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How to Match the Leakage Inductance and Distributed Capacitance of High Frequency Transformer Primary Coil

Feb 20, 2018

How to Match the Leakage Inductance and Distributed Capacitance of High Frequency Transformer Primary Coil

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 surge currents and spike voltages, as well as top oscillations, causing increased losses. Although adding a clamp and an absorption circuit on the drain of the switching transistor can overcome the peak voltage, an excessively large peak leads to an increase in the losses of the clamping and the absorption circuit, resulting in a decrease in the efficiency of the switching power supply and in severe cases causing the power switch tube. damage. The leakage inductance of the transformer is usually controlled to 1% to 3% of the primary inductance.

Primary coil leakage inductance

The leakage inductance of the transformer is caused by the fact that the magnetic flux is not completely coupled between the primary coil and the secondary coil and between the layers. The following measures can be taken in the transformer winding process. Increase the ratio of the height and width of the coil size. Minimize the number of turns of the winding and use magnetic materials with high saturation magnetic induction and low loss. The primary and secondary windings are wound in a layered cross winding manner. Reduce the insulation thickness between the windings as much as possible, but you must ensure that the transformer itself has sufficient dielectric 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, the windings are uniformly distributed along the circumferential circumference. For the ring core transformer under large current working condition, multi-winding is used for winding in parallel, and the wire diameter is reduced as much as possible.

If you want to reduce the leakage inductance of high-frequency transformers, you can sample the sandwich winding method. If it is a primary layered winding, it is more reasonable to split the primary coil in two layers. Why? The usual practice is to wind the primary coil first. The secondary coil is wound again and finally the primary remaining coil is wound. Of course, if you want to minimize the leakage inductance, you can use a three- or four-stage winding method, but if you use a multi-stage winding method will inevitably increase the distributed capacitance of the transformer, if the distributed capacitance is too large, it is very unfavorable to the output efficiency of the transformer. In addition, self-resonance will be generated between leakage inductance and distributed capacitance. The higher the distributed capacitance, the lower the self-resonant frequency! Therefore, the two-stage winding method can ensure the coupling of the primary and secondary coils while ensuring that the leakage inductance meets the requirements. Degree, reducing the primary and secondary leakage inductance of the transformer, but not to produce a relatively large distributed capacitance, to ensure the maximum output power of the transformer! Therefore must be considered comprehensively!

For single-output, it is better to choose Np1+Ns+Np2+Nap, ie, primary 1+secondary+primary 2+ feedback; the best choice for multiple outputs is Np1+(Ns1+Ns2+....)+Np2+Nap, That is primary 1 + all secondary + primary + secondary 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 are maximized, and the magnetic loss is small; the secondary output voltage is stable and reasonable; the distribution due to the primary multilayer is greatly reduced Capacitance; feedback voltage adjustment is convenient, and can adjust the parameters according to the needs of the chip; temperature rise is reduced, more suitable for current social needs; save material cost, more practical.

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