Induction heater and rice cooker

Abstract

An induction heater and a rice cooker. The induction heater includes a heating coil, an inverter circuit, and a control circuit. The inverter circuit generates and outputs power supplied to the heating coil from the power supply circuit. The control circuit causes the inverter circuit to repeatedly output the power supply at a period T, which includes a conduction period Ton during which the power supply is output and a non-conducting period Toff during which the power supply is not output, thereby adjusting the effective value of the power supply. From the time the indicated value of the power supply is set to the target value (P2) of the power supply during the conduction period Ton until the end of the conduction period Ton, the control circuit performs a decreasing process, either by gradually decreasing the indicated value or the slope of the time change of the indicated value, or by continuously decreasing the indicated value.

Description

Technical Field

[0001] This disclosure relates to an induction heater and a rice cooker. More specifically, this disclosure relates to an induction heater that controls the duty cycle of the power supplied to a heating coil, and a rice cooker equipped with the induction heater. Background Technology

[0002] Japanese Patent Application Publication No. 10-113278 (hereinafter referred to as Patent Document 1) discloses a rice cooker as an induction heating cooker. The rice cooker includes: a first heating coil for heating the bottom surface of an inner pot that is detachably housed within a housing; and a second heating coil for heating the lower side of the inner pot. Summary of the Invention

[0003] In the rice cooker described in Patent Document 1, if the power supplied to the first heating coil or the second heating coil is increased, the voltage fluctuation sometimes becomes larger. This increased voltage fluctuation causes flickering. This disclosure provides an induction heater and rice cooker capable of reducing flickering.

[0004] One aspect of the disclosed induction heater includes a heating coil, an inverter circuit, and a control circuit. The inverter circuit generates and outputs power supplied to the heating coil from power from a power supply circuit.

[0005] The control circuit causes the inverter circuit to repeatedly output power in a cycle that includes a conduction period during which power is supplied and a disconnection period during which power is not supplied, thereby adjusting the effective value of the supplied power.

[0006] The control circuit, from the time the indicated value of the supplied power is set to the target value until the end of the conduction period, performs a decreasing process, which either reduces the indicated value or the slope of the time change of the indicated value in stages, or reduces the indicated value continuously.

[0007] Another aspect of the rice cooker disclosed herein includes the aforementioned induction heater and an inner pot for heating by the induction heater.

[0008] According to the method disclosed herein, flickering can be reduced. Attached Figure Description

[0009] Figure 1 This is a schematic diagram showing the structure of a rice cooker according to an embodiment of the present disclosure.

[0010] Figure 2 This is a diagram illustrating the duty cycle control of the rice cooker in the implementation method.

[0011] Figure 3This is a graph illustrating an example of the time variation of the collector-emitter voltage of the first inverter circuit during the rise period of an embodiment.

[0012] Figure 4 This is a graph illustrating an example of the time variation of the collector-emitter voltage of the first inverter circuit during the descent period of the implementation.

[0013] Figure 5 This is a diagram used to illustrate the duty cycle control of the comparative example.

[0014] Figure 6 This is a graph illustrating an example of the time variation of the collector-emitter voltage in the first inverter circuit during the rise period of the comparative example.

[0015] Figure 7 This is a graph illustrating the time variation of the collector-emitter voltage of the first inverter circuit during the falling period of the comparative example.

[0016] Figure 8 This is a graph illustrating an example of the time variation of the supply power (input power) supplied by the induction heater to the first heating coil in an embodiment.

[0017] Figure 9 This is a diagram used to illustrate the duty cycle control of the first variation.

[0018] Figure 10 This is a diagram used to illustrate the duty cycle control of the third variation. Detailed Implementation

[0019] Hereinafter, embodiments will be described in detail with appropriate reference to the accompanying drawings. However, necessary detailed descriptions will be omitted. For example, detailed descriptions of known matters and repeated descriptions of substantially the same structures will be omitted.

[0020] (1) Implementation method

[0021] (1-1)Overview

[0022] Figure 1 This is a schematic diagram showing the structure of a rice cooker 1 according to an embodiment of the present disclosure. The rice cooker 1 is an induction heating cooker. The rice cooker 1 includes an inner pot 100 and an induction heater 10. The inner pot 100 is the object to be heated. The induction heater 10 is a device for induction heating the object to be heated (the inner pot 100).

[0023] In the following embodiment, the induction heater 10 induction heats the inner pot 100 to cook rice. However, the induction heating cooker equipped with the induction heater 10 is not limited to the rice cooker 1, and may also be an IH cooking heater, an electric griddle, etc.

[0024] like Figure 1 As shown, the induction heater 10 includes a heating coil 31, a first inverter circuit 51, and a control circuit 60. The first inverter circuit 51 generates and outputs power supplied to the heating coil 31 from the output power of the power supply circuit 20.

[0025] like Figure 2 As shown, the control circuit 60 causes the first inverter circuit 51 to repeatedly output power at a period T, adjusting the effective value of the power supply. The period T includes the on-time Ton during which power is output and the off-time Toff during which power is not output.

[0026] The control circuit 60 performs a decreasing process. From the time the indicated value of the supplied power is set to the target value P2 until the end of the conduction period Ton, the control circuit 60, as a decreasing process, causes the indicated value or the slope of the time change of the indicated value to decrease in stages, or causes the indicated value to decrease continuously.

[0027] According to this descent process, the change in power per unit time can be reduced compared to the case where no descent process is performed. Therefore, voltage fluctuations, which are a cause of flicker, can be suppressed. As a result, flicker can be reduced.

[0028] (1-2) Details

[0029] The induction heater 10 of this embodiment will now be described in detail. Figure 1 As shown, the induction heater 10 includes a power supply circuit 20, heating coils 31, 32, and 33, a first resonant capacitor 41, a second resonant capacitor 42, a first inverter circuit 51, a second inverter circuit 52, and a control circuit 60.

[0030] Commercial AC power supplies provide AC voltages of 220V to 250V. Power supply circuit 20 generates DC power from the AC power of external power supply 200 and outputs it. Detailed descriptions related to power supply circuit 20 are omitted.

[0031] Heating coils 31, 32, and 33 are used for induction heating of the object to be heated (inner pot 100). The object to be heated is heated by a high-frequency magnetic field generated by passing a high-frequency current through the heating coils 31, 32, and 33. In this embodiment, the object to be heated is the inner pot 100 of the rice cooker 1.

[0032] The heating coil 31, as the main heating coil, is used for induction heating of the object to be heated. The heating coil 31 is disposed at the bottom inner part of the main body of the rice cooker 1 to heat the lower part of the inner pot 100.

[0033] Heating coils 32 and 33, as auxiliary heating coils, are used to induction heat the object being heated with a smaller amount of electricity than heating coil 31. Heating coil 32 is disposed on the inner wall of the main body of the rice cooker 1 to heat the side (main body) of the inner pot 100. Heating coil 33 is disposed on the lid of the rice cooker 1 to heat the upper part of the inner pot 100.

[0034] Heating coil 31 is connected between the output terminals of power supply circuit 20. Heating coil 32 and heating coil 33 are connected in series. The series circuit of heating coils 32 and 33 is connected in parallel with heating coil 31 between the output terminals of power supply circuit 20.

[0035] The first resonant capacitor 41, together with the heating coil 31, forms a resonant circuit. The first resonant capacitor 41 and the heating coil 31 are connected in parallel. The second resonant capacitor 42, together with the series circuit of the heating coils 32 and 33, forms a resonant circuit. The second resonant capacitor 42 and the series circuit of the heating coils 32 and 33 are connected in parallel.

[0036] The first inverter circuit 51 generates and outputs supply power (first supply power) from the output power of the power supply circuit 20 to the heating coil 31. The first inverter circuit 51 has a switching element 510 connected to the heating coil 31. That is, the first inverter circuit 51 outputs supply power to the heating coil 31, which is the corresponding heating coil.

[0037] The heating coil 31 and the first resonant capacitor 41 together form a first resonant circuit. The switching element 510 and the first resonant circuit together form a series circuit. By repeatedly turning the switching element 510 on and off, the first resonant circuit generates a high-frequency resonant current (high-frequency current).

[0038] Specifically, when the switching element 510 is turned on, current flows from the power supply circuit 20 to the heating coil 31. When the current flows in the heating coil 31, if the switching element 510 is turned off, the heating coil 31 resonates with the first resonant capacitor 41. As a result, a high-frequency current is generated in the heating coil 31.

[0039] Thus, a first supply power, serving as high-frequency power, is supplied to the heating coil 31. A series circuit of the first resonant circuit and the switching element 510 is connected between the output terminals of the power supply circuit 20. The switching element 510 is a semiconductor switching element. In this embodiment, the switching element 510 is an N-channel insulated gate bipolar transistor (IGBT).

[0040] The second inverter circuit 52 generates and outputs supply power (second supply power) from the output power of the power supply circuit 20 to the heating coils 32 and 33. The second inverter circuit 52 has a switching element 520 connected to the heating coils 32 and 33. That is, the second inverter circuit 52 outputs supply power to the heating coils 32 and 33, which are corresponding heating coils.

[0041] Heating coils 32 and 33, together with the second resonant capacitor 42, constitute a second resonant circuit. Switching element 520, together with the second resonant circuit, forms a series circuit. By repeatedly switching on and off, switching element 520 causes the second resonant circuit to generate a high-frequency resonant current.

[0042] Specifically, when the switching element 520 is turned on, current flows from the power supply circuit 20 to the heating coils 32 and 33. When the current flows in the heating coils 32 and 33, if the switching element 520 is turned off, the heating coils 32 and 33 resonate with the second resonant capacitor 42. As a result, a high-frequency current is generated in the heating coils 32 and 33.

[0043] Thus, a second power supply, serving as high-frequency power, is supplied to the heating coils 32 and 33. The second resonant circuit and the series circuit of the switching element 520 are connected between the output terminals of the power supply circuit 20. The switching element 520 is a semiconductor switching element. In this embodiment, the switching element 520 is an N-channel IGBT.

[0044] The control circuit 60 may be constituted, for example, a computer system containing one or more processors (microprocessors) and one or more memories. The control circuit 60 may also be constituted by other integrated circuits such as ASICs (application-specific integrated circuits), programmable logic devices, or FPGAs (field-programmable gate arrays).

[0045] The control circuit 60 enables the first resonant circuit and the second resonant circuit to resonate independently, thereby causing the first inverter circuit 51 and the second inverter circuit 52 to operate exclusively.

[0046] Specifically, the control circuit 60 disconnects the switching element 520 of the second inverter circuit 52 while the switching element 510 of the first inverter circuit 51 is turned on. Conversely, the control circuit 60 disconnects the switching element 510 of the first inverter circuit 51 while the switching element 520 of the second inverter circuit 52 is turned on.

[0047] The control circuit 60 causes the first inverter circuit 51 to output the supply power (first supply power) to the heating coil 31. The control circuit 60 performs duty cycle control on the first inverter circuit 51 to adjust the effective value of the first supply power supplied to the heating coil 31.

[0048] The following is for reference Figure 2 The duty cycle control using the first inverter circuit 51 will be explained. Figure 2 This is a diagram used to illustrate the duty cycle control in the rice cooker 1, specifically showing the changes in the indicated values ​​during the rising and falling processes.

[0049] like Figure 2 As shown, the control circuit 60 causes the first inverter circuit 51 to repeatedly output the first supplied power at a period T, adjusting the effective value of the first supplied power. The period T includes the on-time Ton (time t1 to time t2) during which the first supplied power is output and the off-time Toff (time t2 to time t3) during which the first supplied power is not output.

[0050] Time t3 is also time t1 of the subsequent period T. The effective value of the first supplied power depends on the duty cycle. The duty cycle is represented by the conduction period Ton / period T. The effective value of the first supplied power corresponds to the average value of the first supplied power in period T.

[0051] The control circuit 60 drives the switching element 510 of the first inverter circuit 51 at a driving frequency higher than the resonant frequency of the first resonant circuit. This supplies a first power supply to the heating coil 31 of the first resonant circuit. Based on the indicated value of the first power supply, the control circuit 60 sets the duty cycle of the switching element 510 of the first inverter circuit 51. This adjusts the effective value of the first power supply.

[0052] The control circuit 60 controls the gate-emitter voltage of the switching element 510, causing the switching element 510 to operate. Specifically, the control circuit 60 sets the duty cycle for the switching element 510 in such a way that the power determined by the envelope of the waveform of the collector-emitter voltage of the switching element 510 corresponds to an indicated value.

[0053] During the conduction period, the control circuit 60 sets the indicated value of the first supplied power to a target value P2. In this embodiment, the target value P2 is the maximum value of the first supplied power that can be supplied from the first inverter circuit 51 to the heating coil 31. This increases the effective value of the first supplied power. The maximum value of the first supplied power depends on the circuit structure and circuit constants of the induction heater 10. For example, the maximum value of the first supplied power is 1600 W.

[0054] like Figure 2 As shown, during the conduction period Ton, the control circuit 60 increases the indicated value from the initial value P1 to the target value P2. At time t110, the control circuit 60 sets the first power supply indication value to the target value P2 and maintains this setting for a specified period (time t110 to time t120).

[0055] Before setting the indication value of the first power supply to the target value P2, the control circuit 60 performs an upward process from time t1 to time t110. After a predetermined period has elapsed after setting the indication value of the first power supply to the target value P2, the control circuit 60 performs a downward process from time t120 to time t2.

[0056] In the rising process, the slope of the time change of the indicated value is changed in stages to increase the indicated value to the target value P2. Changing the slope of the time change of the indicated value in stages to increase the indicated value to the target value P2 means that the time change up to the target value P2 is represented by multiple straight lines with different slopes.

[0057] In the rise process, the control circuit 60 causes the slope of the time change of the indicated value to decrease in stages. Specifically, the rise process includes multiple (two in this embodiment) increase processes that cause the indicated value to increase along straight lines with different slopes.

[0058] The multiple increment processes include a first increment process (time t1 to time t101) and a second increment process (time t101 to time t110) executed after the first increment process. In the first increment process, the control circuit 60 increments the indication value along a straight line with a first slope, and in the second increment process, increments the indication value along a straight line with a second slope that is smaller than the first slope.

[0059] In a first increment process, the control circuit 60 increases the indicated value from the initial value P1 to P31, and in a second increment process, it increases the indicated value from P31 to P2. The duration of the second increment process is longer than that of the first increment process. For example, P31 is 1000 [W].

[0060] The initial value P1 is the minimum value of the indicator value during the rising process. The initial value P1 is the minimum value of the first supply power that can be supplied from the first inverter circuit 51 to the heating coil 31. This value is the minimum value of the first supply power that ensures the stable operation of the first inverter circuit 51.

[0061] When the indicated value is set to a value smaller than the minimum value of the first supplied power, a large load is applied to the first inverter circuit 51, which may generate noise, etc. According to this embodiment, the load on the first inverter circuit 51 can be reduced. The minimum value of the first supplied power depends on the circuit structure and circuit constants of the induction heater 10. For example, the minimum value of the first supplied power is 700 [W].

[0062] During the descent process, the control circuit 60 causes the indicated value to decrease in stages from the target value P2. Decreasing the indicated value in stages from the target value P2 means setting the indicated value to at least one value lower than the target value P2 and lower than the immediately preceding value.

[0063] During the descent process, the control circuit 60 causes the indicated value to decrease in stages. Specifically, the descent process includes a decrease process that maintains the indicated value at one or more (two in this embodiment) values ​​smaller than the target value P2 for a specified time.

[0064] One or more reduction processes include a first reduction process (time t120 to time t121) and a second reduction process executed after the first reduction process. Figure 2 (Times t121 to t2). In the first reduction process, the control circuit 60 sets the indicator value to a first indicator value P41 that is smaller than the target value P2, and in the second reduction process, sets the indicator value to a second indicator value P42 that is smaller than the first indicator value P41.

[0065] The duration of the first reduction process is equal to the duration of the second reduction process. In the reduction process, the amount of reduction of the indicated value in each step is set to be less than half of the target value P2. Therefore, P2-P41 is less than P2 / 2, and P41-P42 is less than P2 / 2.

[0066] For example, the first indicated value P41 is 1300 [W], and the second indicated value P42 is 1000 [W]. In the reduction process, the minimum value of the indicated value is greater than or equal to the minimum supply power (e.g., 700 [W]) that can be supplied from the first inverter circuit 51 to the heating coil 31. As a result, the load applied to the first inverter circuit 51 can be reduced.

[0067] During the conduction period Ton, the period of rising processing (time t1 to time t110) is set according to the set value of the amount of power supplied to the heating coil 31 during the conduction period Ton. The period of falling processing (time t120 to time t2) is the same.

[0068] In this embodiment, the control circuit 60 adjusts at least one of the rising processing period and the falling processing period. Thus, the cumulative value of the power waveform of Ton during the conduction period (the cumulative value of the supplied power) becomes a set value for the amount of power supplied by Ton to the heating coil 31 during the conduction period.

[0069] As a result, it is possible to suppress the decrease in power caused by rising and falling processing, and to meet the necessary power requirements of Ton during conduction.

[0070] The control circuit 60 causes the second inverter circuit 52 to output the supply power (second supply power) to the heating coils 32 and 33. The control circuit 60 controls the duty cycle of the second inverter circuit 52 to adjust the effective value of the second supply power supplied to the heating coils 32 and 33.

[0071] The control circuit 60 drives the switching element 520 of the second inverter circuit 52 at a driving frequency higher than the resonant frequency of the second resonant circuit. As a result, second power is supplied to the heating coils 32 and 33 of the second resonant circuit.

[0072] The control circuit 60 sets the duty cycle of the switching element 520 for the second inverter circuit 52 based on the indicated value of the second supplied power. This adjusts the value of the second supplied power. The control circuit 60 controls the gate-emitter voltage of the switching element 520, causing the switching element 520 to operate.

[0073] During conduction, control circuit 60 sets the indicated value of the second supplied power to a target value. A lower power level than that supplied to heating coils 31 is supplied to heating coils 32 and 33. The target value of the second supplied power is smaller than the target value P2 of the first supplied power. For example, the target value of the second supplied power is 800 W.

[0074] For the second inverter circuit 52, the control circuit 60 does not perform either the rising or falling processing performed on the first inverter circuit 51. This is because, in this embodiment, the change in power per unit time in the heating coils 32 and 33 is smaller than that in the heating coil 31, and the voltage fluctuation that is a cause of flicker is itself small.

[0075] (1-3) Confirmation of the operation of the induction heater

[0076] To confirm the operation of the induction heater 10 during the rising and falling processes, the time variation of the collector-emitter voltage of the switching element 510 of the first inverter circuit 51 and the time variation of the supply power supplied to the heating coil 31 are measured.

[0077] Changes in indicator values ​​during both rising and falling processing Figure 2 The changes shown are the same. Here, the period of the rising process (time t1 to time t110) is 0.1 [seconds]. The periods of the first decreasing process (time t120 to time t121) and the period of the second decreasing process (time t121 to time t2) of the falling process are both 0.3 [seconds].

[0078] Figure 3 and Figure 4 This indicates the measurement result of the collector-emitter voltage of the switching element 510 in the first inverter circuit 51.

[0079] As a comparative example, the collector-emitter voltage of the switching element 510 of the first inverter circuit 51 and the time variation of the power supplied to the heating coil 31 were measured when the control circuit 60 did not perform the rising and falling processing. Figure 5 This indicates the change in the indicator value in the comparison example.

[0080] like Figure 5 As shown, during the on-period (Ton), control circuit 60 increases the indicated value from the initial value P1 to the target value P2. At time t110, control circuit 60 sets the indicated value to the target value P2 and maintains this setting for a predetermined period (time t110 to time t2). At time t2, control circuit 60 sets the indicated value to 0. Thus, the off-period (Toff) begins.

[0081] Figure 6 as well as Figure 7 This is a diagram illustrating an example of the time variation of the collector-emitter voltage of the switching element 510 in the first inverter circuit 51 of this comparative example.

[0082] Figure 3 This is an example of the time variation of the collector-emitter voltage of the switching element 510 during a specified period including the rise process (time t1 to time t110).

[0083] like Figure 3 As shown, during the rising process (time t1 to time t110), the slope of the envelope E1 of the collector-emitter voltage changes at time t101. That is, at time t101, the control circuit 60 changes the indication value of the first supplied power.

[0084] On the other hand, such as Figure 6 As shown, in the comparative example where no rise processing is performed, the slope of the collector-emitter voltage envelope E2 is fixed from time t1 to time t110.

[0085] Figure 4This is an example of the time variation of the collector-emitter voltage of the switching element 510 during a specified period including the falling process (time t120 to time t2).

[0086] like Figure 4 As shown, during the descent process (time t120 to time t2), the collector-emitter voltage changes in stages. Specifically, at times t120, t121, and t2, the control circuit 60 changes the indication value of the first supplied power. As a result, the collector-emitter voltage is different before time t120, between time t120 and time t121, between time t121 and time t2, and after time t2. On the other hand, as... Figure 7 As shown, in the comparative example where the descent process is not performed, the indication value of the first power supply does not change.

[0087] Figure 8 This represents the measurement result of the power supplied to the heating coil 31 (first power supply). Figure 8 In the diagram, curve G1 represents the measured value of the first power supplied to the heating coil 31. Curve G2 represents an approximation of curve G1.

[0088] According to curve G2, during the rising process (time t1 to time t110), the slope of the time change of the first power supply changes at time t101. That is, at time t101, the control circuit 60 changes the indicated value of the first power supply.

[0089] On the other hand, during the descent process (time t120 to time t2), the first power supply changes in stages. That is, the control circuit 60 changes the indication value of the first power supply at times t120, t121, and t2.

[0090] (1-4) Evaluation results of flickering

[0091] To confirm the flicker reduction effect, Pst (short-term flicker value) was evaluated for modes A through D as shown in Table 1 below. Pst is an indicator of the degree of flicker. Table 1 shows the Pst values ​​for... Figure 2 The indicated values ​​in the rising and falling processing are shown in patterns A to D.

[0092] As the duty cycle control period T [seconds] of the first inverter circuit shortens, the change in power per unit time in the heating coil 31 increases. Therefore, there is a tendency for the voltage fluctuation to increase, which can be a cause of flickering, resulting in Pst easily exceeding 1.

[0093] In the evaluation of flicker, Pst is used to evaluate the extent to which the period can be reduced.

[0094] In Table 1, "rise time" refers to the period of rise processing ( Figure 2 The time interval is t1 to t110 (seconds). "First decrease value" is the first indication value P41 [W] of the first decrease process in the descent process. "First decrease time" is the period of the first decrease process in the descent process (seconds). Figure 2 (Time t120 ~ Time t121) [seconds].

[0095] "Second decrease value" is the second indicator value P42[W] of the second decrease process in the descent process. "Second decrease time" is the period of the second decrease process in the descent process (…). Figure 2 (Time t121 to time t2) [seconds].

[0096] The initial value P1 is 700 [W], and the target value P2 is 1600 [W]. In IEC 61000-3-3, "Limitations of voltage fluctuations, voltage instability, and flicker in common low-voltage power supply systems for equipment rated 16 A / phase or less and not conditionally connected," it is specified that Pst must be 1 or less. In the "Pst" column of Table 1, OK indicates Pst is 1 or less, and NG indicates Pst exceeds 1.

[0097] (Table 1)

[0098]

[0099] Without performing a comparative example of either the rising or falling processing, the extent to which the period can be reduced is evaluated using Pst. The results confirm that if the period becomes 12 seconds or less, Pst exceeds 1.

[0100] On the other hand, as shown in Table 1, it was confirmed that even when the period was reduced to 4 seconds in all modes A to D, Pst remained below 1. In particular, it was also confirmed in mode B that even when the period was reduced to 2 seconds, Pst remained below 1. Therefore, it was confirmed that Pst can be improved by performing both rising and falling processing.

[0101] To confirm the impact of rise time in the rise processing on flicker reduction, Pst was also evaluated for an example where only rise processing is performed during the on-time (Ton). Table 2 shows the evaluation results of Pst.

[0102] (Table 2)

[0103] Ascent time [seconds] Pst 0.04 0.888 0.1 0.836 0.12 0.768

[0104] As shown in Table 2, it was confirmed that Pst was improved by extending the rise time during the rise process.

[0105] (2) Variation

[0106] This disclosure is not limited to the embodiments described above. Various modifications can be made to the above embodiments as needed, as long as they can solve the problems of this disclosure. Hereinafter, variations of the above embodiments are listed. The variations described below can also be appropriately combined.

[0107] The first variation will be explained. Figure 9 This is a diagram illustrating the duty cycle control in the first modified example. In the first modified example, the control circuit 60 is as follows: Figure 9 As shown, the indicator value changes, performing both upward and downward processing.

[0108] like Figure 9 As shown, in the rise process (time t1 to time t110) of the first variation, the indicated value increases along a straight line with a fixed slope from the initial value P1 to the target value P2. The smaller the slope of the time change of the indicated value, the longer the rise time.

[0109] During the rise processing, after a predetermined rise time has elapsed since the start of the conduction period Ton, the indicated value is set to the target value P2. As shown in Table 2, it is confirmed that Pst is improved by extending the rise time during the rise processing.

[0110] When the indicated value is set to the target value P2 at the start of the conduction period Ton, an excessive load is applied to the first inverter circuit 51. The rise time is set to be longer than the shortest time set in a manner that does not apply an excessive load to the first inverter circuit 51.

[0111] exist Figure 5 In the comparative example shown, the slope of the time change of the indicator value is set in such a way that the time required for the indicator value to increase from the initial value P1 to the target value P2 is the shortest time mentioned above. For example... Figure 9 As shown, in the first variation, the slope of the time change of the indicator value in the rising process is compared to... Figure 5 The slope of the time change in the comparative example shown is small.

[0112] For example, the shortest time mentioned above is 0.04 [seconds]. Figure 9 In this case, the rise time is 0.2 seconds. For example, the rise time is longer than the shortest time set in a manner that does not apply excessive load to the first inverter circuit 51, but less than 5 times the shortest time.

[0113] The rise time is set in a manner that prevents the first inverter circuit 51 from being damaged due to temperature rise. If the first inverter circuit 51 is driven for an extended period of time when the power supply from it is relatively low, the first inverter circuit 51 may be damaged due to temperature rise. Therefore, the rise time is set in a manner that prevents the first inverter circuit 51 from being damaged due to temperature rise.

[0114] The rise time is set based on the effective value of the supply power supplied from the first inverter circuit 51 to the heating coil 31 during period T. The effective value of the supply power is the cumulative value of the power during period T.

[0115] like Figure 9 As shown, in the first variation, for example, if the effective value of the supplied power is set as Ps and the time t1 is set as time t1 = 0, then Ps is expressed by the following formula.

[0116] Ps=(P2xt2-(P2-P1)xt110 / 2-(P2-P41)x(t2-t120)-(P41-P42)x(t2-t121)-(P42-P53)x(t2-t122)) / T

[0117] Time t110 corresponds to the rise time. The effective value Ps can be adjusted at time t110. Time t110 is set in such a way that the effective value Ps becomes the value of the power supplied to the heating coil 31 during the period T. In order to ensure that the power supplied from the first inverter circuit 51 during the conduction period Ton is the target value P2, the rise time is set to be less than half the length of the conduction period Ton.

[0118] like Figure 9 As shown, in the first variation, the descent process includes three reduction processes that maintain the indicated value at a value less than the target value P2 for a specified period of time. The three reduction processes are the first reduction process (time t120 to time t121), the second reduction process (time t121 to time t122), and the third reduction process (time t122 ​​to time t2).

[0119] In the first reduction process, the indicator value is set to a first indicator value P41, which is smaller than the target value P2. The second reduction process is executed after the first reduction process. In the second reduction process, the indicator value is set to a second indicator value P42, which is smaller than the first indicator value P41. The third reduction process is executed after the second reduction process. In the third reduction process, the indicator value is set to a third value P43, which is smaller than the second indicator value P42.

[0120] The periods for the first to third reduction processes are equal. In the reduction process, the amount of reduction of the indicated value in each step is set to be less than half of the target value P2.

[0121] Therefore, P2-P41 is below P2 / 2, P41-P42 is below P2 / 2, and P42-P43 is below P2 / 2. For example, the first indicator value P41 is 1350 [W], the second indicator value P42 is 1100 [W], and P43 is 850 [W].

[0122] Preferably, the decrease in the indicated value is uniform. During the decrease process, the minimum value of the indicated value is greater than or equal to the minimum supply power (e.g., 700 W) that can be supplied from the first inverter circuit 51 to the heating coil 31.

[0123] A descent process can also contain more than four decrease processes. Within a descent process, the indicator value of a later decrease process does not necessarily have to be less than the indicator value of a previous decrease process. For example... Figure 9 As shown, in the first variation, the durations of the multiple reduction processes are all equal. However, this is not a limitation.

[0124] The second variation will be described. The rise process of the second variation includes three rise processes that increase the indicator value along straight lines with different slopes, namely the first rise process, the second rise process, and the third rise process.

[0125] In the first increment process, the indicator value is increased along a straight line with a first slope. The second increment process is executed after the first increment process. In the second increment process, the indicator value is increased along a straight line with a second slope that is smaller than the first slope. The third increment process is executed after the second increment process. In the third increment process, the indicator value is increased along a straight line with a third slope that is smaller than the second slope.

[0126] For example, in the first increment process, the indicator value is increased from the initial value P1 to 1100 [W]. In the second increment process, the indicator value is increased from 1100 [W] to 1400 [W]. In the third increment process, the indicator value is increased from 1400 [W] to 1600 [W].

[0127] In the second variation, the later the increment process (from the first to the third), the longer its duration. The initial value P1 is the minimum value of the indicator value in the increment process, for example, 700[W]. The increment process may also contain more than four increment processes.

[0128] In an ascending process, the slope of the time change of the indicator value in subsequent ascending processes does not necessarily have to be smaller than the slope of the preceding ascending processes. Alternatively, in an ascending process, the later the ascending process, the longer the period. However, it is not limited to this.

[0129] The third variation will be explained. Figure 10This is a diagram illustrating the duty cycle control in the third variation. In the third variation, the control circuit 60 makes the indicated value as follows: Figure 10 The changes shown are executed in the manner of both upward and downward processing.

[0130] like Figure 10 As shown, in the rising process of the third variation (time t1 to time t110), the indicator value is increased continuously rather than in stages. The continuous change of the indicator value to increase it to the target value P2 refers to the time change represented by the curve up to the target value P2.

[0131] The control circuit 60 continuously increases the indication value as an ascending process from the start of the conduction period Ton until the indicated value of the supplied power is set to the target value P2 of the supplied power during the conduction period Ton.

[0132] like Figure 10 As shown, the indicator value increases logarithmically from the initial value P1 to the target value P2. For example, P2 is 1600 [W]. The initial value P1 is the minimum value of the indicator value during the ascending process, for example, 700 [W].

[0133] like Figure 10 As shown, the descent process includes a reduction process that maintains the indicated value at a value less than the target value P2 for a specified time. Figure 10 (Times t120 to t2). In the reduction process, the indicator value is set to a first indicator value P41 that is smaller than the target value P2.

[0134] In the descent process, the amount of decrease in the indicated value each time is set to be less than half of the target value P2. Therefore, P2-P41 is less than P2 / 2. For example, P41 is 900 [W].

[0135] During the descent process, the minimum value of the indicated value is greater than or equal to the minimum supply power (e.g., 700 W) that can be supplied from the first inverter circuit 51 to the heating coil 31.

[0136] The fourth variation will be explained. In the rising process of the fourth variation, the indicated value is changed in stages to increase the indicated value to the target value P2. That is, in the rising process, the indicated value of the supplied power is changed discontinuously.

[0137] Alternatively, the indicated value can be continuously changed to increase to the target value P2. In this case, the time change of the indicated value is preferably monotonically increasing. In summary, the control circuit, as an ascending process, increases the indicated value or the slope of the time change of the indicated value in stages, or increases the indicated value continuously, until the indicated value is set to the target value P2.

[0138] The fifth variation will be explained. In the descent process of the fifth variation, the slope of the time change of the indicated value is changed in stages to reduce the indicated value from the target value P2. That is, in the descent process, the indicated value of the supplied power is changed along a straight line with different slopes.

[0139] Alternatively, the indicated value can be continuously changed to decrease from the target value P2. In this case, the time change of the indicated value is preferably monotonically decreasing. In summary, the control circuit, from setting the indicated value to the target value P2 until the end of the conduction period Ton, performs a decreasing process, either by gradually decreasing the indicated value or the slope of the time change of the indicated value, or by continuously decreasing the indicated value.

[0140] The sixth variation will be described. In the sixth variation, the control circuit 60 performs either the rising process or the falling process.

[0141] The seventh variation will be described. In the seventh variation, for the second inverter circuit 52, the control circuit 60 also performs at least one of the rising process and the falling process.

[0142] The induction heater 10 includes multiple heating coils for heating an object (e.g., the inner pot 100 of the rice cooker 1) and multiple inverter circuits for generating and outputting power supplied to the multiple heating coils.

[0143] For at least one of the multiple inverter circuits, the control circuit 60 performs at least one of the rising and falling processes. The induction heater 10 may have a single heating coil and a single inverter circuit. That is, the heating coils 32 and 33, the resonant capacitor 42, and the second inverter circuit 52 are not necessary and can be omitted.

[0144] (3) Method

[0145] The first type of induction heater (10) includes a heating coil (31), an inverter circuit (51), and a control circuit (60). The inverter circuit (51) generates and outputs power supplied to the heating coil (31) from the power supply circuit (20).

[0146] The control circuit (60) causes the inverter circuit to repeatedly output the supplied power with a period (T) that includes an on period (Ton) during which the supplied power is output and an off period (Toff) during which the supplied power is not output, thereby adjusting the effective value of the supplied power.

[0147] The control circuit (60) performs a decreasing process. From the time the indicated value of the supplied power is set to the target value (P2) of the supplied power during the conduction period (Ton) until the end of the conduction period (Ton), the control circuit (60) decreases the indicated value or the slope of the time change of the indicated value in stages, or decreases the indicated value continuously. According to this method, flickering can be reduced.

[0148] The second method is based on the induction heater (10) of the first method. In the descent process of the second method, the control circuit causes the indicated value to decrease in stages, and the descent process includes more than one descent process, which maintains the indicated value at a value (P41, P42, P43) smaller than the target value (P2) for a specified time. According to this method, the amount of power change per unit time during power supply descent can be reduced, and flickering can be reduced.

[0149] The third method is based on the induction heater (10) of the second method. The reduction process of the third method includes a first reduction process and a second reduction process.

[0150] In the first reduction process, the indicated value is set to a first indicated value (P41) that is smaller than the target value (P2). The second reduction process is executed after the first reduction process. In the second reduction process, the indicated value is set to a second indicated value (P42) that is smaller than the first indicated value (P41). According to this method, the amount of power change per unit time during a power supply decline can be reduced, and flickering can be reduced.

[0151] The fourth method is based on the induction heater (10) of the first method. In the descent process of the fourth method, the control circuit causes the indicated value to decrease in stages, with each decrease being less than half of the target value (P2). According to this method, the change in power per unit time during the power supply decrease can be reduced, and flickering can be reduced.

[0152] The fifth method is an induction heater (10) based on any of the first to fourth methods. In the descent process of the fifth method, the minimum value of the indicated value is greater than or equal to the minimum value of the supply power that can be supplied from the inverter circuit (51) to the heating coil (31). According to this method, the load applied to the inverter circuit (51) can be reduced.

[0153] The sixth method is an induction heater (10) based on any of the first to fifth methods. In the sixth method, the control circuit (60) performs the rise process.

[0154] As part of the rise process, the control circuit (60) sets the indicated value to the target value (P2) after a predetermined rise time from the start of the conduction period (Ton). The predetermined rise time is longer than the shortest time set to not apply load to the inverter circuit (51). According to this method, the load applied to the inverter circuit (51) can be reduced.

[0155] The seventh method is an induction heater (10) based on any of the first to fifth methods. In the seventh method, the control circuit (60) performs the rise process.

[0156] The control circuit (60) performs an ascending process from the start of the conduction period (Ton) until the indicated value is set to the target value (P2), by either progressively decreasing the slope of the time change of the indicated value to increase the indicated value, or by continuously increasing the indicated value. According to this method, flickering can be further reduced.

[0157] The eighth method is based on the induction heater (10) of the seventh method. In the eighth method, the rising process includes multiple rising processes that increase the indicated value along straight lines with different slopes. The multiple rising processes include a first rising process and a second rising process.

[0158] In the first increment process, the indication value increases along a straight line with a first slope. The second increment process is executed after the first increment process. In the second increment process, the indication value increases along a straight line with a second slope that is smaller than the first slope. According to this method, the amount of power change per unit time during a power supply increase can be reduced, and flickering can be reduced.

[0159] The ninth method is an induction heater (10) based on the seventh or eighth method. In the rising process of the ninth method, the minimum value of the indicated value is greater than or equal to the minimum value of the supply power that can be supplied from the inverter circuit (51) to the heating coil (31). According to this method, the load applied to the inverter circuit (51) can be reduced.

[0160] The tenth method is an induction heater (10) based on any of the first to ninth methods. In the tenth method, the target value (P2) is the maximum value of the supplied power that can be supplied from the inverter circuit (51) to the heating coil (31). According to this method, the effective value of the supplied power can be increased.

[0161] The eleventh method is an induction heater (10) based on any of the first to tenth methods. In the eleventh method, the length of the descent process is determined according to a set value of the amount of power supplied to the heating coil (31) during the on-time (Ton) period. According to this method, it is possible to suppress the decrease in power caused by the on-time and descent processes.

[0162] The twelfth method is an induction heater (10) based on any one of the first to eleventh methods. In the twelfth method, the inverter circuit (51, 52) includes transistors (510, 520) as switching elements connected to the heating coils (31, 32, 33).

[0163] The control circuit (60) sets the duty cycle for the transistors (510, 520) in such a way that the voltage determined by the envelope of the waveform of the voltage applied between the collector and emitter of the transistors (510, 520) corresponds to the indicated value. According to this method, flicker can be reduced.

[0164] The thirteenth method is an induction heater (10) based on any one of the first to twelfth methods. In the thirteenth method, the induction heater (10) includes a plurality of heating coils (31, 32, 33) and a plurality of the inverter circuits (51, 52).

[0165] The plurality of inverter circuits (51, 52) respectively output the supplied power to the corresponding heating coil of the plurality of heating coils (31, 32, 33). The control circuit (60) performs the descent process for at least one of the plurality of inverter circuits (51, 52). According to this method, flicker can be reduced.

[0166] The rice cooker (1) of the fourteenth type includes: an induction heater (10) of any one of the first to thirteenth types; and an inner pot (100) for heating by said induction heater (10). According to this type, flickering can be reduced.

[0167] This disclosure can be applied to rice cookers equipped with induction heaters.

Claims

1. An induction heater, wherein, The induction heater includes: The first and second coils used for heating; The first inverter circuit and the second inverter circuit are configured to generate and output power supplied to the first coil and the second coil respectively from the power supply circuit. as well as The control circuit is configured to cause the first inverter circuit and the second inverter circuit to repeatedly output the supplied power with a period that includes an on period during which the supplied power is output and an off period during which the supplied power is not output, thereby adjusting the effective value of the supplied power. The change in electrical power of the second coil per unit time is less than the change in electrical power of the first coil per unit time. The control circuit performs a drooping process on the first inverter circuit but not on the second inverter circuit. The control circuit, from the time the indicated value of the supplied power is set to the target value of the supplied power during the conduction period until the end of the conduction period, performs a decreasing process, causing the indicated value or the slope of the time change of the indicated value to decrease in stages in three or more stages, or causing the indicated value to decrease continuously.

2. The induction heater according to claim 1, wherein, In the descent process, the control circuit causes the indicated value to decrease in stages. The descent process includes one or more decrease processes, which maintain the indicated value at a value smaller than the target value for a specified time.

3. The induction heater according to claim 2, wherein, The one or more reduction processes include: The first reduction process sets the indicated value to a first value smaller than the target value; and The second reduction process, which is performed after the first reduction process, sets the indicated value to a second value that is smaller than the first value.

4. The induction heater according to claim 1, wherein, In the descent process, the control circuit causes the indicated value to decrease in stages, with each decrease being less than half of the target value.

5. The induction heater according to claim 1, wherein, In the descent process, the minimum value of the indicated value is greater than or equal to the minimum value of the supply power that can be supplied from the first inverter circuit to the first coil.

6. The induction heater according to claim 1, wherein, As a rising process, the control circuit sets the indicated value to the target value after a predetermined rise time from the start of the conduction period. The specified rise time is longer than the shortest time set to not apply load to the first inverter circuit.

7. The induction heater according to claim 1, wherein, The control circuit performs the rising process on the first inverter circuit, but not on the second inverter circuit. The control circuit, from the start of the conduction period until the indicated value is set to the target value, performs an ascending process by progressively decreasing the slope of the time change of the indicated value to increase the indicated value, or by progressively or continuously increasing the indicated value.

8. The induction heater according to claim 7, wherein, The ascending process includes multiple ascending processes that increase the indicator value along straight lines with different slopes. The plurality of augmentation processes include: The first increment process causes the indicated value to increase along a straight line with a first slope; and A second increment process, performed after the first increment process, causes the indicated value to increase at a second slope that is smaller than the first slope.

9. The induction heater according to claim 7, wherein, In the rising process, the minimum value of the indicated value is greater than or equal to the minimum value of the supply power that can be supplied from the first inverter circuit to the first coil.

10. The induction heater according to claim 1, wherein, The target value is the maximum value of the supplied power that can be supplied from the first inverter circuit to the first coil.

11. The induction heater according to claim 1, wherein, The length of the descent process is determined based on a set value for the amount of power supplied to the first coil during the conduction period.

12. The induction heater according to claim 1, wherein, The first inverter circuit includes a transistor, which is a switching element connected to the first coil. The control circuit is configured to set the duty cycle for the transistor in such a way that the voltage determined by the envelope of the waveform of the voltage applied between the collector and emitter of the transistor corresponds to the indicated value.

13. The induction heater according to claim 1, wherein, The second inverter circuit includes a switching element connected to the second coil. The control circuit sets the duty cycle of the switching element of the second inverter circuit in such a way that the voltage determined by the envelope of the waveform of the voltage applied between the two ends of the switching element of the second inverter circuit becomes a value corresponding to the indicated value.

14. A rice cooker, wherein, The rice cooker includes: an induction heater as described in any one of claims 1 to 13; and an inner pot for heating by the induction heater.

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