How do diodes protect communication circuits from surge currents?

1, The Causes and Hazards of Surge Current
Causes of surge current
The generation of surge current is usually related to sudden changes in the external environment or changes in the internal state of the circuit. For example, during lightning strikes, lightning electromagnetic pulses will couple into the circuit through communication lines, generating instantaneous high voltage and surge current; During power switching, due to the presence of internal resistance in the power supply, significant voltage fluctuations and surge currents will occur at the moment of switching; When switching inductive loads (such as relays, motors, etc.), surge currents are also generated at the moment of switching due to the energy storage characteristics of inductive components.
The hazards of surge current
The harm of surge current to communication circuits is mainly reflected in the following aspects:
Component damage: The amplitude of surge current is much higher than the normal operating current of the circuit, which may cause sensitive components (such as integrated circuits, transistors, etc.) to overload and burn out.
Performance degradation: Voltage fluctuations and electromagnetic interference caused by surge currents may affect the normal operation of circuits, leading to signal distortion, communication interruptions, and other issues.
Shortened lifespan: Circuit components that have been subjected to surge currents for a long time will gradually degrade in performance and greatly shorten their service life.
2, The principle of diode protection for communication circuits from surge currents
The principle of diode protection for communication circuits from surge currents is mainly based on its unidirectional conductivity and fast response characteristics. When surge current occurs, the diode will quickly conduct, guiding the surge current to ground or other safe paths, thereby protecting sensitive components in the circuit. Specifically, the working process of a diode protection circuit can be divided into the following stages:
Surge current detection: When a surge current occurs, the voltage or current in the circuit will quickly rise, triggering the conduction condition of the diode.
Diode conduction: The diode rapidly conducts under forward bias, guiding surge current to ground or other safe paths.
Surge current suppression: As the surge current gradually decays, the voltage across the diode gradually decreases. When the voltage is lower than the conduction voltage of the diode, the diode cuts off and the circuit returns to normal operation.
3, Types of diodes used for surge current protection
Zener diode (Zener diode)
Zener diodes have stable voltage characteristics in reverse breakdown state and can be used to limit overvoltage in circuits. When the surge current causes the voltage to exceed the breakdown voltage of the Zener diode, the diode conducts, clamping the voltage at a safe level. However, the surge current tolerance of Zener diodes is relatively low, and they are usually used for the protection of low-power circuits.
Transient voltage suppression diode (TVS diode)
TVS diode is a semiconductor device specifically designed to suppress surge voltage and surge current. It has an extremely fast response speed (usually in the picosecond range) and a high ability to withstand surge currents. When a surge voltage occurs, the TVS diode quickly conducts, guiding the surge current to ground and protecting sensitive components in the circuit. TVS diodes are widely used in fields such as communication, computer, and automotive electronics.
Schottky diode
Schottky diodes have low forward voltage drop and fast switching speed, making them suitable for protecting high-frequency circuits. Although its surge current tolerance is relatively low, Schottky diodes are still an effective surge current protection component in certain specific applications, such as low-voltage, low-power communication circuits.
4, Design points of diode protection circuit
Choose the appropriate diode type
Choosing the appropriate diode type based on the specific requirements of the circuit is the key to designing a protective circuit. For circuits that need to withstand large surge currents, TVS diodes with high surge current tolerance should be selected; For low-power circuits, it is possible to consider using Zener diodes or Schottky diodes.
Determine the rated parameters of the diode
When selecting a diode, it is necessary to pay attention to its rated parameters, such as reverse breakdown voltage, maximum surge current, forward voltage drop, etc. These parameters should meet the actual needs of the circuit to ensure that the diode can function properly in the event of surge current.
Reasonable circuit layout
The layout of diode protection circuits should be reasonable to reduce electromagnetic interference and signal attenuation. The diode should be placed as close as possible to the protected component to shorten the protection path; Meanwhile, coupling interference between diodes and other high-frequency components should be avoided.
Consider heat dissipation issues
When surge current passes through a diode, it generates heat. If the heat dissipation is poor, it may cause the diode to overheat and be damaged. Therefore, when designing protective circuits, the heat dissipation of diodes should be considered, such as adding heat sinks, optimizing circuit layout, etc.
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