Over current protection in a variable output power supply

A switch mode power supply comprising:

• a transformer comprising:
• a primary winding configured to carry a primary current;
• and
• a secondary winding configured to couple a secondary voltage, induced by a primary voltage, to an output;
• a controller configured to generate a switching signal having a switching period comprising an on phase and an off phase;
• a switching transistor including:
• a control electrode configured to receive the switching signal;
• and
• first and second output electrodes configured to form a current path, in accordance with the control electrode, between the primary winding and a sense node;
• a sense resistor coupled to the sense node;
• and
• a limit circuit configured to:
• sense a voltage of the sense node at a particular point in time of the switching period;
• detect a demagnetizing interval of the switching period;
• generate a limit proxy in accordance with the sense node voltage and a duration of the demagnetizing interval;
• perform a comparison of the limit proxy and a limit reference;
• and
• generate a limit signal in accordance with a result of the comparison, wherein the limit circuit includes a midpoint circuit configured to:
• sample the voltage of the sense node at a midpoint of the on phase;
• charge a sense capacitor from an initial voltage to a charged voltage during a first on phase;
• discharge the sense capacitor from the charged voltage to the initial voltage during a first half of a subsequent on phase;
• generate a midpoint signal upon completion of the discharging of the sense capacitor;
• close a charging switch to couple a charging current source to the sense capacitor during the first on phase;
• and
• close a discharging switch to couple a discharging current source to the sense capacitor during a first half of a second on phase.