Short circuit and soft short protection for data interface charging

1. A flyback converter for charging a mobile device through a data interface cable, comprising:

a primary-side controller configured to regulate the cycling of a power switch in a constant-current mode;

a secondary-side controller configured to communicate through a data channel in the data interface cable with a load device to receive a value of a constant load current consumed by the load device during a soft-short circuit detection mode and configured to communicate a value of the constant load current to the primary-side controller, wherein the primary-side controller is configured to:

adjust the cycling of the power switch until the flyback converter enters the constant-current mode in which it drives a constant output current into the data interface cable while the load device consumes the constant load current;

measure the constant output current using a constant current coefficient; and

determine a difference between a measure of the constant output current and the value of the constant load current to detect a soft-short circuit current.

2. The flyback converter of claim 1 , wherein the secondary-side controller is further configured to communicate the detection of the soft-short current to the load device through the data channel in the data interface cable.

3. The flyback converter of claim 1 , wherein the primary-side controller is configured to detect the soft-short circuit current responsive to the difference between the measure of the constant output current and the value of the constant load current exceeding a soft-short circuit threshold.

4. The flyback converter of claim 1 , wherein the primary-side controller is further configured to shut down the cycling of the power switch responsive to the detection of the soft-short circuit current.

5. The flyback converter of claim 1 , wherein the constant load current equals zero, and wherein the secondary-side controller is configured to detect a soft-short circuit condition responsive to a voltage droop for an output voltage from the flyback converter exceeding a voltage droop threshold.

6. The flyback converter of claim 5 , wherein the secondary-side controller is further configured to:

measure the output voltage droop in a standby mode that delivers zero current to the load;

compare the output voltage droop to the voltage droop threshold; and

detect the soft-short circuit condition responsive to the comparison of the output voltage droop to the voltage droop threshold.

7. The flyback converter of claim 6 , wherein the secondary-side controller is configured to reset the flyback converter responsive to a voltage droop rate being greater than the voltage droop threshold.

8. The flyback converter of claim 1 , wherein the data interface cable is a USB cable.

9. A method of detecting a soft-short circuit, comprising:

at a secondary-side controller for a flyback converter, receiving a load current value for a constant load current to be drawn by a load device coupled to the flyback converter through a data interface cable;

communicating the load current value from the secondary-side controller to a primary-side controller for the flyback converter;

while the load device consumes the constant load current, adjusting a regulation of a power switch in the flyback converter by reducing a constant-current coefficient until the regulation is a constant-current regulation in which the flyback converter drives the data interface cable with a constant output current; and

comparing a difference between a measure of the constant output current and the load current value to a soft-short threshold to detect the soft-short circuit.

10. The method of claim 9 , wherein adjusting the regulation of the power switch begins with a constant-current limit that is greater than the constant load current plus the soft-short threshold, and wherein the adjusting of the regulation of the power switch comprises reducing the constant-current limit until the regulation is the constant-current regulation.

11. The method of claim 9 , wherein communicating the load current value from the secondary-side controller to the primary-side controller comprises communicating the load current value over an optoisolator.

12. The method of claim 9 , wherein communicating the load current value from the secondary-side controller to the primary-side controller comprises pulsing a synchronous rectifier switch.

13. The method of claim 9 , wherein communicating the load current value from the secondary-side controller to the primary-side controller comprises communicating the load current value over a capacitor.

14. The method of claim 9 , further comprising switching off the flyback converter responsive to the detection of the soft-short circuit.

15. The method of claim 9 , wherein the load device comprises a mobile telephone, the method further comprising alerting the mobile telephone of the soft-short circuit detection.

16. The method of claim 9 , wherein the comparison of the difference between the measure of the constant output current and the value of the load current to a soft-short threshold is performed by the primary-side controller, the method further comprising the primary-side controller alerting the secondary-side controller of the detection of the soft-short circuit.