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High-frequency transformer sandwich wrapping introduction
Apr 07, 2018


             High-frequency transformer sandwich wrapping introduction

Two basic methods of high-frequency transformers: sequential winding and sandwich winding. Sequentially around the general single-output power supply, the transformer is divided into three windings, the primary winding Np, the secondary winding Ns, and the auxiliary power winding Nb. The winding sequence is: Np--Ns--Nb. The winding process is simple and easy to control the various parameters of the magnetic core, the consistency is good, the winding cost is low, suitable for mass production, but the leakage inductance is slightly larger, the coupling capacitance is small, and the EMI is relatively good. For low power applications where leakage inductance is insensitive, this type of coiling is commonly used in power supplies with power typically less than 30 to 40W.

Sandwich winding is a common base wrapping method in the manufacture of power transformers. Due to the different windings sandwiched in the sandwich, the sandwich is divided into two types of windings, namely the primary clamp secondary and the secondary clamp primary. Let's introduce them separately.

First of all, let's look at the first kind of common high-frequency power transformer winding method, namely the primary clamp secondary winding. This type of winding is also referred to as the primary average winding method in the actual operation in the ordinary times.

When the power transformer is wound using this primary average winding method, the specific winding process is as shown in the above figure, and the operation sequence is Np/2-Ns-Np/2-Nb.

Next, let's take a look at the second kind of sandwich winding of the high-frequency power transformer, and the primary winding of the secondary clamp. This winding method is also sometimes referred to as the secondary average winding method. The winding diagram is shown in Figure 2 below. When using this winding method to wind the power transformer, the winding sequence is Ns/2-. Np-Ns/2-Nb. When the output is low-voltage and high-current, this type of winding is generally used.

1, sandwich winding can reduce the leakage inductance of the transformer, and the benefit of reducing the leakage inductance is that the voltage peak will be reduced, so that the voltage stress of the MOSFET is reduced.

2. Improve EMI: Common-mode interference currents caused by MOSFETs and heat sinks can also be reduced to improve EMI; due to the addition of a secondary winding in the middle of the primary, the inter-layer distributed capacitance of the transformer primary is reduced, and the interlayer capacitance is reduced. The reduction will reduce the parasitic oscillations in the circuit and also reduce the voltage and current stress of the MOSFET and the secondary rectifier to improve EMI.

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