The present invention relates to an induction heating cooktop wherein it is detected whether the vessel placed thereon is at the appropriate heating position.

The induction heating cooktop functions according to the principle of heating a ferromagnetic cookware like cast iron or steel, for example a pot, with the magnetic field effect generated by the induction coil. In order to drive the induction coils that generate magnetic fields, high amount of electric current is passed through the power switch (IGBT-Insulated Gate Bipolar Transistor, diode or MOSFET) on the circuit board. In the state of the art, half bridge series resonant (HBSR) circuits formed by using two power switches and two resonant capacitors, and single switch quasi-resonant (SSQR) circuits formed by one power switch and one resonant capacitor are used for driving a single induction coil. The single switch quasi-resonant circuits (SSQR) are preferred due to cost advantage; however, they operate in a narrower energy frequency range and can deliver power to the cookware only within a certain voltage and power range. In induction heating cooktops wherein the single switch quasi-resonant circuits (SSQR) are used, problems are encountered in detecting different kinds of cookware and the changes in position of the cookware on the cooktop burner. Furthermore, difficulties arise in detecting the position of the cookware in mains voltage fluctuations and at different temperature conditions. In some induction heating cooktops, multi coil - multi zone structure is used, heating can be maintained on the entire cooktop surface and flexibility is provided for the user. In this type of induction heating cooktops, induction coils of various shapes and sizes are situated on the cooktop surface. Under variable mains voltage, input voltage depending on the power setting and at variable temperature conditions, the detection of the cookware position and furthermore the characteristic features like the diameter, type and the ferromagnetic properties during power transmittance to the cookware is quite critical for products wherein multi coil and also the single switch quasi-resonant circuits (SSQR) are used. The cookware cannot be heated efficiently and the electronic circuit controlling the induction coil can be damaged if sliding or lifting of the cookware from the cooktop cannot be properly detected.

The European Patent Application No. EP2282606 relates to an induction apparatus control method. The presence or absence of the vessel on the induction coil, the resistivity and the dimensions thereof are detected by comparing the resonance voltage with a predetermined fixed reference voltage in the control unit.

In the European Patent No. EP1629698, an induction cooking system comprising a power inverter, a microprocessor, a protection circuit and a pan detection circuit is explained.

In the Japanese Patent Application No. JP4371108, an induction heating cooking device that comprises a cookware detection circuit is explained.

In the Japanese Patent Application No. JP2011023163, a rice cooker according to the preamble of claim 1 is explained wherein existence or nonexistence of a pan on the induction heater or whether or not the pan is located at a designated position is detected under unstable power source voltage conditions.

In the Japanese Patent Application No. JP2007066837, a rice cooker is explained wherein the presence of kitchenware such as spoon, knife on the induction heater is detected under fluctuating power source voltage conditions.

The aim of the present invention is the realization of an induction heating cooktop wherein the position of the vessel placed on the induction coil is detected precisely under variable mains input voltage and temperature conditions.

The induction heating cooktop realized in order to attain the aim of the present invention, explicated in the first claim and the respective claims thereof, comprises : a bridge rectifier that converts the alternating current into direct current, a filter circuit disposed at the outlet of the bridge rectifier and that filters the voltage generated at the DC-line, a resonant circuit having an induction coil that provides the heating of the vessel placed thereon and a resonant capacitor connected in parallel to the induction coil, a power switch that drives the resonant circuit, a freewheeling diode connected in parallel to the power switch and providing continuity of the induction coil current during the non-conduction times of the power switch, a collector node whereon resonance voltage is generated, a control unit that regulates the operation of the power switch, and a current detection circuit connected in series to the induction coil and providing the monitoring of the coil current transferred from the induction coil to the vessel by converting it into voltage data, wherein the control unit determines whether or not the vessel is present on the induction coil or whether alignment of the vessel on the induction coil is appropriate by comparing the phase difference time between the coil current detected by the current detection circuit and the resonance voltage generated on the collector node with a threshold phase difference time recorded in its memory.

The current detection circuit is formed of a current sensing resistor connected in series to the induction coil or formed of a current transformer connected in series to the induction coil and a current sensing resistor connected in parallel to the secondary side of the current transformer.

In an embodiment of the present invention, a first comparator is connected in parallel to the terminals of the current detection resistor. The first comparator generates square wave output signals by detecting the zero crossings of the coil current.

A second comparator, connected to the collector node and the DC-line, generates square wave output signals by comparing the resonance voltage with the DC-line voltage. Voltage dividers are connected to the collector node and the DC-line and the resonance voltage and the DC-line voltage are compared by being decreased to a measurable low level.

A logical AND gate, whereto the first comparator and the second comparator are connected, generates logical-1 output signals when the output signals of both the first comparator and the second comparator are logical-1.

The control unit determines the absence of the vessel on the induction coil or that the vessel has been slid from over the induction coil more than permitted and interrupts the induction coil current if the period of the logical-1 output signal of the logical AND gate is smaller than the threshold signal time recorded in its memory.

In the induction heating cooktop of the present invention, whether or not the vessel is present on the induction coil and the alignment position on the induction coil is detected reliably all through the heating process under unfavorable conditions where the AC mains input voltage is variable.

The induction heating cooktop realized in order to attain the aim of the present invention is illustrated in the attached figures, where:

• Figure 1 - is the schematic view of the control circuit of an induction heating cooktop.
• Figure 2 - is the schematic view of the control circuit of an induction heating cooktop in an embodiment of the present invention.
• Figure 3 - is the graphic showing the change with respect to time in the induction coil current and the voltage generated on the power switch in the control circuit of the induction heating cooktop.
• Figure 4 - is the graphic showing the output signals of the first and second comparators used in the control circuit of the induction heating cooktop.
• Figure 5 - is the graphic showing the output signals of the logical AND gate used in the control circuit of the induction heating cooktop.

The elements illustrated in the figures are numbered as follows:

1. 1. Induction heating cooktop
2. 2. Bridge rectifier
3. 3. Filter circuit
4. 4. Induction coil
5. 5. Resonant capacitor
6. 6. Resonant circuit
7. 7. Power switch
8. 8. Freewheeling diode
9. 9. Collector node
10. 10. Drive circuit
11. 11. Control unit
12. 12. Current detection circuit
13. 13. Current detection resistor
14. 14. Current transformer
15. 15. First comparator
16. 16. Second comparator
17. 17. AND gate
18. 18. First voltage divider
19. 19. Second voltage divider

The induction heating cooktop (1) comprises a bridge rectifier (2) that converts the alternating current received from the mains into direct current, a high frequency filter circuit (3) disposed at the outlet of the bridge rectifier (2) comprising a DC-line capacitor and a DC-line inductor that delivers DC voltage within a certain frequency range by filtering the voltage generated at the DC-line, a resonant circuit (6) having an induction coil (4) that provides the heating of the vessel (K) placed thereon and a resonant capacitor (5) connected in parallel to the induction coil (4), a power switch (7), for example an IGBT (Insulated Gate Bipolar Transistor), having a collector and an emitter, that drives the resonant circuit (6), that is in conducting state in the turned-off position, providing the resonant capacitor (5) to be charged during the turn-off time, that interrupts conduction in the turned-on position, providing the resonant capacitor (5) to be discharged during the non-conduction (turned-on) time and that provides the energy to be delivered from the induction coil (4) to the vessel (K), a freewheeling diode (8) connected in parallel to the power switch (7), that provides continuity of the induction coil (4) current (I_L) (will be referred to as "coil current (I_L)" hereinafter) in the resonant circuit (6) during the non-conduction (turn-on) times of the power switch (7), a collector node (9) whereon resonance voltage (Vce) or in other words the collector-emitter voltage of the power switch (7) is generated during the non-conduction (turn-on) times of the power switch (7), a drive circuit (10) providing the power switch (7) to be driven with the drive voltage at the required level and a control unit (11), for example a microcontroller, that regulates the operation of the power switch (7) by controlling the drive circuit (10).

During the heating process implemented in the induction heating cooktop (1), the conduction times wherein the power switch (7) is in turned-off position are determined by the power scale setting made by the user. The non-conduction times wherein the power switch (7) is in the turned-on position are determined by the control unit (11) depending on the characteristic features of the vessel (K) placed on the induction coil (4), alignment of the vessel (K) on the induction coil (4), AC mains voltage conditions and the temperature of the vessel (K). The resonant capacitor (5) is first charged then discharged during the non-conduction (turned-on) times of the power switch (7) and the coil current (I_L) passes through the freewheeling diode (8) while the resonant capacitor (5) is being discharged. Resonance voltage (Vce) is generated at the collector node (9) and energy is transferred from the induction coil (4) to the vessel (K). At the lowermost level of the resonance voltage (Vce), the power switch (7) is changed from the turned-on position to the turned-off position, in other words from the non-conducting current state to the current conducting state and energy is stored in the induction coil (4) during the conduction time while the power switch (7) is in the turned-off position.

The induction heating cooktop (1) of the present invention comprises a current detection circuit (12) situated in the resonant circuit (6), connected in series to the induction coil (4), that converts the coil current (I_L) transferred from the induction coil (4) to the vessel (K) into voltage data in the non-conduction times wherein the power switch (7) is in the turned-on position and provides the coil current (I_L) to be monitored and the control unit (11) that determines whether or not the vessel (K) is present on the induction coil (4) or whether alignment of the vessel (K) on the induction coil (4) is appropriate by comparing the phase difference time (T) between the coil current (I_L), converted into voltage data received from the current detection circuit (12) and the resonance voltage (Vce) generated on the collector node (9) with a threshold phase difference time (T-threshold) recorded in its memory.

The control unit (11) detects the coil current (I_L) in the resonant circuit (6) as converted into voltage data by means of the current detection circuit (12), and calculates the phase difference time (T) between the coil current (I_L) and the resonance voltage (Vce) during the non-conduction times of the power switch (7) by comparing with the resonance voltage (Vce). The control unit (11) decides that the vessel (K) has been "slid" or "lifted" from over the induction coil (4) depending on the phase difference time (T) and interrupts current transmission to the induction coil (4).

In an embodiment of the present invention, the current detection circuit (12) comprises a current detection resistor (13) that is connected in series to the induction coil (4) in the resonant circuit (6) and that converts the coil current (I_L) into voltage data (Figure 2). The control unit (11) receives the voltage data relating to the coil current (I_L) from the terminals of the current detection resistor (13).

In another embodiment of the present invention, the current detection circuit (12) comprises a current transformer (14) connected in series to the induction coil (4) in the resonant circuit (6) and decreasing the coil current (I_L) to a level that can be detected by the control unit (11) and the current detection resistor (13) connected in parallel to the secondary side of the current transformer (14) (Figure 1).

In another embodiment of the present invention, the induction heating cooktop (1) comprises a first comparator (15) connected in parallel to the current detection resistor (13), generating digital "1" and "0" square wave output signals by detecting the zero crossings (ZC) of the coil current (I_L) and thus providing the coil current (I_L) to be monitored with digital data, a second comparator (16) connected to the collector node (9) and the DC-line at the outlet of the filter circuit (3), that compares the resonance voltage (Vce) with DC-line voltage (Vdc) and provides the resonance voltage (Vce) to be monitored independently from mains voltage fluctuations and a logical AND gate (17) that overlaps the output signals of the first comparator (15) and the second comparator (16) and sends as one signal to the control unit (11) as a single signal.

The first comparator (15) generates logical-1 output signals (S1) between the zero crossing (ZC) points of the coil current (I_L) in the non-conduction (turned-on) times of the power switch (7) when the induction coil (4) transfers energy to the vessel (K) (Figure 4).

The second comparator (16) generates logical-1 output signals (S2) in situations where the resonance voltage (Vce) is equalized with the DC-line voltage (Vdc) and turns to positive with respect to the DC-line voltage (Vdc) by comparing the resonance voltage (Vce) with the DC-line voltage (Vdc) (Figure 4).

The AND gate (17) generates logical-1 output signal (S3) in the case when the output signals (S1, S2) of both the first comparator (15) and the second comparator (16) are logical-1 (Figure 5). The duration of the logical-1 output signal of the AND gate (17) is equal to the phase difference time (T) of the coil current (I_L) and the resonance voltage (Vce).

The AND gate (17) generates logical-0 signal when the output signal of at least one of the first comparator (15) and the second comparator (16) is logical-0.

The control unit (11) determines that the vessel (K) is not present on the induction coil (4) or the vessel (K) is not appropriately aligned on the induction coil (4), in other words, that the vessel (K) is "slid" or "lifted", if the logical-1 output signal time (T) of the AND gate (17) is smaller than the threshold signal time (T-threshold) (T < T-threshold) recorded in its memory and interrupts the current of the induction coil (4).

In another embodiment of the present invention, the induction heating cooktop (1) comprises a first voltage divider (18) having resistors (R1, R2) connected in series to the collector node (9) and applying easily measurable, low level resonance voltage (Vce) to the second comparator (16) by dividing the resonance voltage (Vce) and a second voltage divider (19) having resistors (R3, R4) connected in series to the DC-line and applying easily measurable, low level DC voltage (Vdc) to the second comparator (16) by dividing the DC voltage (Vdc).

By means of the present invention, in induction heating cooktops (1) called "flexi-zone" or "multi-zone", having more than one induction coil (4), each driven by a single power switch (7), the presence or absence of the vessel (K) on the induction coil (4) and whether or not the vessel (K) is in the appropriate position is determined during the heating process and energy is transferred to the vessel (K) in a safe manner under variable mains voltage and temperature conditions. The power switch (7) and the other electronic circuit elements are prevented from being damaged.