Power conversion device for electric stove and control method thereof

Abstract

The present invention discloses a power conversion device for an electric stove and a control method thereof. The control method, in one embodiment of the present invention, receives a user-selected power level through an interface, determines a resonant frequency based on the power level and the object being heated, compares a rectified voltage with the voltage across a switching device to generate a switching time reference signal, determines a delay time based on the difference between the resonant frequency and the frequency of the switching time reference signal, and outputs a switching control signal that turns on after the delay time has elapsed from the falling edge of the switching time reference signal.

Description

Technical Field

[0001] The invention relates to a power conversion device of an electric range and a control method thereof. Background Art

[0002] Since the beginning of the 20th century, with growing awareness of fossil fuels due to global warming and climate change, and the resulting regulation, the use of environmentally friendly induction heating has expanded beyond traditional industrial applications such as metal melting, heat treatment, and welding, and is now also gaining popularity in homes. Induction heating in homes is primarily used in cooking appliances such as rice cookers, induction cooktops, and induction cooktops.

[0003] The development of high-speed switching power semiconductor devices is coinciding with the research and development of high-frequency inverter technology for power converters used in induction heating cooking appliances. While the increasing frequency of power conversion devices offers advantages such as miniaturization and weight reduction for passive components such as inductors and capacitors, the increased operating frequency also increases switching losses in power semiconductors. Consequently, extensive research is underway into resonant inverter technology, which aims to minimize switching losses and reduce switching noise even at high frequencies.

[0004] Resonant inverters used in induction cooking machines are categorized into full-bridge, half-bridge, and single-ended circuits based on capacity and operation method. Single-ended resonant inverters are widely used due to their simple structure, minimal component count, and low price.

[0005] Figure 1 This is a circuit diagram of a single-ended resonant inverter used in induction cooking machines. This single-ended resonant inverter significantly reduces switching losses through zero-voltage switching. Conversely, due to its use of voltage resonance, it switches large voltages across the terminals. Therefore, the switching device primarily uses an insulated gate bipolar transistor (IGBT), which has high withstand voltage, high current rating, and low conduction losses.

[0006] The inverter's input terminals consist simply of a rectifier, a choke coil, and a DC link capacitor. To achieve a high power factor solely through inverter operation, a separate power factor compensation circuit is unnecessary, and a small DC link capacitor is used. Consequently, the DC link voltage becomes an unsmooth ripple current.

[0007] The resonant circuit structure is a series connection of equivalent inductance Lr and equivalent resistance Req to represent the container and the working coil, on which the resonant capacitor Cr is connected in parallel. Figure 2 As shown in the figure, the operation of the single-ended resonant inverter can be divided into four modes for explanation.

[0008] Figure 2 It is an explanation Figure 1An example diagram of the operation of a resonant inverter; Figure 3 The theoretical waveforms of voltage and current in each mode are shown.

[0009] Figure 2 (a) is Figure 3 From t0 to t1, in transfer mode, resonant energy is transferred from the resonant capacitor Cr to the load and equivalent inductance via parallel voltage resonance, regenerating the DC link. At this time, because current flows through the IGBT's antiparallel diode, the voltage across the collector-emitter terminals of the switching device is zero between t0 and t1. Consequently, the IGBT is turned on during this period, achieving zero-voltage switching.

[0010] Figure 2 (b) is Figure 3 From t1 to t2, energy is supplied from the DC link to the load and working coil. The IGBT turns on, changing the current direction, flowing from the DC link power source through the working coil and the IGBT. At t2, the switching device turns off, and the resonant capacitor begins charging. At this point, the low dv / dt increases the IGBT collector-emitter voltage. Therefore, if losses caused by the IGBT tail current are disregarded, this can be considered zero-voltage switching.

[0011] Figure 2 (c) is Figure 3 From t2 to t3, the equivalent inductor Lr and the resonant capacitor Cr enter a parallel resonant mode. Starting at t2, when the IGBT is turned off, the voltage between the collector and emitter of the IGBT increases due to the low dv / dt, reaching the DC link voltage Vdc. Parallel resonance then begins between the equivalent inductor and the resonant capacitor. At this point, the energy stored in the equivalent inductor is transferred to the load and the resonant capacitor.

[0012] Figure 2 (d) is Figure 3 From t3 to t0, the equivalent inductor and resonant capacitor resonate in parallel, transferring energy from the resonant capacitor to the load and equivalent inductor. The resonant voltage is at its lowest at t3 and then gradually increases. If the resonant capacitor voltage rises above the DC link voltage, current begins to flow through the IGBT's antiparallel diode, maintaining the resonant capacitor voltage at the DC link voltage.

[0013] In a single-ended resonant inverter for an induction heating cooking machine, the voltage generated by parallel voltage resonance varies depending on the current flowing through the working coil at the time of switching the device. If the current is small, the resonant voltage decreases, and if the current is large, the resonant voltage increases.

[0014] If the magnitude of the resonant voltage generated at this time is smaller than the DC link voltage, the zero-voltage switching condition is violated, and hard switching is performed. To achieve high-efficiency switching with low current flowing through the working coil, the switching device should be turned on at the time when the voltage across its terminals is lowest. Figure 4 This is a diagram showing an example of a switching operation and a current flowing through a switching device under the condition that the current flowing through the working coil is small.

[0015] At this time, the slope of the resonant voltage decrease changes at the resonant frequency formed by the container, the working coil, and the resonant capacitor and the frequency away from the resonant frequency, and the slope of the relative decrease in the resonant voltage changes, thereby causing hard switching. Figure 5 This is an example diagram used to illustrate the relationship between power and frequency of a single-ended inverter.

[0016] Therefore, there is a problem that although zero voltage switching is achieved at the maximum power at the resonant frequency, the power is relatively low due to hard switching at the minimum power farthest from the resonant frequency, and even then, more heat is generated in the switching device.

[0017] To address the aforementioned issues, Korean Patent No. 10-0692634 (Induction Heating Cooking Machine Driving Current and Driving Method Thereof) proposes a solution that compares the resonant voltage with the compensation voltage to generate a zero-cross pulse signal. The switching device is then turned on after a predetermined delay time from the falling edge of the zero-cross pulse signal, minimizing losses in the resonant current. Specifically, turning on the IGBT at the inflection point of the resonant voltage minimizes losses. Therefore, the time of this inflection point is predicted, and the IGBT is turned on at that time, thereby minimizing losses in the driving current. However, Korean Patent No. 10-0692634 suffers from an issue in which it cannot handle increases in the current flowing through the control and in the resonant voltage. Summary of the Invention

[0018] Problems to be solved by the invention

[0019] The present invention is intended to solve the above-mentioned problem, and an object of the present invention is to change the delay time based on the degree of distance between the frequency of the switching control signal and the resonant frequency, thereby preventing hard switching of the switching device.

[0020] Means used to solve problems

[0021] In order to solve the technical problems mentioned above, a power conversion device according to an embodiment of the present invention is provided as a power conversion device for an electric stove. The electric stove has a panel and an interface portion, the panel is used to place a heated object, and the interface portion receives a power level selected by a user. The power conversion device may include: a power supply portion for supplying a rectified voltage; a switching device; a working coil arranged at the lower portion of the panel, and applying the rectified voltage through switching of the switching device to inductively heat the heated object; a resonant capacitor arranged in parallel with the working coil; a comparison portion for comparing the rectified voltage with the voltage across the switching device to generate a switching time reference signal; a control portion for determining a resonant frequency according to the heated object and the power level input through the interface, determining a delay time according to a difference between the frequency of the switching time reference signal and the resonant frequency, and outputting a switching control signal that is turned on after the delay time starting from the falling edge of the switching time reference signal; and a storage portion for storing the delay time generated according to the difference between the resonant frequency and the switching time reference signal.

[0022] In one embodiment of the present invention, the storage unit stores the delay time generated according to the difference between the resonant frequency and the switching time reference signal based on the frequency band of the resonant frequency and the switching time reference signal; the control unit determines the delay time based on the frequency band of the resonant frequency and the switching time reference signal and the difference between the resonant frequency and the switching time reference signal.

[0023] In one embodiment of the present invention, the off time of the switching control signal may be determined according to the characteristics of the resonant capacitor and the working coil.

[0024] In one embodiment of the present invention, when the frequency of the switching control signal of the switching device is changed to achieve the target power of the power level, the control unit may determine the frequency of the switching control signal as the resonant frequency.

[0025] In one embodiment of the present invention, the control unit applies a damping signal in a standby state, and may determine a frequency of a damped oscillation generated by the resonant capacitor and the working coil as the resonant frequency.

[0026] In order to solve the technical problems mentioned above, a control method for a power conversion device according to an embodiment of the present invention is provided, as a control method for a power conversion device of an electric stove, wherein the electric stove has a panel and an interface portion, wherein the panel is placed with a heated object, and the interface portion receives a power level selected by a user, and the control method may include the following steps: receiving the power level selected by the user through the interface portion; determining a resonant frequency from the power level and the heated object; comparing the rectified voltage with the voltage at both ends of the switching device to generate a switching time reference signal; determining a delay time according to the difference between the resonant frequency and the frequency of the switching time reference signal; and outputting a switching control signal that is turned on after the delay time from the falling edge of the switching time reference signal.

[0027] In one embodiment of the present invention, the delay time may be predetermined and stored according to the frequency bands of the resonant frequency and the switching time reference signal and the difference between the resonant frequency and the switching time reference signal.

[0028] Effects of the Invention

[0029] The present invention as described above prevents hard switching even at low-end power where the frequency of the switching control signal is high, ensures power retention time, and reduces the pressure on the switching device, thereby achieving the effect of realizing linear power design. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a circuit diagram of a single-ended resonant inverter used in an induction heating cooking machine.

[0031] Figure 2 It is an explanation Figure 1 An example diagram of the operation of a resonant inverter.

[0032] Figure 3 The theoretical waveforms of voltage and current in each mode are shown.

[0033] Figure 4 This is a diagram showing an example of a switching operation and a current flowing through a switching device under the condition that the current flowing through the working coil is small.

[0034] Figure 5 This is an example diagram used to illustrate the relationship between power and frequency of a single-ended inverter.

[0035] Figure 6 This is an external structural diagram of an electric stove to which a power conversion device according to an embodiment of the present invention is applied.

[0036] Figure 7 This is a circuit diagram for schematically illustrating the configuration of a power conversion device in which a working coil is energized according to an embodiment of the present invention.

[0037] Figure 8 FIG. 1 is a diagram showing an example of a driving timing diagram of a switching device.

[0038] Figure 9 Is used to illustrate Figure 7 Flowchart of the operation of the control unit.

[0039] Figure 10 FIG. 1 is a diagram showing that soft switching occurs by changing the switching control signal vg according to the present invention. DETAILED DESCRIPTION

[0040] In order to fully understand the structure and effects of the present invention, preferred embodiments of the present invention are described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various forms and various modifications can be made. However, the description of this embodiment is provided to fully disclose the present invention and to fully inform those with ordinary knowledge in the technical field to which the present invention belongs of the scope of the invention. For ease of description, components are shown in the drawings larger than their actual size, and the proportions of each component may be exaggerated or reduced.

[0041] The terms "first", "second", etc. can be applied to describe multiple components, and the components shall not be limited to these terms. The terms are used only to distinguish one component from another. For example, without exceeding the scope of the present invention, the "first component" can be named "second component", and similarly, the "second component" can be named "first component". In addition, for singular expressions, unless there is a clear different expression in the article, the plural expression is included. For the terms used in the embodiments of the present invention, unless there is a different definition, it can be interpreted with the meaning known to people with general knowledge in this technical field.

[0042] Hereinafter, a power conversion device and a control method thereof according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0043] Figure 6 This is an external structural diagram of an electric stove to which a power conversion device according to an embodiment of the present invention is applied.

[0044] Reference Figure 6 An electric stove applicable to an embodiment of the present invention may include: a shell 1 constituting a main body; and a cover plate 2 combined with the shell 1 to close the shell 1 .

[0045] The cover plate 2 is combined with the top of the shell 1 to seal the space formed inside the shell 1 from the outside. It can be made of a material (for example, ceramic glass, etc.) that can well transfer the heat generated by the heating part 3 to the heated object placed in the area corresponding to the heating part 3.

[0046] Multiple heating elements 3 can be configured on the housing 1 for heating objects. Furthermore, an interface unit 4 can be configured on the top of the housing 1. This interface unit 4 allows the user to supply power or adjust the power of the heating elements 3, or display information related to the electric stove. The interface unit 4 can be comprised of a touch screen that allows both touch input and display of information. Depending on the embodiment, other interface unit 4 structures can also be used. For example, the interface unit can be comprised of a button-type interface unit or a dial-type interface unit.

[0047] When a user inputs a power level and the like through the interface unit 4 , this input is transmitted to a control unit (described later) disposed inside the housing 1 .

[0048] In the description of the present invention, the heat generating portion 3 and the interface portion 4 are provided on the housing 1 as an example. However, this is for the convenience of description, and it is obvious that other multiple components for driving the electric stove may also be provided.

[0049] The cover plate 2 may include an operation area 5, which is arranged at a position corresponding to the interface portion 4. In order to facilitate operations by the user, text or patterns may be printed on the operation area 5. The user touches a specific area of the operation area 5 with reference to the pre-printed text or pattern, and then performs the desired operation. In addition, the information output through the interface portion 4 may also be displayed through the cover plate 2. When the interface portion 4 is constructed in other ways other than a touch method, the operation area 5 may be constructed corresponding to the construction method of the interface portion 4. For example, corresponding to a button-type or dial-type interface portion 4, the operation area 5 may also be constructed by a simple structure that exposes the interface portion 4.

[0050] exist Figure 6 In the embodiment of the present invention, an example of configuring three heating parts 3 inside the housing 1 is shown. However, in another embodiment of the present invention, one or two heating parts, or more than three heating parts may be configured inside the housing 1. Figure 6 The structure of the electric range is schematically shown, but it is obvious that the electric range may also have more various structures.

[0051] In one embodiment of the present invention, the heating portion 3 may include a working coil that forms an induced magnetic field using a supplied high-frequency alternating current. That is, if current flows through the working coil, a magnetic field is formed in the working coil, and this magnetic field generates eddy currents in a cooking container that is magnetically coupled to the working coil, thereby heating the heated object to cook food. In this case, the electric stove may be an induction heating cooking device. Alternatively, the heating portion 3 may also include a hot wire that heats the cover plate 2. That is, if electricity is applied to the hot wire, heat can be generated to heat the heated object placed on the cover plate 2 to cook food. In this case, the electric stove may be a ceramic stove cooking device. In this way, the electric stove of the present invention may be an induction heating cooking machine or a ceramic stove cooking machine, but the following description is based on an embodiment in which the heating portion 3 is a working coil.

[0052] Re-reference Figure 1 The control unit described later is configured in the space formed inside the shell 1, receives the input of the user through the interface unit 4, controls the on / off of the switching device described later according to the input of the user, and thus controls the power supply to the working coil.

[0053] Hereinafter, the operation of the power conversion device according to one embodiment of the present invention for supplying power to the heat generating portion 3 serving as the working coil will be described with reference to the drawings.

[0054] Figure 7 This is a circuit diagram for schematically illustrating the configuration of a power conversion device in which a working coil is energized according to an embodiment of the present invention.

[0055] As shown in the figure, a power conversion device according to an embodiment of the present invention may include: a power supply unit 11, a choke coil 12, a rectifier unit 13, a DC link capacitor 14, an equivalent resistor 16a and an equivalent inductor 16b constituting a working coil 16, a resonant capacitor 15 arranged in parallel with the working coil 16, a switching device 17, a control unit 18, a comparison unit 19 and a storage unit 20.

[0056] In the single-ended power conversion device described above, a resonant capacitor 15 is inserted in parallel with the working coil 16 to generate voltage resonance, thereby generating a high resonant voltage. The resonant voltage is designed to be approximately 700V, and the voltage across the switching device 17 exceeds 1000V. Therefore, the switching device 17 used in this power conversion device is primarily an insulated gate bipolar transistor (IGBT) with a rated voltage of 1200V or higher. However, the present invention is not limited to this, and various power semiconductor devices can be used.

[0057] The rectifier 13 can rectify the AC voltage supplied from the power supply 11 and output a rectified voltage. The choke coil 12 smoothes the rectified voltage and removes ripples in the rectified voltage. In other words, the choke coil 12 is connected to block high-frequency signals above a predetermined frequency. Other devices that perform this function may also be configured. Figure 7 The choke coil 12 may be arranged at other positions besides the position of the choke coil 12.

[0058] The DC link capacitor 14 functions as a power source for applying a rectified voltage to the working coil 16. In a single-ended power conversion device, in order to achieve a high power factor solely through the operation of the power conversion device, a separate power factor compensation circuit is not required and a low-capacity capacitor is used, resulting in the DC link voltage becoming an unsmooth ripple current.

[0059] According to the power level of the electric stove input by the user, the control unit 18 generates and outputs a switching control signal for controlling the switching device 17. At this time, the switching control signal can be, for example, a gate drive signal for an IGBT. The switching device 17 receiving the switching control signal switches on or off based on the switching control signal, thereby applying a rectified voltage to the working coil 16. Figure 2 The work can transfer heat to the heated object placed on the cover plate 2 through the induced current generated in the working coil 16.

[0060] The control unit 18 of one embodiment of the present invention is configured to determine the resonant frequency according to the coupling characteristics of the heated object when a user places the heated object on the cover plate 2 and selects a power level through the interface unit 4 .

[0061] In addition, the control unit 18 can determine the switching control signal according to the power level selected by the user. In this case, the control unit 18 can determine the switching control signal by comparing the resonance voltage and the DC link voltage through the comparison unit 19.

[0062] Specifically, the voltage vce across the switching device 17 is represented by the sum of the DC link voltage vdc and the voltage vcr of the resonant capacitor 15. Therefore, the comparator 19 compares the voltage vce across the switching device 17 with the DC link voltage vdc to generate a switching time reference signal, which can be input to the controller 18. In this case, the controller 18 can measure the pulse width of the switching time reference signal corresponding to half a cycle of the resonant voltage to determine the frequency of the switching time reference signal.

[0063] On the other hand, the control unit 18 compares the resonance frequency with the frequency of the switching time reference signal to determine the delay time. Figure 5At high power, the resonance frequency is similar to the frequency of the switching control signal, and soft switching can be achieved. However, at low power, the frequency of the switching control signal is higher than the resonance frequency and the resonance voltage is stretched, so hard switching has to be performed.

[0064] Therefore, the control unit 18 of the present invention can determine the delay time according to the difference between the resonant frequency and the frequency of the switching time reference signal. In this case, the greater the difference between the resonant frequency and the frequency of the switching time reference signal, the longer the delay time is determined.

[0065] In this case, a delay time is predetermined based on the difference between the two frequencies and can be stored in the storage unit 20. The control unit 18 determines the delay time based on the difference between the two frequencies and determines the switching control signal as the PWM signal after the delay time has elapsed from the falling edge of the switching time reference signal. For example, the delay time can be 1 to 5 μs. If the delay time is too long, the switching device 17 may be turned on at the time when the resonant voltage is rising, potentially burning out the switching device 17.

[0066] Figure 8 FIG. 1 is a diagram showing an example of a driving timing diagram of a switching device.

[0067] The comparison unit 19 compares the voltage vce across the switching device 17 with the DC link voltage vdc to generate a switching time reference signal vstr. The control unit 18 can confirm the frequency of the time reference signal determined by the comparison unit 19 .

[0068] Then, control unit 18 determines delay time tg based on the difference between the resonant frequency and the frequency of the switching time reference signal. Based on delay time tg, control unit 18 determines switching control signal vg, which delays the on-time of the switching time reference signal. In this case, the off-time of the switching control signal, which is a PWM signal, is determined by the LC resonance between resonant capacitor 15 and working coil 16. In other words, if the on-time of the switching control signal is determined, the off-time can be determined by the circuit characteristics.

[0069] The switching device 17 receives the switching control signal and is repeatedly turned on or off according to the switching control signal, thereby generating an induced current in the working coil 16 .

[0070] Hereinafter, the operation of the control unit 18 will be described with reference to the drawings.

[0071] Figure 9 Is used to illustrate Figure 7 Flowchart of the operation of the control unit.

[0072] As shown in the figure, in one embodiment of the present invention, the user can input the power level of the electric stove through the interface part 4 of the electric stove (S10); the control part 18 receives the power level selected by the user from the interface part 4, and can determine the resonant frequency (S20) based on the power level and the heated object on the upper part of the cover plate 2.

[0073] The resonant frequency and switching control frequency band of the electric stove can be varied by changing the working coil 16 and the equivalent resistor 16a and equivalent inductor 16b forming the working coil 16 according to the material, manufacturing process, and shape of the heated object. Furthermore, the frequency of the switching control signal can be varied according to the power level input by the user through the interface unit 4.

[0074] In addition, when the same power level is set for the heated object with the same material, shape, and manufacturing process, when the position of the working coil 16 as the container is moved from its original position, the inductance of the equivalent inductor 16b changes, thereby changing the resonant frequency.

[0075] Therefore, when the user inputs a power level, the control unit 18 can determine the resonant frequency at that point in time.

[0076] At this time, the control unit 18 changes the frequency of the switching control signal until the target power of the power level is reached. When the target power is reached, the control unit 18 may determine the frequency of the switching control signal as the resonance frequency.

[0077] Alternatively, the control unit 18 can determine the resonant frequency through the initial damped oscillation. If a switching control signal (damping signal) between 1 and 5 μs is applied once in the standby state, damped oscillation occurs in the resonant circuit (resonant capacitor 15 and working coil 16). At this time, the control unit can determine the frequency of the damped oscillation as the resonant frequency.

[0078] However, the resonant frequency determination is not limited to the one embodiment of the present invention, and the control unit 18 may determine the resonant frequency in more various ways.

[0079] Then, the comparison unit 19 may output a switching time reference signal according to the power level (S30). Figure 8 The comparison unit 19 compares the voltage vce across the switching device 17 with the DC link voltage vdc and outputs a switching time reference signal vstr. At this time, the control unit 18 may determine the frequency of the switching time reference signal.

[0080] Next, the control unit 18 compares the resonant frequency with the frequency of the switching time reference signal and determines the delay time based on the degree to which the switching time reference signal frequency exceeds the resonant frequency (S40). Conventionally, a control signal is output that delays the switching time reference signal by a predetermined time before switching on. However, as the control signal frequency increases relative to the resonant frequency, the resonant voltage falls over a longer time, making a fixed delay time incapable of handling hard switching.

[0081] In one embodiment of the present invention, a table of delay times is prepared and stored in the storage unit 20. The larger the frequency of the switching time reference signal is compared with the resonant frequency, the longer the delay time is. The control unit 18 can determine the delay time generated by the difference between the frequency of the switching time reference signal and the resonant frequency from the storage unit 20.

[0082] The delay time stored in storage unit 20 can vary depending on the resonant frequency and the frequency band of the switching time reference signal. Specifically, the lower the frequency band, the longer the resonant voltage period, and thus the longer the fall time. Therefore, the lower the frequency band, the longer the delay time stored in storage unit 20. Control unit 18 determines the delay time based on the resonant frequency and the frequency band of the switching time reference signal.

[0083] That is, the storage unit 20 stores the delay time in a table based on two variables: the resonance frequency and the frequency band of the switching time reference signal, and the difference between the frequency of the switching time reference signal and the resonance frequency.

[0084] Then, the control unit 18 determines the switching control signal vg to be turned on after the delay time tg has elapsed from the falling edge of the switching time reference signal (S50). In other words, the switching control signal, which is a PWM signal, is determined to be turned on after the delay time tg has elapsed from the falling edge of the switching time reference signal and to be turned off after the pulse width ton determined by the resonant circuits 15 and 16.

[0085] By controlling as described above, Figure 4 The hard switching that occurs becomes Figure 10 Soft switching.

[0086] Figure 10 FIG. 1 is a diagram showing that soft switching occurs by changing the switching control signal vg according to the present invention.

[0087] Through the present invention as described above, hard switching is prevented even at the low-end power where the frequency of the switching control signal is high, the power holding time is ensured, and the pressure on the switching device is reduced, thereby achieving the effect of realizing linear power design.

[0088] The embodiments of the present invention are described above, but they are merely exemplary. A person with ordinary knowledge in the relevant technical field will understand that various modifications and equivalent embodiments are possible. Therefore, the true technical protection scope of the present invention should be defined by the claims.

[0089] Industrial Applicability

[0090] The power conversion device and control method of the electric stove of the present invention can be used to control various household appliances used at home or in industrial sites and can be implemented to control the household appliances, and therefore have industrial applicability.

Claims

1. A power conversion device, as a power conversion device of an electric stove, the electric stove has a panel and an interface part, the panel is placed on the heated object, and the interface part receives the power level selected by the user, wherein: The power conversion device comprises: Power supply unit, supplying rectified voltage; Switching devices; a working coil, disposed at the lower portion of the panel, applying the rectified voltage through switching of the switching device to inductively heat the heated object; a resonant capacitor, arranged in parallel with the working coil; a comparing unit, configured to compare the rectified voltage with the voltage across the switching device to generate a switching time reference signal; a control unit that determines a resonant frequency based on the heated object and a power level input through the interface, determines a delay time based on a difference between the frequency of the switching time reference signal and the resonant frequency, and outputs a switching control signal that turns on after the delay time has elapsed from a falling edge of the switching time reference signal; and The storage unit stores a delay time generated according to a difference between the resonant frequency and the switching time reference signal.

2. The power conversion device according to claim 1, wherein: The storage unit stores a delay time generated according to a difference between the resonance frequency and the switching time reference signal according to a frequency band of the resonance frequency and the switching time reference signal; The control unit determines a delay time according to a frequency band of the resonance frequency and the switching time reference signal and a difference between the resonance frequency and the switching time reference signal.

3. The power conversion device according to claim 1, wherein: The off time of the switching control signal is determined according to the characteristics of the resonant capacitor and the working coil.

4. The power conversion device according to claim 1, wherein: When the frequency of the switching control signal of the switching device is changed to achieve the target power of the power level, the control unit determines the frequency of the switching control signal as the resonance frequency.

5. The power conversion device according to claim 1, wherein: The control unit applies a damping signal in a standby state to determine a frequency of damped oscillation generated by the resonant capacitor and the working coil as a resonant frequency.

6. A control method, as a control method for a power conversion device of an electric range, wherein the electric range comprises a panel and an interface portion, wherein the panel is placed on a heated object, and the interface portion receives a power level selected by a user, wherein: The control method comprises the following steps: receiving a power level selected by a user through the interface portion; determining a resonant frequency from the power level and the heated object; Comparing the rectified voltage with the voltage across the switching device to generate a switching time reference signal; determining a delay time according to a difference between the resonant frequency and the frequency of the switching time reference signal; and A switching control signal that is turned on after the delay time has elapsed from the falling edge of the switching time reference signal is output.

7. The control method according to claim 6, characterized in that: The delay time is predetermined and stored according to a frequency band of the resonance frequency and the switching time reference signal and a difference between the resonance frequency and the switching time reference signal.

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