Patent Publication Number: US-2023147857-A1

Title: LLC Controller and Control Method for Power Converter

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Taiwan Application Series Number 110141881 filed on Nov. 10, 2021, which is incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates generally to an LLC resonant power converter, and more particularly, to methods and apparatuses in use of an LLC resonant power converter with dual output power sources. 
     LLC resonant power converters are excellent among switching mode power supplies in view of power conversion efficiency. It is well known by the manufactures of switching mode power supplies that power switches are major devices that consume significant power when converting power. Theoretically, an LLC resonant converter can control two major power switches, including high-side and low-side switches, to perform ZVS (zero voltage switching), where ZVS refers to a technology that a switch is turned ON at about the moment when the voltage drop across the channel of the switch is 0V. The conduction loss of the high-side and low-side switches is expectedly to be minimized, and the power conversion efficiency is therefore excellent. LLC resonant power converters are particularly suitable for high power applications, those requiring power more than 100 W for example. 
     A single-output LLC resonant power converter, having a single output power source to supply power, usually employs symmetric pulse-width modulation (PWM) control, a technology in which the duty cycles of the high-side and low-side switches are substantially equal, or both are very close to 50%. Nevertheless, a dual-output LLC resonant power converter, having two output power sources to supply power to two different loads respectively for example, may use asymmetric PWM (APWM) control to regulate the two output power sources. The duty cycles of the high-side and low-side switches may correspond to the conditions of the loads supplied by the output power sources respectively. It is of great concern how to provide a variety of protections, such as over-current protection, over-load protection, etc., for a dual-output LLC resonant power converter under asymmetric PWM control 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted. 
       The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG.  1    demonstrates a dual-output LLC resonant power converter according to embodiments of the invention; 
         FIG.  2    shows an LLC controller according to embodiments of the invention; 
         FIG.  3 A  demonstrates the relationship between threshold V CSP-OCP  and duty signal DT H  according to embodiments of the invention; 
         FIG.  3 B  demonstrates the relationship between threshold V CSN-OCP  and duty signal DT H  according to embodiments of the invention; 
         FIG.  4    demonstrates waveforms of signals in  FIGS.  1  and  2    when each of the duty cycles of high-side switch HS and low-side switch LS is about 50%; 
         FIG.  5    demonstrates waveforms of signals in  FIGS.  1  and  2    when the duty cycle of high-side switch HS, duty signal DT H , is about 25%; and 
         FIG.  6    demonstrates a control method in use of the dual-output LLC resonant power converter in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
     Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale. 
     An APWM dual-output LLC resonant power converter is detailed as an example of the invention in this specification, but this invention is not limited to. The invention might be used in a power converter different from the APWM dual-output LLC resonant power converter. 
     An APWM dual-output LLC resonant power converter according to embodiments of the invention has an LLC controller controlling high-side and low-side switches. The LLC controller provides a threshold to determine whether at least one of the loads is too heavy, and whether to stop power conversion to provide OCP. The threshold changes in response to at least one of the duty cycles of the high-side and low-side power switches, to avoid false OCP triggering. 
       FIG.  1    demonstrates dual-output LLC resonant power converter  100  according to embodiments of the invention, converting input power source V IN  at input power line IN into output power sources V O1  and V O2  to supply power to loads  1041  and  1042  respectively. 
     High-side switch HS and low-side switch LS, connected in series between input power line IN and input ground line GND IN , drive resonant circuit RSNT to resonate, where resonant circuit RSNT has transformer TF and capacitor CL. Inside transformer TF, two secondary windings LS 1  and LS 2  are inductively coupled to primary winding LP, which is connected to inductors Lr and Lm. Inductors Lr and Lm refer to the leakage inductors connected in series to and in parallel with primary winding LP respectively. In  FIG.  1   , primary winding LP and capacitor CL are connected in series via node ND. Resonant circuit RSNT is not limited to the circuit shown in  FIG.  1    and might have a different circuit in other embodiments of the invention. 
     When resonant circuit RSNT resonates, induced currents I D1  and I D2  from secondary windings LS 1  and LS 2  respectively can be rectified by diodes D 1  and D 2  to build up output power sources V O1  and V O2  over capacitors CO 1  and CO 2  respectively. 
     Monitoring output power sources V O1  and V O2 , feedback circuits  1061  and  1062  provide feedback signals V FB1  ad V FB2  respectively. Based on feedback signals V FB1  and V FB2  at feedback nodes FB 1  and FB 2 , LLC controller  102  generates high-side control signal S H  and low-side control signal S L  to control the ON times of high-side switch HS and low-side switch LS. An ON time of a switch means a time period when the switch is tuned ON to provide a short circuit connecting two nodes of the switch. 
     Dual-output LLC resonant power converter  100  has detector  108  composed of resistors RA, RB and capacitors CA, CB, connection of which is shown in  FIG.  1   . Detector  108  connects to node ND, detecting the voltage drop across capacitor CL in resonant circuit RSNT to provide current-sense signal V CS . Current-sense signal V CS  can represent a magnitude of resonance in resonant circuit RSNT. Current-sense signal V CS  is just an example of a detection signal, and some embodiments of the invention might provide a detection signal different from current-sense signal V CS  in  FIG.  1   . 
       FIG.  2    shows LLC controller  102  according to embodiments of the invention, including ON-time controller  208 , duty-cycle detector  202 , threshold generators  204 P and  204 N, and over-current protection circuit  206 . 
     Based on feedback signals V FB1  and V FB2 , ON-time controller  208  provides high-side control signal S H  and low-side control signal S L , to control high-side switch HS and low-side switch LS. ON-time controller  208  is also configured to make high-side switch HS and low-side switch LS perform ZVS, where each of high-side switch HS and low-side switch LS is turned ON when its channel voltage is about 0V. 
     In  FIG.  2   , duty-cycle detector  202  provides duty signal DT H  based on high-side control signal S H , representing the duty cycle of high-side switch HS, a number in the range of 0%-100%. Duty signal DT L  represents the duty cycle of low-side switch LS, and can be derived from duty signal DT H  in some embodiments of the invention, because they are basically complementary to each other. For example, if duty signal DT H  is 35%, then duty signal DT L  should be 65%, since the sum of duty signal DT H  and duty signal DT L  is about 100%. According to some embodiments of the invention, duty signal DT L  can be directly obtained from low-side control signal S L , based on the ratio of the ON time of low-side switch LS to the cycle time of low-side switch LS, and duty signal DT H  can be derived from duty signal DT L . 
     In  FIG.  2   , threshold generators  204 P and  204 N provide thresholds V CSP-OCP  and V CSN-OCP  respectively, both based on duty signal DT H . The change of duty signal DT H  may cause threshold V CSP-OCP  and/or V CSN-OCP  to change.  FIG.  3 A  demonstrates the relationship between threshold V CSP-OCP  and duty signal DT H  according to embodiments of the invention. As shown in  FIG.  3 A , threshold V CSP-OCP  is constant V CSP-L  when duty signal DT H  exceeds 35%, is constant V CSP-H  when duty signal DT H  is less than 15%, and varies linearly with the change of duty signal DT H  when duty signal DT H  is between 15% and 35%. Analogously,  FIG.  3 B  demonstrates the relationship between threshold V CSN-OCP  and duty signal DT H  according to embodiments of the invention. As shown in  FIG.  3 B , threshold V CSN-OCP  is constant V CSN-H  when duty signal DT H  exceeds 85%, is constant V CSN-L  when duty signal DT H  is less than 65%, and varies linearly with the change of duty signal DT H  when duty signal DT H  is between 65% and 85%. In an embodiment of the invention, all constants V CSP-H , V CSP-L  V CSN-H , and V CSN-L  are positive. 
     Some embodiments of the invention may provide thresholds V CSP-OCP  and V CSN-OCP  based on duty signal DT L , which is about 100% minus duty signal DT H , as indicated in  FIGS.  3 A and  3 B . 
       FIG.  4    demonstrates waveforms of signals in  FIGS.  1  and  2    when each of the duty cycles of high-side switch HS and low-side switch LS is about 50%. From top to bottom, the waveforms in  FIG.  4    are high-side control signal S H , low-side control signal S L , current I CL  through capacitor CL, current-sense signal V CS , and induced currents I D1  and I D2  through diodes D 1  and D 2  respectively. The waveforms in  FIG.  4    might occur when for example both loads  1041  and  1042  are middle heavy. 
     If deadtimes between ON times of high-side switch HS and low-side switch LS are neglected, the waveforms of high-side control signal S H  and low-side control signal S L  in  FIG.  4    indicate that each of the duty cycles of high-side switch HS and low-side switch LS is about 50%, duty signal DT H  being about 50%. According to the relationships shown in  FIGS.  3 A and  3 B , threshold V CSP-OCP  in  FIG.  4    is constant V CSP-L , and threshold V CSN-OCP  constant V CSN-L . 
     Please refer to  FIGS.  2  and  4   . Over-current protection circuit  206  in  FIG.  2    determines whether an OCP event occurs to output power sources V O1  and V O2  based on thresholds V CSP-OCP , V CSN-OCP  and current-sense signal V CS  from detector  108 , to trigger an over-current protection. 
     For example, if over-current protection circuit  206  finds that current-sense signal V CS  has been exceeding threshold V CSP-OCP  for a predetermined times within a predetermined time window, over-current protection circuit  206  determines that an OCP event is occurring to one of output power sources V O1  and V O2 , so over-current protection is triggered. Similarly, over-current protection circuit  206  could determine that an OCP event is occurring to the other of output power sources V O1  and V O2  and triggers over-current protection if current-sense signal V CS  has been below threshold −V CSN-OCP  for the predetermined times within the predetermined time window. 
     In  FIG.  4   , current-sense signal V CS  vibrates or varies within the acceptable range with borders of thresholds V CSP-OCP  and −V CSN-OCP . Consequentially, in view of the waveforms in  FIG.  4   , over-current protection circuit  206  will not deem an OCP event occurring and will not trigger over-current protection. 
     When over-current protection is triggered, over-current protection circuit  206  in  FIG.  2    sends out protection signal S OCP  to disable ON-time controller  208 , which accordingly controls high-side switch HS and low-side switch LS to stop powering resonant circuit RSNT, so the resonance of resonant circuit RSNT subsides over time, and soon both output power sources V O1  and V O2  receive power no more from resonant circuit RSNT. The resonance of resonant circuit RSNT stops soon if at least one of high-side switch HS and low-side switch LS is kept turned OFF. Some embodiments of the invention might keep both high-side switch HS and low-side switch LS turned OFF when over-current protection is triggered. Other embodiments of the invention might keep one of high-side switch HS and low-side switch LS turned OFF and the other turned ON when over-current protection is triggered. 
       FIG.  5    demonstrates waveforms of signals in  FIGS.  1  and  2    when the duty cycle of high-side switch HS, duty signal DT H , is about 25%. Due to the complementary correlation, duty signal DT L  is about 75% as duty signal DT H  is about 25%. The waveforms in  FIG.  5    might occur when for example load  1041  is middle heavy and load  1042  is light or absent. OCP should not be triggered in  FIG.  5    because none of loads  1041  and  1042  is over heavy. Based on the relationship between threshold V CSP-OCP  and duty signal DT H  shown in  FIG.  3 A , threshold V CSP-OCP  should be in the middle between constants V CSP-H  and V CSP-L . The acceptable range with borders of thresholds V CSP-OCP  and −V CSN-OCP  in  FIG.  5    is extended now because it is wider than the one in  FIG.  4   . The acceptable range in  FIG.  5    still covers the variation of current-sense signal V CS , so over-current protection circuit  206  will not trigger over-current protection. The result that over-current protection is not triggered is correctly expected because none of loads  1041  and  1042  is over heavy. 
     Suppose that threshold V CSP-OCP  in  FIG.  5    is still constant V CSP-L , the same as it is in  FIG.  4   . Even though none of loads  1041  and  1042  is over heavy, over-current protection will be, however, wrongly triggered because peaks of current-sense signal V CS  exceed constant V CSP-L  indeed in  FIG.  5   . In other words, the increasement to threshold V CSP-OCP  based on  FIG.  3 A  when duty signal DT H  becomes below 35% can prevent false triggering of over-current protection. Similarly, the increasement to threshold V CSN-OCP  based on  FIG.  3 B  when duty signal DT H  exceeds 65% can also prevent false triggering of over-current protection. 
     Over-current protection circuit  206  in  FIG.  2    triggers OCP based on current-sense signal V CS  and the acceptable range defined by the borders consisting of thresholds V CSP-OCP  and −V CSN-OCP . It is shown in  FIGS.  3 A and  3 B  that each of thresholds V CSP-OCP  and −V CSN-OCP  is constant, not changing with the change of duty signal DT H  when duty signal DT H  is within a central region from 35% to 65%, and that the acceptable range is a standard range from constant −V CSN-L  to constant V CSP-L . The acceptable range is extended, turning into an extended range covering the standard range when threshold V CSP-OCP , a border of the acceptable range, increases as duty signal DT H  leaves the central region and decreases below 35%, a boundary of the central region. The acceptable range is extended, turning into another extended range covering the standard range when threshold −V CSN-OCP , another border of the acceptable range, decreases as duty signal DT H  leaves the central region and increases over 65%, another boundary of the central region. 
       FIG.  6    demonstrates control method M 10  in use of dual-output LLC resonant power converter  100  in  FIG.  1   . Please refer to  FIGS.  1 ,  2  and  6   . In step S 10 , high-side control signal S H  and low-side control signal S L  are provided to control high-side switch HS and low-side switch LS, respectively. In step S 12 , duty-cycle detector  202  generates duty signal DT H  representing the duty cycle of high-side switch HS, based on high-side control signal S H . In step S 14 , threshold generators  204 P and  204 N provide thresholds V CSP-OCP  and V CSN-OCP  respectively, in response to duty signal DT H . In step  16 , detector  108  detects resonant circuit RSNT to provide current-sense signal V CS . In step S 18 , over-current protection circuit  206  triggers over-current protection in response to current-sense signal V CS , and thresholds V CSP-OCP  and V CSN-OCP . 
     While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.