Induction heating device having plural resonant circuits

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.

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.

FIG. 1shows an induction heating cooker100that has a metal bowl102and a heating coil104that together define a transformer. The turns ratio is n:1, where n is the number of coil turns.FIG. 2illustrates 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.

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.

FIG. 3illustrates a half-bridge type circuit150that could be used as the heating circuit. The circuit150includes a voltage source152, transistors154in a half bridge configuration, a transformer156, and a blocking capacitor158to block DC currents. A resistor R′ represents the resistance of the secondary side, i.e., metal bowl, of the transformer156. 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.

FIG. 4illustrates a class-E converter type circuit180as an induction heating circuit. The circuit180includes a voltage source182, a transformer184, a transistor186, and a capacitor188that is in parallel to the transistor186. The circuit180uses 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 transistor186sees much larger voltage than the DC link voltage. The transistor186needs have a high breakdown voltage.

BRIEF SUMMARY OF THE INVENTION

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.

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.

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.

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.

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an induction heating circuit.FIG. 5Aillustrates an induction heating circuit200according to one embodiment of the present invention. The heating circuit200relates 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.

The heating circuit200includes a voltage source202, a transformer (or heating coil)204, a first capacitor206parallel to the transformer (or heating coil)204, an inductor208, a second capacitor210, and a transistor211. The second capacitor210is parallel to the transistor211. The capacitors206and210are resonant components. The transformer (heating coil)204includes a first heating coil212on the primary side and a metal bowl213on the secondary side. The inductor208may 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”.

FIGS. 5Billustrates an induction heating circuit230according to one embodiment of the present invention. The circuit230has a first capacitor232parallel to the a heating coil234and a second capacitor236parallel to the union of an inductor238and the heating coil234. The heating coil is part of a transformer. A switch240is connected to a node between the second capacitor236and the inductor238. The switch may be an Insulated Gate Bipolar Transistor (IGBT) or Bipolar Junction Transistor (BJT). A diode242is connected anti-parallel to the switch240. The first and second capacitors232and236are resonant capacitors. The above configuration enables capacitors with a lower voltage rating to be used as the first and second capacitors232and236.

FIG. 5Cillustrates an induction heating circuit250according to one embodiment of the present invention. The circuit250has a first capacitor252parallel to a heating coil254and a second capacitor256parallel to the union of an inductor258and the heating coil254. The inductor258is provided above the heating coil254, i.e., closer to the positive rail, in the present embodiment. A switch260is connected to a node common to the first capacitor252, the second capacitor256and the heating coil254. The switch may be an Insulated Gate Bipolar Transistor (IGBT) or Bipolar Junction Transistor (BJT). A diode242is connected anti-parallel to the switch260. The first and second capacitors232and236are resonant capacitors.

FIG. 6illustrates partial view of a rice cooker300having a first heating coil302provided below a metal bowl304and a second heating coil306provided around the side of the metal bowl304according to one embodiment of the present invention.

FIG. 7Aillustrates an equivalent circuit301associated with the rice cooker300ofFIG. 6. The circuit301includes a voltage source312, a first transformer (or first heating coil)314, a capacitor316parallel to the first heating coil314, a second transformer (or second heating coil)318, a capacitor320, and a transistor321. The capacitors316,320are resonant components.

The second heating coil318is wrapped around the metal bowl304to more effectively use the energy consumed in the inductor, i.e., the heat generated by the parasitic resistance therein. The capacitor320is provided in parallel to the transistor321. Alternatively, the first heating coil314may be a heating coil that is wrapped around the metal bowl and the second heating coil318may have a heating coil that is provided below the metal bowl.

FIG. 7Billustrates another equivalent circuit330associated with the rice cooker300ofFIG. 6. The circuit330has a first capacitor332parallel to a first heating coil334and a second capacitor336parallel to the union of the first and second heating coils334and338. The capacitors and the heating coils are resonant components. A switch340is connected to a node between the second capacitor336and the second heating coil338. A diode342is connected anti-parallel to the switch340. The capacitors and the heating coils are resonant components defining resonant loops.

FIG. 7Cillustrates another equivalent circuit350associated with the rice cooker300ofFIG. 6. The circuit350has a first capacitor352parallel to a first heating coil354and a second capacitor356parallel to the union of the first and second heating coils354and358. The second heating coil is provided above the first heating coil in the present embodiment, i.e., the second heating coil358is closer to the positive rail than the first heating coil354. A switch360is connected to a node common to the first capacitor352, the second capacitor356, and the first heating coil354. A diode362is connected anti-parallel to the switch360.

Some of the advantages of the circuit301includes the following. The circuit301has 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.

FIG. 8illustrates an equivalent circuit for the heating coil and the metal bowl ofFIG. 6. Lmrepresents the primary side magnetizing inductance. A transformer352is deemed to be an ideal n:1transformer 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 circuit360including an inductor Lmand a resistor R in parallel, as shown inFIG. 9.

FIG. 10illustrates a heating circuit400according conventional technology. The circuit400is a Class-E type circuit and includes a voltage source402, an inductor Lm, a resistor R, a transistor404, a diode406, and a capacitor408. The inductor Lm and the resistor R are in parallel between the voltage source402and a node410. The transistor404and the capacitor408are in parallel between the node410and the ground. The capacitor408is a resonant component. The diode406may be a body diode of the transistor or a separate diode thereof.

In operation, the switch voltage VSWis not deep negative. The diode410prevents VSWfrom going deep negative.FIG. 11illustrates operating waveforms of the circuit400. The circuit is a Class-E converter type induction heat circuit. The average voltage of VSWis the same as VDCsince 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.

At t0, as inductor current ILmbecomes zero, the diode stops conducting. Between t0˜t1, ILmlinearly increases with the slope of Vdc/Lm. At t1, the switch Sw is turned off. At t1˜t2, ILmincreases and reaches maximum at t2. At t2˜t3, ILmdecreases and reaches zero at t3. Between, t3˜t4, ILmdecreases and reaches the negative peak at t4. Between t4˜t5ILmincreases. At t5, the voltage VSWbecomes zero and the diode starts conducting inductor current. Between t5˜t6, ILmlinearly increases with the slope of VDC/Lm. At t6, the initial state t0is reached.

The voltage VDC-VSWis the output voltage, where VDCis 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 VSWdoes not return to zero at t5. Then, the switch cannot be turned on at zero voltage and would result in much switching loss.

FIG. 12illustrates a heating circuit according to one embodiment of the present invention. The resonant circuit comprising Cr1and Lm1is oscillating at the same frequency of the switching of the switch Sw. The switch Sw is turned on when the voltage VO2across it becomes zero. It is turned off when the current across it starts decreasing. This switching method is one of many possible methods.

FIG. 13illustrates circuits used to heat a rice cooker according a conventional technology. A heating circuit500used to heat the metal bowl of the rice cooker and includes a switch Z1. The switch is an IGBT. A sensor502is used to send a signal G_ON when the output voltage goes to negative. A gate driver504outputs a control signal G that is used to turn on the switch Z1according to the signal G_ON. The gate driver504may be configured to turn-on or turn-off periodically without any input from the sensor502.FIG. 14illustrates waveforms of simulation for with the circuits500,502,504.

FIG. 15illustrates circuits used to heat a rice cooker according to one embodiment of the present invention. A heating circuit600used to heat the metal bowl of the rice cooker. The circuit600includes a switch Z1and an inductor LS1. The switch Z1is controlled by an input G. The inductor LS1is used to detect the derivative of the total current. A sensor602receives a signal D1and determines whether or not the derivative of the switch current has a negative slope. A gate driver604outputs a control signal that is used to turn on the switch Z1of the circuit600according to the signals received from the sensor602. The gate driver604may be configured to turn-on or turn-off periodically without any input from the sensor602.FIG. 16illustrates waveforms of simulation for with the circuits600,602,604.

Below are the results of the simulation on the circuits500,600. The parameters of the circuits were chosen to make output power and operating frequency similar to each other.

The switching power loss is nearly proportional to the peak currents. The conduction loss for unipolar device is the product of (RMS current)2and 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.

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.

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.