Problems that need to be paid attention to in each link of switch power supply design

- Aug 28, 2018-

The physical design of PCB board is the last step in the design of switching power supply. If the design method is not appropriate, PCB may radiate too much electromagnetic interference, resulting in power supply instability.

Design flow from schematic diagram to PCB

Set up the component parameters - "input principle network table -" design parameters setting - "manual layout -" manual wiring - "verification design -" review - "CAM output. 

Problems that need to be paid attention to in each link of switch power supply design

Component layout

Practice has proved that even if the circuit diagram is designed correctly and the printed circuit board is designed improperly, the reliability of the electronic equipment will be adversely affected.For example, if the two thin parallel lines of the printing plate are close together, the signal waveform delay will be generated and reflected noise will be formed at the end of the transmission line.Due to the disturbance caused by the thoughtless consideration of power supply and ground wire, product performance will decline. Therefore, when designing the printed circuit board, the correct method should be adopted.Each switch power supply has four current loops:

(1) power switch ac loop

(2) output rectifier ac loop

(3) input signal source current loop

(4) output load current loop input loop

By charging the input capacitance with an approximate dc current, the filter capacitance mainly plays a role of broadband energy storage.Similarly, the output filter capacitor is used to store high frequency energy from the output rectifier while eliminating dc energy from the output load loop.Therefore, the connection terminal of the input and output filter capacitors is very important. The input and output current loop should only connect to the power source from the connection terminal of the filter capacitors.If the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the capacitor's terminal, the alternating energy will be filtered by the input/output capacitor and radiated to the environment. 

The ac circuit of the power switch ac circuit and the rectifier consists of high-amplitude trapezoidal current, in which the harmonic component is very high, the frequency is much higher than the switching base 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 50 n/s. 

The two circuits are most likely to produce electromagnetic interference, so must the other printed wiring in power source to cloth before these ac circuits, each loop three main components of the filter capacitor, the power switch or rectifier, inductor or transformer shall be placed adjacent to each other, adjust the current path between the element position make them as short as possible.The best way to establish the switch power layout is similar to its electrical design. The best design process is as follows:

Placement transformer

Design power switch current loop

Design the output rectifier current loop

A control circuit connected to an ac power supply circuit

Design input current source circuit and input filter design output load circuit and output filter according to the functional unit of the circuit, layout of all components of the circuit should conform to the following principles:

(1) the PCB size should be considered first.When the PCB size is too large, the length of the printed line increases, the impedance increases, the anti-noise capability decreases, and the cost increases.Too small is not good heat dissipation, and adjacent lines are vulnerable to interference.The optimum rectangular shape of the circuit board, with a length/width ratio of 3:2 or 4:3, is the component on the edge of the circuit board, which is not less than 2mm from the edge of the circuit board. 

(2) when placing devices, consider welding in the future, not too dense. 

(3) center on the core elements of each functional circuit and arrange around it.The components should be uniformly, neatly and compactly arranged on the PCB, and the leads and connections between the components should be minimized and shortened. The decoupling capacitors should be as close to the VCC of the components as possible. 

(4) the distribution parameters between components should be considered for the circuit operating at high frequency.General circuits should be as close to parallel as possible.In this way, not only beautiful, and easy to install welding, easy to mass production. 

(5) arrange the position of each functional circuit unit according to the circuit process, make the layout convenient for signal flow, and keep the signal as consistent as possible. 

(6) the first principle of layout is to guarantee the routing rate of the wiring, pay attention to the connection of flight wires when moving devices, and put the connected devices together. 

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

Parameter Settings

The spacing between adjacent conductors must meet the electrical safety requirements and should be as wide as possible for ease of operation and production.The minimum spacing should at least be able to withstand voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. For the signal lines with large gap between high and low levels, it should be as short and increase the spacing as possible. 

The distance from the inner hole edge of the pad to the edge of the printed plate is greater than 1m m, thus avoiding any defect in the pad during processing.When the wire connecting with the pad is thinner, the connection between the pad and the pad should be designed as water drop. The advantage of this is that the pad does not peel easily, but the wire and the pad do not disconnect easily. 

wiring

The switch power supply contains high-frequency signals. Any printed wire on the PCB can act as an antenna. The length and width of printed wire can affect its impedance and inductance, thus affecting the frequency response.Even the printed wires passing through the dc signal are coupled from the adjacent printed wires to the radio frequency signal and cause circuit problems (or even radiate out the interfering signals again).Therefore, all printed wires passing through the ac current should be designed as short and wide as possible, which means that all components connected to the printed wires and connected to other power cords must be placed close together. 

The length of the printed line is directly proportional to the inductance and impedance, while the width is inversely proportional to the inductance and impedance of the printed line.The length reflects the wavelength of the printed line response. The longer the length, the lower the frequency of the printed line to transmit and receive electromagnetic waves, the more radio-frequency energy it radiates.According to the size of printed circuit board current, increase the power line width and reduce the loop resistance.At the same time, make the direction of the power cord and ground wire and the direction of the current consistent, which helps to enhance the anti-noise ability.Ground is the bottom branch of the four current circuits of switching power supply. As the common reference point of the circuit, it plays an important role. 

Therefore, the placement of grounding wires should be carefully considered in the layout, and mixing various types of grounding can cause power supply instability. 

The following points should be noted in the ground line design:

1. Correct selection of single point grounding in general, filter capacitance public side should be the other ground coupled to the only join points, large current of interact with the primary circuit of the ground should be close to, and the corresponding circuit power filtering capacitance can also on the level ground, mainly considering the current parts back into the ground the circuit is change, due to the actual flow of the impedance of the line can lead to circuit ground potential changes in each part introduce the interference.In this switch power supply, its inductance smaller effect between wiring and devices, and grounding circuit had a greater influence on the formation of the circulation of interference, thus using one-point earthing, the power switch current loop (in the ground of several devices are connected to the ground on his feet, ground of output rectifier current loop of several devices also received corresponding filter capacitance grounded on his feet, the power supply work more stable, not easy self-excitation.If you can't make a single point, you can connect two diodes or a small resistance, but you can connect it to a concentrated piece of copper foil. 

2. Bold grounding line as far as possible if the ground wire is very thin, grounding potential changes over electric current, the timing signal level instability of the electronic equipment, anti-noise performance is bad, so be sure to every large current earthing end USES short and wide printing line as far as possible, try to widen the power supply, ground wire width, it is best to ground wide than the power cord, their relationship is: ground wire power cord, signal lines, if possible, the width of the ground wire should be greater than 3 mm, also can use copper layer on the ground a large area, all the places are not used on the PCB as earth wire connected to ground.When conducting global wiring, the following principles should be followed:

: (1) wiring direction from the surface of welding, the arrangement of the components bearing as far as possible to keep consistent with schematic diagram, wiring should take line direction is consistent with the circuit diagram, because of the production process often need to test the various parameters on surface of the welding, so do check in for production, debug and repair (note: refers to the circuit performance and under the premise of the machine installation and panel layout requirements). 

(2) when designing the wiring diagram, take as few turns as possible. The width of the line on the printing arc should not be abrupt. The wire corner should be more than 90 degrees, and the lines should be simple and clear. 

(3) intersecting circuits are not allowed in printed circuits. For lines that may be intersected, "drill" and "wind" can be used to solve the problem.This means that a lead is "drilled" through the gap under the feet of other resistors, capacitors and transistors, or "wound" around one end of a lead that may be crossed. In special cases, how to make the circuit is very complicated. In order to simplify the design, it is also allowed to use the lead to cross the lead and solve the problem of cross circuit.With a single panel, the directly inserted component is located at the top surface and the surface-attached device is located at the bottom surface. Therefore, it can be overlapped with the surface-attached device when laying, but the solder pad should be avoided.

3. The switching power supply in the input and output area is the low-voltage dc-dc. To feed the output voltage back to the primary level of the transformer, the circuits on both sides should have a common reference ground. 

check

Wiring design is completed, it is necessary to carefully check the wiring design by the designers is in line with the rules, rules at the same time also need to confirm whether accord with the demand of the PCB production process, general inspection line to line, line and element bonding pad, the line and communicating pores, element bonding pad and communicating pores, through hole and the distance between the through hole is reasonable, whether to meet the production requirements.Whether the width of the power cord and ground wire is appropriate, and whether there is room in the PCB for widening the ground wire.Note: some errors can be ignored, for example, parts of the Outline of some connectors are placed outside the frame, and errors can occur when checking spacing.In addition, after every modification of the wire and hole, the copper should be re - coated. 

Review according to the "PCB checklist"

The content includes design rules, layer definition, line width, spacing, pad and hole setting, and the rationality of device layout should be reviewed emphatically. The routing of power supply and ground network, the routing and shielding of high-speed clock network, and the placement and connection of decoupling capacitors. 

Notes for designing output output light drawing file:

A. The output layers shall be the wiring layer (bottom layer), silk screen layer (including top layer and bottom layer), resistance welding layer (bottom layer resistance welding), drilling layer (bottom layer), and in addition, drilling files (NCD rill) shall be generated.

B. When setting the Layer of the screen printing Layer, do not select Part Type, select the Outline, Text and Line of the top (bottom) and screen printing Layer. 

C. Select the Board Outline when setting the Layer of each Layer. When setting the Layer of the screen printing Layer, do not select the Part Type. 

D. Use the default Settings of Power PCB when generating the drill hole file and do not make any changes.