WO2016015971A1

WO2016015971A1 - Induction heating system

Info

Publication number: WO2016015971A1
Application number: PCT/EP2015/065672
Authority: WO
Inventors: Hao Yan; JingBo LU; Jun Han
Original assignee: E.G.O. Elektro-Gerätebau GmbH
Priority date: 2014-07-31
Filing date: 2015-07-09
Publication date: 2016-02-04
Also published as: CN105451384A

Abstract

The invention relates to an induction heating system having a capacitor arrangement with an adjustable capacity. This allows for adjustment of the capacity of the capacitor arrangement according to desired power levels.

Description

Induction heating system

Applicable field and prior art

The invention relates to an induction heating system, comprising an electric drive means, an inductor coil and a capacitor arrangement. The inductor coil and the capacitor arrangement form an inductor circuit, and the electric drive means is configured to drive the induction circuit.

Known induction heating systems are typically employed in order to warm up cookware, for example in an induction cooking field. Transfer of heat is usually accomplished by generating a changing electromagnetic field by the inductor coil, wherein the electromagnetic field induces eddy currents in the cookware, which then heat up the cookware.

In order to control a cooking process, it is desirable to vary the power transferred from the induction heating system to the cookware. Typically, this is done by adjusting an operation frequency with which the electric drive means drives the induction circuit. In most cases, power transfer is maximized when a resonant frequency of the induction circuit is used as the operating frequency. Lower power levels can be implemented by using higher operating frequencies.

In order to get a high maximum transferable power, a capacitor arrangement having a capacitor with a high capacity is normally used. However, this leads to some problems, because operation at low power leads to high switching voltages at the drive means and to long pulse power cycles. This can limit operability of the induction heating system at low power levels and thus causes a raised minimum continuous power. Furthermore, when multiple cooking zones are operated at the same time, frequency differences might create disturbing noise.

Problem and solution

It is thus an object of the invention to create an induction heating system that can easily be operated both at low power and at high power. According to the invention, this is solved by an induction heating system according to claim 1 . Advantageous and preferred configurations of the invention are the subject matter of the other claims and will be explained in more detail below. The wording of the claims is incorporated into the content of the description by explicit reference. The invention relates to an induction heating system, comprising:

an electric drive means,

an inductor coil, and

a capacitor arrangement comprising at least two capacitors,

- wherein the inductor coil and the capacitor arrangement form an induction circuit, and wherein the electric drive means is configured to drive the induction circuit.

According to the invention, the capacitor arrangement has an adjustable capacity. With the induction heating system according to the invention, it is possible to adjust the capacity according to a desired power. For example, if a high power transfer to a cookware is needed, a high capacity can be used. If a low power transfer is desired, a low capacity can be used. Thus, the requirement to set a low power level by drastically increasing the operating frequency below the resonant frequency is relaxed. This allows for a lower minimum continuous power while at the same time providing potential for a high maximum power. According to a preferred embodiment, the capacity of the capacitor arrangement is switchable. This allows for using simple and reliable switches in order to adjust the capacity.

According to an embodiment, the induction circuit is formed as a single-ended parallel resonant circuit. In such a single-ended parallel resonant circuit, an adjustable capacity of the capacitor arrangement can be implemented for example in the following three ways.

According to an embodiment, the capacitor arrangement comprises a first capacitor, a second capacitor and a switch, wherein the second capacitor is connected in series with the switch forming a line being arranged in parallel with the first capacitor. When the switch is open, the capacity of the capacitor arrangement is defined by the capacity of the first capacitor. When the switch is closed, the capacity of the capacitor arrangement is defined by the sum of the capacity of the first capacitor plus the capacity of the second capacitor.

According to a preferred embodiment, the capacitor arrangement comprises a first capacitor, a second capacitor, a first switch and a second switch. The first capacitor is connected in series with the first switch forming a first line, and the second capacitor is connected in series with the second switch forming a second line, wherein the first line and the second line are arranged in parallel. When only the first switch is closed, the capacity of the capacitor arrangement is defined by the capacity of the first capacitor. When only the second switch is closed, the capacity of the capacitor arrangement is defined by the capacity of the second capacitor. When both switches are closed, the capacity of the capacitor arrangement is defined by the sum of the ca- pacity of the first capacitor plus the capacity of the second capacitor. In this embodiment, it may be preferable that the first capacitor and the second capacitor have different capacities. Then it is possible to switch the capacity of the capacitor arrangement to three different capacities.

According to a further embodiment, the capacitor arrangement comprises a first capacitor and a second capacitor arranged in series, wherein a switch is connected parallel to one of the capacitors, for example the second capacitor, in order to bypass this capacitor when closed or the second capacitor, respectively. When the switch is open, the first capacitor and the second capacitor are driven in series, so that the capacity of the capacitor arrangement is lower than the lowest capacity of the capacitors. When the switch is closed, the capacitor with the switch in parallel is bypassed and the capacity of the capacitor arrangement is given by the capacity of the other capacitor.

Preferably, the electric drive means of an induction heating system having a single-ended parallel resonant circuit comprises a transistor connected in series with the induction circuit. This allows for a reliable and simple driving of the induction circuit. According to an embodiment that is alternative to a single-ended resonant circuit, the induction circuit is configured as a series resonant half-bridge circuit. With such a series resonant half- bridge circuit, an adjustable capacity of the capacitor arrangement can, for example, be implemented in the following two ways.

According to a first embodiment, the capacitor arrangement comprises a first capacitor, a sec- ond capacitor, a third capacitor and a fourth capacitor. The first capacitor and the second capacitor are arranged in series forming a first line. The third capacitor and the fourth capacitor are arranged in series forming a second line. The first line and the second line are arranged in parallel, and a switch is arranged between the first line and the second line.

Preferably, a first terminal of the switch is connected with a terminal of the first capacitor and a terminal of the second capacitor. Further preferably, a second terminal of the switch is connected with a terminal of the third capacitor and a terminal of the fourth capacitor.

According to a second embodiment, the capacitor arrangement comprises a first capacitor and a second capacitor. The first capacitor and the second capacitor are arranged in series, and a switch is arranged in series with the second capacitor to deactivate the second capacitor when open. With both embodiments to realize an adjustable capacity in a capacitor arrangement of a series resonant half-bridge circuit, the capacity of the capacitor arrangement can be adjusted by opening or closing the switch, thus selecting which capacitors take part in forming the capacity of the capacitor arrangement. Preferably, the electric drive means of an induction heating system having a series resonant half-bridge circuit comprises a first transistor and a second transistor. The first transistor and the second transistor are arranged in series, and a terminal of the induction coil of the induction circuit is connected with a terminal of the first transistor and with a terminal of the second transistor. This allows for a reliable and easy driving of the induction circuit of the induction heating system.

The switch, the first switch and/or the second switch are preferably one of the group comprising a relay, a TRIAC, an IGBT or a MOSFET. Such types of switches have been proven suitable and reliable for such applications.

According to a further embodiment, the induction heating system also comprises a control unit configured to set the capacity of the capacitor arrangement.

Further preferably, the control unit is configured to set the capacity to a low value if the induction heating system is operated with low power, and the control unit is configured to set the capacity to a high value if the induction heating system is operated with high power. For example, a low power can be assumed if the power is smaller than 35% of the maximum power of the induction heating system. As a further example, a high power can be assumed if the induction heating system is operated with a power being larger than 60% of the maximum power of the induction heating system. With such a control unit, it is possible to automatically adjust the capacity of the capacitor arrangement according to the currently needed power of the induction heating system.

These and other features may furthermore be found in the claims and the description and the drawings. The individual features may be implemented individually or several together in the form of subcombinations in an embodiment of the invention and in other fields, and may represent advantageous and protectable embodiments for which protection is claimed here. The division of the application into subheadings and individual sections does not restrict the comments made therein in their general applicability. Short description of the drawings

Further features and advantages of the invention will be apparent from the following description of several embodiments with reference to the enclosed drawings. The figures show the following:

Fig. 1 shows an induction heating system according to a first embodiment,

Fig. 2 shows an induction heating system according to a second embodiment,

Fig. 3 shows an induction heating system according to a third embodiment,

Fig. 4 shows an induction heating system according to a fourth embodiment, and

Fig. 5 shows an induction heating system according to a fifth embodiment.

The induction heating systems according to the first to third embodiments comprise a single- ended parallel resonant circuit, differing in the respective capacitor arrangements. The induction heating systems according to the fourth and fifth embodiments comprise a series resonant half- bridge circuit, differing in the respective capacitor arrangements.

Detailed description of the embodiments

Fig. 1 shows an induction heating system according to a first embodiment. The induction heating system comprises an induction circuit I that is formed as a single-ended parallel resonant circuit. The induction circuit I comprises a capacitor arrangement C and an inductor coil L. The capacitor arrangement C and the inductor coil L are arranged parallel to each other. The induction coil L is for heating a cookware or a pot, respectively, in an induction cooking field and is constructed in the conventional manner as a conventional induction heating coil.

In addition, the induction heating system comprises a drive means formed by a transistor T that is arranged in series with the induction circuit I. Using the transistor T, it is possible to drive the induction circuit I with a certain frequency. Depending on the frequency, the inductor coil L can transfer a certain amount of energy to a cookware positioned above it.

Opposite to the transistor T, the induction circuit I is connected to a positive supply voltage. Opposite to the induction circuit I, the transistor T is connected to a ground GND.

The capacitor arrangement C comprises a first capacitor C1 and a second capacitor C2. The capacitor arrangement C further comprises a switch K that is arranged in series with the second capacitor C2 to form a line. This line is arranged in parallel with the first capacitor C1 . If the switch K is closed, the capacity of the capacitor arrangement C is given as the sum of the capacity of the first capacitor C1 and the capacity of the second capacitor C2. If the switch K is open, the capacity of the capacitor arrangement C is given as the capacity of the first capacitor C1 . Thus, it is possible to adjust the capacity of the capacitor arrangement C by closing or opening the switch K.

In order to control the switch K, the induction heating system comprises a control unit CU that is configured to set the switch K according to the actually intended amount of electrical power. If the intended power is low, the control unit CU opens the switch K in order to have a lower capacity of the capacitor arrangement C. If the intended power is high, the control unit CU closes the switch K in order to have a higher capacity of the capacitor arrangement C. This allows for a preferable adjustment of the capacity of the capacitor arrangement C in order to minimize currents occurring especially at the transistor T.

Fig. 2 shows an induction heating system according to a second embodiment. The induction heating system according to the second embodiment is similar to the induction heating system according to the first embodiment. Thus, only the differences are discussed.

The capacitor arrangement C of the induction heating system according to the second embodiment comprises not only a switch K, but a first switch K1 and a second switch K2. The first switch K1 is connected in series with the first capacitor C1 in order to form a first line. The second switch K2 is connected in series with the second capacitor C2 in order to form a second line. The first line and the second line are connected parallel to each other. Furthermore, the capacity of the first capacitor C1 is smaller than the capacity of the second capacitor C2.

The control unit CU is configured in order to control both the first switch K1 and the second switch K2. If the desired power is low, the control unit CU closes only the first switch K1 while the second switch K2 is left open, so that the capacity of the capacitor arrangement C is defined by the capacity of the first capacitor C1 . If the desired power is medium, the control unit CU closes only the second switch K2 while the first switch K1 is left open, so that the capacity of the capacitor arrangement C is given by the capacity of the second capacitor C2, which is higher than the capacity of the first capacitor C1 . If the desired power is high, the control unit CU closes both the first switch K1 and the second switch K2, so that the capacity of the capacitor ar- rangement C is given by the sum of the capacity of the first capacitor C1 and the capacity of the second capacitor C2. This allows for a finer adjustment of the capacity of the capacitor arrangement C. Fig. 3 shows an induction heating system according to a third embodiment. As in the induction heating systems according to the first and second embodiments, the induction heating system according to the third embodiment comprises a single-ended parallel resonant circuit I . Only differences are discussed in the following. The capacitor arrangement C of the induction heating system according to the third embodiment comprises a first capacitor C1 and a second capacitor C2 which are connected in series. Parallel to the second capacitor C2 there is arranged a switch K which can be used to bypass the second capacitor C2 if closed.

If the desired power is low, the control unit C closes the switch K so that the capacity of the ca- pacitor arrangement is smaller than the lowest of the capacity of the first capacitor C1 and the capacity of the second capacitor C2. If the desired power is high, the control unit CU opens the switch K so that the capacity of the capacitor arrangement C is given by the capacity of the first capacitor C1 .

Otherwise, the induction heating system according to the third embodiment is similar to the induction heating systems according to the first and second embodiments.

Fig. 4 shows an induction heating system according to a fourth embodiment. In contrast to the induction heating systems according to the first, second and third embodiments, the induction heating system according to the fourth embodiment comprises an induction circuit I that is formed as a series resonant half-bridge circuit. A drive means comprises a first transistor T1 and a second transistor T2 being connected in series between a positive voltage supply and ground GND. Between the first transistor T1 and the second transistor T2, an induction coil L is connected with a first pole. A second pole of the

CI

inductor coil L is connected to a capacitor arrangement C having a first pair of capacitors —

C2 CI

and a second pair of capacitors — . The capacitors — of the first pair are connected in se-

C2

ries, forming a first line. The capacitors —^- of the second pair are also connected in series, forming a second line. The first line and the second line are connected parallel to each other. Between the first line and the second line, a switch K is connected and is controllable by a control unit CU. If the desired power is high, the control unit closes the switch K so that the capacity of the capacitor arrangement C is given as the sum of all capacitors C1 , C2. If the desired power is low, the control unit CU opens the switch K so that the capacity of the capacitor arrangement C is given as the sum of only the capacitors C1 of the first pair of capacitors. This allows for an ad- justment of the capacity of the capacitor arrangement C according to the desired power level.

Fig. 5 shows an induction heating system according to a fifth embodiment. Similar to the induction heating system according to the fourth embodiment, the induction heating system according to the fifth embodiment comprises a series resonant half-bridge circuit as an induction circuit I . However, in contrast to the induction heating system according to the fourth embodiment, the capacitor arrangement C of the induction heating system according to the fifth embodiment comprises a first capacitor C1 and a second capacitor C2 connected in series to each other, wherein a switch K is connected in series with the second capacitor C2.

When the intended power is high, the control unit CU closes the switch K so that the capacity of the capacitor arrangement C is given by the sum of the capacity of the first capacitor C1 and the capacity of the second capacitor C2. If the intended power is low, the control unit CU opens the switch K so that the capacity of the capacitor arrangement C is given by the capacity of only the first capacitor C1 . This allows also for a switching of the capacity of the capacitor arrangement C and an adjustment according to the intended power level.

Applicant has, as an example, tested an induction heating system according to the first embod- iment, with respective capacities of the first capacitor and the second capacitor each having a value of 160 nF, and has measured the following data:

Switch K: closed open

Capacity of capacitor arrangement: 320 nF 160 nF

Maximum power: 2300 W 1400 W

Minimum continuous power: 1200 W 700 W

Transistor switch voltage at 1400 W: 72 V 0 V

Cycle time: 7.5 sec 3.5 sec This test shows that by switching the switch K in an open state, the minimum continuous power can be significantly lowered and the switch voltage at a medium power level can be significantly reduced. This not only allows for a greater flexibility when cooking at different power levels, but also allows for a reduced destructive load on the transistor and other components.

Claims

1 . Induction heating system, comprising:

an electric drive means (T, T1 , T2),

an inductor coil (L), and

a capacitor arrangement (C) comprising at least two capacitors (C1 , C2), wherein the inductor coil (L) and the capacitor arrangement (C) form an induction circuit (I),

wherein the electric drive means (T, T1 , T2) is configured to drive the induction circuit (I), and

wherein the capacitor arrangement (C) has an adjustable capacity.

2. Induction heating system according to claim 1 , wherein the capacity of the capacitor arrangement (C) is switchable.

3. Induction heating system according to claim 1 or 2, wherein the induction circuit (I) is formed as a single-ended parallel resonant circuit.

4. Induction heating system according to claim 3, wherein the capacitor arrangement (C) comprises a first capacitor (C1 ), a second capacitor (C2) and a switch (K), wherein the second capacitor (C2) is connected in series with the switch (K) forming a line being arranged in parallel with the first capacitor (C1 ).

5. Induction heating system according to claim 3, wherein the capacitor arrangement (C) comprises a first capacitor (C1 ), a second capacitor (C2), a first switch (K1 ) and a second switch (K2), wherein the first capacitor (C1 ) is connected in series with the first switch (K1 ) forming a first line, wherein the second capacitor (C2) is connected in series with the second switch (K2) forming a second line, and wherein the first line and the second line are arranged in parallel.

6. Induction heating system according to claim 3, wherein the capacitor arrangement (C) comprises a first capacitor (C1 ) and a second capacitor (C2) arranged in series, wherein a switch (K) is connected parallel to the second capacitor (C2) in order to bypass the second capacitor (C2) when closed.

7. Induction heating system according to one of the claim 3 to 6, wherein the electric drive means (T, T1 , T2) comprises a transistor (T) connected in series with the induction circuit (I).

8. Induction heating system according to claim 1 or 2, wherein the induction circuit (I) is configured as a series resonant half bridge circuit.

9. Induction heating system according to claim 8,

wherein the capacitor arrangement (C) comprises a first capacitor (C1/2), a second capacitor (C1/2), a third capacitor (C2/2) and a fourth capacitor (C2/2), wherein the first capacitor (C1/2) and the second capacitor (C1/2) are arranged in series forming a first line,

wherein the third capacitor (C2/2) and the fourth capacitor (C2/2) are arranged in series forming a second line,

wherein the first line and the second line are arranged in parallel, and

wherein a switch (K) is arranged between the first line and the second line.

10. Induction heating system according to claim 9,

wherein a first terminal of the switch (K) is connected with a terminal of the first capacitor (C1/2) and a terminal of the second capacitor (C1/2), and

wherein a second terminal of the switch (K) is connected with a terminal of the third capacitor (C2/2) and a terminal of the fourth capacitor (C2/2).

1 1 . Induction heating system according to claim 8,

wherein the capacitor arrangement (C) comprises a first capacitor (C1 ) and a second capacitor (C2),

wherein the first capacitor (C1 ) and the second capacitor (C2) are arranged in series, and

wherein a switch (K) is arranged in series with the second capacitor (C2) to deactivate the second capacitor (C2) when opened.

12. Induction heating system according to one of claims 8 to 1 1 ,

wherein the electric drive means (T, T1 , T2) comprises a first transistor (T1 ) and a second transistor (T2),

wherein the first transistor (T1 ) and the second transistor (T2) are arranged in series, and wherein a terminal of the induction coil (L) of the induction circuit (I) is connected with a terminal of the first transistor (T1 ) and with a terminal of the second transistor (T2).

13. Induction heating system according to one of the preceding claims, wherein the switch (K), the first switch (K1 ) and/or the second switch (K2) are one of the following group: a relay, a TRIAC, an IGBT, a MOSFET.

14. Induction heating system according to one of the preceding claims, wherein the induction heating system further comprises a control unit (CU) configured to set the capacity of the capacitor arrangement (C).

15. Induction heating system according to claim 14,

wherein the control unit (CU) is configured to set the capacity to a low value if the induction heating system is operated with low power, and

wherein the control unit (CU) is configured to set the capacity to a high value if the induction heating system is operated with high power.