CN104902600B - Electromagnetic heater and its control method - Google Patents

Abstract

The invention discloses a kind of control method of electromagnetic heater, it comprises the following steps：Detect the operating current of the power switch of the electromagnetic heater；And the pulse width of control signal that the power switch according to the operating current adjustment control turns on and off is so that the operating current changes between default target lower current and default target upper limit electric current.The control method of the electromagnetic heater of the present invention can change the working frequency of electromagnetic heater by the pulse width of adjustment control signal, so as to suppress electromagnetic interference during electromagnetic heater work, reduce electromagnetic interference, influence of the electromagnetic interference to the health of people and the normal work of other electrical equipment is reduced, fully meets people's living needs.The invention also discloses a kind of electromagnetic heater.

Description

Electromagnetic heating device and control method thereof

technical field

The invention relates to the technical field of electromagnetic heating, in particular to a control method of an electromagnetic heating device and an electromagnetic heating device.

Background technique

After the output power of the current induction cooker is stable, the IGBT (Insulated Gate Bipolar Transistor, insulated gate bipolar transistor) drive unit works in the fixed frequency PWM (Pulse Width Modulation, pulse width modulation) control mode, and the PWM square wave signal is shown in the figure 1. Most of the energy is limited to the fundamental wave and lower harmonics. The frequency spectrum of the harmonics is generally narrow in bandwidth and the harmonic amplitude is large, so the electromagnetic interference caused by the fixed frequency is relatively large.

When the electromagnetic interference is large, it will affect people's health and the normal work of other electrical appliances, which will have a great impact on people's lives. Therefore, the control technology of existing induction cooker needs to be improved.

Contents of the invention

The purpose of the present invention is to at least solve the above-mentioned technical drawbacks.

Therefore, an object of the present invention is to provide a control method of an electromagnetic heating device capable of suppressing electromagnetic interference by adjusting the pulse width of a control signal.

Another object of the present invention is to propose an electromagnetic heating device.

In order to achieve the above purpose, a control method of an electromagnetic heating device proposed in an embodiment of the present invention includes the following steps: detecting the operating current of the power switch of the electromagnetic heating device; The pulse width of the control signal for turning on and turning off the power switch is used to change the operating current between a preset target lower limit current and a preset target upper limit current.

According to the control method of the electromagnetic heating device of the embodiment of the present invention, by detecting the operating current of the power switch of the electromagnetic heating device, and then adjusting the pulse width of the control signal for controlling the power switch to be turned on and off according to the operating current so that the operating current is at a preset value The target lower limit current and the preset target upper limit current change, so that the operating frequency of the electromagnetic heating device can be changed by adjusting the pulse width of the control signal, thereby suppressing the electromagnetic interference when the electromagnetic heating device is working, and reducing the electromagnetic interference. Reduce the impact of electromagnetic interference on human health and the normal work of other electrical appliances, and fully meet people's living needs. In addition, the control method is simple and easy to implement, and has strong versatility.

According to an embodiment of the present invention, when the control signal is a pulse width modulation PWM signal, wherein, when the operating current reaches the preset target lower limit current, the pulse width of the PWM signal is increased; when the When the operating current reaches the preset target upper limit current, the pulse width of the PWM signal is reduced.

According to an embodiment of the present invention, the pulse width of the PWM signal is periodically changed according to a preset change rule.

According to an embodiment of the present invention, within a period, the preset change law is: in the first preset time period, the pulse width of the PWM signal is the first preset width, and the PWM signal keeps The first frequency is fixed; during the second preset time period, the pulse width of the PWM signal is increased by a first width threshold on the basis of the first preset width, and the PWM signal keeps the second frequency constant. change, wherein the second frequency is less than the first frequency; during a third preset time period, the pulse width of the PWM signal is increased by the first width threshold on the basis of the second preset width, And the PWM signal keeps the third frequency constant, wherein the third frequency is less than the second frequency; and in the fourth preset time period, the pulse width of the PWM signal is within the third preset The first width threshold is reduced based on the width, and a fourth frequency of the PWM signal is kept constant, wherein the fourth frequency is equal to the second frequency.

According to an embodiment of the present invention, during the first preset time period to the fourth preset time period, the number of pulses of the PWM signal is one or more.

In order to achieve the above purpose, another embodiment of the present invention proposes an electromagnetic heating device, comprising: a power switch; a current detection unit, the current detection unit is connected to the power switch, and the current detection unit is used to detect the The operating current of the power switch; and a controller, the controller is connected to the power switch and the current detection unit respectively, and the controller is used to control the opening and closing of the power switch according to the operating current adjustment The pulse width of the control signal makes the operating current vary between a preset target lower limit current and a preset target upper limit current.

According to the electromagnetic heating device of the embodiment of the present invention, the operating current of the power switch is detected by the current detection unit, and then the controller adjusts the pulse width of the control signal that controls the power switch to be turned on and off according to the operating current so that the operating current is within the preset target Change between the lower limit current and the preset target upper limit current, so that the electromagnetic heating device in the embodiment of the present invention can change the operating frequency by adjusting the pulse width of the control signal, thereby suppressing electromagnetic interference during work, reducing electromagnetic interference, and reducing The impact of electromagnetic interference on human health and the normal operation of other electrical appliances fully meets the needs of people's lives without increasing costs.

According to an embodiment of the present invention, when the power switch is an insulated gate bipolar transistor IGBT, the E pole of the IGBT is connected to the current detection unit, and the electromagnetic heating device further includes an IGBT drive unit, the The IGBT drive unit is connected between the G pole of the IGBT and the controller.

According to an embodiment of the present invention, when the control signal is a pulse width modulation PWM signal, wherein, when the operating current reaches the preset target lower limit current, the controller controls the IGBT drive unit to increase the pulse width of the PWM signal; when the operating current reaches the preset target upper limit current, the controller controls the IGBT driving unit to reduce the pulse width of the PWM signal.

According to an embodiment of the present invention, the pulse width of the PWM signal is periodically changed according to a preset change rule.

According to an embodiment of the present invention, within a period, the preset change law is: in the first preset time period, the pulse width of the PWM signal is the first preset width, and the PWM signal keeps The first frequency is fixed; during the second preset time period, the pulse width of the PWM signal is increased by a first width threshold on the basis of the first preset width, and the PWM signal keeps the second frequency constant. change, wherein the second frequency is less than the first frequency; during a third preset time period, the pulse width of the PWM signal is increased by the first width threshold on the basis of the second preset width, And the PWM signal keeps the third frequency constant, wherein the third frequency is less than the second frequency; and in the fourth preset time period, the pulse width of the PWM signal is within the third preset The first width threshold is reduced based on the width, and a fourth frequency of the PWM signal is kept constant, wherein the fourth frequency is equal to the second frequency.

According to an embodiment of the present invention, during the first preset time period to the fourth preset time period, the number of pulses of the PWM signal is one or more.

Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Description of drawings

The above and/or additional aspects and advantages of the present invention will become apparent and easy to understand from the following description of the embodiments in conjunction with the accompanying drawings, wherein:

Fig. 1 is the schematic diagram of the square wave signal of the PWM of existing control induction cooker;

2 is a schematic block diagram of an electromagnetic heating device according to an embodiment of the present invention;

3 is a schematic diagram of a control circuit of an electromagnetic heating device according to an embodiment of the present invention;

4 is a schematic diagram of a PWM signal of an electromagnetic heating device according to an embodiment of the present invention;

5 is a schematic diagram of a PWM signal of an electromagnetic heating device according to another embodiment of the present invention; and

Fig. 6 is a flowchart of a control method of an electromagnetic heating device according to an embodiment of the present invention.

Reference signs:

A power switch 10 , a current detection unit 20 , a controller 30 , and an IGBT drive unit 40 .

Detailed ways

Embodiments of the present invention are described in detail below, examples of which are shown in the drawings, wherein the same or similar reference numerals designate the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the figures are exemplary only for explaining the present invention and should not be construed as limiting the present invention.

The following disclosure provides many different embodiments or examples for implementing different structures of the present invention. To simplify the disclosure of the present invention, components and arrangements of specific examples are described below. Of course, they are only examples and are not intended to limit the invention. Furthermore, the present invention may repeat reference numerals and/or letters in different instances. This repetition is for the purpose of simplicity and clarity and does not in itself indicate a relationship between the various embodiments and/or arrangements discussed. In addition, various specific process and material examples are provided herein, but one of ordinary skill in the art will recognize the applicability of other processes and/or the use of other materials. Additionally, configurations described below in which a first feature is "on" a second feature may include embodiments where the first and second features are formed in direct contact, and may include additional features formed between the first and second features. For example, such that the first and second features may not be in direct contact.

In the description of the present invention, it should be noted that unless otherwise specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a mechanical connection or an electrical connection, or it can be two The internal communication of each element may be directly connected or indirectly connected through an intermediary. Those skilled in the art can understand the specific meanings of the above terms according to specific situations.

The electromagnetic heating device and the control method of the electromagnetic heating device proposed according to the embodiments of the present invention are described below with reference to the accompanying drawings.

Fig. 2 is a schematic block diagram of an electromagnetic heating device according to an embodiment of the present invention. As shown in FIG. 2 , the electromagnetic heating device includes a power switch 10 , a current detection unit 20 and a controller 30 .

Wherein, the current detection unit 20 is connected with the power switch 10 , and the current detection unit 20 is used to detect the working current of the power switch 10 . The controller 30 is connected to the power switch 10 and the current detection unit 20 respectively, and the controller 30 adjusts the pulse width of the control signal for controlling the power switch to be turned on and off according to the working current so that the working current is within the preset target lower limit current and the preset target current. Change between the set target upper limit current.

In this way, in the embodiment of the present invention, the operating frequency is changed by adjusting the pulse width of the control signal, and according to the change of the operating frequency, the energy of the diffuse spectrum can be used to reduce the harmonic amplitude, thereby suppressing the electromagnetic interference during the operation of the electromagnetic heating device , Reduce electromagnetic interference, reduce the impact on people's health and the impact on other electrical appliances.

According to an embodiment of the present invention, as shown in FIG. 3, when the power switch is an insulated gate bipolar transistor IGBT, the E pole of the IGBT is connected to the current detection unit 20, and the C pole of the IGBT is connected to the resonance circuit of the electromagnetic heating device ( not shown in the figure), and the above electromagnetic heating device further includes an IGBT drive unit 40 , and the IGBT drive unit 40 is connected between the G pole of the IGBT and the controller 30 . Wherein, the controller 30 may be an MCU (Micro Control Unit, microcontroller).

In this way, two target current values Imin are set by the MCU software, that is, the preset target lower limit current and the preset target upper limit current of Imax. When the MCU detects that the operating current value reaches the set target lower limit current value Imin through the current detection unit 20, the MCU By controlling the IGBT drive unit 40 to gradually increase the turn-on pulse width of the IGBT, the operating current gradually increases; When the turn-on pulse width of the IGBT is reduced, the operating current is gradually reduced, so that the operating current can be changed between Imin and Imax.

That is to say, when the control signal is a pulse width modulation PWM signal, wherein, when the operating current reaches the preset target lower limit current, the controller 30 controls the IGBT drive unit 40 to increase the pulse width of the PWM signal; When the preset target upper limit current is reached, the controller 30 controls the IGBT driving unit 40 to reduce the pulse width of the PWM signal.

According to an embodiment of the present invention, as shown in FIG. 4 or FIG. 5 , the pulse width of the PWM signal changes periodically according to a preset change rule.

Wherein, according to an embodiment of the present invention, as shown in FIG. 4 , within a period, the preset change law is: in the first preset time period T1, the pulse width of the PWM signal is the first preset width, such as t1 , and the PWM signal keeps the first frequency constant; in the second preset time period T2, the pulse width of the PWM signal is increased on the basis of the first preset width, such as t1, by the first width threshold Δt, which is t1+Δt, And the PWM signal keeps the second frequency constant, wherein the second frequency is less than the first frequency; in the third preset time period T3, the pulse width of the PWM signal is increased by a first time on the basis of the second preset width t1+Δt The width threshold Δt is t1+2Δt, and the PWM signal keeps the third frequency constant, wherein the third frequency is less than the second frequency; and in the fourth preset time period T4, the pulse width of the PWM signal is within the third preset It is assumed that the first width threshold Δt is reduced on the basis of the width t1+2Δt, that is, t1+Δt, and the fourth frequency of the PWM signal remains constant, wherein the fourth frequency is equal to the second frequency. Therefore, within a cycle, the frequency of the PWM signal changes, so the operating frequency of the electromagnetic heating device also changes, and, in the first preset time period T1, the second preset time period T2, and the third preset time period During the period T3 or the fourth preset period T4, the frequency of the PWM signal is constant, so the operating frequency of the electromagnetic heating device is also constant.

As shown in Figure 4, when the electromagnetic heating device is working, the pulse width of the PWM signal output by the IGBT drive unit 40 is constantly changing according to a certain time interval Δt, and the amount of change of the pulse width each time is based on the fact that the electromagnetic heating device does not generate noise. optimal.

As shown in Figure 4, the solution adopted in this embodiment can be to change the pulse width by Δt such as 0.083us every 10ms (such as T1), then the frequency of the electromagnetic heating device working at each interval of 10ms is around a fixed A change frequency of the frequency, the time interval 0<t<200ms (one cycle). Among them, there are 3 pulses in the T1 time period, and the width of these 3 pulses is the same; and then in the T2 stage, there are also 3 pulses in it, and the width of these 3 pulses is also the same, but it is higher than that in the T1 stage. The pulse width should be 0.083us wide. And, it may be T1=T2=T3=T4=10ms.

According to an embodiment of the present invention, during the first preset time period to the fourth preset time period, the number of pulses of the PWM signal may be one or more. Wherein, as shown in FIG. 4 , during the first preset time period T1 to the fourth preset time period T4 , the number of pulses of the PWM signal is multiple. As shown in FIG. 5 , during the first preset time period T1 to the fourth preset time period T4 , the number of pulses of the PWM signal is one.

Moreover, as shown in FIG. 4 or FIG. 5 , the pulse width of the PWM signal varies from large to small and then from small to large. Of course, it can be understood that, in other embodiments of the present invention, the change law of the pulse width of the PWM signal may also be from small to large and then from large to small.

According to the electromagnetic heating device of the embodiment of the present invention, the operating current of the power switch is detected by the current detection unit, and then the controller adjusts the pulse width of the control signal that controls the power switch to be turned on and off according to the operating current so that the operating current is within the preset target Change between the lower limit current and the preset target upper limit current, so that the electromagnetic heating device in the embodiment of the present invention can change the operating frequency by adjusting the pulse width of the control signal, thereby suppressing electromagnetic interference during work, reducing electromagnetic interference, and reducing The impact of electromagnetic interference on human health and the normal operation of other electrical appliances fully meets the needs of people's lives without increasing costs.

Fig. 6 is a flowchart of a control method of an electromagnetic heating device according to an embodiment of the present invention. As shown in Figure 6, the control method of the electromagnetic heating device includes the following steps:

S1, detecting the working current of the power switch of the electromagnetic heating device.

S2, adjusting the pulse width of the control signal for controlling the power switch to be turned on and off according to the working current so that the working current varies between a preset target lower limit current and a preset target upper limit current.

In this way, in the embodiment of the present invention, the operating frequency is changed by adjusting the pulse width of the control signal, and according to the change of the operating frequency, the energy of the diffuse spectrum can be used to reduce the harmonic amplitude, thereby suppressing the electromagnetic interference during the operation of the electromagnetic heating device , Reduce electromagnetic interference, reduce the impact of electromagnetic interference on human health and the normal work of other electrical appliances.

According to an embodiment of the present invention, when the control signal is a pulse width modulation PWM signal, wherein, when the working current reaches the preset target lower limit current, the pulse width of the PWM signal is increased; when the working current reaches the preset target When the current is upper limit, reduce the pulse width of PWM signal. That is to say, two target current values Imin can be set through the MCU software, that is, the preset target lower limit current and the preset target upper limit current of Imax. When the MCU detects that the operating current value reaches the set target lower limit current value Imin through the current detection unit, The MCU controls the IGBT drive unit to gradually increase the turn-on pulse width of the power switch such as the IGBT, and the working current gradually increases; when the MCU detects that the working current value reaches the set target upper limit current value Imax through the current detection unit, the MCU controls the IGBT drive unit. The turn-on pulse width of the IGBT is gradually reduced, and the working current is gradually reduced, so that the working current can be changed between Imin and Imax.

According to an embodiment of the present invention, as shown in FIG. 4 or FIG. 5 , the pulse width of the PWM signal changes periodically according to a preset change rule.

Wherein, according to an embodiment of the present invention, as shown in FIG. 4 , within a period, the preset change law is: in the first preset time period T1, the pulse width of the PWM signal is the first preset width, such as t1 , and the PWM signal keeps the first frequency constant; in the second preset time period T2, the pulse width of the PWM signal is increased on the basis of the first preset width, such as t1, by the first width threshold Δt, which is t1+Δt, And the PWM signal keeps the second frequency constant, wherein the second frequency is less than the first frequency; in the third preset time period T3, the pulse width of the PWM signal is increased by a first time on the basis of the second preset width t1+Δt The width threshold Δt is t1+2Δt, and the PWM signal keeps the third frequency constant, wherein the third frequency is less than the second frequency; and in the fourth preset time period T4, the pulse width of the PWM signal is within the third preset It is assumed that the first width threshold Δt is reduced on the basis of the width t1+2Δt, that is, t1+Δt, and the fourth frequency of the PWM signal remains constant, wherein the fourth frequency is equal to the second frequency. Therefore, within a cycle, the frequency of the PWM signal changes, so the operating frequency of the electromagnetic heating device also changes, and, in the first preset time period T1, the second preset time period T2, and the third preset time period During the period T3 or the fourth preset period T4, the frequency of the PWM signal is constant, so the operating frequency of the electromagnetic heating device is also constant.

According to an embodiment of the present invention, during the first preset time period to the fourth preset time period, the number of pulses of the PWM signal may be one or more. Wherein, as shown in FIG. 4 , during the first preset time period T1 to the fourth preset time period T4 , the number of pulses of the PWM signal is multiple. As shown in FIG. 5 , during the first preset time period T1 to the fourth preset time period T4 , the number of pulses of the PWM signal is one.

Moreover, as shown in FIG. 4 or FIG. 5 , the pulse width of the PWM signal varies from large to small and then from small to large. Of course, it can be understood that, in other embodiments of the present invention, the change law of the pulse width of the PWM signal may also be from small to large and then from large to small.

In summary, the control method of the electromagnetic heating device according to the embodiment of the present invention can be applied to various electrical appliances using IGBTs as power switches, especially suitable for electromagnetic heating devices such as induction cookers, electromagnetic rice cookers, and electromagnetic pressure cookers. powerful.

According to the control method of the electromagnetic heating device of the embodiment of the present invention, by detecting the operating current of the power switch of the electromagnetic heating device, and then adjusting the pulse width of the control signal for controlling the power switch to be turned on and off according to the operating current so that the operating current is at a preset value The target lower limit current and the preset target upper limit current change, so that the operating frequency of the electromagnetic heating device can be changed by adjusting the pulse width of the control signal, thereby suppressing the electromagnetic interference when the electromagnetic heating device is working, and reducing the electromagnetic interference. Reduce the impact of electromagnetic interference on human health and the normal work of other electrical appliances, and fully meet people's living needs. In addition, the control method is simple and easy to implement, and has strong versatility.

Any process or method descriptions in flowcharts or otherwise described herein may be understood to represent modules, segments or portions of code comprising one or more executable instructions for implementing specific logical functions or steps of the process , and the scope of preferred embodiments of the invention includes alternative implementations in which functions may be performed out of the order shown or discussed, including substantially concurrently or in reverse order depending on the functions involved, which shall It is understood by those skilled in the art to which the embodiments of the present invention pertain.

The logic and/or steps represented in the flowcharts or otherwise described herein, for example, can be considered as a sequenced listing of executable instructions for implementing logical functions, can be embodied in any computer-readable medium, For use with an instruction execution system, device, or device (such as a computer-based system, a system including a processor, or other systems that can fetch instructions from an instruction execution system, device, or device and execute instructions), or in conjunction with such an instruction execution system, device or equipment for use. For the purposes of this specification, a "computer-readable medium" may be any device that can contain, store, communicate, propagate or transmit a program for use in or in conjunction with an instruction execution system, device or device. More specific examples (non-exhaustive list) of computer-readable media include the following: electrical connection with one or more wires (electronic device), portable computer disk case (magnetic device), random access memory (RAM), Read Only Memory (ROM), Erasable and Editable Read Only Memory (EPROM or Flash Memory), Fiber Optic Devices, and Portable Compact Disc Read Only Memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program can be printed, since the program can be read, for example, by optically scanning the paper or other medium, followed by editing, interpretation or other suitable processing if necessary. The program is processed electronically and stored in computer memory.

It should be understood that various parts of the present invention can be realized by hardware, software, firmware or their combination. In the embodiments described above, various steps or methods may be implemented by software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented by any one or combination of the following techniques known in the art: Discrete logic circuits, ASICs with suitable combinational logic gates, Programmable Gate Arrays (PGAs), Field Programmable Gate Arrays (FPGAs), etc.

Those of ordinary skill in the art can understand that all or part of the steps carried by the methods of the above embodiments can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. During execution, one or a combination of the steps of the method embodiments is included.

In addition, each functional unit in each embodiment of the present invention may be integrated into one processing module, each unit may exist separately physically, or two or more units may be integrated into one module. The above-mentioned integrated modules can be implemented in the form of hardware or in the form of software function modules. If the integrated modules are realized in the form of software function modules and sold or used as independent products, they can also be stored in a computer-readable storage medium.

The storage medium mentioned above may be a read-only memory, a magnetic disk or an optical disk, and the like.

In the description of this specification, descriptions referring to the terms "one embodiment", "some embodiments", "example", "specific examples", or "some examples" mean that specific features described in connection with the embodiment or example , structure, material or characteristic is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Although the embodiments of the present invention have been shown and described, those skilled in the art can understand that various changes, modifications and substitutions can be made to these embodiments without departing from the principle and spirit of the present invention. and modifications, the scope of the invention is defined by the appended claims and their equivalents.

Claims (7)

1. a kind of control method of electromagnetic heater, it is characterised in that comprise the following steps：

Detect the operating current of the power switch of the electromagnetic heater；And

According to the operating current adjust pulse width for the control signal for controlling the power switch to turn on and off with The operating current is set to change between default target lower current and default target upper limit electric current, to control the electromagnetism The working frequency of heater is changed, when the control signal is pulse-width modulation PWM signal, wherein,

When the operating current reaches the default target lower current, increase the pulse width of the pwm signal；

When the operating current reaches the default target upper limit electric current, reduce the pulse width of the pwm signal；

Also, the pulse width of the pwm signal carries out cyclically-varying with default changing rule.

2. the control method of electromagnetic heater as claimed in claim 1, it is characterised in that described pre- in a cycle If changing rule be：

In the first preset time period, the pulse width of the pwm signal is the first predetermined width, and the pwm signal keeps the One frequency immobilizes；

In the second preset time period, the pulse width of the pwm signal increases by first on the basis of first predetermined width Width threshold value, and the pwm signal keeps second frequency to immobilize, wherein, the second frequency is less than the first frequency；

In the 3rd preset time period, the pulse width of the pwm signal increases described first on the basis of the second predetermined width Width threshold value, and the pwm signal keeps the 3rd frequency to immobilize, wherein, the 3rd frequency is less than the second frequency； And

In the 4th preset time period, the pulse width of the pwm signal reduces described first on the basis of the 3rd predetermined width Width threshold value, and the pwm signal keeps the 4th frequency to immobilize, wherein, the 4th frequency is equal to the second frequency.

3. the control method of electromagnetic heater as claimed in claim 2, it is characterised in that in first preset time period To the 4th preset time period, the pulse number of the pwm signal is one or more.

A kind of 4. electromagnetic heater, it is characterised in that including：

Power switch；

Current detecting unit, the current detecting unit are connected with the power switch, and the current detecting unit is used to detect The operating current of the power switch；And

Controller, the controller are respectively connected with the power switch and the current detecting unit, the controller according to The operating current adjusts the pulse width for the control signal that controls the power switch to turn on and off so that the work Make electric current and change between default target lower current and default target upper limit electric current, to control the electromagnetic heater Working frequency be changed, when the control signal is pulse-width modulation PWM signal, wherein,

When the operating current reaches the default target lower current, the controller is by controlling IGBT driver elements To increase the pulse width of the pwm signal；

When the operating current reaches the default target upper limit electric current, the controller is by controlling the IGBT to drive Unit is to reduce the pulse width of the pwm signal；

Also, the pulse width of the pwm signal carries out cyclically-varying with default changing rule.

5. electromagnetic heater as claimed in claim 4, it is characterised in that when the power switch is that insulated gate bipolar is brilliant During body pipe IGBT, the E poles of the IGBT are connected with the current detecting unit, and the IGBT driver elements are connected to the IGBT G poles and the controller between.

6. electromagnetic heater as claimed in claim 4, it is characterised in that in a cycle, the default change rule Restrain and be：

In the first preset time period, the pulse width of the pwm signal is the first predetermined width, and the pwm signal keeps the One frequency immobilizes；

In the second preset time period, the pulse width of the pwm signal increases by first on the basis of first predetermined width Width threshold value, and the pwm signal keeps second frequency to immobilize, wherein, the second frequency is less than the first frequency；

In the 3rd preset time period, the pulse width of the pwm signal increases described first on the basis of the second predetermined width Width threshold value, and the pwm signal keeps the 3rd frequency to immobilize, wherein, the 3rd frequency is less than the second frequency； And

In the 4th preset time period, the pulse width of the pwm signal reduces described first on the basis of the 3rd predetermined width Width threshold value, and the pwm signal keeps the 4th frequency to immobilize, wherein, the 4th frequency is equal to the second frequency.

7. electromagnetic heater as claimed in claim 6, it is characterised in that in first preset time period to the described 4th Preset time period, the pulse number of the pwm signal is one or more.