# How to balance the leakage inductance and distributed capacitance of the primary coil of the high-frequency transformer

How to balance the leakage inductance and distributed capacitance of the primary coil of the high-frequency transformer

In the design of High frequency transformers, the leakage inductance and distributed capacitance of the transformer must be minimized because the high-frequency transformer in the switching power supply transmits a high-frequency pulse square wave signal. During transmission transients, leakage inductance and distributed capacitance can cause inrush currents and spikes, as well as top oscillations, resulting in increased losses. Although the addition of clamping and absorbing circuits on the drain of the switching transistor can overcome the spike voltage, excessive spikes can cause an increase in the clamping and absorbing circuit losses, reducing the efficiency of the switching power supply and, in severe cases, the power switching tube. damage. Usually, the leakage inductance of the transformer is controlled to be 1% to 3% of the primary inductance.

Leakage of the primary coil

The leakage inductance of the transformer is caused by the fact that the magnetic flux between the primary and secondary coils is not fully coupled between the layers and between the turns. The following measures can be taken in the winding process of the transformer. Increase the ratio of height to width of the coil size. Minimize the number of turns of the winding, and choose a magnetic material with high saturation magnetic induction and low loss. The primary and secondary windings are wound by a tiered cross-wound method. The insulation thickness between the windings should be reduced as much as possible, but the transformer itself must have sufficient insulation strength. Improve the degree of coupling between the coils. When a toroidal core transformer is used, regardless of the number of turns of the primary and secondary windings, when winding the windings, they are evenly distributed along the circumference of the ring. For the toroidal core transformer under high current operation, multiple windings are used in parallel, and the wire diameter is reduced as much as possible.

The method of reducing the leakage inductance of the high-frequency transformer can sample the sandwich winding method. If it is the primary layered winding, it is reasonable to average the two layers of the primary coil. Why? The usual practice is to wind the primary coil first. The secondary coil is wound again, and finally the remaining coils of the primary are wound up. Of course, if the leakage inductance is the smallest, a three-stage or four-stage winding method can be used. However, if the multi-stage winding method is adopted, the distributed capacitance of the transformer is inevitably increased. If the distributed capacitance is too large, the output efficiency of the transformer is very unfavorable. In addition, self-resonance occurs between the leakage inductance and the distributed capacitance. The larger the distributed capacitance is, the lower the self-resonant frequency is. Therefore, in the case of ensuring the leakage inductance meets the requirements, the two-stage winding method can improve the coupling of the primary and secondary coils. Degree, reduce the primary and secondary leakage inductance of the transformer, and not to produce a relatively large distributed capacitance, to ensure the maximum output power of the transformer! So must be considered comprehensively!

For single output, it is best to choose Np1+Ns+Np2+Nap, ie primary 1+ secondary + primary 2+ feedback; for multi-output, the best choice is Np1+(Ns1+Ns2+....)+Np2+Nap, That is, the primary 1+ all secondary and the + primary 2+ feedback.

The reason for this is that the probability of the circuit in the primary layer is greatly reduced; the coupling characteristics between the primary and secondary layers are maximized, and the magnetic loss is small; the secondary output voltage is stable and reasonable; and the distribution due to the multi-layer is greatly reduced. Capacitance; feedback voltage adjustment is convenient, and parameters can be adjusted according to chip demand; temperature rise is reduced, which is more suitable for current social needs; saving material cost and being more practical.