Method for operating a frequency converter circuit

Abstract

A frequency converter circuit has at least two outputs that are respectively connected to a load, in particular an induction coil. A first output is operated at a first switching frequency and a second output is simultaneously operated at a second switching frequency that is different from the first, in such a way that noise having a frequency generated by the superposition of the first switching frequency and the second switching frequency is produced. The frequency converter circuit is operated in such a way that the frequency of the noise is lower than a first cutoff frequency and/or is higher than a second cutoff frequency.An induction cooking device includes first and second induction heating elements, and a frequency converter circuit including first and second power outputs electrically coupled to the first and second induction heating elements, respectively. The frequency converter circuit generates first and second power output signals with first and second switching frequencies applied to the first and second power outputs, respectively. The second switching frequency is selected to be no greater than a first cut-off frequency and no less than a second cut-off frequency (e.g., higher than the first cut-off frequency). The induction cooking device regulates electrical power applied to the first and/or second outputs by adjusting the switching frequency and/or a duration during which at least one of the first and second switching frequencies are applied in a repeating cycle during which the electrical power applied to the first and/or second outputs.

1. A method of operating a frequency converter circuit having at least two outputs respectively connected to a load, the method which comprises:
 operating a first output at a first switching frequency and simultaneously operating a second output at a second switching frequency different from the first switching frequency to produce noise having a frequency generated by a superposition of the first switching frequency and the second switching frequency; 
 operating the converter circuit to set the first switching frequency and the second switching frequency such that the frequency of the noise is lower than a first cut-off frequency or higher than a second cut-off frequency, the second cut-off frequency being higher than the first cut-off frequency; and 
 regulating an electrical power of at least one of the first and second outputs by adjusting the switching frequency and the relative switch-on time. 
 
     
     
       2. The method according to  claim 1 , wherein the load is an induction coil. 
     
     
       3. The method according to  claim 1 , which comprises determining the first cut-off frequency and/or the second cut-off frequency in dependence on a level of the noise. 
     
     
       4. The method of  claim 3 , further comprising the step of estimating a level of the noise using the first and second switching frequencies of the first and second outputs and the electrical power supplied to the loads. 
     
     
       5. The method according to  claim 1 , which comprises determining the first cut-off frequency and/or the second cut-off frequency in dependence on a total electrical power of the outputs. 
     
     
       6. The method according to  claim 1 , which comprises setting the first cut-off frequency at 2 kilohertz and/or setting the second cut-off frequency at 14 kilohertz. 
     
     
       7. The method of  claim 1 , wherein:
 the first switching frequency is determined according to the required electrical power; 
 the relative switch-on time of 0.5 is used for the first output; and 
 the electrical power of the second load is adjusted using the relative switch-on time and the second switching frequency, and taking into account the first and second cut-off frequencies. 
 
     
     
       8. The method of  claim 1 , wherein the frequency of the noise corresponds to a difference between the second switching frequency and the first switching frequency. 
     
     
       9. A cooktop including:
 a pair of induction coils; and   a frequency converter circuit having a first output connected to a first of the pair of induction coils and a second output connected to a second of the pair of induction coils, the pair of induction coils being either individually or simultaneously operable,   the first output being associated with a first switching frequency and the second output being associated with a second switching frequency, wherein, if the first and second switching frequencies are different from one another, noise having a noise frequency is generated when the pair induction coils is operated simultaneously,   wherein the first switching frequency and the second switching frequency are set or configured to have values such that the noise frequency is either above or below a human audibility threshold range associated with first and second cut-off frequencies, the second cut-off frequency being higher than the first cut-off frequency, and   wherein, when the first switching frequency is set for a user operated power setting and a selected switch-on time, the second switching frequency is selected to be in a range of frequencies extending from the first switching frequency to a sum of the first switching frequency and the first cut-off frequency, and/or above a sum of the first switching frequency and the second cut-off frequency, the second switching frequency being associated with the selected switch-on time.   
     
     
       10. The cooktop of claim 9, wherein the switch-on time is 0.5. 
     
     
       11. A cooktop including:
 a pair of induction coils; and   a frequency converter circuit having a first output connected to a first induction coil of the pair of induction coils and a second output connected to a second induction coil of the pair of induction coils, the pair of induction coils being either individually or simultaneously operable,   the first output being associated with a first switching frequency and the second output being associated with a second switching frequency, wherein noise having a noise frequency is generated by superposition of the first switching frequency and the second switching frequency when the pair induction coils is operated simultaneously;   wherein the first switching frequency and the second switching frequency are set to have values such that the noise frequency is either above or below a human audibility threshold range associated with first and second cut-off frequencies, the second cut-off frequency being higher than the first cut-off frequency, and   wherein the frequency converter circuit is configured to control the first induction coil to operate at a first switching frequency for a user operated power setting at a selected switch-on time of 0.5, and the frequency converter circuit is configured to determine the second switching frequency along the selected switch-on time and taking into account the first and second cut-off frequencies.   
     
     
       12. The cooktop of claim 11, wherein the second switching frequency is (1) equal to a sum of the first switching frequency and the first cut-off frequency, and or (2) above a sum of the first switching frequency and the second cut-off frequency. 
     
     
       13. A cooktop including:
 a pair of induction heating units; and   a frequency converter circuit having a first output connected to a first induction heating unit of the pair of induction heating units and a second output connected to a second induction heating unit of the pair of induction heating units, the pair of induction heating units being either individually or simultaneously operable,   the first output being associated with a first switching frequency and the second output being associated with a second switching frequency different from the first switching frequency in which case noise having a noise frequency is generated by superposition of the first switching frequency and the second switching frequency when the pair induction heating units is operated simultaneously;   wherein the frequency converter circuit is configured to set or determine the first switching frequency and the second switching frequency such that the noise frequency is generally outside a human audibility threshold range, or only insignificantly within it, during simultaneous operation,   wherein the frequency converter circuit is configured to establish the first switching frequency for a first user selected power setting and a selected switch-on time, and to establish the second switching frequency for a second user selected power setting for the selected switch-on time, and wherein the second switching frequency is (1) substantially equal to the first switching frequency, and/or (2) equal to or above a sum of the first switching frequency and an upper cut-off frequency associated with the threshold range.   
     
     
       14. The cooktop of claim 13, wherein the threshold range is associated with a lower cut-off frequency. 
     
     
       15. The cooktop of claim 14, wherein the upper cut-off frequency is higher than the lower cut-off frequency. 
     
     
       16. The cooktop of claim 13, wherein the switch-on time is 0.5. 
     
     
       17. The cooktop of claim 13, wherein each of the first and second heating elements comprises a coil. 
     
     
       18. The cooktop of claim 13, not including a multiplexing unit. 
     
     
       19. The cooktop of claim 13, not including a choking coil. 
     
     
       20. The cooktop of claim 13, further comprising a first power switch and a second power switch associated, respectively, with the first and second inductive heating units. 
     
     
       21. The cooktop of claim 13, herein the first and second switching frequencies lie above a maximum frequency that can be perceived by a human. 
     
     
       22. A converter circuit for an induction cooking device having first and second induction coils, the converter circuit comprising:
 an inlet connectable to a voltage source;   a first circuit having a high frequency switch converting power from the voltage source to a first high frequency output power applied to the first induction coil, wherein the first high frequency output power has a frequency (f1) corresponding to a switching frequency of the first high frequency switch;   a second circuit having a high frequency switch converting power from the voltage source to a second high frequency output power applied to the second induction coil, wherein the second high frequency output power has a frequency (f2) corresponding to a switching frequency of the second high frequency switch,   wherein the difference between the frequencies f2 and f1 is no greater than a predetermined lower threshold and is no less than a predetermined higher threshold, and   wherein the converter circuit regulates the power of the second high frequency output power by selecting the frequency f2 and a duration during which the second high frequency output power has the frequency f2 in a repeating cycle applied to the second high frequency output power.   
     
     
       23. An induction cooking device comprising:
 a first induction heating element;   a second induction heating element;   a frequency converter circuit including a first power output electrically coupled to the first induction heating element and a second power output electrically coupled to the second induction heating element;   the frequency converter circuit generating a first power output signal applied to the first power output, wherein the first power output signal has a first switching frequency;   the frequency converter circuit generating a second power output signal applied to the second power output, wherein the second power output signal has a second switching frequency;   wherein the second switching frequency is selected by the induction cooking device to be no greater than a first cut-off frequency and no less than a second cut-off frequency, wherein the second cut-off frequency is higher than the first cut-off frequency, and   wherein the induction cooking device regulates an electrical power applied to at least one of the first and second outputs by adjusting the switching frequency and a duration during which at least one of the first and second switching frequencies are applied in a repeating cycle during which the electrical power applied to the at least one of the first and second outputs.   
     
     
       24. A cooktop including:
 a pair of induction coils; and   a frequency converter circuit having a first output connected to a first of the pair of induction coils and a second output connected to a second of the pair of induction coils, the pair of induction coils being either individually or simultaneously operable,   the first output being associated with a first switching frequency and the second output being associated with a second switching frequency, wherein, if the first and second switching frequencies are different from one another, noise having a noise frequency is generated when the pair induction coils is operated simultaneously,   wherein the first switching frequency and the second switching frequency are set or configured to have values such that the noise frequency is either above or below a human audibility threshold range associated with first and second cut-off frequencies, the second cut-off frequency being higher than the first cut-off frequency, and   wherein, when the first switching frequency is set for a user operated power setting and the second switching frequency is selected to be in a range of frequencies extending from the first switching frequency to a sum of the first switching frequency and the first cut-off frequency, and/or above a sum of the first switching frequency and the second cut-off frequency, the second switching frequency being associated a selected duration during which the second switching frequency is applied to second output.   
     
     
       25. A cooktop including:
 a pair of induction heating units; and   a frequency converter circuit having a first output connected to a first induction heating unit of the pair of induction heating units and a second output connected to a second induction heating unit of the pair of induction heating units, the pair of induction heating units being either individually or simultaneously operable,   the first output being associated with a first switching frequency and the second output being associated with a second switching frequency different from the first switching frequency in which case noise having a noise frequency is generated by superposition of the first switching frequency and the second switching frequency when the pair induction heating units is operated simultaneously;   wherein the frequency converter circuit is configured to set or determine the first switching frequency and the second switching frequency such that the noise frequency is generally outside a human audibility threshold range, or only insignificantly within it, during simultaneous operation,   wherein the frequency converter circuit is configured to establish the first switching frequency for a first user selected power setting and to establish the second switching frequency for a second user selected power setting, wherein the second switching frequency being associated a selected duration during which the second switching frequency is applied to second output, and   wherein the second switching frequency is (1) substantially equal to the first switching frequency, and/or (2) equal to or above a sum of the first switching frequency and an upper cut-off frequency associated with the threshold range.