I. Why is potting compound used in switching power supplies , and what are the requirements for the compound?
This sounds simple; it could be that a customer requests potting, or they see someone else doing it and are looking for potting solution themselves. To be clear, there are several main objectives and requirements for potting:
1. Waterproofing: The potting compound tightly encapsulates the entire electrical component of the product, completely isolating it from the outside environment. Water, moisture, acids, alkalis, and salts cannot reach the circuit components, thus improving waterproofing, moisture resistance, and corrosion resistance. Therefore, the adhesive you choose must have good flowability; otherwise, how can it deeply penetrate and completely seal the sealant? Furthermore, it must have good adhesion; otherwise, long-term temperature fluctuations after potting will cause it to separate from the components, rendering it useless and affecting the product's lifespan and performance.
2. Thermal conductivity: Power supplies are power-related products, meaning they convert, control, and output energy. These products generate heat during operation; components like switching transistors, transformers, electrolytic capacitors, and inductors are all heat-generating and heat-sensitive. Therefore, the heat generated must be conducted away quickly and as much as possible. Thus, the adhesive must have excellent thermal conductivity; ideally, the higher the thermal conductivity, the better. For example, the lifespan of electrolytic capacitors in a power supply is reduced by about half for every ten-degree Celsius increase in temperature, but you can't add heat sinks to them. Similarly, when transformers and inductors reach high temperatures, many parameters change, making the entire design more difficult, and adding heat sinks is also impractical.
Waterproof power supplies need to be completely sealed, but air has very low thermal conductivity, so they rely primarily on convection and radiation. However, convection is impossible, as the components are all enclosed in a box.
Therefore, applying a thermally conductive adhesive is equivalent to placing each component on the outside for heat dissipation, which not only improves its lifespan but also simplifies the design.
3. Insulation strength: This is relatively easy to determine. Basically, all adhesives are non-conductive. However, be aware that epoxy-based AB adhesives may have some conductivity if the curing reaction is incomplete. Also, adding sand to save money is problematic. First, check if your sand has undergone dehydration or other processing, and whether any other elements in the sand interfere with curing; ensure the AB agent ratio is appropriate, the mixing is sufficient and uniform, and that all other conditions are met.
4. Environmental protection requirements: Generally, there is certification. It depends on whether the supplied products meet the certification requirements. There is a significant price difference between environmentally friendly and non-environmentally friendly materials.
5. Flame retardancy requirements are generally determined by UL's flame retardancy test. A flame retardancy rating of 94-V0 or higher is typically required.
6. There are many other parameters; you'll have to experiment with them yourself. The most important thing is that your supplier is consistent, their supply is stable, and their process is consistent. Otherwise, the variation will be too great.
II. Some observations on several types of adhesives:
1. Epoxy Resin: I believe it is unsuitable for potting power supplies, primarily due to its high shrinkage stress and poor thermal conductivity. It may damage components, pads, or alter the magnetic circuit air gap. Furthermore, if curing is incomplete, it may become partially conductive. The poor thermal conductivity is like covering components with a blanket, trapping them inside and baking them. Practice has shown that potting results in a high failure rate, and the larger the power supply, the more likely it is to fail.
2. Silicone silicone is widely used. It has good thermal conductivity and water resistance, and as a flexible potting compound, it won't damage components. However, it's very expensive, and because of this high price, it's crucial that the supplied products meet testing and certification standards.
3. Polyurethane: I'm not familiar with this and dare not comment. But I've heard it's toxic, which needs further verification.
4. Hot melt black adhesive: This type of adhesive is based on petrochemicals. It is solid at room temperature, and potting is done by heating it to 130-160°C to liquefy it before direct potting. It is a single-component flexible adhesive.
3. Why does the EMI of a switching power supply deteriorate after potting?
The degradation after potting is primarily due to the fact that the dielectric constant of the adhesive is several times greater than that of air, which increases the coupling strength between components. This causes a significant amount of low-frequency switching noise to be coupled to the outside world through the increased capacitance.
Deteriorating EMI after potting may be a long-standing problem for some engineers. Here are some experiences to share:
The deterioration of EMI after potting can be further divided into: conduction (CS) deterioration and radiation (RS) deterioration;
1. Poor conduction, the rectification methods are: (1) Immerse the EMI filter inductor in oil and wrap it with tape (to reduce the impact of potting); (2) Use a slotted frame for the common mode inductor, such as the UU and OTC type frame; (3) Add a Y capacitor to the input to ground or externally.
2. If the radiation deteriorates, the corrective measures are as follows: (1) Place a magnetic ring on the input or output line; (2) Place a magnetic ring on the ground line; (3) Adjust the material of the adhesive.
Special attention should be paid to ensuring that the radiator is grounded, and the grounding process must be good to avoid poor grounding.
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