Line ripple compensation for shimmerless LED driver

Abstract

A power converter controller includes an oscillator, a drive circuit, and a frequency modulator. The drive circuit receives a clock signal from the oscillator and generates a drive signal in response thereto to control switching of a switch of the power converter. The frequency modulator controls the frequency of the clock signal in response to an inductor voltage across an inductor of the energy transfer element to reduce a peak-to-peak ripple value in an output current of the power converter. The frequency modulator controls the frequency of the clock signal during each line half cycle to be a fixed frequency when the inductor voltage is less than or equal to a threshold voltage. When the inductor voltage is greater than the threshold voltage the frequency modulator varies the frequency to be less than the fixed frequency to adjust a shape of an input current of the power converter.

What is claimed is: 
     
       1. A controller for use in an ac-to-dc power converter, the controller comprising:
 an oscillator coupled to generate a clock signal having a frequency; 
 a drive circuit coupled to receive the clock signal and to generate a drive signal in response thereto, the drive signal to control switching of a switch to control a transfer of energy across an energy transfer element from an input of the power converter to an output of the power converter, wherein a switching frequency of the drive signal is based on the frequency of the clock signal and is much greater than a lower frequency of a time-varying inductor voltage across an inductor of the energy transfer element; and 
 a frequency modulator coupled to the oscillator to control the frequency of the clock signal during each line half cycle of the time-varying inductor voltage in response to a magnitude of the time-varying inductor voltage in order to reduce a peak-to-peak ripple value in an output current of the power converter that is due to the lower frequency time variations in the inductor voltage, wherein the frequency modulator controls the frequency of the clock signal during each line half cycle to be a fixed frequency when the magnitude of the inductor voltage is less than or equal to a threshold voltage, and wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to be less than the fixed frequency to adjust a shape of an input current of the power converter when the magnitude of the inductor voltage is greater than the threshold voltage. 
 
     
     
       2. The controller of  claim 1 , wherein the frequency modulator varies the frequency of the clock signal during each line half line cycle to adjust the shape of the input current by decreasing the frequency of the clock signal in response to increases in the magnitude of the inductor voltage and increasing the frequency of the clock signal in response to decreases in the magnitude of the inductor voltage when the magnitude of the inductor voltage is greater than the threshold voltage. 
     
     
       3. The controller of  claim 2 , wherein the clock signal includes a non-zero minimum frequency, and wherein the frequency modulator is configured to stop decreasing the frequency of the clock signal when the minimum frequency is reached. 
     
     
       4. The controller of  claim 2 , wherein the frequency modulator is configured to stop increasing the frequency of the clock signal when the fixed frequency is reached. 
     
     
       5. The controller of  claim 1 , wherein the frequency modulator varies the frequency of the clock signal by an amount that is proportional to a ratio of an amount that the inductor voltage exceeds the threshold voltage to the inductor voltage. 
     
     
       6. The controller of  claim 1 , wherein the frequency modulator continuously adjusts the frequency of the clock signal while the magnitude of the inductor voltage exceeds the threshold voltage such that a ratio of the fixed frequency to the switching frequency is substantially equal to a ratio of the inductor voltage to the threshold voltage. 
     
     
       7. The controller of  claim 1 , further comprising a ripple compensation factor generator coupled to the frequency modulator to control a rate at which the frequency modulator increases and decreases the frequency of the clock signal. 
     
     
       8. The controller of  claim 7 , wherein the frequency modulator sets the frequency of the clock signal to a percentage amount of the fixed frequency when the inductor voltage is greater than the threshold voltage, and wherein the ripple compensation factor generator is coupled to control the rate at which the frequency modulator increases and decreases the frequency of the clock signal such that the frequency modulator gradually changes the frequency of the clock signal over several input voltage line cycles of the power converter until the frequency is equal to the percentage amount of the fixed frequency. 
     
     
       9. The controller of  claim 7 , wherein the ripple compensation factor generator controls the frequency modulator such that the frequency modulator varies the frequency of the clock signal in digital steps. 
     
     
       10. The controller of  claim 9 , wherein digital steps consist of 256 digital steps. 
     
     
       11. The controller of  claim 1 , wherein the controller is configured to detect dimming at the input of the power converter and to enable the frequency modulator in response thereto, wherein when disabled, the frequency modulator controls the frequency of the clock signal to be a fixed frequency regardless of the inductor voltage. 
     
     
       12. The controller of  claim 11 , wherein the controller is configured to enable the frequency modulator in response to an amount of dimming at the input of the power converter exceeding a threshold amount of dimming. 
     
     
       13. The controller of  claim 11 , wherein the controller is configured to enable the frequency modulator in response to a change in the output of the power converter. 
     
     
       14. The controller of  claim 11 , wherein the controller is configured to enable the frequency modulator in response to a change in an error voltage reference level of the controller. 
     
     
       15. The controller of  claim 1 , wherein the controller includes an input voltage block coupled to receive a first signal representative of an instantaneous value of an input voltage of the power converter and to generate a second signal representative of the magnitude of the inductor voltage is response thereto. 
     
     
       16. The controller of  claim 15 , wherein the input voltage block is coupled to receive a third signal that is representative of an output voltage of the power converter, wherein the input voltage block generates the second signal in response to both the first signal and the third signal. 
     
     
       17. The controller of  claim 1 , wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to adjust the shape of the input current of the power converter to be substantially flat when the magnitude of the inductor voltage is greater than the threshold voltage. 
     
     
       18. An ac-to-dc power converter, comprising:
 an energy transfer element to be coupled between an input and an output of the power converter; 
 a switch coupled to an inductor of the energy transfer element; and 
 a controller coupled to the switch to control switching of the switch, wherein the controller includes:
 an oscillator coupled to generate a clock signal having a frequency; 
 a drive circuit coupled to receive the clock signal and to generate a drive signal in response thereto, the drive signal to control switching of the switch to control a transfer of energy across the energy transfer element from the input of the power converter to the output of the power converter, wherein a switching frequency of the drive signal is based on the frequency of the clock signal and is much greater than a lower frequency of a time-varying inductor voltage across the inductor; and 
 a frequency modulator coupled to the oscillator to control the frequency of the clock signal during each line half cycle of the time-varying inductor voltage in response to a magnitude of the time-varying inductor voltage in order to reduce a peak-to-peak ripple value in an output current of the power converter that is due to the lower frequency time variations in the inductor voltage, wherein the frequency modulator controls the frequency of the clock signal during each line half cycle to be a fixed frequency when the magnitude of the inductor voltage is less than or equal to a threshold voltage, and wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to be less than the fixed frequency to adjust a shape of an input current of the power converter when the magnitude of the inductor voltage is greater than the threshold voltage. 
 
 
     
     
       19. The power converter of  claim 18 , wherein the frequency modulator varies the frequency of the clock signal during each line half line cycle to adjust the shape of the input current by decreasing the frequency of the clock signal in response to increases in the magnitude of the inductor voltage and increasing the frequency of the clock signal in response to decreases in the magnitude of the inductor voltage when the magnitude of the inductor voltage is greater than the threshold voltage. 
     
     
       20. The power converter of  claim 19 , wherein the clock signal includes a non-zero minimum frequency, and wherein the frequency modulator is configured to stop decreasing the frequency of the clock signal when the minimum frequency is reached. 
     
     
       21. The power converter of  claim 19 , wherein the frequency modulator is configured to stop increasing the frequency of the clock signal when the fixed frequency is reached. 
     
     
       22. The power converter of  claim 19 , wherein the frequency modulator varies the frequency of the clock signal by an amount that is proportional to a ratio of an amount that the inductor voltage exceeds the threshold voltage to the inductor voltage. 
     
     
       23. The power converter of  claim 18 , wherein the controller further comprises a ripple compensation factor generator coupled to the frequency modulator to control a rate at which the frequency modulator increases and decreases the frequency of the clock signal. 
     
     
       24. The power converter of  claim 23 , wherein the frequency modulator sets the frequency of the clock signal to a percentage amount of the fixed frequency when the inductor voltage is greater than the threshold voltage, and wherein the ripple compensation factor generator is coupled to control the rate at which the frequency modulator increases and decreases the frequency of the clock signal such that the frequency modulator gradually changes the frequency of the clock signal over several input voltage line cycles of the power converter until the frequency is equal to the percentage amount of the fixed frequency. 
     
     
       25. The power converter of  claim 18 , wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to adjust the shape of the input current of the power converter to be substantially flat when the magnitude of the inductor voltage is greater than the threshold voltage. 
     
     
       26. A device, comprising:
 a light emitting diode (LED) load; and 
 an ac-to-dc power converter having an output coupled to the LED load, the power converter comprising:
 an energy transfer element to be coupled between an input and the output of the power converter; 
 a switch coupled to an inductor of the energy transfer element; and 
 a controller coupled to the switch to control switching of the switch, wherein the controller includes:
 an oscillator coupled to generate a clock signal having a frequency; 
 a drive circuit coupled to receive the clock signal and to generate a drive signal in response thereto, the drive signal to control switching of the switch to control a transfer of energy across the energy transfer element from the input of the power converter to the output of the power converter, wherein a switching frequency of the drive signal is based on the frequency of the clock signal and is much greater than a lower frequency of a time-varying inductor voltage across the inductor; and 
 a frequency modulator coupled to the oscillator to control the frequency of the clock signal in response to a magnitude of the time-varying inductor voltage in order to reduce a peak-to-peak ripple value in an output current of the power converter that is due to the lower frequency time variations in the inductor voltage, wherein, wherein the frequency modulator controls the frequency of the clock signal during each line half cycle to be a fixed frequency when the magnitude of the inductor voltage is less than or equal to a threshold voltage, and wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to be less than the fixed frequency to adjust a shape of an input current of the power converter when the magnitude of the inductor voltage is greater than the threshold voltage. 
 
 
 
     
     
       27. The device of  claim 26 , wherein the frequency modulator continuously adjusts the frequency of the clock signal while the magnitude of the inductor voltage exceeds the threshold voltage such that a ratio of the fixed frequency to the switching frequency is substantially equal to a ratio of the inductor voltage to the threshold voltage. 
     
     
       28. The device of  claim 26 , wherein the frequency modulator varies the frequency of the clock signal during each line half cycle to adjust the shape of the input current of the power converter to be substantially flat when the magnitude of the inductor voltage is greater than the threshold voltage.