Transformer and power supply comprising a transformer

Abstract

The utility model discloses a transformer and power supply including transformer. Transformer includes primary winding, secondary winding, power supply auxiliary winding and balance winding. Power supply auxiliary winding is coupled to secondary winding, and provides a power supply voltage and a first balance current. Balance winding is coupled to power supply auxiliary winding, and provides a second balance current. First balance current and second balance current are used for balanceing the common mode current difference between primary winding and secondary winding. The utility model discloses through increasing balance winding on the basis of power supply auxiliary group winding of transformer, can enhance secondary side dynamic point voltage, thereby can balance the common mode current difference between primary winding and secondary winding of transformer. Further, through adjusting the number of turns and line width of balance winding, the purpose of balancing primary and secondary common mode current can be further achieved.

Description

Technical Field

[0001] This utility model relates to the field of power technology, and in particular to a transformer and a power supply including the transformer. Background Technology

[0002] As the power density of power converters continues to increase, their operating frequencies reach hundreds of kHz. High dv / dt and di / dt can generate severe EMI interference. EMI interference is divided into differential-mode interference and common-mode interference. Differential-mode interference is generally suppressed using differential-mode filters, while if common-mode interference is completely suppressed using common-mode filters, the common-mode inductor will be too large, which will limit the power density of the adapter and reduce efficiency.

[0003] To suppress common-mode interference, existing techniques construct moving points in opposite directions on the primary and secondary sides, hoping to cancel out the common-mode currents. However, due to the large voltage difference between the primary and secondary sides and the uncertainty of parasitic capacitance, the common-mode currents usually cannot achieve cancellation. For example, ... Figure 1 As shown, an exemplary structure of a conventional power supply 1000' is illustrated. The power supply 1000' includes a transformer 100', a main switch Q, and a bus capacitor Cp. The transformer 100' includes a primary winding 10' and a secondary winding 20'. When the main switch Q is repeatedly turned on and off at a high frequency, common-mode currents i1 (e.g., generated through parasitic capacitance C1'), i2 (e.g., generated through parasitic capacitance C2'), and i3 (e.g., generated through parasitic capacitance C3') are generated on the primary and secondary windings, forming moving points N1' and N2' (sometimes referred to as "jumping terminals") on the primary and secondary windings, respectively. Due to the large voltage difference between the primary and secondary windings and the uncertainty of parasitic capacitance, the common-mode currents i1 and i2 generated on the secondary winding and i3 generated on the primary winding cannot actually cancel each other out (i.e., i1 + i2 ≠ i3). In other words, the prior art cannot effectively balance the common-mode current difference between the primary and secondary windings of the transformer. Utility Model Content

[0004] The purpose of this invention is to provide a transformer and power supply that can effectively solve at least one defect of the prior art, such as balancing the common-mode current difference between the primary and secondary windings of the transformer.

[0005] To achieve the above objectives, this utility model provides a transformer, which includes a primary winding, a secondary winding, a power supply auxiliary winding, and a balancing winding. The power supply auxiliary winding is coupled to the secondary winding and provides a power supply voltage and a first balancing current; the balancing winding is coupled to the power supply auxiliary winding and provides a second balancing current; the first balancing current and the second balancing current are used together to balance the common-mode current difference between the primary winding and the secondary winding.

[0006] In some embodiments of this utility model, the power supply auxiliary winding includes a primary power supply auxiliary winding located on the primary side and a secondary power supply auxiliary winding located on the secondary side; the balancing winding is coupled to the primary power supply auxiliary winding or the secondary power supply auxiliary winding.

[0007] In some embodiments of this utility model, the balancing winding is coupled to the primary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first or second end of the primary power supply auxiliary winding, while the second end of the balancing winding is suspended.

[0008] In some embodiments of this utility model, the balancing winding is coupled to the secondary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first or second end of the secondary power supply auxiliary winding, while the second end of the balancing winding is left unconnected.

[0009] In some embodiments of this utility model, the turns ratio of the secondary winding to the secondary power supply auxiliary winding is 1:1.

[0010] In some embodiments of this utility model, the first end of the primary power supply auxiliary winding is grounded, and the second end of the primary power supply auxiliary winding is a switching end.

[0011] To achieve the above objectives, this utility model further provides a power supply, which includes the transformer as described above, and the power supply also includes a bus capacitor and a main switch. The first end of the primary winding is connected to the positive terminal of the bus capacitor, the second end of the primary winding is a switching terminal and is connected to the first end of the main switch, and the second end of the main switch is connected to the negative terminal of the bus capacitor.

[0012] In some other embodiments of this invention, the power supply is a flyback converter or a forward converter.

[0013] In some other embodiments of this utility model, the power supply auxiliary winding includes a primary power supply auxiliary winding located on the primary side, the balancing winding is coupled to the primary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first end or the second end of the primary power supply auxiliary winding, and the second end of the balancing winding is suspended; the first end of the balancing winding and the second end of the primary winding are of the same name.

[0014] In some other embodiments of this utility model, the power supply auxiliary winding includes a secondary power supply auxiliary winding located on the secondary side, the balancing winding is coupled to the secondary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first end or the second end of the secondary power supply auxiliary winding, and the second end of the balancing winding is suspended; the first end of the balancing winding and the second end of the primary winding are opposite ends.

[0015] This invention enhances the secondary moving point voltage by adding a balancing winding to the auxiliary power supply winding of a transformer, thereby balancing the common-mode current difference between the primary and secondary windings. Furthermore, by adjusting the number of turns and the wire width of the balancing winding, the common-mode current between the primary and secondary windings can be further balanced.

[0016] Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0017] The above and other features and advantages of this invention will become more apparent from a detailed description of exemplary embodiments with reference to the accompanying drawings.

[0018] Figure 1 The structure of an exemplary power supply 1000' in the prior art is shown;

[0019] Figure 2 The structure of the power supply 1000 in the first preferred embodiment of this utility model is shown;

[0020] Figure 3 The structure of the power supply 1000-1 in the second preferred embodiment of this utility model is shown;

[0021] Figure 4 The structure of the power supply 1000-2 in the third preferred embodiment of this utility model is shown;

[0022] Figure 5 The structure of the power supply 1000-3 in the fourth preferred embodiment of this utility model is shown;

[0023] Figure 6 The structure of the power supply 1000-4 in the fifth preferred embodiment of this utility model is shown;

[0024] Figure 7 The structure of the power supply 1000-5 according to the sixth preferred embodiment of the present invention is shown. Detailed Implementation

[0025] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the present invention will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed description will be omitted.

[0026] In describing the elements / components / etc. described and / or illustrated herein, the terms “a,” “an,” “the,” “the,” and “at least one” are used to indicate the presence of one or more elements / components / etc. The terms “comprising,” “including,” and “having” are used to indicate an open-ended inclusion and to mean that additional elements / components / etc. may exist in addition to those listed. Furthermore, the terms “first,” “second,” etc., in the claims are used only as designations and are not intended to limit the number of objects to which they pertain.

[0027] It should be understood that the wording or terminology used herein is for descriptive purposes and not for limitation, so that those skilled in the art can interpret the terms or wording of this specification based on the teachings herein.

[0028] Furthermore, the present invention may reuse element symbols and / or letters in various embodiments or examples. This repetition is for simplicity and clarity and does not in itself limit the relationship between the various embodiments and / or configurations discussed.

[0029] like Figure 2 The diagram illustrates the structure of a power supply 1000 according to a first preferred embodiment of the present invention. In this invention, the power supply 1000 may include at least a transformer 100, a bus capacitor Cp, and a main switch Q. The transformer 100 may include a primary winding 10, a secondary winding 20, a power supply auxiliary winding 30, and a balancing winding 40. The first end of the primary winding 10 of the transformer 100 is connected to the positive terminal of the bus capacitor Cp, and the second end of the primary winding 10 is a switching terminal connected to the first terminal of the main switch Q. The second terminal of the main switch Q is connected to the negative terminal of the bus capacitor Cp.

[0030] In some embodiments, the auxiliary power supply winding 30 of the transformer 100 is coupled to the secondary winding 20 and can provide a supply voltage (not shown) and a first balancing current (e.g., Figure 2 (i3, i4, i5 in the original text). The balancing winding 40 is coupled to the power supply auxiliary winding 30 and provides a second balancing current (e.g., i3, i4, i5 in the original text). Figure 2 i6 in the middle). Among them, the first balancing current (e.g. Figure 2 (i3, i4, i5) and the second balancing current (e.g. Figure 2i6 in the equation can be used together to balance the common-mode current difference between the primary winding 10 and the secondary winding 20 of transformer 100.

[0031] Preferably, in some embodiments of the present invention, the power supply auxiliary winding 30 may include a primary power supply auxiliary winding 31 located on the primary side and a secondary power supply auxiliary winding 32 located on the secondary side; the balance winding 40 may be coupled to the primary power supply auxiliary winding 31 or the secondary power supply auxiliary winding 32.

[0032] For example, in Figure 2 In the illustrated embodiment, the balancing winding 40 is coupled to the secondary power supply auxiliary winding 32. More specifically, the first end of the balancing winding 40 is electrically connected to the first end of the secondary power supply auxiliary winding 32, and the second end of the balancing winding 40 is left floating. The first end of the secondary power supply auxiliary winding 32 is grounded, for example, and the second end of the secondary power supply auxiliary winding 32 is electrically connected to the first end of the secondary winding 20 and grounded, for example, the second end of the secondary winding 20 is grounded, for example.

[0033] exist Figure 2 In the embodiment shown, moving points (i.e. "jumping ends") N1 (e.g., corresponding to the second end of the primary winding 10), N2 (e.g., corresponding to the first end of the secondary winding 20), N3 (e.g., corresponding to the first end of the secondary power supply auxiliary winding 32), and N4 (e.g., corresponding to the first end of the primary power supply auxiliary winding 31) are respectively formed on the primary and secondary sides.

[0034] When the main switch Q is repeatedly turned on and off at a high frequency, common-mode currents i3 (e.g., generated through the parasitic capacitance C3 between the first end of the secondary winding 20 and the first end of the primary winding 10), i5 (e.g., generated through the parasitic capacitance C5 between the first end of the secondary power supply auxiliary winding 32 and the first end of the primary winding 10), and i6 (e.g., generated through the parasitic capacitance C6 between the second end of the balancing winding 40 and the first end of the primary winding 10) will be generated on the primary and secondary sides; and common-mode currents i1 (e.g., generated through the parasitic capacitance C1 between the primary winding 10 and the second end of the secondary winding 20) and i4 (e.g., generated through the parasitic capacitance C4 between the first end of the primary power supply auxiliary winding 31 and the first end of the secondary power supply auxiliary winding 32) will be generated on the primary side to ground, and common-mode current i2 (e.g., generated through the parasitic capacitance C2 between the second end of the primary winding 10 and the ground) will be generated. Among them, the common-mode currents i3, i5, and i6 are opposite to the common-mode currents i1, i2, and i4. Through appropriate design (including but not limited to designing the number of turns and line width of each winding, etc.), the common-mode currents of the primary and secondary sides can be canceled out (that is, i1+i2-i3+i4-i5-i6=0).

[0035] It should be noted that in actual operation, it may not be possible to truly achieve "i1+i2-i3+i4-i5-i6=0", but only to make it approach 0, that is, to make the primary and secondary common-mode currents cancel each other out as much as possible.

[0036] exist Figure 2 In some variations of the illustrated embodiment, the first end of the balancing winding 40 may, for example, be electrically connected to the second end of the secondary power supply auxiliary winding 32.

[0037] Preferably, in some embodiments of this utility model, such as Figure 2 As shown, the primary winding 10 may include multi-turn windings (e.g., P1, P2, P3 to P14), wherein the first turn winding P1 and the second turn winding P2 are low-turn points and can be used as a shielding layer. The parasitic capacitance can also be adjusted by adjusting the width of the shielding layer. By maximizing the width of the shielding layer, line loss can be reduced on the one hand, and the parasitic capacitance between the primary and secondary sides can be reduced on the other hand, or in other words, the parasitic capacitance between the secondary and primary sides can be increased. The secondary winding 20 may include multi-turn windings (e.g., S1, S2). The primary power supply auxiliary winding 31 may include winding P_aux, and the secondary power supply auxiliary winding 32 may include multi-turn windings (e.g., S3, S4).

[0038] Preferably, in some embodiments of this utility model, such as Figure 2 As shown, the topology of the power supply 1000 may also include capacitors Cs and Co located between the first end of the secondary winding 20 and ground, and a diode D1 is also included between the first end of the secondary winding 20 and capacitor Cs, wherein the anode of the diode D1 is coupled to the first end of the secondary winding 20.

[0039] Preferably, in some embodiments of this utility model, such as Figure 2 As shown, the topology of the power supply 1000 may also include a diode D2 and a capacitor Caux2 located between the first end of the secondary power supply auxiliary winding 32 and the anode of the diode D1, wherein the anode of the diode D2 is coupled to the first end of the secondary power supply auxiliary winding 32.

[0040] Preferably, in some embodiments of this utility model, such as Figure 2 As shown, the topology of the power supply 1000 may also include a diode D3 and a capacitor Caux1 located between the first end of the primary power supply auxiliary winding 31 and the first ground end, wherein the anode of the diode D3 is coupled to the first end of the primary power supply auxiliary winding 31, and the second end of the primary power supply auxiliary winding 31 is coupled to the first ground end (i.e., grounded).

[0041] Preferably, in some embodiments of this utility model, such as Figure 2As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are opposite-named ends.

[0042] Preferably, in some embodiments of this invention, the turns ratio of the secondary winding 20 to the secondary power supply auxiliary winding 32 can be, for example, 1:1. An additional N turns of balancing winding 40 are added to the power supply auxiliary winding 30 to enhance the secondary moving point voltage. Furthermore, by adjusting the number of turns and the line width of the balancing winding 40, the common-mode current of the primary and secondary sides can be balanced.

[0043] Preferably, in some embodiments of this invention, the power supply may be a flyback converter or a forward converter, for example... Figure 2 The power supply 1000 in the text refers to a flyback converter. It's understandable that when the power supply is a forward converter, the direction of its common-mode current is opposite to... Figure 2 The direction of the common-mode current shown is reversed, and the corresponding terminals of the primary winding and the balancing winding also change accordingly.

[0044] like Figure 3 As shown, this illustrates the structure of the power supply 1000-1 according to a second preferred embodiment of the present invention. Figure 3 In the illustrated embodiment, with Figure 2 The power supply 1000 in the illustrated embodiment differs in that the balancing winding 40 is coupled to the primary-side auxiliary power supply winding 31. For example, the first end of the balancing winding 40 is electrically connected to the first end of the primary-side auxiliary power supply winding 31, and the second end of the balancing winding 40 is left floating. Figure 3 In the embodiment shown, the common-mode currents i3, i5, i6 are opposite to the common-mode currents i1, i2, i4. With appropriate design, the common-mode currents on the primary and secondary sides can be canceled out (that is, i1+i2-i3+i4-i5-i6=0).

[0045] Preferably, in some embodiments of this utility model, such as Figure 3 As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are the same end.

[0046] like Figure 4 As shown, this illustrates the structure of the power supply 1000-2 according to a third preferred embodiment of the present invention. Figure 4 In the illustrated embodiment, with Figure 3 The power supply 1000-1 in the illustrated embodiment differs in that the first end of the balancing winding 40 is electrically connected to the second end of the primary-side auxiliary power supply winding 31, and the second end of the balancing winding 40 is left floating. Figure 4 In the embodiment shown, the common-mode currents i3, i5, i6 are opposite to the common-mode currents i1, i2, i4. With appropriate design, the common-mode currents on the primary and secondary sides can be canceled out (that is, i1+i2-i3+i4-i5-i6=0).

[0047] Preferably, in some embodiments of this utility model, such as Figure 4 As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are the same end.

[0048] like Figure 5 As shown, this illustrates the structure of the power supply 1000-3 according to the fourth preferred embodiment of this utility model. Figure 5 In the illustrated embodiment, with Figure 2 Similar to the power supply 1000 in the illustrated embodiment, the balancing winding 40 is coupled to the secondary power supply auxiliary winding 32. Figure 2 The power supply 1000 in the illustrated embodiment differs in that the first end of the secondary power supply auxiliary winding 32 is coupled to the second ground terminal via capacitor Caux2; diode D1 is located between the second end of the secondary winding 20 and capacitor Caux2, with the cathode of diode D1 coupled to the second end of the secondary winding 20 and the anode of diode D1 coupled to capacitor Caux2; diode D2 is located between the second end of the secondary power supply auxiliary winding 32 and the second ground terminal, with the cathode of diode D2 coupled to the second end of the secondary power supply auxiliary winding 32 and the anode of diode D2 coupled to capacitor Caux2 and the second ground terminal. The common-mode current i3 is generated, for example, through the parasitic capacitance C3 between the first end of the primary winding 10 and the second end of the secondary winding 20; the common-mode current i6 is generated, for example, through the parasitic capacitance C6 between the second end of the balancing winding 40 and the first end of the primary power supply auxiliary winding 31.

[0049] exist Figure 5 In the embodiment shown, the common-mode currents i1, i3, i2, i4, and i5 are opposite to the common-mode current i6. With appropriate design, the common-mode currents on the primary and secondary sides can be canceled out (that is, i1+i2+i3+i4+i5-i6=0).

[0050] Preferably, in some embodiments of this utility model, such as Figure 5 As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are opposite-named ends.

[0051] like Figure 6 As shown, this illustrates the structure of the power supply 1000-4 according to the fifth preferred embodiment of this utility model. Figure 6 In the illustrated embodiment, with Figure 5The power supply 1000-3 in the illustrated embodiment differs in that the balancing winding 40 is coupled to the primary-side auxiliary power supply winding 31. For example, the first end of the balancing winding 40 is electrically connected to the first end of the primary-side auxiliary power supply winding 31, and the second end of the balancing winding 40 is left floating. The common-mode current i6 is generated, for example, through the parasitic capacitance C6 between the first end of the secondary-side auxiliary power supply winding 32 and the second end of the balancing winding 40.

[0052] exist Figure 6 In the embodiment shown, the common-mode currents i1, i3, i2, i4, and i5 are opposite to the common-mode current i6. With appropriate design, the common-mode currents on the primary and secondary sides can be canceled out (that is, i1+i2+i3+i4+i5-i6=0).

[0053] Preferably, in some embodiments of this utility model, such as Figure 6 As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are the same end.

[0054] like Figure 7 As shown, this illustrates the structure of the power supply 1000-5 according to the sixth preferred embodiment of this utility model. Figure 7 In the illustrated embodiment, with Figure 6 The power supply 1000-4 in the illustrated embodiment differs in that the first end of the balancing winding 40 is electrically connected to the second end of the primary-side auxiliary power supply winding 31, and the second end of the balancing winding 40 is left floating. The common-mode current i6 is generated, for example, through the parasitic capacitance C6 between the first end of the secondary-side auxiliary power supply winding 32 and the second end of the balancing winding 40.

[0055] exist Figure 7 In the embodiment shown, the common-mode currents i1, i3, i2, i4, and i5 are opposite to the common-mode current i6. With appropriate design, the common-mode currents on the primary and secondary sides can be canceled out (that is, i1+i2+i3+i4+i5-i6=0).

[0056] Preferably, in some embodiments of this utility model, such as Figure 7 As shown, the first end of the balancing winding 40 and the second end of the primary winding 10 are the same end.

[0057] In this invention, to suppress common-mode interference, the phase difference between the primary and secondary moving points is 180°, thereby canceling common-mode noise by designing a reverse common-mode current. However, since the primary moving point voltage is much larger than the secondary moving point voltage and the parasitic capacitance is uncertain, the primary common-mode noise is usually stronger than the secondary common-mode noise, which may lead to the common-mode current not being completely canceled. Based on this, this invention further increases the secondary common-mode current by adding a power supply auxiliary winding on the secondary side. However, this is sometimes difficult to cancel common-mode noise because if the line width of the power supply auxiliary winding is increased to increase the secondary common-mode current, the line width of the secondary winding will be reduced, leading to an increase in winding loss on the secondary side. Therefore, the line width of the secondary power supply auxiliary winding is limited, and the primary common-mode current is still larger. The usual practice is to design the turns ratio of the secondary power supply auxiliary winding to the secondary winding to be 1:1. If the turns ratio is too small, the IC will enter undervoltage protection when the output voltage is low. If the turns ratio is too large, it will increase IC losses or even damage the IC.

[0058] In this invention, it is preferable to add a balancing winding (e.g., N turns) to the auxiliary power supply winding, thereby further enhancing the secondary moving point voltage. Furthermore, by having the auxiliary power supply winding and the balancing winding work together to balance the current, the common-mode current difference between the primary and secondary windings of the transformer can be balanced. Further, by adjusting the number of turns and the line width of the balancing winding, the common-mode current between the primary and secondary windings can also be balanced. In other words, this invention, by reusing the auxiliary power supply winding and extending it further into a balancing winding, can cancel out common-mode interference.

[0059] Exemplary embodiments of the present invention have been specifically shown and described above. It should be understood that the present invention is not limited to the disclosed embodiments; rather, the present invention is intended to cover various modifications and equivalent arrangements contained within the spirit and scope of the appended claims.

Claims

1. A transformer, characterized in that, include: Primary winding, secondary winding, power supply auxiliary winding, and balancing winding; The power supply auxiliary winding is coupled to the secondary winding and provides a power supply voltage and a first balancing current. The balancing winding is coupled to the power supply auxiliary winding and provides a second balancing current. The first balancing current and the second balancing current are used together to balance the common-mode current difference between the primary winding and the secondary winding.

2. The transformer according to claim 1, characterized in that, The power supply auxiliary winding includes a primary power supply auxiliary winding located on the primary side and a secondary power supply auxiliary winding located on the secondary side. The balancing winding is coupled to either the primary-side power supply auxiliary winding or the secondary-side power supply auxiliary winding.

3. The transformer according to claim 2, characterized in that, The balancing winding is coupled to the primary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first or second end of the primary power supply auxiliary winding, while the second end of the balancing winding is left unconnected.

4. The transformer according to claim 2, characterized in that, The balancing winding is coupled to the secondary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first or second end of the secondary power supply auxiliary winding, while the second end of the balancing winding is left unconnected.

5. The transformer according to claim 2, characterized in that, The turns ratio of the secondary winding to the secondary power supply auxiliary winding is 1:

1.

6. The transformer according to claim 2, characterized in that, The first end of the primary power supply auxiliary winding is grounded, and the second end of the primary power supply auxiliary winding is a switching end.

7. A power supply, characterized in that, The power supply includes the transformer as described in claim 1, and the power supply further includes a bus capacitor and a main switch, the first end of the primary winding is connected to the positive terminal of the bus capacitor, the second end of the primary winding is a switching terminal and is connected to the first end of the main switch, and the second end of the main switch is connected to the negative terminal of the bus capacitor.

8. The power supply according to claim 7, characterized in that, The power supply is a flyback converter or a forward converter.

9. The power supply according to claim 7, characterized in that, The power supply auxiliary winding includes a primary power supply auxiliary winding located on the primary side, the balancing winding is coupled to the primary power supply auxiliary winding, and the first end of the balancing winding is electrically connected to the first end or the second end of the primary power supply auxiliary winding, while the second end of the balancing winding is suspended. The first end of the balancing winding and the second end of the primary winding are of the same name.

10. The power supply according to claim 7, characterized in that, The power supply auxiliary winding includes a secondary power supply auxiliary winding located on the secondary side, the balance winding is coupled to the secondary power supply auxiliary winding, and the first end of the balance winding is electrically connected to the first end or the second end of the secondary power supply auxiliary winding, while the second end of the balance winding is suspended. The first end of the balancing winding and the second end of the primary winding are opposite-named ends.

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