Primary resonant flyback converters

Abstract

A primary resonant flyback converter may include a primary winding, a resonant capacitor in series with the primary winding, a secondary winding magnetically coupled to the primary winding, and an output electrically coupled to the secondary winding. A main switch may be operated to energize the primary winding when closed and transfer energy stored in the primary winding to the secondary winding when open. An auxiliary switch may be configured to switch complimentarily to the main switch, thereby allowing a resonant current to circulate through the primary winding and capacitor. Switch timing may be controlled to produce a desired output voltage. The converter may also include an input inductor that receives an input voltage, presenting an improved power factor to an AC input power source and in conjunction with the switching devices boosts a rectified AC input voltage to a DC voltage bus of the converter.

1 . A primary resonant flyback converter comprising:
 a primary winding;   a secondary winding magnetically coupled to the primary winding and electrically coupled to an output rectifier, and an output terminal;   a resonant capacitor coupled to the primary winding;   a main switch configured to switch on to energize the primary winding to switch off to transfer energy stored in the primary winding to the secondary winding;   an auxiliary switch configured to switch on during an off time of the main switch to allow a resonant current to circulate through the primary winding and the resonant capacitor; and   a control circuit configured to operate the main switch and the auxiliary switch to produce a desired voltage at the output terminal.   
     
     
         2 . The primary resonant flyback converter of  claim 1  wherein the control circuit is configured to operate the main switch and the auxiliary switch responsive to a voltage measured across the resonant capacitor. 
     
     
         3 . The primary resonant flyback converter of  claim 2  wherein the voltage measured across the resonant capacitor is an instantaneous voltage. 
     
     
         4 . The primary resonant flyback converter of  claim 2  wherein the voltage measured across the resonant capacitor is an average voltage. 
     
     
         5 . The primary resonant flyback converter of  claim 2  wherein the control circuit is further configured to implement output overvoltage protection based on the voltage measured across the resonant capacitor. 
     
     
         6 . The primary resonant flyback converter of  claim 1  wherein the control circuit is configured to:
 vary a duty cycle of the main switch and the auxiliary switch to produce a desired voltage at the output terminal; and 
 vary a switching frequency of the main switch and the auxiliary switch responsive to a change in output load on the primary resonant flyback converter in combination with at least one of an input voltage into the primary resonant flyback converter and a DC bus voltage of the primary resonant flyback converter. 
 
     
     
         7 . The primary resonant flyback converter of  claim 6  wherein the control circuit is configured to vary the switching frequency by increasing the switching frequency responsive to a decrease in output load or by decreasing the switching frequency responsive to an increase in output load. 
     
     
         8 . The primary resonant flyback converter of  claim 6  wherein the control circuit is configured to vary the switching frequency of the main switch and the auxiliary switch responsive to both the change in output load and the input voltage. 
     
     
         9 . The primary resonant flyback converter of  claim 6  wherein the control circuit configured to vary the switching frequency of the main switch and the auxiliary switch responsive to the DC voltage bus of the primary resonant flyback converter is further configured to vary the switching frequency of the main switch and the auxiliary switch responsive to a voltage ripple on the DC voltage bus. 
     
     
         10 . The primary resonant flyback converter of  claim 1  wherein the control circuit is configured to implement a hysteretic control algorithm to temporarily disable and re-enable operation of the main switch and the auxiliary switch responsive to a decrease in output load on the primary resonant flyback converter. 
     
     
         11 . The primary resonant flyback converter of  claim 10  wherein the control circuit is configured to re-enable operation of the main switch and the auxiliary switch responsive to an increase in output load. 
     
     
         12 . The primary resonant flyback converter of  claim 11  wherein the hysteretic control algorithm references a high threshold corresponding to a load condition at which switching should be enabled and a low threshold corresponding to a load condition at which switching should be disabled. 
     
     
         13 . The primary resonant flyback converter of  claim 12  wherein the high and low thresholds are compared to a feedback signal from the output voltage control loop. 
     
     
         14 . A primary resonant flyback converter comprising:
 a primary winding;   a secondary winding magnetically coupled to the primary winding and electrically coupled to a rectifier, and an output terminal;   a resonant capacitor coupled to the primary winding;   a main switch configured to switch on to energize the primary winding from a DC voltage bus;   an auxiliary switch configured to switch on during an off time of the main switch to allow a resonant current to circulate through the primary winding and the resonant capacitor; and   a control circuit configured to operate the main switch and the auxiliary switch responsive to a sensed output voltage and further configured to sense a current through the primary coil using a current sense circuit coupled in parallel with the resonant capacitor, the current sense circuit comprising a sense capacitor and at least one sense resistor.   
     
     
         15 . The primary resonant flyback converter of  claim 14  wherein a cutoff frequency of the current sense circuit is configured to be higher than a switching frequency of the primary resonant flyback converter. 
     
     
         16 . The primary resonant flyback converter of  claim 14  wherein the control circuit is further configured to use the sensed current through the primary coil for overcurrent protection. 
     
     
         17 . The primary resonant flyback converter of  claim 14  wherein the control circuit is further configured to:
 vary a duty cycle of the main switch and the auxiliary switch to produce a desired voltage at the output terminal; and 
 vary a switching frequency of the main switch and the auxiliary switch responsive to a change in output load on the primary resonant flyback converter in combination with at least one of an input voltage into the primary resonant flyback converter and a DC bus voltage of the primary resonant flyback converter. 
 
     
     
         18 . The primary resonant flyback converter of  claim 17  wherein the control circuit is configured to vary the switching frequency of the main switch and the auxiliary switch responsive to both the change in output load and the input voltage. 
     
     
         19 . The primary resonant flyback converter of  claim 17  wherein the control circuit configured to vary the switching frequency of the main switch and the auxiliary switch responsive to the DC voltage bus of the primary resonant flyback converter is further configured to vary the switching frequency of the main switch and the auxiliary switch responsive to a voltage ripple on the DC voltage bus. 
     
     
         20 . The primary resonant flyback converter of  claim 14  wherein the control circuit is configured to implement a hysteretic control algorithm to temporarily disable and re-enable operation of the main switch and the auxiliary switch responsive to a decrease in output load on the primary resonant flyback converter. 
     
     
         21 . A method of operating a primary resonant flyback converter, the method comprising:
 switching a main switch responsive to a sensed output voltage, wherein closing the main switch energizes a primary coil and opening the main switch transfers energy stored in the primary coil to a load electrically connected to a secondary coil magnetically coupled to the primary coil; and   switching an auxiliary switch complimentarily to the main switch, wherein closing the auxiliary switch allows a resonant current to circulate through the primary coil and a resonant capacitor;   wherein the main switch and the auxiliary switch are operated to produce a desired voltage at an output terminal.   
     
     
         22 . The method of  claim 21  wherein the sensed output voltage is a voltage across the resonant capacitor. 
     
     
         23 . The method of  claim 21  wherein the voltage across the resonant capacitor is an instantaneous voltage. 
     
     
         24 . The method of  claim 21  wherein the voltage across the resonant capacitor is an average voltage. 
     
     
         25 . The method of  claim 21  further comprising implementing output overvoltage protection for the primary resonant flyback converter responsive to the voltage across the resonant capacitor. 
     
     
         26 . The method of  claim 21  further comprising sensing a current through the primary coil using a current sense circuit coupled in parallel with the resonant capacitor, the current sense circuit comprising a sense capacitor and at least one sense resistor. 
     
     
         27 . The method of  claim 26  further comprising implementing overcurrent protection for the primary resonant flyback converter responsive to the sensed current. 
     
     
         28 . The method of  claim 21  further comprising:
 varying a duty cycle of the main switch to produce a desired output voltage; and 
 varying a switching frequency of the main switch responsive to the load on the converter in combination with at least one of an input voltage into the converter and a DC voltage bus of the converter 
 
     
     
         29 . The method of  claim 28  wherein varying the switching frequency of the main switch responsive to at least one of the load on the converter and the input voltage into the converter comprises varying the switching frequency responsive to both the load and the input voltage. 
     
     
         30 . The method of  claim 28  wherein varying the switching frequency of the main switch responsive to the DC voltage bus of the converter further comprises varying a switching frequency of the main switch responsive to a ripple voltage on a DC bus of the converter. 
     
     
         31 . The method of  claim 28  wherein varying a switching frequency of the main switch responsive to the load on the power converter comprises temporarily disabling operation of the main switch responsive to a decrease in output load on the power converter. 
     
     
         32 . The method of  claim 31  further comprising re-enabling operation of the main switch responsive to an increase in output load on the power converter.