Bias supply circuit and a switching power supply employing the same

A method of operating a bias supply circuit for use with a switching power supply employing a main switch and an auxiliary switch and having an input voltage and a voltage across a clamp capacitor coupled to a transformer, comprising:

• providing a first voltage associated with a bias supply winding of said transformer and dependent upon at least one of said input voltage and said voltage across said clamp capacitor during a conduction period of said main switch or said auxiliary switch;
• providing a second voltage associated with a bias supply storage capacitor coupled to said bias supply winding and dependent upon at least another of said input voltage and said voltage across said clamp capacitor during a conduction period of another of said main switch or said auxiliary switch;
• and
• forming a bias supply voltage by summing said first voltage and said second voltage.

A switching power supply, comprising:

• a primary power supply circuit that employs a main switch and an auxiliary switch and that has an input voltage and a voltage across a clamp capacitor coupled to a transformer;
• a secondary power supply circuit coupled to said transformer that provides an output voltage;
• and
• a bias supply circuit, including:
• a bias supply winding associated with said transformer that is configured to provide a first voltage dependent on at least one of said input voltage and said voltage across said clamp capacitor during a conduction period of said main switch or said auxiliary switch;
• and
• a bias supply storage capacitor coupled to said bias supply winding that is configured to provide a second voltage dependent on at least another of said input voltage and said voltage across said clamp capacitor during a conduction period of another of said main switch or said auxiliary switch, thereby allowing a sum of said first voltage and said second voltage to provide a bias supply voltage.