Magnetic core for transformer

Abstract

A magnetic core for a transformer, which includes a closed ring with a thick part and a thin part. The thin part is magnetically saturated before the thick part when excited by the same increasing magnetic fields. The thin part only operates briefly at or near first quadrant saturation point or a third quadrant saturation point and, for the rest of the time, it operates in a state between the first quadrant saturation point and the third quadrant saturation point. The present invention overcomes the drawbacks of the conventional magnetic core for a self-excitation push-pull type converter, and significantly improves the efficiency of the converter when it is under a light load, and further improves its efficiency while under a rated load. As the number of turns of the coil on the magnetic saturation transformer is reduced, the working frequency of the converter is improved while still keeping the loss low.

Claims

1. A magnetic core for a transformer, being of a closed ring shape and comprising a thick part and at least one thin part, said thin part has a sectional area less than 80% and greater than 4% of the sectional area of said thick part, said thin part reaches magnetic saturation before said thick part when excited under an increasing magnetic field, and the thin part only operates briefly at or near a first quadrant saturation point or a third quadrant saturation point and, for all other times, the thin part operates in a state between the first quadrant saturation point and the third quadrant saturation point, wherein a length of the thin part is negatively correlated to a ratio of the sectional area of the thick part to the sectional area of the thin part.

2. The magnetic core of claim 1, which comprises a plurality of thin parts connected in serial.

3. The magnetic core of claim 1, wherein said thin part has the length greater than 0.05 mm and less than one eighth of a length of the total magnetic path of the magnetic core.

4. The magnetic core of claim 1, where said thin part has the sectional area of a size less than 50% and greater than 6.25% of the sectional area of the said thick part.

5. The magnetic core of claim 1, wherein said thick part and said thin part are made of a same material.

6. The magnetic core of claim 1, further comprising a transition section between said thick part and said thin part to facilitate demoulding.

7. The magnetic core of claim 1, wherein said thick part has two or more salient points to prevent winding wires from sliding onto said thin part, or to ensure different windings in given areas.

8. The magnetic core of claim 1, wherein said thick part is wound with a coil, and said thin part is not wound with a coil.

9. The magnetic core of claim 1, wherein a proportion of the length of the thin part in a length of a total magnetic path of the magnetic core is y/(k1), wherein y is an inductance of the magnetic core reduced by the thin part, and k is the ratio of the sectional area of the thick part to the sectional area of the thin part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1-1 is a common application circuit diagram of the Royer circuit of self-excitation push-pull converter,

(2) FIG. 1-2 is a common application circuit diagram of the Jensen circuit of self-excitation push-pull converter;

(3) FIG. 2 is the output side waveform diagram of the Royer circuit winding Ns in FIG. 1-1;

(4) FIG. 3 is the square hysteresis loop of the magnetic core of transformer B1 in the Royer circuit of FIG. 1-1;

(5) FIG. 4 shows a generally known full-wave rectifying circuit;

(6) FIG. 5 is the ring type magnetic core of the existing technology,

(7) FIG. 6 is the principle diagram for conversion efficiency testing in general use in this article;

(8) FIG. 7 is the working area diagram of the magnetic core in a self-excitation push-pull converter;

(9) FIG. 8 is the working area diagram of the magnetic core corresponding to the thick part in this invention in a self-excitation push-pull converter,

(10) FIG. 9-1 is the front view of the magnetic core in embodiments 1 to 3 of this invention;

(11) FIG. 9-2 is the side view of the magnetic core in embodiments 1 to 3 of this invention;

(12) FIG. 9-3 is the top view of the magnetic core in embodiments 1 to 3 of this invention;

(13) FIG. 9-4 is the 3D view of the magnetic core in embodiments 1 to 3 of this invention;

(14) FIG. 10-1 is the schematic diagram of the magnetic path l.sub.w in the thick part of embodiments 1 to 3 of this invention;

(15) FIG. 10-2 is the schematic diagram of the magnetic path l.sub.t in the thin part of embodiments 1 to 3 of this invention;

(16) FIG. 11-1 is the front view of the magnetic core for comparison with the existing technology;

(17) FIG. 11-2 is the side view of the magnetic core for comparison with the existing technology;

(18) FIG. 11-3 is the top view of the magnetic core for comparison with the existing technology;

(19) FIG. 12 is the efficiency comparison diagram of embodiment 4 of this invention with the magnetic core of existing technology fitted into circuit;

(20) FIG. 13-1 is the front view of the magnetic core in embodiment 4 of this invention;

(21) FIG. 13-2 is the side view of the magnetic core in embodiment 4 of this invention;

(22) FIG. 13-3 is the top view of the magnetic core in embodiment 4 of this invention;

(23) FIG. 14 is the efficiency comparison diagram of embodiment 4 of this invention with the magnetic core of existing technology fitted into circuit;

(24) FIG. 15-1 is the front view of the magnetic core in embodiment 5 of this invention;

(25) FIG. 15-2 is the side view of the magnetic core in embodiment 5 of this invention;

(26) FIG. 15-3 is the top view of the magnetic core in embodiment 5 of this invention;

(27) FIG. 15-4 is the 3D view of the magnetic core in embodiment 5 of this invention;

(28) FIG. 16-1 is the front view of the magnetic core in embodiment 6 of this invention;

(29) FIG. 16-2 is the side view of the magnetic core in embodiment 6 of this invention;

(30) FIG. 16-3 is the top view of the magnetic core in embodiment 6 of this invention;

(31) FIG. 16-4 is the 3D view of the magnetic core in embodiment 6 of this invention;

(32) FIG. 17-1 is the front view of the magnetic core in embodiment 7 of this invention;

(33) FIG. 17-2 is the side view of the magnetic core in embodiment 7 of this invention;

(34) FIG. 17-3 is the top view of the magnetic core in embodiment 7 of this invention;

(35) FIG. 17-4 is the 3D view of the magnetic core in embodiment 7 of this invention;

(36) FIG. 18-1 is the front view of the magnetic core in embodiment 8 of this invention;

(37) FIG. 18-2 is the side view of the magnetic core in embodiment 8 of this invention;

(38) FIG. 18-3 is the top view of the magnetic core in embodiment 8 of this invention;

(39) FIG. 18-4 is the 3D view of the magnetic core in embodiment 8 of this invention;

(40) FIG. 19-1 is the front view of the magnetic core in embodiment 9 of this invention;

(41) FIG. 19-2 is the side view of the magnetic core in embodiment 9 of this invention;

(42) FIG. 19-3 is the top view of the magnetic core in embodiment 9 of this invention;

(43) FIG. 19-4 is the 3D view of the magnetic core in embodiment 9 of this invention;

EMBODIMENTS

Embodiment 1

(44) FIGS. 9-1 to 9-4 show the magnetic core of embodiment 1 of this invention, the magnetic core is a magnetic ring of closed magnetic path without air gap, the ring is in cylindrical shape, comprising a thick part and a thin part of the same material, the thin part has a length of over 0.05 mm, and less than one eighth of the total magnetic path length; and the sectional area of the thin part is below 80% and above 4% of the sectional area of the said thick part.

(45) To clearly demonstrate the effect of this embodiment, in embodiment 1 of this invention, the thick part has the same sectional area as that of the magnetic core in the existing technology, and the sectional area of the thin part is smaller than that of the magnetic core in the existing technology. The ratio of the thick part sectional area to thin part sectional area is the reciprocal of the percentage points in the technical plan, denoted as constant k, as the reciprocal of the above-mentioned below 80% and above 4%, it is 1.25 times to 25 times, FIG. 5 shows the ring type magnetic core of the existing technology, with a uniform sectional area 50 as shown, then, according to the existing generally known technology, the inductance of the coil wound over it is:

(46) $\begin{array}{cc}L=\frac{4iAe{N}^2}{l_e}& \mathrm{Formula}\left(3\right)\end{array}$
where: i is magnetic core relative permeability, Ae is the same as in formula, being the effective sectional area (cm.sup.2) of the magnetic core, N is the number of turns of the coil, l.sub.e is the magnetic path length (cm), and the perimeter of the circumference dotted line 51 in FIG. 5 is magnetic path length l.sub.e.

(47) FIGS. 9-1 to 9-4 show the structural diagram of the magnetic core in embodiment 1, FIG. 9-1 is the front view of the magnetic core in embodiment 1 of this invention; FIG. 9-2 is the side view of the magnetic core in embodiment 1 of this invention; FIG. 9-3 is the top view of the magnetic core in embodiment 1 of this invention; and FIG. 9-4 is the 3D view of the magnetic core in embodiment 1 of this invention. In FIG. 9-1, the circumference dotted line 51 is the length of the geometric magnetic path, which is actually divided into two sections, one is in the thick part, its length is denoted as l.sub.w, being the magnetic path inside the thick part 52 in FIG. 9-1, and the other section of the geometric magnetic path is within the thin part, its length is denoted as l.sub.t, being the magnetic path inside the thin part 53 in FIG. 9-1. FIG. 10-1 and FIG. 10-2 are respectively the schematic diagrams of the magnetic path in this invention, in FIG. 10-1, the dotted line 61 shows the magnetic path length 1, within the thick part, in FIG. 10-2, the dotted line 62 shows the magnetic path length l.sub.t within the thin part, then in embodiment 1 of this invention, the inductance of the coil wound on the thick part can be calculated using the Faraday's law, suppose the effective sectional area of the thin part is S.sub.1, then the effective sectional area of the thick part is kS.sub.1, substitute them into formula (3), the inductance L.sub.1 of the N-turn coil on the thick part is:

(48) $\begin{array}{cc}{L}_1=\frac{4i{\mathrm{kS}}_1{N}^2}{l_w+{\mathrm{kl}}_t}& \mathrm{Formula}\left(4\right)\end{array}$
where: i is the relative permeability of the magnetic core; S.sub.1 is the effective sectional area (cm.sup.2) of the thin part of the magnetic core, i.e. kS.sub.1 is effective sectional area of the thick part, the same as S in formula (1); N is the number of turns of coil; the denominator is the total equivalent length of the magnetic path (cm), being the sum of the equivalent length of the magnetic path l.sub.w in the thick part and magnetic path l.sub.e in the thin part, the effective sectional area of the thin part is 1/k that of the thick part, to make equivalent the magnetic path l.sub.t within the thin part to the effective sectional area of the thick part, according to the generally known theory, it should be multiplied with k, so the length is equivalent to kl.sub.t, i.e.:
l.sub.equivalent length=kl.sub.tFormula (5)

(49) The equivalent length of magnetic path lt is l.sub.t times its inherit length. FIGS. 11-1, 11-2 and 11-3 are respectively the front view, side view and top view of the magnetic core for comparison with the existing technology, to facilitate the description of the principle of this invention, the length of the geometric magnetic path 51 of the magnetic core for comparison with the existing technology in FIG. 11-1 is equal to the length of the geometric magnetic path of the circumference dotted line 51 of the magnetic core of this invention in FIG. 9-1, as the effective sectional area magnetic core for comparison with the existing technology in FIG. 11-1 is equal everywhere, the length of its geometric magnetic path 51 is equal to the actual magnetic path length, and the effective sectional area of the magnetic core in FIG. 11-1 is equal to the effective sectional area of the thick part of the magnetic core of this invention in FIG. 9-1, i.e. kS.sub.1, then the magnetic cores in FIGS. 11-1 to 11-3 have:
l.sub.e=l.sub.w+l.sub.tFormula (6)
Ae=k.sub.1Formula (7)

(50) When we substitute the above into formula (3), the inductance L.sub.2 of the coil with the same N turns as the magnetic core for comparison with the existing technology in FIG. 11-1 is:

(51) $\begin{array}{cc}{L}_2=\frac{4i{\mathrm{kS}}_1{N}^2}{l_w+l_t}& \mathrm{Formula}\left(8\right)\end{array}$

(52) Compare formula (4) with formula (8), we obtain:

(53) $\begin{array}{cc}\frac{L_1}{L_2}=.Math.\frac{4i{\mathrm{kS}}_1{N}^2}{l_w+{\mathrm{kl}}_t}.Math..Math.\frac{4i{\mathrm{kS}}_1{N}^2}{l_w+l_t}.Math.=\frac{l_w+l_t}{l_w+l_t}& \mathrm{Formula}\left(9\right)\end{array}$

(54) That is, the inductance of the transformer realized with the magnetic core of this invention and the same number of turns is:

(55) $\begin{array}{cc}{L}_1=\frac{l_w+l_t}{l_w+{\mathrm{kl}}_t}{L}_2& \mathrm{Formula}\left(10\right)\end{array}$

(56) According to formula (10), as the sectional area of the thick part is larger than that of the thin part, i.e. k is constantly greater than 1, the N-turn inductance L.sub.1 of the magnetic core in embodiment 1 of this invention is less than the N-turn inductance L.sub.2 of the magnetic core in the existing technology, as long as k is not taken too big, and the l.sub.t in the thin part is sufficiently short, the inductance L.sub.1 of the N-turn coil of the magnetic core in embodiment 1 of this invention can be quite close to L.sub.2, for example L.sub.1=0.99 L.sub.2, with this, due to the existence of the thin part, when the magnetic core of this invention is used in a self-excitation push-pull converter, it can still trigger the push-pull oscillation in the circuit, because of the similar inductance, and the working frequency changes very little, as the magnetic saturation only occurs in the thin part, the energy loss is low, the no-load input current is small, therefore the conversion efficiency of the self-excitation push-pull converter can be substantially improved.

(57) As the magnetic path lt of the thin part is the smaller the better, so that less part is required for the magnetic ring to reach magnetic saturation, and the loss can be reduced more apparently, the lower limit of the length of the thin part is taken as 0.05 mm because 0.05 mm is the limit of the current mould forming process or cutting process for magnetic cores, in fact, the purpose of this invention can be better realized with a size below 0.05 mm.

(58) The following is the process to prove the dimensional limits in the claims, up to formula (10-10), the methods and processes in this proof are academically initials not published before:

(59) With reference to formula (6), let x be the proportion of thin part length 53 in the total magnetic path length 51, we have:
l.sub.t=xl.sub.eFormula (10-1)
l.sub.e=(1x)l.sub.eFormula (10-2)

(60) According to formula (10), the N-turn inductance L.sub.1 of the magnetic core in embodiment 1 of this invention is smaller than the N-turn inductance L.sub.2 of the existing technology, a constant k is introduced, being a percentage, as it is convenient to express it in decimals, it is required that y be less than 0.1, and it can approach zero infinitely, suppose:
L.sub.t=(1y)L.sub.2Formula (10-3)

(61) Substitute formula (6), formula (10-1), formula (10-2) and formula (10-3) into formula (10):

(62) $\begin{array}{cc}\left(1-y\right)L_2=\frac{l_e}{\left(1-x\right)l_e+{\mathrm{kxl}}_e}{L}_2& \mathrm{Formula}\left(10\text{-}4\right)\end{array}$

(63) Simplify formula (10-4) by dividing out L.sub.2 on both left and right, and dividing out L.sub.e from the numerator and denominator on the right of the equation, we obtain:

(64) $\begin{array}{cc}1-y=\frac{1}{1-x+\mathrm{kx}}=\frac{1}{1+x\left(k-1\right)}& \mathrm{Formula}\left(10\text{-}5\right)\end{array}$

(65) Formula (10-5) is simplified as:

(66) 0$\begin{array}{cc}x\left(k-1\right)=\frac{1}{1-y}-1& \mathrm{Formula}\left(10\text{-}6\right)\end{array}$

(67) Because y is smaller and equal to 0.1, we have the following equations in engineering calculation:
1y.sup.21Formula (10-7)

(68) When y=0.1, formula (10-7) obtains 0.991, with an error of 1%, it has satisfied with the precision for engineering calculation, as y is smaller than and equal to 0.1, formula (10-7) can obtain:

(69) $\begin{array}{cc}\left(1-y\right)\left(1+y\right)1i.e.\text{:}& \mathrm{Formula}\left(10\text{-}8\right)\\ \frac{1}{1-y}1+y& \mathrm{Formula}\left(10\text{-}9\right)\end{array}$

(70) In formula (10-9), y is taken as the maximum value 0.1, then we have:
1.11.1

(71) That is, the circulating decimal 1.1 is approximately equal to 1.1, with an error of 1%, when y drops to 0.05, or by 5%, it is 1.052631.05, with an error of 0.25%, which is already quite accurate. Substitute formula (10-9) into formula (10-6), we have:

(72) $\begin{array}{cc}x\left(k-1\right)=\frac{1}{1-y}-11+y-1=yi.e.\text{:}yx\left(k-1\right)& \mathrm{Formula}\left(10\text{-}10\right)\end{array}$

(73) It can be seen from formula (10-3) that, in this invention, a smaller y value is preferable, as this will have a magnetic core inductance closer to the desired value, in this way, it is not necessary to increase the number of turns of the coil in embodiment 1 of this invention, and it is found in the experimental test that only when the sectional area of the thin part is below 80% of that of the said thick part, can it be ensured that no magnetic saturation will occur in the thick part when magnetic saturation occurs in the thin part. As transistors have a storage time, that is, after the transistor base has received a cutting off signal, the collector current will drop with a time delay till cut-off, the storage time can occur in FIG. 3, the moving line of the magnetic core working point will be from point D to E, correspondingly, the moving line of the magnetic core working point will move from point H to A. When the moving line of the magnetic core working point moves from point D to E, it will increase the magnetic flux in the magnetic core, which will result in saturation within the thick part with an area not quite different, therefore, only when the sectional area of the thin part is below 80% of that of the said thick part, can it be ensured that no magnetic saturation will occur in the thick part when the moving line of the afore-said magnetic core working point moves from point D to E. k is the reciprocal of the afore-said 80%, being 1.25.

(74) When k is taken as 1.25, if y in formula (10-10) is not greater than 3%, then:
x=y/(k1)=0.03/(1.251)=0.12
x is the proportion of the thin part length 53 in the total magnetic path length 51, and 0.12 is approximately one eighth, i.e. the said thin part has a length of over 0.05 mm, less than one eighth of the total magnetic path length.

(75) Of course, it is only an example with y as 3%, Table 2 below gives the tolerable deviation values commonly used in electronic engineering, all obtained using formula (10-10):

(76) TABLE-US-00002 TABLE 2 Percentage of thin part Expected X obtained from length in total magnetic value Y calculation path length 1% 0.04 One 25th 2% 0.08 One 12th 3% 0.12 One 8th 5% 0.2 One 5th 10% 0.4 About one 3rd

(77) It is verified in experimental test that a fairly good implementation effect can be obtained only when y is taken below 3% in this invention.

(78) In actual application, when the value exceeds 25, i.e. the sectional area of thick part is 25 times that of thin part, the thin part is likely to break as its strength cannot be guaranteed, and an air gap will form after break, making it impossible to use in a self-excitation push-pull converter, when the value exceeds 25, as the window area that can be wound with wire in the ring center is too small, it often requires extending the length of the geometric magnetic path 51 in FIG. 9-1 to implement it, it can be seen from formula (3) that, the extension of the length of the geometric magnetic path 51 can reduce the inductance, which in turn requires increasing the number of turns, therefore reducing the implementation effect.

(79) Therefore, the value of k is required to be 1.25 to 25 times, and then the sectional area of the thin part is below 80% and above 4% of that of the thick part.

(80) In the following, a set of measured data will demonstrate the actual effect of the magnetic core in embodiment 1. Take FIG. 1-1 as an example, the plans adopted in the existing technology and presented in background technology are identical.

(81) A circuit as shown in FIG. 1-1 is used to make a converter with input DC at 5V, output DC at 5V and output current of 200 mA, i.e. with an output power of 1 W. The downstream output of the transformer is in the circuit structure as shown in FIG. 4, which is a generally known full-wave rectifying circuit. The main parameters of the circuit are: the capacitor C is 1 uF, resistor R1 is 1K, capacitor C1 is 0.047 uF, and TR1 and TR2 are switching transistors with magnification factor of about 200, with its maximum collector working current being IA; the primary side coils N.sub.P1 and N.sub.P2 have respectively 20 turns, feedback coils N.sub.B1 and N.sub.B2 respectively 3 turns, secondary side coils N.sub.S1 and N.sub.S2 respectively 23 turns, and the magnetic core is a common ferrite loop magnetic core with an outer diameter of 5 mm and sectional area of 1.5 mm.sup.2.

(82) With the above parameters set, at an output of 10 mA as 5% of the full load of 200 mA of the output current, the measured parameters are as shown in Table 1 of background technology, with an efficiency of 38.03%.

(83) In embodiment 1 of this invention, the magnetic core has an outer diameter of 5 mm, with the sectional area of thick part being 1.5 mm.sup.2 and that of thin part being 1.2 mm.sup.2, i.e. the k value is 1.25, and the thin part has a length of 1 mm. No coil will be made on the thin part, a transformer as shown in FIG. 1-1 is made with the same number of turns, when the transformer made as per embodiment 1 of this invention is connected into the circuit, with the output current at 10 mA, or 5% of the full load 200 mA, the efficiency is tested also using the circuit in FIG. 6, and in conjunction with Table 1, the measured data are as shown in Table 3:

(84) TABLE-US-00003 TABLE 3 Input Input Output Output Efficiency current voltage current voltage (calculated Iin Vin Iout Vout value) Actually measured with 28.4 mA 5.060 V 9.96 mA 5.487 V 38.03% existing technology Actually measured with the 25.6 mA 5.066 V 9.95 mA 5.482 V 42.05% transformer made as per embodiment 1 of this invention in the circuit

(85) Obviously, after using this invention, the efficiency of the self-excitation push-pull converter when working with low load has been substantially increased, by (42.05%38.03%)=4.02%.

(86) Further, tests were made from low load to full load, and records were made in Table 4:

(87) TABLE-US-00004 TABLE 4 Load Iin Vin Iout Vout Efficiency rate Product (mA) (V) (mA) (V) Efficiency increase 5% Existing technology 28.4 5.060 9.96 5.487 38.03% 4.02% This invention 25.6 5.066 9.95 5.482 42.05% 100% Existing technology 242.0 5.024 198.00 5.094 82.96% 1.54% This invention 239.1 5.025 199.00 5.102 84.50%

(88) Obviously, after using this invention, the efficiency of the self-excitation push-pull converter when working with both low load and full load has been increased, as the value k is small, ordinary effect is obtained.

(89) The no-load current of the self-excitation push-pull converter has been reduced from 18.0 mA of the existing technology to 14.1 mA of this invention, i.e. the no-load loss has reduced from 90 mW of the existing technology to 70.5 mW of this invention.

Embodiment 2

(90) FIGS. 9-1 to 9-4 show the structure of the magnetic core of embodiment 2 of this invention, the magnetic core is a magnetic ring of closed magnetic path without air gap, the ring is in cylindrical shape, consisting of a thick part and a thin part of the same material, the thin part has a length of over 0.05 mm, and less than one eighth of the total magnetic path length; and the sectional area of the thin part is below 80% and above 4% of the sectional area of the said thick part.

(91) To clearly demonstrate the effect of this embodiment, in the magnetic core of embodiment 2 of this invention, the sectional area of the thin part is 4% that of the thick part, i.e. in embodiment 2 of this invention, the magnetic core has an outer diameter of 5 mm, with the sectional area of thick part being 1.5 mm.sup.2 and that of thin part being 0.06 mm.sup.2, actually, the thin part 53 in FIG. 9-1 is cut using cutting technique, the diameter of the thin part is 0.2760.02 mm, i.e. the k value is 25, due to the restriction of the thickness of the ring cutter blade, the length of the thin part is 0.15 mm, and it cannot be further reduced. No coil will be made on the thin part, a transformer as shown in FIG. 1-1 is made with the same number of turns, when the transformer made as per embodiment 1 of this invention is connected into the circuit, with the output current at 10 mA, or 5% of the full load 200 mA, the efficiency is tested also using the circuit in FIG. 6, and in conjunction with Table 1, the measured data are as shown in Table 5:

(92) TABLE-US-00005 TABLE 5 Input Input Output Output Efficiency current voltage current voltage (calculated Iin Vin Iout Vout value) Actually measured with 28.4 mA 5.060 V 9.96 mA 5.487 V 38.03% existing technology Actually measured with the 15.5 mA 5.066 V 9.95 mA 5.484 V 69.49% transformer made as per embodiment 2 of this invention in the circuit

(93) Obviously, after using this invention, the efficiency of the self-excitation push-pull converter when working with low load has been substantially increased, by (69.49%38.03%)=31.46%. This is the efficiency measured with an output current of 10 mA, or output power of 50 mW.

(94) Further, test was performed with an output current of 20 mA, with the corresponding load rate being (20+200)100%, and the results are recorded in Table 6:

(95) TABLE-US-00006 TABLE 6 Load Iin Vin Iout Vout Efficiency rate Product (mA) (V) (mA) (V) Efficiency increase 5% Existing technology 28.4 5.060 9.96 5.487 38.03% 31.46% This invention 15.5 5.066 9.95 5.484 69.49% 10% Existing technology 40.0 5.045 20.00 5.424 53.76% 27.97% This invention 26.3 5.052 20.00 5.430 81.73%

(96) Obviously, after using this invention, the conversion efficiency of the self-excitation push-pull converter when working with low load below 100 mW, or with an output current below 20 mA has been obviously increased, as a very high value k is taken, the effect is substantial. However, as the thin part diameter is 0.2760.02 mm, it is quite difficult to make it, quite easy to break in experiment, and the finished product rate is extremely low.

(97) The no-load current of the self-excitation push-pull converter has been reduced from 18.0 mA of the existing technology to 4.8 mA of this invention, i.e. the no-load loss has reduced from 90 mW of the existing technology to 24 mW of this invention.

Embodiment 3

(98) The magnetic core shown in structural drawings FIGS. 9-1 to 9-4 are still used in embodiment 3, the magnetic core is a magnetic ring of closed magnetic path without air gap, the ring is in cylindrical shape, consisting of a thick part and a thin part of the same material, the thin part has a length of over 0.05 mm, and less than one eighth of the total magnetic path length; and the sectional area of the thin part is below 80% and above 4% of the sectional area of the said thick part.

(99) In the two embodiments above, the implementation effect is demonstrated with two extreme values of k, to clearly present the effect of this embodiment, in embodiment 3, the median value 2 is taken for constant k. In the application of embodiment 1 of this invention, the magnetic core has an outer diameter of 5 mm, with the sectional area of thick part being 1.5 mm.sup.2 and that of thin part being 0.75 mm.sup.2, i.e. the k value is 2, and the thin part has a length of 1 mm. No coil will be made on the thin part, a transformer as shown in FIG. 1-1 is made with the same number of turns, when the transformer made as per embodiment 1 of this invention is connected into the circuit, with the output current at 10 mA, or 5% of the full load 200 mA, the efficiency is tested also using the circuit in FIG. 6, and in conjunction with Table 1, the measured data are as shown in Table 7:

(100) TABLE-US-00007 TABLE 7 Input Input Output Output Efficiency current voltage current voltage (calculated Iin Vin Iout Vout value) Actually measured with 28.4 mA 5.060 V 9.96 mA 5.487 V 38.03% existing technology Actually measured with the 23.5 mA 5.066 V 9.95 mA 5.480 V 45.80% transformer made as per embodiment 3 of this invention in the circuit

(101) Obviously, after using this invention, the efficiency of the self-excitation push-pull converter when working with low load has been substantially increased, by (45.80%38.03%)=7.77%.

(102) Further, tests were made over the full range from low load to full load at steps of 5%, and at steps of 10% when the load rate is over 40%, records were made in Table 8:

(103) TABLE-US-00008 TABLE 8 Load Iin Vin Iout Vout Efficiency rate Product (mA) (V) (mA) (V) Efficiency increase 5% Existing technology 28.4 5.060 9.96 5.487 38.03% 7.77% This invention 23.5 5.066 9.95 5.480 45.80% 10% Existing technology 40.0 5.045 20.00 5.424 53.76% 7.65% This invention 35.0 5.052 20.00 5.429 61.41% 15% Existing technology 51.1 5.031 30.10 5.381 63.00% 6.21% This invention 46.5 5.037 30.10 5.386 69.22% 20% Existing technology 62.7 5.016 40.00 5.341 67.93% 6.09% This invention 57.7 5.022 40.10 5.349 74.02% 25% Existing technology 73.7 5.002 50.00 5.305 71.95% 4.86% This invention 69.2 5.008 50.10 5.313 76.81% 30% Existing technology 85.1 4.987 59.90 5.269 74.37% 4.73% This invention 80.2 4.994 60.00 5.280 79.10% 35% Existing technology 96.3 4.973 69.90 5.236 76.42% 4.09% This invention 91.6 4.978 70.00 5.245 80.52% 40% Existing technology 107.6 4.960 79.70 5.202 77.68% 4.02% This invention 102.7 4.965 79.90 5.214 81.70% 50% Existing technology 130.1 4.931 99.70 5.138 79.85% 3.24% This invention 125.4 4.936 99.90 5.148 83.09% 60% Existing technology 153.8 4.900 120.50 5.073 81.11% 2.98% This invention 148.7 4.907 120.70 5.084 84.10% 70% Existing technology 174.9 4.873 139.40 5.013 81.99% 2.48% This invention 170.1 4.879 139.60 5.022 84.47% 80% Existing technology 199.8 4.847 161.80 4.950 82.70% 2.16% This invention 195.0 4.845 161.90 4.952 84.86% 90% Existing technology 220.0 5.030 180.00 5.131 83.46% 2.10% This invention 215.0 5.032 180.20 5.137 85.56% 100% Existing technology 242.0 5.024 198.00 5.094 82.96% 2.28% This invention 237.0 5.025 199.00 5.101 85.24%

(104) Obviously, after using this invention, the conversion efficiency of the self-excitation push-pull converter in the whole range from low load and full load has been significantly increased, a comparison chart is made using software, refer to FIG. 12, in which curve 2# is the conversion efficiency curve of the self-excitation push-pull converter after using this invention, and curve 1# is the conversion efficiency curve of the self-excitation push-pull converter with the existing technology.

(105) The no-load current of the self-excitation push-pull converter has also been reduced from 18.0 mA of the existing technology to 12.0 mA of this invention, i.e. the no-load loss has reduced from 90 mW of the existing technology to 60 mW of this invention, or by 30 mW with each product.

(106) According to incomplete statistics, there are at least 1 billion micro power modules with self-excitation push-pull converter currently in use, if they all use the technical plan of this invention, they can save power of over 30 million kWh per hour.

(107) This invention has not only solved the technical issue to be solved in working principle, the above-mentioned many formula derivatives published for the first time also give powerful theoretical support to this invention, meanwhile, the experimental justification has proved that this invention can be fully used in industrial applications to produce the expected effect. Since the self-excitation push-pull converter was made public in 1955, after development and innovation over half a century and more, no one has been able to use the technical means of this invention to solve the technical issue to be solved with this invention, it is because of the insufficient understanding of the theoretical aspect of this circuit that no one has realized that a simple improvement as this invention can be made to produce substantial effects.

Embodiment 4

(108) The transformer used in the self-excitation push-pull converter in embodiment 4 of this invention is slightly different from that in embodiments 1, 2 and 3, but they are identical in essence, the magnetic core is a magnetic ring of closed magnetic path without air gap, the ring is in cylindrical shape, consisting of a thick part and a thin part of the same material, the thin part has a length of over 0.05 mm, and less than one eighth of the total magnetic path length; and the sectional area of the thin part is below 50% and above 6.25% of the sectional area of the said thick part.

(109) In embodiment 1, the thick part of the magnetic core has the same sectional area of magnetic core of the existing technology, but the sectional area of the thin part is smaller than that of the existing technology, at a ratio of 1/k. To fully demonstrate the effect of this embodiment, in the transformer magnetic core used in embodiment 4 of this invention, the sectional area of the thin part is equal to that of the existing technology, that is, the sectional area of the thick part is k times that of the existing technology.

(110) FIGS. 11-1, 11-2 and 11-3 are respectively the front view, side view and top view of the magnetic core for comparison of the existing technology with that in embodiment 4 of this invention, suppose the sectional area of the magnetic core of the existing technology is S.sub.2, when we substitute it into formula (3), the inductance L.sub.3 of the coil with the same N turns as the magnetic core for comparison with the existing technology in FIG. 11-1 is:

(111) $\begin{array}{cc}{L}_3=\frac{4i{S}_2{N}^2}{l_e}& \mathrm{Formula}\left(11\right)\end{array}$

(112) The magnetic core of embodiment 4 of this invention is as shown in FIGS. 13-1 to 13-3, FIG. 13-1 is the front view of the magnetic core in embodiment 4 of this invention; FIG. 13-2 is the side view of the magnetic core in embodiment 4 of this invention; FIG. 13-3 is the top view of the magnetic core in embodiment 4 of this invention; in the magnetic core of embodiment 4, there is a small thin part 53 with the same sectional area as the magnetic core for comparison with the existing technology, i.e. the sectional area of the thin part 53 of the magnetic core in FIG. 13-1 is equal to the above-mentioned S.sub.2, but its length is very short; correspondingly, the sectional area of the thick part 52 in FIG. 13-1 is larger than the sectional area of the magnetic core of the existing technology, equal to kS.sub.2, the ratio is the reciprocal of the percentage in the technical plan, denoted as constant k, for which reciprocal is taken as for the technical plan above, being 1.25 to 25 times, accordingly, the inductance per turn of the coil on the thick part increases, and the inductance L.sub.4 with the same number of turns of coil on the magnetic core of embodiment 4 of this invention is:

(113) $\begin{array}{cc}{L}_4=\frac{4i{\mathrm{kS}}_2{N}^2}{l_w+{\mathrm{kl}}_t}& \mathrm{Formula}\left(12\right)\end{array}$
where: i is the relative permeability of the magnetic core; S.sub.2 the effective sectional area (cm) of the thin part of the magnetic core, i.e. kS.sub.2 is effective sectional area of the thick part, the same as S in formula (1); N is the number of turns of coil; the denominator is the total equivalent length of the magnetic path (cm), being the sum of the equivalent length of the magnetic path l.sub.w in the thick part and magnetic path l.sub.t in the thin part, as the effective sectional area of the thin part is 1/k that of the thick part, to make equivalent the magnetic path l.sub.t within the thin part to the effective sectional area of the thick part, it should be multiplied by k, so the length is equivalent to kl.sub.t;

(114) Compare formula (12) with formula (11), we obtain:

(115) $\begin{array}{cc}\frac{L_4}{L_3}=.Math.\frac{4i{\mathrm{kS}}_2{N}^2}{l_w+{\mathrm{kl}}_t}.Math..Math.\frac{4i{S}_2{N}^2}{l_e}.Math.=\frac{{\mathrm{kl}}_e}{l_w+{\mathrm{kl}}_t}& \mathrm{Formula}\left(13\right)\end{array}$

(116) That is, the inductance of the transformer realized with the magnetic core of embodiment 4 of this invention and the same number of turns is:

(117) $\begin{array}{cc}{L}_4=k\left(\frac{l_e}{l_w+{\mathrm{kl}}_t}{L}_3\right)& \mathrm{Formula}\left(14\right)\end{array}$

(118) According to formula (14), if the magnetic path length in the thin part is sufficiently short, for instance close to 0.05 mm, the sum of the product kl.sub.t and magnetic path l.sub.w in the thick part will approach the magnetic path length l.sub.e of the magnetic core for comparison with the existing technology in FIG. 11-1, i.e. the inductance of the N-turn coil of the magnetic core of this invention L.sub.4kL.sub.3.

(119) With the same number of turns, the inductance can be increased by about k times, that means the number of turns can be reduced as appropriate to realize the same inductance as that with the background technology; in other words, the number of turns in this invention can be reduced as appropriate, and the ratio n of the number of turns with the existing technology to that of this invention is:

(120) $\begin{array}{cc}n=\sqrt{\frac{1}{k}}& \mathrm{Formula}\left(15\right)\end{array}$
i.e., when K is taken as 25, n=1/5=0.2, for the product with input voltage of 24V as mentioned in the background technology, a good working efficiency can be obtained with 96 turns, but with this invention, it only requires 960.2=19.2 turns, or rounded as 20 turns, to realize the same effect. That is why in the technical requirement, the sectional area of the said thin part length is below 80% and above 4% of the sectional area of the said thick part. In the above corresponding to embodiment 1, it is mentioned that: when the value exceeds 25, as the window area in the ring center is too small, it often requires extending the length of the geometric magnetic path 51 in FIG. 9-1 to implement it, it can be seen from formula (3) that, the extension of the length of the geometric magnetic path 51 can reduce the inductance, which in turn requires increasing the number of turns, therefore reducing the implementation effect.

(121) Similarly, when k is taken as 16, n=1/4=0.25, so the number of turns can be reduced to of the original, making it easy to wind it.

(122) In FIG. 13-1, due to the existence of the thin part 53, when the magnetic core of this invention is used in a self-excitation push-pull converter, it can still trigger the push-pull oscillation in the circuit, as the magnetic saturation only occurs in the thin part 53, which is fairly short, the energy loss is low, i.e. the no-load input current of the circuit of self-excitation push-pull converter is small, therefore the conversion efficiency of the self-excitation push-pull converter can be substantially improved; as the energy loss is low, the working frequency of the self-excitation push-pull converter can be further increased, and the resulted benefit is: the number of turns wound on the magnetic core of embodiment of this invention can be further reduced. In the following, a set of measured data will demonstrate the actual effect of the magnetic core in embodiment 4.

(123) In the plan for comparison with existing technology, the plan adopted is the same as that in the background technology and presented in embodiment 1, and it is quoted below for convenience in comparison:

(124) A circuit as shown in FIG. 1-1 is used to make a converter with input DC at 5V, output DC at 5V and output current of 200 mA, i.e. with an output power of 1 W. The downstream output of the transformer is in the circuit structure as shown in FIG. 4, which is a generally known full-wave rectifying circuit. The main parameters of the circuit are: the capacitor C is 1 uF, resistor R1 is 1K, capacitor C1 is 0.047 uF, and TR1 and TR2 are switching transistors with magnification factor of about 200, with its maximum collector working current being 1 A; the primary side coils N.sub.P1 and N.sub.P2 have respectively 20 turns, feedback coils N.sub.B1 and N.sub.B2 respectively 3 turns, secondary side coils N.sub.S1 and N.sub.S2 respectively 23 turns, and the magnetic core is a common ferrite loop magnetic core with an outer diameter of 5 mm and sectional area of 1.5 mm.sup.2.

(125) With the above parameters set, at an output of 10 mA as 5% of the full load of 200 mA of the output current, the measured parameters are as shown in Table 1 of background technology, with an efficiency of 38.03%. For other parameters, refer to the part of corresponding existing technology in Tables 7 and 8.

(126) In embodiment 4 of this invention, the magnetic core has an outer diameter of 5 mm, with the sectional area of thick part being 3 mm.sup.2 and that of thin part being 1.5 mm.sup.2, i.e. the k value is 2, and the thin part has a length of 0.5 mm. No coil will be made on the thin part, the primary side coils N.sub.P1 and N.sub.P2 have respectively 7 turns, the feedback coils N.sub.B1 and N.sub.B2 respectively 2 turns, and the secondary side coils N.sub.S1 and N.sub.S2 respectively 8 turns, when the transformer made as per embodiment 1 of this invention is connected into the circuit, the measured working frequency of the circuit is 139 kHz, with a no-load input current of 6.9 mA.

(127) When the output current is 5% of the full load 200 mA, or 10 mA, the efficiency is tested also using the circuit in FIG. 6, and in conjunction with Table 1, the measured data are as shown in Table 9:

(128) TABLE-US-00009 TABLE 9 Input Input Output Output Efficiency current voltage current voltage (calculated Iin Vin Iout Vout value) Actually measured with 28.4 mA 5.060 V 9.96 mA 5.487 V 38.03% existing technology Actually measured with the 17.5 mA 5.066 V 9.95 mA 5.478 V 61.48% transformer made as per embodiment 4 of this invention in the circuit

(129) Obviously, after using this invention, the efficiency of the self-excitation push-pull converter when working with low load has been substantially increased, by (61.48%38.03%)=23.45%.

(130) Further, tests were made over the full range from low load to full load at steps of 5%, and at steps of 10% when the load rate is over 40%, records were made in Table 10:

(131) TABLE-US-00010 TABLE 10 Load Iin Vin Iout Vout Efficiency rate Product (mA) (V) (mA) (V) Efficiency increase 5% Existing technology 28.4 5.060 9.96 5.487 38.03% 23.45% This invention 17.5 5.066 9.95 5.478 61.48% 10% Existing technology 40.0 5.045 20.00 5.424 53.76% 20.58% This invention 28.9 5.052 20.00 5.427 74.34% 15% Existing technology 51.1 5.031 30.10 5.381 63.00% 16.83% This invention 40.3 5.037 30.10 5.384 79.84% 20% Existing technology 62.7 5.016 40.00 5.341 67.93% 15.12% This invention 51.4 5.022 40.10 5.346 83.05% 25% Existing technology 73.7 5.002 50.00 5.305 71.95% 12.70% This invention 62.8 5.008 50.10 5.314 84.65% 30% Existing technology 85.1 4.987 59.90 5.269 74.37% 11.67% This invention 73.7 4.994 60.00 5.278 86.04% 35% Existing technology 96.3 4.973 69.90 5.236 76.42% 10.31% This invention 85.0 4.978 70.00 5.243 86.74% 40% Existing technology 107.6 4.960 79.70 5.202 77.68% 9.58% This invention 96.1 4.965 79.90 5.211 87.26% 50% Existing technology 130.1 4.931 99.70 5.138 79.85% 8.02% This invention 118.5 4.936 99.90 5.145 87.87% 60% Existing technology 153.8 4.900 120.50 5.073 81.11% 7.21% This invention 141.5 4.907 120.70 5.081 88.33% 70% Existing technology 174.9 4.873 139.40 5.013 81.99% 6.31% This invention 162.7 4.879 139.60 5.021 88.30% 80% Existing technology 199.8 4.847 161.80 4.950 82.70% 5.60% This invention 187.4 4.845 161.90 4.952 88.30% 90% Existing technology 220.0 5.030 180.00 5.131 83.46% 4.84% This invention 208.2 5.032 180.20 5.134 88.31% 100% Existing technology 242.0 5.024 198.00 5.094 82.96% 5.04% This invention 229.1 5.025 199.00 5.091 88.00%

(132) Apparently, after using this invention, the conversion efficiency of the self-excitation push-pull converter in the whole range from low load and full load has been obviously increased, a comparison chart is made using software, refer to FIG. 14, in which curve 2# is the conversion efficiency curve of the self-excitation push-pull converter after using this invention, and curve 1# is the conversion efficiency curve of the self-excitation push-pull converter with the existing technology.

(133) The no-load current of the self-excitation push-pull converter has been reduced from 18.0 mA of the existing technology to 6.9 mA of this invention, i.e. the no-load loss has reduced from 90 mW of the existing technology to 34.5 mW of this invention. At the same time, the working frequency has increased from 97.3 kHz with the existing technology to 139 kHz in embodiment 2 of this invention. The resulted benefit is to reducing the number of turns of the primary side coils N.sub.P1 and N.sub.P2 respectively from 20 to 7, reducing the work time in winding, and also avoiding mistakes.

(134) It can be seen from Table 6 that, at a load of 10%, i.e. an output current of 20 mA, this invention still has an efficiency of 74%, if the magnetic core size is reduced to design a specific micro power DC/DC converter, the efficiency can be further improved. In summary of the above, the overall implementation effect of embodiment 4 is good.

Embodiment 5

(135) FIGS. 15-1 to 15-4 present embodiment 5 of this invention, FIG. 15-1 is the front view of the magnetic core in embodiment 5 of this invention; FIG. 15-2 is the side view of the magnetic core in embodiment 5 of this invention; FIG. 15-3 is the top view of the magnetic core in embodiment 5 of this invention; and FIG. 15-4 is the 3D view of embodiment 5 of this invention; there is also a small thin part 53 with smaller sectional area of magnetic core, on the cylindrical magnetic ring, a cut is made symmetrically to form a flake thin part 53, with a length of over 0.05 mm, and less than one eighth of the total magnetic path length; the sectional area of the thin part is below 80% and above 4% of the sectional area of the thick part. The working principle is identical to that in the above description of the invention and in embodiments 1 to 4, so it will not be repeated here.

Embodiment 6

(136) FIGS. 16-1 to 16-4 present embodiment 6 of this invention, FIG. 16-1 is the front view of the magnetic core in embodiment 6 of this invention; FIG. 16-2 is the side view of the magnetic core in embodiment 6 of this invention; FIG. 16-3 is the top view of the magnetic core in embodiment 6 of this invention; and FIG. 16-4 is the 3D view of embodiment 6 of this invention; there is also a small thin part 53 with smaller sectional area of magnetic core, the thick part 52, and the further improved features of embodiment 6: between the thick part and thin part there is a transition section 54, which can be equivalent as part of the thin part, or be regarded as there are three thin parts in this embodiment, the sectional area of the transition section is changing from big to small, in a differential point of view, actually there are countless number of thin parts, the transition section 54 is provided to facilitate demoulding of the magnetic core after magnetic powder moulding, and actually it is a further improvement to embodiment 1 in FIGS. 9-1 to 9-4. When there is one thin part 53 and two symmetrical thin parts 54 in this embodiment, the sectional area of the thin part 53 and two symmetrical thin parts 54 is not equal, then the thin part 53 with the smallest sectional area functions, and the sectional area of the thin part 53 with the smallest sectional area is below 80% and above 4% of the sectional area of the thick part. The inside of the magnetic core corresponding to thin part 54 will not become magnetic saturated, therefore it will not participate in the magnetic saturation.

(137) Similarly, it is required that the length of the thin part and transition section 54 be short, and the sum of the transition section and the said thin part length be over 0.05 mm and less than one eighth of the total magnetic path length. The working principle is identical to that in the above description of the invention and in embodiments 1 to 4, so it will not be repeated here. Due to the presence of transition section 54, the length of thin part 53 can be zero, and in this case, there is still a part with the minimum sectional area, and this part can reach magnetic saturation first, so the purpose of the invention can still be realized.

Embodiment 7

(138) FIGS. 17-1 to 17-4 present embodiment 7 of this invention, FIG. 17-1 is the front view of the magnetic core in embodiment 7 of this invention; FIG. 17-2 is the side view of the magnetic core in embodiment 7 of this invention; FIG. 17-3 is the top view of the magnetic core in embodiment 7 of this invention; and FIG. 17-4 is the 3D view of embodiment 7 of this invention.

(139) The improvement feature of embodiment 7 of this invention is: on the basis of embodiment 6, two or more salient points 55 are added on the thick part, to prevent the wire on the thick part from sliding to the thin part, and salient points 55 can be at any position on the thick part. Another function of salient points 55 is to determine the zone of different windings, to prevent their mutual intersection.

(140) As there is also a small thin part 53 with smaller sectional area of magnetic core, the thick part 52, and the transition section 54 between the thick part and thin part, the transition section 54 can be equivalent as part of the thin part, the transition section 54 is provided to facilitate demoulding of the magnetic core after magnetic powder moulding, and actually it is a further improvement to embodiment 6 in FIGS. 16-1 to 16-4.

(141) Similarly, it is required that the length of the thin part and the transition section 54 be short. The working principle is identical to that in the above description of the invention and in embodiments 1 to 4, so it will not be repeated here. Due to the presence of transition section 54, the length of thin part 53 can be zero, and the purpose of the invention can still be realized.

Embodiment 8

(142) FIGS. 18-1 to 18-4 present embodiment 8 of this invention, FIG. 18-1 is the front view of the magnetic core in embodiment 8 of this invention; FIG. 18-2 is the side view of the magnetic core in embodiment 8 of this invention; FIG. 18-3 is the top view of the magnetic core in embodiment 8 of this invention; and FIG. 18-4 is the 3D view of embodiment 8 of this invention; the air gap free magnetic path closed magnetic ring consists of the flat thick part 52 and thin part 53 of the same material, the thin part 53 has a length of over 0.05 mm, and less than one eighth of the total magnetic path length; the sectional area of the thin part 53 is below 80% and above 4% of the sectional area of the thick part 52.

(143) The working principle is identical to that in the above description of the invention and in embodiments 1 to 4, so it will not be repeated here.

Embodiment 9

(144) FIGS. 19-1 to 19-4 present embodiment 9 of this invention, FIG. 19-1 is the front view of the magnetic core in embodiment 9 of this invention; FIG. 19-2 is the side view of the magnetic core in embodiment 9 of this invention; FIG. 19-3 is the top view of the magnetic core in embodiment 9 of this invention; and FIG. 19-4 is the 3D view of embodiment 9 of this invention; there is also a small thin part 53 of the magnetic core with smaller sectional area, and a thick part 52.

(145) In embodiment 9, a transition section 54 exists between the thick part and thin part, the transition section 54 can be equivalent as part of the thin part, the transition section 54 is provided to facilitate demoulding of the magnetic core after magnetic powder moulding, and actually it is a further improvement to embodiment 8 in FIGS. 18-1 to 18-4. Due to the presence of transition section 54, the length of thin part 53 can be zero, and the purpose of the invention can still be realized.

(146) The working principle of embodiment 9 is identical to that in the above description of the invention and in embodiments 1 to 4, so it will not be repeated here.

(147) The above are only preferable embodiments of this invention, and it should be pointed out that, the preferable embodiments above should not be regarded as restrictions to this invention, and the scope of protection for this invention shall be that defined by the claims. For ordinary technical personnel in this technological field, within the essence and scope of this invention, some improvements and decorations can be made, and such improvements and decorations shall also be covered in the scope of protection of this invention. For example, the said thin part and thick part can be embodied by using magnetic rings of sectional area in different geometric shapes, or the profile of the whole magnetic core as aforesaid can be embodied with a square or elliptic magnetic ring.