Abstract:
In a first aspect, an LLC resonant converter is provided for driving a plurality of output circuits from a DC input signal. The LLC resonant converter includes: (a) an inverter circuit for converting the DC input signal to a square-wave signal; (b) an inductor network coupled to the inverter circuit; and (c) a plurality of transformers, each transformer including a primary winding and a secondary winding. The primary windings of the transformers are coupled in series, and the series-coupled primary windings are coupled in parallel with the inductor network. The secondary winding of each transformer is coupled to and provides a current to a corresponding one of the output circuits. The secondary winding currents are substantially equal, and power is processed by a single transformer between the DC input signal and each output circuit. Numerous other aspects are also provided.

Description:
BACKGROUND 
     This invention relates generally to power conversion systems. More particularly, this invention relates to LLC resonant converter circuits that include multiple transformers for providing substantially matched currents to multiple output loads. 
     In some electronic circuit applications, to reduce size and weight and to minimize cost, a single power supply may be used to supply matched currents to multiple load circuits. For example, in a light-emitting diode (“LED”) television, a single LLC resonant converter may be used to drive multiple LED strings, where each LED string includes multiple series-connected LEDs, with the same current supplied to each LED string. 
     One such previously known LLC resonant converter circuit, referred to as multi-transformer LLC resonant converter  10 , is illustrated in  FIG. 1 . Multi-transformer LLC resonant converter  10  includes inverter  14 , resonant capacitor  15 , transformers  16   1  and  16   2 , and rectifier/filter circuits  18   1  and  18   2 , respectively, and provides substantially equal output currents I 1  and I 2  to LED strings  12   1  and  12   2 , respectively. Transformer  16   1  includes leakage inductance L s1  and magnetizing inductance L p1 , and transformer  16   2  includes leakage inductance L s2  and magnetizing inductance L p2 . Primary windings P 1  and P 2  of transformers  16   1  and  16   2 , respectively, are coupled together in series, and the series-coupled primary windings are coupled to inverter  14  via resonant capacitor  15 . 
     For high efficiency, LLC resonant converters are typically operated using primary-side zero voltage switching (“ZVS”), which requires large magnetizing currents I 1p1  and I 1p2 . Indeed, I 1p1  and I 1p2  may be a large fraction of primary currents I p1  and I p2 . To supply matched output currents I 1  and I 2  to LED strings  12   1  and  12   2 , primary currents I p1  and I p2  must be matched. As a result, to provide substantially equal primary currents I p1  and I p2 , magnetizing currents I 1p1  and I 1p2  must be substantially equal, which requires that the tolerance of magnetizing inductances L p1  and L p2  must be impractically small. 
     One previously known LLC resonant converter circuit, referred to as LLC resonant converter  10 ′, that attempts to solve this problem is illustrated in  FIG. 2 . In particular, LLC resonant converter  10 ′ includes an additional transformer  16   3  between inverter  14  and resonant capacitor  15  and transformers  16   1 ′ and  16   2 ′. In this circuit, a single magnetizing inductance L p3  of transformer  16   3  provides the necessary shunt inductance of the LLC resonant converter. As a result, the magnetizing inductances of transformers  16   1 ′ and  16   2 ′ can be made very large, which renders the effect of any magnetizing inductance tolerances insignificant. 
     However, the circuit of  FIG. 2  has several significant disadvantages. First, the power from the DC input Vin to each output must be processed through two transformer stages, which degrades efficiency. In addition, because transformer  16   3  must be sized for the full output power, the size and material cost of transformer  16   3  are substantial. 
     Accordingly, improved LLC resonant converter circuits for driving multiple loads with substantially matched output currents are desirable. 
     SUMMARY 
     In a first aspect of the invention, an LLC resonant converter is provided for driving a plurality of output circuits from a DC input signal, the converter including: (a) an inverter circuit for converting the DC input signal to a square-wave signal; (b) an inductor network coupled to the inverter circuit; and (c) a plurality of transformers, each transformer including a primary winding and a secondary winding. The primary windings of the transformers are coupled in series, and the series-coupled primary windings are coupled in parallel with the inductor network. The secondary winding of each transformer is coupled to and provides a current to a corresponding one of the output circuits. The secondary winding currents are substantially equal, and power is processed by a single transformer between the DC input signal and each output circuit. 
     In a second aspect of the invention, an LLC resonant converter is provided for driving a plurality of output circuits from a DC input signal, the converter including: (a) an inverter circuit for converting the DC input signal to a square-wave signal; (b) an inductor network coupled to the inverter circuit; and (c) a plurality of transformers coupled to the inductor network, each transformer including a primary winding, and a secondary winding. The primary windings of the transformers are coupled in series, and the secondary winding of each transformer is coupled to and provides a current to a corresponding one of the output circuits. The secondary winding currents are substantially equal, and the inductor network is separate from the plurality of transformers. 
     In a third aspect of the invention, a method is provided for driving a plurality of output circuits from a DC input signal, the method including providing an LLC resonant converter having: (a) an inverter circuit for converting the DC input signal to a square-wave signal; (b) an inductor network coupled to the inverter circuit; and (c) a plurality of transformers, each transformer including a primary winding and a secondary winding. The primary windings of the transformers are coupled in series, and the series-coupled primary windings are coupled in parallel with the inductor network. The secondary winding of each transformer is coupled to and provides a current to a corresponding one of the output circuits. The secondary winding currents are substantially equal, and power is processed by a single transformer between the DC input signal and each output circuit. The method further includes driving the plurality of output circuits with the LLC resonant converter. 
     Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention can be more clearly understood from the following detailed description considered in conjunction with the following drawings, in which the same reference numerals denote the same elements throughout, and in which: 
         FIG. 1  is a block diagram of a previously known multiple-transformer LLC resonant converter circuit; 
         FIG. 2  is a block diagram of an alternative previously known multiple-transformer LLC resonant converter circuit; 
         FIG. 3  is a block diagram of an exemplary multiple-transformer LLC resonant converter circuit in accordance with this invention; 
         FIG. 4  is a more detailed block diagram of an exemplary multiple-transformer LLC resonant converter circuit in accordance with this invention; 
         FIG. 5  is a block diagram of an alternative exemplary multiple-transformer LLC resonant converter circuit in accordance with this invention; 
         FIG. 6  is a block diagram of another alternative exemplary multiple-transformer LLC resonant converter circuit in accordance with this invention; 
         FIG. 7  is a diagram of an exemplary integrated inductor network for use in multiple-transformer LLC resonant converter circuits in accordance with this invention; and 
         FIGS. 8A-8E  are block diagrams of exemplary rectification configurations for use with multiple-transformer LLC resonant converter circuits in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION 
     In some embodiments, a multiple-transformer resonant converter circuit in accordance with this invention uses a shunt inductor that is separate from the transformers of the circuit. As a result, the magnetizing inductance of each transformer in the circuit can be made very large, thus minimizing the effect of magnetizing inductor mismatches between transformers. Some exemplary embodiments of this invention may also use a series resonant inductor that is separate from the transformers of the circuit. In such embodiments, the leakage inductance of each transformer can be made very small, and approximately “ideal” transformers may be used (e.g., transformers that have approximately zero leakage inductance and infinite magnetizing inductance). In exemplary embodiments of this invention, the separate series resonant inductor and/or shunt inductor may be discrete inductors. In alternative exemplary embodiments of this invention, the separate series resonant inductor and shunt inductor may be integrated inductors. LLC resonant converter circuits in accordance with this invention may be used to drive one, two, or more LED strings (or any other suitable loads) per transformer. 
     Referring to  FIG. 3 , a block diagram of an exemplary multiple-transformer LLC resonant converter in accordance with this invention is described. In particular, exemplary multiple-transformer LLC resonant converter  100  includes inverter  14 , resonant capacitor  15 , inductor network  110 , transformers  116   1 ,  116   2 , . . . ,  116   N , and rectifier/filter circuits  18   1 ,  18   2 , . . . ,  18   N . Any number of transformers and rectifier/filters may be used to drive a corresponding number of loads. Inverter  14  may be a half-bridge inverter as shown in  FIG. 1 , or may be a full-bridge inverter, or other similar circuit, as is known in the art. Under the operation of control circuitry (not shown), inverter  14  converts DC input signal Vin to a square-wave output signal at output nodes VIP and VIN. Inductor network  110  has input terminals coupled to resonant capacitor  15  and inverter output nodes VIP and VIN, and provides an output signal at output nodes VDP and VDN. 
     Transformers  116   1 ,  116   2 , . . . ,  116   N  each have primary windings and secondary windings (not shown), and convert their primary currents I p1 , I p2 , . . . , I pN  to secondary currents I s1 , I s2 , . . . , I sN , respectively. The primary windings of transformers  116   1 ,  116   2 , . . . ,  116   N  are coupled together in series, and the series-coupled primary windings are coupled in parallel across inductor network output nodes VDP and VDN. As described in more detail below, secondary currents I s1 , I s2 , . . . , I sN  are substantially equal to one another. 
     Rectifier/filter circuits  18   1 ,  18   2 , . . . ,  18   N  produce DC output voltages V 1 , V 2 , . . . , V N , respectively, and supply output currents I 1 , I 2 , . . . , I N , respectively, to load circuits  12   1 ,  12   2 , . . . ,  12   N , respectively. Exemplary rectifier/filter circuits  18   1 ,  18   2 , . . . ,  18   N  are described in more detail below. Load circuits  12   1 ,  12   2 , . . . ,  12   N  may be LED strings, or may be any other load circuits. In accordance with this invention, output currents I 1 , I 2 , . . . , I N  are substantially equal to one another. 
     Referring now to  FIG. 4 , an exemplary multiple-transformer LLC resonant converter  100   a  in accordance with this invention is described that includes a shunt inductor separate from the transformers of the circuit. In particular, multiple-transformer LLC resonant converter  100   a  includes inductor network  110   a  and series-coupled transformers  116   a   1  and  116   a   2 , and produces output voltages V 1  and V 2  to load circuits  12   1  and  12   2 . In general, any number of transformers, rectifier/filters and loads may be used. 
     Inductor network  110   a  includes a first inductor L pd  coupled to resonant capacitor  15  and inverter output nodes VIP and VIN and series-coupled transformers  116   a   1  and  116   a   2 . Transformers  116   a   1  and  116   a   2  include leakage inductances L s1  and L s2 , respectively, and also include primary windings P 1  and P 2 , respectively, which are coupled together in series. 
     Unlike previously known LLC resonant converters of  FIGS. 1 and 2 , however, first inductor L pd  is a discrete inductor that is separate from transformers  116   a   1  and  116   a   2 . First inductor L pd  functions as a shunt inductor for multiple-transformer LLC resonant converter  100   a.  As a result, by using a separate shunt inductor (first inductor L pd ), the magnetizing inductances of transformers  116   a   1  and  116   a   2  (not shown in  FIG. 4 ) can be made large to reduce the difference between primary currents I p1  and I p2 , and thus reduce the difference between output currents I 1  and I 2 . 
     The inductance of first inductor L pd  depends on such factors as the voltage gain, quality factor, and switching frequency of the LLC resonant converter. In some embodiments, the inductance of first inductor L pd  may range from about 300 μH to about 1 mH, although other values may be used. 
     Unlike the previously known multiple-transformer LLC resonant converter  10  of  FIG. 1 , secondary winding currents I s1  and I s2  (and output currents I 1  and I 2 ) can be made to substantially equal one another without having to match transformer magnetizing inductances. In addition, such current matching is substantially independent of the relative values of the transformer magnetizing inductances. Further, power from DC input Vin to each output  12   1  and  12   2  is processed through a single transformer stage per output stage. Thus, the efficiency of multiple-transformer LLC resonant converter  100   a  is likely to be greater than that of conventional LLC resonant converter  10 ′. 
     Referring now to  FIG. 5 , a block diagram of an alternative exemplary multiple-transformer LLC resonant converter  100   b  in accordance with this invention is described that includes a shunt inductor L pd  and a series resonant inductor L sd  that are both discrete inductors that are separate from the transformers of the circuit. In particular, multiple-transformer LLC resonant converter  100   b  includes inductor network  110   b  and series-coupled transformers  116   b   1  and  116   b   2 . 
     Inductor network  110   b  includes a first inductor L pd  coupled to resonant capacitor  15  and inverter output nodes VIP and VIN and series-coupled transformers  116   b   1  and  116   b   2 , and a second inductor L sd  coupled in series between inverter output node VIP and inductor network output node VDP. Transformers  116   b   1  and  116   b   2  include primary windings P 1  and P 2 , respectively, which are coupled together in series. Transformers  116   b   1  and  116   b   2  have very small leakage inductances (not shown) and very large magnetizing inductances (not shown), and in this regard may approximate “ideal” transformers. Other numbers of transformers, rectifier/filters and loads may be used. 
     First inductor L pd , and second inductor L sd  are the shunt inductor and series resonant inductor, respectively, of multiple-transformer LLC resonant converter  100   b . Unlike previously known LLC resonant converters of  FIGS. 1 and 2 , however, first inductor L pd  and second inductor L sd  are both separate from transformers  116   b   1  and  116   b   2 . By using a series resonant inductor and shunt inductor that are separate from the transformer components, each magnetic component can be individually controlled and the circuit performance can be optimized. For example, converter  100   b  permits very precise control over the ratio L pd /L sd , which facilitates optimization of the LLC resonant converter. 
     The inductances of first inductor L pd  and second inductor L sd  depend on such factors as the voltage gain, quality factor, and switching frequency of the LLC resonant converter. In some embodiments, the inductance of first inductor L pd  may range from about 300 μH to about 1 mH, and the inductance of second inductor L sd  may range from about 50 μH to about 200 μH, although other values may be used. 
     Referring now to  FIG. 6 , a block diagram of another alternative exemplary multiple-transformer LLC resonant converter is described that includes integrated shunt and series resonant inductors that are both separate from the transformers of the circuit. In particular, multiple-transformer LLC resonant converter  100   c  is similar to LLC resonant converter  100   b  of  FIG. 5 , but includes an inductor network  110   c  that includes a first inductor L pi  and a second inductor L si  that are integrated on a single magnetic core, such as an EE- or EI-core. For example,  FIG. 7  illustrates an exemplary magnetic core  120  in which first inductor L pi  is wound on the center leg  122  of core  120 , and second inductor L si  is wound in two sections on the outer legs  124   a  and  124   b  of core  120 . 
     First inductor L pi  and second inductor L si  are connected in series, with the polarity selected so that flux developed by the windings in center leg  122  cancel flux developed by the windings in outer legs  124   a  and  124   b.  Persons of ordinary skill in the art will understand that second inductor L si  alternatively may be wound on center leg  122  of core  120 , and first inductor L pi  may be wound in two sections on outer legs  124   a  and  124   b  of core  120 . Persons of ordinary skill in the art also will understand that alternative integrated inductor fabrication techniques may be used to form integrated first inductor L pi  and second inductor L si . 
     Referring now to  FIGS. 8A-8E , exemplary rectifier/filter circuit configurations are described that may be used in multiple-transformer LLC resonant converters of this invention. In particular,  FIG. 8A  illustrates a first exemplary rectifier/filter circuit  18   a  that includes a diode full-wave rectifier coupled to the secondary windings S of transformer  116   c . Rectifier/filter circuit  18   a  optionally may include output capacitors C 1  and/or C 2  to smooth out the rectified output voltage and current supplied to LED string  12 . 
     Referring now to  FIG. 8B , an alternative exemplary rectifier/filter circuit  18   b  is described. In particular, rectifier/filter circuit  18   b  includes a diode rectifier coupled to the secondary windings S 1  and S 2  of center-tapped transformer  116   d.  Rectifier/filter circuit  18   b  optionally may include output capacitor C 1  to smooth out the rectified output voltage and current supplied to LED string  12 . 
     Referring now to  FIG. 8C , another alternative exemplary rectifier/filter circuit  18   c  is described that may be used to drive a pair of LED strings  12 A and  12 B. In particular, rectifier/filter circuit  18   c  includes a diode rectifier coupled to the secondary winding S of transformer  116   c.  Rectifier/filter circuit  18   c  optionally may include output capacitors C 1  and/or C 2  to smooth out the rectified output voltage and current supplied to LED strings  12 A and  12 B. 
     Referring now to  FIG. 8D , another alternative exemplary rectifier/filter circuit  18   d  is described that may be used to drive three LED strings  12 A,  12 B and  12 C. In particular, rectifier/filter circuit  18   d  includes a diode rectifier coupled to the secondary winding S of transformer  116   a.  Rectifier/filter circuit  18   d  optionally may include output capacitors C 1 , C 2  and/or C 3  to smooth out the rectified output voltage and current supplied to LED strings  12 A,  12 B and  12 C. 
     Referring now to  FIG. 8E , another alternative exemplary rectifier/filter circuit  18   e  is described that may be used to drive four LED strings  12 A,  12 B,  12 C and  12 D. In particular, rectifier/filter circuit  18   e  includes a diode rectifier coupled to the secondary winding S of transformer  116   a.  Rectifier/filter circuit  18   e  optionally may include output capacitors C 1 , C 2  and/or C 3  to smooth out the rectified output voltage and current supplied to LED strings  12 A,  12 B,  12 C and  12 D. 
     Various modifications may be made to the exemplary LLC resonant converter circuits described above, and all such modifications are within the scope of the claimed invention. For example, a capacitor may be placed in series with the secondary winding of each transformer, coupled between the transformer and the corresponding rectifier circuit. Such a capacitor may block any DC current that results from non-ideal diode rectifiers, and/or from half-wave rectifier circuits used to drive multiple LED strings. 
     The foregoing merely illustrates the principles of this invention, and various modifications can be made by persons of ordinary skill in the art without departing from the scope and spirit of this invention.