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PCB design specifications switching power supply solutions

Issuing time2015-07-21
SHENZHEN Branch Power Technology Co., Ltd.-PCB design specifications switching power supply solutions

In any switching power supply design, the physical design of the PCB is the last link, if improper design, PCB can radiate excessive electromagnetic interference, resulting in an unstable power supply, perform the following steps for each of the required items to note Analysis:

First, from schematic to PCB design flow

   Establishing component parameters -> Enter principle netlist -> Design Parameters Settings -> manual layout -> manual routing -> Verify Design -> Review -> CAM output.

Second, the parameters

   Spacing between adjacent wires must meet the electrical safety requirements, and in order to facilitate the operation and production, the spacing should be as wide as some minimum distance of at least fit to be able to withstand the voltage, at a lower wiring density, spacing of signal lines can be properly applied large, high, low disparity signal lines as short as possible and increase the distance, under normal circumstances would take the line spacing set 8mil.

   The inner edge of the hole from the pad to the PCB edge is greater than 1mm, to avoid defects lead to the pad when working. When traces and pads connected to smaller, designed to connect between the pads and traces a teardrop-shaped, so that the benefits are not easy pads from the skin, but is not easy to trace and pad off.

Third, the component layout

   Practice has proved that, even if the circuit schematic design properly, improperly printed circuit board design, can also adversely affect the reliability of electronic equipment. For example, if the PCB two thin parallel lines are very close, it will signal waveform forming delays in the transmission line terminal to form a reflection noise; interference due to power, ground caused by thoughtless, product performance will decline, therefore, in the design of printed circuit boards, they should pay attention to the correct approach.

Each switching power supply has four current paths:

(1) The power switch AC circuit

(2) The output of the rectified AC circuit

(3) The input signal current source circuit

(4) the output load current circuit

   An approximate DC input circuit through the input capacitor charging current, filter capacitor energy storage mainly plays a role in broadband; similarly, the output filter capacitor is also used to store high-frequency energy output from the rectifier circuit while eliminating the output load current energy Therefore, the terminal input and output filter capacitor is very important, input and output current loop should be separately connected only from the terminal to the power supply filter capacitor; if the connection between the input / output circuit and power switch / rectifier circuit can not and terminal capacitor is directly connected to AC power from the input or output filter capacitor and radiation into the environment.

   AC power switch circuit and the rectifier circuit includes high-amplitude trapezoidal alternating current, the high harmonic component of the current, which is much larger than the switching frequency of the fundamental frequency, the peak amplitude of up to continuous input / output DC current of 5 times the magnitude of the transition time is usually about 50ns. These two circuits most prone to electromagnetic interference, so they must be good cloth AC circuit wiring before the other tracks in the power supply, the three main components of each loop filter capacitors, power switches or rectifiers, inductors or transformer should be placed adjacent to one another, adjusting the position of the current element path between them as short as possible.

Establish the best way of switching power supply layout similar to its electrical design, best design process is as follows:

Place transformer

? Design power switch current loop

? Design output rectifier current loop

Connected to AC power circuit control circuit

? Design input current source circuit and the input filter

Design output load circuit and output filter circuitry according to functional unit, when all the components of the circuit layout, subject to the following principles:

(1) The first thing to consider PCB size .PCB size is too large, the printing of lines long, resistance increases, decreased noise immunity, cost increases;. Is too small, heat is not good, and adjacent lines susceptible to interference circuit board Best rectangle shape, an aspect ratio of 3: 2 or 4: 3, located on the edge of the circuit board components, from the edge of the board is generally not less than 2mm.

(2) to be considered when placing the device after welding, not too dense.

(3) the core elements of each functional circuit centered around it for layout components should be uniform, neatly, compactly arranged in the PCB, minimize and shorten the lead and connections between the various components, decoupling VCC capacitor as close to the device.

(4) circuit at high frequency, to consider the distribution parameter between components. General circuit should be as far as possible parallel components. This not only beautiful, but also easy assembly and welding, easy to mass production.

(5) Process according to each functional position of the circuit arrangement of the circuit elements, enable easy layout of signal flow, and as far as possible consistent signal direction.

(6) The primary principle is to ensure that the wiring layout routability, pay attention to connect fly line when moving the device, the device has a connection relationship together.

(7) the loop area as small as possible in order to limit the radiation noise switching power supply.

Fourth, the wiring

   Switching power supply includes a high frequency signal, any traces on the PCB can act as an antenna, length and width of the printed lines will affect the impedance and inductance, thus affecting the frequency response. Even a DC signal through printing Thread will be coupled from adjacent tracks to the RF signal and cause electrical problems (or even re-radiated interfering signals.) So should all tracks by AC current designed to be as short and wide, which means that you must all traces connected to the power supply line and connected to other components placed very close. inductance and impedance is proportional to the length of the tracks of its exhibit, and the width is the trace inductance and impedance into inversely proportional to the wavelength response length reflects the traces, the length of the longer traces capable of transmitting and receiving electromagnetic waves of frequency more. Low, it can radiate radio frequency energy more depending on the size of current printed circuit board, be bold power line width, reduce loop resistance. At the same time, so that the direction of the power cord to the ground and the current agreement, This helps to enhance noise immunity.

   Ground is the bottom branch switching power four current paths, as a common reference point of the circuit plays an important role, it is an important method of controlling interference. Therefore, the layout should carefully consider placing ground wire, all kinds of Ground mixing will cause instability in the ground power supply design should note the following:

1. Select the correct single-point ground

   Typically, the filter capacitor common ground should be further coupled to a unique connection point to the large current exchange, the same level circuits should be as close to the ground, and power supply filter capacitor of the present stage of the circuit should also be connected to the ground level on, mainly on account of the various parts of the circuit back to the earth current is changing, due to the impedance of the line of actual flows will lead to changes in various parts of the ground potential of the circuit and the introduction of interference.

   In the switching power supply, its wiring and inductance of the device between the small and the large circulation grounding circuit formation of interference, thus grounded in one point, is about the power switch current loop (in the ground are several devices connected to the ground pin, ground output rectifier current loop several devices are also connected to a respective filter capacitor ground feet, so a more stable power supply, easy self-excited. When do single point, common ground a pin diode or two a small resistance, in fact, connected to the more concentrated at a copper can.

2. Try bold grounding wire

   If the grounding line is very thin, the ground potential varies with the current changes, resulting in an electronic device timing signal level instability, anti-noise performance deterioration, so make sure the ground terminal of each high current use as short and wide printing thread, try to widen the power, ground width, it is better than the power ground line width, their relationship is: Ground> power cables> Signal line, if possible, the width of the ground wire should be greater than 3mm, can also be used a large area of copper layer used as a ground wire on the PCB to not be of use in places connected to ground with a ground.

When wiring globally, we must abide by the following principles:

(1) wiring direction: from the welding surface, the arrangement orientation of the elements as far as possible consistent with the schematics, best direction and a circuit diagram of the wiring traces consistent direction, due to the production process usually requires various parameters in the welding surface testing, so it facilitates the production of inspection, commissioning and maintenance (Note: refers to the premise of meeting circuit performance and the whole installation and panel layout requirements).

(2) Design wiring diagram traces as little time turning a linewidth not printed on the arc mutation leads corners should 90 degrees, and strive to simple lines.

(3) are not allowed in the printed circuit cross circuit, may cross the line, you can use the "drill", "around" the two solutions. That let a lead from the gap at the other resistors, capacitors, transistors foot " drill "in the past, or may cross from one end of a certain wire" around "in the past, how the circuit is very complex, in exceptional circumstances, in order to simplify the design also allows wire jumper to resolve cross-circuit problem. because a single panel, line element is positioned top surface, surface-mount device located bottom surface, so when the layout of the line device can be used with surface mount components overlap, but to avoid overlapping pads.

3. Input with the output ground

   The switching power supply for the low-voltage DC-DC, wishing output voltage fed back transformer primary, both sides of the circuit should be a common reference, and thus on both sides of the ground after the shop copper respectively, also connected together to form common ground.

Fifth, check

   After wiring design is complete, you need to check carefully whether the wiring design rules established by the designer, but also need to confirm whether the rules established by the PCB production process to meet the needs of the general inspection line and line, line and component pad, line and through hole component pads and the through-hole distance, between the through-hole and through-hole is reasonable, whether to meet production requirements. power line and ground the width is appropriate, if there is to make the PCB ground widen The place.

Note: Some errors can be ignored, such as a part of some connectors on the Outline of the frame, the spacing error checking; in addition each modified traces and vias after copper must re once.

Six Review

   According to "PCB Checklist", including design rules, the alignment is reasonable, power, ground network layer definitions, line width, spacing, pads, vias setting, also the focus of the review device layout, high-speed clock networks traces and shielding, decoupling capacitor placement and connections.

Seven design output

Gerber files output Notes:

a. need to output layer wiring layer (lower layer), silkscreen (including the top screen and bottom screen), the solder layer (solder resist layer), drilling layer (bottom layer), while also generating drill files (NC Drill )

b. When setting Layer silkscreen, do not choose Part Type, select the top (bottom) and silkscreen of the Outline, Text, Line

c. When the time set in each of Layer, to the Board Outline option, set Layer silkscreen, do not choose Part Type, select the top (bottom) and Outline of silkscreen, Text, Line.

d. When generating drill files, using the default settings PowerPCB, do not make any changes.

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