In electronic circuit design, PCB (printed circuit board) is a key part of realizing the function of electronic equipment. In the PCB design process, the distribution of current is a very important issue.
1. Concept of electric current
Current is the amount of charge passing through A cross section of a conductor per unit time, usually in amperes (A). In a circuit, current can be divided into DC current and AC current. Dc current refers to the current whose direction is constant, while AC current refers to the current whose direction is periodically changed.
2. Classification of current on PCB board
On the PCB board, the current can be divided into large current and small current. High current usually refers to a circuit with a large current value, such as a power supply circuit and a motor drive circuit. Small current usually refers to circuits with small current values, such as signal processing circuits, microcontroller circuits, etc.
3. Characteristics of large current and small current
Characteristics of high current
(1) The current value is large: the current value of a large current circuit is usually between a few amps and tens of amps.
(2) High power consumption: due to the large current value, the power consumption of the large current circuit is correspondingly high.
(3) Large heat: large current circuit will generate large heat during the working process, and heat dissipation measures need to be taken.
(4) Strong electromagnetic interference: the high-current circuit will produce strong electromagnetic interference during the working process, which may have an impact on other circuits.
Characteristic of small current
(1). Small current value: The current value of a small current circuit is usually between a few milliamps and tens of milliamps.
(2) Low power consumption: due to the small current value, the power consumption of the small current circuit is relatively low.
(3) Small heat: The heat generated by the small current circuit in the working process is small, and the heat dissipation requirements are relatively low.
(4) Weak electromagnetic interference: The electromagnetic interference generated by the small current circuit during the working process is weak, and the impact on other circuits is small.
4. Distribution principle of large current and small current
Power distribution principle
(1) Power distribution should follow the principle of "from large to small", that is, first distribute the power supply of the high-current circuit, and then distribute the power supply of the small-current circuit.
(2) Power distribution should be as close to the load as possible to reduce the length of the power line and reduce resistance and voltage drop.
(3) Power distribution should consider the stability of the power supply and ripple requirements to ensure the normal operation of the circuit.
Ground distribution principle
(1) The ground wire distribution should follow the principle of "from small to large", that is, the ground wire of the small current circuit is distributed first, and then the ground wire of the large current circuit is distributed.
(2) Ground distribution should be as short and thick as possible to reduce ground resistance and reduce ground noise.
(3) The ground wire distribution should consider the influence of the ground wire loop to avoid a large ground wire loop.
Signal line distribution principle
(1) The distribution of signal lines should follow the principle of "from high to low", that is, the high-speed signal line is distributed first, and then the low-speed signal line is distributed.
(2) The distribution of signal lines should be avoided as far as possible in parallel with large current lines to reduce electromagnetic interference.
(3) Signal line allocation should consider the transmission characteristics of the signal line, such as impedance matching, transmission delay, etc.
5. Wiring methods for high current and low current
High current wiring method
(1) Choose the appropriate wire width: large current wiring should choose a wider wire to reduce resistance and voltage drop.
(2) Using multiple parallel wires: For large currents, multiple parallel wires can be used to reduce the current density of a single wire.
(3) Star wiring: For high-current circuits with multiple loads, star wiring can be used to reduce wire length and resistance.
(4) Consider the heat dissipation problem: high current wiring should consider the heat dissipation problem, such as the use of heat sinks, fans and other heat dissipation measures.
Low current wiring method
(1) Choose the appropriate wire width: small current wiring can choose a narrower wire to save PCB space.
(2) The use of snake wiring: for small current signal lines, snake wiring can be used to reduce the interference of signal lines to other signal lines.
(3) Avoid parallel with large current lines: small current signal lines should be avoided as far as possible to parallel with large current lines to reduce electromagnetic interference.
(4) Consider signal integrity: The wiring of small current signal lines should consider signal integrity, such as impedance matching, transmission delay, etc.
6. PCB layout method of high current and low current
High current layout method
(1) Place the high-current circuit in the center of the PCB to reduce the length of the power cord and ground wire.
(2) The high-current circuit is arranged separately from other circuits to reduce electromagnetic interference.
(3) Put the power supply and ground wire of the high-current circuit on the edge of the PCB to facilitate heat dissipation.
(4) Consider the problem of heat dissipation, such as setting heat dissipation holes and fins near the large current circuit.
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