Dual mode over-current protection for switching mode power converter

1. A switched mode power converter, comprising:

a transformer having a primary winding and a secondary winding;

a primary side power switch coupled to said primary winding and being adapted to periodically apply an input voltage to said primary winding;

an output filter operatively coupled to said secondary winding to provide an output voltage and output current;

a forward synchronous rectification device operatively coupled in series between said secondary winding and said output filter;

a free-wheeling synchronous rectification device operatively coupled in shunt with said secondary winding and said output filter; and

a control circuit adapted to receive a current sense signal reflecting the output current and a feedback signal reflecting the output voltage, the control circuit including a first over-current protection circuit adapted to shut off operation of the switched mode power converter if a sum of the current sense signal and the feedback signal exceed a first predetermined limit, and a second over-current protection circuit adapted to regulate operation of said primary side power switch responsive a comparison of the sum of the current sense signal and the feedback signal to a second predetermined limit, wherein said second predetermined limit is less than the first predetermined limit.

2. The switched mode power converter of claim 1 , wherein said control circuit further comprises a soft start circuit operatively coupled to the second-over current protection circuit, the soft start circuit holding the second predetermined limit at a constant level during a starting condition of the switched mode power converter.

3. The switched mode power converter of claim 1 , wherein the second over-current protection circuit further comprises a shunt regulator adapted to increase a level of the second predetermined limit upon a short circuit condition of the switched mode power converter.

4. The switched mode power converter of claim 1 , wherein the first over-current protection circuit further comprises a first comparator having a first input receiving said sum of the current sense signal and the feedback signal and a second input receiving a reference defining the first predetermined limit.

5. The switched mode power converter of claim 1 , wherein the second over-current protection circuit further comprises a second comparator having a first input receiving said sum of the current sense signal and the feedback signal and a second input receiving a variable reference defining the second predetermined limit.

6. The switched mode power converter of claim 5 , wherein the variable reference is provided by a voltage divider circuit and a shunt regulator adapted to vary a voltage defined by the voltage divider circuit.

7. The switched mode power converter of claim 5 , wherein the second over-current protection circuit further comprises a third comparator having a first input receiving the feedback signal and a second input receiving a threshold reference.

8. In a switched mode power converter comprising a transformer having a primary winding and a secondary winding, a primary side power switch coupled to said primary winding and being adapted to periodically apply an input voltage to said primary winding, an output filter operatively coupled to said secondary winding to provide an output voltage and output current, a forward synchronous rectification device operatively coupled in series between said secondary winding and said output filter, and a free-wheeling synchronous rectification device operatively coupled in shunt with said secondary winding and said output filter, a control circuit comprises:

a first over-current protection circuit adapted to shut off operation of the switched mode power converter if a sum of a current sense signal reflecting the output current and a feedback signal reflecting the output voltage exceed a first predetermined limit; and

a second over-current protection circuit adapted to regulate operation of said primary side power switch responsive a comparison of the sum of the current sense signal and the feedback signal to a second predetermined limit, wherein said second predetermined limit is less than the first predetermined limit.

9. The control circuit of claim 8 , further comprising a soft start circuit operatively coupled to the second-over current protection circuit, the soft start circuit holding the second predetermined limit at a constant level during a starting condition of the switched mode power converter.

10. The control circuit of claim 8 , wherein the second over-current protection circuit further comprises a shunt regulator adapted to increase a level of the second predetermined limit upon a short circuit condition of the switched mode power converter.

11. The control circuit of claim 8 , wherein the first over-current protection circuit further comprises a first comparator having a first input receiving said sum of the current sense signal and the feedback signal and a second input receiving a reference defining the first predetermined limit.

12. The control circuit of claim 8 , wherein the second over-current protection circuit further comprises a second comparator having a first input receiving said sum of the current sense signal and the feedback signal and a second input receiving a variable reference defining the second predetermined limit.

13. The control circuit of claim 12 , wherein the variable reference is provided by a voltage divider circuit and a shunt regulator adapted to vary a voltage defined by the voltage divider circuit.

14. The control circuit of claim 12 , wherein the second over-current protection circuit further comprises a third comparator having a first input receiving the feedback signal and a second input receiving a threshold reference.