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(11) | EP 0 981 141 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | High-voltage transformer |
| (57) A high-voltage transformer comprises an E-shaped core (100) and an I-shaped core.
A high-voltage secondary winding (310) is located around the root of the central arm
(111) of the E-shaped core (100). A low-voltage secondary winding (320) is also located
around the central arm (111) and is separated from the high-voltage secondary winding
(310) by magnetic shunts. The primary winding (300) is wound around the free end of
the central arm (111) adjacent to the second secondary winding (320). This arrangement has been found to reduce radiated noise when used to drive the magnetron of a microwave oven. |
[0001] The present invention relates to a high-voltage transformer comprising a primary winding, a first secondary winding spaced from the primary winding and a second secondary winding.
[0002] Transformers are used in microwave ovens to raise the mains voltage to a level suitable for driving a magnetron.
[0003] In such a microwave oven, 110V/220V/240V mains AC is applied across the primary winding of a high-voltage transformer. The turns ratio of the high-voltage transformer is such that approximately 4000V is produced by a half-wave doubler from output of the transformer's secondary winding. This high voltage is then applied to a magnetron which generates microwaves for cooking food.
[0005] Figure 1 is a perspective view of a known microwave oven. Referring to Figure 1, the microwave oven comprises a body 20, a door 21, a cooking chamber 22, an electrical component chamber 23, a rotatable tray 24, a control panel 25, a lamp 26, a fan motor 27 and an air guide 28.
[0006] The body 20 houses the cooking chamber 22 and the device chamber 23 which are respectively located at left and right positions in the body 20. The control panel 25 is provided on the front of the electrical component chamber 23 so that a user can control the microwave ove, and to indicate the status of the microwave oven. A magnetron MGT, a high-voltage transformer HVT, a high-voltage diode HVD, a high-voltage capacitor HVC, the lamp 26, the fan motor 23 and the air guide 26 are installed in the electrical component chamber 23. The lamp 26 illuminates the interior of the cooking chamber 22 and the fan motor 27 drives a fan which drives air into the electrical component chamber 23. The air guide 28 guides the air driven by the fan toward the interior of the cooking chamber 22. The cooking chamber 22 is opened and closed by the door 21 which is installed on the body 20.
[0007] The rotatable tray 24 is disposed in the cooking chamber 22 and food is placed on it for cooking. The rotatable tray 24 is rotated by a driving motor (not shown) which is installed at the bottom of the cooking chamber 22, so that food being cooked is evenly irradiated.
[0008] Figure 2 is a perspective view of the high-voltage transformer of the known microwave oven, Figure 3 is a longitudinal sectional view of the high-voltage transformer, Figure 4 is a perspective view of a second secondary winding and a shunt core of the high-voltage transformer, Figure 5 is a detailed perspective view of the high-voltage transformer, and Figure 6 is a circuit diagram of a circuit including the high-voltage transformer.
[0009] A shunt core 11 is disposed between a primary winding 30 and a second secondary winding 32. The shunt core 11 shunts a portion of the flux from the first and secondary windings 31, 32. Further, when the magnetron MGT is in operation, the shunt core 11 shunts the flux thereby limiting the electric current which is available to the secondary windings 31, 32. As a result, the overdriving of the magnetron MGT is prevented.
[0010] The structure and arrangement of the high-voltage transformer HVT are as follows. The shunt core high-voltage transformer HVT includes a core 10 which has three arms. The core 10 is laminated and has an 'E-I' shape. Figure 5 shows the 'E' shaped core only. After the assembly, the 'I' shaped core is coupled to across the ends of the arms of the 'E' shaped core. Imaginary lines (a) and (b) respectively indicate the direction of the shunted magnetic flux and the direction of the non-shunted normal magnetic flux. The three arms comprise a central leg 10b and two outer legs 10a. The primary winding 30 and the secondary windings 31, 32 are wound around the central leg 10b.
[0011] The high-voltage transformer HVT of the microwave oven receives mains AC power across its the primary winding 30. 3.3V AC is generated across the second secondary winding 32 for energising the heater filament 40 of the magnetron MGT. Approximately 2000V is generated across the first secondary winding 31 in accordance with the turns ratio between it and the primary winding 30. 2000V AC transformer output is rectified and doubled by means of a half-wave doubler circuit which comprises the high-voltage capacitor HVC and the high-voltage diode HVD. The high DC voltage is applied between the anode and the cathode of the magnetron MGT. The magnetron MGT is driven by the high-voltage of 4000V and generates microwaves at approximately 2450MHz. The microwaves heat/cook food which is in the cooking chamber 22.
[0012] In the process of heating/cooking the food, electrical noise is generated by the electric devices, particularly from the magnetron MGT and the high-voltage transformer HVT. The noise causes electromagnetic interference (hereinafter referred to as EMI). The EMI noise is radiated and conducted. The radiated EMI noise can severely affect the other electric/electronic devices. Also, a study has revealed that the EMI noise has a harmful effect on the human body. Accordingly, a regulations have been made in order to ensure that the EMI noise is kept within a predetermined limit. The radiated EMI noise in the frequency band of 30MHz-1000MHz is of particular concern. Measurements of the radiation noise are shown in Figures 7 and 8.
[0013] Figures 7 and 8 are graphs showing EMI noise, measured using antennas to the side of and above a high-voltage transformer HVT respetively, which occurs in the situation where the second secondary winding is wound between the first secondary winding and the primary winding of the high-voltage transformer in a known microwave oven
[0014] The line (b) denotes the noise which is radiated from the microwave oven, which is obtained within the frequency band of 30MHz-1000MHz Line (a) denotes the limit for EMI noise. As shown in Figures 7 and 8, large amounts of noise are generated from the known microwave oven, and in some areas, the amount of noise exceeds the limit shown in line (a).
[0015] In order to prevent the noise, the conventional microwave oven is provided with a noise shielding member such as a ferrite ring around a wire extending from the high-voltage transformer HVT or an expensive EMI filter which is connected to the circuit of the microwave oven.
[0016] One example may be found in the KR-A- 91-6366 which discloses a 'noise cutoff transformer'. Referring to this, the secondary winding is disposed at the center of a core, and primary windings are disposed above and below the secondary winding. The primary and secondary windings are sealed by a metal layer.
[0017] Another example may be found in the KR-U-98039017 which discloses a transformer which accommodates a Ferrite bead (noise filter). Referring to this, a lead wire of the transformer is fed through the ferrite bead which serves as a noise filter, so that the space required for the noise filter on a printed circuit board is reduced and construction is simplified.
[0018] High-voltage transformers commonly used in microwave ovens, however, requires a separate shielding member, attached to the high-voltage transformer, or a separate, expensive EMI filter.
[0019] A high-voltage transformer according to the present invention is characterised in that the second secondary winding is adjacent to the primary winding.
[0020] The tranformer arrangement of the present invention has been found to reduce radiated noise when used for driving a magnetron of a microwave oven.
[0021] Preferably, the first secondary winding is a high-voltage winding and the second secondary winding is a low-voltage winding. More preferably, the windings are located about a central arm of an E-shaped core. Still more preferably, magnetic shunt paths are arranged between the central arm and the outer arms of the E-shaped core, the shunt paths being between the first and second secondary windings.
[0022] According to the present invention, there is also provided a method of manufacturing a transformer according to the present invention in its most preferred form, the method comprising locating the first secondary winding about the central arm, inserting magnetic shunt means between the central arm and the outer arms, locating the second secondary winding about the central arm so as to be separated from the first secondary winding by the magnetic shunt means and locating the primary winding about the central arm so as to be separated from the magnetic shunt means by the second secondary winding. The windings may be wound onto the central arm or preformed and slid over the central arm.
[0023] An embodiment of the present invention will now be described, by way of example with reference to Figures 9 to 13 of the accompanying drawings, in which:-
Figure 1 is a perspective view of a known microwave oven;
Figure 2 is a perspective view of a known high-voltage transformer;
Figure 3 is a longitudinal sectional view of the high-voltage transformer of Figure 2;
Figure 4 is a perspective view of the second secondary windng and the shunt core of the high-voltage transformer of Figure 2;
Figure 5 is a detailed perspective view of the high-voltage transformer of Figure 2;
Figure 6 is a circuit diagram of a known magnetron driving circuit;
Figures 7 and 8 are graphs showing the noise radiated from the high-voltage transformer of Figure 2;
Figure 9 is a perspective view of a high-voltage transformer according to the present invention;
Figure 10 is a longitudinal sectional view of the high-voltage transformer of Figure 9;
Figure 11 is a circuit diagram of a magnetron driving circuit including the high-voltage transformer of Figure 9; and
Figures 12 and 13 are graphs the noise radiated from the high-voltage transformer of Figure 9.
[0024] Referring to Figures 9 to 11, a high-voltage transformer according to the present invention includes a core 100, a shunt core 110 for shunting the magnetic flux produced in the core and for preventing a overdriving of a magnetron MGT, a primary winding 300 wound on the core 100, a first secondary winding 310, one end of which is connected to a high-voltage capacitor HVC, for generating a high-voltage and a second secondary winding 320 which is wound around the core 100 below the shunt core 110 for generating a low-voltage for heating the magnetron MGT.
[0025] The core 100 has an 'E-I' shape, including one central arm 111 and two outer arms 112, 113. The primary winding 300 and the secondary windings 310, 320 are wound around the central arm 111. The core 100 is comprised of plates of soft magnetic material and the primary winding 300 and the secondary windings 310, 320 are comprised of insulated copper wire. The first secondary winding 310 is connected to the high-voltage capacitor HVC in series, and branched so as to be connected with the high-voltage diode HVD and the magnetron MGT. The primary winding 300 is connected to a mains input and the second secondary winding 320 is connected to the filament 40 of the magnetron MGT. The second secondary winding 320 is preferably disposed between the shunt core 110 and the primary winding 300.
[0026] The shunt core 110 is disposed between the first secondary winding 310 and the second secondary winding 320. The shunt core 110 shunts a portion of the magnetic flux as in the above description of a prior art high-voltage transformer. The high-voltage transformer HVT receives mains power across its primary winding 300. The second secondary winding 320 outputs 3.3V AC to the filament 40 of the magnetron MGT so that the filament is heated.
[0027] The first secondary winding 310 outputs a high-voltage of approximately 2000V in accordance with the turns ratio between it and the primary winding 300. This high-voltage is rectified and doubled to approximately 4000V by a half-wave doubler circuit, which is comprised of the high-voltage capacitor HVC and the high-voltage diode HVD, and then applied between the anode and the cathode of the magnetron MGT. Accordingly, the magnetron MGT is driven by the high-voltage supplied thereto and generates microwaves at approximately 2450MHz The microwaves are transmitted into the cooking chamber 22 (see Figure 1), and heat/cook food placed therein.
[0028] In the process of heating/cooking the food, EMI noise is generated by electronic devices, including the magnetron MGT and the high-voltage transformer HVT, in the electronic component chamber 23. The radiated EMI noise is significantly reduced due to the structure of the shunt core 110, the primary winding 300, and the second secondary winding 320, in which the second secondary winding 320 is disposed between the shunt core 110 and the primary winding 300 (see Figure 10). The reduction of noise due to the position of the second secondary winding 320 shown in Figures 12 and 13.
[0029] Figures 12 and 13 are graphs showing radiated EMI noise at positions respectively to the side of the high-voltage transformer HVT and above the high-voltage transformer HVT. The line (b) denotes the noise radiated from the microwave oven within the frequency band of 30MHz-1000MHz, and line (a) denotes the limit for EMI noise. As shown in Figures 12 and 13, the amount of noise is substantially reduced. Also, by comparison with Figures 7 and 8, the noise is kept within a safer limit, since the noise does not exceed the limit (a) set forth for the amount of EMI noise.
[0030] A method of assembling the high-voltage transformer HVT of Figure 9 will now be described. First, the first secondary winding 310 is wound around the central arm 111 of the core 100. The shunt core 110 is then placed into the core 100 at a position spaced from the first secondary winding 310. Next, the second secondary winding 320 is wound in a manner such that it is positioned near to the shunt core 110 on the central arm 111 of the core 100. Then the primary winding 300 is wound in a manner such that it is positioned near to the second secondary winding 320 on the central arm 111 of the core 100.
[0031] As described above, by shifting the position of the windings, that is by positioning the second secondary winding at a position separated from the first secondary winding, the EMI noise, especially the radiation noise occurring in the frequency band of 30MHz-1000MHz is reduced, without additional devices such as a shielding member or an EMI filter.
1. A high-voltage transformer comprising a primary winding (300), a first secondary winding
(310) spaced from the primary winding (300) and a second secondary winding (320),
characterised in that the second secondary winding (320) is adjacent to the primary winding (300).
2. A transformer according to claim 1, wherein first secondary winding (310) is a high-voltage
winding and the second secondary winding (320) is a low-voltage winding.
3. A transformer according to claim 2, wherein the windings (300, 310, 320) are located
about a central arm (111) of an E-shaped core (100).
4. A transformer according to claim 3, wherein magnetic shunt paths (110) are arranged
between the central arm (111) and the outer arms (112, 113) of the E-shaped core (100),
the shunt paths (110) being between the first and second secondary windings (310,
320).
5. A method of manufacturing a transformer according to claim 4, the method comprising
locating the first secondary winding (310) about the central arm (111), inserting
magnetic shunt means (110) between the central arm (111) and the outer arms (112,
113), locating the second secondary winding (320) about the central arm (111) so as
to be separated from the first secondary winding (310) by the magnetic shunt means
(110) and locating the primary winding (300) about the central arm (111) so as to
be separated from the magnetic shunt means (110) by the second secondary winding (320).
6. A high voltage transformer for use in a microwave oven, comprising a second secondary
coil to be disposed near to a primary coil which is separated from a first secondary
coil.
7. The high voltage transformer as claimed in claim 6, wherein the second secondary coil
is wound between the primary coil and the first secondary coil.
8. The high voltage transformer as claimed in claim 6, wherein a shunt core is disposed
between the second secondary coil and the first secondary coil.
9. The high voltage transformer as claimed in claim 6, wherein the second secondary coil
preheats a filament of a magnetron.
10. A high voltage transformer for use in a microwave oven comprising:
a core for inducing a magnetic field flux generated by an electric current;
a primary coil, wound around the core, for generating the magnetic field flux;
a first secondary coil, wound around the core, for generating a first outputted voltage through the magnetic field flux within the core which is generated by the primary coil;
a shunt core, disposed between the primary coil and the first secondary coil, for shunting the magnetic field flux induced from the core; and
a second secondary coil, wound around the primary coil, for generating a second outputted voltage in accordance with the magnetic field flux within the core.
11. The high voltage transformer as claimed in claim 10, wherein the second secondary
coil is disposed between the shunt core and the primary coil.
12. The high voltage transformer as claimed in claim 10, wherein the second secondary
coil preheats a filament of a magnetron.
13. The high voltage transformer as claimed in claim 10, wherein the core comprises one
central leg and two external legs, and the primary coil and the two secondary coils
are wound around the central leg.
14. in a method for manufacturing a high voltage transformer for use in a microwave oven
which comprises a core for inducing a magnetic field flux generated by an electric
current; a primary coil, wound around the core, for generating the magnetic field
flux; a first secondary coil, wound around the core, for generating a first outputted
voltage through the magnetic field flux within the core which is generated by the
primary coil; a shunt core, disposed between the primary coil and the first secondary
coil, for shunting the magnetic field flux induced from the core; and a second secondary
coil for generating a second outputted voltage in accordance with the magnetic field
flux within the core, the second secondary coil is disposed between the shunt core
and the primary coil.
15. In a method for manufacturing a high voltage transformer for use in a microwave oven
which comprises a core having one central leg and two external legs, and a shunt core
disposed at the core, the method comprises the steps of:
(1) winding a first secondary coil around an inner side of the central leg;
(2) coupling the shunt core to the first secondary coil in a manner the shunt core is spaced apart from the first secondary coil at a predetermined distance;
(3) winding a second secondary coil in a manner that the second secondary coil is positioned near to the shunt core on the central leg; and
(4) winding a primary coil in a manner that the primary coil is positioned near to the second secondary coil on the central leg.