Safety distance requirements
Including electrical clearance (spatial distance), creepage distance (surface distance), and insulation penetration distance.
1. Electrical clearance: The shortest distance measured along the air between two adjacent conductors or between a conductor and the surface of an adjacent motor housing.2. Creepage distance: The shortest distance measured along the insulating surface between two adjacent conductors or between a conductor and the surface of an adjacent motor housing.1. Creepage distance and electrical clearance requirements: 1. Creepage distance: For input voltage 50V-250V, L—N ≥ 2.5mm before the fuse; for input voltage 250V-500V, L—N ≥ 5.0mm before the fuse; electrical clearance: for input voltage 50V-250V, L—N ≥ 1.7mm before the fuse; for input voltage 250V-500V, L—N ≥ 3.0mm before the fuse; after the fuse, there are no requirements, but it is advisable to maintain a certain distance to avoid short-circuit damage to the power supply;2. The primary side AC to DC part ≥ 2.0mm;3. The primary side DC ground to ground ≥ 4.0mm if the primary side ground is to the earth;4. The primary side to secondary side ≥ 6.4mm, such as phototransistor, Y capacitor, etc. If the component pin spacing ≤ 6.4mm, grooves should be opened;5. The gap between the two levels of the transformer should be ≥ 6.4mm, and for reinforced insulation, it should be ≥ 8mm.
Part 2 Interference resistance and EMC section
1. Long line interference resistance
In Figure 2, during PCB layout, the driver resistor R3 should be close to Q1 (MOSFET), and the current sampling resistor R4 and C2 should be close to the 4th pin of IC1, as mentioned in Figure 1, R should be as close as possible to the operational amplifier to shorten the high impedance line. Since the input impedance of the operational amplifier is very high, it is easily interfered with. The output impedance is relatively low, making it less susceptible to interference. A long line acts like a receiving antenna, easily introducing external interference.
In layout A of Figure 3, R1 and R2 should be placed close to the transistor Q1, as Q1’s input impedance is very high, and if the base line is too long, it is prone to interference, so R1 and R2 should not be far from Q1.In layout B of Figure 3, C2 should be close to D2 because Q2’s transistor input impedance is very high, and if the line from Q2 to D2 is too long, it is prone to interference, so C2 should be moved close to D2.2. Small signal lines should be kept as far away from large current lines as possible, avoiding parallelism, D >= 2.0mm.
3. Small signal line handling: Circuit board wiring should be concentrated as much as possible to reduce board area and improve interference resistance.4. The area enclosed by a current loop should be minimized.
For example: the current sampling signal line and the signal line from the optocoupler
5. Optocoupler devices are easily interfered with and should be kept away from strong electric fields and strong magnetic field devices, such as large current lines, transformers, high potential pulsed devices, etc.6. For multiple ICs, pay attention to Vcc and ground lines.
Series multi-point grounding, mutual interference
7. Noise requirements1. Try to minimize the area surrounded by high-frequency pulse currents, as shown below (Figures 1 and 2)
General board layout:
2. Filter capacitors should be placed as close as possible to the switching transistor or rectifier diode as shown in Figure 2, C1 should be as close to Q1 as possible, and C3 close to D1, etc.3. The area where pulse currents flow should be kept away from the input and output terminals, separating the noise source from the input and output ports.
Figure 3: MOSFET and transformer too close to the entrance, the electromagnetic radiation energy directly affects the input terminal, thus failing EMI testing.Figure 4: MOSFET and transformer far from the entrance, the electromagnetic radiation energy is further away from the input terminal, thus EMI conduction can pass.4. Control circuits should be separated from power circuits, using single-point grounding, as shown in Figure 5.
Ground components around the control IC to the IC’s ground pin; then lead from the ground pin to the large capacitor ground line. The ground of the third pin of the optocoupler should connect to the first pin of the IC, and the fourth pin connects to the second pin of the IC. As shown in Figure 6.5. If necessary, output filter inductors can be placed on the ground loop.6. Use multiple capacitors with low ESR in parallel for filtering.7. Use copper foil for low inductance, low resistance wiring, and avoid long parallel lines between adjacent ones. Routing should avoid parallel and crossing, using vertical connections, and the line width should not have sudden changes, and routing should not have sudden angles (i.e., ≤ right angles). (For the same current loop, parallel routing can enhance resistance to interference.)8. Interference resistance requirements:1. Try to shorten the connecting lines between high-frequency components as much as possible, trying to reduce their distribution parameters and mutual electromagnetic interference. Components that are easily interfered with should not be too close to strong interference devices, and input and output components should be kept as far away as possible.
2. Some components or wires may have a high potential difference, and their distance should be increased to avoid discharge leading to accidental short circuits.
Part 3Overall layout and routing principles
1. Overall layout
1. Heat sinks should be evenly distributed, and airflow should be good.
Figure 1: Heat sink blocking airflow is not conducive to heat dissipation; Figure 2: Good ventilation is conducive to heat dissipation.2. Capacitors, ICs, etc., should maintain distance from heat-generating components (heat sinks, rectifier bridges, continuous current inductors, power resistors) to avoid being affected by heat.3. Current loops: For easy threading, the distance between the lead holes should not be too far or too close.4. Input/output, AC/socket should satisfy the consistency of wire lengths, leaving some space margin, paying attention to the position occupied by the plug wire buckle, making it easy to insert and remove, and the output wire holes should be neat for soldering.5. Components should not touch each other, and the screw positions of MOSFETs and rectifier diodes should not collide with other components to simplify the assembly process. Capacitors and resistors should not collide with screws or bars, and the positions of screws and bars should be considered during board layout. As shown in Figure 3:
6. Except for temperature switches, thermistors, etc., key components sensitive to temperature (such as ICs) should be kept away from heat-generating components. Components that generate significant heat should maintain a certain distance from components that affect the lifespan of the entire machine.7. For adjustable components such as potentiometers, variable inductors, variable capacitors, and micro switches, the layout should consider the structural requirements of the entire machine. If adjustments are made inside the machine, they should be placed conveniently on the PCB; if adjustments are made outside the machine, their positions should correspond to the position of the adjustment knob on the front panel.8. Leave room for the positioning holes of the printed PCB board support.9. Components located at the edge of the circuit board should be at least 2mm away from the edge of the circuit board.10. Output wires, indicator wires, and fan wires should be arranged in a row, with consistent polarity corresponding to the panel.11. General layout: Do not introduce high voltage on small boards; place high voltage components on larger boards. If there are special circumstances, safety regulations must be considered. For example, place R1 and R2 on the larger board and introduce a low voltage line.12. The primary heat sink should maintain a distance of more than 5mm from the shell (except for the mica sheet).13. During board layout, pay attention to the height of components on the back side. As shown in Figure 5:
14. The primary and secondary Y capacitors and the transformer core should pay attention to safety regulations.
2. Layout requirements for unit circuits
1. Arrange the positions of various functional circuit units according to the flow of the circuit, making the layout convenient for signal flow and keeping the signal in the same direction as much as possible.2. Layout should be centered around the core components of each functional circuit, with components evenly and compactly arranged on the PCB to minimize and shorten the connecting leads between components.3. When working at high frequencies, consider the distribution parameters of components. General circuits should arrange components in parallel as much as possible, which is not only aesthetically pleasing but also facilitates soldering and mass production.3. Routing principles1. Wires used for input and output should avoid being adjacent and parallel as much as possible; it is best to add ground lines between them to avoid feedback coupling.2. The width of the routing is mainly determined by the adhesion strength between the wire and the insulating substrate and the current flowing through them. When the copper foil thickness is 50μm and the width is 1mm, a current of 1A will not cause a temperature rise above 3℃. Based on this, for 2 oz (70μm) thick copper foil, 1mm width can carry 1.5A current without exceeding a 3℃ temperature rise (Note: natural cooling).3. The electrical clearance width between the input control circuit part and the output current and control part (i.e., the distance between small current lines and output lines) should be 0.75mm–1.0mm (Min 0.3mm). The reason is that if the copper foil and pads are too close, it can easily cause short circuits and adverse reactions due to electrical interference.4. The ROUTE line corners should generally be rounded; right angles and sharp angles will affect electrical performance in high-frequency circuits.5. Power lines should be thickened according to the size of the line current to reduce loop impedance, while ensuring that the directions of the power lines and ground lines are consistent with the data transmission direction, minimizing the enclosed area, which helps enhance noise immunity.A: The grounding of heat sinks is mostly done using single-point grounding to improve noise suppression capability as shown in the following figure:
Before the change: multi-point grounding forms a magnetic field loop, and EMI testing fails.
After the change: single-point grounding has no magnetic field loop, EMI testing is OK.7. Routing of filter capacitorsA: Noise and ripple are completely filtered by the filter capacitor.
B: When the ripple current is too large, multiple capacitors in parallel, the ripple current passing through the first capacitor generates more heat than the second and third capacitors, making it easier to damage. When routing, try to evenly distribute the ripple current to each capacitor, as shown in routing diagrams A and B. If space permits, the routing can also be done in the style of diagram B.
8. The lead of high-voltage high-frequency electrolytic capacitors has a rivet, as shown in the following figure, which should maintain a distance from the top layer routing copper foil and comply with safety regulations.
9. Weak signal routing should not pass under inductors, current loops, and other devices.
The current sampling line may collide with the magnetic core and line copper foil during mass production, causing failures.10. High-voltage lines should not run under metal film resistors, and low-voltage lines should run in the middle of the resistor as much as possible to avoid short circuits if the resistor is damaged.11. Adding solder:
A: Add solder where the copper foil of the power line is narrower;
B: The RC absorption circuit not only requires solder due to larger current but also aids in heat dissipation;
C: Add solder under heat-generating components for heat dissipation, but do not press the solder pad.12. Signal lines should not pass through transformers, heat sinks, or MOSFET pins.13. If the output is superimposed, the differential mode inductor front capacitor should connect to the front ground, and the differential mode inductor back capacitor should connect to the output ground.
14. The area where high-frequency pulse currents flow:
A: Try to minimize the area surrounded by high-frequency pulse currents, as marked in the five loops in the figure above.B: The power line and ground line should be kept close together to reduce the area they enclose, thus minimizing electromagnetic interference generated by external magnetic fields cutting through the loop and reducing the electromagnetic radiation of the loop to the outside.C: Large capacitors should be kept close to the MOSFET, and the output RC absorption circuit should be kept as close to the rectifier as possible.D: The routing of power lines and ground lines should be thickened and shortened to reduce loop resistance, with smooth corners and no sudden changes in width, as shown in the figure below:
E: The area where pulse currents flow should be kept away from the input and output terminals, separating the noise source from the output.
F: The oscillation filtering decoupling capacitor should be close to the IC ground, and the ground line should be short.
15. Manganese copper wire vertical transformer core I-shaped inductors power resistors heat sinks magnetic rings should not have the first layer of lines.16. Slots and copper foil routing should have a distance of more than 10MIL, paying attention to the safety regulations of the upper and lower metal parts.
17. The same name terminals of drive transformers, inductors, and current loops should be consistent.18. In double-sided boards, add more vias at large current routing points, and the vias should be soldered to increase current-carrying capacity.19. In single-sided boards, jump wires should not touch other components, and if jump wires connect to high voltage components, they should maintain a certain distance from low voltage components according to safety regulations. They should also maintain a distance of more than 1mm from heat sinks.4. Case analysisAs switch power supplies become smaller, their operating frequencies increase, and the density of internal components becomes higher, the interference resistance requirements for PCB routing become stricter. The problems found in some case routings and their solutions are as follows:1. Overall layoutCase 1 is a six-layer board, where the initial layout places the component side for the control part and the solder side for the power part. During debugging, significant interference was discovered, caused by the unreasonable placement of the PWM IC and optocoupler, as shown:
As shown in the figure, the PWM IC and optocoupler are placed under the MOSFET, with only a 2.0mm PCB separating them, causing direct interference from the MOSFET to the PWM IC. The improvement made was to:
Move the PWM IC and optocoupler away from each other, ensuring that there are no components carrying pulsed currents above them.2. Routing issuesPower routing should be minimized to reduce the area enclosed by the loop and avoid interference. The area enclosed by small signal lines should be minimized, such as in current loops:
The larger the area enclosed by lines A and B, the more interference it receives because it is feedback. The feedback coupling line should be short and should not intersect or run parallel with pulsed signals.The current sampling line of the PWM IC chip should be routed as far away as possible from the drive line and synchronization signal line, avoiding parallel routing to prevent mutual interference. The current waveform is as follows:
Part 4Thermal design section
Note: The small board should not be too close to the transformer
The small board being too close to the transformer can lead to semiconductor components on the small board being easily heated and affected.Part 5Process handling section
Each PCB must have an arrow indicating the direction through the soldering furnace:
In layout, the direction of DIP packaged ICs must be perpendicular to the direction of the soldering furnace, not parallel, as shown below; if layout difficulties arise, horizontal placement of ICs is allowed (the direction of SOP packaged ICs is opposite to that of DIP).
The routing direction should be horizontal or vertical, and transitions from vertical to horizontal should enter at a 45-degree angle. If the width of the copper foil entering the round pad is smaller than the diameter of the round pad, a teardrop should be added. Routing should be as short as possible, especially noting that clock lines, low-level signal lines, and all high-frequency circuit routing should be shorter.Analog and digital circuit ground lines and power supply systems should be completely separated.If there are large area ground and power supply regions on the printed board (area exceeding 500 square millimeters), local windows should be opened. As shown in the figure:
The center distance between the legs of horizontally inserted components (resistors, diodes, etc.) must be 300mil, 400mil, and 500mil apart. (If not necessary, 240mil can also be used, but only for IN4148 type diodes or 1/16W resistors. 1/4W resistors start from 10.0mm).The diameter of heat dissipation holes on the PCB should not exceed 140mil.
If there are Φ12 or square holes larger than 12MM on the PCB, a cover hole should be made to prevent solder from flowing out, as shown in the figure (the hole size is 1.0MM).
On PCBs with surface-mounted components, to improve the mounting accuracy of surface-mounted components, the PCB must have calibration marks (MARKS), with at least two marks on each board, located at a set of diagonally opposite corners, as shown in the figure:
Spacing of surface-mounted components:
Distance between surface-mounted components and through-hole components:
SMD component leads must be slightly larger than the lead diameter, too large pads can lead to cold solder joints. The outer diameter D of the pad should generally not be less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum pad diameter can be (d+1.0) mm, and pads with a hole diameter greater than 2.5mm should be appropriately enlarged. Components should be neatly arranged, and their directions should be as consistent as possible.For surface-mounted components on the PCB, the long axis centerline should be arranged perpendicular to the long axis centerline of the PCB to avoid breaking.
Source: PCBword
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