JPH05159944A - High frequency booster transformer - Google Patents

Abstract

PURPOSE:To enhance an efficiency by winding a primary winding and a secondary winding in multilayers at adjacent positions in a central axis direction of a central leg in such a manner that the widths of windings are substantially equal and a ratio of number of turns of one layer of the secondary winding to the total number of turns and a ratio of a radius of the central leg to the winding widths of both the windings become special values. CONSTITUTION:A high frequency booster transformer has a primary winding L1 and a secondary winding L2 wound around a central leg 13 of a substantially columnar shape of a core 10 having an E-shaped section. The windings L1 and L2 are so wound in multilayers at adjacent positions in a central axis C direction of the leg in such a manner that the winding widths in a direction perpendicular to the axis C of the leg 13 are substantially the same in size, number of turns per one layer of the winding L2 of a high voltage side is 1/10 or less of the total number of turns of the winding L2 and Rm/W falls in a range of 0.6-1.7, where Rm is a radius of the leg 13, and both the widths of the windings L1, L2 are W. Thus, the transformer having a small copper loss and a low profile can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact and thin high-frequency step-up transformer suitable for use in an inverter for lighting a cold cathode tube for illuminating the back surface of liquid crystal.

[0002]

2. Description of the Related Art A conventional high frequency boosting transformer for an inverter is constructed as shown in FIG. A low-voltage side primary winding L ₁ and a high-voltage side secondary winding L ₂ are separately wound by the collar 1 on a hollow bobbin 2 having a plurality of collars 1. The secondary winding L ₂ in which a high voltage is generated is further divided into a plurality of stages by the collar 1 and wound so as to reduce the potential difference between the overlapping lines to prevent dielectric breakdown. In the hollow portion 3 of the bobbin 2, the central legs 6, 7 of the two E-shaped cores 4, 5 are respectively provided.
Is inserted, and the core 4 and the core 5 are fixed to each other.

[0003]

A step-up transformer of this type needs to be thin so that it can be mounted in a narrow gap. For this reason, the bobbin 2 has a flat rectangular cross section, and even if the number of turns is the same, the winding length becomes long, the conductor resistance increases, and the efficiency decreases. The conventional rectangular transformer has a limit of about 5 mm in terms of thinning, and there is a problem that copper loss is significantly increased when the thickness is further compressed to widen the width. In addition, since the secondary winding L ₂ has to be divided and wound, the winding work is complicated and the total volume is increased.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high efficiency, low profile, high frequency boosting transformer having a small copper loss.

[0005]

SUMMARY OF THE INVENTION The present invention is a high frequency boosting transformer in which a low voltage side primary winding and a high voltage side secondary winding are wound around a substantially cylindrical central leg of an E-shaped core. The secondary winding and the secondary winding are wound in multiple layers at positions adjacent to each other in the central axis direction of the central leg, and the winding widths of the secondary winding and the secondary winding are almost the same. It is 1/10 or less of the total number of turns of the winding, and the radius of the center leg is Rm
When both the winding widths of the primary winding and the secondary winding are W,
A feature is that the Rm / W is in the range of 0.6 to 1.7.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described with reference to FIGS. The core 10 having an E-shaped cross section has a flat plate portion 11, outer legs 12 integrally formed at both ends thereof, and a cylindrical central leg 13 integrally formed at the center of the flat plate portion 11. 20
Is a plastic bobbin provided with three flanges 21, two winding grooves 24 and 25, and a hollow winding shaft 22, as shown in FIG. The central leg 13 is inserted. The low-voltage side primary winding L ₁ is placed in the winding groove 24, and the high-voltage side secondary winding L ₂ is placed in the winding groove 25 adjacent in the central axis C direction of the central leg 13.
Are respectively wound in multiple layers. Above the core 10,
A flat core 30 having an I-shaped cross section is abutted and fixed. Although illustration is omitted, between the core 30 and the central leg 13,
Alternatively, by inserting a plastic film or providing a gap between the core 30 and the central leg 13 and the outer leg 12, magnetic saturation is less likely to occur.

As shown in FIG. 5, when the wire rod 8 is wound in multiple layers in the direction of the arrow, in each layer, the winding start portion and the winding end portion of the next layer are adjacent to each other. However, even if the voltage across the secondary winding L ₂ is ₂ kV,
For example, if the secondary winding L ₂ is wound in 20 layers, the voltage applied between the two is 2000/10, which is 200 V. Therefore, as in the conventional example in which the secondary winding L ₂ is divided into a plurality of stages with a collar, the overlapping wire materials are overlapped. The potential difference between the two becomes low and dielectric breakdown is prevented. Some high frequency boosting transformers for inverters have a small voltage of the secondary winding L ₂ of 1,000 V or less, but in order to reliably prevent dielectric breakdown, one layer of the secondary winding L ₂ is used. It is desirable that the number of turns per hit is 1/10 or less of the total number of turns of the secondary winding L ₂ .

Next, let us consider conditions under which the required inductance L ₀ and saturation current I ₀ can be obtained in the inductance element. Now, assuming that the number of turns of the coil is N and the magnetic resistance is R, the inductance is given by N ² / R, so R ≦ N ² / L ₀ ────────── (1) In addition, Supposing that the cross-sectional area of the magnetic circuit having an area is S and the saturation magnetic flux density is Bm, the saturation current is given by BmSR / N, so R ≧ I ₀ N / BmS ────────── (2) That is, The magnetic resistance R needs to be small in terms of the inductance L ₀ , and the magnetic resistance R needs to be large in terms of the saturation current I ₀ .

The number of turns N is plotted on the horizontal axis and the magnetic resistance R is plotted on the vertical axis.
FIG. 6 is a plot of the relationship between the equations (1) and (2). In the figure, the shaded area indicates the range that satisfies the inequalities of the equations (1) and (2). As shown in FIG. 6, the minimum number of turns that satisfies the required characteristics is N ₀ , and the minimum magnetic resistance is R ₀ . When the average magnetic path length is l and the magnetic permeability of the core is μ, the magnetic resistance R is generally given by R = 1 / μS. Therefore, if the cross-sectional area S is constant, the magnetic resistance R corresponds to the volume of the magnetic body. It is considered to be proportional. Therefore, N
_{It is considered that 0} and R ₀ are solutions that satisfy the required characteristics and minimize the volume of the inductance element. That is, N ₀ = L ₀ I ₀ / BmS ────── (3) R ₀ = L ₀ I ₀ ² / (BmS) ² ─── (4) is the optimum solution of the number of turns and the magnetic resistance.

Next, regarding the rectangular transformer as shown in FIG. 1 and the round transformer as shown in FIG. 3, the specifications of both transformers are defined as follows, and the respective characteristics are compared. Primary inductance 200μH or more Primary winding saturation current 1A or more Primary / secondary winding ratio 1:50 Primary winding diameter 0.25mmΦ Secondary winding diameter 0.07mmΦ Winding insulation voltage 100V _PP or less

The relative permeability μ _r of the magnetic material is 3000, the saturation magnetic flux density Bm is 0.3 T, and the minimum cross-sectional area S of the magnetic circuit is 15.
When N ₀ and R ₀ are calculated from the equations (3) and (4) as mm ² , N ₀ = L ₀ I ₀ / BmS = 200 × 10 ^-6 × 1 / (0.3 × 15 × 10 ^-6 ) = 45 (turns) R ₀ = L ₀ I ₀ ² / (BmS) ² = 200 × 10 ⁻⁶ × 1 ² /(0.3×15×10 ⁻⁶ ) ² = 9.88 (AT / Wb) Therefore, Since the primary winding has 45 turns and the winding ratio is 1:50, the secondary winding has 2250 turns. When the rectangular and round transformers were designed under the above conditions, the following results were obtained.

Comparison of square transformer and round transformer

[Table 1]

As is clear from Table 1, when the same performance is realized with the same magnetic material, the round transformer shape is advantageous in terms of copper loss and volume. In particular, when used as an inverter transformer, a large amplitude resonance current of several tens of kHz flows in the primary winding, and therefore reduction of copper loss in the primary winding is a factor that can greatly contribute to improvement in conversion efficiency of the transformer.

FIG. 7 shows a core 10 and a core 30, and a primary winding L.
₁ is a diagram showing a side cross section of only a primary winding L ₂ ; Figure 3
In such a transformer, the condition that the sum of the sectional areas of the central leg 13 and the primary winding L ₁ and the secondary winding L ₂ on the plane orthogonal to the central axis C of the central leg 13 is minimized, that is, in FIG. Next, the condition for minimizing the sum of the radius Rm of the center leg 13 and the winding width W of the primary winding and the secondary winding will be obtained. Now, the radius of the central leg 13 is Rm, and the turn ratio between the primary winding and the secondary winding is 1:
n, the thickness of the flat plate portion 11 of the core 10 and the core 30 is t, and the primary winding L
₁ of the height h _1, the secondary winding L ₂ of the height and h _2, the primary winding and each d ₁ diameter of the wire of the secondary winding,
Assuming d ₂ , the number of turns per layer of the primary winding L ₁ is h ₁
/ D ₁ and the number of turns per layer of the secondary winding L ₂ is h ₂ / d ₂
Becomes Therefore, the number of layers to be wound on each side is N ₀ d ₁ / h _{1 on} the primary side and nN ₀ d ₂ / h ₂ on the secondary side, but the height h of the secondary winding L ₂ and the primary winding L ₁ is ₂ , h ₁
By setting the ratio of h ₂ / h ₁ = nd ₂ ² / d ₁ ² ───── (5), the primary winding L ₁ and the secondary winding L ₂
The winding widths of are equal. If this winding width W is expressed by the specifications of the primary winding L ₁ , then W = N ₀ d ₁ ² / h ₁ ───────── (6)

The surface of the core extending from the side surface of the central leg 13 intersects with the flat plate portion 11, that is, a cylindrical portion having the same diameter as the central leg 13 and the same height as the thickness t of the flat plate portion 11, , I will call it a connecting part. In small and low profile transformers, the minimum cross-sectional area S of the magnetic circuit is generally limited by this joint of the core. In this case, since the area of the connecting portion of the core is equal to or smaller than the cross-sectional area of the center leg 13, 2πRmt ≦ πRm ² ∴Rm ≧ 2t ──────────────────────────────────── (7) Is S = 2πRmt ────────── (8).

Total cross-sectional area ΣS including the central leg 13 and the winding portion
Is ΣS = π (Rm + W) ² ─────────────────────────────────────────────────────────── (9) that the minimum of this be given by equation (9). Put P in parentheses, and P = Rm + W ─────────── (10) should be the minimum. Substituting W in equation (6) into this equation, P = Rm + d ₁ ² / h ₁ × N ₀ Therefore, from equation (3), P = Rm + d ₁ ² / h ₁ × (L ₀ I ₀ / BmS) Equation (8) Substituting S for P = Rm + (d ₁ ² L ₀ I ₀ / 2πBmh ₁ t) × 1 / Rm where K = d ₁ ² L ₀ I ₀ / 2πBmh ₁ t, P = Rm + K / Rm ─ (11) The value Rm ₀ of the radius Rm of the central leg 13 that minimizes P is dP /
From dRm = 0, dP / dRm = 1-K / Rm ² (12) Therefore, Rm ₀ = K ^1/2 ──────────── (13). From equations (11) and (12), the minimum value of P is P = K ^1/2 + K / K ^1/2 = 2K ^1/2 ── (14) That is, Rm = K ^1/2 , W = K ^{1 When / 2} , that is, when Rm = W, P becomes the minimum.

Now, assuming that W = K ^1/2 is fixed and Rm is X times this, Rm = X · K ^1/2 Substituting this into the equation (11), P = X · K ^1/2 + K / X · K ^1/2 = X · K ^1/2 + K ^1/2 / X ∴ P = K ^1/2 (X + 1 / X) ────── (15) Figure 8 shows the relationship between P and X. It is the plotted figure. P
Becomes minimum when X = 1, and increases with the curve of X + 1 / X with respect to X. As a practical range of the value of P, from the minimum value to plus 15%, the value of X at this time, that is, the ratio of Rm and W is 0.6 as shown in the figure.
The range is from to 1.7.

[0018]

According to the present invention, a high-frequency step-up transformer having a low profile and a small bottom area can be constructed, and copper wire loss is reduced by shortening the winding wire material. Further, since it is not necessary to divide the secondary winding into a plurality of stages, there is an effect that the winding process is simplified and the productivity is improved.

[Brief description of drawings]

FIG. 1 is an exploded perspective view of a conventional transformer

FIG. 2 is a front sectional view showing an embodiment of the transformer of the present invention.

FIG. 3 is a partially cutaway perspective view of the transformer main part.

FIG. 4 is a front sectional view of a bobbin in the transformer.

FIG. 5 is an explanatory view of a cross-section of a wire wound in multiple layers.

FIG. 6 is a diagram showing the relationship between the number of turns and the magnetic resistance.

FIG. 7 is a side sectional view showing the dimensional relationship between the core and the winding.

FIG. 8 is a diagram showing a relationship between X and P.

[Explanation of symbols]

10 core 13 center leg L ₁ primary winding L ₂ secondary winding

Claims (1)

[Claims] 1. A high-frequency step-up transformer in which a low-voltage side primary winding and a high-voltage side secondary winding are wound around a substantially cylindrical central leg of a core having an E-shaped cross section. Is wound in multiple layers at positions adjacent to each other in the central axis direction of the central leg, and the winding widths in the direction orthogonal to the central axis of the central leg are substantially the same, and When the number of turns per unit is 1/10 or less of the total number of turns of the secondary winding, the radius of the central leg is Rm, and the winding widths of the primary winding and the secondary winding are both W, Rm / W is A high-frequency boosting transformer characterized by being in the range of 0.6 to 1.7.