Abstract:
A full-bridge quasi-resonant DC-DC converter is provided, including a transformer having a primary winding and a secondary winding, a full-bridge converting circuit electrically connected with the primary winding of the transformer, a resonant capacitor provided between the full-bridge converting circuit and the primary winding, a rectifier circuit electrically connected with the secondary winding of the transformer, and a resonant inductor connected in series with the rectifier circuit. Therefore, the full-bridge quasi-resonant DC-DC converter reduces the switching losses of the switching elements and effectively reduces the size of the converter, while increases the conversion efficiency.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to DC-DC converters, and, more particularly, to a full-bridge quasi-resonant DC-DC converter. 
         [0003]    2. Description of Related Art 
         [0004]    Due to the growing international energy demand, increasing scarcity of non-renewable energy such as oil and climate changes and other issues, the development of clean and renewable energy source, such as solar energy, wind energy, ocean (tidal or temperature difference), hydro, geothermal, biomass energy and the like, is imminent. In general, clean renewable energy is a less stable source of energy. Therefore, the conversion of this unstable energy through energy conversion device to energy that can be used for households or industries is a key for the development of renewable energy. 
         [0005]    Full-bridge DC-DC converters have a wide operating range (maximum input voltage to minimum input voltage), making the circuit more flexible in design, and therefore are common application architectures. However, traditional full-bridge DC-DC converters adopt hard switching, and since switching is in an ideal state, the switching elements will create switching losses when they are turned on and turned off. Also, line equivalent inductance tends to produce surge voltage on the switching elements, reducing the life of the switching elements. 
         [0006]    In order to solve the problems associated with hard switching, resonant circuits are used to reduce the voltage and current stress of the switching elements. Nevertheless, the resonant inductor of a traditional resonant circuit is typically at the primary winding of the converter. When the primary side is in a state of low-voltage and high current, the design of the resonant inductor can be difficult and inefficient. 
         [0007]    Therefore, how to provide a DC-DC converter that enables soft switching and reduces line losses of the resonant inductor has become an important issue to be solved by those skilled in the art. 
       SUMMARY 
       [0008]    The present disclosure provides a full-bridge quasi-resonant DC-DC converter, which includes: a transformer, a full-bridge converting circuit and a resonant inductor, wherein the transformer includes a primary winding and a secondary winding, the full-bridge converting circuit includes a first arm circuit having a first switching element disposed at a first upper arm and a second switching element disposed at a first lower arm and connected in series with the first switching element, a second arm circuit connected in parallel with the first arm circuit, and having a third switching element disposed at a second upper arm and a fourth switching element disposed at a second lower arm and connected in series with the third switching element, an input end, an output end electrically connected with the primary winding of the transformer, and a resonant capacitor provided between the first arm circuit or the second arm circuit and the primary winding, the rectifier circuit is electrically connected with the secondary winding of the transformer for rectifying signals generated by the secondary winding to produce output signals, the resonant inductor connected in series with the rectifier circuit, and the resonant inductor and the resonant capacitor form a resonant circuit. 
         [0009]    The present disclosure further provides a driving method of soft-switching the full-bridge quasi-resonant DC-DC converter. The method includes the steps of: (1) turning on the first switching element at the first upper arm and the fourth switching element at the second lower arm so as to transfer electrical energy received at the input end of the full-bridge converting circuit from the primary winding of the transformer to the secondary winding, and charging the resonance capacitor; (2) when no current flows through the first switching element and the fourth switching element, turning off one of the first switching element and the fourth switching element to achieve zero-current switching, and turning on one of the second switching element and the third switching element in order to transfer the electrical energy stored in the resonant capacitor during step (1) to the secondary winding; (3) after going through a preset power-off output region, turning on the third switching element or the second switching element that is not yet turned on during step (2) to again transfer the electrical energy received at the input end from the primary winding of the transformer to the secondary winding, and charging the resonant capacitor; and (4) when no current flows through the second switching element and the third switching element, turning off one of the second switching element and the third switching element to achieve zero-current switching, and turning on the first switching element or the fourth switching element in order to again transfer the electrical energy stored in the resonant capacitor during step (3) to the secondary winding. 
         [0010]    In the driving method of soft switching the full-bridge quasi-resonant DC-DC converter, the method may alternatively include, in step (1), first turning on the second switching element and the third switching element, and then in step (2) turning off one of the second switching element and the third switching element to achieve zero-current switching, and turning on one of the first switching element and the fourth switching element. Then, after going through a preset power-off output region, turning on the fourth switching element or the first switching element that is not yet turned on during step (2), and then when no current flows through the first switching element and the fourth switching element, turning off one of the first switching element and the fourth switching element to achieve zero-current switching, and turning on the second switching element or the third switching element. This can achieve a result similar to the above driving method of the above full-bridge quasi-resonant DC-DC converter. 
         [0011]    The present disclosure further provides another driving method of soft-switching the full-bridge quasi-resonant DC-DC converter. The method includes the steps of: (1) turning on the first switching element at the first upper arm and the fourth switching element at the second lower arm so as to transfer electrical energy received at the input end of the full-bridge converting circuit from the primary winding of the transformer to the secondary winding, and charging the resonance capacitor; (2) when no current flows through the first switching element and the fourth switching element, turning off the first switching element and the fourth switching element to achieve zero-current switching, and turning on the second switching element and the third switching element in order to continuously transfer the electrical energy received at the input end of the full-bridge converting circuit from the primary winding of the transformer to the secondary winding, and charging the resonant capacitor; and (3) when no current flows through the second switching element and the third switching element, turning off one of the second switching element and the third switching element to achieve zero-current switching, and turning on the first switching element or the fourth switching element in order to transfer the electrical energy stored in the resonant capacitor during step (1) and step (2) to the secondary winding. 
         [0012]    In the another driving method of soft switching the full-bridge quasi-resonant DC-DC converter, the method may alternatively include, in step (1), first turning on the second switching element and the third switching element, and then in step (2) turning off the second switching element and the third switching element to achieve zero-current switching, and turning on the first switching element and the fourth switching element, and then when no current flows through the first switching element and the fourth switching element, turning off one of the first switching element and the fourth switching element to achieve zero-current switching, and turning on the second switching element or the third switching element. This can achieve a result similar to the above another driving method of the full-bridge quasi-resonant DC-DC converter. 
         [0013]    Compared to the prior art, the full-bridge quasi-resonant DC-DC converter of the present disclosure shifts the resonant inductor to the secondary side, and together with the soft-switching driving methods, effectively reduces the switching losses of the switching elements as well as the size of the resonant inductor, and lowers the average peak current, thereby improving the overall conversion efficiency of the converter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a circuit diagram of a full-bridge quasi-resonant DC-DC converter in accordance with an embodiment of the present disclosure; 
           [0016]      FIGS. 2A to 2D  are waveform diagrams illustrating a driving method of soft switching the full-bridge quasi-resonant DC-DC converter in accordance with an embodiment of the present disclosure; and 
           [0017]      FIGS. 3A to 3D  are waveform diagrams illustrating a driving method of soft switching the full-bridge quasi-resonant DC-DC converter in accordance with another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a through understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
         [0019]      FIG. 1  is a circuit diagram of a full-bridge quasi-resonant DC-DC converter in accordance with an embodiment of the present disclosure. The full-bridge quasi-resonant DC-DC converter includes a transformer  10 , a full-bridge converting circuit  11 , a rectifier circuit  12 , and a resonant inductor  13 . 
         [0020]    The transformer  10  has a primary winding  101  and a secondary winding  102 . In an embodiment, the primary winding  101  of the transformer  10  has a coil, and the secondary winding  102  of the transformer  10  has two coils connected in series with each other and is a center-tapped winding or divided winding. 
         [0021]    The full-bridge converting circuit  11  includes a first arm circuit  11   a  and a second arm circuit  11   b . The first arm circuit  11   a  has a first switching element Q 1  disposed at a first upper arm and a second switching element Q 2  disposed at a first lower arm and connected in series with the first switching element Q 1 . The second arm circuit  11   b  has a third switching element Q 3  disposed at a second upper arm and a fourth switching element Q 4  disposed at a second lower arm connected in series with the third switching element Q 3 . The full-bridge converting circuit  11  further includes an input end  110  and an output end  111 . The output end  111  is electrically connected with the primary winding  101  of the transformer  10 . A resonant capacitor  14  is provided between the first arm circuit  11   a  or the second arm circuit  11   b  and the primary winding  101 . 
         [0022]    The rectifier circuit  12  is electrically connected with the secondary winding  102  of the transformer  10  for rectifying signals generated by the secondary winding  102  to produce output signals. 
         [0023]    In an embodiment, the rectifier circuit  12  is a full-bridge rectifier circuit having four switching elements that can be, for example, diodes or metal oxide-semiconductor field-effect transistors (MOSFET). 
         [0024]    In an embodiment, the full-bridge quasi-resonant DC-DC converter  1  further includes a fifth switching element Q 5  and a sixth switching element Q 6  connected in series with the fifth switching element Q 5 . The rectifier circuit  12  is connected in parallel with the fifth switching element Q 5  and the sixth switching element Q 6 . The output signals of the rectifier circuit  12  are converted into AC signals through the fifth switching element Q 5  and the sixth switching element Q 6  in order to be in sync with the mains. 
         [0025]    The resonant inductor  13  is connected in series with the rectifier circuit  12 . The resonant inductor  13  and the resonant capacitor  14  form a resonant circuit. The resonant inductor  13  is located at a secondary side of the full-bridge quasi-resonant DC-DC converter  1 . Since the current at the secondary side is less than that at a primary side, it does not affect the resonant status, and the design of the resonant inductor  13  is relatively simpler and smaller in size. 
         [0026]    In an embodiment, the resonant inductor  13  is provided at the secondary side of the transformer  10 , and also serves as a filter inductor for filtering the output signals of the rectifier circuit. 
         [0027]    In an embodiment, the full-bridge quasi-resonant DC-DC converter  1  further includes a first capacitor  15  electrically connected with the input end  110  of the full-bridge converting circuit  11  for stabilizing the voltage inputted into the full-bridge converting circuit  11 . 
         [0028]    In an embodiment, the full-bridge quasi-resonant DC-DC converter  1  further includes a second capacitor  16  electrically connected with the rectifier circuit  12  for filtering the output signals of the rectifier circuit  12 . 
         [0029]    In an embodiment, the first to sixth switching elements are power MOSFETs. 
         [0030]      FIGS. 2A to 2D  are waveform diagrams illustrating a driving method of soft switching the full-bridge quasi-resonant DC-DC converter in accordance with an embodiment of the present disclosure. As shown in  FIGS. 2A and 2B , based on the conduction states of the switching elements, the operations of the circuit is divided into time periods t 0  to t 6 , and the operations in each of the periods t 0  to t 6  are described as follow: 
         [0031]    (1) t 0  to t 1   
         [0032]    The first switching element Q 1  disposed at the first upper arm and the fourth switching element Q 4  disposed at the second lower arm are turned on, so as to transfer the electrical energy received at the input end  110  of the full-bridge converting circuit  11  from the primary winding  101  to the secondary winding  102 , and the resonance capacitor  14  is being charged. 
         [0033]    (2) t 1  to t 2   
         [0034]    When no current flows through the first switching element Q 1  and the fourth switching element Q 4 , the first switching element Q 1  (see  FIG. 2A ) or the fourth switching element Q 4  (see  FIG. 2B ) is turned off to achieve zero-current switching, and the second switching element Q 2  or the third switching element Q 3  is correspondingly turned on in order to again the electrical energy stored in the resonant capacitor  14  to the secondary winding  102 . 
         [0035]    (3) t 2  to t 3   
         [0036]    This is a preset power-off output region. 
         [0037]    (4) t 3  to t 4   
         [0038]    After the preset power-off output region, the third switching element Q 3  or the second switching element Q 2  that is not yet turned on during the period of t 1  to t 2  is turned on to again transfer the electrical energy received at the input end  110  from the primary winding  101  to the secondary winding  102 , and the resonant capacitor  14  is being charged. 
         [0039]    (5) t 4  to t 5   
         [0040]    When no current flows through the second switching element Q 2  and the third switching element Q 3 , the second switching element Q 2  (see  FIG. 2A ) or the third switching element Q 3  (see  FIG. 2B ) is turned off to achieve zero-current switching, and the first switching element Q 1  or the fourth switching element Q 4  is correspondingly turned on in order to again transfer the electrical energy stored in the resonant capacitor  14  to the secondary winding  102 . 
         [0041]    (6) t 5  to t 6   
         [0042]    Similarly, this is another preset power-off output region. 
         [0043]      FIGS. 2C and 2D  are different from the first embodiment shown in  FIGS. 2A and 2B  in that the driving method in  FIGS. 2C and 2D  involves first turning on the second switching element Q 2  at the first lower arm and the third switching element Q 3  at the second upper arm. Apart from this, other operations are similar to the driving method shown in  FIGS. 2A and 2B , so they will not be further described. 
         [0044]      FIG. 3A to 3D  are waveform diagrams illustrating a driving method of soft switching the full-bridge quasi-resonant DC-DC converter in accordance with another embodiment of the present disclosure. As shown in  FIGS. 3A and 3B , based on the conduction states of the switching elements, the operations of the circuit are divided into time periods t 0  to t 4 , and the operations in each of the periods t 0  to t 4  are described as follow: 
         [0045]    (1) t 0  to t 1   
         [0046]    The first switching element Q 1  disposed at the first upper arm and the fourth switching element Q 4  disposed at the second lower arm are turned, so as to transfer the electrical energy received at the input end  110  of the full-bridge converting circuit  11  from the primary winding  101  to the secondary winding  102 , and the resonance capacitor  14  is being charged. 
         [0047]    (2) t 1  to t 2   
         [0048]    When no current flows through the first switching element Q 1  and the fourth switching element Q 4 , the first switching element Q 1  and the fourth switching element Q 4  are turned off to achieve zero-current switching, and the second switching element Q 2  and the third switching element Q 3  are correspondingly turned on in order to continuously transfer the electrical energy received at the input end  110  of full-bridge converting circuit  11  from the primary winding  101  to the secondary winding  102 , and the resonance capacitor  14  is being charged. 
         [0049]    (3) t 2  to t 3   
         [0050]    When no current flows through the second switching element Q 2  and the third switching element Q 3 , the second switching element Q 2  (see  FIG. 3A ) or the third switching element Q 3  (see  FIG. 3B ) is turned off, and the first switching element Q 1  or the fourth switching element Q 4  is correspondingly turned on in order to transfer the electrical energy stored in the resonant capacitor  14  to the secondary winding  102 . 
         [0051]    (4) t 3  to t 4   
         [0052]    This is a preset power-off output region. In an embodiment, since one process of releasing energy from the resonant capacitor  14  is eliminated, the root mean square (RMS) current of the full-bridge quasi-resonant DC-DC converter of the present disclosure can be reduced, thereby enhancing the overall efficiency. 
         [0053]      FIGS. 3C and 3D  are different from the first embodiment shown in  FIGS. 3A and 3B  in that the driving method in  FIGS. 3C and 3D  involves first turning on the second switching element Q 2  at the first lower arm and the third switching element Q 3  at the second upper arm. Apart from this, other operations are similar to the driving method shown in  FIGS. 3A and 3B , so they will not be further described. 
         [0054]    Moreover, the driving method disclosed in the above embodiments are used for soft switching the full-bridge quasi-resonant DC-DC converter in accordance with the present disclosure; however, the present invention is not limited to this, but can be used to drive other types of DC-DC converters with a full-bridge converting circuit. 
         [0055]    It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.