The varistor is often used as an overvoltage protection device, but its current capacity is large, but its energy capacity is not large. In addition, its impulse current maximum pulse width is much smaller than the actual pulse current width of large and medium power semiconductor systems. Short circuits or explosions often occur.
Varistor characteristic parameters
1 varistor voltage UN (U1mA): A flag voltage that is normally turned on by a voltage at a mA current of 1 mA across a varistor. This voltage is called a varistor voltage UN. The varistor voltage is also commonly referred to by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In test and practical use, the varistor voltage is usually reduced by 10% from the normal value as a criterion for varistor failure.
2 Maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or maximum DC voltage Udc that the varistor can withstand for a long time. Generally Uac ≈ 0.64U1mA, Udc ≈ 0.83U1mA.
3-way flow (maximum inrush current) IP: refers to the maximum inrush current peak of the 8/20μs wave that the varistor can withstand. The meaning of "sustainable" is that the rate of change of the varistor voltage after the impact is not more than 10%. The IP value of the impact is usually given in the current technical specifications.
4Maximum clamp voltage (limit voltage) VC: The maximum clamp voltage value given in the technical specification refers to the voltage appearing on the varistor when the specified 8/20μs wave impulse current IX(A) is applied to the varistor. .
In actual use, the higher the varistor voltage, the larger the applied inrush current, and the higher the limiting voltage (or residual voltage), which can be found on the V-I curve given by the product.
5 Rated energy E: Rated energy refers to the maximum energy that the varistor can withstand the impact current of the specified waveform (the rate of change of the varistor voltage after impact is not more than 10%), which can be expressed by the following formula:
E=K*IP*VC*T
Where: IP, VC see above, T is the pulse width, and K is the constant related to the waveform. For 8/20 μs and 10/100 μs waves, K = 1.4; for a 2 ms square wave, K = 1.
6 rated power (maximum average power) Pm: refers to the maximum average power that the varistor can withstand multiple impacts at room temperature, and the interval between each impact is short, so that there is heat accumulation effect. . Although the varistor can withstand large pulse power, the average power that can be withstood is small.
7 Capacitor C0: refers to the capacitance between the two electrodes of the varistor, ranging from a few pF to several hundred nF. The smaller the volume, the higher the varistor voltage and the smaller the capacitance.
8 Leakage current Il: Current flowing when a maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, the voltage of Udc=0.83U1mA is usually applied to the varistor (sometimes also 0.75U1mA). Static leakage current is generally required to be ≤ 20μA (also required to be ≤ 10μA). In actual use, it is not the magnitude of the static leakage current value itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. The rate of change is not more than double after the impact test or under high temperature conditions, which is considered to be stable.
9 nonlinear index α: refers to the ability of the voltage to change the current, which can be expressed as:
I=KUα or α=loglog
It can be seen from the former formula that the larger α indicates that the influence of the voltage change on the current is larger, and the nonlinear characteristic is better. It can be seen from the following formula that α is the reciprocal of the slope of each point on the volt-ampere characteristic, and the flatter the characteristic, the larger α (the leakage current region and the saturation region α=1, also called the low α region). When measuring with an instrument, generally set I2 = 1 mA, I1 = 0.1 mA, so
αT=1/log (U1mA/U0.1mA)
Reasons for varistor burnout:
1. Aging failure, manifested as an increase in leakage current, and the varistor voltage drops significantly until it reaches zero;
Aging failure means that the low-resistance linearization of the resistor gradually increases, the leakage current increases malignantly and concentrates into the weak point, and the weak point material melts, forming a short-circuit hole of about 1kΩ, and the power supply continues to push a large current into the short-circuit point. Forming high heat and catching fire. This type of accident can usually be avoided by a hot-melt joint in series with the varistor. Transient overvoltage destruction refers to a strong transient overvoltage that causes the resistor body to be perforated, resulting in a larger current and a high heat fire. The entire process takes place in a short period of time, so that the heat fusion joints provided on the resistor body are not as fast as the fuse.
2. Transient overvoltage damage.
3. The number of overvoltage protections;
4, the surrounding working temperature;
5. Whether the varistor is squeezed;
6. Whether to pass the quality certification;
7. The surge energy is too large and exceeds the absorbed power;
8, the pressure is not enough;
9, current and surge are too large and so on.