Monday, April 25, 2011

【 Weak current College 】 switching power supply for PCB design specifications 】

In any switching power supply design, PCB physical design are the last link, if improper PCB design methods may be radiation excessive electromagnetic interference, leading to the power supply is unstable, the following for each step in the matter of:

1. from the schematics to PCB design process to build component parameters-> input principle netlist-> design parameters settings-manual layout-> > > verifying manually routing-design-review-> > CAM output.

Second, parameters setting adjacent wire spacing must be able to meet the electrical safety requirements and for ease of operation and production, space should be as wide as possible. The minimum spacing is at least to be suitable for the accommodation of voltage, wiring density is low, the signal line spacing can be appropriately increased, on the high and low level of disparity signal cable should be as short as possible and increase the spacing, under normal circumstances will go line spacing set to 8mil.

Welding plate hole edge to PCB edge distance to greater than 1 mm, this prevents machining defect: when the pads. When the connection with pad alignment to the finer, the pads and alignment between connections are designed as drops of water, this benefit is not easy to pad up skin, but the alignment and pads are not easily broken.

3. components layout proved that even if the correct circuit schematic design, printed circuit board design, also on the reliability of electronic devices. For example, if the printed circuit board with two fine parallel lines are very close, will form the signal waveform delays in the transmission line terminal formation reflection noise; because of the power supply, ground for think not thoughtful and interference, makes the product performance, therefore, in the design of printed circuit board, it should be noted that using the right methods. Each switching power supply has four current loop:

(1). the power switch on the ACcircuit

(2). output rectifiers AC loop

(3). the input signal source current loop

(4). the output load current circuit input circuit through an approximate DC current to the input filter capacitor capacitance charging, mainly played a role in broadband energy storage; Similarly, the output filter capacitor is also used to store the output of high-frequency rectifiers from energy, while eliminating the output load circuit of DC energy. Therefore, input and output filter capacitor's terminals, input and output current loop should only from filter capacitor terminals connected to a power source; if the input/output circuit and power switch/rectifier connection cannot and capacitor Terminal is directly connected to the exchange of energy will be input or output filter capacitor and radiation into the environment. Power switch AC circuit and rectifier circuit contains high-amplitude AC ladder-current, the current harmonic constituents in very high, their frequency is much larger than switch base frequency, the peak amplitude up to continuous input/output DC current ranges of 5 times, the transition time is typically around $ 50ns. The loop is the easiest to produce electromagnetic interference, it must supply the other printed line wiring cloth well before these AC circuits, each loop of three major components filter capacitors, power switch or rectifiers, inductance or the transformer should be placed next to each other, enabling them to adjust component position between the current path as short as possible. Establishment of switching power supply layouts best method and electrical design, best design process are as follows:

· Placement of transformer

· Designed power switch current loop

· Design output rectifier current loop

· Connection to AC power circuit of the control circuit

· Design input current source circuit and input filter design output load circuit and an output filter function according to the circuit on the circuit, all the components for layout, to comply with the following principles:

(1) it is first necessary to consider the PCB size. PCB size is too large, printed lines long, impedance, anti-noise reduced capacity, cost also increases; too small, the heat is not good, and the adjacent line susceptible to interference. Circuit board best shape rectangle, aspect ratio 3: 2 or 4: 3, located on the edge of the circuit board, circuit board edge components are generally not less than 2mm.

(2) place the devices to be considered when subsequent welding, not too dense.

(3) to each function circuit core components as the Center, around it for layout. Components should be uniform, neat, compact and arrange in the PCB, minimize and reduce the components and connections between the leader, decoupling capacitor as close as possible to devices VCC.

(4) in the high-frequency circuit working, consider the components of the distribution parameters. General circuit should, as far as possible, so that the components are arranged in parallel. In this way, not only beautiful, but Assembly welding easy, easy-to-volume production.

(5) in accordance with the arrangements for the various circuit processes function circuit unit position which makes the layout for the signal flow and make the signal as a consistent direction.

(6) layout, first and foremost, ensure that the wiring layout pass rate, mobile devices, note the fly line connections, a connection between the devices together.

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

4. wiring switching power supply includes a high-frequency signal, PCB printed lines on any can play the role of the antenna, the printed line length and width affect its impedance and inductance, resistance to impact frequency response. Even through the DC signal of printed lines also be printed from the adjacent line coupled to the RF signals and cause circuit issues (even radiation from interfering signal again). It should be all by AC current printed lines designed to be as much as possible short, wide, which means that all connections to must be printed and attached to other components of the power cable is very near placement. Length of printed lines and the inductance and resistance, while the width is proportional to the printed line inductance and resistance inversely. Length reflects the printed line response in the wavelength, the longer length, the printed line to send and receive electromagnetic waves of frequencies lowerIt can radiate more RF energy. According to the size of the printed circuit current, to increase the power line width, reduce the loop resistance. At the same time, the power cord, Earth trends and current direction consistent, this helps to enhance the ability of noise. Grounding is switching power supply four current circuit of underlying slip as circuit public reference point plays an important role, it is important to control interference. Therefore, in the layout should carefully consider the placement of the grounding, the grounding will cause power work mix. The design of the ground should note the following:

1. the right to select a single point of grounding in General, the public-side filter capacitor should be other bonding coupled to high current communication and the only connection point, the same level circuit grounding should be as close as possible to, and at the circuit of a power filter capacitors should be received in the line received the mainly consider circuit parts flow back into the current is changing, because the actual flow of line impedance can cause circuits to partially potentials and the introduction of interference. During the switching power supply, cabling, and devices of inductance less drastic and earthing circuit formation in circulating to disturb a greater impact and, therefore, adopt the point grounding, is about power switchcurrent loop (in the device's ground lead are attached to the grounding pin, output voltage rectifier current circuit of several devices in the ground also received a corresponding filter capacitance of earthing pin, so the power work more stable, less exciting. Do not to the point, in total located 2 diode or a small resistance, the add-in are more concentrated at a copper foil.

2. as far as possible, bold Earth wire to the Earth wire is very thin, ground potential with current changes, the electronic equipment of timed signal level instability, anti-noise performance deterioration and, therefore, to ensure that every large current ground-end uses as far as possible short, wide printing lines, try to expand the power and ground wire width, preferably ground than power line width, their relationship is: ground > power cord > signal lines, if possible, grounding line width should be greater than the available area 3mm, also for ground wire copper layers in PCB with, on not being used in places connected with the land as a ground wire. For global distribution, you must follow the following principles:

(1). the wiring direction: from the welding surface, the components of the arrangement position as possible consistent with schematics, wiring diagrams and best direction alignment direction, because of the production process is typically required in welding surface detection of various parameters, so doing facilitate production check, debug and repair (Note: refers to meet performance and overall machine installed circuit and the Panel layout requirements).

(2). the design diagram is alignment as few turn printing arc on line width not mutations, the wire should be ≥ 90 ° corner, seeking to line simple and straightforward.

(3). the printed circuits are not allowed in a cross-circuit, may cross the line, you can use the "drill" and "around" solution. That is, let a lead from the level of resistance, capacitance, transistors at the foot ofthe fills "drilled" in the past, or from may cross a leader at the other end of the "round" in the past, in special circumstances on how circuit complexity to simplify design also allows the use of wire jumper, cross circuit. Due to adoption of the single-panel, direct plug in components located on the top surface, the table attached devices located at the bottom surface, so when the layout directly plugged devices can overlap with the table attached devices, but to avoid overlapping the pads.

3. input to and output to the switching power supply for low-voltage of DC-DC, wishes to output voltage feedback back to transformer primary, there should be common on either side of the circuit of the reference to the ground on either side of the shop copper respectively, have joined together to form a common ground.

5. check the wiring design is completed, you will need to carefully check the wiring design conforms to the rules set by the designers also need to verify that the rules are consistent with the printed circuit board production process requirements, check the cables and wires, cables and component pads, line and through-hole, component pads and through-hole, through holes and the distance between the through hole is reasonable, whether to meet production requirements. The power and ground wire width is appropriate, whether there are in the PCB can make ground widened. Note: some errors can be ignored, such as some of the plug part of the Outline of a plate and frame, check the spacing is wrong; in addition each modified line and through-hole, to copper.

6. review under the "PCB Checklist", including design rules, of the definition, line width, spacing, pads, through-hole settings, also focused on reviewing the reasonableness of the layout of the device, power supply, ground network alignment, high-speed clock network alignment and shield, decoupling capacitor of a substantial and connections, etc.

7. Design output export-painted paperconsiderations:

A. need output layer wiring layer (bottom), printing (including top screen printing, underlying screen printing), solder mask layer (layer mask), drilling layer (bottom), and also generate drill files (NCDrill)

B. setting screen printing layer for Layer, PartType, choose not to select the top level (bottom) and screen printing layer Outline, Text, Linec. in the settings of each layer, the Layer will be selected, set the BoardOutline silk-screen printing layer for Layer, PartType, choose not to select the top level (bottom) and screen printing layer Outline, Text, Line. D. generate drill file, use the default settings, PowerPCB not to make any changes.

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