High-frequency transformers are one of the key electronic components for electronic products. If an abnormality occurs during use, the electronic products will explode, and in severe cases, it will threaten human life. According to the test specifications of high-frequency transformers, withstand voltage is a very critical test item.
When the transformer factory encounters poor withstand voltage, it is generally mainly a problem of safety distance.
It is generally closely related to factors such as the width of the retaining wall, the number and thickness of the tape, the insulation degree of the varnish, the insertion depth of the PIN pin, and the position of the wire joint during the production of the skeleton.
However, to solve the problem of poor withstand voltage, we cannot simply ask the skeleton manufacturer to improve, but consider all materials and processes related to the insulation system.
Today we will explain in detail the reasons for high voltage poor caused by the skeleton.
01
The safety thickness of the skeleton does not meet the requirements. For example: the thinnest thickness of the UL test PM-9630 is 0.39mm. If your wall thickness is lower than this thickness, it is reasonable to have poor withstand voltage. If the mold is OK during mass production and NG during the process, it may be caused by uneven thickness due to mold eccentricity or misalignment.
02
Poor debugging during molding causes poor pressure resistance and (temperature resistance). Usually these two problems occur at the same time, mainly due to improper molding parameter debugging.
If the temperature of the bakelite mold is too low (too high) or uneven, it may cause the bakelite to fail to fully react chemically, the molecular chain is not complete, resulting in poor pressure resistance and temperature resistance. When the injection pressure and injection speed are too low, it may cause the product to be insufficiently dense, resulting in poor pressure resistance and temperature resistance.
03
During the pin insertion process, if the pin insertion mold design is not scientific enough and the workmanship is not good, the die head is very likely to cause “internal injuries” to the product when it moves upward. The product is seriously cracked, and the quality control will generally see it and judge it as NG, but slight cracks cannot be seen by the naked eye, even a magnifying glass cannot see it.
And after the skeleton is inserted, the OA random inspection cannot be measured by a high-voltage tester. It is necessary to wait for the transformer manufacturer to wind and tighten the wire before the cracks are pulled open to generate arcs. (This requires high pin debugging technology and high requirements for pin mold design and manufacturing).
04
Poor mold design and workmanship lead to poor HIPOT. This accounts for a large proportion of this defect. The mold joint line is too thick, the step difference is large, and the eccentricity may lead to poor pressure resistance.
If the mold flow uniformity is not considered during the design or workmanship of some products, the unbalanced glue feeding will cause the density of some areas (especially the tail of the product) to be too loose, resulting in poor pressure resistance.
Some molds, especially the VED joint, have a large step difference. When the transformer manufacturer winds the wire, there are gaps in the rubber coating, which often cause breakdown. I have handled such customer complaints many times. In addition, the depth of the outlet groove is designed too deep, resulting in gaps after the rubber coating, which often cause breakdown.
05
Wear of the molding machine, insufficient internal energy, and wear of the screw may also lead to poor pressure resistance.
Everyone knows that if the alloy layer on the screw falls off and is injected into the cavity with the raw material to make a product, then this product is naturally conductive. Of course, if there are metal impurities in the raw material, it will also cause poor pressure resistance.
06
The proportion of inferior materials added to plastic materials is too high, the raw materials are not dried enough, there are too many additives, and too much color powder containing heavy metals is added, which may lead to poor withstand voltage.
07
The most important thing in pin debugging: almost inserting through. This often happens. The insertion depth is too deep when inserting the pin, and the PIN hole is too deep, which may cause poor withstand voltage.
08
When punching the burrs, the projection pressure is too high, and the beads are not cleaned and there are too many CP lines, which may also cause slight cracks in the product and lead to poor withstand voltage.
There are often various problems in the manufacturing process, and specific problems must be analyzed specifically. Some HIPOT defects are often caused by a combination of several reasons.
A comprehensive analysis is required to solve the problem, which requires us not only to be proficient in the manufacturing technology of this profession, the characteristics of the raw materials, the structure of the mold, and the performance of the machine, but also to understand the manufacturing process of the transformer manufacturer, the characteristics of varnish, the way of encapsulation, etc., in order to solve the problem more effectively.
Post time: Aug-16-2024