Constant current mode ripple attenuation method in flyback power supply

1. A method for limiting an output current of a power supply over multiple switching cycles in a constant current mode using pulse width modulation and attenuating a voltage ripple on a digital feedback signal, said power supply having a primary and secondary side, the method comprising the steps of:

generating a sense voltage signal on the primary side that is proportional to an output voltage signal on the secondary side;

measuring a reset time on the primary side using said sense voltage signal, said reset time representing a duration of a current pulse on the secondary side;

generating a ripple compensation signal representing a output voltage ripple signal;

generating a first digital voltage signal used to maintain a substantially constant current on the secondary side, said first digital voltage signal representing the on-time for a subsequent switching cycle, wherein a product of said on-time for a subsequent switching cycle and said reset-time for a present switching cycle is a first value, wherein said first value is substantially constant for multiple switching cycles;

generating the digital feedback voltage signal by combining said first digital signal and said ripple compensation signal.

2. The method of claim 1 , wherein said step of generating a ripple compensation signal includes the steps of:

filtering said sense voltage signal using a low pass filter to generate a filtered sense signal representing the digital feedback signal without said voltage ripple; and

subtracting said filtered sense signal from said sense voltage signal to generate said voltage ripple signal.

3. The method of claim 2 wherein said step of generating a ripple compensation signal further comprises the step of:

scaling said voltage ripple signal to a first level that is compatible with said first digital voltage signal.

4. A power supply control system for limiting an output current of a power supply over multiple switching cycles in a constant current mode using pulse width modulation and attenuating a voltage ripple on a digital feedback signal, said power supply having a primary and secondary side, the method comprising the steps of:

sense voltage means for generating a sense voltage signal on the primary side that is proportional to an output voltage signal on the secondary side;

reset time means for measuring a reset time on the primary side using said sense voltage signal, said reset time representing a duration of a current pulse on the secondary side;

ripple detection means for generating a ripple compensation signal representing a voltage ripple signal;

intermediary means for generating a first digital voltage signal used to maintain a substantially constant current on the secondary side, said first digital voltage signal representing the on-time for a subsequent switching cycle, wherein a product of said on-time for a subsequent switching cycle and said reset-time for a present switching cycle is a first value, wherein said first value is substantially constant for multiple switching cycles;

feedback means for generating the digital feedback voltage signal by combining said first digital signal and said ripple compensation signal.

5. The power supply control system of claim 4 , wherein said ripple detection means comprises:

low pass filtering means for filtering said sense voltage signal using a low pass filter to generate a filtered sense signal representing the digital feedback signal without said voltage ripple; and

ripple identification means for subtracting said filtered sense signal from said sense voltage signal to generate said voltage ripple signal.

6. The power supply control system of claim 5 wherein said ripple detection means further comprises:

scaling means for scaling said voltage ripple signal to a first level that is compatible with said first digital voltage signal.

7. A power supply control system for limiting an output current of a power supply over multiple switching cycles in a constant current mode using pulse width modulation and attenuating a voltage ripple on a digital feedback signal, said power supply having a primary and secondary side, the method comprising the steps of:

a sense voltage device having a winding on the primary side to generate a sense voltage signal on the primary side that is proportional to an output voltage signal on the secondary side;

a counter, to determine a reset time on the primary side using said sense voltage signal, said reset time representing a duration of a current pulse on the secondary side;

a ripple detector, for generating a ripple compensation signal representing a voltage ripple signal;

a preliminary feedback signal, disposed to receive said reset time, to generate a first digital voltage signal used to maintain a substantially constant current on the secondary side, said first digital voltage signal representing the on-time for a subsequent switching cycle, wherein a product of said on-time for a subsequent switching cycle and said reset-time for a present switching cycle is a first value, wherein said first value is substantially constant for multiple switching cycles;

a feedback device, disposed to receive said ripple compensation signal and said first digital voltage signal to generate the digital feedback voltage signal by combining said first digital signal and said ripple compensation signal.

8. The power supply control system of claim 7 , wherein said ripple detector comprises:

a low pass filter for filtering said sense voltage signal to generate a filtered sense signal representing the digital feedback signal without said voltage ripple; and

a ripple identifier, coupled to said low pass filter, for subtracting said filtered sense signal from said sense voltage signal to generate said voltage ripple signal.

9. The power supply control system of claim 8 wherein said ripple detector further comprises:

a signal scalar for scaling said voltage ripple signal to a first level that is compatible with said first digital voltage signal.