Input voltage dependent control for active clamp flyback

A control circuit for a power converter, comprising:

• a valley detector for providing valley pulse signals at valley points in a resonant waveform in the sensing signal during the discontinuous time;
• a peak detector for providing peak pulse signals at peak points in a resonant waveform in the sensing signal during the discontinuous time;
• a power controller for turning on a power switch to maintain a desired output voltage, wherein the power switch is coupled to a primary winding of the power converter to control a primary current flow, and the output voltage is provided by a secondary winding, and a sensing circuit provides a sensing signal through an auxiliary winding;
• a modulation controller, coupled in parallel to the power switch, configured to activate a modulation capacitance at a time determined by the magnitude of an input voltage of the power converter and to turn on a modulation switch that is coupled in series with a capacitor, wherein the capacitor and the modulation switch are coupled in parallel with the power switch, wherein:
• when the power converter is operated in a low input voltage mode, the modulation switch is turned on in a demagnetization-time during which the secondary winding discharges;
• and
• when the power converter is operated in a high input voltage mode, the modulation switch is turned on in a discontinuous time.

A control circuit for a power converter, comprising:

• a power controller for turning on a power switch to maintain a desired output voltage, wherein the power switch is coupled to a primary winding of the power converter to control a primary current flow, and the output voltage is provided by a secondary winding, and a sensing circuit provides a sensing signal through an auxiliary winding;
• and
• a modulation controller, comprising a delay circuit and a one-shot circuit, is configured to activate a modulation capacitance, coupled in parallel to the power switch, at a time determined by the magnitude of an input voltage of the power converter, wherein:
• when the power converter is operated in a low input voltage mode, the modulation switch is turned on in a demagnetization-time during which the secondary winding discharges, wherein, in the low input voltage mode, the modulation controller is further configured to turn on the modulation switch after a pre-set delay time after the power switch is turned off;
• and
• when the power converter is operated in a high input voltage mode, the modulation switch is turned on in a discontinuous time.

A power converter, comprising:

• a transformer having a primary winding for coupling to an input voltage of the power converter, a secondary winding providing an output voltage of the power converter, and a sensing circuit for providing a sensing signal through auxiliary winding,
• a power switch for coupling to the primary winding of the power converter to control a primary current flow;
• a capacitor and a modulation switch coupled in parallel to the power switch, the modulation switch being coupled in series with the capacitor;
• and
• a control circuit, comprising:
• a power controller for turning on the power switch to maintain a desired output voltage;
• a mode selection circuit for determining the operating mode depending on the magnitude of the input voltage of the power converter, in which the mode selection circuit compares the input voltage of the power converter with a reference voltage and provides a mode selection signal that indicates a high input voltage mode if the input voltage of the power converter is higher than the reference voltage;
• and a low input voltage mode if the input voltage of the power converter is lower than the reference voltage;
• and
• a modulation controller configured to turn on the modulation switch to activate the capacitor according to the mode selection signal, wherein:
• when the power converter is operated in the low input voltage mode, the modulation switch is turned on in a demagnetization-time during which the secondary winding discharges.

A power converter, comprising:

• a transformer having a primary winding for coupling to an input voltage of the power converter, a secondary winding providing an output voltage of the power converter, and a sensing circuit for providing a sensing signal through auxiliary winding,
• a power switch for coupling to the primary winding of the power converter to control a primary current flow;
• a capacitor and a modulation switch coupled in parallel to the power switch, the modulation switch being coupled in series with the capacitor;
• and
• a control circuit, comprising:
• a power controller for turning on the power switch to maintain a desired output voltage;
• a mode selection circuit for determining the operating mode depending on the magnitude of the input voltage of the power converter, in which the mode selection circuit compares the input voltage of the power converter with a reference voltage and provides a mode selection signal that indicates a high input voltage mode if the input voltage of the power converter is higher than the reference voltage;
• and a low input voltage mode if the input voltage of the power converter is lower than the reference voltage;
• and
• a modulation controller configured to turn on the modulation switch to activate the capacitor according to the mode selection signal, wherein:
• when the power converter is operated in the high input voltage mode, the modulation switch is turned on in a discontinuous time during which no current flows in the transformer and the sensing signal is characterized by a resonant waveform.