Abstract:
A half-bridge resonant converter including a half-bridge switching circuit, a resonant circuit and a rectifier circuit is provided. The half-bridge switching circuit is controlled by two control signals for alternatively coupling two terminals of a DC power source to an input of a resonant circuit. The control signals have the same frequency and duty cycle, and one of the control signals is delayed a period by the other one, in which the frequency of the control signals is constant and the duty cycle of the control signals is variable according to a load. The rectifier circuit generates an output voltage across the load from an output of the resonant circuit. Therefore, the resonant converter of the present invention has the advantage of constant frequency operating and at least one of the two switches operated in ZVS.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power converters, and more particularly to half-bridge resonant converters. 
     2. Description of the Related Art 
     In recent years, miniaturization and power saving have become important issues in electronic equipments, especially in portable electronic equipments, and hence power converters used in electronic equipments are required to reduce the size and weight and increase the power conversion efficiency. This is achieved by using switching-mode power converters with semiconductor power switches. In reducing power converter size, designers have turned to increased switching frequencies. Higher switching frequencies allow for smaller, lighter inductive/capacitive energy storage and filter components, but also bring with them increased switching losses. 
     Switching losses include the power loss which results from the simultaneous presence of voltage and current in the semiconductor switch during turn-on and turn-off transitions. The semiconductor switch, for example, may be implemented by metal-oxide-semiconductor field-effect transistor (MOSFET). In addition, switching losses further include the power loss which results from the charging and discharging of the parasitic capacitance across the drain and source of the MOSFET switch. As the switching frequency increases, so do the switching losses. Excessive switching losses can result in damage to the switch and poor power conversion efficiency. 
     In order to reduce switching losses, resonant concepts are applied to switching power converters to allow zero-voltage switching (ZVS) and/or zero-current switching (ZCS) so as to minimize switching losses. All resonant converters operate in essentially the same way: a square pulse of voltage or current is generated by the power switches and this is applied to a resonant circuit. Energy circulates in the resonant circuit and some or all of it is then tapped off to supply the output. 
     Referring to  FIG. 1 , it is a schematic diagram of a conventional half-bridge series resonant converter  100 . Power provided by a DC power source  200  is delivered to the resonant converter  100  at an input voltage V IN  and is delivered to a load  300  at an output voltage V OUT . The resonant converter  100  includes a half-bridge switching circuit  110  for alternatively coupling two terminals of the DC power source  200  to an input of a resonant circuit  120 . The resonant circuit  120  includes resonant inductor L R  and resonant capacitor C R  coupled in series. In general, the resonant inductance L R  may comprise, in whole or in part, the leakage inductance of the transformer  130 . The rectifier circuit  140  includes diodes D 3 , D 4  and an output capacitor C O  to generate the output voltage V OUT  from an output of the resonant circuit  120 . 
     In  FIG. 1 , the switching circuit  110  includes a half-bridge circuit  112  with two switches Q 1 , Q 2  controlled by a control circuit  114 . The switches Q 1 , Q 2  are implemented by MOSFET. As switching current reversal is required, all switches must have freewheeling diodes. In the switching circuit  110 , the switches Q 1 , Q 2  employ external freewheeling diodes D 1 , D 2 . The control circuit  114  provides 50% duty cycle symmetrical control signals, which are identical to each other with 180-degree phase shift, to drive the switches Q 1 , Q 2  in a substantially complementary fashion such that the two terminals of the DC power source  200  are alternatively coupled to the input of the resonant circuit  120 . In practice, a short dead time should be set up to avoid the simultaneous conduction (or turn-on) of the two switches; hence, the duty cycle of the two control signals is not 50% but generally close to 50%. During the dead time, both of the switches Q 1 , Q 2  are turned off. 
     Referring to  FIG. 2 , it is a timing diagram of the control signals of the switches Q 1 , Q 2  and voltages across the switches Q 1 , Q 2  in the resonant converter  100  of  FIG. 1  for full load or light load. In the present embodiment, the control signals are two gate-to-source voltages V GS1 , V GS2 , and the voltages across the switches Q 1 , Q 2  are two drain-to-source voltages V DS1 , V DS2 . Here, the dead time τ D  is exaggerated for display. It is obvious that the switch Q 1  is turned on when the voltage V DS1  across the switch Q 1  is zero, and the switch Q 2  is turned on when the voltage V DS2  across the switch Q 2  is zero. In other words, both switches Q 1 , Q 2  are operated under ZVS so as to minimize switching losses. In such resonant converter  100 , as the load  300  is changed, the output voltage V OUT  (or output power) muse be regulated by controlling the switching frequency f S  (or the reciprocal of the switching period T). 
     Referring to  FIG. 3 , it is a diagram of output power v.s. frequency of the resonant converter  100  of  FIG. 1 . Under a light load (see curve  31 ), the resonant converter  100  operates at the operating point  31 ′ whose frequency is above and further away from the resonant frequency f R1 . When the load becomes heavy (see curve  32 ), the resonant converter  100  must provide more output power to drive the load, hence it operates at the operating point  32 ′ whose frequency is above and closer to the resonant frequency f R2 . The dashed curve  33  is the locus of operating points. It is shown that the switching frequency (or frequency of operating point) is changed as load is changed. The characteristic of variable (switching) frequency operating has the disadvantage of making the control and the electromagnetic interference (EMI) filter design more complicated. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a half-bridge resonant converter with a constant (switching) frequency operating and low switching losses. 
     Another object of the present invention is to provide a half-bridge resonant converter with a near-constant (switching) frequency operating and low switching losses. 
     In accordance with the present invention, a half-bridge resonant converter including a half-bridge switching circuit, a resonant circuit, a rectifier circuit and a control circuit is provided. Power provided by a DC power source is delivered to the resonant converter at an input voltage and is delivered to a load at an output voltage. The half-bridge switching circuit is coupled to the resonant circuit, and the resonant circuit is coupled to the rectifier circuit. The half-bridge switching circuit includes two switches which are controlled by two corresponding control signals for alternatively coupling two terminals of the DC power source to an input of the resonant circuit. The control signals have the same frequency and duty cycle, and one of the control signals is delayed a period by the other one, in which the period prevents simultaneous conduction of the switches. The frequency of the control signals is constant and the duty cycle of the control signals is variable according to the load, in which the duty cycle is less than or equal to 50%. The resonant circuit includes a resonant inductance and a capacitance. The rectifier circuit generates the output voltage from an output of the resonant circuit. The control circuit coupled to the two switches and the output voltage across the load generates the two control signals. 
     In accordance with the present invention, a half-bridge resonant converter including a half-bridge switching circuit, a resonant circuit, a rectifier circuit, a valley detector and a control circuit is provided. The valley detector is coupled to one of the two switches, in which the one of the two switches isn&#39;t operated under zero voltage switching. The control circuit is coupled to the valley detector, the two switches and the output voltage across the load. The valley detector detects a relatively minimum of a voltage across the one of the two switches and sends a signal representing the detection of the relatively minimum. The control circuit generates the two control signals. By the two control signals, the control circuit delays the turn-ons of the two switches until the control circuit receives the signal representing the detection of the relatively minimum. 
     Therefore, the half-bridge resonant converter of the present invention has the advantage of constant or near-constant frequency operating and at least one of the two switches operated in ZVS. The constant or near-constant frequency operating makes the control and the EMI filter design simpler. At least one of the two switches operated in ZVS makes switching losses acceptable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the disclosure will be apparent and easily understood from a further reading of the specification, claims and by reference to the accompanying drawings in which like reference numbers refer to like elements and wherein: 
         FIG. 1  is a schematic diagram of a conventional half-bridge series resonant converter; 
         FIG. 2  is a timing diagram of control signals of switches and voltage across the switches in the resonant converter of  FIG. 1 ; 
         FIG. 3  is a diagram of transfer function of output power of the resonant converter of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a resonant converter in accordance with one embodiment of the present invention; 
         FIG. 5  is a timing diagram of control signals of switches in the resonant converter of  FIG. 4 ; 
         FIG. 6  is a timing diagram of simulation of control signals of switches and voltage across the switches in the resonant converter of  FIG. 4 ; 
         FIG. 7  is a schematic diagram of a resonant converter in accordance with another embodiment of the present invention; and 
         FIG. 8  is a timing diagram of simulation of control signals of switches and voltage across the switches in the resonant converter of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. These embodiments are provided so that this application will be thorough and complete. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. 
       FIG. 4  is a schematic diagram of a resonant converter in accordance with one embodiment of the present invention. Referring to  FIGS. 1 and 4 , the circuit topology of the resonant converter  400  is similar to the resonant converter  100  of  FIG. 1 . The only difference between the resonant converters  100  and  400  is the control signals applied to the switches Q 1 , Q 2  of the half-bridge switching circuit  110 . The control circuit  114  of  FIG. 1  provides two (symmetrical) control signals with the same frequency which is variable and the same duty cycle which is constant and equal to 50%, as a result both switches Q 1 , Q 2  are operated under ZVS but the resonant converters  100  is operated at a variable frequency according to the load  300 . 
     Instead, the control circuit  414  of  FIG. 4  provides two control signals with the same frequency which is constant and the same duty cycle which is variable. Referring to  FIG. 5 , it is a timing diagram of the control signals V GS1 , V GS2  of the switches Q 1 , Q 2  in the resonant converter  400  of  FIG. 4 . Under full load condition, the control signals V GS1 , V GS2  have the same frequency f S1  (=1/T 1 ) and the same duty cycle d 1  (=τ 1 /T 1 ), and one of the control signals is delayed a period by the other one, for example, the control signal V GS1  is delayed the period τ D  by the control signal V GS2 . Under light load condition, the control signals V GS1 , V GS2  have the same frequency f S1  (=1/T 1 ) and the same duty cycle d 2  (=τ 2 /T 1 ), and one of the control signals is delayed a period by the other one, for example, the control signal V GS1  is delayed the period τ D  by the control signal V GS2 . Note that the frequency is constant and the duty cycle is variable according to the load. In the present embodiment, the frequency is always fixed at f S1 . The duty cycle is τ 1 /T 1  under full load or τ 2 /T 1  under light load. 
     Referring to  FIG. 6 , it is a timing diagram of simulation of the control signals V GS1 , V GS2  of the switches Q 1 , Q 2  and the voltages V DS1 , V DS2  across the switches Q 1 , Q 2  in the resonant converter  400  of  FIG. 4 . Under full load condition, the switch Q 1  is turned on when the voltage V DS1  across the switch Q 1  is zero, and the switch Q 2  is turned on when the voltage V DS2  across the switch Q 2  is zero. Both switches Q 1 , Q 2  are operated under ZVS so as to minimize switching losses. It is similar to the timing diagram of the conventional resonant converter  100  as shown in  FIG. 2 . Under light load condition, the switch Q 1  is turned on when the voltage V DS1  across the switch Q 1  is zero, but the switch Q 2  is turned on when the voltage V DS2  across the switch Q 2  is not zero, wherein the voltage V DS2  across the switch Q 2  is about V IN /2 because the switches Q 1 , Q 2  have the same duty cycle. Even through the switch Q 2  isn&#39;t operated under ZVS, the switching loss resulted from the switch Q 2  is small because the voltage V DS2  across the switch Q 2  is small (about V IN /2) when the switch Q 2  is turned on. 
     In the resonant converter  400  of the present invention, the switching frequency f S1  is constant no matter how load is changed. As the load  300  is changed, the duty cycle of the control signals V GS1 , V GS2  of the switches Q 1 , Q 2  is changed to regulate the output voltage V OUT  to match the load  300 . The characteristic of constant (switching) frequency operating has the advantage of making the control and the electromagnetic interference (EMI) filter design simpler. The price is the low switching loss because one of the switches Q 1 , Q 2  isn&#39;t operated under ZVS when the load is under light load condition, for example, Q 2  isn&#39;t operated under ZVS as shown in  FIG. 6 . The switching loss resulted from the switch Q 2  is small because the voltage V DS2  across the switch Q 2  is small (about V IN /2). Therefore, the total switching losses of the resonant converter  400  of the present invention are small and acceptable. 
     Referring again to  FIG. 6 , there are four stages I-IV during the switching period T 1 . During the stage I, the switches Q 1 , Q 2  are turned off, and the duration of the stage I is long enough for the resonant inductor L R  and the resonant capacitor C R  to generate signal oscillation of the voltages V DS1 , V DS2 . The signal oscillation damps as the energy is almost transferred to the rectifier circuit  140  through the transformer  130 . When the signal oscillation of the voltage V DS2  damps approximately to V IN /2, the switch Q 2  is turned on so that the switch Q 2  is not operated under ZVS. The switching loss resulted from the switch Q 2  is small because the voltage V DS2  across the switch Q 2  is small (about V IN /2). However, it may happen that the switch Q 2  is turned on at a relatively maximum of the voltage V DS2  when the signal oscillation of the voltage V DS2  damps approximately to V IN /2, as illustrated in  FIG. 6 . In order to further reduce the switching loss resulted from the switch Q 2 , a valley detector is employed in the resonant converter  400 . 
     Referring to  FIG. 7 , a valley detector  416  is employed in the resonant converter  400  to constitute the resonant converter  700 . The inputs of the valley detector  416  are coupled to two terminal of the switch Q 2  to detect a relatively minimum of the voltage V DS2  when the signal oscillation of the voltage V DS2  damps approximately to V IN /2. The output of the valley detector  416  is coupled to the control circuit  414  to send a signal representing the detection of the relatively minimum of the voltage V DS2  when the signal oscillation of the voltage V DS2  damps approximately to V IN /2. The control circuit  414  delays the turn-ons of the switches Q 1 , Q 2  until it receives the signal sent from the valley detector  416 , as illustrated in  FIG. 8 . Referring to  FIG. 8 , the switching period will change slightly because of the delay of the turn-ons of the switches. For example, the switching period changes from T 1  to T 2  when the load becomes light. Note that the switching period T 2  is close to the switching period T 1 , so the resonant converter  700  has a “near-constant” (switching) frequency operating. The resonant converter  700  guarantees that the switch Q 2  is turned on at a relatively minimum of the voltage V DS2  when the signal oscillation of the voltage V DS2  damps approximately to V IN /2, so as to further reduce the switching loss resulted from the switch Q 2 . 
     In summary, the present invention employs two control signals having the same constant or near-constant frequency and the same variable duty cycle, and one of the control signals is delayed a period by the other one, to drive the switches of the half-bridge switching circuit of the resonant converter. Therefore, the resonant converter of the present invention has the advantage of constant or near-constant frequency operating and at least one of the two switches operated in ZVS. The constant or near-constant frequency operating makes the control and the EMI filter design simpler. At least one of the two switches operated in ZVS makes switching losses acceptable. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.