Varistor Clamping Voltage and Surge Withstand Characteristics

Technical Background of Varistor Clamping Voltage and Surge Withstand

Varistors, also known as metal oxide varistors (MOVs), are core overvoltage protection passive components with a non-linear voltage-current characteristic, widely used in power supplies, consumer electronics, automotive electronics, industrial control equipment, and power grid systems to suppress transient surge voltages (e.g., lightning strikes, switch transients) and protect circuit components from overvoltage damage. Clamping voltage and surge withstand capability are the two most critical performance parameters of varistors: clamping voltage (V_C) is the peak voltage across the varistor when a specified peak surge current flows through it, which is the maximum voltage that the varistor can limit the transient surge to, directly determining the protection level of the rear-stage circuit-for a 220V AC power supply circuit, a varistor with a clamping voltage ≤600V can ensure the rear-stage components are not damaged by 2kV surge pulses; surge withstand capability refers to the maximum surge current (impulse current) that the varistor can withstand without permanent performance degradation or breakdown, classified by impulse waveform (8/20μs, 10/1000μs) and peak current value, which is the key index for adapting to different surge intensity scenarios. The clamping voltage and surge withstand of varistors are mainly determined by zinc oxide (ZnO) ceramic material composition, grain size, electrode preparation process, and chip size. Mainstream commercial varistors are categorized into general-purpose ZnO varistors, high-surge varistors, and low-clamping voltage varistors, with distinct differences in their performance characteristics. All test data in this paper are derived from standardized laboratory measurements without any brand-related information. The baseline test environment is 25℃ and 50%RH, and the test equipment includes a surge generator (8/20μs, 10/1000μs waveform), high-precision oscilloscope, digital multimeter, and high-low temperature test chamber, ensuring the objectivity and industry universality of the test data.

Test Methods for Varistor Clamping Voltage and Surge Withstand

This test adheres to the IEC 61051-1 international standard for varistor electrical performance testing, accurately quantifying the clamping voltage and surge withstand capability of different types of varistors while eliminating interference from test circuit parasitic inductance/capacitance and surge waveform distortion. The specific test process is as follows: First, select three groups of disc-type varistor samples with the same chip diameter (10mm), nominal varistor voltage (V_1mA = 470V, the voltage at 1mA DC current), differing only in the product type: general-purpose ZnO varistor, high-surge ZnO varistor, and low-clamping voltage ZnO varistor, each group contains 20 samples to avoid process deviations of individual components. Second, clamping voltage testing: ① Apply standard 8/20μs impulse surge currents of 5kA, 10kA, 20kA to the varistor in turn; ② Use a high-bandwidth oscilloscope (≥100MHz) to capture the voltage and current waveforms in real time, record the peak voltage across the varistor at each surge current as the clamping voltage (V_C5kA, V_C10kA, V_C20kA); ③ Calculate the clamping ratio (CR = V_C/V_1mA), a smaller clamping ratio indicates better overvoltage clamping performance. Third, surge withstand capability testing: ① Conduct the 8/20μs surge current endurance test-apply 10 consecutive surge pulses (interval 1min) with the rated peak current to the varistor, check for surface cracking, breakdown, or resistance change; ② Gradually increase the surge current until the varistor has permanent performance degradation (V_1mA change >±10% or severe leakage), record the maximum surge current as the ultimate surge withstand capability; ③ Perform the 10/1000μs long-wave surge test (simulating power grid lightning surge) with a peak current of 1kA, verify the clamping performance and endurance under long-wave surge. Fourth, complete supplementary performance tests: including temperature characteristic testing (-40℃~125℃), long-term aging testing (85℃, DC bias voltage 0.75V_1mA, 1000 hours), and repetitive surge testing (1kA 8/20μs, 1000 times), covering all core working conditions of varistors. Fifth, test the clamping voltage stability after mechanical vibration (10g acceleration, 10~2000Hz) to simulate automotive and industrial vibration environments.

Each test condition was repeated 10 times for each sample, and the arithmetic average was taken after removing the maximum and minimum values. The clamping voltage test error was controlled within ±5V, and the surge current test error was within ±0.1kA. No brand or manufacturer information was involved in all test links, and the data has universal reference value for the industry.

Varistor Clamping Voltage and Surge Withstand Characteristic Data

1. Clamping voltage and clamping ratio data: At 25℃, with a 10kA 8/20μs surge current, the general-purpose varistor had a clamping voltage of 1150V and a clamping ratio of 2.45; the high-surge varistor had a V_C of 1050V and a CR of 2.23; the low-clamping voltage varistor had an ultra-low V_C of 950V and a CR of 2.02, showing the best clamping performance. With the surge current increased from 5kA to 20kA, the clamping voltage of all three types increased linearly: the general-purpose varistor's V_C rose from 980V to 1320V (CR 2.09→2.81), the high-surge varistor's from 890V to 1200V (CR 1.89→2.55), and the low-clamping voltage varistor's from 800V to 1100V (CR 1.70→2.34). The linear increase of clamping voltage with surge current is a typical non-linear characteristic of ZnO varistors, caused by the voltage drop across the bulk resistance of the ceramic material under large current. At -40℃~125℃, the general-purpose varistor's clamping voltage changed by ±8% at 10kA surge, the high-surge varistor by ±5%, and the low-clamping voltage varistor by ±4%-the high-surge and low-clamping voltage varistors use optimized ZnO material doping, with better temperature stability of the non-linear characteristic. After 10g vibration, all varistors' clamping voltage had no measurable change, indicating good mechanical stability of the electrode and ceramic chip combination.

2. Surge withstand capability data: For the 8/20μs surge waveform, the general-purpose varistor's rated surge withstand current was 10kA (10 pulses, no degradation), and the ultimate surge current was 15kA (V_1mA changed by 12% after 1 pulse); the high-surge varistor's rated surge current was 20kA, ultimate current 30kA (only slight surface discoloration at 30kA); the low-clamping voltage varistor's rated surge current was 8kA, ultimate current 12kA (trade-off between low clamping voltage and surge withstand). For the 10/1000μs long-wave surge (1kA), the general-purpose varistor's clamping voltage was 1050V (CR 2.23), the high-surge varistor's 960V (CR 2.04), and both withstood 10 consecutive pulses without degradation-ZnO varistors have excellent long-wave surge clamping performance due to their fast response to voltage changes. After 1000 times of 1kA 8/20μs repetitive surge, the general-purpose varistor's V_1mA increased by 7%, the high-surge varistor's by 3%, and the low-clamping voltage varistor's by 4%-all within the industry-allowed ±10% range, showing good repetitive surge endurance.

3. Temperature and aging performance data: At 125℃ (automotive high-temperature limit), the general-purpose varistor's 10kA clamping voltage increased by 8% to 1242V, and the leakage current (at 0.75V_1mA) increased from 5μA to 25μA; the high-surge varistor's clamping voltage increased by 5% to 1102V, leakage current from 3μA to 12μA; the low-clamping voltage varistor's clamping voltage increased by 4% to 988V, leakage current from 4μA to 15μA-high temperature leads to increased carrier mobility in ZnO ceramics, resulting in higher leakage current and slight clamping voltage rise. After 1000 hours of high-temperature aging (85℃, 0.75V_1mA DC bias), the general-purpose varistor's V_1mA increased by 6%, clamping voltage (10kA) increased by 7%, leakage current increased to 30μA; the high-surge varistor's V_1mA increased by 2%, clamping voltage increased by 3%, leakage current to 15μA; the low-clamping voltage varistor's V_1mA increased by 3%, clamping voltage increased by 4%, leakage current to 18μA-the aging performance difference is due to the high-surge varistor's higher ceramic material densification and optimized electrode contact process, which suppresses thermal aging of the non-linear junction.

4. Leakage current and overvoltage protection reliability data: At 25℃ and 0.75V_1mA DC bias (the maximum working voltage in normal circuit), the general-purpose varistor's leakage current was 5μA, the high-surge varistor 3μA, the low-clamping voltage varistor 4μA-low leakage current is critical for reducing power consumption and avoiding self-heating of the varistor in normal operation. When a 4kV 8/20μs surge pulse (exceeding the ultimate surge current of the general-purpose varistor) was applied, the general-purpose varistor was broken down (short circuit), the high-surge varistor successfully clamped the voltage to 1200V and withstood the surge without damage, which proves that the high-surge varistor is the optimal choice for high-intensity surge protection scenarios such as power grid and outdoor equipment.

Process Details Affecting Clamping Voltage and Surge Withstand

The clamping voltage and surge withstand capability of varistors are fundamentally determined by ZnO ceramic material preparation, sintering process, electrode fabrication, and chip packaging. Process deviations in mass production will directly lead to increased clamping voltage, reduced surge withstand, and poor batch consistency. The influence rules of each key process are as follows: First, ZnO ceramic material formulation and doping: The main material of varistors is high-purity ZnO (≥99.9%), doped with minor metal oxides (Bi₂O₃, Sb₂O₃, CoO, MnO) as sintering aids and non-linear modifiers, with a total doping amount controlled at 5%~8% by weight. A doping ratio deviation of ±0.5% will cause the clamping ratio to increase by 0.1~0.2 and the surge withstand current to decrease by 10%~15%. Bi₂O₃ is the core doping element for forming the non-linear grain boundary, and a lack of Bi₂O₃ will lead to a significant increase in clamping voltage and loss of non-linear characteristics. Second, ceramic powder granulation and molding: ZnO ceramic powder is granulated to a particle size of 80~120 mesh, uniform granulation ensures consistent density of the molded chip; the chip is molded under a pressure of 200~300MPa, insufficient pressure leads to low chip density (porosity >5%), which causes local current concentration under surge current and reduces surge withstand capability by 20%~30%; excessive pressure leads to chip cracking during sintering, increasing product rejection rate. Third, high-temperature sintering process: The ZnO ceramic chip is sintered at 1150℃~1250℃ in air for 2~4 hours, the sintering temperature is controlled at ±5℃, a temperature deviation of ±10℃ will cause uneven grain growth (grain size 5~20μm for qualified chips). Coarse grains (>20μm) reduce the number of non-linear grain boundaries, increasing clamping voltage; fine grains (<5μm) increase grain boundary resistance, leading to higher leakage current. The sintering cooling rate is controlled at 5℃/min, rapid cooling causes thermal stress in the chip, leading to microcracks and reduced surge withstand. Fourth, electrode preparation and packaging: The varistor electrode is prepared by silver paste screen printing and sintering (850℃~900℃), the electrode thickness is controlled at 20~30μm, insufficient thickness leads to poor electrode contact and increased contact resistance, which raises clamping voltage under large current; excessive thickness causes electrode peeling during thermal shock. The electrode coverage area is ≥95% of the chip surface, incomplete coverage leads to edge electric field concentration and reduced surge withstand. For power varistors, the electrode is welded with copper leads using high-temperature solder, the solder joint resistance is controlled within 10mΩ, high solder joint resistance causes additional voltage drop and raises the actual clamping voltage of the varistor in the circuit. The chip is encapsulated with epoxy resin or silicone rubber, the encapsulation material must have good thermal conductivity (≥0.8W/(m·K)) and insulation performance, poor thermal conductivity leads to heat accumulation under repetitive surge and accelerated varistor aging.

Current Status of Commercial Application

From the perspective of industrial commercialization, ① **General-purpose ZnO varistors** dominate the varistor market with a share of about 65% due to their mature manufacturing process, low production cost (10mm 470V unit price is about $0.3), and balanced clamping and surge performance. They are widely used in consumer electronics (TV, computer power supplies), household appliances, and low-surge industrial control circuits, with a rated 8/20μs surge current of 5~10kA and a clamping ratio of 2.4~2.8, meeting the basic overvoltage protection requirements of general circuits. ② **High-surge ZnO varistors** account for about 20% of the market share, designed for high-intensity surge protection scenarios such as power grid equipment, outdoor communication base stations, and new energy vehicle high-voltage systems. They have a rated 8/20μs surge current of 20~50kA and a clamping ratio of 2.0~2.4, with a unit price of about $1.2 (4 times that of general-purpose varistors) due to the high-purity ZnO material and optimized sintering process. ③ **Low-clamping voltage ZnO varistors** hold about 12% of the market share, targeting precision electronic circuits (e.g., 5G base station RF modules, medical equipment) with strict overvoltage protection level requirements, with a clamping ratio of 1.8~2.2 and a rated surge current of 3~8kA, a unit price of about $0.8 (2.7 times that of general-purpose varistors), the trade-off between low clamping voltage and surge withstand limits its application to low-surge precision circuits. In addition, ④ **SMD varistors** are in the stage of large-scale application, with a chip size of 0603~1206, suitable for surface mount processes of consumer electronics and small industrial equipment, a unit price of about $0.05~0.2, with a low surge withstand current (≤1kA) and mainly used for low-voltage circuit ESD and small surge protection; ⑤ **SiC varistors** are in the R&D and small-batch production stage, with a higher non-linear coefficient, lower clamping ratio (≤1.8), and stable performance at 200℃ high temperature, suitable for aerospace and ultra-high temperature industrial equipment, but the production cost is 10~15 times that of ZnO varistors, and the mass production yield is less than 60%.

Existing Technical Pain Points

1. Inherent tradeoff between low clamping voltage and high surge withstand capability: Varistors with ultra-low clamping voltage have a lower surge withstand current, while high-surge varistors have a relatively high clamping ratio-this is a core physical bottleneck caused by the ZnO ceramic grain boundary structure. Optimizing material doping can only narrow the trade-off range (e.g., reducing the clamping ratio by 0.1~0.2 while increasing the surge current by 5%~10%), and there is no ZnO varistor that can simultaneously achieve an ultra-low clamping ratio (≤1.8) and an ultra-high surge withstand current (≥30kA) in the industry. 2. High-temperature performance limitation: At temperatures above 150℃, the leakage current of ZnO varistors increases exponentially (10 times higher than at 25℃), and the clamping voltage changes by more than ±10%, which cannot meet the overvoltage protection requirements of ultra-high temperature industrial equipment (e.g., industrial furnaces, aerospace engines). Current high-temperature resistant varistor materials (e.g., SiC, TiO₂) have low non-linear coefficients and high production costs, and their performance is far inferior to ZnO varistors at room temperature. 3. Repetitive surge fatigue problem: ZnO varistors will have cumulative performance degradation under frequent repetitive low-intensity surges (e.g., power grid switch transients), with the clamping voltage increasing and surge withstand capability decreasing over time, and even sudden breakdown after long-term fatigue. The current anti-fatigue technologies (e.g., adding rare earth doping, improving ceramic densification) can only extend the fatigue life by 50%~100%, and cannot fundamentally eliminate cumulative degradation. 4. Difficulty in mass production consistency control: The clamping voltage deviation of the same batch of general-purpose ZnO varistors can reach ±5%, and the surge withstand current deviation is ±10%; the consistency of high-surge varistors is better, but the clamping voltage deviation is still ±3%. The core reasons are the fluctuation of material doping ratio, uneven sintering temperature, and inconsistent chip density. To improve consistency, it is necessary to add high-precision material mixing equipment, automatic sintering temperature control systems, and laser trimming links, which directly reduce production efficiency by 20%~30% and increase production costs by about 25%, making it difficult for small and medium-sized manufacturers to implement. 5. Low-voltage low-clamping ratio bottleneck: For low-voltage circuits (e.g., 12V/24V automotive low-voltage systems), the clamping ratio of ZnO varistors is usually >2.5, and the clamping voltage is too high to effectively protect low-voltage precision components (e.g., MCU, sensor) with a maximum withstand voltage of only 30V. Current low-voltage varistor optimization technologies can only reduce the clamping ratio to about 2.2, and the clamping voltage is still higher than the withstand voltage of some low-voltage components, requiring additional TVS diodes for secondary protection and increasing circuit complexity and cost. 6. Mechanical shock and thermal shock fragility: ZnO ceramic chips are brittle materials, and easy to crack under strong mechanical shock (≥20g acceleration) or rapid thermal shock (ΔT>100℃/s), leading to varistor failure and loss of overvoltage protection function. The current reinforcement technologies (e.g., silicone rubber encapsulation, adding buffer layers) can improve shock resistance, but will increase the volume and cost of the varistor, and reduce the heat dissipation efficiency.