Circuits and methods for controlling power converters including transformers

A controller for controlling a transformer, said controller comprising:

• a first signal generator operable for receiving a first feedback signal indicating an output voltage of a secondary winding of said transformer and for generating a first signal;
• and
• a pulse signal generator coupled to said first signal generator and operable for generating a pulse signal based on said first signal, wherein said pulse signal is configured to control power of said transformer,
• wherein said first signal increases from a predetermined reference voltage level to a first level during a charging period, wherein said first signal decreases from said first level to a second level that is lower than said predetermined reference voltage level during a discharging period, and wherein said first signal increases from said second level to said predetermined reference voltage level during an adjusting period,
• wherein a ratio of a duration of said discharging period to a total duration of said charging period, said discharging period and said adjusting period is constant,
• wherein during said charging period a current flowing through a primary winding of said transformer increase, wherein during said discharging period a current flowing through said secondary winding decreases.

A method for controlling a transformer, comprising:

• operating said transformer in a plurality of switching cycles, wherein at least one of said switching cycles comprises a charging period, a discharging period, and an adjusting period;
• powering said transformer during said charging period;
• discharging said transformer to power a load during said discharging period;
• and
• determining a duration of said adjusting period, comprising:
• generating a sawtooth wave signal;
• increasing said sawtooth wave signal from a predetermined reference voltage level to a first level during said charging period;
• decreasing said sawtooth wave signal from said first level to a second level that is lower than said predetermined reference voltage level during said discharging period;
• increasing said sawtooth wave signal from said second level during said adjusting period;
• and
• terminating said adjusting period when said sawtooth wave signal increases to said predetermined reference voltage level,
• wherein a ratio of a duration of said discharging period to a total duration of said charging period, said discharging period and said adjusting period is constant.