Logarithmic power compensation for a switching power supply

A circuit for controlling an output voltage, the circuit comprising:

• a transformer having a primary winding and a secondary winding;
• a first switch coupled in series with the primary winding, the first switch including a control terminal;
• a second switch coupled in series with the secondary winding, the second switch including a control terminal;
• a capacitor coupled in parallel with the second switch;
• a drive circuit for generating a first control signal and a second control signal, the first control signal being applied to the control terminal of the first switch and the second control signal being applied to the control terminal of the second switch, wherein the first control signal is a complement of the second control signal.

A method of controlling an output voltage, the method comprising the steps of:

• applying an input voltage to a primary winding of a transformer;
• allowing current to flow through the primary winding during a first time period;
• charging a capacitor during the first time period, the capacitor coupled to a secondary winding of the transformer;
• preventing current from flowing through the primary winding during a second time period;
• and
• during the second time period, rendering substantially conductive a switch coupled in parallel with the capacitor.

An electrical circuit comprising:

• a transformer having a primary and a secondary winding, each of the windings having first and second terminals, the first terminal of the primary winding coupled to an input voltage node;
• a reference potential node;
• a first resistor with a current path coupled between the second terminal of the primary winding and the reference potential node;
• a first switch with a current path coupled between the second terminal of the primary winding and the reference potential node, the current path of the first switch coupled in series with the current path of the first resistor, the first switch further having a control input terminal;
• a diode with a first terminal coupled to the second terminal of the secondary winding;
• a second resistor with a first terminal coupled to a second terminal of the diode;
• a capacitor with a first terminal coupled to the reference potential node and a second terminal coupled to a second terminal of the second resistor;
• a second switch with a current path coupled between the second terminal of the second resistor and the reference potential node, the second switch also including a control terminal;
• a comparator for comparing a voltage at the second terminal of the capacitor with a control voltage;
• and
• a drive circuit for receiving an output of the comparator and for controlling the first and second switches.