Industry experts explain the winding of transformers in inverters

This article will introduce the winding method of high-frequency transformer in high-frequency inverter .

If you use high-frequency magnetic cores such as EE55 to make high-frequency inverters, it is best to refer to the winding method of the audio output transformer in the power amplifier of the electronic tube audio system for the wire wrapping of the high-frequency transformer. This kind of transformer strives to achieve a flat response in the audio frequency range of 20Hz~20KHz, and the winding method is very particular. The frequency response range of the top electronic tube audio output transformer is even 10Hz~100KHz, and the magnetic core used is just a high-silicon silicon steel sheet.

Taking the most commonly used combination of "SG3525A (or KA3525A, UC3525) + field tube IRF3205 (or MTP75N06, etc.) + EE55 core transformer" as an example, the power can reach more than 500W, and the operating frequency is generally 20~50KHz. Among them, the EE55 core transformer is generally a low-voltage winding (primary) 3T+3T, center tap, and a high-voltage winding (secondary) 75T.

To make it well, you need to pay attention to two points:

1. Each winding should be wound with multiple strands of thin copper wires, not a single thick copper wire, because high-frequency alternating current has a skin effect. The so-called skin effect, in simple terms, means that high-frequency alternating current only flows along the surface of the wire, and no current flows inside the wire (in fact, the closer to the center axis of the wire, the weaker the current, and the closer to the surface of the wire, the stronger the current). Using multiple strands of thin copper wires together to wind is actually to increase the surface area of ​​the wire, so as to use the wire more efficiently. For example, for the primary 3T+3T, if you use a single enameled wire with a diameter of 2.50mm, the cross-sectional area of ​​the wire is 4.9 square millimeters, and if you use 38 enameled wires with a diameter of 0.41mm (a single cross-sectional area of ​​0.132 square millimeters) and wind them together, the total cross-sectional area also meets the requirements. However, the surface area of ​​the wire in the second method is much larger (the surface area of ​​the wire in the first method is: the circumference of the cross-section of a single-strand wire × the number of strands × the total length of the winding = 2.5 × 3.14 × 1 × L = 7.85L, and the surface area of ​​the wire in the second method is: the circumference of the cross-section of a single-strand wire × the number of strands × the total length of the winding = 0.41 × 3.14 × 38 × L = 48.92L, the latter is 48.92L/7.85L = 6.2 times the former), the wire is more effectively used, the current is smoother, and because the thin wire is softer, it is easier to wind. The secondary 75T high-voltage winding can be wound with 3 to 5 strands.

Second, it is best to use a layered and segmented winding method. The main purpose of this winding method is to reduce high-frequency leakage inductance and distributed capacitance. For example, the winding method of the above transformer is divided into two layers in the primary and three layers and three sections in the secondary. Specifically: ① Wind the first section of the secondary high-voltage winding. Connect the lead-out wire (head), first use 5 wires to wind the secondary high-voltage winding 25T in parallel, do not cut the wire, and then wrap a layer of insulating paper (the insulating paper should be thin, just one layer, otherwise, because the insulating paper will be used many times below, it may not be able to accommodate the entire wire package), and prepare to wind half of the primary low-voltage winding; ② Wind half of the primary low-voltage winding. Reserve the lead-out wire (head), note that it is reserved, because it will be connected uniformly and then connected to the lead-out wire. The same applies to the word "reserved" in the following primary. Use 19 wires to wind 3T in parallel, reserve the center tap, and then wind 3T in parallel, reserve the lead-out wire (tail), and cut the wire. There is another trick in the specific operation. Because of the large number of strands, it is not convenient to wind 19 strands at a time, and the torque tension is also large. It can be divided into multiple times. For example, it can be divided into three times, each time with 6 to 7 strands of wire, so that it can be wound more smoothly. Note that the head, middle and tail of the three times are placed together, and the winding direction should be the same. Then wrap another layer of insulation paper and prepare to wind the second section of the secondary high-voltage winding; ③ Wind the second section of the secondary high-voltage winding. Turn over the secondary high-voltage winding wire that has not been cut off before (be careful not to touch the previous primary winding wire, and use insulation paper to separate it if necessary), and wind 25T again. Note that the winding direction should be the same as the first section, and the wire is still not cut. Wrap another layer of insulation paper and prepare to wind the other half of the primary low-voltage winding; ④ Wind the other half of the primary low-voltage winding. Wind the primary low-voltage winding again in the same way as step ②, and note that the winding direction should be the same as the previous half. Cut the same wire, wrap a layer of insulation paper, and prepare to wind the third section of the secondary high-voltage winding; ⑤ Wind the third section of the secondary high-voltage winding. Then follow the method in step ③ to wind the remaining secondary high-voltage winding 25T, and still pay attention to the same winding direction as the previous two sections. Connect the lead wire (tail) and cut the wire. At this point, all the windings are wound; ⑥ Combine the primary low-voltage windings. Connect the heads of the primary low-voltage windings wound twice in parallel, connect the center taps in parallel, and connect the tails in parallel (so that the number of winding turns is still 3T+3T, and the total number of parallel wires is 38), connect the lead wires, and you will get the head, middle, and tail lead ends of the primary low-voltage winding. Finally, wrap a layer of insulating tape, and the wire package is completed.

The above description seems very complicated, but it is not difficult after getting familiar with it. The high-frequency transformer wound in this way is definitely easy to use; if you refer to the symmetrical cross-winding method of high-end electronic tube audio transformers and pay attention to the fine craftsmanship in production, as long as the magnetic core is adapted, the operating frequency can be increased to more than 100KHz. However, the symmetrical cross-winding method is the most complicated and difficult to handle (the winding segments are finer, each layer is symmetrically divided into two groups, the connection method is complicated, and the phase of a certain section of the winding will be connected incorrectly if you are careless), so I will not introduce it. Why is it that the frequency of the high-frequency transformers made by some people can never be increased, the power cannot be increased (increasing the power requires increasing the frequency), and the heat is serious, it is because of the large leakage inductance, large distributed capacitance, and serious high-frequency current skin phenomenon, etc.

The winding method of self-excited high-frequency transformer is the same.