Abstract:
A switching power supply with a resonant converter has an AC to DC converter and a DC to DC converter. The AC to DC converter converts an inputted AC power into a DC power. The DC to DC converter has a resonant converter determining a current operating state according to waveforms of a transformer voltage and a driving signal actually measured and further controlling a switching frequency of the resonant converter to approach or to be equal to a resonant frequency for operational efficiency enhancement. Accordingly, the failure to accurately calculate a resonant frequency beforehand can be solved and the issue of accurately keeping the switching frequency consistent with the resonant frequency can be tackled.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a switching power supply with a resonant converter and a method controlling the same, and more particularly to a switching power supply with a resonant converter compensating a switching frequency thereof to approach a resonant frequency thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    With reference to  FIG. 12 , a conventional switching power supply with a resonant converter has an AC (Alternating Current) to DC (Direct Current) converter  70  and a DC to DC converter  80  formed by a resonant converter. The AC to DC converter  70  converts an AC power into a high-voltage DC power, such as 380 V DC power. The DC to DC converter  80  then converts the high-voltage DC power into a DC power with a desired voltage. The DC to DC converter  80  is formed by an LLC converter (two inductors and one capacitor). With reference to  FIG. 13 , an LLC converter  90  has a half-bridge circuit  91 , a resonant circuit  92 , a transformer  93 , and an output circuit  94 . 
         [0005]    The half-bridge circuit  91  is connected to the primary side of the transformer  93  through the resonant circuit  92 . The secondary side of the transformer  93  is connected to the output circuit  94 . 
         [0006]    The resonant circuit  92  has a resonant capacitor Cr, an excited inductor Lm and a resonant inductor Lr of the transformer  93 . The resonant circuit  92  has two resonant frequencies. One of the resonant frequencies Fr 1  is determined by the resonant capacitor Cr, the excited inductor Lm, and the resonant inductor Lr of the transformer  93 . The other resonant frequency Fr 2  is determined by the resonant capacitor Cr and the resonant inductor Lr of the transformer  93 . 
         [0007]    When a load of the foregoing switching power supply is relatively light or an input voltage of the LLC converter  90  is relatively high, a switching frequency Fs of the LLC converter  90  is greater than the resonant frequency Fr 2  and a gain obtained by a ratio between an output voltage and an input voltage of the LLC converter  90  is lowered. When the load of the LLC converter  90  is relatively heavy or an input voltage of the LLC converter  90  is relatively low, the resonant converter  90  lowers the switching frequency Fs to acquire a higher gain, thereby satisfying the load demand. Under the circumstance, the switching frequency Fs is lower than the resonant frequency Fr 2 . 
         [0008]    From the foregoing, the LLC converter  90  adjusts the switching frequency thereof according to how the load or the input voltage varies. Generally, when the switching frequency Fs approaches or is equal to the resonant frequency Fr 2 , the switching power supply has an optimal working efficiency. As mentioned, the resonant frequency of the LLC converter  90  is determined by resonant elements, such as the resonant capacitor Cr and the resonant inductor. It means that the resonant frequency is a preset value calculated according to the specifications of the resonant elements, and the switching frequency is adjusted according to the preset value. However, the reality is that specification error of the resonant elements oftentimes exists upon production of the resonant elements, and the resonant frequency generated by the resonant elements with specification error is hard to be the same resonant frequency as indicated in the specification. Hence, even though the LLC converter  90  is accurately controlled for the switching frequency thereof to approach or be equal to the resonant frequency, an optimal working efficiency fails to be effectively achieved. 
       SUMMARY OF THE INVENTION 
       [0009]    An objective of the present invention is to provide a switching power supply with a resonant converter and a method controlling the same. The switching power supply and the method determine an operating state of the switching power supply, and adjust a switching frequency according to the operating state to make the switching frequency approach an actual resonant frequency and enhance operational efficiency. 
         [0010]    To achieve the foregoing objective, the switching power supply with a resonant converter has an AC (Alternating Current) to DC (Direct Current) converter and a DC to DC converter. 
         [0011]    The AC to DC converter has an AC power input terminal, a DC power output terminal and a control terminal. 
         [0012]    The DC to DC converter has a resonant converter, a resonant controller, and a phase detector. 
         [0013]    The phase detector is connected to the resonant converter and the resonant controller to respectively acquire a transformer voltage and a driving signal and generate a conversion voltage signal based on the transformer voltage and the driving signal. 
         [0014]    The resonant controller generates a feedback voltage control signal according to the conversion voltage signal and sends the feedback voltage control signal to the control terminal of the AC to DC converter to adjust a DC voltage outputted from the AC to DC converter and further control a switching frequency of the resonant converter of the DC to DC converter. 
         [0015]    The foregoing switching power supply respectively acquires waveforms of a transformer voltage and a driving signal from the resonant converter and the resonant controller with the phase detector, and calculates with the waveforms to generate a conversion voltage signal in response to a current operating state. When the conversion voltage signal is nonzero, it indicates that the switching frequency is greater than or less than the resonant frequency. The resonant controller then generates a feedback voltage control signal according to calculation of the conversion voltage signal and the Dc power voltage outputted from the AC to DC converter, and sends the feedback voltage control signal to the AC to DC converter to adjust an output voltage of the AC to DC converter, that is, an input voltage to the resonant converter. The switching frequency varies with the input voltage of the resonant converter so as to approach or to be equal to the resonant frequency. 
         [0016]    To achieve the foregoing objective, the method controlling a switching power supply having a resonant converter has steps of: 
         [0017]    acquiring a transformer voltage and a driving signal from a resonant converter to generate a present conversion voltage signal; 
         [0018]    determining if the present conversion voltage signal is zero; 
         [0019]    determining if a difference value between the present conversion voltage signal and a previous conversion voltage signal is greater than zero when the present conversion voltage signal is nonzero, wherein the previous conversion voltage signal is generated by a transformer voltage and a driving signal previously obtained; and 
         [0020]    determining if a switching frequency of the resonant converter is reduced when the difference value is not greater than zero, decreasing the switching frequency when the switching frequency is reduced, and increasing the switching frequency when the switching frequency is not reduced. 
         [0021]    The foregoing method first decreases or increases the switching frequency of the resonant converter, and determines a current operating state of the switching power supply according to the actually measured waveforms of the transformer voltage and the driving signal. When the present conversion voltage signal generated by the waveforms of the transformer voltage and the driving signal is nonzero, it indicates that the switching frequency of the switching power supply and the resonant frequency are inconsistent. The method further determines if the conversion voltage signal is less than a previously obtained conversion voltage signal, and if positive, indicating that the compensation direction is correct, the method further decreases or increases the switching frequency. The foregoing steps are performed continuously until the conversion voltage signal is zero. In other words, the switching frequency and the resonant frequency are consistent. 
         [0022]    Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a circuit diagram of a first embodiment of a switching power supply in accordance with the present invention; 
           [0024]      FIG. 2  is a circuit diagram of a second embodiment of a switching power supply in accordance with the present invention; 
           [0025]      FIG. 3  is a circuit diagram of a third embodiment of a switching power supply in accordance with the present invention; 
           [0026]      FIG. 4  is a waveform diagram of a resonant circuit of the switching power supply in each of  FIGS. 1 to 3  when the resonant circuit is operated at a switching frequency less than a resonant frequency; 
           [0027]      FIG. 5  is a waveform diagram of a resonant circuit of the switching power supply in each of  FIGS. 1 to 3  when the resonant circuit is operated at a switching frequency greater than a resonant frequency; 
           [0028]      FIG. 6  is a circuit diagram of a phase detector in a DC to DC converter of the switching power supply in each of  FIGS. 1 to 3 ; 
           [0029]      FIG. 7  is a waveform diagram associated with transformer voltage and driving signal when the phase detector in  FIG. 6  is operated at a light load; 
           [0030]      FIG. 8  is a waveform diagram associated with transformer voltage and driving signal when the phase detector in  FIG. 6  is operated at a heavy load; 
           [0031]      FIG. 9  is a flow diagram of a first embodiment of a method compensating a switching frequency of the switching power supply in each of  FIGS. 1 to 3 ; 
           [0032]      FIG. 10  is a flow diagram of a second embodiment of a method compensating a switching frequency of the switching power supply in each of  FIGS. 1 to 3 ; 
           [0033]      FIG. 11  is a circuit diagram of a control module built in an AC to DC converter in the switching power supply in each of  FIGS. 1 to 3 ; 
           [0034]      FIG. 12  is a functional block diagram of a conventional, switching power supply; and 
           [0035]      FIG. 13  is a circuit diagram of an LLC circuit in the conventional switching power supply in  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    With reference to  FIG. 1 , a first embodiment of a switching power supply in accordance with the present invention has an AC to DC converter  10  and a DC to DC converter  20 . 
         [0037]    The AC to DC converter  10  has an AC power input terminal AC IN, a DC power output terminal DC OUT and a control terminal BC, and serves to convert a mains power inputted from the AC power input terminal AC IN into a relatively high DC voltage Vbulk and output the DC voltage Vbulk through the DC power output terminal DC OUT. The control terminal BC affects the DC voltage Vbulk outputted from the DC power output terminal DC OUT. 
         [0038]    In the present embodiment, the DC to DC converter  20  has a resonant converter, a resonant controller  25  and a phase detector  30 . The resonant converter is formed by an LLC converter, and has a full-bridge circuit  21 , a resonant circuit  22 , a transformer  23  and an output circuit  24 . 
         [0039]    The full-bridge circuit  21  has multiple paired electronic switches QA˜QD being alternately turned on. Each electronic switch QA˜QD is connected to the resonant controller  25 , and is turned on by a driving signal provided by the resonant controller  25 . The resonant circuit  22  is formed by a resonant capacitor Cr, an excited inductor Lm and a resonant inductor Lr of the transformer  23 , and is connected between the DC power output terminal of the AC to DC converter  10  and the primary side of the transformer  23 . The secondary side of the transformer  23  is connected to the output circuit  24 . 
         [0040]    In the present embodiment, the transformer  23  has at least one transformer voltage-measuring point, such as at a coupling winding at the secondary side of the transformer  23  as shown in  FIG. 1 , at a coupling winding at the primary side of the transformer  23  as shown in  FIG. 2 , and at the secondary side of the transformer  23  as shown in  FIG. 3 , to provide a transformer voltage Vtr to the phase detector  30  for the phase detector  30  so as to determine a current operating state according to the transformer voltage Vtr and the driving signal (a gate-source voltage of the electronic switch QB (Vgs_QB) in the present embodiment) provided by the resonant controller  25 , and to further generate a conversion voltage signal Vturn based on the transformer voltage Vtr and the driving signal. In the present embodiment, the transformer voltage is measured at the coupling winding at the secondary side of the transformer  23 . 
         [0041]    The conversion voltage signal Vturn is used to determine a current operating state. Specifically, the conversion voltage signal Vturn determines that the switching frequency Fs of the resonant converter  22  is identical to the resonant frequency Fr. The concept of determination is described as follows. 
         [0042]    According to actual test results, when the switching frequency Fs and the resonant frequency of the LLC circuit are not the same, waveforms of the electronic switches of the full-bridge circuit  21  and the transformer  23  are illustrated in  FIG. 4 . It can be seen that the switching frequency Fs is less than the resonant frequency Fr 2  from the observation of the waveform of the transformer voltage Vtr. Under such operating state, the switching frequency of the electronic switches QA˜QD needs to be raised. With reference to  FIG. 5 , an operating state when the switching frequency Fs is greater than the resonant frequency Fr 2  is shown. Under such operating state, the switching frequency of the electronic switches QA˜QD needs to be lowered. 
         [0043]    With further reference to the waveforms shown in  FIGS. 4 and 5 , no matter if the switching frequency Fs is greater or less than the resonant frequency Fr 2 , a phase difference between the waveform of the transformer voltage Vtr and that of the driving signal appears as long as the resonant frequency Fr 2  and the switching frequency Fs are not equal. The present invention employs the phase detector  30  to measure the waveforms of the transformer voltage Vtr and the driving signal so as to determine whether there is inconsistency between the resonant frequency Fr 2  and the switching frequency Fs for the resonant controller  25  to compensate the switching frequency Fs. 
         [0044]    With reference to  FIG. 6 , the phase detector  30  has a comparator  31 , a logic gate  32  and a low-pass filter  33 . An input terminal of the comparator  31  is connected to any one of the at least one voltage-measuring point on the transformer  23  to acquire the waveform of the transformer voltage Vtr. A reference terminal of the comparator  31  is connected to a DC power source to serve as a DC reference voltage level. An output terminal of the comparator  31  is connected to an input terminal of the logic gate  32 . 
         [0045]    In the present embodiment, the logic gate  32  is an XOR (Exclusive OR) gate. The other input terminal of the logic gate  32  is connected to the resonant controller  25  to obtain a driving signal. The driving signal in the present embodiment is the gate-source voltage of the electronic switch QB (Vgs_QB). When operated under a light-load condition in  FIG. 7  and a heavy-load condition in  FIG. 8 , the phase detector  30  transmits a voltage signal V PHASE  generated by comparing the transformer voltage Vtr with the DC reference signal to the logic gate  32  to perform an exclusive OR operation with the driving signal (Vgs_QB) and generate a pulse signal Vx. To ensure signal accuracy, the pulse signal is further filtered by the low-pass filter  33  to obtain a conversion voltage signal Vturn transmitted to the resonant controller  25  for the resonant controller  25  to determine if the resonant frequency Fr 2  is inconsistent with the switching frequency Fs and perform compensation according to the determination result. Such compensation allows the resonant frequency Fr 2  and the switching frequency Fs to approach to consistency. Depending on the operating state of the switching power supply, the definition of “approach to consistency” may be a condition that the switching frequency Fs approaches the resonant frequency Fr 2  or that the switching frequency Fs is equal to the resonant frequency Fr 2 . 
         [0046]    In the present embodiment, the resonant controller  25  has an operator  251  and a control unit  252 . The operator  251  performs a subtraction operation between the conversion voltage Vturn and a reference voltage V REF  and sends an error value Verror out of the subtraction operation to the control unit  252  for the control unit  252  to determine if the compensation is necessary to be performed. The control unit  252  has a compensation process built therein. With reference to  FIG. 9 , the compensation process has the following steps. 
         [0047]    Step  701 : Determine if the error value Verror is equal to zero. If the error value Verror is zero, indicating a state that the resonant frequency Fr 2  approaches or is equal to the switching frequency Fs, end the determination process. If the error value Verror is nonzero, indicating a state that the resonant frequency Fr 2  and the switching frequency Fs are inconsistent, go to next step. 
         [0048]    Step  702 : Determine if a difference value (ΔVerror) between a present error value and a previous error value is greater than zero. If the difference value is not greater than the previous error value, go to next step (Step  703 ). Otherwise, perform step  706 . 
         [0049]    Step  703 : Determine if a present switching frequency is less than a previous switching frequency. If the present switching frequency Fs(n) is less than the previous switching frequency Fs(n-1), perform Step  704  and return to Step  701 . Otherwise, perform Step  705  and return to Step  701 . 
         [0050]    Step  704 : Decrease the switching frequency Fs. 
         [0051]    Step  705 : Increase the switching frequency Fs. 
         [0052]    Step  706 : Determine if a present switching frequency is less than a previous switching frequency. If the present switching frequency Fs(n) is less than the previous switching frequency Fs(n-1), perform Step  705  and return to Step  701 . Otherwise, perform Step  704  and return to Step  701 . 
         [0053]    After returning to Step  701 , the compensation process continues operation until the error value Verror is equal to zero, indicating that the resonant frequency Fr 2  and the switching frequency Fs are consistent. 
         [0054]    With reference to  FIG. 10 , the control unit  252  further has another compensation process built therein. The compensation process has the following steps. 
         [0055]    Step  801 : Determine if the error value Verror is equal to zero. If the error value Verror is zero, indicating a state that the resonant frequency Fr 2  approaches or is equal to the switching frequency Fs, end the compensation process. If the error value Verror is nonzero, indicating a state that the resonant frequency Fr 2  and the switching frequency Fs are inconsistent, go to next step. 
         [0056]    Step  802 : Determine if a difference value (ΔVerror) between a present error value and a previous error value is equal to zero. If the difference value is nonzero, go to next step (Step  803 ). Otherwise, perform step  808  and return to Step  801 . 
         [0057]    Step  803 : Determine if the difference value is greater than zero. If the difference value is greater than zero, indicating that a previous pre-adjustment compensates the switching frequency in an opposite direction, perform Step  807 . Otherwise, perform step  804 . 
         [0058]    Steps  804 ˜ 807  are substantially the same as Steps  703 ˜ 706  in the foregoing determination process except returning to Step  801  after decreasing or increasing the switching frequency Fs. 
         [0059]    Step  804 : Determine if a present switching frequency is not greater than a previous switching frequency. As the previous pre-adjustment compensates the switching frequency in a correct direction, if the present switching frequency Fs(n) is less than the previous switching frequency Fs(n-1), perform Step  805  and return to Step  801 . Otherwise, perform Step  806  and return to Step  801 . 
         [0060]    Step  805 : Decrease the switching frequency Fs. 
         [0061]    Step  806 : Increase the switching frequency Fs. 
         [0062]    Step  807 : Determine if a present switching frequency is less than a previous switching frequency. As the previous pre-adjustment compensates the switching frequency in an opposite direction, if the present switching frequency Fs(n) is less than the previous switching frequency Fs(n-1), perform Step  806  and return to Step  801 . Otherwise, perform Step  805  and return to Step  801 . 
         [0063]    Step  808 : Perform a pre-adjustment on the switching frequency Fs. 
         [0064]    After returning to Step  801 , the compensation process continues operation until the error value Verror is equal to zero, indicating that the resonant frequency Fr 2  and the switching frequency Fs are consistent. 
         [0065]    There are several ways of adjusting the switching frequency in the following. As the switching frequency Fs is related to a ratio of the output voltage and the input voltage (Vo/Vin) of the switching power supply or the gain, adjustment to any of the output voltage and the input voltage can change the switching frequency Fs. Furthermore, when the switching power supply is operated under an open-loop mode, fixed input voltage and variable output voltage are applied to adjust the switching frequency Fs. When the switching power supply is operated under a close-loop mode, variable input voltage is applied to adjust the switching frequency Fs. 
         [0066]    According to the illustration of  FIG. 1 , an input voltage of the DC to DC converter  20  is supplied by the AC to DC converter  10 , and when an output voltage of the AC to DC converter  10  varies, the switching frequency Fs of the DC to DC converter  20  is also changed. Hence, the resonant controller  25  of the DC to DC converter  20  generates a feedback voltage control signal (Bulk Control) and sends it to the control terminal BC of the AC to DC converter  10  to change the output voltage of the AC to DC converter  10 . The input voltage of the DC to DC converter  20  is changed by varying the output voltage of the AC to DC converter  10 , and the switching frequency Fs is thus adjusted. To one having ordinary skill in the art, it is understandable that both of the feedback DC voltage Vbulk and the feedback voltage control signal (Bulk Control) serve to adjust the output voltage of the AC to DC converter  10 . A corresponding substantial implementation is described as follows. 
         [0067]    With further reference to  FIG. 1 , the AC to DC converter  10  has a control module  100 . With reference to  FIG. 11 , the control module  100  has a superposition circuit  101  and a controller  102 . The superposition circuit  101  has two input terminals and an output terminal. The two input terminals are respectively connected to the DC power output terminal DC OUT and the control terminal BC of the AC to DC converter  10  to acquire the feedback DC voltage Vbulk and the feedback voltage control signal (Bulk Control) for signal superposition. The superposing signal is sent to an input terminal of the controller  102  for the controller  102  to generate a driving signal for adjusting the DC voltage Vbulk on the DC power output terminal DC OUT of the AC to DC converter  10 . 
         [0068]    In sum, because of the production error of the resonant elements, the LLC circuit fails to accurately calculate the resonant frequency Fr 2  beforehand. The uncertainty about the resonant frequency Fr 2  results in difficulty in effective adjustment of a desired relationship between the switching frequency Fs and the resonant frequency Fr 2 . Instead of using a preset resonant frequency Fr 2  as an adjustment basis, the present invention employs actually measured values to determine actual states of the switching frequency Fs and the resonant frequency Fr 2 . After the switching power supply enters a steady state, the switching frequency Fs is dynamically adjusted to increase operational efficiency and resolve the problem that the LLC circuit fails to accurately calculate the resonant frequency arising from the production error of the resonant elements. 
         [0069]    Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only. Changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.