Power PCB Design Rules

A. Power PCB Design Safety Distance Requirements

The safety distance includes electrical clearance (spatial distance), creepage distance (creepage distance), and insulation penetration distance.

Electric clearance: The shortest distance measured along with the air between two adjacent conductors or the surface of the adjacent motor housing.

Creepage distance: The shortest distance between two adjacent conductors or a conductor and the surface of the adjacent motor housing measured along the insulating surface.

For the requirements of creepage distance and clearance distance, please refer to NE61347-1-2-13

(1) Creepage distance: When the input voltage is 50V-250V, the LN before the fuse is ≥2.5mm; when the input voltage is 250V-500V, the LN before the fuse is ≥5.0mm; Clearance: when the input voltage is 50V~250V, L-N before the fuse≥1.7mm; when the input voltage is 250V~500V, L-N≥3.0mm before the fuse; there is no requirement after the fuse, but a certain distance should be kept to avoid short-circuit damage to the power supply.

(2) The AC and DC part of the primary side≥2.0mm

(3) Primary side DC grounding to ground≥4.0mm, such as primary side grounding to ground

(4) From the primary side to the secondary side ≥6.4mm, such as optocouplers with a pitch ≤6.4mm, Y capacitors and other components should be slotted.

(5) Between the two layers of the transformer ≥6.4mm, ≥8mm reinforced insulation.

B. Power PCB Design Rules for Anti-interference, EMC

a. Longline anti-interference

 

In the PCB layout, the driving resistor R3 should be close to Q1 (MOS tube), and the current sampling resistors R4 and C2 should be close to the fourth pin of IC1. As shown in the figure, R should be as close as possible to the op-amp to shorten the high impedance line. Due to the high impedance at the input of the operational amplifier, it is susceptible to interference. The output end has low impedance and is not susceptible to interference. The long wire is equivalent to the receiving antenna and is easy to introduce external interference.

When typesetting in A in the figure, R1 and R2 should be placed close to transistor Q1. Since the input impedance of Q1 is very high and the baseline is too long, it is susceptible to interference, so R1 and R2 should not be too far away from Q1.

In the figure, C2 should be close to D2, because the input impedance of the Q2 transistor is very high. If the line from Q2 to D2 is too long and is susceptible to interference, then C2 should be moved near D2.

b. The small-signal trace should be as far as possible from the high current trace, avoid parallel, D>=2.0mm.

c. Small signal line processing: Try to concentrate the wiring of the circuit board to reduce the layout area and improve the anti-interference ability.

d. Current loop routing minimizes the enclosed area. Current sampling signal line and the signal line of an optocoupler

e. The photoelectric coupling device is susceptible to interference. They should be kept away from equipment with strong electric and magnetic fields, such as high-current wiring, transformers, and high-potential pulse equipment.

f. For the power supply of multiple ICs, please pay attention to the Vcc and ground wires.

g. Multi-point grounding in series, mutual interference.

h. Noise requirements

1. Minimize the area surrounded by high-frequency pulse current.

2.The filter capacitor should be as close as possible to the switch tube or rectifier diode.

3. The area where the pulse current flows is far away from the input and output terminals to separate the noise source from the input and output ports.

MOS tube and transformer are too close to the entrance, electromagnetic radiation energy directly acts on the input terminal. Therefore, it is easy to cause the EMI test to fail.

MOS tube and transformer are far from the entrance, the distance between electromagnetic radiation energy and the input terminal increases, and cannot directly act on the input terminal, so EMI can be conducted.

4. The control loop is separated from the power loop, and single-point grounding is adopted.

Connect the components around the control IC to the ground pin of the IC; then lead from the ground pin to the large capacitor ground. The third pin of the optocoupler is connected to the first pin of the IC, and the fourth pin is connected to the second pin of the IC.

5. If necessary, the output filter inductor can be placed on the ground loop.

6. Use multiple low ESR capacitors for parallel filtering.

7. Use copper foil for wiring with low inductance and low resistance. There should not be too long parallel lines between adjacent parallel lines. Try to avoid parallel and perpendicular crossing. Do not change the line width. Don’t turn suddenly (ie: ≤right angle). (Parallel wiring of the same current loop can enhance the anti-interference ability)

i. Anti-interference requirements

1. Shorten the connections between high-frequency components as much as possible, minimize their distribution parameters and mutual electromagnetic interference, components that are vulnerable to interference should not be too close to strong interference components and input and output components should be as far away as possible.

2. There may be a large potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuits caused by the discharge.

C. Power PCB Design Rules General Layout and Wiring

a. Overall layout

1. The radiating fins are evenly distributed and the ventilation channels are well ventilated.

The radiator will block the air duct, which is not conducive to heat dissipation.

Good ventilation is conducive to heat dissipation.

2. Keep a certain distance between capacitors, ICs, etc., and thermal components (heat sinks, rectifier bridges, freewheeling inductors, power resistors) to avoid thermal influence.

3. Current loop: To facilitate threading, the lead hole should not be too far or too close.

4. The length of the two wires of input/output and AC/socket should be equal, and a certain space should be reserved. Pay attention to the position of the plug cord to facilitate plugging and unplugging. The output cord hole should be clean and suitable for welding.

5. The components should not be in contact with each other, and the screw positions of the MOS tube and the rectifier tube, and the pressure belt should not be in contact with other components to simplify the assembly process as much as possible. Capacitors and resistors may collide with pressure bars or screws. When laying out the circuit board, consider the screws first. And the location of the layering.

6. Except for temperature switches and thermistors, temperature-sensitive key components (such as IC) should be kept away from heating components, and components with higher heat generation should be kept away from capacitors and other components that affect their service life.

7. For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, micro switches, etc., the structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be placed on a PCB board that is easy to adjust. For external adjustment, its position should match the position of the adjustment knob on the chassis panel.

8. The position occupied by the positioning hole bracket of the printed circuit board should be reserved.

9. The components located on the edge of the circuit board are usually not less than 2mm away from the edge of the circuit board.

10. The output wires, lamp wires, and fan wires should be arranged in a row as much as possible, and the polarity is the same as the panel.

11. General layout: Do not connect the small board to the high voltage, and place the high voltage components on the big board. If there are special circumstances, safety regulations must be considered.

12. The main radiator should keep a distance of more than 5mm from the shell (except for the polyester film heat sink).

13. When laying out the circuit board, please pay attention to the height of the components on the reverse side.

14. Pay attention to the safety regulations for primary and secondary Y capacitors and transformer cores.

b. Layout requirements of the unit circuit

1. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction.

2. Take the core component of each functional circuit as the center and arrange it around it. Components should be uniformly and neatly arranged on the PCB in a compact manner to minimize and shorten the connection leads between components.

3. The distribution parameters of the components should be considered when working at high frequencies. Generally, circuits should be connected in parallel as much as possible. This is not only beautiful, but also easy to install and weld, and easy to mass-produce.

c. Wiring principle

1. Try to avoid placing wires for input and output terminals adjacent to parallel wires. It is best to add a ground wire between the two wires to avoid feedback coupling.

2. The width of the wiring mainly depends on the bonding strength between the wire and the insulating substrate and the value of current flowing through them. When the thickness of the copper foil is 50μm and the width is 1mm, the temperature rise will not exceed 3°C when the current flows through 1A. Based on this, it is estimated that a 2oz (70μm) thick copper foil and a width of 1mm wire can flow 1.5A current, and the temperature rise will not exceed 3°C (Note: natural cooling).

3. The width of the electrical gap between the input control loop part and the output current and control part (that is, the distance between the small current traces and the respective distances between the output traces): 0.75mm–1.0mm (Min0.3 mm ). The reason is that if the copper foil and the pad are too close, it is easy to cause a short circuit, and it is also easy to cause an adverse reaction of electrical interference.

4. The corners of the ROUTE line are usually arc-shaped. Right angles and acute angles will affect the electrical performance of high-frequency circuits.

5. According to the size of the line current, the power line should be as thick as possible to reduce the loop impedance. At the same time, the direction of the power line and the ground line is consistent with the data transmission direction, which reduces the enclosed area and helps to enhance the anti-noise ability.

6: Most radiator grounds also use single-point grounding to improve noise suppression.

Before the change: Multi-point grounding forms a magnetic field loop, and the EMI test fails.

After modification: Single point grounding, no magnetic field loop, EMI test is normal.

7. Wiring of the filter capacitor

a. Noise and ripple are completely filtered by the filter capacitor.

b. When the ripple current is too large, multiple capacitors are connected in parallel, and the ripple current flowing through the first capacitor generates too much ripple current, multiple capacitors are connected in parallel, and the heat generated by the first capacitor flows through Will generate more heat than the second capacitor and the third capacitor. It is easy to be broken. When wiring, please try to distribute the ripple current to each capacitor.

8. The pins of high-voltage and high-frequency electrolytic capacitors have rivets. Keep away from the copper foil on the top layer and follow safety regulations.

9. When routing weak signals, do not lay the wires under inductors, current loops, and other devices.

During the mass production of the current sampling line, the magnetic core of the circuit collided with the copper foil, which caused a malfunction.

10. Do not lay the high-voltage wire under the metal film resistor, but place the low-voltage wire in the middle of the resistor as much as possible. If the resistor is broken, it is easy to short-circuit with the copper wire below.

11. Kathy

a: Put the tin on the narrow part of the copper foil of the power cord.

b: The RC absorption circuit, not only requires high current and Tin but also helps to dissipate heat.

c: Add tin under the thermal element to dissipate heat. The tin cannot be pressed onto the pad.

12. The signal line cannot pass through the transformer, heat sink, and MOS pin.

13. If the output is superimposed, connect the front capacitor of the differential mode inductor to the front ground, and then connect the capacitor after the differential mode inductor to the output ground.

14. The flow channel area of high-frequency pulse current.

a: Minimize the area surrounded by high-frequency pulse current.

b: The power line and the ground line are as close as possible to reduce the enclosed area, thereby reducing the electromagnetic interference generated by the external magnetic field loop cutting, and reducing the electromagnetic radiation outside the loop.

c: The large capacitor should be as close as possible to the MOS tube, and the output RC absorption circuit should be as close as possible to the rectifier tube.

d: The wiring between the power cord and the ground wire should be as thick as possible and as short as possible to reduce loop resistance. The corners should be round and the line width should not change suddenly.

d: The area where the pulse current flows is far away from the input and output terminals to separate the noise source from the socket.

f: The decoupling capacitor of the oscillation filter is grounded close to the IC, and the grounding wire is required to be short.

Vertical transformer cores, I-shaped inductors, power resistors, radiators, and magnetic rings, the first layer of lines cannot pass under them.

15. The distance between the slot and the copper foil of the wiring should be greater than 10MIL. Pay attention to the safety regulations for upper and lower metal parts.

16. The drive transformer, inductance, and current loop must use the same name.

17. For double-sided circuit boards, more vias are usually added to high current traces. Through holes should be tinned to increase current carrying capacity.

18. In a panel, jumpers should not touch other components. If jumpers are connected to high-voltage components, they should be kept at a safe distance from low-voltage components. At the same time, the distance to the radiator should be greater than 1mm.

D. Power PCB Design Rules Case Study

The size of the switching power supply is getting smaller and smaller, its operating frequency is getting higher and higher, and the density of internal components is getting higher and higher. This requires more and more stringent anti-interference requirements for PCB wiring. For some cases of wiring, the problems and solutions are as follows:

1. Overall layout:

Case 1 is a six-layer board. The first layout is the control part on the component surface and the power supply part on the solder surface. During the debugging process, it was found that the interference was very large. The reason is that the positions of the PWM IC and the optocoupler are unreasonable. The PWM IC and the optocoupler are placed under the MOS tube, and there is only a 2.0mm PCB layer between them. The MOS tube directly interferes with the PWM IC, so it is improved to remove the PWM IC and the optocoupler, and there is no component with pulsating components flowing above it.

2. Wiring problem:

The shorter the power trace, the better, to reduce the area enclosed by the loop and avoid interference. The area around the smaller signal line is smaller, and the larger the current loop area, the greater the interference. Because it is a feedback circuit, the larger the area covered by the A-line and the B line, the greater the interference received. Because it is a feedback coupling, the feedback line should be short, and there should be no pulsation signals that cross or parallel to it.

The current sampling line and driveline of the PWM IC chip and the synchronization signal line should be routed as far as possible. They cannot be routed in parallel, otherwise, they will interfere with each other.

E. Power PCB Design Rules Thermal Design

a. The small board should not be too close to the transformer.

If the small board is too close to the transformer, the semiconductor components on the small board are easily affected by heat.

b. Try to avoid using large-area copper foil, otherwise, it will easily generate heat when heated for a long time. This will help eliminate the adhesion between the copper foil and the substrate. Volatile gases are generated by heating the mixture.

F. Power PCB Design Rules Process Processing 

1. Each PCB must be marked with an arrow in the direction of the tin furnace.

2. During the layout, the placement direction of the DIP packaged IC must be perpendicular to the direction through the soldering furnace, not parallel, as shown in the figure below; if the layout is difficult, the IC can be placed horizontally (on the contrary, the placement direction of the SOP packaged IC DIP is the same).

3. The wiring direction is horizontal or vertical. It takes 45 degrees to enter from vertical to horizontal.

4. If the width of the copper foil entering the round pad is smaller than the diameter of the round pad, teardrops should be added.

5. The wiring should be as short as possible, especially the clock line, the wiring of the low-level signal line and all high-frequency loops needs to be shorter.

6. The ground wire and power supply system of the analog circuit and the digital circuit should be completely separated.

7. If there is a large area of ground wire and power wire on the printed circuit board (more than 500 square millimeters), it should be partially.

8. The distance between the centers of horizontally inserted components (resistors, diodes, etc.) must be 300mil, 400mil and 500mil. (If not necessary, 240mil can be used, but it can be used with IN4148 diode or 1/16W resistor. 1/4W resistor starts from 10.0mm) The distance between the centers of jumper pins must be 200mil, 300mil, 500mil, 600mil, 700mil , 800mil, 900mil, 1000mil.

9. The diameter of the heat dissipation holes on the PCB board should not be greater than 140mil.

10. If there is a hole over Φ12 or a square 12MM on the PCB, a hole cover must be made to prevent the solder from flowing out (the hole is 1.0MM)

11. In order to improve the placement accuracy of the SMD components, the PCB board must be equipped with calibration marks (MARKS), and each board must have at least two marks, which are respectively set on the PCB One set on the opposite corner.

12. Pitch of SMD components: 0.75 MM or more.

13. The distance between the chip component and the plug-in component foot should be in accordance with the requirements.

14. When the pins of SMD devices are connected with a large area of copper foil, thermal isolation is required.

15. The center hole of the component pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a virtual solder. The outer diameter of the pad D is generally not less than (d+1.2) mm. d is the lead diameter. For digital circuits, the minimum diameter of the pad can be (d+1.0) mm, and the pad with an aperture greater than 2.5 mm should be appropriately enlarged. The components are placed neatly and in the same direction as possible.

16. The long axis line of the patch components on the PCB board should be arranged in a direction perpendicular to the long axis line of the PCB board as far as possible, and it is not easy to break.

Power PCB Design Rules Application 

• Switching Mode Power Supply
• Inverter power supply
• AC voltage stabilized power supply
• DC regulated power supply
• DC/DC power supply
• Communication power supply
• Module power supply
• Variable frequency power supply
• UPS power supply
• EPS Emergency Power Supply
• Purifying power supply
• PC power supply
• Rectifier power supply
• Heating power supply
• Welding Power/Arc Power Supply
• Electroplating power supply
• Network power supply,
• Power Operating Power Supply
• Adapter power supply
• Linear power supply
• Power Controller/Driver
• Inverter power supply
• Parametric power supply
• The voltage regulating power supply
• Transformer Power Supply
• Security power supply
• High-voltage power supply
• Medical power supply
• Military power supply
• Aerospace Power Supply
• Laser power supply
• Electroplating
• Anodic oxidation
• Induction heating
• Medical equipment
• Electric power test
• Environmental protection dust removal
• Air purification
• Food destroy
• Laser infrared
• Photoelectric display
• Radar navigation
• High energy physics
• Research on plasma physics and nuclear technology

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