Abstract:
Apparatus for providing electrical power to at least one inductor wherein the at least one inductor is disposed to inductively couple the electrical power to an electrically conductive object to heat the electrically conductive object by inducing electrical current to flow in the electrically conductive object, the apparatus comprising at least one inductor, at least one first half-bridge converter coupled across a DC bus and having an output coupled to a first terminal of the at least one inductor; and at least one second half bridge converter coupled across the DC bus and having an output coupled to a second terminal of the at least one inductor, each half bridge converter having first and second alternately conductive series connected switches connected between the DC bus, the at least one inductor being energized by alternately turning on respective ones of the first and second switches of each converter to cause current to flow from the DC bus alternatingly through the at least one inductor from the first and second converters.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit and priority of U.S. Provisional Patent Applications Ser. No. 60/506,189 filed Sep. 25, 2003 entitled “INDUCTION HEATING APPARATUS OPTIMIZED TO DRIVE MULTIPLE COILS IN A MATRIX CONFIGURATION”, the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   Present induction cooktop designs (mainly but not only for domestic use) implement a “concentrated hobs” topology: similar to gas cooktops, there are a number of main heating sources, typically four, (with different maximum power levels in general to simulate the different gas outlets) and the cooking utensil, e.g. a pan or pot, is heated only if put in the proper position above one of the heating sources, typically disposed under a glass cooktop plate. Each heating source or hob is realized by a coil which, driven by a series resonant converter running at several tens kHz, couples electromagnetic energy with the cooking utensil (which must have a metallic, e.g., ferromagnetic base), thus heating it by inducing electrical currents in the cooking utensil. By changing the frequency according to a reference power signal, the actual power delivered to the coil (and to the cooking utensil) is changed accordingly. 
   To work properly, the relative position between the coil and the utensil must be defined. Otherwise, electromagnetic coupling will be weak and the resonant converter will be overstressed without delivering the proper power to the utensil. 
   According to another concept, called the “distributed” induction heating concept, the utensil is to be heated independently from its position over the cooktop glass plate. This requires the use of a large number of smaller coils, whose number defines the “resolution” of the cooktop versus the size of the utensil, each coil driven by a properly sized adjustable high frequency power source, and an intelligent utensil detection system, which activates only those coils which are covered by the utensil. This poses the issue of how many electrical power sources are needed and how they should be connected to the coils. The number of electrical power sources and their connections to the coils must be made in accordance with the following requirements:
         a) The heating sources (hobs) must be activated in any possible subset configuration, except that in each subset the elements (coils) are contiguous to each other; the subset number may vary from 1 to a maximum. In fact, the utensil can be square or round or long or of any other shape.   b) Connections must be achieved with the minimum possible number of “switching” elements (arranged or not arranged in a switching matrix).   c) Each subset, constituted by “M” elements, must be independently controllable in power level.   d) The cost of the power conversion must be minimized.       

   The simplest way to achieve goals a) to c) is to have one power converter driving each coil; in this case no switching matrix is needed on the power side, and each converter would be optimized to drive the coil and to control it, and the coil power control could be simply realized through a low level reference signal network arranged in a switching matrix (for an N×N coil set, 2×N lines would be sufficient, for example, for ON/OFF control and another 2×N lines for fine power tuning) and controlled by a centralized controller. This solution has possibly never been described but it may be very expensive, so its industrial validity is questionable. 
   Another possible solution would be to use a single converter sized for the maximum whole cooktop power, while a switching matrix properly connects the various coils to the converter&#39;s outputs. 
   Since such a switching matrix should act directly on the power connections of the coils, very expensive high power relays or high power solid state switches would be needed. Such a solution, described for example, in U.S. Pat. No. 5,714,739 for the case of 4 hobs, has several drawbacks. In particular, such an optimized switching matrix (hence having a “minimum number of switches”) would put the coils in parallel to each other as the cooking utensil&#39;s size increases. This means the “equivalent” inductance seen by the converter would go down as the number of coils increases, (which also means a higher power level is required from the converter). In turn, this would result in an increase of the switching frequency of the converter as the power level increases, which is not acceptable. 
   Furthermore, to independently control the power of more subsets each composed by “m 1 ”, “m 2 ” . . . “mn” coil elements, low frequency PWM operation through the switching relays would be required, as described in previous patents, having the converter tuned to the overall load at the maximum power. However, this would mean the converter would be connected to a dynamically changing load, i.e., every time a number of relays is commutated to select a new load or simply to PWM an activated load, the equivalent load seen by the converter would change. 
   The relays in such a design would have a very limited lifetime, due to the enormous number of operations required (for example, switching at a period of 2.5 sec, and considering an average cooktop utilization of 4 hours/day×365 days/year×10 years, the relays would be required to perform 30 million operations). 
   The number of subsets of independent coils would reduced in such a system. It can be shown that only the elements above (or below) the center diagonal of the rectangular matrix of coils may be independently controlled through the action of the relays, and, in fact, not in all cases. If the elements above the center diagonal are independently controllable, the elements below the center diagonal of the matrix would receive a non controllable amount of power; i.e., with 2×N relays, for a square N×N matrix, a maximum of 2N−1 elements can be independently controlled. In a rectangular matrix, the number of independently controllable coils is N+M−1, where N+M equal the total number of converters (N=number of columns, M=number of rows). 
   SUMMARY OF THE INVENTION 
   The above and other problems are solved by the present invention. According to the invention, an induction heating apparatus is provided comprising a plurality of converters, without the need of any relays or solid state relay switches, connected to each other and to series resonant circuits each comprising a coil and a resonant capacitor, such that, by switching on any two of the converters, a single coil, at the cross point between them, is activated. 
   The invention, according to one aspect, comprises an apparatus for providing electrical power to at least one inductor wherein the at least one inductor is disposed to inductively couple the electrical power to an electrically conductive object to heat the electrically conductive object by inducing electrical current to flow in the electrically conductive object, the apparatus comprising at least one inductor, at least one first half-bridge converter coupled across a DC bus and having an output coupled to a first terminal of the at least one inductor; and at least one second half bridge converter coupled across the DC bus and having an output coupled to a second terminal of the at least one inductor, each half bridge converter comprising first and second alternately conductive series connected switches connected between the DC bus, the at least one inductor being energized by alternately turning on respective ones of the first and second switches of each converter to cause current to flow from the DC bus alternatingly through the at least one inductor from the first and second converters. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
       FIG. 1  is a block diagram of a portion of an induction heating apparatus according to the invention. 
       FIG. 2  is a schematic diagram of two of the plurality of converters of  FIG. 1  showing their connections to an induction heating coil element; 
       FIG. 3  is a timing diagram illustrating phase control showing how power to an induction heating element may be controlled; and 
       FIG. 4  shows one arrangement of the induction heating apparatus according to the invention; and 
       FIG. 5  is a block diagram showing another arrangement of the induction heating apparatus according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to the drawings,  FIG. 1  shows a block diagram of the invention (only limited to 6 coil-capacitor networks for sake of clarity). 
   The arrangement includes a plurality of connectors n 1  to nN and m 1  to mM and mm. Each converter (n 1  to nN and m 1  to mM) is of the half-bridge type, and is operated at a fixed frequency near the resonant frequency of the network constituted by a single coil L 12 , L 13  . . . LMN (with its equivalent cooking utensil resistance in parallel not shown) and the resonant capacitor C in series. Although the converters are shown arranged in rows (m) and columns (n), this need not be the case. Any arrangement can be employed, and in fact, the coils are preferably arranged and connected to the converters in such a way as described later to provide the maximum permissible independent control of the coils via the N+M converters. 
   Each resonant network is connected between two converters, in such a way that a full bridge configuration is provided with the resonant network coupled into the full bridge between the half bridges. 
   Each pair of converters is then operated at constant frequency, but in phase shift between each other, so that the power delivered to a coil may be changed according to a reference power control signal. 
   A low level signal network, issuing an ON/OFF command and a power reference level, will be provided by a central controller unit (not shown in  FIG. 1 ) to each of the converters. 
   Such a network will comprise a number of (double) lines which is equal to the number of converters N+M (such network is not shown in  FIG. 1 ). 
   When operating more than one converter at the same time, even if they are contiguous to each other, each converter will “see” its own series resonant circuit, and will not interfere with any other converter. There are no coils being placed in parallel to each other thereby not changing the resonant circuit seen by any converter, as in the prior art solutions. 
   To better explain what a single coil “sees” when activated, and how the phase shift between the two converters, e.g., (one in a column and the other in a row) is able to change the power level of the coil at the cross point between two converters,  FIG. 2  show the equivalent schematic diagram of two converters operating to control the power in a single coil L, while  FIG. 3  show the high frequency PWM strategy implemented in the two converters, showing the phase shift between the two. 
   In  FIG. 2 , S 1  and S 2 , together with D 1  and D 2 , C 2  and C 3 , and V 2  and V 3 , represent any one of the converters of the switching matrix, while S 3  and S 4 , together with D 3  and D 4 , C 6  and C 7 , and V 4  and V 5 , represent any other converter of the switching matrix. 
   R 2 , R 3 , R 7  and R 8  are dummy resistances which represent the equivalent series resistance of the switches S 1 , S 2 , S 3 , S 4 , while V 1  is a DC voltage which represents the rectified AC line (not necessarily filtered). 
   TX 1 , R 1  and C 4 , together with R 4 , represents the model of the resonant circuit comprising the series resonant capacitor C 4  in series with the coil, shown as a transformer TX 1  representing the primary coil L and the secondary coil P and its resistance R 1  (comprising the equivalent circuit of the cooking utensil being heated.) 
   Since S 1  and S 2  are operated at constant frequency, with 50% duty cycle, V 2  and V 3  are simply 180° phase shifted between each other, as are V 4  and V 5 . The coil&#39;s power control variable is the phase shift between V 2  and V 5 , for example, and between V 3  and V 4  (which is the same). 
     FIG. 3  shows one example of the phase arrangement of voltages V 2 ( 3 A), V 3 ( 3 B), V 4 ( 3 C) and V 5 ( 3 D), each at 25 kHz where V 4  is 90° out of phase with V 3  and V 5  is 90° out of place with V 2 . This results in half the full power being provided to the coil, as shown by the corresponding hatched lines. 
   For 0° phase shift between V 2  and V 5 , and V 3  and V 4 , the power delivered to the coil will be maximum. For 180° phase shift between V 2  and V 5 , and V 3  and V 4 , the power will be zero. This is shown in  FIG. 3E , where V 5  is shown with a 180° phase shift from V 2  and in  FIG. 3F , where V 4  is shown with a 180° phase shift from V 3 . In both cases, the pulses do not overlap so no power is delivered to the coil. For intermediate values of the phase shift, intermediate values of the coil&#39;s maximum power will be obtained. 
   The operation of each series resonant converter is such that the operating frequency must be above the resonant frequency of the series resonant network, otherwise excessive overstress will be applied to the switches. 
   The zero power condition may be achieved by switching off one like switch (high side or low side) of each converter, by 180° phase shift as described above or by switching off both devices of one of the pair of converters (three-state condition). 
   In case of multiple coils being activated at the same time, it is sufficient to activate the converter pairs connected to those coils. 
   Each converter or the converter pair will be driven with a different phase delay with respect to the other converters of the pair depending on the power level selected for the particular coil. 
   From  FIG. 1 , it could appear the converters are connected to the coil in a standard matrix configuration. Such a configuration can present some of the drawbacks already described for the prior art solutions unless the coils are arranged so as to optimize their independent control. 
   An example, with reference to  FIG. 4 , will better explain this. 
   Suppose the converters are connected in a standard matrix configuration (M rows and N columns) and n 1 , n 2  . . . nN define the converters in the columns and m 1 , m 2  . . . mM defines the converters in the rows. 
   N and M can be any number above 1.  FIG. 4  shows converters n 1  to nN in columns and converters m 1  to M in rows. However, only coils above the center diagonal are shown. 
   Also suppose the following pairs of converters are activated:n 2  and m 1  energizing coil L 21 , n 4  and m 1  energizing coil L 41 , n 2  and m 4  energizing coil L 24 , n 4  and m 4  energizing coil L 44 . 
   Suppose further that each coil requires a different power level: P 1 , P 2 , P 3  and P 4 , that is, coil L 21  receives power P 1 , and L 41  receives power P 2 , and L 24  receives power P 3  and coil L 44  receives power P 4 , where each coil L is in the form Lnm. 
   Now, to deliver power P 1  to L 21 , shown in  FIG. 4 , converters n 2  and m 1  will be activated with a proper phase shift Phn 2 −m 1  between each other. 
   For sake of clarity, a “virtual” zero phase is defined. Converter n 2  will have phase Phn 2  and converter m 1  will have phase Phm 1 . The coil network will see the difference Phn 2 −Phm 1 =Phn 2 −m 1 . 
   Then, to deliver power P 3  to the coil network at the cross point between n 2  and m 4 , converter n 2  is already activated, so it will be only necessary to activate converter m 4 , with a phase Phm 4  such that Phn 2 −Phm 4  will provide the needed power P 3 . 
   Again, to deliver power P 2  to the coil network at the cross point between n 4  and m 1 , m 1  already being ON, it will be sufficient to activate converter n 4 , with a phase Phn 4  such that Phn 4 −Phm 1  provides the power P 2 . 
   If we now try to control also coil L 44  to provide power P 4  to it independently, we discover that this is not possible. In fact, converters n 4  and m 4  are already activated and their phase shift is fixed by the power they need to deliver to the other coil networks L 24  and L 41 . 
   So, similar to the case of a relay switching matrix, a first limit to this topology is that only the upper (or lower) semi-diagonal matrix of coils has elements whose power may be independently selected, and not in all cases. This is shown in  FIG. 4 . In this figure, only the coils above the semi-diagonal are shown (with the exception of coil L 44 , which is shown, and it can be observed that only the coils above the semi-diagonal can be independently controlled, but not in all cases. Coil L 44 , below the semi-diagonal cannot be independently controlled. If coils L 11 , L 21  and L 14  are activated, coil L 24  also can not be independently activated, for example, even though it also is above the semi-diagonal. 
   To show this further limitation, suppose only the upper semi-diagonal is actually wired and N=M=5. Also suppose 4 adjacent coils must be activated to heat a large pot or pan, and those coils are on the upper left corner of the cook-top, that is to say, activated by column converters n 1  and n 2  and row converters m 1  and m 2 . This would comprise coils L 11 , L 12 , L 21  and L 22  of  FIG. 4 . 
   At this point, we also want to heat a smaller pan, which may be heated by a single coil-driven by converter n 1  and m 4 . 
   Since n 1  and n 2  are active converters, not only the coil at the cross point between n 1  and m 4  will be activated, but also the adjacent coil (the one at the cross point between n 2  and m 4 ). Since N=M=5, L 14  as well as coil L 24  are above the semi-diagonal. See  FIG. 4 . 
   The theoretical number of coil networks which can be truly independently controlled in a rectangular matrix of N by M converters is only N+M−1. 
   To reduce the impact of these limitations, another concept has to be used: the concept of “adjacent coils”. The basic idea is that adjacent coils cannot have the need to be driven at different power levels. 
   While this seems a limitation (and it is from a purely theoretical point of view) it is not from a practical point of view, because:
         a) in the case of large cooking utensils, covering more than one coil, adjacent coils share, of course, the same power level because they are heating the same utensil.   b) in the case of small utensils, only covering one coil, adjacent coils should simply not be activated at all.       

   At this point a modified switching matrix can be introduced, which takes advantage of the “adjacent coils” concept. 
   Such a matrix is schematically shown in  FIG. 5  in a tree and branch structure. 
   The blocks in  FIG. 5  represents the converters, the circles represent the coils. 
   An example of 6 converters is shown. It can be shown that the maximum number of coils which may be driven by 6 converters is 15. Each block (converter) may drive up to 5 coils (circles). 
   The formula (if the principle of “adjacent coils” is applied) is:
 
 N coils= N converter*( N converter−1)/2, where  N converter is the total number of converters
 
   Out of the 15 coils shown in  FIG. 5 , only 5 are actually truly independent from each other according to the formula 2N−1 for a square matrix or N+M−1 for a rectangular matrix, where N and M represent the number of columns and rows respectively. By proper wiring of the connections, however, the non-independent coils can be placed “adjacent” each other and adjacent to an independent coil in a way such that a small cooking utensil which needs a single coil can be independently controlled, while a bigger pan which covers several adjacent coils will require adjacent coils that are not independent to be activated but with the same power level. 
   If the “adjacent coils” principle is not applied, the general formula for the number of coils that can be driven truly independently is 2N−1 for a square matrix or N+M−1 for a general rectangular matrix, as discussed above. 
   Another benefit of the switching matrix of  FIG. 5  is that every converter is attached to an equal number of coils, so its power loading (which affects its size and cost) is the same. 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention should be limited not by the specific disclosure herein, but only by the appended claims.