Hold-up time enhancement circuit for llc resonant converter

Abstract

An open loop half-bridge LLC power converter includes circuitry to reliably increase hold-up time without sacrificing efficiency. An LLC resonant circuit includes resonant inductance, a primary transformer winding, and resonant capacitance. An auxiliary circuit includes an auxiliary transformer winding, an inductor, and a third switching element coupled in series. A controller is coupled across a voltage sensor and effective thereby to determine a holdup time condition. In a “normal” operating condition the controller generates switch driver signals to turn OFF the third switching element and disable the auxiliary circuit, and in a hold-up time condition the controller turns ON the third switching element and enables the auxiliary circuit wherein the output voltage is increased via current supplied from the auxiliary winding. In various embodiments the auxiliary winding may be an auxiliary primary or secondary, or a secondary to an auxiliary primary winding of a second transformer.

What is claimed is: 
     
         1 . A power conversion system comprising:
 first and second switching elements coupled in series across first and second power input terminals;   an LLC resonant circuit coupled between the first and second switching elements and comprising a first inductor, a primary winding of a transformer, and one or more resonant capacitors;   an output circuit comprising one or more secondary transformer windings and a bulk capacitor coupled across first and second output terminals;   a voltage sensor;   a hold-up time enhancement circuit further comprising an auxiliary primary winding of the transformer, a second inductor coupled on a first end to the auxiliary transformer winding, and a third switching element coupled to a second end of the second inductor;   a controller coupled to the voltage sensor and effective to determine a holdup time condition based on a voltage across the voltage sensor;   the controller is further effective in a first operating mode associated with a normal condition to generate switch driver signals to turn OFF the third switching element and disable the hold-up time enhancement circuit; and   the controller is further effective in a second operating mode associated with a hold-up time condition to generate switch driver signals to turn ON the third switching element and enable the holdup time enhancement circuit wherein the output voltage is increased via current supplied from the auxiliary winding.   
     
     
         2 . The system of  claim 1 , wherein the auxiliary winding is coupled on a first end to the second inductor and on a second end to circuit ground and the third switching element is coupled on a first end to the second inductor and on a second end to circuit ground. 
     
     
         3 . The system of  claim 2 , further comprising a fourth switching element coupled between the second end of the auxiliary winding and circuit ground. 
     
     
         4 . The system of  claim 2 , the hold-up time enhancement circuit further comprising a diode having an anode coupled to the second end of the second inductor and a cathode coupled to the first end of the third switching element. 
     
     
         5 . The system of  claim 1 , the hold-up time enhancement circuit further comprising first and second diodes in each of first and second branches of a diode bridge,
 the anodes of each of the first diodes are coupled to circuit ground,   the cathodes of each of the second diodes are coupled to the first end of the second inductor, and   opposing ends of the auxiliary winding are coupled between the first and second diodes of the first and second branches, respectively.   
     
     
         6 . The system of  claim 1 , the output circuit comprising first and second secondary windings coupled in series, opposing ends of the secondary windings coupled to the first output terminal, and a center tap between the secondary windings coupled to the second output terminal. 
     
     
         7 . A power conversion system comprising:
 first and second switching elements coupled in series across first and second power input terminals;   an LLC resonant circuit coupled between the first and second switching elements and comprising a first inductor, a primary winding of a transformer, and one or more resonant capacitors;   an output circuit comprising one or more secondary transformer windings, and a bulk capacitor coupled across first and second output terminals;   a voltage sensing circuit;   a hold-up time enhancement circuit further comprising an auxiliary secondary winding of the transformer, a second inductor coupled on a first end to the auxiliary transformer winding, and a third switching element coupled to a second end of the second inductor; and   a controller coupled to the voltage sensor and effective to determine a holdup time condition based on a voltage across the voltage sensor,   the controller is further effective in a first operating mode associated with a normal condition to generate switch driver signals to turn OFF the third switching element and disable the hold-up time enhancement circuit, and   the controller is further effective in a second operating mode associated with a hold-up time condition to generate switch driver signals to turn ON the third switching element and enable the holdup time enhancement circuit wherein the output voltage is increased via current supplied from the auxiliary winding.   
     
     
         8 . The system of  claim 7 , wherein the auxiliary winding is coupled on a first end to the second inductor and on a second end to the second output terminal, further wherein the third switching element is coupled on a first end to the second inductor and on a second end to the second output terminal. 
     
     
         9 . The system of  claim 8 , further comprising a fourth switching element coupled between the second end of the auxiliary winding and the second output terminal. 
     
     
         10 . The system of  claim 7 , the hold-up time enhancement circuit further comprising a diode having an anode coupled to the second end of the second inductor and a cathode coupled to the first end of the third switching element. 
     
     
         11 . The system of  claim 7 , the hold-up time enhancement circuit further comprising first and second diodes in each of first and second branches of a diode bridge,
 the anodes of each of the first diodes are coupled to the second output terminal,   the cathodes of each of the second diodes are coupled to the first end of the second inductor, and   opposing ends of the auxiliary winding are coupled between the first and second diodes of the first and second branches, respectively.   
     
     
         12 . The system of  claim 7 , the output circuit comprising first and second secondary windings coupled in series, opposing ends of the secondary windings coupled to the first output terminal, and a center tap between the secondary windings coupled to the second output terminal. 
     
     
         13 . The system of  claim 12 , wherein the auxiliary winding is coupled on a first end to the second inductor and on a second end to the first output terminal, further wherein the third switching element is coupled on a first end to the second inductor and on a second end to the second output terminal. 
     
     
         14 . A power conversion system comprising:
 first and second switching elements coupled in series across first and second power input terminals;   an LLC resonant circuit coupled between the first and second switching elements and comprising a first inductor, a primary winding of a first transformer, and one or more resonant capacitors;   a primary winding of a second transformer coupled in parallel with the primary winding of the first transformer;   an output circuit comprising one or more secondary transformer windings of the first transformer and a bulk capacitor coupled across first and second output terminals;   a voltage sensor;   a hold-up time enhancement circuit further comprising a secondary winding of the second transformer as an auxiliary winding, a second inductor coupled on a first end to the auxiliary winding, and a third switching element coupled to a second end of the second inductor; and   a controller coupled to the voltage sensor and effective to determine a holdup time condition based on a voltage across the voltage sensor,   the controller is further effective in a first operating mode associated with a normal condition to generate switch driver signals to turn OFF the third switching element and disable the hold-up time enhancement circuit, and   the controller is further effective in a second operating mode associated with a hold-up time condition to generate switch driver signals to turn ON the third switching element and enable the holdup time enhancement circuit wherein the output voltage is increased via current supplied from the auxiliary winding.   
     
     
         15 . The system of  claim 14 , wherein the auxiliary winding is coupled on a first end to the second inductor and on a second end to circuit ground, further wherein the third switching element is coupled on a first end to the second inductor and on a second end to circuit ground. 
     
     
         16 . The system of  claim 15 , further comprising a fourth switching element coupled between the second end of the auxiliary winding and circuit ground. 
     
     
         17 . The system of  claim 15 , the hold-up time enhancement circuit further comprising a diode having an anode coupled to the second end of the second inductor and a cathode coupled to the first end of the third switching element. 
     
     
         18 . The system of  claim 17 , the output circuit comprising first and second secondary windings coupled in series, opposing ends of the secondary windings coupled to the first output terminal, and a center tap between the secondary windings coupled to the second output terminal. 
     
     
         19 . The system of  claim 18 , wherein the auxiliary winding is coupled on a first end to the second inductor and on a second end to the first output terminal, further wherein the third switching element is coupled on a first end to the second inductor and on a second end to circuit ground. 
     
     
         20 . The system of  claim 14 , the hold-up time enhancement circuit further comprising first and second diodes in each of first and second branches of a diode bridge,
 the anodes of each of the first diodes coupled to circuit ground,   the cathodes of each of the second diodes coupled to the first end of the second inductor, and   opposing ends of the auxiliary winding coupled between the first and second diodes of the first and second branches, respectively.