During the application of communication products, overvoltage and overcurrent caused by lightning strikes and other reasons will cause damage to the device ports, so corresponding protection circuits should be designed. Next, 2PCB will introduce the components in the lightning protection circuit and the requirements for the protection circuit in PCB routing.
Components in lightning protection circuits
Gas discharge tube: a switch-type protection device, the working principle is gas discharge. When the voltage between the two poles is large enough, the gap between the poles will discharge and break down, and the original insulation state will be converted to a conductive state, similar to a short circuit. The voltage maintained between the two poles in the conductive state is very low, generally 20~50V, so it can protect the subsequent circuit.
Varistor: a voltage-limiting protection device. Using the nonlinear characteristics of the varistor, when the overvoltage appears between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thereby protecting the subsequent circuit.
Voltage clamping transient suppression diode (TVS): a voltage-limiting protection device, the function is very similar to that of a varistor. It also uses the nonlinear characteristics of the device to clamp the overvoltage to a lower voltage value to protect the subsequent circuit.
Voltage switching transient suppression diode (TSS): Like the TVS tube, it is also a voltage limiting protection device made by semiconductor technology, but its working principle is similar to that of the gas discharge tube, and different from that of the varistor and TVS tube.
Positive temperature coefficient thermistor (PTC): A current limiting protection device, it has an action temperature value Ts, and its resistance remains basically constant. At this time, the resistance is called cold resistance. Because its resistance can increase rapidly with the increase of temperature, it is generally connected in series on the line for transient large current overcurrent protection.
Insurance tube, fuse, air switch: All are protection devices, used in the event of short circuit, overcurrent and other faults inside the equipment, can disconnect the short-circuit load or overcurrent load on the line, prevent electrical fires and ensure the safety of the equipment.
Inductor, resistor, wire: They are not protection devices themselves, but they can play a coordinated role in the protection circuit composed of multiple different protection devices.
Transformers, optocouplers, and relays: They are also not protection devices, but the isolation of these devices can be used in the design of port circuits to improve the port circuit’s ability to resist overvoltage.
Reminder: In the process of selecting devices for protection circuits, gas discharge tubes, varistors, thermistors, insurance tubes, fuses, air switches, etc. must be selected with safety certification.
A common mistake in the design of protection circuits is that the protection devices in the protection circuits meet the requirements of the design indicators, but problems occur during the PCB layout process, reducing the protection effect of the protection circuit.
The protection circuit has the following requirements in terms of PCB routing:
1. When routing the interface circuit, be careful not to make the printed routing too thin. Generally, the routing on the surface of the printed board can withstand an 8/20us impact current of about 1kA with a line width of 15mi.
2. When using isolation devices such as transformers for protection design, special attention should be paid to the selection of devices and the insulation design of PCB. The primary circuit should be separated from other circuits and ground printed lines on the single board, and there should be sufficient insulation distance. There should be no unexpected discharge path.
3. The protection device should be placed close to the output end or connector. The connection between the protection device and the protected line and the connection between the protection device and the ground should be as short as possible.
4. The connection from the port to the gas discharge tube and varistor, such as large-flow protection devices, should be routed on the surface of the PCB as much as possible to prevent PCB damage due to overheating.
5. The PCB connection through the impact current should have as few vias as possible, and the line width should be kept consistent to avoid the reflection of the impact current waveform caused by impedance mismatch.
6. The input and output of the protection device are separated, and the routing does not cross or parallel each other to avoid the input impact current coupling to the output end and reduce the performance of the protection device.