DE102013221145B4 - induction heating - Google Patents

Abstract

Induction heater with - a first inductively acting device in the form of a first coil (L1), which is designed as an induction coil, - a capacitor (C1) which is connected in parallel to the first coil (L1), wherein the first coil (L1) and the Condenser (C1) form a parallel resonant circuit (7), - a controllable switching means (T1) connected in series with the parallel resonant circuit (7) between a terminal pole, at which a positive intermediate circuit potential (UZ) is present, and a terminal pole at which a DC link reference potential (GND), is looped in and is controlled such that during a heating operation, a vibration of the parallel resonant circuit (7) is effected, and - a second inductance device (L2) connected in series with the switching means (T1) and the parallel resonant circuit ( 7) between the terminal pole at which the positive DC link potential (UZ) is present, and the terminal pole at which the DC link reference potential ( GND), is looped, wherein the second inductively acting device (L2) is inductively coupled to a power receiving circuit (3) for receiving stored in the second inductively acting device (L2) energy, wherein the energy receiving circuit (3) has a third inductively acting Device (L3), which is inductively coupled to the second inductively acting device (L2), wherein the third inductively acting device (L3) is polarized in opposite directions to the second inductively acting device (L2).

Description

The invention relates to an induction heating device according to the preamble of claim 1.

Generic induction heaters are typically used in cooktops. In this case, an (induction) coil of the induction heating device is typically arranged below a cooking plate so that it can induce eddy currents in a cookware located above the cooking plate by means of a generated alternating magnetic field. In this way, energy is transferred to the cookware, which is thus heated. A generic induction heater is, for example, the document DE 10 2005 050 036 A1 refer to.

To generate the alternating magnetic field of the resonant circuit, in which the corresponding coil is integrated, set in vibration. Typically, such a resonant circuit is designed as a parallel resonant circuit, which also has a capacitor in addition to the coil. To power the resonant circuit typically a DC link voltage is used, which is generated for example by means of a bridge rectifier from the AC line voltage. By means of a switching means, such as an IGBT, which is periodically switched between high and low resistance, ie non-conductive and conductive states, can be achieved that the intermediate circuit voltage relative to the DC link reference potential periodically applied to the parallel resonant circuit, the resonant circuit can oscillate during the high-impedance states ,

Due to the inductive covering of the coil and the cookware, a magnetizing reactive power is always required for the transmission of a desired active power to the cookware. This reactive power loads in the underlying topology with a parallel resonant circuit and the switching means, which is typically designed as a power semiconductor, directly in the form of magnetizing current and indirectly in the form of voltage peaks between the collector and emitter. Furthermore, in the transition to a respective switching period in which the switching means is turned on, often a certain residual voltage across the switching means, since in cookware with a correspondingly large attenuation, the voltage amplitude of the resonant circuit voltage is too much attenuated and thus no longer on the DC link reference potential of 0 V can drop.

A similar problem occurs at small cooking levels, d. H. With a small power circuit, since due to the shortened switching periods, the energy in the induction coil is not maximum and thus the amplitude of the resonant circuit voltage in the oscillation can not decay to the DC link reference potential.

If, for one of the reasons described, the resonant circuit voltage can not decay to the intermediate circuit reference potential, this will result when the switching device is switched back on, ie. H. at the transition to the conducting state, to a capacitive short circuit of the series connection of the capacitor of the parallel resonant circuit and a usually existing intermediate circuit capacitor. This leads to a high power loss at the switching means and electromagnetic emissions. The energy advantage of the parallel resonant circuit compared to a series resonant circuit, which consists in particular in that only one switching means is required instead of two switching means and that only little reactive power is required in the switching means for energy transfer, is at least partially neutralized or possibly completely neutralized by the high power loss.

So far, to limit the maximum power dissipation and to avoid destruction of the switching means, the smallest possible power level for continuous operation has been limited to about half the peak power. Even at this operating point, however, large residual stresses and thus high heat losses occur in the switching means and electromagnetic emissions. Even lower power levels are therefore implemented in ED operation, skipping entire oscillation cycles. However, this mode of operation adversely affects the cooking behavior.

The DE 2 231 339 A shows an induction heater and a switching arrangement therefor.

The US 2007/0241101 A shows an induction heating circuit.

The DE 10 2006 017 801 A shows a power supply unit.

The DE 29 01 326 A1 shows a sine power generator.

The DE 28 36 610 A1 shows an induction heater with an inverter, wherein the induction coil of the resonant circuit is the heating coil for the cookware.

The DE 26 48 758 A1 shows a sine power generator.

Task and solution

It is therefore an object of the invention to reduce the power loss on the switching element and preferably additionally fed back into the resonant circuit.

This is inventively achieved by an induction heater according to claim 1. Embodiments of the invention are claimed, for example, in the subclaims. The content of the claims is hereby incorporated by express reference to the content of the description.

• – ein erstes induktiv wirkendes Bauelement in Form einer ersten Spule, welche als Induktionsspule ausgebildet ist,
• – einen Kondensator, welcher der ersten Spule parallelgeschaltet ist, wobei die erste Spule und der Kondensator einen Parallelschwingkreis bilden, und
• – ein ansteuerbares Schaltmittel, das in Serie mit dem Parallelschwingkreis zwischen einem Anschlusspol, an dem ein positives Zwischenkreispotential ansteht, und einem Anschlusspol, an dem ein Zwischenkreisbezugspotential ansteht, eingeschleift ist und derart angesteuert ist, dass während eines Heizbetriebs eine Schwingung des Parallelschwingkreises bewirkt wird.

The invention relates to an induction heater comprising:

• A first inductively acting component in the form of a first coil, which is designed as an induction coil,
• A capacitor which is connected in parallel with the first coil, the first coil and the capacitor forming a parallel resonant circuit, and
• - A controllable switching means, which is connected in series with the parallel resonant circuit between a terminal pole, at which a positive DC potential, and a terminal pole, at which a DC link reference potential is present, is looped in and is driven such that during a heating operation, a vibration of the parallel resonant circuit is effected.

According to the invention, a second inductively acting component is connected in series with the switching means and the parallel resonant circuit between the terminal pole, at which the positive DC link potential is present, and the terminal pole, at which the DC link reference potential is present.

With the induction heating device according to the invention, the capacitive inrush current is limited by the switching means at the transition of the switching means from a non-conductive to a conductive state, which is effected by the inductively acting component looped in series with the switching element. The on-switching losses occurring at the switching means, often referred to as E _ON , are thus also limited.

The first inductively acting component is a coil which is designed as an induction coil. This induction coil is typically arranged horizontally below a cooking plate of a hob, so that it can heat a resting on the hotplate cookware, such as a pot or a pan. This is done by the induction coil generates a time-varying magnetic field.

The parallel resonant circuit, which is formed from the capacitor and the first coil, is excited by means of the controllable switching means. This preferably takes place in that the controllable switching means is switched on during periodically recurring switching periods. The controllable switching means is preferably an insulated gate bipolar transistor (IGBT). However, other suitable switching means such as JFET or MOSFET or normal bipolar transistors may be used.

Each time the switching means is switched on, the difference between the DC link potential and the DC link reference potential essentially drops across the capacitor and the first inductively acting component, which is also referred to as an inductor. As a result, the capacitor is charged and the current through the first inductively acting device increases.

After the end of a respective switching period oscillates the parallel resonant circuit, which means that in the first inductively acting device in the form of electricity, so magnetically stored energy is converted into electrical field energy on the capacitor. Typically, the maximum achievable voltage across the capacitor is much higher than it was at the end of the previous switching period. If the voltage drop across the capacitor is again minimal after one end of the oscillation cycle, the switching means is preferably switched on again. This can be minimized when switching over the switching means dropping power loss.

The second inductively acting component is preferably a discrete component, particularly preferably a coil, which has the properties of an inductive effect required here. It prevents the capacitive inrush current peaks in the switching means with residual voltage at the capacitor and stores the energy magnetically.

The second inductively acting component is preferably arranged between the parallel resonant circuit and the switching means.

The second inductively acting component is inductively coupled to an energy absorption circuit for receiving energy stored in the second inductively acting component or is a component of the energy absorption circuit. The energy absorption circuit is preferably designed to absorb energy immediately after a transition of the switching means into a non-conductive state.

With such an energy absorption circuit, the energy which is usually converted into heat in the switching means during a switch-on process can be stored in an advantageous manner. If the switching means is switched off, ie transferred to a non-conducting state, this energy would become a burden on the switching means. In other words, the second inductively acting device would try to maintain the flow of current, which should just be prevented by a switched-off switching means.

The energy absorption circuit is preferably designed such that the absorbed energy is at least partially returned to a DC link or the parallel resonant circuit. It will be explained in more detail below how the energy absorption circuit can be designed for this purpose.

The energy absorption circuit has a third inductively acting component which is inductively or transformer-coupled to the second inductively acting component. Particularly preferably, the third inductively acting component is a coil. In the typical case where the second inductor is also a coil, namely a second coil, the third inductor is a third coil. The second and third coils may form a transformer.

With such an embodiment, a dissipation of the energy contained in the second inductively acting component can be achieved in an advantageous manner, since two inductively acting components, particularly preferably two coils, can be inductively or transformerically coupled for energy transfer in a known manner.

The third inductively acting component preferably has an inductance similar or identical to the second inductively acting component. This causes a particularly effective coupling.

The third inductively acting component is poled in opposite directions or in opposite directions to the second inductively acting component.

Preferably, the third inductively acting device is coupled via a diode to the intermediate circuit or the parallel resonant circuit. With such a circuit, the energy from the second inductively acting device in the DC link or the resonant circuit can be fed back. Such a diode further causes that during the switching periods, d. H. while the switching means is turned on, the third inductively acting device and thus also the second inductively acting device are decoupled from the intermediate circuit voltage.

According to one embodiment, the diode is arranged between the third inductively acting component and a connection node between the capacitor or the first coil and the second inductively acting component.

According to an alternative embodiment, the diode is arranged between the third inductively acting component and the intermediate circuit reference potential. The anode of the first diode may be connected to the DC link reference potential.

The induction heater according to the invention can be summarized as follows:
When switching the switching means of the increase of the current and thus the power loss are limited by the second inductively acting device. When the switching means is switched off, the energy stored in the second inductively acting component is inductively transmitted to the third inductively acting component. In this case, the diode is conductive and the energy is directly transferred back into the DC link or parallel resonant circuit.

It is understood that the processes described run partially simultaneously.

Short description of the drawing

The invention will be described in detail below with reference to the drawing. Here are shown schematically:

1 a circuit diagram of an induction heater according to a first embodiment and

2 a circuit diagram of an induction heater according to a second embodiment.

Detailed description of the embodiment

1 shows a first embodiment of an induction heater according to the invention. It should be noted that the inductively acting components are each designed as coils in the described embodiment. This also justifies to refer to these components as coils with corresponding enumeration. However, in each case also other inductively acting components could be used.

The induction heating device shown is powered by an AC mains voltage UN, which at two poles 1 is applied. The mains AC voltage UN becomes a bridge rectifier 2 fed, which is carried out in a known manner. The bridge rectifier 2 has four diodes D _B1 , D _B2 , D _B3 and D _B4 . These serve to rectify the mains AC voltage UN.

At the output of the bridge rectifier 2 a _DC link capacitor C _{ZK is} arranged. This smoothes the of the bridge rectifier 2 rectified voltage. Over the intermediate circuit capacitor C _ZK thus drops an intermediate circuit voltage UZ.

As a reference potential for both the bridge rectifier 2 as well as for the DC link capacitor C _ZK is a DC link reference potential in the form of a ground voltage GND.

One pole of a capacitor C _{1 is} connected to one pole of the intermediate circuit capacitor C _ZK . The capacitor C ₁ forms a parallel resonant circuit together with a first coil L ₁ 7 , From the first coil L ₁ can energy by means of an alternating magnetic field on a cookware 5 are transmitted, which is presently shown with an inductance L _T and a resistor connected in series R _T. A current induced in the inductance L _T is thus converted into heat by the resistor R _T and thus serves to heat the cookware 5 ,

At that pole of the capacitor C ₁ which is opposite to the pole connected to the intermediate circuit capacitor C _ZK , there is a resonant circuit voltage. This pole also has a second coil L ₂ and an insulated gate bipolar transistor (IGBT) T _{1 connected} in series, the transistor T ₁ in turn being connected to the ground voltage GND. The transistor T _{1 in the} present case forms the switching means. Parallel to this, a freewheeling diode D _{F is} connected.

The transistor T ₁ can assume a conducting and a non-conducting state. In the conducting state, the transistor T _{1 is} low-resistance, in the non-conducting state, it is high-impedance. It is controlled by a control device, not shown, which ensures that the transistor T _{1 is turned on} within periodically recurring switching periods, ie assumes its conducting state. During these switching periods, a conductive connection is thus produced from the capacitor C ₁ via the second coil L ₂ through the transistor T ₁ to the ground voltage GND. During such switching periods, the capacitor C ₁ and the first coil L ₁ can thus absorb energy. After the end of such a switching period, the transistor T _{1 is} turned off again, thus assumes its non-conductive state. Then oscillates from the capacitor C ₁ and the first coil L ₁ existing parallel resonant circuit 7 , wherein the already mentioned above magnetic alternating field is generated.

A switching frequency of the transistor T ₁ may for example be in the range between 10 kHz and 100 kHz.

Preferably, the transistor T _{1 is} exactly turned on again when the voltage across the transistor T ₁ has reached its minimum. However, this will not necessarily fall back to the ground voltage GND after a period of oscillation. This results in turning on the transistor T _1, a voltage drop across the transistor T ₁ , which leads to turn-on.

When the transistor T ₁ is turned on, a current or current increase flowing through the transistor T _{1 is} reduced by the second coil L ₂ .

By the current, which then flows through the second coil L ₂ , ₂ energy is stored in the second coil. When the transistor T _{1 is} turned off, that is, in a state in which it is no longer conductive, the circuit basically tends to the current flow, which is just to be prevented by turning off the transistor T ₁ , by means of the second coil L ₂ stored energy to maintain.

To dissipate the stored energy in the second coil L ₂ is a Energieabführschaltung 3 provided, which has a plurality of components, which are explained in more detail below.

First, a third coil L _{3 is} provided, which is inductively coupled to the second coil L ₂ . In this case, the third coil L _{3 is} opposite to the second coil L ₂ wound or poled. When the transistor T _{1 is} switched off, the energy stored in the second coil L ₂ causes a positive voltage to be present at a connection node between the second coil L ₂ and the transistor T ₁ in comparison to the opposite pole of the second coil L ₂ . Due to the already mentioned inductive coupling of the second and third coils, however, the energy stored in the second coil L ₂ is at least partially transmitted to the third coil L ₃ .

The third coil L ₃ is connected in series with a diode D ₁ . Due to the reverse polarity of the third coil L ₃ compared to the second coil L ₂ is located at the connection node between the third coil L ₃ and the diode D ₁ immediately after turning off the transistor T _{1 to} a positive voltage. The corresponding pole of the third coil L ₃ is connected to the anode of the diode D ₁ , so that the diode D ₁ becomes conductive precisely in this state. Thus, the energy transferred from the second coil L ₂ to the third coil L ₃ can be directly transferred to the parallel resonant circuit 7 be fed back. A current flow in the reverse direction is prevented by the diode D _1, however.

2 shows a second embodiment of an induction heater according to the invention. This is similar to the first embodiment. Identical features and components will not be discussed again below. Rather, in this regard, reference is made to the above description of the first embodiment.

In contrast to the first embodiment is in the in 2 illustrated second embodiment, the diode D ₁ between the DC link reference potential GND and the third inductively acting device L ₃ arranged. The anode of the diode D ₁ is connected directly to the DC link reference potential GND. This allows a current flow in the same direction as in the first embodiment in the parallel resonant circuit 7 for the return of energy. Such an embodiment according to the second embodiment has certain advantages in the construction of the circuit.

The dissipated energy is thus at least partially back into the parallel resonant circuit 7 returned and made usable in this way.

The illustrated induction heater has significantly lower losses compared to prior art induction heaters. Assuming a voltage of 140 V across the transistor T ₁ at power-up, the total losses in all power components involved decrease to about 10 W, compared to about 30 W in a prior art embodiment. At the same time, the suppression of capacitive switching results in a significant reduction of electromagnetic emissions as well as an improvement in user comfort.

Claims (12)

Induction heater with - a first inductively acting device in the form of a first coil (L ₁ ), which is designed as an induction coil, - a capacitor (C ₁ ) which is connected in parallel to the first coil (L ₁ ), wherein the first coil (L ₁ ) and the capacitor (C ₁ ) a parallel resonant circuit ( 7 ), - a controllable switching means (T ₁ ), in series with the parallel resonant circuit ( 7 ) between a terminal pole, at which a positive intermediate circuit potential (UZ) is present, and a terminal pole, at which a DC link reference potential (GND) is present, is looped in and is controlled such that during a heating operation, a vibration of the parallel resonant circuit ( 7 ), and - a second inductively acting device (L ₂ ) connected in series with the switching means (T ₁ ) and the parallel resonant circuit ( 7 ) between the terminal pole, at which the positive intermediate circuit potential (UZ) is present, and the terminal pole, at which the intermediate circuit reference potential (GND) is present, is looped, wherein the second inductively acting component (L ₂ ) inductively with an energy absorption circuit ( 3 ) is coupled to receive stored in the second inductance device (L ₂ ) stored energy, wherein the energy absorption circuit ( 3 ) has a third inductively acting device (L ₃ ), which is inductively coupled to the second inductively acting device (L ₂ ), wherein the third inductively acting device (L ₃ ) in opposite directions to the second inductively acting device (L ₂ ) is poled. An induction heater according to claim 1, wherein said second inductance device (L ₂ ) is a second coil. Induction heater according to one of claims 1 or 2, wherein the second inductively acting device (L ₂ ) between the parallel resonant circuit ( 7 ) and the switching means (T ₁ ) is arranged. Induction heater according to one of the preceding claims, characterized in that the energy absorption circuit ( 3 ) is formed for receiving energy immediately after a transition of the switching means (T ₁ ) in a non-conductive state. Induction heater according to claim 4, wherein the energy absorption circuit ( 3 ) for the at least partial return of the absorbed energy into the parallel resonant circuit ( 7 ) is trained. Induction heater according to claim 4 or 5, characterized in that the third inductively acting component (L ₃ ) is designed in the form of a third coil. Induction heater according to claim 6, wherein the third inductively acting device (L ₃ ) has an inductance similar or identical to the second inductively acting device (L ₂ ). Induction heater according to one of claims 6 to 7, wherein the third inductively acting device (L ₃ ) via a diode (D ₁ ) to the resonant circuit ( 7 ) is coupled. Induction heater according to claim 8, wherein the diode (D ₁ ) between the third inductively acting device (L ₃ ) and a Connection node between the capacitor (C ₁ ) and the second inductively acting device (L ₂ ) is arranged. Induction heater according to claim 8, wherein the diode (D ₁ ) between the third inductively acting device (L ₃ ) and the intermediate circuit reference potential (GND) is arranged. Induction heater according to one of claims 8 to 10, wherein energy from the third inductively acting device (L ₃ ) via the diode (D ₁ ) after a transition of the switching means (T ₁ ) in a non-conductive state in the parallel resonant circuit ( 7 ) is transferred. Induction heater according to one of claims 8 to 11, wherein the diode (D ₁ ) is arranged such that its anode is connected to the intermediate circuit reference potential (GND).

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