Multi-winding planar transformer and flyback switching power supply
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- NANJING KANGNI NEW ENERGY AUTO PARTS CO LTD
- Filing Date
- 2025-06-10
- Publication Date
- 2026-06-12
Smart Images
Figure CN224355086U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of transformer technology, and in particular to a multi-winding planar transformer. Background Technology
[0002] In switch-mode power supplies (SMPS), the transformer is one of the largest components in terms of physical size, thus limiting the overall size of the power supply. Therefore, reducing the size of the transformer allows the power supply to have higher power density.
[0003] Planar transformers, due to their unique planar structure and tight coupling of windings, significantly reduce high-frequency parasitic parameters and have therefore been widely used in recent years. For an ideal transformer, the magnetic flux of the primary winding is completely coupled to the secondary winding, with no leakage flux. However, leakage inductance is unavoidable in practical applications. Planar transformers use copper traces on printed circuit boards instead of the original multi-strand excitation wires. This method significantly increases the current density compared to the latter, enabling the realization of the desired high power density.
[0004] In traditional planar transformers, the basic winding structure typically consists of two windings: a primary winding and a secondary main feedback winding. The routing of the primary and secondary windings employs a parallel-swapping and alternating (PSPS) design to achieve good interlayer magnetic coupling, low leakage inductance, and minimal inter-turn interference. Specifically, the secondary winding of the transformer has several coils connected in parallel, each coupled to the same primary winding. In multi-secondary winding applications, the uniform distribution of the planar transformer windings, especially for the secondary winding which is not the main feedback winding, is particularly important in applications where inter-turn interference is sensitive, such as under no-load or load shedding conditions. Planar transformers have low window utilization, few turns, large magnetic flux, and unbalanced magnetic flux distribution. Especially in applications with multiple secondary windings, the uneven distribution of the wiring leads to incomplete coupling of magnetic flux between turns. When the magnetic flux in the magnetic circuit is large, it will increase the leakage inductance of the transformer, resulting in an increase in back electromotive force, which can easily break down switching devices. At the same time, the planar structure has a high capacitive effect and other problems. The leakage inductance and the distributed capacitance in the circuit form an oscillating loop, which increases electromagnetic interference. Summary of the Invention
[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a multi-winding planar transformer and flyback switching power supply, thereby solving the technical problems of traditional planar transformers in multi-winding application scenarios mentioned in the background art.
[0006] To achieve the above objectives, this utility model employs the following technical solution:
[0007] In a first aspect, this utility model provides a multi-winding planar transformer, including a magnetic core, coil windings surrounding the magnetic core, an upper multi-layer circuit board, a middle multi-layer circuit board, and a lower multi-layer circuit board;
[0008] The coil winding includes a primary driving coil winding, a primary coil winding, a primary auxiliary coil winding, a secondary main feedback winding, a first secondary coil winding, and a second secondary coil winding.
[0009] The upper multilayer circuit board includes circuit boards from layer 1 to layer 6, the middle multilayer circuit board includes circuit boards from layer 7 to layer 10, and the lower multilayer circuit board includes circuit boards from layer 11 to layer 16.
[0010] The primary drive coil windings are distributed on the 1st, 6th, 11th and 16th layers of the circuit board; the primary coil windings are distributed on the 2nd and 15th layers of the circuit board; the primary auxiliary coil windings are distributed on the 5th and 12th layers of the circuit board; the secondary main feedback windings are distributed on the 3rd and 14th layers of the circuit board; the first secondary coil winding is distributed on the 4th and 13th layers of the circuit board; and the second secondary coil winding is distributed on the 8th and 9th layers of the circuit board.
[0011] The same coil winding distributed on the same multilayer circuit board is connected by layer switching through vias, and the same coil winding distributed on different multilayer circuit boards is connected by layer switching through half-hole technology on the edge of the printed circuit board.
[0012] Optionally, the first end of the primary drive coil winding is introduced through via PW1 on the first layer circuit board. A counter-clockwise n-turn trace runs from the outer circumference to the inner circumference via V12. From via V12, a trace is led to the inner circumference of the sixth layer circuit board. A counter-clockwise n-turn trace runs to the outer circumference and then to via PW3. From via PW3, a trace runs to the outer circumference of the eleventh layer circuit board, then a counter-clockwise n-turn trace runs to the inner circumference via V34. From via V34, a trace runs to the inner circumference of the sixteenth layer. A counter-clockwise n-turn trace runs to the outer circumference and then to via PW2. Via PW2 is the corresponding end of the primary drive coil winding.
[0013] Optionally, the first end of the primary coil winding is introduced through via PW7 on the second layer circuit board, and a circumferential line is run counterclockwise for one turn to via PW8. From via PW8, a line is led to the 15th layer circuit board and then runs counterclockwise for one turn to via PW9. Via PW9 is the same-name end of the primary coil winding. The shadow areas of the conductive coils of the primary coil winding and the primary drive coil winding on each layer circuit board overlap.
[0014] Optionally, the first end of the primary auxiliary coil winding is introduced through via PW4 on the 5th layer circuit board, and a circumferential line is run counterclockwise for 1 turn to via PW5. From via PW5, a line is led to the 12th layer circuit board and then runs counterclockwise for 1 turn to via PW6. Via PW6 is the same-name end of the primary auxiliary coil winding. The shadow areas of the conductive coils of the primary auxiliary coil winding and the primary drive coil winding on each layer circuit board overlap.
[0015] Optionally, the corresponding end of the secondary main feedback winding is introduced through via SW1 on the third layer circuit board, and a circumferential clockwise trace is run for one turn to via SW2. From via SW2, a trace is led to the 14th layer circuit board and then a clockwise trace is run for one turn to via SW3. The via SW3 is the tail end of the secondary main feedback winding. The shadow areas of the conductive coils of the secondary main feedback winding and the primary drive coil winding on each layer circuit board overlap.
[0016] Optionally, the corresponding end of the first secondary coil winding is introduced through via SW4 on the fourth layer circuit board, and a circumferential clockwise trace is run for one turn to via SW5. From via SW5, a trace is led to the 13th layer circuit board and then a clockwise trace is run for one turn to via SW6. The via SW6 is the tail end of the first secondary coil winding. The shadow areas of the conductive coils of the first secondary coil winding and the primary driving coil winding on each layer circuit board overlap.
[0017] Optionally, the corresponding end of the second secondary coil winding is introduced through via SW7 on the 8th layer circuit board, runs clockwise for one turn along the outer circumference to via V56 on the outer circumference, is led from via V56 to the 9th layer circuit board, runs clockwise for one turn to via V78, is led from via V78 back to the inner circumference of the 8th layer circuit board, runs clockwise for one turn along the inner circumference to via SW8, and via SW8 is the tail end of the second secondary coil winding; therefore, the shadow areas of the conductive coils of the second secondary coil winding and the primary driving coil winding on each layer circuit board overlap.
[0018] Optionally, the upper multilayer circuit board, the middle multilayer circuit board, and the lower multilayer circuit board are stacked and soldered sequentially.
[0019] Vias PW1 to PW9 on each layer of the circuit board are in the same position and are set on one side of each layer of the circuit board in sequential order; vias SW1 to SW8 on each layer of the circuit board are in the same position and are set on the other side of each layer of the circuit board in sequential order; via PW2 is removed on the circuit boards of layers 1 to 6.
[0020] The vias V12 on the 1st to 6th layers of the circuit board and V34 on the 11th to 16th layers of the circuit board are in the same position; the vias V56 and V78 on the 7th to 10th layers of the circuit board are in the same position.
[0021] Optionally, the magnetic core is an ECI type magnetic core, including an E-type base and an I-type cover plate combined together. The E-type base includes a base plate, a winding post in the middle of the base plate, and side posts on both sides of the base plate. Each layer of circuit board is provided with a magnetic post window corresponding to the winding post, and each layer of circuit board is disposed between the winding post and the side posts.
[0022] Secondly, this utility model provides a flyback switching power supply, including the multi-winding planar transformer as described above.
[0023] Compared with the prior art, the beneficial effects achieved by this utility model are as follows:
[0024] This utility model provides a multi-winding planar transformer and flyback switching power supply, which includes 6 independent coil windings. The primary drive coil winding is layered through holes V12 and V34, while the remaining coil windings are layered through half-holes distributed outside the shadow of the ECI magnetic core window. This greatly improves the utilization rate of the magnetic core window and thus improves the integration of the planar coil.
[0025] The selection of the half-hole entry and exit distribution method of the conductive coil of the primary drive coil in layers 6 and 11, as well as the unique design of the half-hole PW2 of the printed circuit board where layers 1 and 6 are located, minimize the loop envelope of the external drive circuit fitted by the entire drive winding, greatly optimize the EMI of the flyback switching power supply, reduce the use of external EMI filtering devices, and thus reduce production costs.
[0026] The second secondary coil winding is distributed on layers 8 and 9, both of which belong to the middle multilayer circuit board. Its TOP layer and BOTTOM layer are electrical isolation layers because they do not have copper covering for conductive coils. This meets the soldering requirements for direct stacking and assembly of the upper and lower 6 printed circuit boards, reduces the process difficulty of electrical isolation, and simplifies the process. Attached Figure Description
[0027] Figure 1 This is a schematic diagram of the structure of the multi-winding planar transformer provided in this embodiment of the utility model;
[0028] Figure 2 This is a schematic diagram of the structure of the first to third layers of the circuit board provided in this embodiment of the utility model;
[0029] Figure 3 This is a schematic diagram of the structure of the 4th to 6th layers of the circuit board provided in this embodiment of the utility model;
[0030] Figure 4 This is a schematic diagram of the structure of the 7th to 10th layers of the circuit board provided in this embodiment of the utility model;
[0031] Figure 5This is a schematic diagram of the structure of the 11th to 13th layers of the circuit board provided in this embodiment of the utility model;
[0032] Figure 6 This is a schematic diagram of the structure of the 14th to 16th layers of the circuit board provided in this embodiment of the utility model;
[0033] Figure 7 This is a simplified topology diagram of the flyback switching power supply provided in this embodiment of the present invention. Detailed Implementation
[0034] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.
[0035] Example 1:
[0036] This embodiment provides a multi-winding planar transformer, including a magnetic core, coil windings surrounding the magnetic core, an upper multilayer circuit board, a middle multilayer circuit board, and a lower multilayer circuit board.
[0037] Specifically, in this embodiment, six independent coil windings are provided, namely, primary driving coil winding P1, primary coil winding P2, primary auxiliary coil winding P3, secondary main feedback winding S1, first secondary coil winding S2, and second secondary coil winding S3.
[0038] Based on the aforementioned six independent coil windings, an upper multilayer circuit board, a middle multilayer circuit board, and a lower multilayer circuit board are designed. For example... Figure 1 As shown, the upper multilayer circuit board includes circuit boards from layers 1 to 6, the middle multilayer circuit board includes circuit boards from layers 7 to 10, and the lower multilayer circuit board includes circuit boards from layers 11 to 16. The primary-side drive coil winding P1 is distributed on circuit boards from layers 1 to 6, 11, and 16; the primary-side coil winding P2 is distributed on circuit boards from layers 2 to 15; the primary-side auxiliary coil winding P3 is distributed on circuit boards from layers 5 to 12; the secondary-side main feedback winding S1 is distributed on circuit boards from layers 3 to 14; the first secondary-side coil winding S2 is distributed on circuit boards from layers 4 to 13; and the second secondary-side coil winding S3 is distributed on circuit boards from layers 8 to 9.
[0039] The same coil winding distributed on the same multilayer circuit board is connected by layer switching through vias, and the same coil winding distributed on different multilayer circuit boards is connected by layer switching through half-hole technology on the edge of the printed circuit board.
[0040] like Figures 2 to 6As shown, the primary drive coil winding P1 is introduced through via PW1 on the first layer circuit board. It runs counterclockwise for 5 turns from the outer circumference to the inner circumference via V12. From via V12, it runs to the inner circumference of the sixth layer circuit board. It runs counterclockwise for 5 turns to the outer circumference and then runs to via PW3. From via PW3, it runs counterclockwise for 5 turns to the outer circumference of the eleventh layer circuit board and then runs counterclockwise for 5 turns to the inner circumference via V34. From via V34, it runs to the inner circumference of the sixteenth layer. It runs counterclockwise for 5 turns to the outer circumference and then runs to via PW2. Via PW2 is the same-name end of the primary drive coil winding P1.
[0041] The primary coil winding P2 is introduced through via PW7 on the second layer of the circuit board. It runs counterclockwise for one turn to via PW8, and then runs counterclockwise for one turn to via PW9 on the 15th layer of the circuit board. The same-name end of the primary coil winding P2 is located at via PW9. The shadow areas of the conductive coils of the primary coil winding P2 and the primary drive coil winding P1 on each layer of the circuit board overlap.
[0042] The primary auxiliary coil winding P3 is introduced through via PW4 on the 5th layer circuit board. It runs counterclockwise for one turn to via PW5, and then runs counterclockwise for one turn to via PW6 from via PW5 to the 12th layer circuit board. Via PW6 is the same-name end of the primary auxiliary coil winding P3. The shadow areas of the primary auxiliary coil winding P3 and the primary drive coil winding P1 on each layer circuit board overlap.
[0043] The corresponding terminal of the secondary main feedback winding S1 is introduced through via SW1 on the 3rd layer circuit board. One turn of the trace is run clockwise to via SW2. From via SW2, the trace is led to the 14th layer circuit board and then one turn of the trace is run clockwise to via SW3. The via SW3 is the tail end of the secondary main feedback winding S1. The shadow areas of the conductive coils of the secondary main feedback winding S1 and the primary drive coil winding P1 on each layer circuit board overlap.
[0044] The corresponding end of the first secondary coil winding S2 is introduced through via SW4 on the 4th layer circuit board. One turn of the trace is run clockwise to via SW5. From via SW5, the trace is led to the 13th layer circuit board and then one turn of the trace is run clockwise to via SW6. The tail end of the first secondary coil winding S2 is at via SW6. The shadow areas of the conductive coils of the first secondary coil winding S2 and the primary drive coil winding P1 on each layer circuit board overlap.
[0045] The second secondary winding S3 is introduced through via SW7 on the 8th layer circuit board. It runs clockwise for one turn along the outer circumference to via V56. From via V56, it runs clockwise for one turn to via V78 on the 9th layer circuit board. From via V78, it runs back to the inner circumference of the 8th layer circuit board and runs clockwise for one turn to via SW8. Via SW8 is the tail end of the second secondary winding S3. Therefore, the shadow areas of the conductive coils of the second secondary winding S3 and the primary driving winding P1 on each layer circuit board overlap.
[0046] The upper multilayer circuit board, the middle multilayer circuit board, and the lower multilayer circuit board are stacked and soldered in sequence; the vias PW1 to PW9 on each layer of the circuit board are in the same position and are arranged in sequence on one side of each layer of the circuit board; the vias SW1 to SW8 on each layer of the circuit board are in the same position and are arranged in sequence on the other side of each layer of the circuit board; via PW2 is removed on the 1st to 6th layers of the circuit board; the vias V12 on the 1st to 6th layers of the circuit board and the vias V34 on the 11th to 16th layers of the circuit board are in the same position; the vias V56 and V78 on the 7th to 10th layers of the circuit board are in the same position.
[0047] In terms of physical distance, via PW2 is closer to via PW1. In the intermediate transition layer of the primary drive coil winding P1, the lead is led out from via PW3, which is farther from via PW1, instead of from via PW2. There are two main purposes for this. First, in order to match the design of a highly integrated, small, modular flyback power supply, the low-end drive MOS of the primary drive coil winding P1 is mostly designed to be close to the edge to meet the requirement of minimizing electrical clearance. In order to achieve better EMI characteristics, the envelope of the input and output lines should be as small as possible. Based on the above two requirements, under the premise of meeting the electrical creepage distance, the distance between the input and output ends of the primary drive coil winding P1 should be as small as possible to meet the requirement of minimizing the loop envelope and achieving optimal EMI (electromagnetic interference). Second, as the primary drive coil winding P1 is used in this solution at a maximum voltage of 500V, the withstand voltage between adjacent turns of the primary drive winding P1 is 25V, and the withstand voltage of the input and output lines is 500V.
[0048] Specifically, in this embodiment, the vias PW1 and PW3~PW9 of the upper multilayer circuit board are half-hole designs, the vias PW1~PW9 of the middle multilayer circuit board are through-hole designs, and the vias PW1~PW9 of the lower multilayer circuit board are half-hole designs. The through-hole design of the middle multilayer circuit board facilitates the routing of wires from the upper and lower multilayer circuit boards to the middle multilayer circuit board.
[0049] Unlike vias V12 and V34, vias V56 and V78 are not located under the mounting shadow area of the magnetic core, thus not occupying the limited magnetic post window utilization, which is beneficial for the high integration of the planar transformer windings. The creepage clearance between the conductive coil windings of each layer of the upper and lower multilayer circuit boards and vias V12 and V34 is a preset value. The 7th and 10th layers of the middle multilayer circuit board are empty layers, which are used for electrical isolation between the primary winding coil and the secondary winding coil.
[0050] The magnetic core uses an ECI type core, which includes an E-type base and an I-type cover plate. The E-type base includes a base plate, a winding post in the middle of the base plate, and side posts on both sides of the base plate. Each circuit board layer has a magnetic post window corresponding to the winding post, and each circuit board layer is located between the winding post and the side posts. The ECI type core has the characteristics of: reduced loss in the cylindrical center post + uniform magnetic circuit in the arc-shaped side posts + leakage magnetic field suppression in the closed structure + heat dissipation through the open window. Its design performs well in high power density, high frequency applications, and automated production, and is the preferred solution for balancing performance and cost.
[0051] In summary, this planar transformer contains six independent primary and secondary windings. The primary drive coil winding P1 is layer-changed through vias V12 and V34. The layer-change method for the remaining windings is to use half-holes distributed outside the shadow of the ECI core window for layer-change winding, which greatly improves the utilization rate of the magnetic column window and thus improves the integration of the planar coil. The second secondary coil winding S3 is distributed on layers 8 and 9, which are the middle four layers of the printed circuit board. Its TOP layer and BOTTOM layer are electrical isolation layers because they are not covered with copper for the conductive coils. This meets the soldering requirements for direct stacking and assembly of the upper and lower six printed circuit boards, reduces the process difficulty of electrical isolation, and simplifies the process.
[0052] Example 2:
[0053] like Figure 7 As shown, this utility model provides a flyback switching power supply, including a multi-winding planar transformer as provided in Embodiment 1.
[0054] The first end of the primary drive coil winding P1 is connected to the power supply V. PFC The terminals of the primary winding P2, the primary auxiliary winding P3, the secondary main feedback winding S1, the first secondary winding S2, and the second secondary winding S3 are connected to the corresponding terminals of the Schottky diodes.
[0055] Power MOSFETs and Schottky diodes, among other related components, can be surface-mounted on the middle multilayer circuit board for integrated design, reducing the size of the flyback switching power supply.
[0056] The selection of the half-hole entry and exit distribution method of the primary drive coil winding P1 in layers 6 and 11, and the unique design of the half-hole PW2 in the printed circuit board where layers 1 and 6 are located, minimize the loop envelope of the external drive circuit fitted by the entire drive winding, greatly optimize the EMI of the flyback switching power supply, reduce the use of external EMI filtering devices, and thus reduce production costs.
[0057] The above description is only a preferred embodiment of the present utility model. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present utility model, and these improvements and modifications should also be considered within the protection scope of the present utility model.
Claims
1. A multi-winding planar transformer, characterized in that, It includes a magnetic core, a coil winding surrounding the magnetic core, an upper multilayer circuit board, a middle multilayer circuit board, and a lower multilayer circuit board; The coil winding includes a primary driving coil winding, a primary coil winding, a primary auxiliary coil winding, a secondary main feedback winding, a first secondary coil winding, and a second secondary coil winding. The upper multilayer circuit board includes circuit boards from layer 1 to layer 6, the middle multilayer circuit board includes circuit boards from layer 7 to layer 10, and the lower multilayer circuit board includes circuit boards from layer 11 to layer 16. The primary drive coil windings are distributed on the 1st, 6th, 11th and 16th layers of the circuit board; the primary coil windings are distributed on the 2nd and 15th layers of the circuit board; the primary auxiliary coil windings are distributed on the 5th and 12th layers of the circuit board; the secondary main feedback windings are distributed on the 3rd and 14th layers of the circuit board; the first secondary coil winding is distributed on the 4th and 13th layers of the circuit board; and the second secondary coil winding is distributed on the 8th and 9th layers of the circuit board. The same coil winding distributed on the same multilayer circuit board is connected by layer switching through vias, and the same coil winding distributed on different multilayer circuit boards is connected by layer switching through half-hole technology on the edge of the printed circuit board.
2. The multi-winding planar transformer according to claim 1, characterized in that, The primary drive coil winding begins at the first end through via PW1 on the first layer circuit board. A counter-clockwise n-turn trace runs from the outer circumference to via V12 on the inner circumference. From via V12, a trace runs to the inner circumference of the sixth layer circuit board. A counter-clockwise n-turn trace runs to the outer circumference and then to via PW3. From via PW3, a trace runs to the outer circumference of the eleventh layer circuit board, then a counter-clockwise n-turn trace runs to via V34 on the inner circumference. From via V34, a trace runs to the inner circumference of the sixteenth layer. A counter-clockwise n-turn trace runs to the outer circumference and then to via PW2. Via PW2 is the corresponding end of the primary drive coil winding.
3. The multi-winding planar transformer according to claim 2, characterized in that, The primary winding is introduced through via PW7 on the second layer circuit board, and runs counterclockwise for one turn to via PW8. From via PW8, it runs counterclockwise for one turn to via PW9 on the 15th layer circuit board. Via PW9 is the same-name end of the primary winding. The shadow areas of the primary winding and the primary drive winding on each layer of circuit board overlap.
4. The multi-winding planar transformer according to claim 3, characterized in that, The primary auxiliary coil winding is introduced through via PW4 on the 5th layer circuit board, and runs counterclockwise for one turn to via PW5. From via PW5, it runs counterclockwise for one turn to via PW6 on the 12th layer circuit board. Via PW6 is the same-name end of the primary auxiliary coil winding. The shadow areas of the conductive coils of the primary auxiliary coil winding and the primary drive coil winding on each layer circuit board overlap.
5. The multi-winding planar transformer according to claim 4, characterized in that, The corresponding end of the secondary main feedback winding is introduced through via SW1 on the third layer circuit board, and a circumferential clockwise line is run for 1 turn to via SW2. From via SW2, a line is led to the 14th layer circuit board and then a clockwise line is run for 1 turn to via SW3. The via SW3 is the tail end of the secondary main feedback winding. The shadow areas of the conductive coils of the secondary side main feedback winding and the primary side drive coil winding on each layer of the circuit board overlap.
6. The multi-winding planar transformer according to claim 5, characterized in that, The corresponding end of the first secondary coil winding is introduced through via SW4 on the fourth layer circuit board, and one turn of the wire is run clockwise to via SW5. The wire is then led from via SW5 to the 13th layer circuit board and one turn of the wire is run clockwise to via SW6. The via SW6 is the tail end of the first secondary coil winding. The shadow areas of the conductive coils of the first secondary winding and the primary driving winding on each layer of the circuit board overlap.
7. The multi-winding planar transformer according to claim 6, characterized in that, The second secondary coil winding is introduced through via SW7 on the 8th layer circuit board, and runs clockwise for one turn along the outer circumference to via V56. From via V56, it runs clockwise for one turn to via V78 on the 9th layer circuit board, and then runs clockwise for one turn back to via V78. From via V78, it runs clockwise for one turn to via SW8 on the inner circumference of the 8th layer circuit board. Via SW8 is the tail end of the second secondary coil winding. Therefore, the shadow areas of the conductive coils of the second secondary coil winding and the primary drive coil winding on each layer circuit board overlap.
8. The multi-winding planar transformer according to claim 7, characterized in that, The upper multilayer circuit board, the middle multilayer circuit board, and the lower multilayer circuit board are stacked and soldered in sequence. Vias PW1 to PW9 on each layer of the circuit board are in the same position and are set on one side of each layer of the circuit board in sequential order; vias SW1 to SW8 on each layer of the circuit board are in the same position and are set on the other side of each layer of the circuit board in sequential order; via PW2 is removed on the circuit boards of layers 1 to 6. The vias V12 on the 1st to 6th layers of the circuit board and V34 on the 11th to 16th layers of the circuit board are in the same position; the vias V56 and V78 on the 7th to 10th layers of the circuit board are in the same position.
9. The multi-winding planar transformer according to claim 1, characterized in that, The magnetic core is an ECI type magnetic core, which includes an E-type base and an I-type cover plate. The E-type base includes a base plate, a winding post in the middle of the base plate, and side posts on both sides of the base plate. Each layer of circuit board has a magnetic post window corresponding to the winding post, and each layer of circuit board is disposed between the winding post and the side posts.
10. A flyback switching power supply, characterized in that, Including the multi-winding planar transformer as described in any one of claims 1-9.