Switch mode power supply systems

A system for estimating an output current of a Switch Mode Power Supply (“SMPS”), said SMPS including a transformer with at least a primary winding coupled to an input of the SMPS and a secondary winding coupled to an output of the SMPS, the system comprising:

• an input to sense a current in said primary winding of said transformer;
• an averager to average a signal from said sensed current over a period when current is flowing in said primary winding to provide an average current signal;
• a first subsystem to determine an estimate of a discharge time of current through said secondary winding of said transformer;
• and
• a second subsystem to scale said average current signal using said discharge time to estimate said SMPS output current;
• and
• wherein said averager comprises a low-pass filter including a resistor and a capacitor, wherein said resistor is implemented by a switched capacitor resistor, said switched capacitor resistor comprising:
• a first switched capacitor resistor terminal;
• a second switched capacitor resistor terminal;
• a common terminal;
• an intermediate terminal;
• a capacitor coupled between said intermediate terminal and said common terminal;
• a first controllable switch coupled between said first terminal and said intermediate terminal;
• a second controllable switch coupled between said second terminal and said intermediate terminal;
• and
• at least one resistor connected in series with one of said first and second switches between one of said first and second terminals and said intermediate terminal.