Electromagnetic induced heating device

Abstract

To provide an electromagnetic induction heating device that can efficiently supply the desired power to a heated object.SOLUTION: An electromagnetic induction heating device includes a heating coil, a DC power supply, a step-down chopper circuit, and an inverter. The step-down chopper circuit includes: a first upper and lower arms being a series body of two switching elements; a step-down choke coil with one end connected to the output terminal of the first upper and lower arms; and a smoothing capacitor connected between the other end of the step-down choke coil and one end of the DC power supply. The inverter includes: second upper and lower arms being a series body of two switching elements; a heating coil with one end connected to the output terminal of the second upper and lower arms; a first resonant capacitor connected between the other end of the heating coil and one end of the smoothing capacitor; and a series body of a relay connected between the output terminal of the first upper and lower arms and the other end of the heating coil and a second resonant capacitor. The first upper and lower arms, when not operating as a part of the inverter, operates as a part of the step-down chopper circuit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an inverter-type electromagnetic induction heating device that supplies a desired amount of power to an object to be heated, such as a metal pot, to perform induction heating. [Background technology]

[0002] In recent years, inverter-type electromagnetic induction heating devices that heat objects such as metal pots without using fire have become widely used. Electromagnetic induction heating devices pass a high-frequency current through a heating coil, generating eddy currents in a metal object (such as a pot) placed close to the heating coil, causing heat to be generated by the electrical resistance of the object itself. Generally, magnetic objects such as iron, which have a high specific resistance, are easy to heat, while non-magnetic objects such as copper and aluminum, which have a low resistance, are difficult to heat.

[0003] A conventional technique for solving such problems is the induction heating device disclosed in Patent Document 1. As shown in Figure 6 of the document, this device uses an inverter that converts the DC voltage output from a step-down circuit into AC voltage, and switches the inverter between a full-bridge circuit configuration and a half-bridge circuit configuration depending on the state and type of the object to be heated.When heating an object made of a non-magnetic material such as aluminum, one leg of the full-bridge circuit is disconnected from the circuit and a half-bridge circuit using only the other leg is driven to supply current to the heating coil. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4910004 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, when a high-resistance magnetic pot is heated, a switch (relay) that switches the circuit type of the high-frequency inverter and the capacity of the resonant capacitor is turned on, the inverter is switched to a full-bridge circuit type, and the magnetic pot is induction-heated using a first resonant capacitor (Figure 1 of the same document, reference number 12) and a second resonant capacitor (same document, reference number 13).

[0006] On the other hand, when heating a low-resistance non-magnetic pot, the switch (relay) is turned off, the inverter is switched to a half-bridge circuit configuration, the large-capacity second resonant capacitor is completely disconnected, and the small-capacity first resonant capacitor is used to induction heat the non-magnetic pot. In this way, when heating a low-resistance non-magnetic pot, the inverter operates in a half-bridge circuit configuration using only one of the upper and lower arms, so the other upper and lower arms are idle and not used effectively.

[0007] The object of the present invention is to address the above-mentioned problems, and in particular to provide an inverter-type electromagnetic induction heating device that effectively utilizes all of the upper and lower arms of the inverter and can efficiently supply the desired power to heated objects made of different materials. [Means for solving the problem]

[0008] In order to achieve the above object, the electromagnetic induction heating device of the present invention is an electromagnetic induction heating device comprising: a heating coil for induction heating an object to be heated; a DC power supply for outputting a DC voltage; a step-down chopper circuit for stepping down the DC voltage output by the DC power supply; and an inverter for converting the output voltage of the step-down chopper circuit into an AC voltage and supplying it to the heating coil, wherein the step-down chopper circuit comprises a first upper and lower arm which is a series body of two switching elements; a step-down choke coil having one end connected to an output terminal of the first upper and lower arm; and an inverter between the other end of the step-down choke coil and one end of the DC power supply. and a smoothing capacitor connected to the output terminal of the inverter, the inverter having a second upper and lower arm which is a series body of two switching elements, the heating coil having one end connected to the output terminal of the second upper and lower arm, a first resonant capacitor connected between the other end of the heating coil and one end of the smoothing capacitor, and a series body of a relay and the second resonant capacitor connected between the output terminal of the first upper and lower arm and the other end of the heating coil, the first upper and lower arm being an electromagnetic induction heating device which operates as part of the step-down chopper circuit when not operating as part of the inverter. [Effects of the Invention]

[0009] According to the electromagnetic induction heating device of the present invention, despite the small number of parts, the optimum inverter voltage and drive frequency can be set even under conditions of large load fluctuations, and the desired power can be supplied to the load efficiently. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a circuit diagram of an electromagnetic induction heating device according to a first embodiment. [Figure 2] FIG. 2 is an explanatory diagram of a half-bridge circuit of the electromagnetic induction heating device of the first embodiment. [Figure 3A] Operating waveform of the electromagnetic induction heating device in Figure 2. [Figure 3B] Operating waveform of the electromagnetic induction heating device in Figure 2. [Figure 4] 3 is a diagram showing the operating characteristics of the electromagnetic induction heating device shown in FIG. 2. [Figure 5] 3 is a diagram showing the operating characteristics of the electromagnetic induction heating device shown in FIG. 2. [Figure 6] FIG. 2 is an explanatory diagram of a full-bridge circuit of the electromagnetic induction heating device according to the first embodiment. [Figure 7] Figure 6 shows the operating waveform of the electromagnetic induction heating device. [Figure 8] FIG. 7 is a diagram showing the operating characteristics of the electromagnetic induction heating device of FIG. 6. [Figure 9] Figure 6 shows the operating waveform of the electromagnetic induction heating device. [Figure 10] FIG. 7 is a diagram showing the operating characteristics of the electromagnetic induction heating device of FIG. 6. [Figure 11] FIG. 7 is a diagram showing the operating characteristics of the electromagnetic induction heating device of FIG. 6. [Figure 12] Figure 6 shows the operating waveform of the electromagnetic induction heating device. [Figure 13] FIG. 7 is a diagram showing the operating characteristics of the electromagnetic induction heating device of FIG. 6. [Figure 14] FIG. 7 is a diagram showing the operating characteristics of the electromagnetic induction heating device of FIG. 6. [Figure 15] FIG. 10 is a circuit diagram of an electromagnetic induction heating device according to a second embodiment. [Figure 16] FIG. 10 is a circuit diagram of an electromagnetic induction heating device according to a third embodiment. [Figure 17] FIG. 10 is a circuit diagram of an electromagnetic induction heating device according to a fourth embodiment. [Figure 18] FIG. 10 is a circuit diagram of an electromagnetic induction heating device according to a fifth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the electromagnetic induction heating device of the present invention will be described with reference to the drawings. In each drawing, the same reference numerals indicate the same components or components with similar functions, and redundant explanations will be omitted where appropriate. [Example]

[0012] 1 is a circuit diagram of an electromagnetic induction heating device according to a first embodiment of the present invention. The electromagnetic induction heating device of this embodiment has a heat-resistant glass top plate installed on top of a metal housing, and a high-frequency current is supplied to a heating coil located below the top plate to induction heat a metal object to be heated (such as a pot) placed at a predetermined position on the top surface of the top plate, but a description of this well-known configuration will be omitted below.

[0013] 1, first upper and lower arms 3, each having power semiconductor switching elements (hereinafter simply referred to as "switching elements") 5a and 5b connected in series, are connected between the positive and negative electrodes of a DC power supply 35. The switching elements 5a and 5b of the first upper and lower arms 3 are IGBTs (insulated gate bipolar transistors) suitable for large currents. Diodes 6a and 6b are connected in parallel in the opposite directions to the switching elements 5a and 5b, respectively.

[0014] One end of the step-down choke coil 41 is connected to the connection point (output terminal) of the switching elements 5a, 5b of the first upper and lower arms 3, and a smoothing capacitor 44 is connected between the other end of the step-down choke coil 41 and the negative electrode of the DC power supply 35.

[0015] The second upper and lower arms 4, each having switching elements 5c and 5d connected in series, are connected in parallel to the smoothing capacitor 44. IGBTs (insulated gate bipolar transistors) suitable for large currents are used as the switching elements 5c and 5d of the second upper and lower arms 4. Diodes 6c and 6d are connected in parallel in the opposite direction to the switching elements 5c and 5d, respectively.

[0016] One end of the heating coil 11 is connected to the connection point (output terminal) of the switching elements 5c and 5d of the second upper and lower arms 4, and first resonant capacitors 12a and 12b are connected between the other end of the heating coil 11 and the positive and negative electrodes of the smoothing capacitor 44, respectively.

[0017] Furthermore, a series circuit of a second resonant capacitor 13 and a relay 20 is connected between the output terminal of the first upper and lower arms 3 and the other end of the heating coil 11 .

[0018] Here, because a metal pot (not shown) placed on the top plate and heating coil 11 are magnetically coupled, when the metal pot is converted into an equivalent circuit viewed from the heating coil 11 side, the equivalent resistance and equivalent inductance of the metal pot are connected in series. The equivalent resistance and equivalent inductance vary depending on the material of the metal pot; both are small for non-magnetic materials such as low-resistance copper or aluminum, and both are large for magnetic materials such as high-resistance iron. Below, we will explain the differences between the circuit method used when heating non-magnetic materials (step-down chopper circuit + half-bridge inverter) and the circuit method used when heating magnetic materials (step-down full-bridge inverter).

[0019] <Heating method for non-magnetic materials> When the object to be heated is a non-magnetic material such as a copper or aluminum pot, the metal pot is induction heated using a step-down chopper circuit and a half-bridge inverter. Specifically, as shown in Figure 2, heating is performed using a step-down chopper circuit 40 (composed of the upper-arm switching element 5a and lower-arm diode 6b of the first upper and lower arms 3, the step-down choke coil 41, and the smoothing capacitor 44) that is formed when the relay 20 is turned off, and a half-bridge inverter 100 (composed of the second upper and lower arms 4, the heating coil 11, and the first resonant capacitors 12a and 12b).

[0020] As mentioned above, low-resistivity non-magnetic materials have low equivalent resistance, so a large current must be passed through them to obtain the desired output. The skin resistance of the heated object is proportional to the square root of the frequency, so increasing the frequency is effective when heating low-resistivity heated objects such as copper or aluminum. Therefore, the combined capacitance of the first resonant capacitors 12a and 12b is set so that the second upper and lower arms 4 can be driven at a frequency of, for example, approximately 90 kHz.

[0021] Figure 3 shows the operating waveforms of the circuit in Figure 2. In Figure 3, the gate signals of switching elements 5a, 5c, and 5d are vg(5a), vg(5c), and vg(5d), respectively, the current flowing through step-down choke coil 41 is i(41), the voltage of smoothing capacitor 44 is v(44), the currents flowing through switching elements 5a, 5c, and 5d are i(5a), i(5c), and i(5d), respectively, the currents flowing through diodes 6b, 6c, and 6d are i(6b), i(6c), and i(6d), respectively, the output voltage of inverter 100 is v(100), and the current flowing through heating coil 11 is i(11), and the direction from left to right in Figure 2 is defined as positive.

[0022] In FIG. 3, when the switching element 5a of the step-down chopper circuit 40 is in the on state, a current i(41) flows from the DC power supply 35 through the switching element 5a, the step-down choke coil 41, and the smoothing capacitor 44. When the switching element 5a is in the off state, the stored energy in the step-down choke coil 41 causes the current i(41) to circulate through the smoothing capacitor 44 and the diode 6b, thereby performing a step-down operation.

[0023] Inverter 100 applies a rectangular wave whose amplitude is the output voltage of step-down chopper circuit 40, i.e., voltage v(44) of smoothing capacitor 44, to heating coil 11 and a resonant load circuit including first resonant capacitors 12a and 12b by complementarily turning on and off switching elements 5c and 5d. By driving inverter 100 at a switching frequency higher than the resonant frequency, current i(11) flows through heating coil 11, lagging behind inverter 100's output voltage v(100). Zero-voltage switching is achieved by turning on the gates of switching elements 5c and 5d of upper and lower arms 4 while diodes 6c and 6d are conducting, eliminating switching loss during turn-on. Figure 3A shows the case where switching element 5a has an on-time duty cycle of 0.5, while Figure 3B shows the case where switching element 5a has an on-time duty cycle of 0.25. Comparing the two figures, it can be seen that when the on-time duty of the switching element 5a is reduced, the voltage v(44) of the smoothing capacitor 44 decreases and the current i(11) of the heating coil 11 decreases.

[0024] The power control method for the circuit in FIG. 2 will be described using FIGS. 4 and 5. As mentioned above, the step-down chopper circuit 40 can control the voltage of the smoothing capacitor 44 by changing the on-time duty of the switching element 5a. FIG. 4 shows the relationship between the on-time duty and the output voltage of the step-down chopper circuit 40. The output voltage is the voltage of the DC power supply 35 multiplied by the on-time duty, and therefore changes in proportion to the on-time duty. The output voltage of the step-down chopper circuit 40 is the input voltage to the inverter 100. FIG. 5 shows the relationship between the input voltage and power of the inverter 100. To reduce the interruption current and minimize switching losses, the inverter 100 operates at a nearly constant frequency near the resonant frequency. This also maintains a nearly constant impedance of the resonant load circuit, including the heating coil 11 and the first resonant capacitors 12a and 12b. Therefore, the power can be controlled arbitrarily by adjusting the input voltage of the inverter 100. In other words, power can be controlled by pulse amplitude control.

[0025] <Magnetic material heating method> When the object to be heated is a magnetic material such as an iron pot, the metal pot is induction-heated using a step-down full-bridge inverter. Specifically, as shown in FIG. 6, heating is performed using a step-down chopper circuit 40 (comprising an upper-arm switching element 5a and a lower-arm diode 6b of the first upper and lower arms 3, a step-down choke coil 41, and a smoothing capacitor 44) formed when the relay 20 is turned on, and a step-down full-bridge inverter 200 (comprising a second upper and lower arms 4 to which the output voltage of the step-down chopper circuit 40 is applied, a first upper and lower arms 3 to which the voltage of the DC power supply 35 is applied, a heating coil 11, and a second resonant capacitor 13). Therefore, the switching elements and diodes of the first upper and lower arms 3 function as both the switching elements and diodes of the step-down chopper circuit 40 and the switching elements and diodes of the inverter 200.

[0026] As described above, a high-resistivity magnetic material has a large equivalent resistance, making it difficult for current to flow through the heating coil 11. Therefore, by using the first upper and lower arms 3 as one of the upper and lower arms of a full-bridge system, the output voltage of the inverter 200 is applied to a resonant load circuit including the heating coil 11 and the first and second resonant capacitors 12a, 12b, and 13, and a rectangular wave having an amplitude equal to the voltage of the DC power supply 35 and the output voltage of the step-down chopper circuit 40 is applied. In this way, the output voltage of the inverter 200 of the full-bridge circuit system can be made higher than the output voltage of the inverter 100 of the half-bridge circuit system, making it possible to obtain the desired output required for heating the magnetic material.

[0027] In the case of copper and aluminum, as mentioned above, the resistance is low, so the inverter frequency is increased to approximately 90 kHz to increase the skin resistance. However, in the case of iron, the resistance is inherently high, so the first upper and lower arms 3 and the second upper and lower arms 4 are driven at a frequency of approximately 20 kHz. For this reason, the capacitance of second resonant capacitor 13 is set to match the drive frequency of approximately 20 kHz. Because the drive frequencies are significantly different, the capacitance of second resonant capacitor 13 is significantly larger than that of first resonant capacitors 12a and 12b. Therefore, the resonant frequency of the full-bridge inverter is mainly set by second resonant capacitor 13.

[0028] Figure 7 shows the operating waveforms of the circuit in Figure 6. In Figure 7, the gate signals of switching elements 5a to 5d are vg(5a) to vg(5d), respectively, the current flowing through step-down choke coil 41 is i(41), the voltage of DC power supply 35 is v(35), the voltage of smoothing capacitor 44 is v(44), the currents flowing through switching elements 5a to 5d are i(5a) to i(5d), respectively, the currents flowing through diodes 6a to 6d are i(6a) to i(6d), respectively, the output voltage of inverter 200 is v(200), the current flowing through heating coil 11 is i(11), and the direction from left to right in Figure 6 is defined as positive.

[0029] 7, during the period when switching elements 5a and 5d are simultaneously on, switching element 5a serves both as an element of step-down chopper circuit 40 and inverter 200, and a combined current of current i(41) of step-down choke coil 41 and current i(11) of heating coil 11 flows. Therefore, current i(5a) of switching element 5a is larger than current i(5d) of switching element 5d, and conduction loss increases. However, when turned on, current i(11) of heating coil 11 circulates via diode 6b, enabling zero-voltage switching operation and no turn-on loss. In other words, in this embodiment of the present invention, the upper and lower arms 3 also serve as elements of step-down chopper circuit 40, and by combining this with inverter operation, zero-voltage switching operation can be achieved and an increase in switching loss can be suppressed.

[0030] Next, several control methods that can be considered as a power control method when heating a magnetic material will be explained in order.

[0031] <<First control method>> The first control method is to change the switching frequency of inverter 200. Figure 8 shows the relationship between the switching frequency of inverter 200 and power. When inverter 200 is driven at a switching frequency higher than the resonant frequency of the resonant load circuit, the resonant load circuit becomes inductive. Therefore, increasing the switching frequency increases the reactance, reducing the current in the heating coil and resulting in a drop in power, as shown in Figure 8. In other words, power can be easily controlled using pulse frequency modulation control.

[0032] <<Second control method>> The second control method is to change the on-time duty of the upper-arm switching element 5a of the upper and lower arms 3. A dead time is provided for the lower-arm switching element 5b of the upper and lower arms 3, allowing it to operate complementarily with the switching element 5a. Figure 9 shows the operating waveforms when the on-time duty of the switching element 5a is set to 0.25. In Figure 9, when the on-time duty of the switching element 5a is reduced, the voltage v(44) of the smoothing capacitor 44 decreases compared to Figure 7. The output voltage v(44) of the step-down chopper circuit 40 is applied to the second upper and lower arms 4 of the inverter 200, and the voltage v(35) of the DC power supply 35 is applied to the first upper and lower arms 3 of the inverter 200, resulting in operation as a full-bridge inverter. As shown in Figure 9, the output voltage vout(200) of the inverter 200 turns off earlier than the switching element 5d. Therefore, there are periods when the output voltage is 0V, and positive and negative voltages with different voltage amplitudes are applied to the resonant load circuit.

[0033] Figure 10 shows the relationship between the upper arm on-time duty and the output voltage of the step-down chopper circuit 40. As mentioned above, the output voltage of the step-down chopper circuit 40 is the voltage of the DC power supply 35 multiplied by the on-time duty, and therefore changes in proportion to the on-time duty. Figure 11 shows the relationship between the upper arm on-time duty and power. Because the voltage applied to the second upper and lower arms 4 changes, it is possible to control power by combining pulse amplitude control of the inverter input voltage and pulse width modulation control of the first upper and lower arms 3.

[0034] <<Third control method>> The third control method is a method of changing the phase difference between the first upper and lower arms 3 and the second upper and lower arms 4 of the inverter 200. Fig. 12 shows operating waveforms under conditions in which the phase of the switching elements (5c, 5d) of the second upper and lower arms 4 is advanced relative to the phase of the switching elements (5a, 5b) of the first upper and lower arms 3. Note that the on-time of each arm is the same, and only the phase difference is changed.

[0035] In Figure 12, the on-time of switching element 5a is under the same conditions as in Figure 7, so the voltage v(44) of smoothing capacitor 44 is approximately the same value as in Figure 7. Because there is a phase difference between upper and lower arms 3 and 4, during the period when both upper arms are in the on state, the output voltage vout(200) of inverter 200 is the difference between the voltage v(35) of DC power supply 35 and the voltage v(44) of smoothing capacitor 44 of step-down chopper circuit 40. Conversely, during the period when both lower arms are in the on state, the output voltage vout(200) of inverter 200 is 0 V. Because the current of step-down choke coil 41 is superimposed on switching elements 5a and 5b, they can be turned off while ensuring the interruption current required for zero voltage switching, so no switching loss occurs when they are turned on.

[0036] 13 shows the relationship between the phase difference between the first upper and lower arms 3 and the second upper and lower arms 4 and the power. As shown here, as the phase difference increases, the output voltage of the inverter 200 decreases, and the power decreases. In this way, it is possible to control the power by phase shift control as well.

[0037] <<Fourth control method>> The fourth control method is to change the voltage of the DC power supply 35. Figure 14 shows the relationship between the voltage and power of the DC power supply 35. By increasing the voltage of the DC power supply 35, the output voltage of the step-down chopper circuit 40 also increases, so the output voltage of the inverter 200 increases and the power increases. In other words, power can be easily controlled by pulse amplitude control.

[0038] In this way, in this embodiment, it is possible to switch between a circuit configuration consisting of a step-down chopper circuit and half-bridge inverter for heating non-magnetic materials, and a circuit configuration consisting of a step-down full-bridge inverter for heating magnetic materials, by turning relay 20 on and off. In either case, all of the upper and lower arms are effectively utilized to adjust the inverter output voltage, and heating can be performed at an optimum frequency according to the material of the object to be heated. Therefore, with the electromagnetic induction heating device of this embodiment, despite having a small number of parts, the optimum inverter voltage and drive frequency can be set even under conditions of large load fluctuations, and the desired power can be efficiently supplied to the load. [Example]

[0039] Fig. 15 is a circuit diagram of an electromagnetic induction heating device according to Example 2. The same parts as those in Fig. 1 are given the same reference numerals, and duplicated explanations will be omitted.

[0040] 15, one end of a step-down choke coil 41 is connected to the output terminal of the first upper and lower arms 3, and a smoothing capacitor 44 is connected between the other end of the step-down choke coil 41 and the positive electrode of the DC power supply 35. This differs from the first embodiment in FIG. 1 in that the smoothing capacitor 44 is connected to the high side (positive electrode side). In this case, the switching element of the step-down chopper circuit 40 is the lower arm switching element 5b of the upper and lower arms 3, and the free wheel diode is the upper arm diode 6a. [Example]

[0041] Fig. 16 is a circuit diagram of an electromagnetic induction heating device according to Example 3. The same parts as those in Fig. 1 are given the same reference numerals, and duplicated explanations will be omitted.

[0042] In FIG. 16 , the switching elements 5c and 5d of the second upper and lower arms 4 are power semiconductor switching elements with a faster switching speed than the switching elements 5a and 5b of the first upper and lower arms 3. As described above, increasing the frequency is effective when heating a low-resistance object such as copper or aluminum, and the second upper and lower arms 4 are driven at a higher frequency than the first upper and lower arms 3. Therefore, by using SJ (super junction)-MOSFETs (MOS field-effect transistors) for the switching elements 5c and 5d of the second upper and lower arms 4, faster switching is possible than with existing MOSFETs, which is effective in reducing switching losses due to higher frequencies. Furthermore, since low-resistance non-magnetic materials have low equivalent resistance, a large current must be passed to obtain the desired output. In the present invention, the voltage applied to the second upper and lower arms 4 can be lower than the voltage applied to the first upper and lower arms 3, so the switching elements 5c and 5d of the second upper and lower arms 4 can be power semiconductor switching elements with a lower withstand voltage than the switching elements 5a and 5b of the first upper and lower arms 3. The lower the breakdown voltage of a power semiconductor device, the smaller its on-resistance, which is also effective in reducing conduction loss. [Example]

[0043] Fig. 17 is a circuit diagram of an electromagnetic induction heating device according to Example 4. The same parts as those in Fig. 1 are given the same reference numerals and redundant explanations will be omitted.

[0044] 17, an AC voltage supplied from a commercial AC power supply 1 is rectified by diodes 2a to 2d, and the DC power smoothed by a boost chopper circuit 30 composed of a boost choke coil 31, a switching element 32, a diode 33, and a smoothing capacitor 34 is applied to a step-down chopper circuit 40. In the first embodiment of FIG. 1, the voltage of a DC power supply 35 is applied to the step-down chopper circuit 40, but in the fourth embodiment, the DC power smoothed by the boost chopper circuit 30 is applied to the step-down chopper circuit 40. The step-up chopper circuit 30 has a function of controlling the input power factor of the commercial AC power supply 1 and also a function of controlling the voltage of the smoothing capacitor 34.

[0045] Furthermore, inverter 300 is connected to the rear of boost chopper circuit 30 via a normal filter composed of inductor 45 and capacitor 46. Inverter 300 is a half-bridge inverter that mainly heats a magnetic pot. Inverter 300 has upper and lower arms, each of which has switching elements 5e and 5f connected in series, connected between the positive and negative electrodes of capacitor 46. One end of heating coil 11A is connected to the connection point (output terminal) of switching elements 5e and 5f, and resonance capacitors 12c and 12d are connected between the other end of heating coil 11A and the positive and negative electrodes of capacitor 46, respectively. Diodes 6e and 6f are connected in parallel in the opposite directions to switching elements 5e and 5f, respectively.

[0046] Furthermore, inverter 400 is connected to the rear of boost chopper circuit 30 via a normal filter composed of inductor 47 and capacitor 48. Like inverter 300, inverter 400 is also a half-bridge inverter and mainly heats a magnetic pot. In inverter 400, upper and lower arms, each consisting of switching elements 5g and 5h connected in series, are connected between the positive and negative electrodes of capacitor 48. One end of heating coil 11B is connected to the connection point (output terminal) of switching elements 5g and 5h, and resonant capacitors 12e and 12f are connected between the other end of heating coil 11B and the positive and negative electrodes of capacitor 48, respectively. Diodes 6g and 6h are connected in parallel in the opposite directions to switching elements 5g and 5h, respectively.

[0047] Here, electromagnetic induction cookers currently available on the market have two or three heating elements. For example, one of the heating elements can heat all metal pans, including non-magnetic pans such as aluminum and copper, while the remaining heating elements are designed to heat only magnetic pans, resulting in a more affordable cooker. In such affordable cookers, when primarily heating magnetic pans, the AC voltage supplied from a commercial AC power source is rectified by a diode bridge, and the resulting unsmooth power is applied to the inverter via a normal filter consisting of an inductor and a capacitor. As a result, the current flowing through the heating coil pulsates at twice the frequency of the commercial AC power source, which can cause excitation noise depending on the material of the pan. However, in this embodiment, DC power smoothed by a boost chopper circuit 30 is applied to the inverters 300 and 400 via a normal filter, thereby suppressing current pulsation in the heating coil and preventing excitation noise. [Example]

[0048] Fig. 18 is a circuit diagram of an electromagnetic induction heating device according to Example 5. The same parts as those in Fig. 16 are given the same reference numerals, and duplicated explanations will be omitted.

[0049] 18 differs from the fourth embodiment in that inverter 300 is connected downstream of step-down chopper circuit 40. When inverter 100 or inverter 200 is stopped, inverter 300 is operated at a substantially constant frequency near the resonant frequency to reduce the cutoff current, and the output voltage of step-down chopper circuit 40 can be adjusted to control power. When inverter 100 or inverter 200 is operating, step-down chopper circuit 40 is controlled with priority given to the operation of inverter 100 or inverter 200, which causes the input voltage of inverter 300 to fluctuate. However, this does not pose a problem because inverter 300 can control power using pulse frequency modulation control. [Explanation of symbols]

[0050] 1 Commercial AC power supply, 2a~2d, 6a~6h, 33 diodes, 3 first upper and lower arms, 4 second upper and lower arms, 5a to 5h, 32 switching elements, 11 heating coil, 12, 13 Resonant capacitor, 20 relays, 30 Boost chopper circuit, 31 Boost choke coil, 34, 44 smoothing capacitor, 35 DC power supply, 40 Step-down chopper circuit 41 Step-down choke coil, 45, 47 Inductors, 46, 48 capacitors, 100, 200, 300, 400 inverter

Claims

1. a heating coil for induction heating an object to be heated; a DC power supply that outputs a DC voltage; a step-down chopper circuit that steps down the DC voltage output by the DC power supply; an inverter that converts the output voltage of the step-down chopper circuit into an AC voltage and supplies the AC voltage to the heating coil; An electromagnetic induction heating device comprising: The step-down chopper circuit comprises: a first upper and lower arm which is a series body of two switching elements; a step-down choke coil having one end connected to the output terminal of the first upper and lower arms; a smoothing capacitor connected between the other end of the step-down choke coil and one end of the DC power supply, The inverter is a second upper and lower arm which is a series body of two switching elements; the heating coil having one end connected to the output terminal of the second upper and lower arms; a first resonant capacitor connected between the other end of the heating coil and one end of the smoothing capacitor; a series circuit of a relay and a second resonant capacitor connected between the output terminals of the first upper and lower arms and the other end of the heating coil, The electromagnetic induction heating device according to claim 1, wherein the first upper and lower arms operate as part of the step-down chopper circuit when not operating as part of the inverter.

2. a heating coil for induction heating an object to be heated; a DC power supply that outputs a DC voltage; a step-down chopper circuit that steps down the DC voltage output by the DC power supply; an inverter that converts the output voltage of the step-down chopper circuit into an AC voltage and supplies the AC voltage to the heating coil; An electromagnetic induction heating device comprising: The step-down chopper circuit comprises: a first upper and lower arm, which is a series body of two switching elements connected in parallel to the DC power supply; a step-down choke coil having one end connected to the output terminal of the first upper and lower arms; a smoothing capacitor connected between the other end of the step-down choke coil and one end of the DC power supply, The inverter is a second upper and lower arms each including two switching elements connected in series and connected in parallel to the smoothing capacitor; the heating coil having one end connected to the output terminal of the second upper and lower arms; a first resonant capacitor connected between the other end of the heating coil and one end of the smoothing capacitor; a series circuit of a relay and a second resonant capacitor connected between the output terminals of the first upper and lower arms and the other end of the heating coil, Each switching element has a diode connected in reverse parallel. The first upper and lower arms include: When the relay is off, it operates as a part of the step-down chopper circuit, When the relay is turned on, it operates as a part of the step-down chopper circuit and also as a part of the inverter, The electromagnetic induction heating device is characterized in that the second upper and lower arms operate as part of the inverter regardless of whether the relay is on or off.

3. 3. The electromagnetic induction heating device according to claim 2, An electromagnetic induction heating device, characterized in that the second upper and lower arms use switching elements that are faster than those of the first upper and lower arms.

4. 3. The electromagnetic induction heating device according to claim 2, An electromagnetic induction heating device characterized in that, when the object to be heated is a non-magnetic material, the relay is turned off, thereby causing the inverter to be of a half-bridge type.

5. 5. The electromagnetic induction heating device according to claim 4, The electromagnetic induction heating device is characterized in that the step-down chopper circuit adjusts the power of the inverter by controlling the amplitude of the voltage applied to the second upper and lower arms.

6. 3. The electromagnetic induction heating device according to claim 2, An electromagnetic induction heating device characterized in that, when the object to be heated is a magnetic body, the relay is turned on to cause the inverter to be in a full-bridge mode.

7. 7. The electromagnetic induction heating device according to claim 6, The electromagnetic induction heating device is characterized in that the inverter controls a switching frequency to adjust power.

8. 7. The electromagnetic induction heating device according to claim 6, the step-down chopper circuit controls the amplitude of a voltage applied to the second upper and lower arms; The electromagnetic induction heating device is characterized in that the inverter adjusts power by controlling the pulse width of the voltage applied to the heating coil.

9. 7. The electromagnetic induction heating device according to claim 6, The electromagnetic induction heating device is characterized in that the inverter adjusts power by controlling a phase difference between a first upper and lower arm and a second upper and lower arm.

10. 7. The electromagnetic induction heating device according to claim 6, The electromagnetic induction heating device is characterized in that the inverter adjusts power by controlling the amplitude of the voltage applied to the first upper and lower arms.

11. 11. The electromagnetic induction heating device according to claim 1, the DC voltage output from the DC power supply is a voltage obtained by rectifying an AC voltage supplied from a commercial AC power supply and then smoothing it by a boost chopper circuit; An electromagnetic induction heating device, characterized in that an inverter different from the inverter is connected to the subsequent stage of the step-up chopper circuit.

12. The electromagnetic induction heating device according to claim 11, An electromagnetic induction heating device, characterized in that an inverter different from the inverter is connected to the subsequent stage of the step-down chopper circuit.

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