Abstract:
A heating circuit includes a first heating coil provided adjacent to an object to be heated. A first capacitor is provided in parallel to the first heating coil, the first capacitor being a resonant component. An inductor is coupled to the first heating coil and the first capacitor.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention claims benefits of U.S. Provisional Application No. 60/792,154, filed on Apr. 14, 2006, which is incorporated by reference. 
     
       [0002]    The induction heating circuit may be used in many electronics devices. One of its use is in an induction heating cooker, e.g., a rice cooker. The rice cooker has a housing enclosing a metal bowl, a heating coil, and a ceramic provided between the heating coil and the metal bowl. 
         [0003]      FIG. 1  shows an induction heating cooker  100  that has a metal bowl  102  and a heating coil  104  that together define a transformer. The turns ratio is n:1, where n is the number of coil turns.  FIG. 2  illustrates a simple equivalent circuit of the heating coil and the metal bowl as a transformer, where R′ is the resistance of the metal bowl. The heating coil is on the primary side, and the metal bowl is on the secondary side of the transformer. A power circuit applies AC voltage across the coil. The AC voltage is transferred to the secondary side, i.e., to the metal bowl. The AC voltage applied to the secondary side is reduced by 1/n. High current flows through the metal bowl since it has low resistance. This current is reduced to 1/n at the primary side, which is the coil. 
         [0004]    There are several methods of applying AC voltage to the primary side of the transformer, i.e., the heating coil. The circuit is generally comprised of switching semiconductor devices (transistors or switch), capacitor, and inductors. The semiconductor devices are operated in a switch mode, not in a linear mode. 
         [0005]      FIG. 3  illustrates a half-bridge type circuit  150  that could be used as the heating circuit. The circuit  150  includes a voltage source  152 , transistors  154  in a half bridge configuration, a transformer  156 , and a blocking capacitor  158  to block DC currents. A resistor R′ represents the resistance of the secondary side, i.e., metal bowl, of the transformer  156 . Since the voltage applied on the switching device does not exceed the DC link voltage by much, the device having low breakdown voltage can be used. The output voltage is symmetrical upon voltage polarity. One disadvantage of the half-bridge type circuit is that it requires two transistors and a complicated driver for controlling the high side switching device. Since the output voltage is small, the required number of turns of coil is small and the primary current is large. 
         [0006]      FIG. 4  illustrates a class-E converter type circuit  180  as an induction heating circuit. The circuit  180  includes a voltage source  182 , a transformer  184 , a transistor  186 , and a capacitor  188  that is in parallel to the transistor  186 . The circuit  180  uses one transistor so the manufacturing cost is lower. The output voltage is substantially fixed and larger than that of the half bridge type circuit. The transistor  186  sees much larger voltage than the DC link voltage. The transistor  186  needs have a high breakdown voltage. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In one embodiment, a heating circuit includes a first heating coil provided adjacent to an object to be heated. A first capacitor is provided in parallel to the first heating coil, the first capacitor being a resonant component. An inductor is coupled to the first heating coil and the first capacitor. 
         [0008]    In one embodiment, a second heating coil is coupled to the first heating coil, the second heating coil including a heating coil provided around the conductive bowl. A switch is coupled to the inductor. A second capacitor is provided in parallel to the switch. The heating circuit is coupled to a sensor to detect if a current slope goes negative and a gate driver configured to output a control signal to turn on or off the switch. 
         [0009]    In one embodiment, an induction heating circuit includes a first heating coil provided below a conductive bowl to heat the conductive bowl; a second heating coil provided around a body of the conductive bowl; and at least one capacitor defining a resonant loop with the first heating coil, the second heating coil, or both. 
         [0010]    In one embodiment, the heating circuit includes first, second, and third nodes, wherein the first capacitor and the first heating coil are provided between the first and second nodes, wherein the second heating coil is provided between the second and third nodes. The heating circuit includes a second capacitor having an end connecting the first node and another end connecting the third node. 
         [0011]    In one embodiment, the heating circuit includes first, second, third and fourth nodes, wherein the first heating coil is provided between the first and second nodes, and the second heating coil is provided between the second and third nodes. A first capacitor has one end connected to the first node and another end connected to the third node. A second capacitor has one end connected to the second node and another end connected to the fourth node. A switch is provided between the third node and the fourth node, the third node being between the second heating coil and the switch. 
         [0012]    In one embodiment, the heating circuit includes first, second, third and fourth nodes. A first capacitor has one end connected to the second node that is provided between the first and second heating coils and another end connected to the fourth node. A second capacitor has one end connected to the third node and another end connected to the fourth node. A switch is provided between the third and fourth nodes. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows an induction heating cooker that has a metal bowl and a heating coil that together define a transformer. 
           [0014]      FIG. 2  illustrates a simple equivalent circuit of the heating coil and the metal bowl as a transformer. 
           [0015]      FIG. 3  illustrates a half-bridge type circuit that could be used as the heating circuit. 
           [0016]      FIG. 4  illustrates a class-E converter type circuit as an induction heating circuit. 
           [0017]      FIGS. 5A-5C  illustrate induction heating circuits according to embodiments of the present invention. 
           [0018]      FIG. 6  illustrates partial view of a rice cooker having a first heating coil provided below a metal bowl and a second heating coil provided around the side of the metal bowl according to one embodiment of the present invention. 
           [0019]      FIGS. 7A-7C  illustrate equivalent circuits associated with the rice cooker of  FIG. 6 . 
           [0020]      FIG. 8  illustrates an equivalent circuit for the heating coil and the metal bowl of  FIG. 6 . 
           [0021]      FIG. 9  illustrates a circuit illustrating a heating coil and a metal bowl. 
           [0022]      FIG. 10  illustrates a heating circuit according to conventional technology. 
           [0023]      FIG. 11  illustrates operating waveforms of the circuit of  FIG. 10 . 
           [0024]      FIG. 12  illustrates a heating circuit according to one embodiment of the present invention. 
           [0025]      FIG. 13  illustrates circuits used to heat a rice cooker according a conventional technology. 
           [0026]      FIG. 14  illustrates waveforms of simulation for with the circuits  FIG. 13 . 
           [0027]      FIG. 15  illustrates circuits used to heat a rice cooker according to one embodiment of the present invention. 
           [0028]      FIG. 16  waveforms of simulation for with the circuits of  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The present invention relates to an induction heating circuit.  FIG. 5A  illustrates an induction heating circuit  200  according to one embodiment of the present invention. The heating circuit  200  relates to the class-E converter type circuit having one or more resonant components, e.g., a capacitor and inductor. These resonant components increase the power output and enables the circuit to be operated using a smaller current. 
         [0030]    The heating circuit  200  includes a voltage source  202 , a transformer (or heating coil)  204 , a first capacitor  206  parallel to the transformer (or heating coil)  204 , an inductor  208 , a second capacitor  210 , and a transistor  211 . The second capacitor  210  is parallel to the transistor  211 . The capacitors  206  and  210  are resonant components. The transformer (heating coil)  204  includes a first heating coil  212  on the primary side and a metal bowl  213  on the secondary side. The inductor  208  may be another heating coil. Herein the heating circuit of the present embodiments will be described primarily from the perspective of the primary side of the transformer, so the term “heating coil” will be used where possible instead of the term “transformer”. 
         [0031]      FIGS. 5B  illustrates an induction heating circuit  230  according to one embodiment of the present invention. The circuit  230  has a first capacitor  232  parallel to the a heating coil  234  and a second capacitor  236  parallel to the union of an inductor  238  and the heating coil  234 . The heating coil is part of a transformer. A switch  240  is connected to a node between the second capacitor  236  and the inductor  238 . The switch may be an Insulated Gate Bipolar Transistor (IGBT) or Bipolar Junction Transistor (BJT). A diode  242  is connected anti-parallel to the switch  240 . The first and second capacitors  232  and  236  are resonant capacitors. The above configuration enables capacitors with a lower voltage rating to be used as the first and second capacitors  232  and  236 . 
         [0032]      FIG. 5C  illustrates an induction heating circuit  250  according to one embodiment of the present invention. The circuit  250  has a first capacitor  252  parallel to a heating coil  254  and a second capacitor  256  parallel to the union of an inductor  258  and the heating coil  254 . The inductor  258  is provided above the heating coil  254 , i.e., closer to the positive rail, in the present embodiment. A switch  260  is connected to a node common to the first capacitor  252 , the second capacitor  256  and the heating coil  254 . The switch may be an Insulated Gate Bipolar Transistor (IGBT) or Bipolar Junction Transistor (BJT). A diode  242  is connected anti-parallel to the switch  260 . The first and second capacitors  232  and  236  are resonant capacitors. 
         [0033]      FIG. 6  illustrates partial view of a rice cooker  300  having a first heating coil  302  provided below a metal bowl  304  and a second heating coil  306  provided around the side of the metal bowl  304  according to one embodiment of the present invention. 
         [0034]      FIG. 7A  illustrates an equivalent circuit  301  associated with the rice cooker  300  of  FIG. 6 . The circuit  301  includes a voltage source  312 , a first transformer (or first heating coil)  314 , a capacitor  316  parallel to the first heating coil  314 , a second transformer (or second heating coil)  318 , a capacitor  320 , and a transistor  321 . The capacitors  316 ,  320  are resonant components. 
         [0035]    The second heating coil  318  is wrapped around the metal bowl  304  to more effectively use the energy consumed in the inductor, i.e., the heat generated by the parasitic resistance therein. The capacitor  320  is provided in parallel to the transistor  321 . Alternatively, the first heating coil  314  may be a heating coil that is wrapped around the metal bowl and the second heating coil  318  may have a heating coil that is provided below the metal bowl. 
         [0036]      FIG. 7B  illustrates another equivalent circuit  330  associated with the rice cooker  300  of  FIG. 6 . The circuit  330  has a first capacitor  332  parallel to a first heating coil  334  and a second capacitor  336  parallel to the union of the first and second heating coils  334  and  338 . The capacitors and the heating coils are resonant components. A switch  340  is connected to a node between the second capacitor  336  and the second heating coil  338 . A diode  342  is connected anti-parallel to the switch  340 . The capacitors and the heating coils are resonant components defining resonant loops. 
         [0037]      FIG. 7C  illustrates another equivalent circuit  350  associated with the rice cooker  300  of  FIG. 6 . The circuit  350  has a first capacitor  352  parallel to a first heating coil  354  and a second capacitor  356  parallel to the union of the first and second heating coils  354  and  358 . The second heating coil is provided above the first heating coil in the present embodiment, i.e., the second heating coil  358  is closer to the positive rail than the first heating coil  354 . A switch  360  is connected to a node common to the first capacitor  352 , the second capacitor  356 , and the first heating coil  354 . A diode  362  is connected anti-parallel to the switch  360 . 
         [0038]    Some of the advantages of the circuit  301  includes the following. The circuit  301  has a lower peak and RMS current for the transistor at larger output power. This circuit can use less expensive transistors. The conduction loss and switching loss at the transistor is reduced. 
         [0039]      FIG. 8  illustrates an equivalent circuit for the heating coil and the metal bowl of  FIG. 6 . L m  represents the primary side magnetizing inductance. A transformer  352  is deemed to be an ideal n:  1  transformer having infinite magnetizing inductance. R′ represents the resistance of the metal bowl. The resistance R′ is seen as R=n*n*R′ at the primary side. Hence, the heating coil and the metal bowl can be treated a circuit  360  including an inductor L m  and a resistor R in parallel, as shown in  FIG. 9 . 
         [0040]      FIG. 10  illustrates a heating circuit  400  according conventional technology. The circuit  400  is a Class-E type circuit and includes a voltage source  402 , an inductor L m , a resistor R, a transistor  404 , a diode  406 , and a capacitor  408 . The inductor Lm and the resistor R are in parallel between the voltage source  402  and a node  410 . The transistor  404  and the capacitor  408  are in parallel between the node  410  and the ground. The capacitor  408  is a resonant component. The diode  406  may be a body diode of the transistor or a separate diode thereof. 
         [0041]    In operation, the switch voltage V SW  is not deep negative. The diode  410  prevents V SW  from going deep negative.  FIG. 11  illustrates operating waveforms of the circuit  400 . The circuit is a Class-E converter type induction heat circuit. The average voltage of V SW  is the same as V DC  since the average voltage across the inductor Lm should be zero in steady state, repetitive operation. If the R is not too small, the switch can be turned-on while the diode is conducting current. 
         [0042]    At t 0 , as inductor current I Lm  becomes zero, the diode stops conducting. Between t 0 ˜t 1 , I Lm  linearly increases with the slope of Vdc/Lm. At t 1 , the switch Sw is turned off. At t 1 ˜t 2 , I Lm  increases and reaches maximum at t 2 . At t 2 ˜t 3 , I Lm  decreases and reaches zero at t 3 . Between, t 3 ˜t 4 , I Lm  decreases and reaches the negative peak at t 4 . Between t 4 ˜t 5  I Lm  increases. At t 5 , the voltage V SW  becomes zero and the diode starts conducting inductor current. Between t 5 ˜t 6 , I Lm  linearly increases with the slope of V DC /L m . At t 6 , the initial state t 0  is reached. 
         [0043]    The voltage V DC -V SW  is the output voltage, where V DC  is deemed zero voltage. The output voltage is reduced by (1/n). A large current flows through the secondary side of the transformer (or heating coil), i.e., through the bowl. If the current is too large, the resonant circuit loses much of its energy and V SW  does not return to zero at t 5 . Then, the switch cannot be turned on at zero voltage and would result in much switching loss. 
         [0044]      FIG. 12  illustrates a heating circuit according to one embodiment of the present invention. The resonant circuit comprising C r1  and L m1  is oscillating at the same frequency of the switching of the switch Sw. The switch Sw is turned on when the voltage V O2  across it becomes zero. It is turned off when the current across it starts decreasing. This switching method is one of many possible methods. 
         [0045]      FIG. 13  illustrates circuits used to heat a rice cooker according a conventional technology. A heating circuit  500  used to heat the metal bowl of the rice cooker and includes a switch Z 1 . The switch is an IGBT. A sensor  502  is used to send a signal G_ON when the output voltage goes to negative. A gate driver  504  outputs a control signal G that is used to turn on the switch Z 1  according to the signal G_ON. The gate driver  504  may be configured to turn-on or turn-off periodically without any input from the sensor  502 .  FIG. 14  illustrates waveforms of simulation for with the circuits  500 ,  502 ,  504 . 
         [0046]      FIG. 15  illustrates circuits used to heat a rice cooker according to one embodiment of the present invention. A heating circuit  600  used to heat the metal bowl of the rice cooker. The circuit  600  includes a switch Z 1  and an inductor L S1 . The switch Z 1  is controlled by an input G. The inductor L S1  is used to detect the derivative of the total current. A sensor  602  receives a signal D 1  and determines whether or not the derivative of the switch current has a negative slope. A gate driver  604  outputs a control signal that is used to turn on the switch Z 1  of the circuit  600  according to the signals received from the sensor  602 . The gate driver  604  may be configured to turn-on or turn-off periodically without any input from the sensor  602 .  FIG. 16  illustrates waveforms of simulation for with the circuits  600 ,  602 ,  604 . 
         [0047]    Below are the results of the simulation on the circuits  500 ,  600 . The parameters of the circuits were chosen to make output power and operating frequency similar to each other. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 First Circuits 
                 Second Circuits 
               
               
                   
                 500, 502, 504 
                 600, 602, 604 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Frequency 
                 23 
                 kHz 
                 23 
                 kHz 
               
               
                 Output power 
                 1.23 
                 kW 
                 1.28 
                 kW 
               
               
                 Peak voltage of switch 
                 920 
                 V 
                 930 
                 V 
               
             
          
           
               
                 Peak current 
                 Switch 
                 38 
                 A 
                 18.5 
                 A 
               
               
                   
                 Diode 
                 19 
                 A 
                 4.0 
                 A 
               
               
                 RMS current 
                 Switch 
                 12.3 
                 A 
                 7.9 
                 A 
               
               
                   
                 Diode 
                 4.5 
                 A 
                 0.6 
                 A 
               
               
                 Average current 
                 Switch 
                 5.8 
                 A 
                 4.3 
                 A 
               
               
                   
                 Diode 
                 1.5 
                 A 
                 0.1 
                 A 
               
               
                   
               
             
          
         
       
     
         [0048]    The switching power loss is nearly proportional to the peak currents. The conduction loss for unipolar device is the product of (RMS current) 2  and on-resistance. The conduction loss for bipolar device is the product of average current and on-voltage. In both case of IGBT (Insulated Gate Bipolar Transistor) and anti-parallel diode, the conduction loss will be between these two conduction loss equations. The following table compares the power losses. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Re- 
               
               
                   
                   
                   
                 duc- 
               
               
                   
                 First Circuits 
                 Second Circuits 
                 tion 
               
               
                   
                 500, 502, 504 
                 600, 602, 604 
                 Ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Switching 
                 Switch, A * Pz 
                 38 
                 18.5 
                 45% 
               
               
                 Loss 
                 Diode, A * Pd 
                 19 
                 4.0 
                 21% 
               
               
                 Conduction 
                 Switch, A * A * 
                 151 
                 62 
                 41% 
               
               
                 Loss A 
                 Ronz 
               
               
                 (unipolar) 
                 Diode, A * A* 
                 20 
                 0.36 
                 2% 
               
               
                   
                 Rond 
               
               
                 Conduction 
                 Switch, A * Vonz 
                 5.8 
                 4.3 
                 74% 
               
               
                 Loss B 
                 Diode, A * Vond 
                 15 
                 0.1 
                 1% 
               
               
                 (bipolar) 
               
               
                   
               
             
          
         
       
     
         [0049]    In the above, Pz is switching loss per peak current for the switch; Pd is switching loss per peak current for the diode; Ronz, is on-resistance for the switch; Rond is on-resistence for the diode; Vonz is on-voltage for the switch; Vond is on-voltage for the diode. 
         [0050]    The present invention has been described in terms of specific embodiments. As will be apparent to those skilled in the art, various changes and modifications may be made without departing from the spirit and scope of the invention. For example, the heating circuit has been described in the context of a rice cooker but is not limited to such a device. The scope of the invention should be interpreted using the appended claims.