What are the selection parameters for varistors?

Detailed explanation of core parameters for selecting varistors

Varistors (MOVs) are key protective devices used in circuits to suppress transient overvoltages such as surges and ESD. Its selection is not simply based on "voltage", it requires a comprehensive consideration of a series of interrelated parameters to ensure that it can effectively protect the downstream circuit and work reliably on its own. The main selection parameters are as follows:

1. Varistor Voltage (VN)
Definition: The voltage value across a varistor at a specific DC current (usually 1mA). This is a milestone where varistors begin to exhibit significant nonlinear characteristics.
Key selection points:
Basic basis: This is the starting point for selection. The varistor voltage must be higher than the maximum continuous operating voltage of the protected circuit (including the allowed upper limit of fluctuations). This is the key to ensuring that the varistor exhibits a high resistance state under normal operating conditions and does not affect the circuit function.
Rule of thumb: For alternating current (AC) applications, it is common to choose a voltage sensitive voltage that satisfies: VN ≥ (1.2-1.5) Vrms 2 (i.e. approximately 1.2-1.5 times the peak AC voltage). For direct current (DC) applications: VN ≥ (1.2~1.5) Vdc.
Consideration of margin: Leaving a certain margin is to cope with power grid fluctuations, device aging (varistor voltage will slowly decrease with the number of uses), and the influence of environmental temperature (varistor voltage has a negative temperature coefficient, and VN will slightly decrease when the temperature rises).

2. Maximum Continuous Operating Voltage (MCOV or VC)
Definition: The maximum AC RMS or DC voltage that a varistor can withstand safely and reliably for a long time without deterioration or damage.
Key selection points:
Practical work constraint: MCOV must be greater than or equal to the maximum continuous operating voltage that the protected circuit may actually experience (including the worst-case steady-state overvoltage situation). This is the core parameter that ensures the long-term reliability of varistors, and it can directly reflect their ability to withstand steady-state voltage more than the varistor voltage VN.
Relationship with VN: MCOV is usually smaller than VN (for AC, MCOV ≈ VN/√ 2~VN/1.4). When selecting, both VN and MCOV requirements must be met simultaneously.

3. Current carrying capacity/Surge Current Rating (Ip or Imax)
Definition: The maximum peak current that a varistor can withstand a specified number of times (usually 1 or 2 times) under a specific waveform (usually a standard 8/20 μ s current wave) without damage (such as cracking or ignition) or performance parameters exceeding the allowable range of variation (such as a change in varistor voltage of ≤± 10%).
Key selection points:
Core protection capability: This directly determines how much transient energy the varistor can absorb. It is necessary to estimate or refer to standards (such as IEC 61000-4-5) to determine the maximum surge current (peak and waveform) that the circuit may experience.
Strict derating: The selected current capacity must be significantly greater than the expected maximum surge current peak. It is usually recommended to choose a current capacity that is 1.5 to 2 times or more the expected surge. Insufficient derating is one of the main reasons for MOV failure.
Frequency and lifespan: Pay attention to the impact frequency (single or multiple) corresponding to the nominal current capacity in the specification sheet. If you need to withstand repeated surges, you need to pay attention to its lifespan curve or choose a higher specification.

4. Maximum Clamping Voltage (Vc)
Definition: The highest voltage value exhibited at both ends of a varistor when a specified waveform (such as 8/20 μ s) and peak surge current are applied.
Key selection points:
The safety threshold of the protected object: Vc must be lower than the maximum safe voltage that the protected circuit or component can withstand (i.e. its withstand voltage value). This is a direct indicator to ensure that the downstream circuit is not damaged by overvoltage.
Related to IP: Vc is tested under a specific IP. The specification sheet usually provides Vc curves or tables corresponding to different IP levels. When selecting, the corresponding Vc value should be checked based on the expected maximum surge current Ip.
The lower the better (under other conditions): Choosing MOVs with lower Vc can provide better protection while ensuring current carrying capacity and voltage level.

5. Energy Absorption (W)
Definition: The maximum energy (in joules J) that a varistor can absorb without damage during a surge event of a specific waveform (usually 10/1000 μ s or 2ms square wave).
Key selection points:
Energy perspective: The essence of surges is energy impact. The current carrying capacity Ip reflects the instantaneous current stress, while the energy tolerance integrates current, voltage, and time.
Complex surge assessment: For non-standard waveforms or long duration surges (such as overvoltage caused by certain switch operations), energy tolerance is a more direct evaluation basis.
Calculation and derating: Estimate surge energy (E ≈ Vc Ip pulse duration factor) and ensure that the selected MOV has sufficient energy tolerance margin (usually more than twice).

6. Response Time
Definition: The time required for a varistor to detect an overvoltage exceeding its threshold voltage and begin clamping action.
Key selection points:
Usually non critical limitation: The response time of MOVs is usually in the ns range (<25ns), much faster than the rising edge of most threatening surges (μ s range). Special attention should be paid to ESD protection (ns level rising edge), but in general power surge protection, the response speed of MOVs is fast enough.
Coordination with other devices: In situations where extremely fast response is required (such as high-speed data line ESD protection), MOVs may need to be used in conjunction with other faster devices (such as TVS diodes).

7. Rated power dissipation
Definition: The maximum power loss that a varistor can sustain at a specific ambient temperature (primarily due to leakage current).
Key selection points:
Steady state reliability: Under continuous operating voltage, MOV has a small leakage current (μ A), which will generate power consumption. Ensure that the actual power consumption (≈ V ²/dynamic resistance) is less than the rated dissipated power at the highest operating voltage and highest operating ambient temperature.
High temperature impact: Leakage current increases with temperature, and power consumption also increases accordingly. Special attention should be paid to derating in high-temperature application environments.

8. Capacity value
Definition: The inherent capacitance value exhibited by a varistor at low voltages (such as 1V, 1kHz).
Key selection points:
High frequency signal impact: For protecting high-frequency signal lines (such as communication lines, USB, HDMI), larger junction capacitors (up to nF level) may cause signal attenuation, distortion, or reflection, affecting signal integrity.
Choice: When protecting high-frequency lines, low capacitance MOVs (or consider using low capacitance TVS diodes) should be selected. In low-frequency applications such as power lines, the influence of capacitance is usually negligible.

9. Physical dimensions and packaging
Key selection points:
Flow and heat dissipation: Generally, the larger the size (diameter or thickness) of a MOV, the greater its flow capacity and energy tolerance, and the better its heat dissipation. Select the appropriate size according to the surge level requirements.
Installation methods: There are various packaging options including radial lead (direct insertion), axial lead, and surface mount (SMD). Choose packaging forms that meet PCB layout, space requirements, and production processes.
Voltage Creepage Distance: In high-voltage applications or situations with strict safety requirements (such as safety regulations), it is necessary to consider whether the pin spacing provided by the package meets the insulation and creepage distance requirements.

Selection Summary and Key Logic

1. Determine the working environment: Clearly define the maximum continuous operating voltage (ACrms/Vdc) of the protected circuit and the potential surge sources (type, intensity, waveform, frequency) that may be encountered.
2. Set voltage threshold: Based on the operating voltage, calculate and select MOVs that meet the requirements of MCOV ≥ actual maximum operating voltage and VN with sufficient margin.
3. Evaluate surge threats: Estimate or determine the maximum expected surge current 'Ip' and energy 'W' based on standards.
4. Choose protection capability: Select MOVs with a current carrying capacity (Ip) and energy endurance (W) significantly greater than the expected surge value (fully derated). Simultaneously verify whether the clamping voltage (Vc) at the surge level is lower than the withstand voltage value of the protected object.
5. Consider application details:
High frequency circuit: Pay attention to the capacitance value.
High temperature environment: Verify the rated dissipated power and temperature derating curve.
Space limitations/process: Choose appropriate packaging and size.
Reliability requirement: For applications that require multiple surges, pay attention to the lifespan curve or choose higher specifications.
Safety certification: If there are mandatory safety requirements in the application scenario (such as UL, TUV, CQC), the corresponding certified model must be selected.

Remember: The selection of varistors is a systematic balancing process, where various parameters are interrelated (such as size affecting current/heat dissipation, voltage level affecting clamping). Be sure to refer to the detailed datasheet of the target model, understand its testing conditions and characteristic curves, and conduct actual surge testing verification if possible. Reasonable derating design is the key to ensuring long-term reliability.