2. What is the common withstanding test voltage?
There are three common test voltages for the withstanding testing: AC, DC, and Impulse. The AC voltage test is most accepted by standard units, because the AC voltage is the same as our actual electricity environment. Both positive and negative half cycles are tested. For the stray capacitance of the DUT, the AC voltage test does not have the issue of charging and discharging. These are the advantages of AC voltage test. However, AC voltage test also has disadvantages. We will explain the disadvantages in the fourth question with the equivalent circuit of the test. During production line testing, leakage current is usually used as judgment criterion for Pass/Failure. The purpose of using DC voltage test is to solve the problem of larger leakage current measurement error in AC voltage test.
The last test voltage is Impulse. Impulse is mainly focused on testing circuits or components for transient overvoltage limiting devices by simulating the actual appearance on transmission and distribution lines. Transient voltage is divided into pulse and oscillation. Impulse simulates pulse transient voltage such as lightning strike as well as devices used in substation and transmission and distribution systems, such as circuit breakers, isolators, voltage isolators which require to be tested with this applied voltage. In sum, the test voltage is to restore the appearance of the actual use environment or solve the test issues.
3. How long should the test time be set?
The withstanding voltage test is divided into the Type test in the R&D stage and the Routine test in the mass production. Type test is tested in the R&D stage from the selection of insulation materials, the composition of modules to prototype unit. The Type test time is 60 seconds for the prototype. If the material or module is subjected to Margin verification, a destructive experiment will be performed. The test voltage and test time will stop when the material or module appears insulation breakdown.
Routine test takes into account the production capacity and test cost, so the test time is mostly 1~3 seconds. In order to make up for the problem that the test time may not be able to detect defective products, the test voltage is usually increased by 10~20%. For some applications that are not allowed mistakes or manufacturers who have high requirements for their own quality, the 60 seconds of the Type test will still be used as the Routine test time.
Note: The above test time refers to the premise that sufficient test voltage has been established on the DUT.
Establish sufficient test voltage: AC withstanding voltage must consider the time required to establish the set test voltage from zero start, and DC withstanding voltage must consider the charging time of stray capacitance and filter capacitance. The characteristics of capacitors are that the time-varying voltage will generate current. Capacitors do not allow transient voltage changes.
The capacitor does not allow transient voltage changes. From the above formula (2), it can be seen that the voltage change is affected by two variables. The first variable is the charging current (I charge), and the second variable is the capacitance value (C).
Assuming that the charging current in a line is 1uA and the stray capacitance is 0.0025uF, if we want to establish a voltage of 1500V on this capacitance, after applying the formula (2), we calculate that the rising voltage every 1 second is 400V. Therefore, to establish 1500 volts takes 3.75 seconds, which is a physical limitation.
Hence, it is best to use an LCR meter to measure the equivalent capacitance of the DUT before the test, which helps to accurately confirm the required rising time. From this elaboration, a disadvantage of the DC withstanding voltage test can be obtained that is the charging and discharging time of the capacitors will affect the test throughput of the production test.
4. How to choose the output capacity of the withstanding voltage tester?
Now, let's discuss the issue left in question 2. Why is the DC withstanding voltage test necessary? And the reason why the leakage current measurement error is larger during the AC withstanding voltage test.
Figure 4 - the Equivalent circuit of the DUT
IR is the current flowing through the insulation resistance; IC is the current flowing through the equivalent capacitance (including stray capacitance and filter capacitance); IT is the leakage current measured by the withstanding voltage tester, IC=VTest/XC, XC=1/2πfC. During the AC withstanding voltage test, the frequency (f) is 60Hz. When the capacitance (C) is larger, XC will be smaller, IC will be larger, and the error of the leakage current IT will be larger.
What if DC is used instead? The frequency (f) is 0Hz, XC is infinite, and IC is 0, so the measured IT leakage current = IR, which will not cause errors.
It is necessary to establish sufficient test voltage on the insulation resistance, and the current required for testing with a DC waveform is smaller, which is relatively safe compared to AC.
After explaining the accuracy of AC/DC withstanding voltage from the equivalent circuit, let’s get back to the topic. How to choose the output capacity of the withstanding voltage tester? The output capacity of the withstanding voltage tester is in VA, which refers to the product of the maximum rated AC test voltage and the maximum rated current. At present, 100VA/200VA/250VA/500VA withstanding voltage testers are available on the market. Taking AC 5000V as an example, 500VA can provide 100mA, and 200VA can provide 40mA. The output capacity actually depends on how much current is required to maintain the test voltage. We expect to establish enough test voltage on the insulation resistance to be tested to confirm whether the insulation is good. When the insulation breaks down, the insulation resistance will reduce. If the current at this time is insufficient (insufficient capacity), it cannot establish a sufficient voltage in this resistance with a decreasing value.
This phenomenon is similar to the DC power supply switching from CV mode to CC mode. When the insulation resistance is very large (the load current is small), and when the insulation breaks down, the insulation resistance becomes smaller (the load current becomes large), and the capacity is needed to support it. Therefore, the destructive experiment of materials in the R&D unit or the laboratory of the third-party manufacturer will require a capacity of 500VA. The routine test of the production test commonly utilizes 100VA model. The reason is that the yield rate of the product is very high during mass production. Taking the insulation resistance of 100MΩ as an example, to establish a test voltage of 5000V, the load current is only 50μA and to establish a test voltage of 1500V, the load current is only 15μA, which is more than enough for the capacity of 100VA. Of course, if it is AC withstanding voltage, the influence of equivalent capacitance must be considered.
We summarize the advantages and disadvantages of AC withstanding voltage and DC withstanding voltage testing.
5. What is the definition of insulator and insulation breakdown?
In electrical textbooks, resistivity is often used to define conductors (below 10-5Ω˙m), semiconductors (resistivity is between conductors and insulators), and insulators (higher than 108Ω˙m). Under this definition, copper is a conductor, and air is an insulator, but from the phenomenon of lightning/lightning strikes in nature, it can be known that insulation is conditional, as long as the voltage is high enough, the insulator can also become a conductor. From a microscopic point of view, copper is a conductor, but nano-copper is an insulator; carbon is an insulator, but nano-carbon is a conductor. Hence, the physical properties of macro and micro may change. Below is the Wikipedia definition of an insulator: An insulator, also known as a dielectric or electrical insulator, is a material that impedes the flow of electric charge. In an insulator, valence band electrons are tightly bound around its atoms. This material is used as an insulator, or insulation, in electrical equipment. Its role is to support or separate individual electrical conductors so that no current can flow through. This description can be reduced to the best definition that applies everywhere: an insulator does not allow current to flow.
After understanding the definition of insulator, next we will explain the definition of insulation breakdown. The following is the original description of insulation breakdown in UL/IEC 60950-1 chapter 5.2.2: Insulation breakdown is considered to have occurred when the current that flows as a result of the application of the test voltage rapidly increases in an uncontrolled manner that is the insulation does not restrict the flow of the current.
The determination of insulation breakdown is: the current flowing through the insulation to be tested can already generate a corresponding current with the rise of the test voltage (uncontrollable steep rising). That is, the insulation to be tested can no longer effectively limit current growth under the test electric field strength. Gaseous and liquid insulation substances have the reversibility of insulation breakdown. When the high voltage causing the insulation breakdown disappears, they can return to the insulation state, while solid insulation does not have this reversibility. Once the insulation breaks down, it will cause permanent damage to its insulation ability.