# Patent

## Title
Asymmetrical resonant power converters

## Abstract

A resonant power converter is provided and includes a capacitor, an inductive device, a first transistor, a second transistor, and a control circuit. The capacitor and the inductive device develop a resonant tank. The first transistor and the second transistor are coupled to switch the resonant tank. The control circuit generates a first signal and a second signal to control the first transistor and the second transistor respectively. Frequencies of the first signal and the second signal are changed for regulating output of the resonant power converter. The control circuit is further coupled to detect an input voltage of the resonant power converter. A pulse width of the second signal is modulated in response to change of the input voltage.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of U.S. provisional application entitled “Asymmetrical Resonant Power Converter”, Ser. No. 61/207,353, filed Feb. 10, 2009.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power converters, and more particularly, relates to the asymmetrical resonant power converter.
2. Description of the Related Art
The resonant power converter is a high efficiency power converter. However, the narrow operation range is its drawback. When the input voltage has a significantly change, its operation might fall into a none-linear region. The object of the present invention is to provide a control scheme to solve this problem. It allows the resonant power converter can be operated in a wide input voltage range.
BRIEF SUMMARY OF THE INVENTION
An exemplary embodiment of a resonant power converter comprises a capacitor, an inductive device, a first transistor, a second transistor, and a control circuit. The capacitor and the inductive device develop a resonant tank. The first transistor and the second transistor are coupled to switch the resonant tank. The control circuit generates a first signal and a second signal to control the first transistor and the second transistor respectively. Frequencies of the first signal and the second signal are changed for regulating output of the resonant power converter. The control circuit is further coupled to detect an input voltage of the resonant power converter. A pulse width of the second signal is modulated in response to change of the input voltage.
Another exemplary embodiment of a power converter comprises a capacitor, an inductive device, two transistors, and a control circuit. The capacitor and the inductive device developing a resonant tank. The two transistors are coupled to switch the resonant tank. The control circuit generates switching signals coupled to control the transistors. Frequencies of the switching signals are varied for regulating output of the power converter. The control circuit is coupled to detect a voltage of the capacitor for modulating the switching signals and change a DC bias of the capacitor.
A detailed description is given in the following embodiments with reference to the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 is a preferred embodiment of an asymmetrical resonant power converter of the present invention.
FIG. 2 is a schematic diagram of the control circuit of the invention.
FIG. 3 shows a preferred embodiment of the amplification circuit.
FIG. 4 and FIG. 5 show the circuit schematic of the voltage-to-current converter.
FIG. 6 is a preferred embodiment of the oscillator.
FIG. 7 is a preferred embodiment of the output circuit.
FIG. 8 shows the circuit schematic of delay circuits in FIG. 7.
FIG. 9 shows key waveforms of an asymmetrical resonant power according to the present invention.
FIG. 10 shows key waveforms of an asymmetrical resonant power according to the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.
FIG. 1 is a preferred embodiment of an asymmetrical resonant power converter 1 of the present invention. A capacitor 50 and an inductive device (such as a transformer 30 and its parasitic inductor 35) develop a resonant tank. Transistors 10 and 20 are coupled to switch the resonant tank. Two rectifiers 71 and 72 are connected from the secondary winding of the transformer 30 to an output capacitor 75 for generating an output VO at the output capacitor 75. A control circuit 100 generates a switching signal (first signal) SH and a switching signal (second signal) SL to control the transistors 10 and 20 respectively. The switching frequencies of the switching signals SH and SL are changed in accordance with a feedback signal VFB for regulating the output VO of the power converter. A Zener diode 80, a resistor 81 and an optical-coupler 85 form a feedback circuit coupled to the output VO of the power converter to generate the feedback signal VFB. Furthermore, the control circuit 100 is coupled to detect an input voltage VIN of the power converter via resistors 61 and 62, and an input voltage Vx which is correlated to the input voltage VIN is generated via the resisters 61 and 62. The pulse width of the switching signal SL is modulated in response to the change of the input voltage VIN. The control circuit 100 is further coupled to detect the voltage of the capacitor 50 through resistors 51 and 52 and a capacitor 53 for modulating the switching signals SH, SL and change the DC bias of the capacitor 50. A DC bias voltage Vy which is correlated to the DC bias of the capacitor 50 is generated via the resisters 51 and 52. The capacitor 50 has a DC bias correlated to the input voltage VIN, which will allow the resonant power converter operating in a wide input voltage range. Since the DC bias of the capacitor 50 is correlated to the input voltage VIN, the control circuit 100 can detect the input voltage VIN for determining the DC bias of the capacitor 50.
FIG. 2 is a preferred embodiment of the control circuit 100. The control circuit 100 includes a feedback-input circuit 301 coupled to the output VO of the power converter through the optical-coupler 85 to receive the feedback signal VFB. A level-shift circuit and a voltage-to-current converter (V-I) 300 develop the feedback-input circuit 301. A transistor 114 and resistors 112, 115, 116 form the level-shift circuit, and a shifted signal VF is generated at the output of the level-shift circuit in accordance with the feedback signal VFB. An amplification circuit (AMP) 200 and a voltage-to-current converter (V-I) 400 develop a differential circuit 401, which is coupled to detect the input voltage VIN and the voltage of the capacitor 50 for generating a modulation signal VM. An oscillator (VCO) 500 generates an oscillation signal SW in response to the feedback signal VFB. An output circuit (OUT) 600 generates the switching signals SH and the SL in accordance with the oscillation signal SW. The pulse width of the switching signal SL is modulated by the modulation signal VM. The pulse width of the switching signal SL is decreased in response to the increase of the input voltage VIN. Therefore, the DC bias of the capacitor 50 will be increased in response to the increase of the input voltage VIN.
The feedback-input circuit 301 generates a charge current IC and a discharge current ID in accordance with the feedback signal VFB through the voltage-to-current converter 300. The charge current IC and the discharge current ID are connected to the oscillator 500. The charge current IC determines the on time of the oscillation signal SW, and the discharge current ID determine the off time of the oscillation signal SW. The voltage-to-current converter 400 generates a modulation current IM according to the modulation signal VM. The modulation current IM is coupled to the oscillator 500 to modulate the off time of the oscillation signal SW.
FIG. 3 shows a preferred embodiment of the amplification circuit 200. An operational amplifier (op-amp) 210 is coupled to receive the input voltage VX via the positive input. The negative input of the op-amp 210 is coupled to the output of the op-amp 210. A resister R215 is disposed between the output of the op-amp 210 and the positive input of an op-amp 230, and a resister R216 is disposed between the resister R215 and ground. The negative input of the op-amp 230 is coupled to receive a reference voltage VR through a resistor R235. In the embodiment, the reference voltage VR is 1.5V. A resistor R236 is connected from the negative input of the op-amp 230 to the output of the op-amp 230. The output of the op-amp 230 is coupled to the positive input of an op-amp 250 through a resistor R251, and another resistor R252 is connected between the resistor R251 and the ground. The negative input of the op-amp 250 is coupled to the output of an op-amp 240 through a resistor R241. Another resistor R242 is further coupled from the negative input to the output of the op-amp 250. The modulation signal VM is output from the output of the op-amp 250 through a resistor R270 and a capacitor 275 which provide filtering.
Thereby, the modulation signal VM can be expressed according the following equations:




  
    
      
        
          V
          
            X
            ⁢
            
              
            
            ⁢
            1
          
        
        =
        
          
            V
            X
          
          ×
          
            
              R
              216
            
            
              
                R
                215
              
              +
              
                R
                216
              
            
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        
          V
          
            X
            ⁢
            
              
            
            ⁢
            2
          
        
        =
        
          
            
              V
              
                X
                ⁢
                
                  
                
                ⁢
                1
              
            
            ⁢
            
              
                
                  R
                  235
                
                +
                
                  R
                  236
                
              
              
                R
                235
              
            
          
          -
          
            1.5
            ×
            
              
                R
                236
              
              
                R
                235
              
            
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          V
          
            X
            ⁢
            
              
            
            ⁢
            3
          
        
        =
        
          
            (
            
              V
              
                X
                ⁢
                
                  
                
                ⁢
                2
              
            
            )
          
          ×
          
            
              R
              252
            
            
              
                R
                251
              
              +
              
                R
                252
              
            
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          V
          
            X
            ⁢
            
              
            
            ⁢
            4
          
        
        =
        
          
            
              
                
                  R
                  241
                
                +
                
                  R
                  242
                
              
              
                R
                241
              
            
            ⁢
            
              V
              
                X
                ⁢
                
                  
                
                ⁢
                3
              
            
          
          -
          
            
              
                R
                242
              
              
                R
                241
              
            
            ⁢
            
              V
              y
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




VX1, VX2, VX3, and VX4 in the equations (1) ̃(4) represent the voltages at the positive input of the op-amp 230, at the output of the op-amp 230, at the positive input of the op-amp 250, and at the output of the amplification circuit 200 respectively.
According to the equation (1) ̃(3), we write:




  
    
      
        
          V
          
            X
            ⁢
            
              
            
            ⁢
            4
          
        
        =
        
          
            
              (
              
                
                  
                    
                      R
                      241
                    
                    +
                    
                      R
                      242
                    
                  
                  
                    R
                    241
                  
                
                ×
                
                  
                    R
                    252
                  
                  
                    
                      R
                      251
                    
                    +
                    
                      R
                      252
                    
                  
                
                ×
                
                  
                    
                      R
                      235
                    
                    +
                    
                      R
                      236
                    
                  
                  
                    R
                    235
                  
                
                ×
                
                  
                    R
                    216
                  
                  
                    
                      R
                      215
                    
                    +
                    
                      R
                      216
                    
                  
                
              
              )
            
            ×
            
              V
              X
            
          
          -
          
            (
            
              
                
                  
                    R
                    241
                  
                  +
                  
                    R
                    242
                  
                
                
                  R
                  241
                
              
              ×
              
                
                  R
                  252
                
                
                  
                    R
                    251
                  
                  +
                  
                    R
                    252
                  
                
              
              ×
              
                
                  R
                  236
                
                
                  R
                  235
                
              
              ×
              1.5
            
            )
          
          -
          
            
              
                R
                242
              
              
                R
                241
              
            
            ⁢
            
              V
              y
            
          
        
      
    
    
      
        (
        5
        )
      
    
  




As we define the



  
    
      
        
          
            
              R
              241
            
            +
            
              R
              242
            
          
          
            R
            241
          
        
        ×
        
          
            R
            252
          
          
            
              R
              251
            
            +
            
              R
              252
            
          
        
        ×
        
          
            
              R
              235
            
            +
            
              R
              236
            
          
          
            R
            235
          
        
        ×
        
          
            R
            216
          
          
            
              R
              215
            
            +
            
              R
              216
            
          
        
      
      =
      
        k
        1
      
    
    ;
  




  
    
      
        
          
            
              R
              241
            
            +
            
              R
              242
            
          
          
            R
            241
          
        
        ×
        
          
            R
            252
          
          
            
              R
              251
            
            +
            
              R
              252
            
          
        
        ×
        
          
            R
            236
          
          
            R
            235
          
        
      
      =
      
        k
        2
      
    
    ;
  




  
    
      
        
          R
          242
        
        
          R
          241
        
      
      =
      
        k
        3
      
    
    ;
    and
  




  
    
      
        V
        M
      
      =
      
        V
        
          X
          ⁢
          
            
          
          ⁢
          4
        
      
    
    ;
  



we obtain:

VM=(k1VX−1.5k2)−k3Vy  (6)

FIG. 4 and FIG. 5 show the circuit schematic of the voltage-to-current converter 300 and 400 respectively. In FIG. 4, the voltage-to-current converter 300 comprises a V-to-I circuit, a first current mirror, and a second current mirror. The V-to-I circuit which is developed by operational amplifiers 310 and 314, a transistor 311, and a resistor 312 generates a V-to-I current IR at the transistor 311 in accordance with a constant voltage VR and the shifted signal VF shifted from the feedback signal VFB. The V-to-I current IR is generated in response to




  
    
      
        V
        R
      
      -
      
        V
        F
      
    
    
      R
      312
    
  
  ,




which represents the differential of the constant voltage VR. Accordingly, the shifted signal VF is converted to the V-to-I current IR. The first current mirror is formed by the transistors 315, 316, and 317. The first current mirror generates charge current IC in response to the V-to-I current IR. The charge current IC is proportional to the V-to-I current IR. The second current mirror is formed by the transistors 320 and 321. The second current mirror generates the discharge current ID in response to the V-to-I current IR. In the embodiment, since the charge current IC and the discharge current ID are both mirrored from the V-to-I current IR, the charge current IC is thus proportional to the discharge current ID. The charge current IC and the discharge current ID are decreased in response to the increase of the feedback signal VFB.

In FIG. 5, the voltage-to-current converter 400 comprises a V-to-I circuit, and a current mirror. An operational amplifier 410, a transistor 411, and a resistor 312 develop the V-to-I circuit. The operational amplifier 410 is coupled to receive the modulation signal VM for generating a V-to-I current II at the transistor 411. The current mirror is formed by transistors 415, 416, 420, and 421. The V-to-I current II is mirrored by the current mirror, and the modulation current IM is generated in response to the V-to-I current II.
FIG. 6 is a preferred embodiment of the oscillator 500. Two switches 510 and 520, two comparators 540 and 545, two NAND gates 560 and 565, and an inverter 570 form the oscillator 500. When the switch 510 is turned on by the oscillation signal SW, the charge current IC charges the capacitor 530, and a ramp signal IRAMP is generated. Once the voltage across the capacitor 530 reaches a high-trip point V1 of the comparator 540, the comparator 540 and the NAND gates 560 and 565 are coupled to turn on the switch 520, and the discharge current ID and the modulation current IM discharge the capacitor 530. In the meantime, the oscillation signal SW is also disabled, and the switch 510 is turned off. The discharging is performed continuously until the voltage across the capacitor 530 is lower than a low trip-point voltage V2. The comparator 545 and the NAND gates 565 and 560 are coupled to turn off the switch 520. The oscillation signal SW generated through the inverter 570 becomes enabled, so that the switch 510 is turned on again.
FIG. 7 is a preferred embodiment of the output circuit 600. Two delay circuits 700 and 701, two AND gates 650 and 660, an inverter 610, and two buffers 670 and 680 form the output circuit 600. One input of the AND gate 650 is coupled to receive the oscillation signal SW, and another input of the AND gate 650 is coupled to receive the oscillation signal SW through the delay circuit 700. The switching signal SH is generated in response the two inputs of the AND gate 650 through the buffer 670. As for the AND gate 660, one input of the AND gate 660 is coupled to receive the oscillation signal SW through the inverter 610. Another input of the AND gate 660 is coupled to receive the oscillation signal SW through the inverter 610 and the delay circuit 701. The switching signal SL is generated in response to the two inputs of the AND gate 660 through the buffer 680.
FIG. 8 shows the circuit schematic of delay circuits 700 and 701 in FIG. 7. Each of the delay circuits 700 and 701 includes an inverter 715, a current source 725, a transistor 720, a capacitor 750, and an AND gate 790. An input terminal IP of each of the delay circuits 700 and 701 is coupled to receive the oscillation signal SW. The oscillation signal SW is supplied to the inverter 715 and one input of the AND gate 790. The output of the inverter 715 is coupled to a gate of the transistor 720. A drain of the transistor 720 and one terminal of the capacitor 750 are coupled to another input of the AND gate 790. A source of the transistor 720 and the other terminal of the capacitor 750 are coupled to the ground. A delay signal OP is enabled in response to the rising edge of the oscillation signal SW after a delay time Td. The delay time Td is determined according to the current value provided by the current source 725 and the capacitance of the capacitor 750.
In practice, referring to FIGS. 1, 2, 4, 6, 7, 8, and 9, when the power converter is converted from heavy load to light load, the output voltage VO increases, and the level of the feedback voltage VFB decreases. The charge current IC and the discharge current ID are increased in response to the decrease of the feedback signal VFB. As long as the charge current IC and the discharge current ID increases, the charging slope and the discharging slope of the ramp signal IRAMP is also increased. The on time and the off time of the oscillation signal SW decreases, and the switching period of the oscillation signal SW is decreased. The switching period of the switching signals SH and the SL is also decreased. In other word, the switching frequencies of the switching signals SH and the SL are increased.
Referring to FIGS. 1, 2, 3, 5, 6, 7, 8, and 10, when the input voltage VIN increases, the DC bias voltage Vy is increased as well. According to the equation (6) described in FIG. 3, the modulation signal VM is increased in accordance to the increase of the input voltage VIN. The modulation current IM is also increased in accordance to the increased modulation signal VM. As the modulation current IM increases, the discharge current ID+IM is increased, too. The slope of the signal Iramp is therefore increased at the discharging part of the waveform, and the pulse width of the switching signal SL (second signal) is decreased in response to the increase of the input voltage VIN. As for the decreased pulse width of the switching signal SL (second signal), the ratio of the switching signal SH (first signal) of the oscillation signal SW will be increased, and the DC bias of the capacitor 50 will be increased in response to the increase of the input voltage VIN.
According to the invention, when the input voltage of the power converter has a significantly change, the pulse width of the switching signal SL can be modulated, and the operation might not fall into a none-linear region.
While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. 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.

## Claims

1. A resonant power converter comprising:
a capacitor and an inductive device developing a resonant tank;
a first transistor and a second transistor coupled to switch the resonant tank; and
a control circuit generating a first signal and a second signal to control the first transistor and the second transistor respectively; in which the frequencies of the first signal and the second signal are changed in accordance with a feedback signal for regulating output of the resonant power converter; and
wherein the control circuit is further coupled to detect an input voltage of the resonant power converter, and a pulse width of the second signal is modulated in response to change of the input voltage; and the control circuit comprising:
a feedback-input circuit coupled to the output of the resonant power converter to receive the feedback signal;
an oscillator generating an oscillation signal in response to the feedback signal;
an output circuit generating the first signal and the second signal in accordance with the oscillation signal; and
a differential circuit coupled to input of the resonant power converter and the capacitor for generating a modulation signal;
wherein the pulse width of the second signal is modulated by the modulation signal; and
wherein the feedback-input circuit generates a charge current and a discharge current in response to the feedback signal, the charge current and the discharge current are coupled to the oscillator, the charge current determines an on time of the oscillation signal, and the discharge current determines the off time of the oscillation signal.

2. The resonant power converter as claimed in claim 1, wherein the differential circuit generates a modulation current in accordance with the modulation signal, and the modulation current is coupled to the oscillator to modulate the off time of the oscillation signal.

3. The resonant power converter as claimed in claim 1, wherein the pulse width of the second signal is decreased in response to increase of the input voltage.

4. The resonant power converter as claimed in claim 1, wherein a DC bias of the capacitor is increased in response to the increase of the input voltage.

5. A power converter comprising:
a capacitor and an inductive device developing a resonant tank;
two transistors coupled to switch the resonant tank; and
a control circuit generating switching signals coupled to control the transistors;
wherein frequencies of the switching signals are varied for regulating output of the power converter; and
wherein the control circuit is coupled to detect a voltage of the capacitor for modulating the switching signals and change a DC bias of the capacitor and the control circuit comprises:
a feedback-input circuit coupled to the output of the power converter to receive a feedback signal; and
an oscillator generating an oscillation signal in response to the feedback signal, wherein the feedback-input circuit generates a charge current and a discharge current in response to the feedback signal, the charge current and the discharge current are coupled to the oscillator, the charge current determines an on time of the oscillation signal, and the discharge current determines the off time of the oscillation signal.

6. The power converter as claimed in claim 5, wherein the control circuit is further coupled to detect an input voltage of the power converter for determining the DC bias of the capacitor.

7. The power converter as claimed in claim 5, wherein pulse widths of the switching signals are varied for determining the DC bias of the capacitor.

8. The power converter as claimed in claim 5, wherein the control circuit further comprising:
an output circuit generating the switching signals in accordance with the oscillation signal; and
a differential circuit coupled to an input of the power converter and the capacitor for generating a modulation signal;
wherein pulse widths of the switching signals are modulated by the modulation signal.

9. The power converter as claimed in claim 5, wherein a pulse width of one of the switching signals is changed in response to the change of the input voltage.

10. The power converter as claimed in claim 5, in which the DC bias of the capacitor is changed in response to the change of the input voltage.