As a crucial component of modern electronic devices, the safety and stability of power adapters directly impact the operational performance and lifespan of these devices. Lightning, a common natural disaster, poses a significant threat to power adapters. Therefore, lightning protection measures are paramount in the design of power adapters.
I. The hazards of lightning to power adapters
Lightning is a powerful natural phenomenon containing immense energy; the voltage and current generated by its instantaneous discharge can reach tens of thousands of volts and thousands of amperes. Lightning primarily damages power adapters through two pathways: direct lightning strikes and induced lightning. Direct lightning strikes occur when lightning directly hits the power adapter; although relatively rare, their destructive power is devastating when they do happen. Induced lightning strikes, on the other hand, are transient overvoltages generated in the power adapter's circuitry through electromagnetic induction when lightning discharges nearby; this is more common.
Transient overvoltages generated by lightning can severely impact the circuitry of power adapters, potentially causing damage to electronic components, short circuits, or even fires. Therefore, implementing effective lightning protection measures to safeguard power adapters from lightning strikes is crucial for ensuring the normal operation of equipment.
II. Application of Varistors in Lightning Protection
A voltage-dependent resistor (VDR) is a resistive device with non-linear current-voltage characteristics. Its main function is to clamp the voltage and absorb excess current when a circuit is subjected to overvoltage, thus protecting sensitive components. The resistive material of a voltage-dependent resistor is usually a semiconductor, therefore it belongs to the category of semiconductor resistors.
The working principle of a varistor can be summarized as follows: When the voltage applied across its terminals is below its threshold voltage (UN), the current flowing through it is extremely small, equivalent to a resistor with infinite resistance; at this time, the varistor acts as an open switch. When the voltage exceeds UN, the resistance of the varistor decreases rapidly, and the current flowing through it surges, equivalent to a resistor with infinite resistance; at this time, the varistor acts as a closed switch. Utilizing this characteristic, the varistor can clamp the voltage to a relatively fixed value when an overvoltage occurs in the circuit, thereby protecting downstream circuits from damage.
In power adapters, varistors are commonly used as surge protectors. Their main function is to absorb transient overvoltages generated by lightning and prevent them from damaging the circuit. When a high voltage from a lightning strikes the power adapter, the varistor responds quickly, clamping the overvoltage to a lower level. At the same time, the overcurrent caused by a short circuit will burn out the fuse or force the circuit breaker to trip, thereby forcibly cutting off the power supply and protecting the circuit from damage.
III. Selection and Parameters of Varistors
When selecting a varistor for surge protection of a power adapter, several factors need to be considered, including parameters such as maximum continuous operating voltage, varistor voltage, maximum limiting voltage, maximum energy, and maximum peak current.
Maximum continuous operating voltage: This is the maximum voltage that the varistor can withstand under normal operating conditions. When selecting a varistor, ensure that the maximum continuous operating voltage is slightly lower than the varistor's voltage rating to avoid affecting its lifespan.
Varistor voltage: This refers to the voltage across a varistor when a pulse current (typically 1mA, duration less than 400ms) passes through it for a specified duration. In power adapters, the varistor voltage should be selected based on the device's operating voltage and potential overvoltage conditions.
Maximum limiting voltage: The peak voltage across the varistor under the maximum pulse peak current and specified waveform that the varistor can withstand. This parameter determines the varistor's protection capability under overvoltage surges.
Maximum energy: The maximum energy a varistor can absorb when subjected to an overvoltage surge. This parameter is closely related to the lifespan and reliability of the varistor.
Maximum peak current: The maximum surge current that a varistor can withstand under a specific pulse current waveform (e.g., 8/20μs). This parameter reflects the varistor's ability to withstand extreme conditions.
In addition, the surge resistance characteristics of the varistor must be considered, meaning the varistor itself should be able to withstand the specified surge current, surge energy, and average power when multiple surges occur consecutively. Furthermore, the continuous operating voltage life and surge life of the varistor are also important selection criteria.
IV. Practical Applications of Varistors in Power Adapters
In the surge protection design of power adapters, varistors are typically used in conjunction with components such as fuses and overvoltage protectors to form a complete surge protection circuit. For example, in a three-phase four-wire power supply system, the first level of surge protection usually uses a high-energy surge arrester to short-circuit and discharge transient overvoltages generated by lightning to the ground; the second level of surge protection uses an overvoltage protector, which includes a varistor element to further absorb overvoltage energy and protect the circuit from damage.
In the internal circuitry of a power adapter, a varistor is typically installed at the input terminal, connected in series with a fuse. When a high voltage surge from lightning strikes the power adapter, the varistor responds quickly, clamping the overvoltage to a lower level. Simultaneously, the overcurrent caused by a short circuit burns out the fuse at the front, thus cutting off the power supply and protecting the circuit from damage.
V. Conclusion
Varistors, as crucial surge protection components in power adapters, offer advantages such as fast response time, large current capacity, and no follow-up current. They effectively absorb transient overvoltages generated by lightning, protecting the circuit from overvoltage damage. In the surge protection design of power adapters, the model and parameters of the varistor should be selected appropriately based on the actual conditions of the equipment and the potential overvoltage scenarios to ensure the effectiveness and reliability of the surge protection circuit. Furthermore, regular inspection and maintenance of the power adapter are essential to promptly identify and address potential surge protection hazards, ensuring the normal operation and safe use of the equipment.
