the demand for the printing board, the general inspection line and lines, lines The components pad, line and through holes, element pads and through holes, the distance between the through holes and through holes is not reasonable, is it to meet the processing requirements?.

Is the width of the power cord and the ground wire properly, is there where the floor is widening in the PCB. Pay attention to: Some errors can be ignored, for example, some of the plug-in's OUTLINE is placed outside the board box, check the spacing error; additionally, after each modify the traverse and via holes, you must re-coating once. Sixth, review according to the "PCB Checklist", including design rules, layer meaning, line width, spacing, pad, via setting, and focus on the rationality of the device layout, power, ground line network, high speed The trace of the clock network and shield, the placement and connection of decoupling capacitance, etc.

7. Tips for designing output output light painting documents: a. The layer to be output has a wiring layer (under layer), silk print layer (including top-layer screen printing, underlayer screen), solder resist layer (underlayer soldering), drilling layer (underlayer) Also generate a drilling file (ncdrill) b.

When setting the Layer of the silk print layer, do not select PartType, select the top layer (underde) and the silk screen Outline, Text, Linec. When setting the Layer of each layer, boardoutline Optionally, when setting the Layer of the silk print, do not select PartType, select the top layer (underlying) and the silk screen Outline, Text, Line. d.

When generating a drilling file, use PowerPCB's default settings, don't make any changes. .