KR20060020083A - Induction heating apparatus being capable of operating in wide bandwidth frequencies - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating apparatus, and more particularly, to an induction heating apparatus for supplying optimal power to an object in a wide range of frequency operating conditions by adjusting values of resonance capacitance and series inductance in an induction heating circuit.

Induction heating apparatus of the present invention AC power; A work coil which receives electric power from the AC power source and heats the object; A load cable connecting the AC power supply and the work coil in series; First and second capacitances connected at both ends to the load cable; And a series inductor Ls connected in series between the AC power supply and the load cable.

In addition, the AC power source is characterized by consisting of a switching full bridge configured to obtain a large output while minimizing switching losses.

The induction heating apparatus of the present invention has a series inductor between an AC power supply and a parallel resonant circuit portion. AC power supplies have built-in output transformers with variable frequency inverters and leakage inductance, and the output transformers have small leakage inductance due to the structure of hollow hollow windings inside and outside that are essentially coaxial with each other. .

Induction heating, resonant capacitance, series inductor, power factor, operating frequency, work coil, resonant circuit, full bridge.

Description

Induction heating apparatus being capable of operating in wide bandwidth frequencies}             

1 is a block diagram of the configuration of a conventional general induction heating apparatus.

2 is a block diagram of a configuration of an induction heating apparatus according to an embodiment of the present invention.

3 is a circuit diagram used in the induction heating apparatus of the present invention.

Figure 4 is a circuit diagram of an induction heating apparatus coupled to a full bridge circuit according to another embodiment of the present invention.

5 is a waveform diagram of a current iLw flowing in a work coil by the circuit operation of FIG. 4;

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating apparatus. In particular, an induction heating apparatus for supplying an optimum power to an object in a wide range of frequency operating conditions by adjusting the resonance capacitance and the value of a series inductor in an induction heating circuit while minimizing switching loss. Is about

A typical induction heating apparatus heats a workpiece of an electrical conductor with an eddy current induced magnetically, where a loss (IR ² ) due to electrical resistance occurs in the eddy current path of the workpiece. One form of induction heating apparatus includes a power supply inverter of an alternating voltage output having a required operating frequency. The output of the inverter is connected to a series connected output supply terminal pair of a series inductor and a resonant circuit part by a step-down transformer. The resonant circuit portion includes a work coil coupled in parallel with the resonant capacitor. The work coil is located in the vicinity of the object and generates an oscillating magnetic field that induces an eddy current in the object.

1 is a block diagram showing the configuration of a general induction heating apparatus operating on the same principle. Referring to Figure 1 describes the operating state of the induction heating apparatus as follows.

 When the heating is started by the operation of the internal or external switch, the control board 12 drives the inverter module 16 to output RF (Radio Frequency) power to the work coil. The control panel 12 then calculates the output voltage and current values to control the oscillation frequency of the PLL circuit so that the output power matches the set value.

 The RF output uses a Q characteristic of the L / C resonant circuit 18 to control the frequency to obtain a desired output level. In addition, the control board 12 (Control Board) to control the output voltage or current value does not exceed the limit, and if the following conditions occur during the heating trips (Trip) immediately to prevent damage to the device.

-The current value exceeds the limit

-If the output is short

-FET control of inverter module 16 does not operate normally

-When interlock occurs from outside

The device can be used in manual or automatic mode. In manual mode, the heating cycle is controlled by switch operation. In automatic mode, the job is pre-programmed in the device. Heating cycle is controlled by the parameter.

Job parameter is a combination of the output power level and heating time.When using the device in the automatic mode, the heating cycle is automatically terminated after outputting the RF power during the heating time. do.

Meanwhile, the functions of each component are briefly described as follows.

First, the display device 10 (Display Board) controls the heating cycle by giving a command to the Control Board by the internal or external operation switch input, and the operation state information of the device from the control panel 12 (Control Board), For example, it has a function of receiving and displaying output voltage, inverter current, set and output power, frequency, and the like. The display device 10 is composed of UP and operation switches, status indicators, and the like.

The control board 12 is a device that generates an RF power by driving an inverter module 16. The control board 12 includes a PLL oscillation circuit, a power calculator, a control logic circuit, and the like.

RF output calculates output voltage and current value and controls PLL frequency to make constant output. The RF output can be selectively controlled by the power, voltage or current level (Level), and the control panel 12 controls the output voltage or current value does not exceed the limit. If the limit value is exceeded, it is tripped immediately to prevent damage to the device.

The inverter module 16 includes an FET driver, a half bridge, and a snubber circuit. The bridge circuit uses FETs to operate at frequencies above 450KHz, and consists of multiple FETs connected in parallel according to the power capacity.

The L / C resonant circuit 18 is composed of an inductor and a capacitor, and functions to match the output of the unit and the work coil. Matching with the work coil is performed by adjusting the tap of the inductor.

However, the conventional general induction heating apparatus described so far requires various operating conditions according to its use field. For example, induction heating is generally used at frequencies in the range of about 20 kHz to about 400 kHz, but in some applications other ranges of operating frequencies will be required. In addition, the voltage applied to the work coil may vary. Furthermore, the power factor of the energy delivered to the work coil can vary widely, depending on the configuration and placement of the product. As another example, if an induction heater was designed to heat paint on an automobile surface, it would be designed to operate at only one particular frequency, voltage and power factor. As such, most induction heating apparatuses are designed to operate at a specific frequency or a specific voltage according to an application field.

Accordingly, there is a need for an induction heating apparatus that can be used in various fields in various environments. An apparatus is required.

  In addition, a device that supports a set of working conditions must be tuned before the user can operate it. The tuning procedure of such a device is morphologically complex and it is possible to understand the technical principles of induction heating device operation. Therefore, it is a situation performed by a skilled user in operating an induction heating apparatus supporting a change in operating conditions. Therefore, there is a need for an induction heating apparatus that simplifies the tuning process.

On the other hand, the conventional magnetic and nonmagnetic combined induction heating apparatus has a low output by forming a half-bridge resonant circuit by the alternate switching operation of a simple inverter, there is a problem in the efficiency of heating.

 In order to solve the above problems, an object of the present invention is to provide an induction heating apparatus applicable to various fields by adjusting the operating frequency of the induction heating apparatus through a resonant capacitor.

Another object of the present invention is to provide an induction heating apparatus that can be easily tuned even by an unskilled person by simply tuning in adjusting the operating frequency of the induction heating apparatus.

In addition, the AC power inverter is characterized in that it comprises a switching full bridge configured to obtain a large output while minimizing switching losses.

Still other objects of the present invention will become apparent from the following detailed description and the accompanying drawings.

Induction apparatus of the present invention for achieving the above object, AC power source, the work coil for heating the object by receiving power from the AC power source, the load cable for connecting the AC power source and the work coil in series, the And a series inductor (Ls) connected in series between the AC power supply and the load cable, and first and second capacitances connected to both ends of the load cable.

More preferably, the AC power source is composed of a switching full bridge that outputs power from the smoothing means by alternately switching in response to a plurality of switching control signals to obtain a large output while minimizing switching losses. .

Hereinafter, an induction heating apparatus of the present invention will be described in detail with reference to the accompanying drawings. For convenience of explanation, the description is divided into the first embodiment and the second embodiment. In the first embodiment, the AC power supply unit is composed of a half bridge, and the second embodiment is composed of a full bridge circuit.

<First Embodiment>

The first embodiment is a case where the AC power supply unit is composed of a half bridge circuit. 2 is a block diagram of a circuit configuration of the induction heating apparatus of the present invention.

As shown, the induction heating apparatus of the present invention is provided with an AC power supply unit 110 and 120 and a work coil (Lw), there is a load cable 132 connecting between the AC power supply unit and the work coil and both ends are The first and second capacitances Cs and C _L connected to the load cables, respectively, and a series inductor Ls connected in series between the AC power supply and the load cable are used as basic components.

3 is a detailed circuit diagram of the induction heating apparatus of the present invention. As shown, the output of the AC power source is connected in series to the series inductor Ls and the resonant circuit unit 130. Here, the resonant circuit unit 130 includes a work coil Lw connected in parallel with a resonant capacitance composed of a first capacitor and a second capacitor connected in parallel.

On the other hand, the output of AC power is Vs measured with the effective voltage. The value of the series inductor is Ls, the value of the work coil is Lw, the voltage across the capacitor is V _L , and the resonant capacitor is Cr divided into two capacitors (Cs, C _L ) connected at each end of the load cable 132.

The AC power supply includes a half-bridge inverter 110, which includes a pair of switches 114 and 116 connected in series to a DC power supply that is V _dc / 2, and these switches include a controller 112. Controlled by

On the other hand, the AC power source of the induction heating apparatus of the present invention preferably includes an output transformer 120. The primary terminal of the transformer 120 is connected to the output of the inverter 110, the secondary voltage output terminal is the output terminal of the AC power supply. Such output transformers are commonly included in induction heating devices for electrical isolation, stepwise impedance matching, and safety.

On the other hand, the transformer is an ideal transformer in which the ratio of the number of turns of the secondary side to the primary side is N: 1. The capacitor connected across the primary side of the transformer is the capacitor C _iv between the windings.

In FIG. 3, the leakage inductance L _R is connected in series between the capacitor C _iv between the windings and the primary side terminal 136, and the magnetizing inductance L _M of the transformer 120 is connected in parallel to the primary side input terminal.

Capacitors between the windings, components such as leakage inductance, and magnetization inductance, represent conversion factors related to the turns ratio of the ideal transformer on behalf of the two-side of the ideal transformer. For example, the leakage inductance shown on the secondary side of transformer 120 is L _R / N ² . In order to clarify the content description, the resistor indicating the power loss in the inductor and the core of the transformer 120 are omitted.

The series inductor Ls present between the AC power supplies 110 and 120 and the work coil Lw determines the power distributed by the device at a particular frequency and power factor. Thus, in order to obtain maximum power over a wide range of operating frequencies and power factors, this series inductor Ls needs to be adjusted over a wide range. The composite has a series inductor multi-tap and the user can select a specific inductor value Ls.

The series inductor (Ls) has several taps, which allows the user to select the appropriate inductor value. The inductance between the AC power supply and the work coil 120 is not just a series inductor Ls. The inductance also has a leakage inductance of the load cable 132 and the transformer 120. The leakage inductance value converted to the secondary side of the transformer 120 is L _R / N ² , and is represented by the output inductance of the AC power supply.

For induction heating to support a wide range of operating conditions in Figure 3, it is desirable to make the leakage inductance of the transformer 120 as small as possible. Because the user may replace the series inductor (Ls) with a short circuit, or if there is no other stray inductance in the device, the total series inductance between the AC power supply (110, 120) and the work coil (Lw) is greater than the L _R / N ² value. It's never small.

The worst case operating condition in the device of FIG. 3 occurs when the user selects a maximum specific operating frequency f _max , an allowable maximum output and a specific minimum power factor. That is, the user selects the minimum specific output voltage V _Lmin , and the DC link voltage V _dc becomes the minimum value V _dcmin in the inverter 110 (outputting the minimum effective value voltage to the transformer output V _pmin ).

Therefore, even if the series inductor Ls is replaced with a bus-bar in FIG. 3, the leakage inductance of the transformer output of the AC power supply should be smaller when observed at the secondary terminal. When observed from the secondary side to allow stray inductance and manufacturing and operating errors in the load cable, the leakage inductance of the transformer output should be less than 0.25L _seffmax .

In general, when an electrically continuous winding extends to the T-turn, the electrically discontinuous winding is cut into S-segments (split segments), with each part extending essentially to the T-turn of the T-turn, resulting in coaxial The transformer has a winding ratio of S: 1. The number of turns of the continuous winding need not be an integer and may also be less than one.

The number of breaks in the discontinuous winding is an integer greater than one. The number of parts in which the discontinuous winding is expanded is basically an integer value, allowing the error of the gap between the end and the next end in one part. Also, if the cable length is minimized, leakage inductance is minimized. However, the minimum value of the cable length is limited by the magnetizing inductance value required by the transformer.

In the inverter 110 of FIG. 3, the minimum allowable magnetizing inductance L _M is determined by the maximum magnetizing current I _mpeak required at the minimum switching frequency f _min .

Since the inductance value can be reduced by connecting an additional inductor to the primary terminal of the transformer 120 if necessary, higher inductance is not a problem.

Under certain circuit conditions and substrate layout, the capacitors between the windings resonate with parasitic inductance values that vary in the device, resulting in oscillating circuits. The oscillation also changes as a function of power factor.

Cylindrical coaxial transformers with air cores large enough to achieve the magnetizing inductance L _M required in this situation cannot be configured with a capacitor between the winding inductance small enough to support the desired operating conditions and the winding small enough to prevent oscillation. .

Like air core transformers, leakage inductance is minimized by thinning the insulation between the windings. Thin insulation between the windings increases the capacitor between the windings, but the length between the shorter windings tends to decrease the capacitor between the windings. When the number of turns of the primary side winding is 4 and the number of secondary winding portions connected in parallel is 4, the winding ratio is 4: 1. The leakage inductance value is small enough to allow an induction heating device with a wide range of operating conditions required, and small enough to prevent oscillation of the capacitor between windings.

In FIG. 3, the resonant circuit unit 130 includes a work coil L _W. Hereinafter, the word "capacitance" expresses a value, and the word "capacitor" expresses a specific component having a capacitance value.

The maximum efficiency of the operation in the induction heating device is generated at a place where the operating frequency is slightly larger than the resonant frequency f _res of the resonant circuit part. In some applications it is desirable to arrange the AC power supply at a location far away from the work position of the work coil. In this case, the load cable 132 is installed to allow a current to flow from the AC power supply to the work coil (L _W ).

The series inductor Ls is connected between the AC power supply units 110 and 120 and the ends of the load cable 132. When a large current is applied to the load cable 132, the price is expensive and difficult to install. Therefore, in order to reduce the amount of current in the load cable 132, the capacitor Cs is placed near the AC power supply and another capacitor C _L is located near the work coil L _W. These two capacitors are selected to be optimized to have a constant power factor in the load cable 132.

The induction heating apparatus of FIG. 3 is tuned to operate under a wide range of operating conditions. Tuning basically involves selecting the resonant capacitor _Cr and the series inductor Ls. The series inductor Ls is _chosen by the formula Ls = L _seff -L _O. Where L _O is the output inductance of the AC power supply.

The procedure for tuning an induction heating apparatus is as follows. First, the user selects the required operating frequency depending on the application. For example, the user selects a higher operating frequency for surface heating, while selecting a lower operating frequency for deep heating from the surface. The user also selects the required load voltage V _L. The user then selects the basic series inductor L _S.

 The preliminary selection is made from a chart, equation or table provided by the manufacturer of the induction heating apparatus, which selection relates to the rough load voltage for the various operating frequencies with which the series inductor Ls is supported. Basic series inductors need not be precise because they catch errors in sequential steps of the tuning process.

When the user runs the device at full power, the operating frequency is reduced to a small value, but not more than about 10%. The user increases the series inductor (Ls) value when the current limit indicator (not shown) shows the current limit value, and the user decreases the Ls value when the induction heating device is resonance limited. This procedure is repeated until the device is current limited or not resonance limited.

 The user must choose the Ls value within the resonance limit because these values provide optimum efficiency (best power factor PF). In this regard, the apparatus of FIG. 3 is tuned and ready for operation.

  The tuning procedure of the device is very simple and the tuning procedure of this device allows the use of the induction heating device of FIG. 3 in various required operating conditions without a detailed understanding of the principle of operation.

 Manufacturers of induction heating units educate users on these tuning procedures. The tuning procedure is not limited to the apparatus of FIG. 3 but applies to any induction heating device in which the series inductor Ls and the resonant capacitor have a topology (inductance in series with the parallel resonant circuit section) that can be changed or adjusted by the user. The aforementioned tuning procedure can be extended for use in implementing a split capacitor as shown in FIG. 3. This is especially true for split capacitor implementations.

_Cr and Ls are first determined according to the previous procedure for the case of non-partitioning. In this case, all capacitances are located at the load ends of the load cable 132. At this time, the capacitance is moved from the end of the load cable 132 to the power supply end of the load cable (the position of Cs) until the power factor value of the load current reaches a maximum value (eg, to match the maximum possible value). All of the rest of the capacitance remains at the load end of the load cable 132. In another implementation, the magnitude of the transferred capacitance is determined by a power factor meter (not shown) located on the load cable 132.

The capacitance value is shifted until the power factor displayed on the instrument is at its maximum (as close as possible). The magnitude of the capacitance transferred in the third implementation is then determined by a current detector (not shown) that reacts to the current in the ammeter or load cable.

 The precision of the instrument is not critical and any signal proportional to the current is sufficient. According to a third implementation, the capacitance is repeatedly moved from the load end of the load cable 132 to the power supply end of the load cable 132. The current measured with an ammeter decreases repeatedly until it starts to increase at some point. In that respect, the transfer of the last amount of capacitance from the load end to the power supply end of the load cable 132 returns to the load end and the correct division has been made.

Second Embodiment

Another second embodiment as an induction device of the present invention is shown in FIG. The induction heating apparatus of FIG. 4 is an induction heating apparatus combined with a soft switching full bridge circuit capable of obtaining a large output in order to reduce switching loss. That is, the second embodiment employs a full bridge circuit, whereas the first embodiment employs a half bridge circuit.

As shown, the full-bridge circuit of the present invention, the smoothing unit 210 for increasing the power factor of the input power, and alternately switching according to a plurality of switching control signals input from the outside to switch the power from the smoothing unit 210. The switching unit 220 is configured to output.

The smoothing unit 210 includes a smoothing capacitor C _f for smoothing the DC power rectified by the rectifying unit (not shown).

The switching unit 220 includes first and second inverters S1 and S2 coupled in series to the output terminal of the smoothing unit 210, and third and fourth inverters S3 coupled in series to the output terminal of the smoothing unit 210. , S4, the first to fourth damping diodes D1 to D4 and the first to fourth inverters S1 to S4 coupled in reverse parallel to the first to fourth inverters S1 to S4, respectively. Corresponding first to fourth auxiliary resonant capacitors Ca1 to Ca4 coupled in parallel.

The first to fourth inverters S1 to S4 alternately perform a pair of switching operations in response to a plurality of corresponding switching control signals SC1 to SC4, respectively. By this switching operation, the resonance circuit unit 130 receives power smoothed by the smoothing unit in time division.

FIG. 5 is a waveform diagram illustrating switching control signals SC1 to SC4 of the first and fourth inverters and an inductor current i _Lw flowing in the work coil in the nonmagnetic heating mode according to an embodiment of the present invention.

First, the operation in the nonmagnetic heating mode will be described with reference to FIG. 5. The first and fourth switching elements S1 and S4 of the switching unit 220 are turned off, and the second and third switching elements S2 are turned off. When S3 is turned on, the resonant circuit part forms a resonant capacitor Cr full bridge resonant circuit, and switches the voltage Vd induced across the smoothing capacitor C _f of the smoothing part 210 to the switching part 220. Licensed through As a result, the resonance circuit 130 increases the inductor current i _Lw in the work coil as well as the resonance.

At this time, when the first and fourth inverters S1 and S4 are turned off before the resonant half cycle passes, the work coils of the first to fourth auxiliary resonant capacitors Ca1 to Ca4 and the resonant circuit unit 130 of the switching unit 220 are turned off. Lw and the resonant capacitor Cr co-resonate so that the voltages of both ends of the second and third auxiliary resonant capacitors Ca2 and Ca3 fall from the input voltage Vd to zero, during which time the first and fourth auxiliary resonant capacitors The voltage at both ends of Ca1 and Ca4 rises from zero to the level of the input voltage Vd.

Thereafter, as the second and third antiparallel diodes D2 and D3 of the second and third inverters S2 and S3 of the switching unit 220 become conductive, the first and second work coils of the resonance circuit unit 130 are conducted. The input voltage Vd is inversely applied to the Lw and the resonant capacitor Cr, whereby the resonance of the resonant circuit unit 130 occurs continuously.

In this state, the second and third inverters S2 and S3 of the switching unit 220 are turned on under the condition of zero voltage.

Then, the inductor current i _Lw drops to zero by the continuous resonance, and this time the inductor current i _Lw increases by resonance in the opposite direction.

In such a state, to turn off the second and third inverters S2 and S3 of the switching unit 220 before the resonant half cycle passes, the first to fourth auxiliary resonant capacitors Ca1 to Ca4 of the switching unit 220 are turned off. ) And the work coil Lw and the resonant capacitor Cr of the resonant circuit unit 130 are secondary resonant so that the voltages at both ends of the first and fourth auxiliary resonant capacitors Ca1 and Ca4 fall from the input voltage Vd to zero, In the meantime, the voltages of the second and third auxiliary resonance capacitors Ca2 and Ca3 are increased from zero to the input voltage Vd level.

Thereafter, the first and fourth antiparallel diodes D1 and D4 of the switching unit 220 are turned on, and an input voltage Vd is applied to the work coil Lw and the resonant capacitor Cr of the resonance circuit unit 130. As a result, the resonance of the resonance circuit unit 130 continuously occurs, and when the resonance current drops to zero, the operation of the heating mode of one cycle is terminated.

By adopting the alternating switching method as described above, switching loss can be minimized.

As described above, the induction heating apparatus of the present invention can supply optimal power to an object under a wide range of operating frequency conditions by using a series inductor connecting an AC power source and a resonant circuit part and resonant capacitance in the resonant circuit part.

In addition, the induction heating apparatus of the present invention can be easily tuned even by an unskilled person by simply adjusting the tuning process to adjust the operating frequency.

In addition, the switching unit of the present invention can minimize the switching loss by adopting a method of alternately switching the switching elements.

Claims (5)

In the induction heating apparatus for supplying the optimum power to the object in a wide range of frequency operating conditions by adjusting the value of the resonant capacitance and series inductor in the induction heating circuit while minimizing switching loss, AC power supply for supplying power; A work coil which receives electric power from the AC power source and heats the object; A load cable connecting the AC power supply and the work coil in series; First and second resonant capacitances connected at both ends to the load cable; And Induction heating device, characterized in that it comprises a series inductor (Ls) connected in series between the AC power supply and the load cable. The induction heating apparatus according to claim 1, further comprising a switching full bridge which alternately switches in response to a plurality of switching control signals input from the outside to output power from the smoothing means. The switching full bridge of claim 1, wherein the switching full bridge comprises: first and second inverters coupled in series with the output terminal of the smoothing means, third and fourth inverters coupled in series with the output terminal of the smoothing means, and the first through the second inverters. An induction heating device comprising: first to fourth damping diodes reversely coupled in parallel to each of the four inverters; and first to fourth capacitors coupled in parallel to each of the first to fourth inverters. . A method for tuning an induction heating apparatus including an AC power supply and a work coil, and a load cable connected in series between the AC power supply and the work coil, Selecting a total resonant capacitance Cr value connected to the work coil; Connecting a capacitor Cr connected to one end of the load cable; And And repetitively changing the value of the capacitance from one end of the load cable to the other until the required electrical condition is satisfied. The method of claim 4, wherein the induction heating apparatus has a series inductor connected between the above-described AC power supply and the above-described load cable.

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