Patent Publication Number: US-9906135-B2

Title: Multiphase DC/DC converters and control circuits for controlling converters using fixed and/or variable frequencies

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/800,143 filed Mar. 13, 2013. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to multiphase DC/DC converters. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     A multiphase DC/DC converter includes multiple converters connected in parallel and operated at the same frequency with a phase shift therebetween. The converters may be independent resonant converters that produce overlapping currents at the output of the multiphase DC/DC converter. These overlapping currents may assist in reducing ripple current in the output capacitor. Additionally, the multiphase DC/DC converter may include an inductor coupled to the converters to promote current sharing between the converters. This also assists in reducing ripple current in the output capacitor. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     According to one aspect of the present disclosure, a multiphase DC/DC power converter includes an input, an output, at least a first converter and a second converter coupled in parallel between the input and the output, an inductor coupled to the first and second converters, an output capacitor coupled between the output and a reference potential, and a control circuit coupled to the first converter and the second converter. The first and second converters each include a power switch. The control circuit is configured to switch the power switches at a frequency with a phase shift therebetween, and to vary the frequency to regulate a voltage at the output. 
     According to another aspect of the present disclosure, a multiphase DC/DC power converter includes an input, an output, at least a first converter and a second converter coupled in parallel between the input and the output, an inductor coupled to the first and second converters, an output capacitor coupled between the output and a reference potential, and a control circuit coupled to the first converter and the second converter. The first and second converters each include a power switch. The control circuit is configured to switch the power switches at a fixed frequency with substantially no phase shift therebetween during startup of the multiphase DC/DC power converter, and to switch the power switches at a variable frequency with a defined phase shift therebetween after startup of the multiphase DC/DC power converter. 
     According to another aspect of the present disclosure, a control circuit for a multiphase DC/DC power converter is disclosed. The multiphase DC/DC power converter has an input, an output, at least a first converter and a second converter coupled in parallel between the input and the output, an inductor coupled to the first and second converters, and an output capacitor coupled between the output and a reference potential. The first and second converters each including a power switch. The control circuit is configured to switch the power switches at a fixed frequency with substantially no phase shift therebetween during startup of the multiphase DC/DC power converter, and to switch the power switches at a variable frequency with a defined phase shift therebetween after startup of the multiphase DC/DC power converter. 
     Further aspects and areas of applicability will become apparent from the description provided herein. It should be understood that various aspects of this disclosure may be implemented individually or in combination with one or more other aspects. It should also be understood that the description and specific examples herein are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a block diagram of a multiphase DC/DC power converter including three converters and a control circuit according to one example embodiment of the present disclosure. 
         FIG. 2  is a schematic of a multiphase DC/DC power converter including two half bridge converters according to another example embodiment. 
         FIG. 3  is a graph illustrating triangular shaped waveforms of current in the two half bridge converters of  FIG. 2 . 
         FIG. 4  is a graph illustrating a waveform of ripple current in an output capacitor of the power converter of  FIG. 2 . 
         FIG. 5  is a schematic of a multiphase DC/DC power converter including two half bridge converters according to another example embodiment. 
         FIG. 6  is a graph illustrating an example of a zero current switching region and a zero voltage switching region for one of the half bridge converters of  FIG. 2 . 
         FIG. 7  is a graph illustrating example DC characteristics of one of the half bridge converters of  FIG. 2 . 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     A multiphase DC/DC power converter according to one example embodiment of the present disclosure is illustrated in  FIG. 1  and indicated generally by reference number  100 . As shown in  FIG. 1 , the multiphase DC/DC power converter  100  includes an input Vin, an output Vout, multiple converters  102 ,  104 ,  106  coupled in parallel between the input Vin and the output Vout, an inductor L, an output capacitor C, and a control circuit  108  coupled to each converter  102 ,  104 ,  106 . Each converter includes a power switch S. 
     As further explained below, the control circuit  108  switches the power switches S at a frequency with a phase shift therebetween. Additionally, the control circuit  108  varies the frequency to regulate a voltage at the output Vout. 
     Additionally, the control circuit  108  may switch the power switches S at a fixed frequency with substantially no phase shift therebetween during startup of the multiphase DC/DC power converter  100 . The control circuit  108  then switches the power switches S at a variable frequency with the phase shift therebetween after startup of the multiphase DC/DC power converter  100 . 
     As shown in  FIG. 1 , the inductor L is coupled between the output capacitor C and the converters  102 ,  104 ,  106 . Alternatively, the inductor L may be coupled on an input side of the converters  102 ,  104 ,  106 . For example, in some embodiments, the inductor L may be coupled between the input Vin and the converters  102 ,  104 ,  106 . As apparent to those skilled in the art, the value of inductor L may be different depending on, for example, the location of the inductor L, other resonant elements within the power converter  100 , etc. 
     As further explained below, providing the inductor L in the multiphase DC/DC power converter  100  (e.g., coupled between the converters  102 ,  104 ,  106  and the output capacitor C) may reduce the ripple current in the output capacitor C, balance the currents in the multiple converters  102 ,  104 ,  106 , and/or facilitate lossless switching transitions. This may improve efficiency of the multiphase DC/DC converter  100 . 
     In the example of  FIG. 1 , the output capacitor C is coupled between the output and a reference potential. The reference potential may include a reference voltage such as earth ground, chassis ground, etc. or another suitable reference potential. As shown in  FIG. 1 , the output capacitor is coupled to the inductor L to form an LC filter. In particular, the output capacitor C is coupled on the output side of the inductor L so that the output capacitor C is isolated from the converters  102 ,  104 ,  106  by the inductor L. Alternatively, if the inductor L is coupled on an input side of the converters  102 ,  104 ,  106 , the output capacitor C may be coupled to an output of each converter  102 ,  104 ,  106 . Further, although one output capacitor C is shown, any suitable number of capacitors may be employed. 
     As shown in  FIG. 1 , the converters  102 ,  104 ,  106  are coupled in parallel between the input Vin and the output Vout. Additionally, each converter  102 ,  104 ,  106  may include a rectification circuit (not shown in  FIG. 1 ) coupled to the power switch S. The rectification circuit may include, for example, a center tapped full wave rectifier, a bridge rectifier, or any other suitable rectification circuit for producing DC power from AC power. 
     Additionally, the phase shift between converters  102 ,  104 ,  106  may be varied to control the overlap of rectified pulses depending upon the number of converters employed. For example, the phase shift may be equal to 360/(2*N), where N is the number of converters in the multiphase DC/DC power converter  100 . 
     As shown in  FIG. 1 , the control circuit  108  provides a control signal  110 A,  110 B,  110 C to the converters  102 ,  104 ,  106 , respectively. The control circuit  108  switches (via the control signal  110 A,  110 B,  110 C) the power switches S at a variable frequency to regulate a voltage at the output Vout. In this way, a pre-regulator circuit is not needed to regulate the voltage at the output Vout. Accordingly, as shown in  FIG. 1 , the input Vin is not coupled to a pre-regulator circuit for regulating the voltage at the output Vout. 
     Additionally, in the example of  FIG. 1 , the control circuit  108  receives a signal  112  representing the voltage at the output Vout of the multiphase DC/DC power converter  100 . In this way, the control circuit  108  may provide closed loop control for regulating the output voltage of the multiphase DC/DC power converter  100 . Additionally and/or alternatively, although not shown in  FIG. 1 , the control circuit  108  may receive other signals representing parameters of the multiphase DC/DC power converter  100  including, for example, a voltage at the input Vin. 
     As shown in the example of  FIG. 1 , the output of each converter  102 ,  104 ,  106  are shorted together on the input side of the inductor L. Such interconnection with the appropriate phase shift as mentioned above results in overlapping outputs. The overlapping outputs as well as the inductor L and/or the presence of resonant elements in each converter  102 ,  104 ,  106  may help force current from each converter  102 ,  104 ,  106  to increase or decrease in a resonant fashion. In this way, a resonant condition of one converter may be forced by the other converters. Thus, the rate of which current from one or more converters increases or decreases is substantially the same as the rate of which current in the other converter(s) decreases or increases. Therefore, the current through the inductor L (i.e., the sum of current provided by each converter  102 ,  104 ,  106 ) is substantially constant. 
     Additionally, as explained above, the converters  102 ,  104 ,  106  operate simultaneously as resonant converters. However, if one or more of the converters is disabled, the other converter(s) will not operate as a resonant converter(s). This is due to the inductor L coupled to the converters  102 ,  104 ,  106  as explained above. In such a condition, the control circuit  108  would provide the active converter(s) a pulse width modulated signal at a fixed frequency to achieve voltage regulation at the output Vout. 
     As explained above, this feature of controlling converters  102 ,  104 ,  106  with a pulse width modulated signal at a fixed frequency may be employed to control the power converter  100  during other conditions as well. For example, the control circuit  108  switches the power switches S at the fixed frequency (with substantially no phase shift) during startup of the multiphase DC/DC power converter  100 . Therefore, to regulate the voltage at the output Vout, duty cycles of the power switch(es) S of each converter  102 ,  104 ,  106  may be varied. 
     By employing a fixed frequency during startup, the power converter  100  may be soft started to reduce current stress on the power switches S. For example, during startup of the power converter  100 , the output capacitor C may be discharged. By switching the power switches S at a fixed frequency, a closed loop, monotonic rise in the charge of the output capacitor C may be achieved. In this way, the power converter  100  is soft started. 
     Then, at some point in time after startup of the power converter  100 , the control circuit  108  may switch the power switches S at a variable frequency with a defined phase shift therebetween after the voltage at the output Vout is substantially regulated. In some embodiments, this transition may include increasing a phase shift between the power switches S from the substantially no phase shift (as explained above) to the defined phase shift over a defined time period. The defined time period may be any suitable time, including, for example, one switching cycle of the power converter  100 , etc. Alternatively, this transition may include several incrementing phase shifts of some of the converters until the defined phase shift is achieved. 
     In the example of  FIG. 1 , the multiphase DC/DC power converter  100  include three converters  102 ,  104 ,  106 . However, it should be understood that two or more converters may be employed in any given application of this disclosure. For example, additional converters may be added to a particular implementation to achieve higher efficiencies and/or for increased output power. Regardless of the number, the multiple converters preferably employ the same topology. For example, the multiple converters may be forward converters, bridge converters (including full bridge converters, half bridge converters as shown in  FIG. 2 , etc.), push-pull converters, etc. 
       FIG. 2  is one example of a multiphase DC/DC power converter  200  including an input Vin, two converters  202 ,  204 , an inductor L 2 , an output capacitor C 6  and an output Vout. The converters  202  and  204  are coupled in parallel between the input Vin and the inductor L 2 . Further, the output capacitor C 6  is coupled between the inductor L 2  and the output Vout. 
     As shown in  FIG. 2 , the power converter  200  may include a filter coupled to the input Vin. In the example of  FIG. 2 , the filter is a capacitor C 1 . Alternatively, the filter may include different and/or additional filtering elements if desired. 
     In the example embodiment of  FIG. 2 , the converters  202 ,  204  employ the same topology. Each is a half bridge converter having its own isolation transformer TX 1 , TX 2  coupled to a rectification circuit  206 ,  208 . Each rectification circuit  206 ,  208  is coupled to the inductor L 2 . 
     As shown in  FIG. 2 , the rectification circuits  206  and  208  are center tapped full wave rectifiers and include diodes D 1 , D 2  and D 3 , D 4 , respectively. Although the example of  FIG. 2  illustrates the rectification circuits  206 ,  208  as including diode rectifiers, synchronous rectifier FETs (e.g., MOSFETs) may be used to further improve efficiency. 
     Additionally, the converter  202  includes power switches Q 1 , Q 2  coupled to a primary winding P 1  of transformer TX 1 . Secondary windings S 1 , S 2  of transformer TX 1  are coupled to the output Vout via the rectification circuit  206 . The converter  204  includes power switches Q 3 , Q 4  coupled to a primary winding P 2  of transformer TX 2 . Secondary windings S 3 , S 4  of transformer TX 2  are coupled to the output Vout via the rectification circuit  208 . 
     In addition to the transformers TX 1 , TX 2 , the converter  202  may employ additional resonant elements coupled to the primary windings P 1 , P 2 . In the example of  FIG. 2 , the converter  202  includes capacitors C 2 , C 3  coupled to power switches Q 1 , Q 2 , respectively, and an inductor L 1  coupled between the capacitors C 2 , C 3  and the primary winding P 1  of transformer TX 1 . Likewise, the converter  204  includes capacitors C 4 , C 5  coupled to power switches Q 3 , Q 4 , respectively, and an inductor L 3  coupled between the capacitors C 4 , C 5  and the primary winding P 2  of transformer TX 2 . 
     For clarity, inductors L 1 , L 3  are shown as separate external inductors from the transformers TX 1 , TX 2 . However, it should be apparent that the inductors L 1 , L 3  may include separate external inductances (e.g., from an inductor) as well as leakage and/or parasitic inductances from the transformers TX 1 , TX 2 . 
     The resonant elements may be any suitable value depending on design and desired results. For example, values of corresponding resonant elements may be chosen to be substantially equal. 
     Depending on the design, the power switches Q 1 , Q 2 , Q 3 , Q 4  may be capable of soft switching. For example, values of the capacitors C 2 , C 3 , C 4 , C 5 , the inductors L 1 , L 3  and the magnetizing inductance of each transformer TX 1 , TX 2  may be selected to achieve substantially zero voltage and/or current switching of the power switches Q 1 , Q 2 , Q 3 , Q 4 . In this way, soft switching of the power switches Q 1 , Q 2 , Q 3 , Q 4  may be capable while varying the frequency if the frequency variation is below a resonant frequency. However, if the frequency variation is above the resonant frequency, zero current switching of the power switches Q 1 , Q 2 , Q 3 , Q 4  may be compromised.  FIG. 6  illustrates an example of a zero current switching region and a zero voltage switching region for one of the converters  202 ,  204 . A resonant frequency is shown by the dashed line  600 . 
     Referring back to  FIG. 2 , despite possible inequalities (e.g., from tolerance related variations, etc.) in the resonant elements, the inductor L 2  may assist in achieving current balance in the converters  202 ,  204 . In this way, the inductor L 2  may stabilize a load current flowing to the output Vout in steady state conditions. To stabilize the load current, the sum of the current from each converter  202 ,  204  remains substantially constant (as described above). Therefore, when current from the converter  202  to the inductor L 2  is ramping up, current from the converter  204  is ramping down at the same rate (and vice versa) to maintain the substantially constant load current through the inductor L 2 . This relationship between current from each converter  202 ,  204  results in a current waveform having a substantially triangular shape in the primary and secondary windings of each transformer TX 1 , TX 2 . For example,  FIG. 3  illustrates triangular shaped waveforms of current in the primary winding of each transformer TX 1 , TX 2 . 
     Referring again to  FIG. 2 , as a result of the current relationship from the converters  202 ,  204 , ripple current in the output capacitor C 6  may be reduced and in some cases, substantially zero. In some embodiments, the ripple may be about 2% rms of the output load current. Likewise, this same result may be realized by the capacitor C 1  coupled between the input Vin and the converters  202 ,  204 . It should be understood, however, that the ripple current may vary depending on the values of the resonant elements of the converters  202 ,  204 . 
       FIG. 4  illustrates an example waveform of ripple current in the output capacitor C 6  for a 12V/180 A rated power rail. As shown in  FIG. 4 , the ripple current is about 4.7 A rms. 
     Although not shown in  FIG. 2 , the power switches Q 1 , Q 2 , Q 3 , Q 4  are switched by a control circuit (e.g., similar to the control circuit  108  of  FIG. 1 ). For example, the control circuit may switch the power switches Q 1 , Q 2 , Q 3 , Q 4  at a variable frequency to regulate the voltage at the output Vout. Therefore, and as shown in  FIG. 2 , the input Vin of the power converter  200  is not coupled to a pre-regulator circuit to regulate the voltage at the output Vout. 
     Additionally and/or alternatively, the control circuit may switch the power switches Q 1 , Q 2 , Q 3 , Q 4  at a fixed frequency with substantially no phase shift therebetween during startup of the multiphase DC/DC power converter  200 . Then, at some point in time after startup, the control circuit may switch the power switches Q 1 , Q 2 , Q 3 , Q 4  at a variable frequency with a defined phase shift therebetween. 
     To regulate the output voltage while having a variable frequency, a relationship is formed between the values of the resonant elements (e.g., the capacitors C 2 , C 3 , C 4 , C 5 , the inductors L 1 , L 3 , etc.) of each converter  202 ,  204  and the value of the magnetizing inductance of the transformers TX 1 , TX 2 . For example, the values of the resonant elements and the magnetizing inductances of the transformers TX 1 , TX 2  may be selected based on the design requirements including, for example, maximum efficiency required (e.g., at 50% load, 100% load, etc.), increased hold-up capacity, soft switching (explained above), etc. 
     Additionally, by employing a variable frequency, the power converter  200  may operate at a gain of one. Alternatively, if desired, the power converter  200  may be operated at a gain of less or more than one. For example, the gain and the frequency depend on a load coupled to the output Vout and the voltage at the input Vin. In this way, one skilled in the art can design the converters  202 ,  204  to achieve a desired gain. For example,  FIG. 7  illustrates example gain curves for one of the converters  202 ,  204 . 
     In addition, and as explained above, the converters  202 ,  204  are operated simultaneously as resonant converters. However, when one converter (e.g., converter  202 ) is disabled, the other active converter (e.g., converter  204 ) does not operate as a resonant converter due to the presence of the inductor L 2 . In such a condition, the control circuit (not shown) may provide the active converter a pulse width modulated signal at a fixed frequency to achieve voltage regulation at the output Vout. Additionally, current in the primary and secondary windings of the transformer in the active converter would have a substantially square wave (e.g., a trapezoidal, square, etc.) shape instead of the triangular shape as described above. 
       FIG. 5  illustrates an example multiphase DC/DC power converter  500  substantially similar to the multiphase DC/DC power converter  200  of  FIG. 2 . However, as shown in  FIG. 5 , the multiphase DC/DC power converter  500  includes converters  502 ,  504  and an inductor L 2  coupled on an input side of the converters  502 ,  504 . For example, in the example of  FIG. 5 , the inductor L 2  is coupled between an input capacitor C 1  and the converters  502 ,  504   
     Additionally, by coupling the inductor L 2  on the input side of the converters  502 ,  504 , substantially the same benefits as described above with respect to  FIGS. 1 and 2  (which include an inductor coupled on the output side of converters) may be obtained. For example, coupling the inductor L 2  on the input side of the converters  502 ,  504  may reduce ripple current in the input capacitor C 1 . 
     The multiphase DC/DC power converters disclosed herein may include high performance converters having an output power of 3 kW or more. Additionally, although a half bridge converter is shown in the example of  FIG. 2 , any suitable converter topology may be employed including forward converters, other bridge converters (e.g., full bridge), push-pull converters, etc. 
     Additionally, the power switches disclosed herein may be any suitable power switch including, for example, Gallium Nitride (GaN) power switches that are switched at a frequency in the range of 500 kHz to 1 MHz. 
     Further, the teachings disclosed herein may be employed in many different power applications, including, for example, high power converters, server power supplies, etc. 
     By employing the teachings disclosed herein, power converters may have low electromagnetic interference (EMI), reduced ripple current stress for input and output capacitors, increased efficiency, lossless switching transitions, etc. Additionally, by employing the teachings disclosed herein, the converters may handle practical tolerance related variations in the resonant elements without degrading performance, achieve substantially monotonic rise of the output voltage during startup without high levels of stress on switching devices, include lower current stresses during short circuit conditions, etc. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.