In the design of switching power supply, the design of power transformer is extremely critical. Especially after the working frequency is increased, it is necessary to achieve the highest power density of the power supply, and it is difficult to meet the better EMC index. The following describes a transformer design method that can greatly improve the EMI noise of the switching power supply.
Due to the existence of parasitic capacitance between the primary side and the secondary side winding, when the transformer is "switched", there is dV/dt at the boundary, and the "quiet layer" is arranged on both sides of the insulating tape, that is, the end side of the primary side and the Vin connected. One end of the secondary side and the ground is disposed on both sides of the insulating tape, which can reduce the dV/dt of the parasitic capacitance at the boundary. Since the drain voltage of the FET is fluctuating, it is wound into the first layer of the skeleton so that the outer layer shields the electromagnetic field emitted by the inner layer. Figures 1 and 2 show two low-noise winding techniques that can be used in a typical flyback converter transformer.
figure 1
Method 1: As shown in Figure 1, the end of the secondary winding connected to the diode must be in close proximity to the insulating tape, so both ends of the insulating tape will have a certain size of dV/dt, but the dV/dt is lower than the drain of the primary winding. The contiguous end is much smaller when adjacent to the insulating tape. The advantage of this transformer is that the "quick end" of the secondary winding is located at the outermost layer, which itself shields the transformer from radiation.
figure 2
Method 2: As shown in Figure 2, the two silent ends are adjacent to the insulating tape. The advantage of this transformer is that the common mode noise across the boundary is reduced, but the external noise of the transformer is relatively large, and the copper shield is required to be wound on the outside. (ie Faraday shielding).
image 3
In the case where the transformer skeleton window has sufficient margin, it is usually necessary to use copper skin "isolation" between the primary side and the secondary side inside the transformer to shield the noise generated by the winding. If the window margin is not sufficient, consider the primary side IC supply winding as a Faraday shield. As shown in FIG. 3, both ends of the winding are AC-coupled to the primary side ground, so that the capacitive noise emitted by the primary side main winding is reduced, and thus the common mode noise conducted to the secondary side is greatly reduced. A disadvantage of using Faraday shields is that the leakage inductance is greatly increased, thereby reducing efficiency. Therefore, we do not generally use any conventional shielding inside the flyback converter transformer, but Faraday shielding is often used.
When designing custom products for customers, the customer requires that CLASS B be transmitted without the addition of peripheral auxiliary circuits. According to Method 1, the traditional "sandwich winding method" not only solves the conduction problem, but also solves the problem. Only the method 1 brings about the problem of large leakage inductance of the transformer, thereby improving the stability of the power supply and meeting the requirements of the customer. The winding sequence of the customized product is shown in Table 1. The conduction result is shown in Figure 4. The peak has a margin of at least 20 dB, and the average has a margin of 10 dB.
Table 1
Figure 4
For switching power supplies with transformer topologies, the electromagnetic compatibility (EMC) design of the transformer has a large impact on the EMC level of the entire switching power supply. Normally, the addition of a power line filter is necessary to suppress conducted EMI. However, simply relying on a filter at the input of the power supply to suppress interference tends to result in increased inductance and increased capacitance of the components in the filter. The increase in inductance increases the volume and increases the capacitance by the leakage current safety standard. This paper presents a new transformer design method. It not only reduces the volume of the power line filter, but also has a stronger ability to suppress conducted electromagnetic interference (EMI), and can reduce the manufacturing cost and process complexity of the transformer.
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Installation Steps
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