Magnetron drive circuit for a microwave oven

Abstract: In a magnetron driving control apparatus of a microwave oven, the voltage at output side of a high voltage transformer (10) and the voltage across a magnetron (30) are detected by a first and second voltage detecting units (62, 64) and status of a cooking process is determined according to changes in the sensed voltages by a control unit (70). The control unit (70) controls the driving of the magnetron, such that foodstuff is optimally cooked and automatic cooking is performed without the use of high-priced sensors by using the outputs of the voltage detecting units (63, 64).

Description: The present invention relates to a microwave oven comprising a magnetron, a high-voltage transformer having a secondary winding arranged to supply high-voltage power to the magnetron via a rectifying circuit, switching means for selectively applying ac power to a primary winding of the transformer and control means for controlling the switching means to effect cooking. The present invention also relates to a method of operating a microwave oven comprising a magnetron, a high-voltage transformer having a secondary winding arranged to supply high-voltage power to the magnetron via a rectifying circuit, and switching means for selectively applying ac power to a primary winding of the transformer for effecting cooking. A known microwave oven is shown in Figures 1 and 2 and comprises a body 1, a cooking chamber with a hinged door 3, a key input unit 40 for setting cooking conditions, such as the cooking mode, the cooking time, cooking start/stop and the like, and a display unit 4 for displaying information relating to the cooking process. An internal wall structure 5 within the body 1 defines the cooking chamber 2 and a waveguide 6 is welded to the wall structure 5. The waveguide 6 leads from a magnetron 30 to an opening 7 through which microwaves, generated from the magnetron 30, are fed into the cooking chamber 2. Referring to Figure 3, which is a schematic block diagram illustrating the electrical circuitry of the microwave oven shown in Figures 1 and 2, the microwave oven includes a relay unit 8, a high voltage transformer 10, a high voltage diode 20, a high voltage capacitor 25, a magnetron 30, a key input unit 40, a weight sensor 42, a gas sensor 44 and a control unit 50. The relay unit 8 operative to control the supply of mains alternating current AC (for example at 220-240V) according to a control signal from the control unit 50. The high voltage transformer 10 receives the mains AC and outputs a high voltage (approximately 2,200V) and a low voltage (approximately 3.4V). The high voltage transformer includes a first coil 12 for receiving the mains AC supply, a second coil for generating the high voltage and a filament coil 16 for outputting the low voltage. The high voltage diode 20 and the high voltage capacitor 25 are disposed between the high voltage transformer 10 and the magnetron 30 so as to rectify and smooth the high voltage output by the transformer. The magnetron's heater is supplied with the low voltage output by the high voltage transformer. When a user operates the key input unit 40, it outputs key signals corresponding to the keys pressed. The weight sensor 42 weighs foodstuffs placed in the cooking chamber 2 and outputs a weight signal. The gas sensor 44 detects the amount of gas generated by the foodstuffs being cooked and outputs a gas signal corresponding thereto. The control unit 50 outputs control signals for driving the magnetron 30 according to the key signals, times cooking periods and establishes cooking periods according to the weight signals and the gas signals. The control unit 50 terminates cooking by stopping the magnetron 30 when the cooking period has elapsed. When a user puts foodstuff in the cooking chamber 2 and establishes a cooking mode using the key input unit 40 to start cooking, a key signal corresponding thereto is output from the key input unit 40 to the control unit 50 and the control unit 50 outputs a control signal to the relay unit 8 so that power is applied to the magnetron 30 and times the cooking period. The mains AC voltage is supplied to the first coil 12 of the high voltage transformer 10. A low voltage is then generated across the filament coil 16 and supplied to the magnetron's heater. At the same time, a high voltage is generated across the second coil 14, rectified, smoothed and applied to the magnetron's cathode. Thus energised, the magnetron 30 generated microwaves which are guided to the cooking chamber 2 by the waveguide 6 to heat and cook the foodstuff therein. The weight sensor 42 outputs a weight signal to the control unit 50 and the gas sensor 44 outputs a gas signal to the control unit 50. The control unit 50 the weight of the foodstuff and amount of gas with pre-established weight and amount of gas reference values, using the weight and gas signals. The control unit 50 establishes a cooking period according to the result of these comparisons. The control unit 50 determines whether the cooking time has exceeded the established cooking time, and if the counted cooking time is above the established cooking time, the control unit 50 outputs a control signal to the relay unit 8 to stop driving of the magnetron 30. The commercial AC voltage supplied to the high voltage transformer 10 is now cut off by the control signal from the control unit 50, thereby stopping driving the magnetron 30 and completing the cooking operations of the microwave oven. If it is assumed that the output power of the magnetron 30 is P in , and the power at a predetermined position in the cooking chamber 2 is P out ,, then P out can be obtained by following formulae 1, 2 and 3. Formula 1 P in = E 2 s Formula 2 E y = E s sin(χ) Formula 3 P out = (E y ) 2 = {E s sin(χ)} 2 = E 2 s sin(χ) 2 where, E s is a field energy formed by microwaves generated from the magnetron 30, that is, input field energy, and E y is the field energy at the predetermined position in the cooking chamber 2, that is, output field energy. Accordingly, the output power of the magnetron 30 is obtained by a squared value of E s , the field magnitude formed of the microwaves generated by the magnetron 30. At this time, the microwaves generated from the magnetron 30 are sine waves, so that the field energy E y at a particular position in the cooking chamber 2 is the field energy E s multiplied by sin(χ), where sin( χ ) is varied in value or phase thereof according to the states of foodstuff (by way of example; kind, quantity, cooking process status of the foodstuff and the like). In other words, absorbed quantity of microwaves differs according to the kind and quantity of the foodstuff and gas quantity, such that the output power P out at the particular position in the cooking chamber 2 differs according to the kind and quantity of the foodstuff. Particularly, because the quantity of gas generated from the foodstuff according to the cooking process status differs, P out also changes according to the cooking process status. For reference, impedance characteristic of the waveguide 6 according to the quantity of foodstuff are shown by the polar chart in Figure 4. The voltage standing wave ratio (VSWR), which gives a measure of the impedance match between the load and the waveguide 6, decreases when the quantity of water forming the load increases. It has been proposed to control the cooking time simply according to weight of the foodstuff and quantity of gas detected by a weight sensor and a gas sensor without consideration of impedance variations which affect the magnetron's output. This can result in foodstuff remaining uncooked or being overcooked at the end of the cooking period because the magnetron 30 may not be outputting its full power level. A microwave oven according to the present invention is characterised by first voltage sensing means for sensing the output voltage of said secondary winding and second voltage sensing means for sensing the voltage across the electrodes of the magnetron, wherein the control means is responsive to the outputs of the voltage sensing means for controlling the switching means. The control means may compare the outputs of the voltage sensing means with a threshold value and controls the switching means in dependence on said comparisons. Alternatively, the control means may control the switching means in dependence on the rates of change of the outputs of the voltage sensing means. A method according to the present invention is characterised by controlling the switching means in dependence on the output voltage of said secondary winding and the voltage across the electrodes of the magnetron. The method may comprise comparing the said voltages with a threshold value and controlling the switching means in dependence on said comparisons or controlling the switching means in dependence on the rates of change of said voltages. An embodiment of the present invention will now be described, by way of example, with reference to Figures 5 to 8 of the accompanying drawings, in which:- Figure 1 is a schematic perspective view of a known microwave oven; Figure 2 is a schematic sectional view of the microwave oven of Figure 1; Figure 3 is a schematic block diagram of the microwave oven of Figure 1; Figure 4 is a is a plot of the impedance characteristics of a waveguide according to quantity of foodstuff; Figure 5 is a schematic block diagram illustrating a magnetron control apparatus of a microwave oven according to the present invention; Figure 6 is a schematic diagram illustrating the output voltages of the first and second detecting units in Figure 5; Figure 7 is a schematic diagram illustrating the output voltages of the first and second detecting units; and Figure 8 is a schematic diagram illustrating operational procedures for controlling a magnetron in a microwave oven according to the present invention. Throughout the drawings, like reference numerals and symbols are used for designation of like or equivalent parts or portions. Referring to Figure 5, the magnetron control apparatus includes a relay unit 8, a high voltage transformer 10, a high voltage diode 20, a high voltage capacitor 25, a magnetron 30, a key input unit 60, a first voltage detecting unit 62, a second voltage detecting unit 64 and a control unit 70. The first voltage detecting unit 62 serves to detect the voltage across a secondary coil 14 of the high voltage transformer 10 and includes a first resistor R1a and a second resistor R1b. Reference numeral R1 in the drawing represents a combined resistance value of the first and second resistors R1a, R1b in parallel. The second voltage detecting unit 64 serves to detect the voltage applied to the cathode of the magnetron 30 and includes third to sixth resistors R2a, R2b, R3a, R3b. Reference numerals R2 and R3 represent the combined resistance values of the third and fourth resistors R2a, R2b in parallel and the fifth and sixth resistors R3a, R3b in parallel, respectively. In order to avoid an electric shock, resistors R1a and R1b of the first voltage detecting unit 62 are connected in parallel, and resistors R3a and R3b of the second voltage detecting unit 64 are also connected in parallel. The ratio of R2 to R3 is preferably 1000:1. The control unit 70 outputs a control signal for driving the magnetron 30 according to a key signal output from the key input unit 60, determines whether cooking is completed according to the voltages of the high voltage transformer 10 and the magnetron 30 detected by the first and second voltage detecting units 62, 64, and outputs a control signal for stopping the driving of the magnetron 30 when it is determined that cooking is completed. Figure 6 shows the average output voltages (C) of the high voltage transformer 10 and the magnetron 30 detected by the first and second voltage detecting units 62, 64. Figure 7 shows the output voltages of the first and second detecting units 62, 64 according to the cooking process status in the exemplary cases of load, no load and defrosting. According to one example of the determination of whether cooking has been completed, if the output voltages of the first and second detecting units 62,64 conform to a pre-established cooking completion voltage (by way of example; the portion "D" in Figure 7), it is determined that cooking is complete. According to another example of the determination of whether cooking has been completed, if there is no change of in voltage, as illustrated by portion "D" in Figure 7, detected by the first and second detecting units 62, 64 for a predetermined period of time, the control unit 70 determines that the cooking is completed. The operation of the magnetron control apparatus will be described in detail with reference to Figures 5, 6, 7 and 8, where S denotes steps. First, when a user inserts foodstuff into the cooking chamber, establishes a cooking course via the key input unit 60 and starts the cooking, a key signal corresponding thereto is supplied from the key input unit 60 to the control unit 70, step S10. The control unit 70 determines whether the user has input a start cooking instruction from the key signal from the key input unit 60, step S20, and when it is determined that a cooking start has been instructed, a control signal is output to the relay unit 8 so that power will be applied to the magnetron 30, step S30. The relay unit 8 is activated according to the control signal output by the control unit 70 and mains AC voltage is supplied from outside via input terminals A and B to the first coil 12 of the high voltage transformer 10. Consequently, a low voltage is produced across the filament coil 16 and simultaneously a high voltage is produced across the secondary coil 14. The low voltage is supplied to the heater filament of the magnetron 30 to pre-heat the filament and the high voltage is divided in voltage by the capacitor 25 and the diode 20 and rectified to be converted to a DC-type high voltage. The divided and rectified DC-type high voltage is now supplied across the magnetron 30 to cause microwaves to be generated thereby and the microwaves are fed into the cooking chamber 2 via the opening 7 of the waveguide 6 to heat and cook the foodstuff in the cooking chamber 2. The quantity of gas generated by the foodstuff varies according to the cooking process status and changes the output of the magnetron 30. The output voltage of the secondary coil 14 is detected by the first voltage detecting unit 62 which outputs a corresponding signal to the control unit 70. At the same time, the voltage across the magnetron 30 is detected by the second voltage detecting unit 64 and a corresponding signal output to the control unit 70. The control unit 70 therefore indirectly detects the output voltage of the secondary coil 14 and the voltage across the magnetron 30 (step S40). The control unit determines whether cooking has been completed according to the voltage (to be more specific, the average output voltage illustrated as "C" portion in Figure 6) and the output voltage of the secondary coil 14 of the high voltage transformer 10 detected at step S40, step S60. Here, by way of example for determining whether cooking has been completed, the output voltage of the secondary coil 14 and the voltage across the magnetron 30 are compared with a pre-established voltage (to be more specific, the average output voltage of the first and second voltage detecting units over time, as illustrated in Figure 7), and as a result of the comparison, a determination as to whether the cooking has been completed is performed In other words, if the output voltage of the secondary coil 14 and the voltage across the magnetron 30 conform to a pre-established cooking completion voltage (by way of example, "D" portion in Figure 7), it is discriminated that the cooking has been completed. Meanwhile, by way of another example for determining whether cooking has been completed, if there are no changes of output voltage of the secondary coil 14 and the voltage of the magnetron 30 for a predetermined period of time as illustrated in "D" portion in Figure 7, it is determined by the control unit 70 that the cooking has been completed. As a result of the determination at step S60, if it is determined that the cooking has not been completed, flow returns to step S30 to maintain the driving status of the magnetron 30, and if it is determined that the cooking has been completed, the control unit 70 outputs a control signal to the relay unit 8 to de-energise the magnetron 30, step S60. Subsequently, the relay unit 8 is deactivated according to the control signal from the control unit 70 and the supply of mains AC voltage applied to the first coil 12 of the high voltage transformer 10 via input terminals A and B is cut off. Consequently, the outputs of filament coil 16 and the secondary coil 14 are stopped to thereby cease the driving of the magnetron 30 and to complete the overall cooking operations of the microwave oven. As apparent from the foregoing, there is an advantage in the magnetron driving control apparatus of a microwave oven and method thereof according to the present invention, in that a cooking process status is discriminated by voltages of a high voltage transformer and a magnetron and a cooking operation is completed as a result of the discrimination to optimally cook the foodstuff. There is another advantage in that by way of a simple construction without recourse to high priced sensors such as weight sensor, gas sensor and the like, automatic cooking can be performed, to thereby reduce a manufacturing cost.

Claims: A microwave oven comprising a magnetron (30), a high-voltage transformer (10) having a secondary winding (14) arranged to supply high-voltage power to the magnetron (30) via a rectifying circuit (20), switching means (8) for selectively applying ac power to a primary winding (12) of the transformer (10) and control means (70) for controlling the switching means (8) to effect cooking, characterised by first voltage sensing means (62) for sensing the output voltage of said secondary winding and second voltage sensing means (64) for sensing the voltage across the electrodes of the magnetron (30), wherein the control means (70) is responsive to the outputs of the voltage sensing means (62, 64) for controlling the switching means (8). A microwave oven according to claim 1, wherein the control means (70) compares the outputs of the voltage sensing means (62, 64) with a threshold value and controls the switching means (8) in dependence on said comparisons. A microwave oven according to claim 1, wherein the control means (70) controls the switching means (8) in dependence on the rates of change of the outputs of the voltage sensing means (62, 64). A method of operating a microwave oven comprising a magnetron (30), a high-voltage transformer (10) having a secondary winding (14) arranged to supply high-voltage power to the magnetron (30) via a rectifying circuit (20), and switching means (8) for selectively applying ac power to a primary winding (12) of the transformer (10) for effecting cooking, the method being characterised by controlling the switching means in dependence on the output voltage of said secondary winding and the voltage across the electrodes of the magnetron (30). A method according to claim 4, comprising comparing the said voltages with a. threshold value and controlling the switching means (8) in dependence on said comparisons. A method according to claim 4, comprising controlling the switching means (8) in dependence on the rates of change of said voltages.

Inventors: HOH JUNG-EUI, HOH, JUNG-EUI
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1998
Main classification: H05B6
IPC codes: H05B6