Llc controller with programmable fractional burst frequency

Abstract

A method of controlling an LLC resonant converter includes programming a burst stop frequency and a burst start frequency in response to a maximum switching frequency of the LLC resonant converter. The burst stop frequency and the burst start frequency are fractions of the maximum switching frequency. The LLC resonant converter is switched in response to a feedback signal to regulate an output of the LLC resonant converter. The steps of switching the LLC resonant converter in a run state in response to the feedback signal reaching a value corresponding to the programmed burst start frequency, and stopping the switching of the LLC resonant converter in a stop state in response to the feedback signal reaching a value corresponding to the programmed burst stop frequency are repeated.

What is claimed is: 
     
         1 . A method of controlling an LLC resonant converter, comprising:
 programming a burst stop frequency and a burst start frequency in response to a maximum switching frequency of the LLC resonant converter, wherein the burst stop frequency and the burst start frequency are fractions of the maximum switching frequency;   switching the LLC resonant converter in response to a feedback signal to regulate an output of the LLC resonant converter;   repeating the steps of:
 switching the LLC resonant converter in a run state in response to the feedback signal reaching a value corresponding to the programmed burst start frequency; and 
 stopping the switching of the LLC resonant converter in a stop state in response to the feedback signal reaching a value corresponding to the programmed burst stop frequency. 
   
     
     
         2 . The method of  claim 1  wherein the programmed burst start frequency is less than the programmed burst stop frequency. 
     
     
         3 . The method of  claim 1  further comprising clamping a switching frequency of the LLC resonant converter at the maximum switching frequency of the LLC resonant converter during a soft start of the LLC resonant converter. 
     
     
         4 . The method of  claim 1  further comprising programming a maximum frequency signal by selecting an external resistance coupled between a maximum frequency pin of an LLC resonant controller and ground, wherein the maximum switching frequency of the LLC resonant converter is set in response to the maximum frequency signal. 
     
     
         5 . The method of  claim 1  further comprising further programming the burst stop frequency and the burst start frequency in response to programming internal registers of a state machine of an LLC resonant controller. 
     
     
         6 . The method of  claim 1  wherein stopping the switching of the LLC resonant converter in response to the feedback signal reaching a value corresponding to the programmed burst stop frequency comprises comparing a feedback current with a current through a fractional current mirror. 
     
     
         7 . The method of  claim 6  wherein programming the burst stop frequency and the burst start frequency in response to the maximum switching frequency comprises programming the current through the fractional current mirror. 
     
     
         8 . The method of  claim 7  wherein programming the current through the fractional current mirror comprises selectively activating or deactivating each of a plurality of transistors included in the fractional current minor. 
     
     
         9 . A method of controlling a resonant converter, comprising:
 switching the resonant converter at a switching frequency in response to a feedback signal to control a transfer of energy to an output of the resonant converter;   varying the switching frequency between a minimum switching frequency and a maximum frequency in response to a load at the output of the resonant converter;   switching the resonant converter at a burst mode frequency of a burst mode of operation of the resonant converter in response to a low load or no load condition at the output of the resonant converter, wherein the burst mode frequency is a fraction of the maximum switching frequency; wherein the burst mode frequency includes a burst start frequency and a burst stop frequency, wherein the burst start frequency is less than the burst stop frequency; wherein the burst start frequency and the burst stop frequency are fractions of the maximum switching frequency; and programming the burst stop frequency and the burst start frequency in response to the maximum switching frequency.   
     
     
         10 . The method of  claim 9  further comprising programming the burst stop frequency and the burst start frequency in response to programming internal registers of a state machine of a resonant converter controller of the resonant converter. 
     
     
         11 . The method of  claim 9  further comprising programming the burst stop frequency and the burst start frequency comprises programming currents through a plurality of fractional current mirrors. 
     
     
         12 . The method of  claim 11  wherein programming the currents through the plurality of fractional current minors comprises selectively activating or deactivating each of a plurality of transistors included in the plurality of fractional current mirrors. 
     
     
         13 . The method of  claim 9  further comprising programming the maximum switching frequency by selecting an external resistance circuit coupled to a maximum frequency pin of a resonant converter controller of the resonant converter. 
     
     
         14 . The method of  claim 9  wherein the step of varying the switching frequency includes decreasing the switching frequency as low as the minimum switching frequency in response to an increase in the load at the output of the resonant converter. 
     
     
         15 . The method of  claim 9  wherein the step of varying the switching frequency includes increasing the switching frequency as high as the maximum switching frequency in response to a decrease in the load at the output of the resonant converter.