# Patent

## Title
Half-bridge flyback power converter

## Abstract

A half-bridge flyback power converter is provided. The half-bridge flyback power converter comprises a high-side switch and a low-side switch periodically conducting an input voltage to a primary winding of a transformer. When the high-side switch and the low-side switch are switched off, energy stored in the transformer is transmitted to a secondary circuit and serves to charge a bulk capacitor. A forward diode facilitates to combine the input voltage with a voltage across the bulk capacitor for powering the secondary circuit. Therefore, the power consumption of the leakage inductance is reduced and circuit topology is simplified.

## Background
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a power converter, and more particularly to a flyback power converter using half-bridge structure.
2. Description of the Related Art
Power converters have been used to convert an unregulated power source to a regulated voltage or a regulated current. Referring to FIG. 1, it illustrates a schematic circuit diagram of a conventional flyback power converter. A transistor 15 controlled by a control signal PWM switches a transformer 50 to transmit the power from the primary-side of the transformer 50 to a secondary circuit and to regulate an output voltage VO. A capacitor 42 and a snubber resistor 41 are used to store and consume energy of a leakage inductor of the transformer 50. The energy of the leakage inductor of the transformer 50 must be completely dissipated; otherwise, a drain voltage of the transistor 15 will be continuously increased until the energy of the leakage inductor is fully dissipated by the snubber resistor 41. The energy therein is given as,




  
    
      
        
          
            V
            R41
            2
          
          
            R
            41
          
        
        =
        
          
            1
            2
          
          ×
          
            L
            I
          
          ×
          
            I
            P
            2
          
          ×
          f
        
      
    
    
      
        (
        1
        )
      
    
  




where VR41 is the voltage across the snubber resistor 41; R41 is the resistance of the snubber resistor 41; f is the switching frequency.

One of the drawbacks of the conventional flyback power converter is that the leakage inductor of the transformer 50 consumes considerable power and thus reduces the efficiency of the power converter. Besides, an operating voltage VDS of the transistor 15 is increased because of the energy involving the leakage inductance, to which the voltage VR41 adds the operating stress of the transistor 15 (VDS=VIN+VR41). Another drawback of the conventional flyback power converter is a NTC (negative temperature coefficient) thermistor 11 that is disposed for to restricting an inrush current of the flyback power converter. When the flyback power converter is turned on, an inrush current is induced when charging the capacitor 18. The inrush current reduces reliability of the flyback power converter. In addition, the NTC thermistor 11 increases the power consumption and operating temperature of the flyback power converter.
Therefore, it is desirable to improve the efficiency of the flyback power converter by eliminating the necessity of the thermistor and reducing the power consumption by the leakage inductor of the transformer. Furthermore, it is also desirable to reduce the operating stress of the power converter for higher reliability.
SUMMARY OF THE INVENTION
The present invention is directed to a half-bridge flyback power converter. Energy stored in a leakage inductor of a transformer is recycled for powering a secondary circuit. No snubber component is required for dissipating the energy in the leakage inductor. Furthermore, thermistor is inessential in the present invention since no inrush current is generated. Therefore, efficiency and reliability of the power converter in the present invention is substantially improved. According to one embodiment of the present invention, the half-bridge flyback power converter includes a transformer having a primary winding coupled in series with a high-side switch and a low-side switch. The transformer further comprises a secondary winding coupled to the secondary circuit. The high-side switch and the low-side switch periodically conduct an input voltage to the primary winding of the transformer controlled by specific control signals. When the high-side switch and the low-side switch are switched off, the magnetized energy stored in the transformer is transmitted to the secondary circuit and serves to charge a bulk capacitor via two flyback diodes. Meanwhile, the energy in the leakage inductor is stored in the bulk capacitor as well. Since no path is provided between the input voltage and the bulk capacitor, no inrush current will be generated. A forward diode facilitates combination of the input voltage and the voltage of the bulk capacitor for transmitting the power to the secondary circuit.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic circuit diagram illustrating a conventional flyback power converter.
FIG. 2 is a schematic circuit diagram illustrating a half-bridge flyback power converter according one embodiment of the present invention.
FIG. 3a is a schematic circuit diagram illustrating an operation of a half-bridge flyback power converter when a high-side switch and a low-side switch thereof are switched on according to one embodiment of the present invention.
FIG. 3b is a schematic circuit diagram illustrating an operation of a half-bridge flyback power converter when the high-side switch and the low-side switch are switched off according to one embodiment of the present invention.
FIG. 4 is a schematic circuit diagram illustrating the current flow of the high-side switch and the low-side switch of a half-bridge flyback power converter when an input voltage is lower than a voltage across a bulk capacitor according to one embodiment of the present invention.



DESCRIPTION OF THE EMBODIMENTS
Referring to FIG. 2, it illustrates a schematic circuit diagram of a half-bridge flyback power converter according to one embodiment of the present invention. The half-bridge flyback power converter comprises a transformer 55 having a primary winding NP and a secondary winding NS, a secondary circuit, a high-side switch 30 and a low-side switch 35, flyback diodes 25 and 26, a forward diode 20 and a bulk capacitor 60. An input voltage VIN is supplied from a bridge rectifier 12, or from a DC voltage source, for example. The high-side switch 30 is coupled between the input voltage VIN and a second terminal of the primary winding NP. A first terminal of the primary winding NP is coupled to a terminal of the low-side switch 35. Another terminal of the low-side switch 35 is coupled to a ground reference level. The secondary winding NS of the transformer 55 is coupled to the secondary circuit. The secondary circuit includes an output rectifier 51 and an output capacitor 52. The output rectifier 51 has an anode coupled to a first terminal of the secondary winding NS. The output capacitor 52 is coupled from a cathode of the output rectifier 51 to a second terminal of the secondary winding NS. The output capacitor 52 is also coupled in parallel with output terminal pairs of the half-bridge flyback power converter.
The first terminal of the primary winding NP and the first terminal of the secondary winding NS have the same polarity. The high-side switch 30 and the low-side switch 35 periodically conduct the input voltage VIN to the primary winding NP of the transformer 55 in response to control signals S2 and S1 respectively. When the input voltage VIN is applied to the primary winding NP, the magnetized energy εP is thus stored in the transformer 55. The magnetized energy εP is given by,




  
    
      
        
          ɛ
          P
        
        =
        
          
            1
            2
          
          ×
          
            L
            P
          
          ×
          
            I
            P
            2
          
        
      
    
    
      
        (
        2
        )
      
    
  




Referring to FIG. 3a, it illustrates a schematic circuit diagram of the half-bridge flyback power converter where the transformer 55 is operatively magnetized by a primary current IP when the high-side switch 30 and the low-side switch 35 are switched on. In the mean time, a leakage inductor energy εI is stored in the leakage inductor LI of the transformer 55. The leakage inductor LI is associated with a magnetic flux that is not transmittable to the secondary side of the transformer 55. The leakage inductor energy εI is given by,




  
    
      
        
          ɛ
          I
        
        =
        
          
            1
            2
          
          ×
          
            L
            I
          
          ×
          
            I
            P
            2
          
        
      
    
    
      
        (
        3
        )
      
    
  




In FIG. 3a, an anode of the flyback diode 25 is coupled to the ground reference level. A cathode of the flyback diode 25 is coupled to the second terminal of the primary winding NP. A positive terminal of the bulk capacitor 60 is coupled to a cathode of the flyback diode 26. An anode of the flyback diode 26 is coupled to the first terminal of the primary winding NP. A negative terminal of the bulk capacitor 60 is coupled to the ground reference level. A cathode of the forward diode 20 is coupled to the input voltage VIN. An anode of the forward diode 20 is coupled to the positive terminal of the bulk capacitor 60.
Referring to FIG. 3b, it illustrates a schematic circuit diagram of a half-bridge flyback power converter where the magnetized energy εP stored in the transformer 55 is transmitted to the secondary circuit through the secondary winding NS when the high-side switch 30 and the low-side switch 35 are both switched off. In the meantime, the bulk capacitor 60 is charged with the energy εP and εI through the flyback diodes 25 and 26.
When the input voltage VIN is higher than a voltage VB across the bulk capacitor 60, the forward diode 20 is reverse-biased. Therefore, a switching current is only supplied from the input voltage VIN. The forward diode 20 facilitates to combine the input voltage VIN with energy of the bulk capacitor 60. When the input voltage VIN is lower than the voltage VB of the bulk capacitor 60, the energy stored in the bulk capacitor 60 will be used to power the secondary circuit. Regardless of the energy in the leakage inductor of the transformer 55, a turn-ratio of the transformer 55 and an output voltage VO of the power converter can determine the voltage VB across the bulk capacitor 60. The voltage VB is given by,




  
    
      
        
          V
          B
        
        =
        
          
            
              N
              P
            
            
              N
              S
            
          
          ×
          
            V
            O
          
        
      
    
    
      
        (
        4
        )
      
    
  




Referring to FIG. 4, it illustrates a schematic circuit diagram of a half-bridge flyback power converter, wherein the current flow of the high-side switch 30 and the low-side switch 35 thereof are depicted. The forward diode 20 facilitates to combine the switching current from input voltage VIN and the bulk capacitor 60.
In the half-bridge flyback power converter according to the embodiment of the present invention, the energy of the leakage inductor is stored in the bulk capacitor 60. No snubber component is required for dissipating the energy of the leakage inductor. The energy of the leakage inductor is recycled for powering the secondary circuit. Therefore, the efficiency of the power converter is improved. Moreover, the input voltage VIN and the bulk capacitor 60 clamp the operating voltage of the high-side switch 30 and the low-side switch 35. The voltage stress is thus reduced.
According to the circuit topology of the present invention, no inrush current is generated. The high-side switch 30 blocks a path from the input voltage VIN to the bulk capacitor 60. The bulk capacitor 60 is only charged with the magnetized energy εP and the leakage inductor energy εI, such that a negative temperature coefficient (NTC) thermistor is not required, and further the efficiency and the reliability of the power converter are improved.
While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.

## Claims

1. A half-bridge flyback power converter, comprising:
a transformer, having a primary winding and a secondary winding, wherein said secondary winding is coupled to a secondary circuit; wherein a first terminal of said primary winding and a first terminal of said secondary winding have same polarity;
a high-side switch;
a low-side switch, wherein said high-side switch is coupled between an input voltage and a second terminal of said primary winding; wherein said low-side switch is coupled between said first terminal of said primary winding and a ground reference level;
a first flyback diode, wherein an anode of said first flyback diode is coupled to said ground reference level; wherein a cathode of said first flyback diode is coupled to said second terminal of said primary winding;
a second flyback diode, having an anode coupled to said first terminal of said primary winding;
a bulk capacitor, wherein a positive terminal of said bulk capacitor is coupled to a cathode of said second flyback diode; wherein a negative terminal of said bulk capacitor is coupled to said ground reference level; and
a forward diode, having a cathode coupled to said input voltage and an anode coupled to said positive terminal of said bulk capacitor.

2. The half-bridge flyback power converter as recited in claim 1, wherein said high-side switch and said low-side switch periodically conduct said input voltage to said primary winding of said transformer in response to at least a control signal.

3. The half-bridge flyback power converter as recited in claim 1, wherein when said high-side switch and said low-side switch are switched off, energy stored in said transformer is transmitted to said secondary circuit and serves to charge said bulk capacitor through said first flyback diode and said second flyback diode.

4. The half-bridge flyback power converter as recited in claim 1, wherein said forward diode facilitates to combine said input voltage with a voltage of said bulk capacitor; wherein when said input voltage is lower than a voltage across said bulk capacitor, energy stored in said bulk capacitor serves to power said secondary circuit.

5. The half-bridge flyback power converter as recited in claim 1, wherein energy of the leakage inductor of said transformer is stored in said bulk capacitor when said high-side switch and said low-side switch are switched off.

6. The half-bridge flyback power converter as recited in claim 1, wherein said bulk capacitor is only charged by magnetized energy and leakage inductor energy of said transformer.