Input voltage detection for flyback converter
A switch detection circuit is provided that senses the voltage on a rectifying component for rectifying a secondary winding current through a secondary winding of a flyback converter's transformer to determine whether a power switch transistor attached to a primary winding of the transformer has ceased cycling.
1. A secondary-side controller for a flyback converter, comprising:
an output voltage terminal for an output voltage in a power lead within a data cable for powering a mobile device;
a plurality of data terminals for coupling to corresponding data channels in the data cable;
a switch detection circuit configured to sense a voltage of a rectifying circuit connected to a secondary winding of the flyback converter to determine whether a power switch transistor connected to a primary winding for the flyback converter has stopped cycling on and off, wherein the rectifying circuit is a synchronous rectifier switch transistor, and wherein the switch detection circuit comprises a comparator configured to compare a gate voltage of the synchronous rectifier switch transistor to a reference voltage and further comprises a timer configured to time a time-out period, and wherein the comparator is configured to reset the timer responsive to a cycling on of the synchronous rectifier switch transistor; and
a logic circuit configured to switch off a voltage bus switch transistor to isolate the power lead from an output voltage rail in the flyback converter responsive to a detection through the plurality of data terminals that the mobile device is connected to the flyback converter through the data cable while the switch detection circuit senses that the power switch transistor has stopped cycling on and off.
2. The secondary-side controller of claim 1 , wherein the data cable comprises a universal serial bus (USB) cable.
3. The secondary-side controller of claim 2 , wherein the plurality of data terminals comprises a Configuration Channel 1 (CC1) terminal and a Configuration Channel 2 (CC2) terminal.
4. The secondary-side controller of claim 2 , wherein the logic circuit is further configured to notify the mobile device through at least one of the data terminals that the voltage bus switch transistor has been switched off.
5. The secondary-side controller of claim 1 , wherein the data cable comprises a universal serial bus (USB) cable.
6. The secondary-side controller of claim 5 , wherein the plurality of data terminals comprises a Configuration Channel 1 (CC1) terminal and a Configuration Channel 2 (CC2) terminal.
7. The secondary-side controller of claim 5 , wherein the logic circuit is further configured to notify the mobile device through at least one of the data terminals that the voltage bus switch transistor has been switched off.
8. A method, comprising:
while a flyback converter is attached to an AC mains, cycling on and off a power switch transistor attached to a primary winding of a transformer for the & flyback converter;
in a rectifying component attached to a secondary winding of the transformer, rectifying a secondary winding current to prevent the secondary winding current from flowing while the power switch transistor is on;
in response to a detachment of the flyback converter from the AC mains, ceasing the cycling of the power switch transistor;
detecting the ceasing of the cycling of the power switch transistor by comparing a gate voltage for a synchronous rectifier switch transistor to a threshold voltage to determine whether the synchronous rectifier switch transistor is cycling; and
switching off a voltage bus switch transistor to isolate a mobile device attached to the flyback converter through a data cable from an output voltage rail for the flyback converter responsive to a detection that the power switch transistor has ceased cycling.
9. The method of claim 8 , wherein switching off the voltage bus switch transistor occurs responsive to a time-out period expiring following the detection that the power switch transistor has ceased cycling.
10. The method of claim 8 , further comprising notifying the mobile device through a data channel in the data cable that the voltage bus switch transistor has been switched off.
11. A secondary-side controller for a flyback converter, comprising:
an output voltage terminal for an output voltage in a power lead within a data cable for powering a mobile device;
a plurality of data terminals for coupling to corresponding data channels in the data cable;
a switch detection circuit configured to sense a voltage of a rectifying circuit connected to a secondary winding of the flyback converter to determine whether a power switch transistor connected to a primary winding for the flyback converter has stopped cycling on and off, wherein the rectifying circuit is an output diode, and wherein the switch detection circuit comprises a comparator configured to compare a cathode voltage for the output diode to an anode voltage for the output diode; and
a logic circuit configured to switch off a voltage bus switch transistor to isolate the power lead from an output voltage rail in the flyback converter responsive to a detection through the plurality of data terminals that the mobile device is connected to the flyback converter through the data cable while the switch detection circuit senses that the power switch transistor has stopped cycling on and off.
12. The secondary-side controller of claim 11 , wherein the switch detection circuit is further configured to compare the cathode voltage to a threshold voltage to detect whether an input voltage to the flyback converter has a brown-out condition, and wherein the logic circuit is further configured to switch off the voltage bus switch transistor responsive to a detection of the brown-out condition.