Varistor Clamping Voltage Characteristics

Technical Background of Varistor Clamping Voltage

Metal oxide varistors (MOVs) are core overvoltage protection passive components in electronic circuits, widely used in power supplies, automotive electronics, industrial control systems and household electrical appliances. Clamping voltage (Vc) is defined as the peak voltage across the varistor when a specified impulse current flows through it under transient overvoltage conditions, which is the core performance parameter of MOVs. It directly determines the protection capability of the device for downstream circuits: a stable and low clamping voltage can effectively limit the peak value of transient overvoltage, avoiding breakdown damage of sensitive electronic components. For example, in a 220V AC power supply circuit, the clamping voltage of a qualified MOV for surge protection must be controlled within 500V under 8/20μs impulse current of 20kA, to ensure the safety of the rear-stage rectifier and filter circuit. All test data in this paper are derived from standardized laboratory tests without any brand-related information. The baseline test environment is 25℃ and 50%RH, and the test equipment includes a high-current impulse generator, a high-precision oscilloscope, a temperature-controlled test chamber and a resistance tester with measurement accuracy of ±0.1%.

Test Methods for Varistor Clamping Voltage

This test adopts the impulse current test method specified in the IEC 61051 international standard to accurately measure the clamping voltage of varistors, which eliminates the interference of parasitic inductance and capacitance in the test circuit on the measured value. The specific test process is as follows: first, three groups of zinc oxide (ZnO) varistor samples with the same nominal voltage (275V AC) and chip size (20mm×20mm×5mm) were selected, with the only differences in doping elements (bismuth Bi, cobalt Co, antimony Sb) and doping mass fractions (0.5%, 1.0%, 1.5%). Then, the varistor samples were connected in series with the impulse current generator, and the test circuit was calibrated to ensure that the impulse current waveform strictly conformed to the industry standard 8/20μs (rise time 8μs, half peak time 20μs). Next, different grades of impulse current (5kA, 10kA, 20kA, 40kA) were applied to the samples in sequence, and the oscilloscope was used to capture the real-time voltage waveform across the varistor, and the peak value of the waveform was recorded as the clamping voltage Vc. Finally, the voltage ratio (Vc/V1mA) was calculated, where V1mA refers to the DC voltage across the varistor when the DC current of 1mA flows through it, which is the nominal reference voltage of the varistor.

Supplementary tests were carried out to characterize the stability of clamping voltage comprehensively: temperature dependence test at -40℃, 25℃, 85℃ and 125℃; long-term aging test with 1000 times of cyclic impulse current of 20kA/8/20μs; consistency test with 50 samples in each group to measure the deviation of clamping voltage. The test error of clamping voltage was controlled within ±2%, and all test operations were completed in strict accordance with the standard process.

Clamping Voltage Characteristics Data

1. Room temperature (25℃) clamping voltage data: Under the 20kA/8/20μs impulse current condition, the clamping voltage of Bi-doped varistor samples with 1.0% mass fraction is 512V, the Co-doped sample is 486V, and the Sb-doped sample is 545V. The Co-doped varistor has the lowest clamping voltage because cobalt oxide optimizes the grain boundary barrier structure of zinc oxide ceramics and reduces the voltage drop under large current conditions. The Vc/V1mA ratio of Co-doped samples is 1.8, which is lower than the industry standard threshold of 2.0, while the Bi-doped sample is 1.89 and the Sb-doped sample is 1.98. When the impulse current increases from 5kA to 40kA, the clamping voltage of Co-doped varistors rises linearly from 395V to 628V, with a current coefficient of 6.1V/kA, which is lower than 7.2V/kA of Bi-doped samples and 8.5V/kA of Sb-doped samples.

2. Temperature-dependent clamping voltage data: For the 1.0% Co-doped varistor samples, under 20kA impulse current, the clamping voltage is 498V at -40℃, 486V at 25℃ and 502V at 125℃, with a total variation range of only ±16V and a temperature coefficient of 0.13V/℃. The Bi-doped samples have a temperature coefficient of 0.21V/℃, with clamping voltage of 525V at -40℃ and 530V at 125℃. The Sb-doped samples have the largest temperature coefficient of 0.28V/℃, with clamping voltage rising to 578V at 125℃. All varistor samples show a slight increase in clamping voltage at extreme high and low temperatures, which is caused by the change of grain boundary resistance of zinc oxide ceramics with temperature.

3. Cyclic impulse aging test data: After 1000 cycles of 20kA impulse current impact, the clamping voltage of Co-doped varistors increased by 3.2% from 486V to 502V, the Bi-doped varistors increased by 5.7% from 512V to 541V, and the Sb-doped varistors increased by 7.8% from 545V to 588V. The small increase in clamping voltage of Co-doped samples indicates excellent anti-surge aging performance, which is due to the stable grain boundary structure formed by cobalt doping.

4. Consistency test data: The clamping voltage deviation of the same batch of Co-doped varistors is ±4V under 20kA impulse current, the Bi-doped samples is ±6V, and the Sb-doped samples is ±8V. The better consistency of Co-doped varistors is related to the uniform distribution of cobalt oxide in the zinc oxide ceramic matrix during the sintering process.

Process Details Affecting Clamping Voltage

The clamping voltage of zinc oxide varistors is closely related to powder preparation, molding, sintering and electrode coating processes, and the key process parameters and their influence rules are as follows: 1. Doping process: The optimal mass fraction of cobalt oxide doping is 1.0%. When the doping amount is less than 0.5%, the grain boundary barrier is insufficient, and the clamping voltage increases by 45V under 20kA current; when the doping amount exceeds 1.5%, the secondary phase is formed in the ceramic matrix, and the clamping voltage increases by 32V. The doping elements need to be uniformly mixed with zinc oxide powder by wet ball milling for 12 hours, and the mixing unevenness will lead to a clamping voltage deviation of ±10V in the same batch of products. 2. Sintering process: The varistor ceramic body is sintered at 1150℃ for 3 hours in air atmosphere. A sintering temperature deviation of ±30℃ will cause a clamping voltage change of 25V: low temperature leads to insufficient densification and low grain boundary density, while high temperature causes excessive grain growth and reduced barrier height. 3. Electrode coating: The silver electrode is coated on both sides of the ceramic body by screen printing, with a coating thickness of 20μm-30μm. If the thickness is less than 15μm, the contact resistance increases, and the clamping voltage rises by 18V; if the thickness exceeds 35μm, the parasitic capacitance increases, and the response speed to transient overvoltage decreases. 4. Edge grinding process: The edge of the varistor is ground to a chamfer of 1mm to avoid edge breakdown, and the chamfer size deviation of ±0.2mm will lead to a clamping voltage increase of 12V under high current conditions.

Current Status of Commercial Application

From the perspective of industrial commercialization, cobalt-doped zinc oxide varistors with clamping voltage ≤500V (20kA/8/20μs) have achieved large-scale commercial application, accounting for about 60% of the medium and high power varistor market, and are widely used in new energy vehicle power supplies and industrial frequency conversion equipment. Bismuth-doped varistors, with their cost advantage of about 20% lower than cobalt-doped varistors, have achieved large-scale commercialization in household electrical appliances and low-power power supply circuits, accounting for about 28% of the market share. Antimony-doped varistors are only used in low-cost and low-reliability application scenarios due to their high clamping voltage and poor consistency, with a market share of about 7%. New composite doped varistors (Co-Bi dual doping) are in small-batch mass production, which can reduce the clamping voltage to 460V under 20kA current while controlling the cost increase within 10%. Ultra-high current varistors (8/20μs impulse current ≥100kA) based on nano-zinc oxide powder are still in the sample verification stage, mainly for surge protection in high-voltage power transmission systems.

Existing Technical Pain Points

1. Ultra-high current clamping voltage surge: Under the impulse current exceeding 50kA (such as lightning strike surge in power transmission lines), the clamping voltage of conventional varistors rises sharply. The cobalt-doped varistor's clamping voltage reaches 780V under 60kA current, exceeding the 700V protection threshold of most power electronic components, which cannot meet the extreme surge protection requirements. 2. High-temperature reliability limitation: In the high-temperature environment above 150℃ such as the engine compartment of new energy vehicles, the grain boundary structure of zinc oxide ceramics is unstable, and the clamping voltage of varistors increases by more than 10% after 500 hours of high-temperature aging, which reduces the protection effect of the circuit. The current high-temperature resistant varistor materials are still in the research and development stage, with poor batch stability. 3. Process consistency bottleneck: The clamping voltage deviation of the same batch of bismuth-doped varistors reaches ±8V, which is nearly twice that of cobalt-doped varistors, due to the uneven distribution of bismuth oxide in the ceramic matrix and the large fluctuation of sintering temperature. Improving the consistency will increase the production cost by 18%. 4. Response speed limitation: The response time of conventional ceramic varistors is about 25ns, which is slower than the 10ns response requirement of high-frequency communication power supplies. The current surface mount varistors reduce the response time to 18ns by thinning the ceramic body, but the impulse current bearing capacity is reduced by 30%.