Patent Publication Number: US-6713965-B2

Title: Microwave oven

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 2001-84398, filed Dec. 24, 2001 in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to microwave ovens, and more particularly, to a microwave oven which blocks high frequency noise flowing through ground and power lines. 
     2. Description of the Related Art 
     Generally, a microwave oven carries out a cooking operation by converting commercial alternating current (AC) power into a high voltage to drive a magnetron. Since the microwave oven uses the commercial AC power, it is difficult to install and use the microwave oven in transportation vehicles such as cars. 
     Recently, a microwave oven has been developed to use direct current (DC) power, such as a battery, to cook foods. This type of microwave oven employs an inverter circuit to convert a DC voltage into an AC voltage required to drive a magnetron. 
     FIG. 1 shows a conventional microwave oven using DC power comprising a DC power source  1 , a low voltage unit including a switching unit  2  and an inverter unit  3  connected to a primary side of a high voltage transformer  4 , a high voltage unit including a magnetron driving unit  5  connected to a secondary side of the high voltage transformer  4 , a magnetron filter box  6 , and a controller  10 . The magnetron filter box  6  includes a magnetron  7 , and a low pass filter which comprises choke coils  8  and feed-through condensers  9 . 
     The switching unit  2  includes a door switch, a low voltage transformer, and a typical noise filter, and supplies operating power, which is voltage-dropped by the low voltage transformer, through a power line connected to the controller  10 . The inverter unit  3  comprises a push-pull circuit having switching devices, such as Field Effect Transistors (FETs), and is switched by a control signal from the controller  10  to convert DC power into AC power. 
     The high voltage transformer  4  induces the AC power converted according to a turn ratio of a primary coil L 1  to a secondary coil L 2  as a high AC voltage, and supplies the high AC voltage to the magnetron driving unit  5 . The magnetron driving unit  5  supplies a high DC voltage of 4000V to the magnetron  7  using a half-wave voltage doubler circuit (not shown) having a high voltage diode and a high voltage condenser. 
     One end of each of the switching unit  2  of the low voltage unit and the magnetron driving unit  5  of the high voltage unit is grounded through a ground line. 
     In the conventional microwave oven using the DC power as a power source, since the controller  10  drives the inverter unit  3  at a high usable frequency of 20 to 50 KHz, electrical energy flows are exchanged between the low and high voltage units, thus causing high frequency noise to be widely generated. Furthermore, the high frequency noise flows through the ground lines and radiates to the outside. Moreover, the high frequency noise flows through a power line connected to the controller  10 , thereby causing the high frequency noise to disturb a control operation of the controller  10 . 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a microwave oven which blocks high frequency noise flowing through ground and power lines. 
     Additional objects and advantages of the 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. 
     To achieve the above and other objects of the present invention, there is provided a microwave oven comprising a low voltage unit which converts direct current (DC) power into alternating current (AC) power and supplies operating power, a controller which receives the operating power from the low voltage unit, a high voltage transformer having primary and secondary coils, wherein the high voltage transformer induces the AC power converted by the low voltage unit as a high AC voltage according to a turn ratio of the primary coil to the secondary coil, a high voltage unit which generates a high DC voltage from the high AC voltage induced by the high voltage transformer and supplies the high DC voltage to a magnetron, and a filter unit which blocks high frequency noise while passing a signal of a DC voltage through ground lines to which the low and high voltage units are grounded and a power line connected between the low voltage unit and the controller. 
     According to an aspect of the present invention, the filter unit includes at least one choke coil which increases impedances of the ground and the power lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a block diagram of a conventional microwave oven using DC power; and 
     FIG. 2 is a block diagram of a microwave oven using DC power according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     The present invention can be applied to microwave ovens using AC power for a home use as well as microwave ovens using DC power as a power source. Furthermore, the present invention can be applied to microwave ovens usable with a power source selected from DC and AC power sources. For convenience and to avoid unnecessary repetition, the present invention as applied to microwave ovens using DC power is described. 
     FIG. 2 shows a microwave oven using DC power according to an embodiment of the present invention. The microwave oven comprises a DC power source  1 , a low voltage unit including a switching unit  2  and an inverter unit  3  connected to a primary side of a high voltage transformer  4 , a high voltage unit including a magnetron driving unit  5  connected to a secondary side of the high voltage transformer  4 , a magnetron filter box  6 , and a controller  10 . 
     The magnetron filter box  6  includes a magnetron  7 , and a low pass filter which comprises choke coils  8  and feed-through condensers  9 . The switching unit  2  includes a door switch, a low voltage transformer, and a typical noise filter, and supplies operating power, which is voltage-dropped by the low voltage transformer, through a power line connected to the controller  10 . The inverter unit  3  comprises a push-pull circuit having switching devices, such as Field Effect Transistors (FETs), and is switched by a control signal from the controller  10  to convert DC power into AC power. 
     The high voltage transformer  4  induces the AC power converted according to a turn ratio of a primary coil L 1  to a secondary coil L 2  as a high AC voltage, and supplies the high AC voltage to the magnetron driving unit  5 . The magnetron driving unit  5  supplies a high DC voltage of, for example, 4000V to the magnetron  7  using a half-wave voltage doubler circuit (not shown) having a high voltage diode and a high voltage condenser. 
     One end of each of the switching unit  2  of the low voltage unit and the magnetron driving unit  5  of the high voltage unit is grounded through a ground line. 
     The microwave oven of the present invention further includes second, third and fourth choke coils  11 ,  13  and  12  as a filter unit which removes high frequency noise flowing through the ground and power lines during an operation in which electrical energy flows are exchanged between the low and high voltage units. 
     The second and third choke coils  11  and  13  are disposed on ground lines to which the switching unit  2  of the low voltage unit and the magnetron driving unit  5  of the high voltage unit are respectively grounded. The fourth choke coil  12  is disposed on the power line connected between the switching unit  2  and the controller  10 . 
     The second, third and fourth choke coils  11 ,  13  and  12  comprise corresponding inductances, and serve to increase the impedances of the ground and power lines, respectively. Accordingly, a signal of a DC voltage flows through the power and ground lines, while the high frequency noise is blocked. Therefore, when the controller  10  drives the inverter unit  3  at a high usable frequency (higher than 20 KHz), even though the high frequency noise is generated due to electrical energy flows between the low and high voltage units, radiation of the high frequency noise to the outside through the ground lines can be prevented. Furthermore, the flow of the high frequency noise to the controller  10  through the power line can be blocked. 
     As described above, in the microwave oven of the present invention, high frequency noise can be blocked by connecting choke coils to ground lines, to which low and high voltage units are grounded, and to a power line connected to a controller. Use of the choke coils increases impedances of the ground and power lines. Therefore, the microwave oven of the present invention can prevent noise from being radiated to the outside through the ground lines, and from flowing to the controller through the power line, thus allowing the controller to perform a stable operation. 
     Although an embodiment of the present invention has been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.