Abstract:
A resonant DC/DC power converter is provided with isolated primary and secondary circuits. The primary circuit includes at least four switches as first and second pairs in a bridge configuration, an isolation transformer having at least one primary and at least one secondary winding, and a resonant tank including a resonant capacitor and a split resonant inductor having two separate windings. In one embodiment, the split resonant inductor windings are substantially identical. Synchronous switching of diagonally opposed switch pairs in the bridge configuration thereby produces a center point voltage of the primary transformer winding is substantially free of stepwise voltage changes.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/936,535, filed Feb. 6, 2013, and which is hereby incorporated by reference. 
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of electrical power conversion, particularly to isolated resonant converters required to provide low common mode noise. 
     Isolated DC/DC converters often experience common mode (CM) noise that is present between the primary and secondary terminals. The common mode noise is typically generated by unbalanced currents flowing through the inter-winding capacitances of the isolation transformer generated during switching operation of the converter. 
     One example of a prior art converter  10  is shown in  FIG. 1 . The converter includes an isolation transformer T with depicted inter-winding capacitances Ci 1  and Ci 2  representing lumped components of the real structure. The inter-winding capacitances typically define a capacitive coupling between the primary and secondary circuits of the converter. Because of the capacitive coupling, a primary center point voltage Vm can be used to predict common mode voltage Vcm present between the primary and secondary circuits. This can be predicted by means of experimental measurement or by circuit model simulation of the circuit. 
     For a converter  10  according to  FIG. 1 , the primary center point voltage Vm voltage is graphically represented as  21  on  FIG. 2 , where the voltage of an input source V 1  is marked for reference. As shown on  FIG. 2 , the center point voltage Vm features low frequency components and voltage transients with higher dv/dt. The high dv/dt voltage transients cause high capacitive currents through the inter-winding capacitances Ci 1 /Ci 2  which are difficult to damp. Therefore the center point voltage Vm and consequently the common mode voltage Vcm tend to oscillate at high frequency. 
     It can be demonstrated that if the primary center point voltage Vm is stable, then noise currents flowing through the inter-winding capacitances Ci 1  and Ci 2  cancel each other and the common mode voltage Vcm between the primary and secondary circuits is minimized. 
     One solution as has previously been known in the art is to decrease the common mode voltage of a DC/DC converter  30  having a topology as represented in  FIG. 3 . This topology is useful in applications where the resonant choke of the converter is represented by a leakage inductance of the isolation transformer. However, it does not offer a solution for applications where a discrete resonant choke is used. 
     Therefore, a solution which provides low common mode voltage and includes a discrete resonant choke is required. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one exemplary embodiment of an invention according to the present invention, a resonant DC/DC converter includes a primary circuit and a secondary circuit. The primary circuit has at least four switches configured as primary switches in a bridge configuration, an isolation transformer with at least one primary and one secondary winding, and a resonant tank having a resonant capacitor and a split resonant inductor with two separate windings. The secondary circuit is coupled to a secondary winding of the isolation transformer whereby the center voltage of the primary transformer winding is substantially free of stepwise voltage changes. 
     In an exemplary aspect of such an embodiment, the two separate windings of the split resonant inductor may be structurally identical or otherwise comprise substantially equivalent parasitic capacitances. 
     In another exemplary aspect, the split resonant inductor substantially shares a common path of magnetic flux. 
     In another exemplary aspect, the split resonant inductor may further be characterized by an m:n inductance ratio of substantially 1:1. Alternatively, the m:n inductance ratio may be different from 1:1, having a value provided to further optimize a common mode voltage between the primary and secondary circuits. 
     In another exemplary aspect, the split resonant inductor may be comprised of two independent inductors. 
     In another exemplary aspect, the resonant capacitor may be a single component, or in certain embodiments may be split into two separate components, wherein the split resonant inductor may further substantially share the common path of magnetic flux. 
     In another exemplary aspect, the secondary circuit may include a rectifier circuit coupled across the secondary isolation transformer winding and configured to produce a DC output to a load. In one embodiment, the rectifier circuit may include a plurality of synchronous rectifier switching elements arranged in a bridge configuration. In another embodiment, the rectifier circuit may include a plurality of synchronous rectifier switching elements arranged in a center-tap configuration. 
     In another exemplary aspect, the resonant circuit may be defined as including first and second impedance circuits coupled to nodes between the respective first and second pairs of switching elements. The impedance circuits include respective impedance values that are substantially identical, wherein synchronous switching of opposing pairs of switching elements in the inverter bridge produces a center point voltage of the isolation transformer that is substantially a DC voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram representing an exemplary resonant converter as conventionally known in the art. 
         FIG. 2  is a graphical diagram representing a primary center point voltage Vm of the prior art converter according to  FIG. 1 . 
         FIG. 3  is a circuit diagram representing a resonant converter of the prior art with a suppressed common mode voltage present between a primary side and a secondary side of the converter. 
         FIG. 4  is a circuit diagram representing an exemplary embodiment of a power converter according to the present invention. 
         FIG. 5  is a graphical diagram representing exemplary switching voltage waveforms to emphasize an operation description of the topology according to an exemplary embodiment of a power converter according to the present invention. 
         FIG. 6  is a graphical diagram depicting a primary center point voltage Vm of the converter according to embodiments according to the present invention. 
         FIG. 7  is a circuit diagram representing another topology of a power converter according to an embodiment according to the present invention, where the magnetic flux of both resonant chokes Lr 1  and Lr 2  share the same path. 
         FIG. 8  is a circuit diagram representing another topology of a power converter according to an embodiment according to the present invention, where a split resonant capacitor is integrated as a single component. 
         FIG. 9  is a circuit diagram representing another topology of a power converter according to an embodiment according to the present invention, wherein a resonant capacitor is connected between a switching branch and the Lr 1  inductor. 
         FIG. 10  is a circuit diagram representing another topology of a power converter according to an embodiment according to the present invention, with a single resonant capacitor and two resonant inductors sharing the same magnetic path. 
         FIG. 11  is a circuit diagram representing another topology of a power converter according to an embodiment according to the present invention, where the resonant capacitor is connected between a switching branch and the Lr 1  inductor, and wherein the resonant chokes share the same magnetic path. 
         FIG. 12  is a circuit diagram representing an example of a secondary circuit that may be used with various embodiments of a power converter according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. 
     The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. 
     The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. Terms such as “wire,” “wiring,” “line,” “signal,” “conductor,” and “bus” may be used to refer to any known structure, construction, arrangement, technique, method and/or process for physically transferring a signal from one point in a circuit to another. Also, unless indicated otherwise from the context of its use herein, the terms “known,” “fixed,” “given,” “certain” and “predetermined” generally refer to a value, quantity, parameter, constraint, condition, state, process, procedure, method, practice, or combination thereof that is, in theory, variable, but is typically set in advance and not varied thereafter when in use. 
     The terms “switching element” and “switch” may be used interchangeably and may refer herein to at least: a variety of transistors as known in the art (including but not limited to FET, BJT, IGBT, JFET, etc.), a switching diode, a silicon controlled rectifier (SCR), a diode for alternating current (DIAC), a triode for alternating current (TRIAC), a mechanical single pole/double pole switch (SPDT), or electrical, solid state or reed relays. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa. 
     The terms “power converter” and “converter” unless otherwise defined with respect to a particular element may be used interchangeably herein and with reference to at least DC-DC, DC-AC, AC-DC, buck, buck-boost, boost, half-bridge, full-bridge, H-bridge or various other forms of power conversion or inversion as known to one of skill in the art. 
     Terms such as “providing,” “processing,” “supplying,” “determining,” “calculating” or the like may refer at least to an action of a computer system, computer program, signal processor, logic or alternative analog or digital electronic device that may be transformative of signals represented as physical quantities, whether automatically or manually initiated. 
     The terms “controller,” “control circuit” and “control circuitry” as used herein may refer to, be embodied by or otherwise included within a machine, such as a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed and programmed to perform or cause the performance of the functions described herein. A general purpose processor can be a microprocessor, but in the alternative, the processor can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     Referring generally to  FIGS. 4-12 , various embodiments of a power converter according to the present invention may now be described in greater detail. Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Referring first to  FIG. 4 , one exemplary embodiment  40  of a symmetrical resonant converter (SyRC) according to the present invention includes a full-bridge inverter stage with switching elements Q 1 -Q 4 , a primary circuit  12  disposed along a lateral branch  12  (e.g., including a primary winding of isolation transformer T, a split resonant choke Lr 1 /Lr 2 , a split resonant capacitor Cr 1 /Cr 2 ) and a secondary circuit  14  (e.g., including or coupled to a secondary winding of isolation transformer T). The components in the lateral branch may define first and second impedance circuits, with the first impedance circuit in the example shown including the first split resonant inductor winding Lr 1  and the first split resonant capacitance Cr 1  and having an impedance value Z 1 , and with the second impedance circuit including the second split resonant inductor winding Lr 2  and the second split resonant capacitance Cr 2  and having an impedance value Z 2 . 
     Many forms of secondary circuits  14  may be used, such as for example that illustrated in  FIG. 12 . In the example shown, a rectifier circuit is coupled across the secondary winding of the isolation transformer T, which may include a bridge of synchronous rectifier switching elements Qsr 1 -Qsr 4 . A filtering capacitor Cf and a load R L  are coupled across an output of the rectifier circuit. 
     If the full-bridge inverter is controlled symmetrically, e.g. wherein diagonally opposed switching elements Q 1  and Q 4  are controlled for switching synchronously, and likewise diagonally opposed switching elements Q 2  and Q 3  are controlled for switching synchronously but with a 180 degrees shift with respect to Q 1  and Q 4 , then the resultant switching voltages  51  and  52  may be characterized by the symmetrical waveforms depicted on  FIG. 5 . Assuming further that the respective impedance values Z 1  and Z 2  are equivalent, then the primary center point voltage Vm of the isolation transformer T is substantially a DC voltage  62  as depicted on  FIG. 6 . Consequently, the capacitive currents flowing through the inter-winding capacitances of the isolation transformer T will be minimized because there are no stepwise voltage changes, resulting in a low common mode noise present between the primary  12  and secondary  14  circuits. 
     Note that within switching transients, impedance values Z 1  and Z 2  are mainly defined or otherwise associated with the split inductor windings Lr 1  and Lr 2  because the impedances of resonant capacitors Cr 1  and Cr 2  are relatively, and generally substantially, lower. However, the parasitic capacitances of Lr 1  and Lr 2  may typically be kept substantially equal. Therefore, equivalent physical structures of chokes Lr 1  and Lr 2  are preferred to avoid unbalanced inter-winding capacitance currents. 
     Another embodiment of a DC-DC converter topology may be as shown in  FIG. 7 . This converter topology  70  includes a split resonant choke Lr 1 /Lr 2  which utilizes the same magnetic core, i.e. the magnetic fluxes share the same path. This embodiment allows for a higher power density of the converter while still gaining benefits of the embodiment as previously described according to  FIG. 4 . 
     Taking into account that the voltage across the resonant capacitors Cr 1  and Cr 2  remains substantially unchanged during the switching transitions, any ratio between capacitance of Cr 1  and capacitance of Cr 2  can be chosen providing that Cr 1 *Cr 2 /(Cr 1 +Cr 2 ) is kept constant. Therefore, capacitors Cr 1  and Cr 2  can be integrated into a single resonant capacitor. In this case, the center point voltage Vm is still free of voltage transients with higher dv/dt. However, the center point voltage Vm may be characterized by an AC voltage  61 , as depicted on  FIG. 6 , having mainly a fundamental frequency component given by the switching frequency of the converter. Because the center point voltage Vm essentially governs the common mode voltage between the primary and secondary circuits, the common mode voltage features a similar shape  61  as well. If an application allows for such a low frequency common mode voltage, or if a common mode filter implementation is efficient to dampen low frequency common mode voltages, then the converter can be simplified and a single resonant capacitor can be positioned anywhere within the lateral branch of the converter. As a result, other embodiments of a power converter topology  80 ,  90 ,  100 ,  110  may be obtained as represented in  FIGS. 8-11 , respectively. 
     If an application requires a single resonant capacitor and if it is further required to decrease the RMS value of the common mode voltage between the primary and secondary circuits, then the chokes Lr 1  and Lr 2  can be intentionally designed with an inductance ratio other than 1:1. Another reason to do this is the eventual existence of asymmetry of the isolation transformer T characterized by unequal inter-winding capacitances. In each of these cases, the inductance ratio of Lr 1  and Lr 2  may be different than 1:1 and provides optimal common mode voltage in the respective application. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of a new and useful invention, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.