What are the cases of varistors in power supply protection?

Varistors (MOVs) are widely used in the field of power protection. Their nonlinear characteristics and high energy absorption capacity make them the core components of overvoltage protection. This article is written by a voltage protector manufacturer to tell you about specific application cases and technical analysis:


1. Switching power supply surge protection
At the input end of the switching power supply, varistors are often used in combination with EMC filters and gas discharge tubes (GDTs) to form multi-level protection. For example:

- Application scenarios: laptop adapters, industrial switching power supply modules.
- Function: Suppress transient overvoltages caused by lightning surges or switching operations, and protect components such as rectifier bridges and capacitors. Varistors can handle surge currents of 100A~25kA, while modular varistors are used in situations with higher energy (>25kA).
- Typical circuit: The front end of the EMC filter is connected in parallel with a varistor, and the back end is connected in series with a GDT to achieve a combination of fast response and large energy discharge.

2. LED lighting system protection
LED drive circuits are susceptible to voltage fluctuations, and varistors can effectively suppress electrostatic discharge (ESD) and power surges:
- Application scenarios: outdoor LED display screens and indoor smart lighting systems.
- Function: SMD varistors are used for port electrostatic protection and absorb high-voltage pulses; lead varistors protect driver ICs from damage caused by power grid fluctuations. For example, after an outdoor LED screen uses an 8000V varistor, the lightning failure rate is reduced by 30%.
- Combination design: The varistor is connected in parallel with the TVS diode to take into account high-frequency response and energy absorption.


3. Back-EMF suppression of inductive loads
Inductive loads such as motors and relays will generate high back-EMF when the power is off, and the varistor can absorb this energy:
- Application scenarios: industrial motor control, household appliances (such as air conditioning compressors).
- Function: Connected in parallel at both ends of the motor coil to clamp the back-EMF within a safe range (such as 470V varistors for 220V systems). For example, in AC motors with electromagnetic brakes, varistors protect switch contacts from arc erosion.
- Parameter selection: The varistor voltage needs to be 1.5~2 times higher than the operating voltage, and the flow rate is selected according to the motor power (usually 3kA~10kA).


4. Lightning protection for solar power generation systems
The DC side of the photovoltaic system is susceptible to induced lightning strikes, and the combination of varistors and GDTs can improve reliability:
- Application scenarios: Solar connection boxes, inverter input/output terminals.
- Function: The varistor absorbs DC side surges and limits the voltage to a safe level; the GDT handles the remaining energy. In one case, this design enabled the system to pass the ISO10605 immunity test.
- Key parameters: The DC system needs to select a high-voltage model (such as above 1000V) and consider temperature stability.


5. Household appliances and lightning protection sockets
Smart home appliances and lightning protection sockets rely on varistors for basic protection:
- Application scenarios: smart refrigerators, air conditioners, lightning protection sockets.
- Function: Varistors (385V~470V) suppress grid fluctuations and lightning-induced voltages. For example, the MOV inside the lightning protection socket can limit a 6000V surge to less than 600V to protect back-end equipment.
- Safety design: A temperature fuse is connected in series to prevent the varistor from short-circuiting and causing a fire.


6. Communication equipment and UPS power protection
Communication base stations and UPS need to cope with complex surge environments:
- Application scenarios: 5G base station power modules, data center UPS.
- Function: Varistors (such as the Sunlord series) form a three-level protection with TVS and GDT to suppress lightning strikes and switching surges. In a UPS case, the MOV module (20kA/6kV) reduced the lightning failure rate by 50%.

- Note: Good grounding is required, and the grounding resistance is less than 10Ω.

Varistors (such as zinc oxide varistors) have the following core advantages in voltage protectors, which make them key components in the field of overvoltage protection:

1. Excellent nonlinear volt-ampere characteristics
The varistor is in a high resistance state under normal voltage and conducts almost no current; when the voltage exceeds the threshold (varistor voltage), its resistance value drops sharply, quickly absorbing the overvoltage energy, thereby effectively clamping the voltage peak. This characteristic originates from the electron tunneling effect of the zinc oxide grain boundary, which can respond to overvoltage in nanoseconds.

2. High energy absorption capacity
The varistor can withstand large current surges (such as transient energy generated by lightning strikes or switching operations), and the current capacity can reach thousands of amperes (such as more than 10kA), which is suitable for high-energy overvoltage scenarios.

3. Fast response time
The varistor has a short response time and can quickly suppress overvoltage and protect the back-end sensitive devices from damage. This feature makes it superior to slow-response components such as traditional gas discharge tubes.

4. Bidirectional protection function
Unlike unidirectional TVS diodes, varistors have bidirectional symmetrical protection characteristics, which can cope with positive and negative polarity overvoltages at the same time, and can be connected to the circuit without distinguishing polarity.

5. Modular integration and multifunctional combination
Modern voltage protectors often integrate varistors with current-limiting devices and degradation alarm functions to form a modular design.

6. Wide application adaptability
- Power supply system: Suitable for multi-level protection of industrial frequency AC and DC power supplies.
- Signal lines: Such as RS-232 interfaces, coaxial cables, etc., require low capacitance design to avoid signal attenuation.
- Industrial equipment: Used for surge suppression of inductive loads such as motors and SSR relays to prevent back electromotive force from damaging contacts.

7. Reliability and stability
Varistor maintains stable performance in the range of -40℃ to +85℃ and has passed UL, AEC-Q200, and other certifications, suitable for harsh environments. However, it should be noted that frequent overvoltage may cause aging, and it needs to be used with fuses.

Summary

The core advantages of varistors are their nonlinear characteristics, high energy absorption, fast response, and modular design, making them an ideal choice for voltage protectors. However, in practical applications, parameters (such as varistor voltage and current capacity) must be selected according to the scenario and auxiliary circuits must be used to optimize reliability and life.