Induction heating device

Abstract

To detect resonance current flowing in a heating coil without using a current transformer and a shunt resistor, in induction heating equipment in which an inverter is PWM-controlled.SOLUTION: Induction heating equipment is provided, comprising: an inverter circuit that supplies a high-frequency AC voltage to a heating coil; a control circuit that controls upper and lower arms of the inverter circuit by frequency control or PWM control; a resonance circuit configured by serially connecting the heating coil and a resonance capacitor; a resonance capacitor voltage differentiation current that differentiates a both-end voltage of the resonance capacitor generated by the current flowing in the resonance capacitor; a rectification circuit that rectifies the output of the resonance capacitor voltage differentiation current; and an envelope detection circuit that outputs an output voltage detecting an envelope of the output of the rectification circuit. In a case where the upper and lower arm control of the inverter circuit is PWM control, the control circuit calculates resonance current value flowing in the heating coil on the basis of a corrected output voltage obtained by correcting the output voltage of the envelope detection circuit.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The present invention relates to an induction heating device (IH cooking heater) equipped with a current detection circuit that detects a heating coil current. [Background technology]

[0002] In recent years, inverter-type induction heating devices (IH cooking heaters) that heat objects such as pots without using fire have become widely used. IH cooking heaters run high-frequency current through a heating coil, generating eddy currents in a pot made of iron or stainless steel placed close to the coil, causing the pot to generate heat through its own electrical resistance. In this way, IH cooking heaters are rapidly becoming more and more popular, replacing gas ranges, because they enable cooking without using fire, are safe, and provide a comfortable cooking environment.

[0003] In an IH cooking heater, a heating coil is placed under a glass top plate, and an inverter that supplies high-frequency current is connected to the heating coil. A high-frequency current of about 40 Arms, about 20k to 100kHz, flows through the heating coil. Conventionally, a current transformer or shunt resistor was used to detect the current in the heating coil.

[0004] However, current transformers come in two types: one with a primary winding and a secondary winding on a core, and one with a secondary winding on a through-type core, but both types have the problem of large component size and high cost.In addition, shunt resistors have the problem of Joule loss occurring in the shunt resistor and a rise in temperature because a large current flows through the resistor.

[0005] Therefore, the induction cooker shown in Patent Document 1 is equipped with a voltage detection circuit that detects the voltage across the resonant capacitor, a differentiation circuit that differentiates the output from this voltage detection circuit, and an envelope detection circuit that rectifies the output from this differentiation circuit and outputs current information that follows the peak value of the current flowing through the heating coil, thereby solving the above-mentioned problems while detecting the heating coil current. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2007-026906 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, when the inverter is PWM controlled (Pulse Width Modulation controlled), the peak value of the AC current flowing through the heating coil differs depending on whether it is positive or negative. As can be seen from Figures 6 and 7 of Patent Document 1, the configuration is such that the output voltage of the differentiation circuit is half-wave rectified and the peak value is detected, so when the inverter is PWM controlled, Patent Document 1 cannot accurately detect the effective current flowing through the heating coil. As a result, the type of pot cannot be accurately determined, which not only fails to provide the user with the correct value, but also poses the problem of not maintaining the safety of the product.

[0008] Therefore, the present invention solves the above-mentioned problems of the conventional art. In an induction heating device in which an inverter is PWM-controlled, the voltage across the resonant capacitor caused by the current flowing through the resonant capacitor is differentiated to detect the voltage generated across a resistor, and the detected value is corrected by a microcomputer according to the control state, thereby making it possible to accurately detect the effective current flowing through the heating coil. [Means for solving the problem]

[0009] In order to solve the above problems, the induction heating device of the present invention is an induction heating device that uses a heating coil to inductively heat an object to be heated, and includes a DC power source that supplies a DC voltage, an inverter circuit that converts the DC voltage into a high-frequency AC voltage and supplies it to the heating coil, a control circuit that controls upper and lower arms of the inverter circuit with frequency control or PWM control, a resonant circuit in which the heating coil and a resonant capacitor are connected in series, a resonant capacitor voltage differentiation circuit that differentiates the voltage across the resonant capacitor generated by the current flowing through the resonant capacitor, a rectifier circuit that rectifies the output of the resonant capacitor voltage differentiation circuit, and an envelope detection circuit that outputs an output voltage obtained by detecting the envelope of the output of the rectifier circuit, and the control circuit determines whether the control of the upper and lower arms of the inverter circuit is frequency control or PWM control, and if it is frequency control, calculates the resonant current value flowing through the heating coil based on the output voltage of the envelope detection circuit, and if it is PWM control, calculates the resonant current value flowing through the heating coil based on a corrected output voltage obtained by correcting the output voltage of the envelope detection circuit. Effect of the Invention

[0010] According to the induction heating device of the present invention, when the inverter is PWM controlled, the voltage across the resonant capacitor caused by the current flowing through the resonant capacitor is differentiated and rectified to detect the envelope. The current information obtained by detection is corrected on the microcomputer side according to the control state of the inverter, so that the effective current flowing through the heating coil can be accurately detected not only in frequency control but also in PWM control. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a block diagram of an induction heating device according to an embodiment. [Diagram 2] FIG. 2 is a diagram showing the configuration of an inverter circuit of the induction heating device according to the embodiment; [Diagram 3] FIG. 13 is a diagram showing the resistance value of each heated object and its inductance ratio to iron. [Figure 4] 4 shows inverter operation waveforms of the induction heating device according to the embodiment. [Diagram 5]4 shows frequency characteristics of input power of an induction heating device according to an embodiment. [Figure 6] 4 shows the duty characteristics of the input power of an induction heating device according to an embodiment. [Figure 7] FIG. 2 is a circuit diagram of a current detection circuit according to an embodiment. [Figure 8] 4 is an operational waveform of a current detection circuit of an induction heating device according to an embodiment of the present invention; [Figure 9] 4 is a graph showing the relationship between the current detection circuit and the heating coil current of the induction heating device according to the embodiment. [Figure 10A] FIG. 4 is a diagram showing a drive waveform of a first PWM control. [Figure 10B] FIG. 4 is a diagram showing a drive waveform of a second PWM control. [Figure 11] FIG. 4 is a diagram showing the output voltage Vout for each control method. [Figure 12] 5A and 5B are explanatory diagrams of correction strategies of the first correction process and the second correction process. [Figure 13] 5 is a flowchart showing a correction process after a current detection process according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] An induction heating device according to an embodiment of the present invention will now be described with reference to the drawings.

[0013] <Configuration of induction heating device> FIG. 1 is a block diagram of an induction heating device according to a first embodiment. The induction heating device shown here is a heating device capable of simultaneously heating two or more objects to be heated, such as a pan placed on a top plate, and includes a power supply circuit 10 that converts AC voltage from a commercial power supply 1 to output DC voltage, an input current detector 11, an input current detection circuit 12, two or more inverters 100 (100a, 100b, 100c), a drive circuit 21, an envelope detection circuit 51, a control circuit 60, and an input power setting unit 61. The objects to be heated can be induction-heated by the heating coils 31 of each inverter. Since the configurations of the inverters are the same, the first inverter 100a will be described below as a representative example.

[0014] The inverter 100a is composed of an inverter circuit 20, a resonant circuit 30, and a differential circuit 40. The inverter circuit 20 is connected between a positive electrode point p and a negative electrode point n of a power supply circuit 10, and converts a DC voltage supplied from the power supply circuit 10 into a high-frequency AC voltage and applies it to the resonant circuit 30. The resonant circuit 30 is a series circuit of a heating coil 31 and a resonant capacitor Cr, and high-frequency power is supplied to the heating coil 31 from the inverter circuit 20. The differential circuit 40 differentiates the voltage that changes due to the current flowing through the resonant capacitor Cr.

[0015] The output value of the differentiation circuit 40 of each inverter is sent to a control circuit 60 via an envelope detection circuit 51. An input power setting unit 61 is an interface that allows the user to set the input power (heating power), and sends a signal corresponding to the set heating power to the control circuit 60. The control circuit 60 generates a drive signal corresponding to the calculation result from the envelope detection circuit 51 and the signal from the input power setting unit 61. Based on the drive signal from the control circuit 60, a drive circuit 21 generates a drive signal waveform that controls the inverter circuit 20 of each inverter.

[0016] Next, the operation of the inverter 100a will be described. In general, an IH cooking heater uses a resonant inverter. A resonant inverter is an inverter that controls the current flowing through the resonant circuit 30 to have a delayed phase with respect to the output voltage of the inverter circuit 20 by setting the drive frequency fs of the inverter circuit 20 > the resonant frequency fr of the resonant circuit 30 and making the characteristics of the resonant load inductive. This suppresses an increase in loss in the inverter circuit 20. That is, in FIG. 1, the resonant current I L However, by controlling the inverter circuit 20 to have a delayed phase with respect to the voltage at the output terminal t, which is the connection point between the inverter circuit 20 and the resonant circuit 30, the loss in the inverter circuit 20 can be suppressed.

[0017] However, when the drive frequency fs of the inverter circuit 20 is fixed and the conduction period of the inverter circuit 20 is changed to perform power control (PWM control), a resonant current I LThe polarity of the resonant current I L In some cases, the output voltage of the inverter circuit 20 may transition to a leading-phase mode in which the output voltage of the inverter circuit 20 leads the output voltage of the inverter circuit 20. The leading-phase mode increases losses in the inverter circuit 20, and is therefore a mode that must be avoided in a resonant inverter.

[0018] 2 shows a more specific circuit configuration of the induction heating device of this embodiment. This is a circuit diagram of the induction heating device in which the power supply circuit 10 is a full-wave rectifying passive filter type and the inverter circuit 20 is a half-bridge circuit configuration.

[0019] 2, the power supply circuit 10 converts AC voltage from a commercial power supply 1 into DC voltage and supplies it to an inverter 100a, and is composed of a diode bridge 13 that rectifies the AC voltage, an inductor 14, and a filter capacitor Cf. An inverter circuit 20 of the inverter 100a is connected between a positive electrode point p and a negative electrode point n of the filter capacitor Cf.

[0020] The inverter circuit 20 of the inverter 100a is configured by connecting in series IGBTs (hereinafter referred to as switching elements SW (SW1, SW2)) which are power semiconductor switching elements. A diode D (D1, D2) is connected in parallel in the opposite direction to each switching element SW, and the cathode terminal of the diode D is connected to the collector terminal of the switching element SW, and the anode terminal is connected to the emitter terminal. Hereinafter, the circuit formed by the switching element SW1 and the diode D1 is referred to as the upper arm, and the circuit formed by the switching element SW2 and the diode D2 is referred to as the lower arm. In addition, snubber capacitors Cs (Cs1, Cs2) are connected in parallel to the switching elements SW1 and SW2, respectively. The snubber capacitors Cs1 and Cs2 are charged or discharged by the interruption current when the switching element SW1 or the switching element SW2 is turned off. Since the capacitance of snubber capacitors Cs1, Cs2 is sufficiently larger than the output capacitance between the collector and emitter of switching elements SW1, SW2, the change in voltage applied to both switching elements SW at turn-off is reduced, and turn-off loss is suppressed.

[0021] A resonant circuit 30 is connected between an output terminal t, which is the connection point between the switching elements SW1 and SW2, and the positive electrode p and negative electrode n of the power supply circuit 10. This resonant circuit 30 is composed of a heating coil 31 and resonant capacitors Cr1 and Cr2. Here, the direction flowing from the output terminal t to the heating coil 31 is called a resonant current I L The differential circuit 40 is a resonant current I L is supplied to the resonant capacitors Cr1 and Cr2, causing the voltage to change. Differentiate the voltage across the resonant capacitor Cr1, one end of which is connected to GND.

[0022] Moreover, the input current detector 11 detects a current input from the commercial power supply 1. The input current detection circuit 12 converts the output signal level of the input current detector 11 into a signal suitable for the input level of the control circuit 60.

[0023] The control circuit 60 detects the input current detected by the input current detection circuit 12 and the resonant current I L Based on the relationship between the material and the state of the object to be heated, the heating operation is started or stopped. The object to be heated is distinguished between magnetic and non-magnetic. The method for distinguishing between them is to pass a low power current (about 300W) through the object before heating. The resonant current I L Alternatively, the current values ​​(Ic1 and Ic2 described below) of the switching elements SW1 and SW2 are detected, and the material of the heated object is determined based on the detected current value. If the current value is small, the object is determined to be a magnetic material such as iron, and if the current value is large, the object is determined to be a non-magnetic material such as non-magnetic stainless steel, aluminum or copper. Figure 3 shows the resistance value of each heated object at a frequency of 20 kHz. As shown here, the resistance value of non-magnetic stainless steel is 1 / 3 of that of iron, 1 / 20 of aluminum, and approximately 1 / 25 of copper.

[0024] Furthermore, the control circuit 60 sets the conduction period (hereinafter also referred to as "Duty") of the switching elements SW1 and SW2 of the inverter circuit 20 via the drive circuit 21 in response to a signal from the input power setting unit 61, and performs PWM control of the input power. Material detection needs to be performed with low power and in a short time to prevent the occurrence of overcurrent or overvoltage.

[0025] As shown in FIG. 2, the current flowing through the upper arm of the inverter circuit 20 is Ic1, the current flowing through the lower arm is Ic2, and the resonant current is I L Let the collector-emitter voltage of the upper arm switching element SW1 be Vc1, the collector-emitter voltage of the lower arm switching element SW2 be Vc2, and the power supply voltage of the inverter be Vp.

[0026] Next, the operation will be described. Figure 4 shows the operating waveforms of the inverter of this embodiment in modes 1 to 4. In any mode, the switching elements SW1 and SW2 are provided with a dead time period (a period when both switching elements are off) and are driven complementarily.

[0027] As shown in FIG. 4, a sinusoidal resonant current I L The resonant frequency fr is determined by the inductance value L of the heating coil 31 as shown in Equation 1. 31 , is determined from the combined value of resonant capacitor Cr1 and resonant capacitor Cr2.

[0028]

number

[0029] The operations in modes 1 to 4 will be described in detail below.

[0030] (Mode 1) Mode 1 begins when the current Ic1 of switching element SW1 increases from negative to 0 A. No current flows through switching element SW1 when mode 1 begins, but since switching element SW1 is already on, current Ic1 begins to flow through switching element SW1 immediately after mode 1 begins. At this time, the voltage across switching element SW1 (voltage Vc1 between the collector terminal and emitter terminal) is 0 V, so the switching element SW1 undergoes ZVZCS turn-on, which generates no loss.

[0031] (Mode 2) When the switching element SW1 is shut off and mode 2 is entered, the resonant current I L flows through the path of the power supply circuit 10, snubber capacitor Cs1, heating coil 31, and resonant capacitor Cr1, the path of the heating coil 31, resonant capacitor Cr2, and snubber capacitor Cs1, and the path of the heating coil 31, resonant capacitor Cr1, and snubber capacitor Cs2. At this time, the snubber capacitor Cs1 is charged, and the snubber capacitor Cs2 is discharged. As a result, the voltage across the switching element SW1 rises gradually, resulting in ZVS turn-off, and the switching loss can be reduced.

[0032] When the voltage Vc1 of the snubber capacitor Cs1 becomes equal to or higher than the power supply voltage (voltage between pn), the voltage Vc2 of the snubber capacitor Cs2 becomes 0 V, the diode D2 turns on, and the resonant current I L While a current is flowing through diode D2, an ON signal is input to switching element SW2.

[0033] (Mode 3) Mode 3 begins when the current Ic2 of switching element SW2 increases from negative to 0 A. No current flows through switching element SW2 when mode 3 begins, but since switching element SW2 is already on, current Ic2 begins to flow through switching element SW2 immediately after mode 3 begins. At this time, the voltage across switching element SW2 (voltage Vc2 between the collector terminal and emitter terminal) is 0 V, so the switching element SW2 undergoes ZVZCS turn-on, which generates no loss.

[0034] (Mode 4) When the switching element SW2 is shut off and mode 4 is entered, the resonant current I L flows through the path of the heating coil 31, snubber capacitor Cs2, power supply circuit 10, and resonant capacitor Cr2, the path of the heating coil 31, snubber capacitor Cs2, and resonant capacitor Cr1, and the path of the heating coil 31, snubber capacitor Cs1, and resonant capacitor Cr2. At this time, the snubber capacitor Cs2 is charged, and the snubber capacitor Cs1 is discharged. As a result, the voltage across the switching element SW2 rises gradually, resulting in ZVS turn-off, and the switching loss can be reduced.

[0035] When the voltage Vc2 of the snubber capacitor Cs2 becomes equal to or higher than the power supply voltage (voltage between pn), the voltage Vc1 of the snubber capacitor Cs1 becomes 0 V, the diode D1 turns on, and the resonant current I L While a current is flowing through diode D1, an ON signal is input to switching element SW1.

[0036] The above operations in modes 1 to 4 are repeated to pass a high-frequency current through heating coil 31, generating magnetic flux from heating coil 31. This magnetic flux causes eddy currents to flow in the pot placed above heating coil 31, causing the pot itself to heat up through induction heating.

[0037] <Method of controlling power of induction heating device> Next, we will explain the power control method. Figure 5 shows the relationship between frequency and input power. IH cooking heaters use the resonance phenomenon to pass a large high-frequency current through the heating coil. For this reason, the frequency characteristics of the input power show resonance characteristics. As shown in Figure 3, the resistance of an iron pot is high, so the resonance Q is small and it shows gentle resonance characteristics. On the other hand, low-resistance materials such as aluminum and copper have a large resonance Q and show steep resonance characteristics. For iron pots and other items with a small resonance Q, it is possible to control power by frequency by taking advantage of the gentle resonance characteristics.

[0038] In addition, the relationship between the duty cycle of the switching element SW1 and the input power is shown in Figure 6. For iron pots and other items with a small resonance Q, power control using the duty cycle (PWM control) is possible. On the other hand, for materials with steep resonance characteristics such as aluminum, frequency control or duty cycle control is difficult, and power can be controlled by controlling the output voltage of the power supply circuit 10.

[0039] Next, the differentiation circuit 40 and the envelope detection circuit 51 which constitute the resonant current detection circuit will be described in detail with reference to FIG. 2 and FIG.

[0040] As shown in FIG. 2, the differentiation circuit 40 includes a capacitor C 40 and resistor R 40 It is composed of a series circuit.

[0041] 7, the output voltage VR_OUT of the differentiation circuit 40 is connected to an envelope detection circuit 51 via a diode D15 that corresponds to a rectifier circuit 50. The voltage of a capacitor C51b of the envelope detection circuit 51 becomes the output voltage Vout.

[0042] Next, the operation of the current detection circuit of this embodiment will be described with reference to FIG. 8. The operational waveforms shown here are, from top to bottom, (a) the current I Cr1 , (b) the output voltage VR_OUT of the differentiation circuit 40, (c) the diode D 15 and the output voltage Vh of the envelope detection circuit 51. out It is.

[0043] When the inverter 100a is operated under the above conditions, the peak current of the resonant capacitor Cr1 of the resonant circuit 30 is 30 A / −23 A, as shown in FIG. 40 470 pF, resistor R 40 is 150Ω, the resonant capacitor Cr1 and the capacitor C 40 Due to the capacitance ratio (approximately 1 / 2000), the current I Cr1 Approximately 1 / 2000 of the capacitor C 40 As shown in Fig. 8(b), 40 The peak of the output voltage VR_OUT of the differentiation circuit 40 generated at this point is 3.0 V / −3.6 V. This output voltage VR_OUT is input to the envelope detection circuit 51.

[0044] Then, as shown in FIG. 8(c), the output voltage V out The resonant current I flowing through the heating coil 31 is approximately 2.75V. L is the total current of the resonant capacitors Cr1 and Cr2. If the resonant capacitors Cr1 and Cr2 have the same capacitance, the resonant current I L is the current I of the resonant capacitor Cr1 Cr1 In other words, the resonant current I L The voltage is proportional to

[0045] Figure 9 shows the resonant current I L and the output voltage V of the envelope detector circuit 51. out As shown here, the resonant current I L and the output voltage V out In frequency control, where the heat output is controlled by changing the drive frequency fs while keeping the conductive periods of the switching elements SW1 and SW2 equal, a constant proportional relationship is shown, but in PWM control, where the heat output is controlled by changing the conductive periods of the switching elements SW1 and SW2 while keeping the drive frequency fs constant, a different proportional relationship is shown.

[0046] Here, the details of the PWM control will be described with reference to Figures 10A and 10B. As described above, when heating an iron pan or the like with a small resonance Q, the PWM control can be used. In this case, the first PWM control (Figure 10A) that makes the conduction period U of the switching element SW1 of the upper arm longer than the conduction period L of the switching element SW2 of the lower arm, or the second PWM control (Figure 10B) that makes the conduction period U of the switching element SW1 of the upper arm shorter than the conduction period L of the switching element SW2 of the lower arm can be used. However, if only one of the PWM controls is used continuously, the loss is biased to one of the switching elements SW, so it is desirable to perform an inversion process that switches between the first PWM control and the second PWM control at intervals of, for example, 3 seconds, to level out the loss of the switching element SW.

[0047] <Output voltage V OUT Correction process> In the first PWM control of FIG. 10A and the second PWM control of FIG. 10B, the relationship in the magnitude of the conduction period of each switching element SW is reversed. Therefore, compared to the case of adopting frequency control in which the conduction periods of each switching element SW are equal, the output voltage V OUT The value goes up and down.

[0048] Figure 11 shows the output voltage V for each control type when the drive frequency fs is 38 kHz. OUT As shown here, the output voltage V OUT (solid line) vs. the output voltage V when the first PWM control is used OUT (Dash line) is large, and the output voltage V OUT (dashed line) tends to be small. Therefore, the output voltage V OUT Assuming that the resonant current I L The output voltage V OUT (Dash line) and the output voltage V when the second PWM control is used OUT If the (dashed line) is input as is, the control circuit 60 will generate the appropriate resonant current I L Therefore, it is not possible to calculate

[0049] Therefore, in the control circuit 60 of this embodiment, as shown in FIG. 12, the output voltage V OUT When the voltage is input, the first correction process is performed to reduce the voltage value, and the output voltage V OUT The corrected output voltage V is approximately equal to OUT ', and the output voltage V OUT When the voltage is input, the output voltage V OUT The corrected output voltage V is approximately equal to OUT This allows the appropriate resonant current I L can be calculated.

[0050] Here, for example, the following formula 2 can be used in the above-mentioned first correction process, and for example, the following formula 3 can be used in the second correction process.

[0051]

number

[0052]

number

[0053] In addition, K in Equation 2 and Equation 3 is the output voltage V OUT The output voltage V OUT and an appropriate value is determined in advance by experiment or simulation for each combination of the conduction period U of the upper arm switching element SW1 and the conduction period L of the lower arm switching element SW2, and the control circuit 60 holds a table format and calculation formula such as Table 1. For example, if the conduction period U of the upper arm is within the range of U1 to U2 and the conduction period L of the lower arm is within the range of L1 to L2, K 1,1 is selected as a constant K and used in the calculation of Equation 2 and Equation 3.

[0054] [Table 1]

[0055] <Flowchart of Resonant Current Detection in the Present Embodiment> Next, the calculation process of the resonance current IL in the control circuit 60 will be described with reference to the flowchart of FIG.

[0056] First, in step S1, the control circuit 60 judges whether the current control method is frequency control or PWM control. Specifically, the drive circuit 21 compares the conduction period set in the upper arm switching element SW1 with the conduction period set in the lower arm switching element SW2, and if the conduction periods of the upper and lower arms are the same, it judges that frequency control is being executed, and if the conduction periods of the upper and lower arms are different, it judges that PWM control is being executed. If PWM control is being executed, the process proceeds to step S2, and if frequency control is being executed, the process proceeds to step S5.

[0057] Next, in step S2, control circuit 60 determines whether the conduction period U of the upper arm is longer than the conduction period L of the lower arm. If the conduction period U of the upper arm is longer than the conduction period L of the lower arm, the process proceeds to step S3, and if the conduction period L of the lower arm is longer than the conduction period U of the upper arm, the process proceeds to step S4.

[0058] In step S3, the control circuit 60 detects the output voltage V out As described above, during the first PWM control in which the conduction period U of the upper arm is longer than the conduction period L of the lower arm (see FIG. 10A), the output power V OUT Since the voltage value of increases (see Figure 11), the corrected output voltage V OUT ' is calculated.

[0059] Similarly, in step S4, the control circuit 60 detects the output voltage V outAs described above, during the second PWM control in which the conduction period L of the lower arm is longer than the conduction period U of the upper arm (see FIG. 10B), the output power V OUT Since the voltage value of becomes smaller (see Figure 11), the corrected output voltage V OUT ' is calculated.

[0060] In step S5, the control circuit 60 controls the resonant current I L Specifically, during frequency control, the control circuit 60 calculates the output voltage V out Using this as is, the resonant current I L (See FIG. 9.) Also, during PWM control, the control circuit 60 calculates the output voltage V out The corrected output voltage V OUT ' to find the resonant current I L As a result, the control circuit 60 can calculate an appropriate resonant current I L can be calculated.

[0061] As described above, by differentiating the voltage related to the resonant capacitor Cr1, rectifying the voltage generated by the resistor and performing envelope detection, and performing correction inside the microcomputer according to the control state of the inverter, it is possible to detect the resonant current flowing through the heating coil without using a current transformer or shunt resistor in an induction heating device in which the inverter is PWM controlled as shown in Figures 10A and 10B. As a result, the output voltage V detected depending on the control state can be varied even with the same heating power as shown in Figure 11. OUT This makes it possible to accurately detect the resonant current even during PWM control, where there was a problem of the resonant current varying, and can contribute to making resonant current detection circuits more compact and less costly. [Explanation of symbols]

[0062] 1 Commercial power supply 10 Power circuit 11 Input Current Detector 12 Input current detection circuit 13 Diode Bridge 14 Inductor 20 Inverter circuit 21 Drive circuit 30 Resonant circuit 31 Heating coil 40 Differential circuit 50 Rectifier circuit 51 Envelope detection circuit 60 Control circuit 61 Input power setting section 100a, 100b, 100c inverter Cf Filter capacitor Cr Resonant Capacitor Cs snubber capacitor C Capacitor D Diode R resistance SW Switching element

Claims

1. An induction heating device that uses a heating coil to inductively heat an object to be heated, A DC power source for supplying a DC voltage; an inverter circuit that converts the DC voltage into a high-frequency AC voltage and supplies the AC voltage to the heating coil; A control circuit that controls the upper and lower arms of the inverter circuit by frequency control or PWM control; a resonant circuit in which the heating coil and a resonant capacitor are connected in series; a resonant capacitor voltage differentiation circuit that differentiates a voltage across a resonant capacitor generated by a current flowing through the resonant capacitor; a rectifier circuit that rectifies the output of the resonant capacitor voltage differentiation circuit; an envelope detection circuit that detects an envelope of the output of the rectifier circuit and outputs an output voltage; The control circuit includes: determining whether the upper and lower arm control of the inverter circuit is frequency control or PWM control; In the case of frequency control, a resonant current value flowing through the heating coil is calculated based on the output voltage of the envelope detection circuit; In the case of PWM control, the induction heating device calculates a resonant current value flowing through the heating coil based on a corrected output voltage obtained by correcting the output voltage of the envelope detection circuit.

2. 2. The induction heating device according to claim 1, The resonant capacitor voltage differentiation circuit is composed of a series circuit of a capacitor and a resistor, The induction heating device according to claim 1, wherein the voltage generated across the resistor is used as the output of the resonant capacitor voltage differentiation circuit.

3. In the induction heating device according to claim 1 or 2, The envelope detection circuit includes: a parallel circuit of a first capacitor and a first resistor; a series circuit of a second resistor and a second capacitor connected to both ends of the first resistor; An induction heating device, characterized in that the voltage across the second capacitor is used as the output voltage of the envelope detection circuit.

4. In the induction heating device according to any one of claims 1 to 3, The control circuit includes: During a first PWM control in which a conduction period of the upper arm is longer than a conduction period of the lower arm, a first correction process is performed to reduce an output voltage of the envelope detection circuit, thereby generating the corrected output voltage; An induction heating device characterized in that during a second PWM control in which the conduction period of the lower arm is longer than the conduction period of the upper arm, the corrected output voltage is generated by performing a second correction process that increases the output voltage of the envelope detection circuit.

5. 5. The induction heating device according to claim 4, The induction heating device, wherein the control circuit alternately performs the first PWM control and the second PWM control.

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