Patent Publication Number: US-6335520-B1

Title: Microwave oven and a method for controlling the same

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits according under 35 U.S.C. §119 from an application for MICROWAVE OVEN AND CONTROL METHOD THEREOF earlier filed in the Korean Industrial Property Office on the Jul. 27, 2000 and there duly assigned Serial No. 43478/2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a microwave oven and a method for controlling the same, and more particularly, to a microwave oven and a method for controlling the same, capable of stabilizing a circuit system therein by controlling a conversion control signal. 
     2. Description of the Related Art 
     FIG. 6 is a circuit diagram schematically showing a figuration of a conventional microwave oven. As shown therein, the conventional microwave oven is comprised of a power supply part  51 , a high voltage transformer  53  generating a high voltage by means of the power supplied from the power supply part  51 , a magnetron  55  generating electromagnetic waves by means of the high voltage generated by the high voltage transformer  53  to heat food within a cooking compartment of the microwave oven, a relay switch  57  switching on and off the supply of the power and a frequency, and a control part  59  controlling the high voltage transformer  53 , the magnetron  55  and the relay switch  57  when the power is supplied from the power supply part  51 . 
     With this configuration, if the power is supplied from the power supply part  51  and the relay switch  57  turns on by means of control of the control part  59 , an electric current starts to flow at the primary winding of the high voltage transfer, thereby generating a high voltage at the secondary winding of the high voltage transformer  53 . In the secondary winding of the high voltage transformer  53  are provided a voltage having a few volts to heat filaments of the magnetron  55  and a voltage of thousands volts to oscillate the magnetron  55 . In order to apply a direct current to a negative pole of the magnetron  55 , a rectifying and filtering means for rectifying and filtering the electric current is also provided therein. 
     However, since the core of the high voltage transformer  53  used in the conventional microwave oven is made of a silicon steel sheet, it is heavy and bulky, and it is inconvenient for consumers to handle it. Because the number of turns for the secondary winding of the high voltage transformer should increase in order to generate a high voltage from the high voltage transformer  53 , this causes a problem that the high voltage transformer  53  must further increases in dimension. 
     In addition, to adjust an output voltage from the secondary winding of the high voltage transformer, the conventional microwave oven employs a method of controlling a duty cycle, because it is not possible to perform an analog control from a low output to a high output. The duty cycle control method controls the maximum rated output supplied from the power supply part  51  with a ratio of an on time and an off time of the high voltage transformer. In the duty cycle control method, if the on-time of the maximum rated output is short and the off-time thereof is long, the low output is generated, whereas the high output is generated if the on-time of the maximum rated output is long and the off-time thereof is short. Where the output is adjusted by the duty cycle control method, there is a great variation in temperature affecting cooking of food, which may lower an efficiency in cooking and further cause the food to be ill-tasting. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in view of the above-described shortcomings, and it is an object of the present invention to provide a microwave oven able to facilitate an output control by allowing a high voltage transformer to continuously and variably generate a high voltage output from the secondary winding thereof in an analog form. 
     This and other objects of the present invention may be achieved by a provision of a microwave oven, comprising a microwave oven comprising a power supply part supplying a commercial alternating current (AC) power, a rectifying and filtering part rectifying and filtering the commercial AC power, a high voltage transformer generating a high voltage by means of direct s current (DC) power from the rectifying and filtering part; and a magnetron generating electromagnetic waves based on the high voltage from the high voltage transformer, further comprising a control signal generator part generating a control signal; an inverter part converting the DC power from the rectifying and filtering part into a high voltage AC power based on the control signal from the control signal generator part, and a control part blocking the control signal converted through the inverter part from being applied to the magnetron if the converted control signal is not within a predetermined range. 
     The control part includes a D/A converter part converting the control signal from the control signal generator part into an analog signal, a detector part detecting whether the control signal converted by the D/A converter part is within the predetermined range, an output control part controlling an output of the control signal passing through the detector part, and an oscillator part varying the control signal outputted by the output control part and inputting the varied control signal into the inverter part. 
     The control part further includes an on-off and soft starter part controlling an on-off operation and a soft start operation of the oscillator part depending upon the control signal. 
     The control part further includes a low voltage off part supplying a stop signal to the on-off and soft starter part and the D/A converter part where an abnormal power is inputted through the power supply part, to stop an operation of the on-off and soft starter part and the D/A converter part. 
     The control signal detected by the detector part is applied to an input terminal of the output control part. 
     The output control part uses a resistance property between a drain and a source of a field effect transistor (FET). 
     The oscillator part includes a switching part switching the DC power into an AC power. 
     An oscillating frequency of the oscillator part is given an expression Fo=1/(1.4×(external resistance+75)×capacitor). 
     The on-off and soft starter part uses a resistance property between a drain and a source of an FET. 
     The low voltage off part includes a transistor and a photo coupler which are connected in series to each other, to form a logical product (AND) circuit element. 
     The high voltage transformer includes a ferrite core to minimize a loss in a high frequency. frequency. 
     A method controlling a microwave oven including a power supply part supplying a commercial alternating current (AC) power, a rectifying and filtering part rectifying and filtering the commercial AC power, an inverter part converting a DC power from said rectifying and filtering part into an AC power of a high frequency, a high voltage transformer generating a high voltage by means of the AC power from the inverter part, and a magnetron generating electromagnetic waves based on the high voltage from the high voltage transformer, includes the steps of generating a control signal, applying the control signal to the inverter part so that the inverter part converts the DC power from the rectifying and filtering part into the high frequency AC power, detecting whether the control signal converted through the inverter part is within a predetermined range, and preventing the control signal from being applied to the magnetron if the control signal is not within the predetermined range. 
     The method further includes the steps of determining whether the control signal to be applied to the inverter part is within the predetermined range and preventing the control signal from entering into the inverter part if the control signal is not within the predetermined range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a microwave oven according to the present invention; 
     FIG. 2 is a detailed circuit diagram for FIG. 1; 
     FIG. 3 shows graphs for electric potentials and waveforms of several points in FIG. 2; 
     FIG. 4 shows graphs for waveforms obtained by overlapping direct currents (DC) with source signal for improving a power factor; 
     FIG. 5 is a graph showing operational characteristics of a detector part; and 
     FIG. 6 is a block diagram of a conventional microwave oven. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a microwave oven according to the present invention includes a power supply part  7  supplying a commercial alternating current(AC) power, a rectifying and filtering part  8  rectifying and filtering the electric power supplied from the power supply part  7 , a high voltage transformer  24  generating a high voltage based on the commercial AC power, and a magnetron  25  generating electromagnetic waves by means of the high voltage generated by the high voltage transformer  24 . 
     A reactor  9  (see FIG. 2) and a filtering capacitor  10  (also see FIG. 2) are connected to the rectifying and filtering part  8 , to thereby prevent a noise from the inverter part from being discharged outside. A resistor  19  and a filtering capacitor  20  connected to the rectifying and filtering part  8  reduces the high voltage approximately 310V rectified in the rectifying and filtering part  8  to a voltage having about 15V in order to use it as a semiconductor driving power. 
     The microwave oven according to the present invention includes a control signal generator part  26  generating a control signal, and an inverter part  30  connected to a primary winding of the high voltage transformer  24 , the inverter part  30  converting a DC power rectified and filtered through the rectifying and filtering part  8  into a high voltage AC power based on the control signal inputted through the control signal generator part  26 . In the inverter part  30  is provided a resonator part  6  connected in series to the primary winding of the high voltage transformer  24 , performing an operation of resonance. 
     In addition, the microwave oven according to the present invention includes a control part  40  controlling the control signal converted through the resonator part  6  of the inverter part  30 , if the converted control signal is not within a predetermined range of the control signal, so that the converted control signal is within the predetermined range. 
     The control part  40  receives the control signal from the control signal generator part  26  and determines whether the control signal is within the predetermined range. Where the control signal is determined not to be within the predetermined range, the control part  40  prevents the control signal from being applied to the inverter part  30 . 
     The control part  40  is provided with a D/A converter part  2  converting the control signal from the control signal generator part  26  into an analog signal, a detector part  5  detecting the control signal converted by the D/A converter part to determine whether the control signal is within a predetermined range, an output control part  4  controlling and outputting the control signal detected by the detector part  5 , and an oscillator part  21  varying the control cycle of the control signal outputted by the output control part  4  and applying it to the inverter part  30 . The oscillator part  21  is comprised of a switching part  27  converting the DC power into an AC power; and the switching part  27  is provided with a pair of switching power elements. 
     The control part  40  includes an on-off and soft starter part  3  controlling an on-off and soft start operation of the oscillator part  21  according to the control signal inputted by the control signal generator part  26 , and a low voltage off part  21  outputting a stop signal to the on-off and soft starter part  3  and the D/A converter part  2  where the commercial DC power inputted through the power supply part is determined to be abnormal. The control part  40  divides the control signal generated by the control signal generator part  26  and inputs the divided control signals into the D/A converter part  2  and the on-off and soft starter part  3 , respectively. 
     The flow of the control signal divided into the D/A converter part  2  will be described in more detail. 
     The control signal divided into the D/A converter  2  are converted into an analog signal and the converted analog signal is applied to the detector part  5 . Where the control signal applied to the detector part  5  is determined to be within the predetermined range by the control part  40 , the control part  40  applies the control signal to an input terminal of the output control part  4 . The control signal applied to the output control part  4  is applied to the input terminal of the oscillator part  21 , varied by the oscillator part  21  and then inputted into the inverter part  30 . The control signal inputted into the inverter part  30  is converted into the AC power having a high frequency and supplied into the magnetron  25  through the primary and secondary windings of the high voltage transformer  24 , thereby generating electromagnetic waves. 
     The control part  40  determines whether the converted control signal through the inverter part  30  is within the predetermined range. Where the control part  40  determines that the converted control signal is not within the determined range, the control part  40  blocks the converted control signal from being applied to the magnetron  25 . If the converted signal control signal is determined to be within the predetermined range, the control part  40  applies the converted control signal to the magnetron  25  via the output control part  4  and the inverter part  30 . 
     Further, as described above, where the control part  40  determines that the control signal passing through the D/A converter part is not within the predetermined range, the control part  40  blocks the control signal from being applied to the output control part  4 , thereby protecting the circuit in a more stable manner. 
     Because the high voltage transformer  24  is driven with a high frequency (about 20 Kilo-Hertz) through the semiconductor oscillation, it is effective to use a ferrite core minimizing the loss of the high frequency, thereby having no need to increase the number of turns of the secondary winding of the high voltage transformer  24 . The high voltage transformer employing the ferrite core is less in dimension and weight compared with the conventional core type high voltage transformer. 
     The D/A converter part  2 , the on-off and soft starter part  3 , the oscillator part  21 , the output control part  4 , etc. constituting the control part  30  will be described in more detail, referring to FIG.  2 . 
     When the power is initially supplied to the microwave oven from the power supply part  7  or when the microwave oven is on standby, the control signal is not inputted to the input terminal of a photo coupler  18  connected to the control signal generator part  26  from the signal generator part  26 , and therefore, the inverter part  30  is in no operation. This means that the oscillation from the inverter part  30  does not occur. To allow the inverter part  30  to oscillate, pulse width modulation (PWM) waveforms should be continuously applied through an input terminal (P 1 ) of the photo coupler  18  from the control signal generator part  26 . 
     The PWM waves applied to the photo coupler  18  functions to operate (start oscillation) of the inverter part  30  and to control an output of the inverter part  30  by varying oscillation frequencies of the oscillator part  21  depending upon changes in pulse width of the PWM waveforms. 
     When the PWM waveforms are not applied to the on-off and soft starter part  3 , a transistor  306  constituting the on-off and soft starter part  3  turns on with a base thereof biased by a resistor  302  and a capacitor  303 . If the transistor  306  turns on, a gate potential of a field effect transistor (FET)  310  becomes minimum and the resistance between a drain and a source of the FET  310  becomes infinitely great. When the resistance between the drain and the source of the FET becomes infinitely great, a capacitor  311  results in being separated from the oscillator part  21 , thereby allowing the oscillation of the oscillator part  21  to stop. Thus, the inverter part  30  stops operating 
     Conversely, where the PWM waveforms are applied to the on-off and soft starter part  3 , the base bias of the transistor  306  is drained out through an orientation diode  301 , thereby allowing the transistor  306  to turn off. A zener diode  304  interrupts the residue base bias of the transistor  306 , allowing the transistor to maintain the state. If the transistor  306  turns off, a filter capacitor  308  is slowly charged with a VCC voltage through the resistor  305  and the gate resistor  307 . Accordingly, the resistance between the drain and the source of the FET  310  slowly becomes decreased, and the oscillating capacitor  311  result in being combined with the oscillator part  21 , thereby initiating the oscillation. 
     Where the PWM waveforms are applied to the input terminal of the photo coupler  18 , the values of the analog voltage of the D/A converter  2  are determined depending upon the relation between high values and low values in the PWM waveforms. 
     Where the voltage value (P 2 ) is lowered, the value of resistance between the drain and the source of the FET  402  is increased to allow the oscillating frequencies to be lowered and the output of the inverter part  30  to be increased. A resistor  201  is for a gate bias voltage of the FET  402 ; and the resistors  203  and  205  and a capacitor  204  are π types filters, converting digital PWM waveforms into analog waveforms, which are applied to the FET  310  through a gate resistor  401 . 
     As described above, the element coupling and separating the oscillator part  21  and the oscillating capacitor  311  is the resistor between the drain and the source of the FET  310 . Where the resistor between the drain and the source is high, the capacitor  311  results in having a lower capacity, thereby increasing the oscillating frequencies. Conversely, where the resistor between the drain and the source is so low as to be ignored, the oscillation occurs for the whole capacity of the capacitor  311 . 
     Where the oscillating frequency is high, the output of the inverter part  30  becomes decreased. Thus, when the inverter part  30  starts to oscillate, it is desirable to increase the oscillating frequency as high as possible to allow the output to be the minimum, and then to slowly lower the frequency until the desired output is obtained, thereby giving no burden to the various electric elements. The soft start operation considers all the properties of the oscillating frequency and the inverter part  30 . The present invention realizes the soft start by means of the resistance property between the drain and the source of the FET  310 . 
     Hereinbelow, the output control part of the present invention will be described in more detail. 
     The oscillator part  21  oscillates by itself, when and external resistor (RT) and a capacitor (CT) are connected structurally, generating gate pulses of the switching elements  22  and  23 . 
     The oscillating frequency Fo of the oscillator part  21  is obtained by the equation of Fo=1/4(1.4*(RT+75)*CT), wherein the external resistance(RT)=resistance( 404 )/{resistance( 403 )+the resistance ( 402 ) between the drain and the source and the capacitor (CT)=capacitor( 311 ). 
     The oscillating frequency can vary by changing the value of external resistance (RT). The inverter part according to the present invention uses the resistance properties between the drain and the source of the FET  402  to change the external resistance value. 
     The variation of the oscillating frequency aims at improving a power factor of the inverter part  30 , in addition to controlling the output of the inverter part  30 . Where an output is made from the inverter part  30  considering no improvement of the power factor, the voltage of the secondary winding of the high-voltage transformer  24  is determined in proportion to the voltage supplied through the power supply part. The supplied voltage has a waveform resulting from rectification of the commercial AC power, the secondary high voltage has also the same waveform as the rectified waveform. Consequently, the magnetron  25  is operated in proximity to top points (90° and 270° of the commercial AC signal) of the secondary high voltage. In reverse, the operation of the magnetron  25  stops in proximity to zero crossing points (0° and 180° of the commercial AC signal) because the secondary high voltages is low, which shortens the durability of the oscillating element of the magnetron and deteriorates the efficiency of electric energy. Therefore, it is preferable to provide the oscillating element of the magnetron with a load property similar to that of the possible resistance over the whole range of the commercial AC power waveforms. through the power supply part. The supplied voltage has a waveform resulting from rectification of the commercial AC power, the secondary high voltage has also the same waveform as the rectified waveform. Consequently, the magnetron  25  is operated in proximity to top points (90E and 270E of the commercial AC signal) of the secondary high voltage. In reverse, the operation of the magnetron  25  stops in proximity to zero crossing points (0E and 180E of the commercial AC signal) because the secondary high voltages is low, which shortens the durability of the oscillating element of the magnetron and deteriorates the efficiency of electric energy. Therefore, it is preferable to provide the oscillating element of the magnetron with a load property similar to that of the possible resistance over the whole range of the commercial AC power waveforms. 
     As shown in FIG. 3 which shows graphs for an electric potentials and waveforms of several points of FIG. 2, the improvement of the power factor is to allow the magnetron  25  to have a uniform load over the whole section of the AC signal. However, it is not easy for the magnetron  25  to have a uniform load over the whole section of the DC signal under the non-linear load structure, which is merely possible in pure resistance load. Thus, to operate the magnetron  26  to have the uniform load properties, the operational voltage should be calibrated reversely. 
     The reverse calibration of the operational voltage is accomplished by lowering the high voltage supplied to the magnetron, in proximity to 90° and 270°, at which the magnetron is the most actively operated, and enhancing the high voltage in proximity to 0° and 180°, at which the magnetron is the least actively operated. Hence, electric current approximate to the pure resistance load may be obtained. 
     Diodes  11  and  12  are full wave rectifier circuit elements to obtain an AC signal waveform necessary for improving the power factor and operating the low voltage off part  1 . The obtained waveform signal is converted into low voltage by attenuator resistance elements  13  and  14  and transmitted into the gate of the output control part  4  through the capacitor  17 . The capacitor  17  can transmit only the AC signal without lowering gate bias voltage of the output control part  4 , thereby allowing the FET  402  to be always in the operable range. 
     Where the phase angles are 90° and 270°, the strength of the gate bias voltage(P 4 ) is obtained by weighing a sign wave over the reference bias voltage(P 2 ), so that the resistance value between the drain and the source of the FET  402  is changed, allowing the output of the inverter part  30  to vary. That is, where the phase angles are 90° and 270°, the resistance value between the drain and the source of FET  402  becomes the least and the oscillating frequency of the oscillator unit  21  becomes the maximum accordingly, thereby lowering the output of the inverter part. FIG. 4 shows graphs for waveforms of source signals for improving the power factor with DC being overlapped. As described above, the reference source for improving the power factor is obtained from the commercial AC power; and to improve the power factor, the variation in resistance between the drain and the source of the FET is used. 
     The low voltage off part  1  is used so as to protect the various power elements by suspending the operation of the inverter part  30 , where the AC input voltage is extremely low because of abnormal power lines or falling of a thunderbolt. The filter capacitor  103  is charged with the AC signal converted into low voltages by the attenuation resistors  15  and  16  through the diode  101  of the low voltage off part  1 . When the AC signal charging the filter capacitor  103  are lower than the predetermined value of the zener diode  102 , the transistor  104  is off, to erase the PWM waveforms applied to the photo coupler  18  and suspend the oscillation of the inverter part  30 . The photo coupler  18  and the transistor  104  of the low voltage off part  1  are connected in series to each other, and thus these elements are in the form of logic product, that is, AND, so that the resultant turns off if either of them turns off. 
     Where a resonance voltage generated in the resonance part  6  is higher than a predetermined value, the detector part  5  applies the resonance voltage to the base of the transistor  504  through divided voltage resistors  601  and  505 . After an emitter resistor  503  and a charging capacitor  502  are charged with the resonance voltage applied to the transistor  504 , the resonance voltage is applied to the input terminal of the output control part  4  through the diode  501 . 
     The resonance voltage of the resonance part  6  is abnormally risen because it is affected by surge noises entering over the power line. To protect the circuits from the surge noises, according to the present invention, the abnormal resonance voltage is converted into normal voltage by means of a transistor employing an emitter-floor mechanism, and the converted normal voltage is fed back to the input terminal of the output control part  4 , thereby allowing the resonance part to operate in a closed-loop. 
     As shown in FIG. 5 which is a graph showing operational characteristics of a detector part, before the inverter part  30  starts to operate, that is, when the central voltage (P 6 ) of the resonance part  6  is V/2 during suspension of the inverter part  30 , the optimum soft start is realized. Here, V means the DC voltage applied to a collector of the switching power element  22  and a resonance capacitor  602  through a reactor  9 . Where the commercial AC power supply is 220V, V is about 310V, and thus, V/2 is about 155V. AV@ means the DC voltage applied to a collector of the switching power element  22  and a resonance capacitor  602  through a reactor  9 . Where the commercial AC power supply is 220V, V is about 310V, and thus, V/2 is about 155V. 
     To adapt the voltage (P 6 ) to the level of V/2, a value of a pull-up resistor  502  should be equal to a sum of a value of the resistor  601  and the resistor  505 . However, the value of the resistor  505  is so small as to be ignorable, in comparison with the resistor  601 , the resistor  502  have the same value as that of the resistor  601 , thereby allowing the DC bias of V/2 level to be supplied the central point (P 6 ) of the resonance part  6 . 
     The main feature of the inverter for the microwave oven according to the present invention is to generate a high voltage through an oscillation of semiconductor, and further, to enhance or lower the strength of the high voltage obtained from the semiconductor oscillation by varying the oscillating frequencies. If the oscillating frequencies are lowered, the resonance current is increased, thereby increasing the high voltage. Conversely, if the oscillating frequencies are heightened, the secondary high voltage is lowered. 
     The output of the microwave oven, that is, of the magnetron, is proportional to the strength of the secondary high voltage of the high voltage transformer, and therefore, the output of the microwave oven is controlled by controlling the secondary high voltage. 
     As stated above, the microwave oven according to the present invention enables precision control and output control by feeding back a control signal to the microwave oven. By detecting an abnormal status of the control signal, the circuit system is protected, thereby enhancing the stability thereof. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.