Switching controller and method for operating a flyback converter in a critically continuous conduction mode

A method of operating a power converter having a controllable switch in a critically continuous conduction mode (CCCM), comprising the steps of:

• generating an error signal that is a result of comparing an electrical characteristic of said power converter to a reference electrical characteristic;
• comparing said error signal to a reference waveform and providing, in response thereto, a drive signal to said controllable switch;
• and
• sensing a voltage across said controllable switch and comparing said sensed voltage to a sensed time-varying input voltage;
• and
• modifying said reference waveform utilizing a waveform shaping circuit and turning-on said controllable switch in response to said sensed voltage being lower than said input voltage.

A flyback power converter, comprising:

• a power transformer having primary and secondary windings;
• a controllable switch coupled to said primary windings;
• a power diode coupled to said secondary windings;
• an output capacitor coupled to said power diode;
• and
• a switching controller, including:
• an error amplifier for sensing an electrical characteristic of an output of said flyback power converter and comparing said electrical characteristic to a reference electrical characteristic to generate an error signal;
• a drive signal generator, coupled to said controllable switch, for comparing said error signal to a reference waveform and providing, in response thereto, a drive signal to said controllable switch;
• and
• a waveform shaping circuit, coupled to said flyback power converter, for comparing a sensed voltage across said controllable switch to a sensed time-varying input voltage and providing, in response to said sensed voltage being lower than said input voltage, a modifying signal to said drive signal generator, said modifying signal modifying said reference waveform and causing said drive signal generator to turn-on said controllable switch, said waveform shaping circuit including:
• an operational amplifier configured as an comparator, wherein said sensed voltage and said input voltage are coupled to an inverting node and a noninverting node, respectively, of said operational amplifier;
• an exclusive OR gate, said gate inputs are coupled to an output of said operational amplifier and to said drive signal of said drive signal generator;
• and
• a second controllable switch coupled to an output of said gate.