EMI becomes worse after the switching power supply is filled with glue? Why? How to solve it?

1、 Glue pouring and requirements for glue

This is simple. It may be that the customer asked for glue filling, or that someone else was looking for glue filling. To be sure, there are several main purposes and requirements for potting adhesive:

1. Waterproof, the potting glue tightly wraps the electrical part of the whole product, completely isolating it from the outside world; What water, moisture, acid, alkali and salt can not reach the circuit components, which can improve the ability of waterproof, moisture-proof and anti-corrosion. Therefore, the glue you choose should have good fluidity. How can it penetrate deeply and be completely sealed if it has poor fluidity; Moreover, the adhesive force should also be provided, otherwise, it will be useless to separate the components with cold and hot alternations for a long time after potting, and the product life performance will also be affected.

2. Thermal conductivity. Power supply is a power product, which is used to convert, control and output energy. The product will generate heat when it is working, such as switch tubes, transformers, electrolytic capacitors, inductors, which are not heat generating devices and are all heat fearing devices. What to do? Be sure to transmit the heat as soon as possible. Therefore, the adhesive must have good thermal conductivity. It can be said that the higher the thermal conductivity of the adhesive is expected, the better. The life of the electrolytic capacitor in the power supply is reduced by half when the temperature rises by 10 degrees, but you cannot add a radiator to it; When the temperature of transformer and inductor is high, many parameters are changing, which makes the whole design more difficult. It's not easy for you to add a radiator to it.

The whole waterproof power supply needs to be sealed, and the thermal conductivity of air is very low, mainly relying on convection and radiation. However, convection is impossible, and the components are all enclosed in the box.

Therefore, filling with glue with good thermal conductivity can be equivalent to placing each component outside for heat dissipation, which not only improves the service life but also simplifies the design.

3. The insulation strength is easy to handle. Basically, all adhesives do not conduct electricity. However, it should be noted that the curing reaction of epoxy type AB adhesive is not complete and may be a bit conductive. In addition, it is hard to say that sand is mixed in order to save money. First of all, whether your sand has been dehydrated or processed, or whether there are other elements interfering with the solidification of your sand; Whether the proportion of AB agent is reasonable, whether the mixture is sufficient and uniform, and whether other conditions are met.

4. Environmental requirements are generally certified. It depends on whether the products provided are consistent with those certified. There is still a lot of price difference between environmentally friendly and non environmentally friendly materials.

5. Flame retardant requirements generally refer to the flame retardant test in UL. It is generally required to be above 94-V0.

6. There are many other parameters. You should try them yourself. The main thing is that your suppliers should be constant, their supply should be stable, and their processes should be stable. Otherwise, the dispersion is too large.

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2、 Some Opinions on Several Glues

1. Epoxy resin: I think it is not suitable to encapsulate the power supply, mainly because of large shrinkage stress and poor thermal conductivity. It may strain components, pads or change the magnetic circuit air gap. In addition, if the curing is incomplete, it may conduct electricity locally. Poor thermal conductivity is like covering a quilt on a component and baking it inside. Practice has proved that the bad rate is high after potting, and the bigger the power supply is, the more bad it is.

2. Organic silica gel is widely used. Good thermal conductivity, water resistance, etc. It is a flexible potting adhesive and will not strain the components. It is only very expensive and because of its high price, it is important whether the products supplied are consistent with the inspection and certification.

3. Polyamine, I'm not familiar with this, and I dare not talk about it. But it is said that it is poisonous and needs to be verified.

4. Hot melt black glue, which belongs to petrochemical base material. It is solid at room temperature, and it is directly sealed after being heated to 130-160 for liquefaction. It belongs to one component flexible adhesive

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3、 Why does the EMI become worse after the switching power supply is filled with glue?

The dielectric constant of the glue is several times larger than that of air, which increases the coupling strength between components. This makes a lot of low-frequency switching noise more coupled to the outside world by the increased capacitance.

The deterioration of EMI after glue injection may be a problem that has puzzled some engineers for a long time. Some experience can be shared:

EMI variation after glue injection is divided into:

Conduction (CS) variation and radiation (RS) variation

1. The methods of rectification are as follows:

(1) The EMI filter inductor is immersed in oil and covered with adhesive tape (to reduce the influence of glue pouring on it);

(2) Slotted skeletons for common mode inductors, such as UU and OTC skeletons;

(3) Input Y capacitance to ground or surgery.

2. The radiation becomes worse. The rectification methods include:

(1) Magnetic ring is sleeved on input or output line;

(2) The ground wire is sheathed with a magnetic ring;

(3) Material of glue.

Special attention shall be paid to the radiator grounding. The grounding process shall be good, and no poor grounding is allowed!