Hipot Tester



6. According to the definition of insulation breakdown, how do we avoid misjudgment?


Most of the standards only specify the test voltage in the acceptance standard of the withstanding voltage test, and the voltage and test time cannot cause insulation breakdown. Only a few production test standards define how much the leakage current cannot exceed certain current. The mnemonic phrase for insulation and insulation breakdown is: the insulator does not allow current to flow. When the overvoltage on the insulator becomes larger and larger, the uncontrollable sudden rise of the current is called insulation breakdown. The V-I characteristic curve of this description is shown as below, Figure 5.


Figure 5 - V-I characteristic curve of insulation breakdown


The possible misjudgment effect of the conventional use of absolute leakage current: How to determine the leakage current acceptance standard of the production line? Some companies use the average method, through the test of the sample, calculate the average value and add 25% as the upper limit of acceptance, or 30mA leakage current value that will cause harm to the human body is used as the upper limit of control (the leakage circuit breaker on the power distribution generally also uses 30mA as the operating current). Please see Table 2.


If your DUT is valuable, using the absolute value leakage current method may cause false failures. Since the definition of insulation breakdown is determined by the uncontrollable steep rising current, the V-I characteristic curves of the test results are very important to material verification during R&D stage and production line testing of expensive products. At present, the GPT-9900 series and GPT-12000/15000 series electrical safety analyzers of GW Instek all have the V-I characteristic curve scanning function, which will not cause losses from misjudgment. The actual measurement results are shown in Figure 7.



Figure 6 - Impact of false failures


Figure 7 - V-I insulation breakdown case of GW Instek GPT-10000 series


7. Does the appearance of ARC and Flashover mean that the insulation has broken down?


UL/IEC 60950-1 chapter 5.2.2 also has a paragraph about ARC and Flashover in the original text for insulation breakdown. The original text is as follows: Corona discharge or a single momentary flashover is not regarded as insulation breakdown. Corona discharge and single instant flashover are not considered insulation breakdown.


You must have the correct concept. The ARC test is actually an industry standard, not an international regulation. ARC detection and insulation breakdown are two different things.


In the past, when the withstanding voltage tester could not display the V-I characteristic curve to determine the breakdown of the insulation, the conventional method was to watch and listen to see if there was an arc. It was believed that the insulation damage and the arc were cause-and-effect relationship, because the lighting and illumination in the room would affect the arc observation. A dark room was built to observe the arc. Or listening to determine if there is any squeaking sound of the arc. Some of the sound of the arc occurs in the ultrasonic frequency band that the human ear cannot hear. Purchase an ultrasonic converter to convert it into audible audio. However, from the standard definition, we know that arc and insulation breakdown are two different things. We should think about why an arc occurs and the possible harm caused by the arc. The generation of arc is the relationship between voltage and distance.


Taking the application of daily life as an example, the piezoelectric material on the lighter generates a voltage through the pressure to generate an arc to ignite the gas. From this example, we can see that the possible harm caused by the arc is fire or dust explosion if there is flammable gas or dust near the electrical facilities. The second hazard is high temperature. For example, industrial electric welding machines deliberately generate arcs through high voltage in order to use its high temperature characteristics to cut hard metals. The maximum temperature of the arc in this application is four times the temperature of the surface of the sun (Celsius 20,000 degrees). The arc energy unit of arc flash is generally expressed in cal/cm2 or J/cm2. Cal/cm2 (calories per square centimeter) refers to the total energy on a unit area, which is used as the unit of arc magnitude. 1 cal/cm2 of energy is equivalent to a lit cigarette butt on the fingertip for 1 second. It only takes 1-2 cal/cm2 to cause second-degree burns to human skin.


8. In the test sequence of insulation resistance and withstanding voltage, can the withstanding voltage be excluded after passing the insulation resistance test?


Many standards require the test sequence to be insulation resistance (IR) + withstanding voltage (ACW or DCW) + insulation resistance (IR). The first thing to consider is why the insulation resistance should be measured twice? Furthermore, if the insulation resistance is high, does it mean that there is no problem with the insulation, so there is no need to measure the withstanding voltage? Insulation resistance is measured by normal working voltage (non-destructive test), and withstanding voltage test is to measure leakage current (destructive test) by several times higher than the working voltage. Next, don't be too thrilled when you measure a very high insulation resistance!


Figure 8 - Insulation defect and its equivalent circuit


The left diagram (a) of Figure 8 is a schematic diagram of an air gap caused by a defect in the insulation material. This air gap will reflect a large insulation resistance when measuring the insulation resistance, but when we measure the insulation resistance, we perform a withstand voltage test. , from the equivalent circuit of the right figure (b) of Figure 8, it is known that the air gap of the insulation defect is due to the high resistance, and most of the voltage will be divided on it as a result of the series voltage division, so it will be destroyed first, and wait until the first When the insulation resistance is measured twice, it will show a lower insulation resistance.


Therefore, the order of the test is meaningful starting from insulation resistance (IR) + withstanding voltage (ACW or DCW) + insulation resistance (IR). If the difference between the two insulation resistances is too large, it means that there is a prior insulation defect, and the defective insulation material is destroyed after the destructive test of the withstanding voltage, so the second insulation resistance is very different from the first measurement.


9. Will the high voltage withstanding voltage test damage the expensive chips in the circuit?


If your withstanding voltage test passes, there must be no damage. Because damage to the chip is either overvoltage or overcurrent, neither of which will happen when the insulation is good. However, in the application of withstanding voltage test, there is a very special test called ion migration. The test method of ion migration is to set the relative humidity of the environment to 85% through the environment test equipment, and the temperature is usually 80° C or above, and then apply the high voltage on the copper foil of the PCB. Figure 9 Under the electron microscope, the short-circuit phenomenon caused by the dendrite in the insulating layer between the two traces is called ion migration.


Figure 9 - Ion migration phenomenon under electron microscope


Carrying out this test around key components of the PCB helps to detect layout issues early. Once the problem in Figure 9 occurs around the key components, it may cause a short circuit and burn the key components.



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