First, why is the glue? What is the requirement for glue?
This is simple to say. It may be that the customer asks for glue. It may be that someone else is looking for glue in the glue. The potting glue has the following major purposes and requirements:
1. Waterproof. The potting compound tightly wraps the electrical part of the entire product and is completely isolated from the outside world. Water, moisture, and acid-base salts do not meet circuit components, thereby improving water, moisture, and corrosion resistance. Therefore, the selected glue should have good fluidity. If the fluidity is not good, it can not be deeply penetrated and completely sealed. Then it is necessary to have the adhesive force. Otherwise, the long-term cold and hot alternately after the potting is separated from the components is not useful, and the product life performance is also affected. influences.
2. Thermal conductivity. Power is a power product that converts, controls, and outputs energy outward. Product work will be hot, such as switching tubes, transformers, electrolytic capacitors and inductor are heating devices and are afraid of thermal devices. How to do? Be sure to send it out as quickly and as much as possible. Therefore, the glue should have good thermal conductivity. It can be said that the higher the thermal conductivity of the glue, the better. The electrolytic capacitor in the power supply is about half the life of the temperature rise, but you can't add heat sink to it; the transformer and inductor temperature are high, many parameters are changing, the whole design is difficult to improve, you can't add it. Heat sink.
The waterproof power supply needs to be sealed as a whole, and the thermal conductivity of the air is very low, mainly by convection and radiation. But convection is impossible, and the components are stuffy in the box.
Therefore, the glue with good thermal conductivity can be equivalent to heat-dissipating each component to the outside, which not only improves the life but also simplifies the design.
3. Insulation strength. Basically all the glues are not electrically conductive, but it should be noted that the curing reaction of the AB glue like epoxy may not be completely conductive. Also, in order to save sand in it, it is hard to say. First, is your sand processed by dehydration treatment; whether sand will interfere with curing by other elements; whether the ratio of AB agent is reasonable, whether the mixture is sufficiently uniform, and whether other conditions are available.
4. Environmental protection requirements. Generally, there are certifications. It depends on whether the products provided are consistent with the certification. The price difference between environmentally friendly and non-environmental materials is still quite a lot.
5. Flame retardant requirements. Generally look at the flame retardant test in UL, generally requires 94-V0 or more.
6. Other parameters are more, you have to fill it yourself. The main thing is that your supplier should be constant, his supply should be stable, and the process should be stable. Otherwise the dispersion is too large.
Second, some thought of glue.
1. Epoxy resin: I think it is not suitable for potting the power supply, mainly because of large shrinkage stress and poor thermal conductivity. When the epoxy shrinks, it may scratch the component, the pad, or change the magnetic path air gap. If the curing is not complete, it may be locally conductive. Its thermal conductivity is as good as covering the component with a quilt and stuffing it inside. Practice has proved that the defect rate after filling is high, and the more the power supply is broken.
2. Organic silica: a lot of use, thermal conductivity and water resistance are good, is a flexible potting glue will not pull the component. It is only very expensive, and it should be noted that the products supplied by many unscrupulous merchants in the market may not be consistent with the testing and certification.
3. Polyurethane: I heard that it is toxic, pending research.
4. Hot melt vinyl: This category of rubber is a petrochemical substrate. Solid at room temperature, potting is heated to 130-160, directly sealed after liquefaction, is a one-component flexible adhesive
Third, why does EMI deteriorate after switching power supply?
After the glue is poor, the dielectric constant of the glue is several times larger than that of the air, which increases the coupling strength between the components, which makes a large amount of low-frequency switching noise more coupled to the outside by this increased capacitance.
The EMI deterioration after filling can be a problem that has plagued some engineers for a long time. Some experiences are shared for everyone:
The EMI degradation after filling is divided into: conduction (CS) variation and radiation (RS) variation.
1. The conduction is worse, the methods of rectification are:
(1) EMI filter inductor immersion oil, wrapping tape (to reduce the impact of glue filling);
(2) Slotted skeletons for common mode inductor, such as UU and OTC type skeletons;
(3) Input ground or surgical plus Y capacitor.
2. The radiation is worse, the methods of rectification are:
(1) Input or output line magnetic ring;
(2) A magnetic ring on the ground;
(3) The material of the glue.
Special attention: The heat sink must be grounded, and the grounding process should be good and the grounding should not be bad.
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