Y-capacitor used in transformers

In the process of rectification of switching power supply EMI, I believe that many engineers are very familiar with the use of X capacitors, Y capacitors, common mode inductors and related properties. Here, I also use the Y capacitor for the initial and secondary use of the transformer.

During the test, I usually carefully observe the adjustment of the Y capacitor for each customer during the conduction test. Among them, the Y capacitor is increased, the Y capacitor is reduced, and the Y capacitor is placed on both feet. There is also a conduction rectification in which the Y capacitor is connected in series with a 6uH feedthrough inductor.

Why is it that the different adjustments to the Y capacitor will or will be able to solve the problem of conduction? There are often many customers who blindly increase, increase, and increase the Y capacitor until the safety test fails. At this time, the EMI problem is still not solved. At this time, I let the customer reduce or decrease the Y capacitance. Small, and then reduced, until the EMI problem is solved, at this time some engineers are confused! Why does this happen?

I briefly quote here that the presence of distributed capacitance between the coil winding layers and layers of the transformer results in a voltage that is bound to be stored during operation, which creates a source of disturbance. If you do not deal with this source of interference, it will be very harsh for EMI testing. Then at this time, after connecting a Y capacitor between the initial and secondary ground of the transformer, it is bound to provide a bleed path for the disturbing voltage source of the distributed capacitance storage of the transformer, which will reduce its connection to the AC terminal and outward. The source of the disturbance.

The addition of this Y capacitor is extremely counter-productive. It also provides another way for the secondary Schottky and the secondary coil to disturb the invisible source, and will inevitably lead to poor EMI test results. Therefore, this creates the different EMI effects caused by the size of the Y capacitor and related usage.

Therefore, no matter which EMI component, including the Y capacitor, is not necessarily used, it will work well. We must analyze the device characteristics, as well as the working principle of the circuit, disturb the modeling analysis of the source, and analyze the path of the disturbance, so that the EMC problem can be solved faster and better, and the certificate can be obtained more quickly.