What problems should be paid attention to when PCB proofing components are used?
In PCB proofing, components with high sensitive voltage threshold should be used as much as possible on the premise of meeting the requirements of circuit functions. Because the electrostatic and discharge sensitivity of a circuit board depends on the components with the lowest sensitive voltage threshold in the circuit board.
1, limiting the output current can avoid the locking effect of CMOS circuits.
Lock-in effect means that there are parasitic PNP transistors and NPN transistors in the internal structure of CMOS circuits, and they just form a parasitic PNPN thyristor structure. This interlocking positive feedback structure may be triggered by external factors (such as electrostatic discharge), and the current will flow through the PNP tube (or NPN tube), and then the current will be amplified by another parasitic NPN tube (or PNP tube), and the current will become larger and larger due to the positive feedback, which will eventually burn out. It can be seen that limiting the current so that it can’t reach the level of maintaining the locked state is one of the problems to be considered for CMOS devices in PCB proofing.
The common solution is to use a resistor to separate each output terminal from its cable, and use two high-speed switching diodes to clamp the cable to VDD (drain power supply) and VSS (source power supply).
2. Use filter network.
Sometimes a long input cable is needed between the CMOS circuit system and the mechanical contacts, which increases the possibility of electromagnetic interference. Therefore, a filter network should be considered. At the same time, a long input line is bound to be accompanied by a large distributed capacitance and inductance, which is easy to form LC self-excited oscillation, resulting in diode burning of the protection network. The solution is to connect a resistor in series at the input end, and its resistance value can be selected according to the formula R=VDD/1mA.
3. RC network
Where it is really feasible, for the sensitive input of bipolar devices, the RC network composed of resistors with larger resistance and capacitors with at least 100pF can reduce the influence of electrostatic discharge.
4. Avoid dangling input pins of CMOS devices.
In PCB proofing, it is necessary to avoid the input terminal of CMOS devices welded to the circuit board being suspended. At the same time, it should be noted that all unused redundant input leads on CMOS devices are not allowed to float. This is because once the input terminal is suspended, the input potential will be in an unstable state, which will not only destroy the normal logic relationship of the circuit, but also easily lead to electrostatic breakdown and external noise interference. The redundant input terminals should be treated separately according to the function of the circuit.
