On-time control for constant current mode in a flyback power supply
View Patent ↗A primary side sensing power control system and method for constant current control that utilizes a relationship that involves the measured reset-time from the previous cycle to determine the primary side peak current and off-time for the next cycle. This control mechanism does not need the knowledge of input voltage or magnetizing inductance. Therefore, it removes the sensitivities of input voltage and magnetizing inductance to the output current limit. Furthermore, it uses a time measurement instead of a voltage measurement for the current calculation which in many cases is easier to perform.
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, said power supply having a primary side and a secondary side wherein an inductor, having a magnetizing inductance, electrically separates said primary and secondary sides, 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; and
generating a digital feedback voltage signal on the primary side that is used to maintain a substantially constant current on the secondary side, said digital feedback voltage signal representing the on-time for a subsequent switching cycle, wherein said digital feedback voltage signal is generated without any representation of the input voltage or any representation of the magnetizing inductance.
2. The method of claim 1 , wherein said digital feedback voltage signal is based upon a comparison between a sensed peak primary side voltage signal and a target voltage signal.
3. The method of claim 2 , wherein said target voltage signal is generated using said reset time.
4. A power supply control system for limiting an output current of the power supply over multiple switching cycles in a constant current mode using pulse width modulation, said power supply having a primary side and a secondary side wherein an inductor, having a magnetizing inductance, electrically separates said primary and secondary sides, 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 determining 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; and
feedback means for generating a digital feedback voltage signal on the primary side that is used to maintain a substantially constant current on the secondary side, said digital feedback voltage signal representing the on-time for a subsequent switching cycle, wherein said digital feedback voltage signal is generated without any representation of the input voltage or any representation of the magnetizing inductance.
5. The system of claim 4 , wherein said digital feedback voltage signal is based upon a comparison between a sensed peak primary side voltage signal and a target voltage signal.
6. The system of claim 5 , wherein said target voltage signal is generated using said reset time.
7. A power supply control system for limiting an output current of the power supply over multiple switching cycles in a constant current mode using pulse width modulation, said power supply having a primary side and a secondary side wherein an inductor, having a magnetizing inductance, electrically separates said primary and secondary sides, comprising:
a sense voltage device having a winding on the primary side, to generate a sense voltage 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; and
a feedback device, disposed to receive said reset time, to generate a digital feedback voltage signal on the primary side that is used to maintain a substantially constant current on the secondary side, said digital feedback voltage signal representing the on-time for a subsequent switching cycle, wherein said digital feedback voltage signal is generated without any representation of the input voltage or any representation of the magnetizing inductance.
8. The system of claim 7 , wherein said digital feedback voltage signal is based upon a comparison between a sensed peak primary side voltage signal and a target voltage signal.
9. The system of claim 8 , wherein said target voltage signal is generated using said reset time.