Switch mode power supply systems
This invention relates to methods and apparatus for sensing the output current in a switch mode power supply (SMPS) using primary side sensing. We describe a module which senses a current in a primary winding of a transformer and a voltage on a primary or auxiliary winding of the transformer, and which includes a multiplier coupled to an output of a signal averager averaging a primary winding current and to an output of a timing signal generator using the sensed voltage to signal when a secondary winding is powering an output of the SMPS, to multiply an averaged current sense signal by a fraction of a total cycle period of said SMPS during which the secondary winding is providing power to provide a signal estimating an output current of the SMPS.
1. 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.
2. A switched capacitor resistor as claimed in claim 1 , comprising two resistors one connected in series with each of said first and second switches between said respective first and second terminals and said intermediate terminal.
3. A switched capacitor resistor as claimed in claim 1 , comprising a controller to provide control signals to said first and second switches to control charge flow from said first terminal onto said capacitor and then from said capacitor to said second terminal, through said at least one resistor.
4. A switched capacitor resistor as claimed in claim 3 , comprising two resistors one connected in series with each of said first and second switches between said respective first and second terminals and said intermediate terminal, and wherein said controller is configured to magnify a value of each of said two resistors by a factor of at least two.
5. A switched capacitor resistor as claimed in claim 4 , wherein said controller is configured to magnify a value of a said resistor by a factor of at least ten.
6. The switched capacitor resistor of claim 1 , wherein the at least one resistor is to operate to time-slice at a duty cycle of less than 100% on time to increase an effective value of said at least one resistor.
7. A method of estimating an output current of a Switch Mode Power Supply (“SMPS”), comprising:
sensing a current in a primary winding of a transformer;
low-pass filtering a signal from said sensed current over a period when current is flowing in a primary winding, which is connected to an input of the SMPS, to provide an average current signal;
determining an estimate of a discharge time of current through a secondary winding of said transformer that is connected to an output of the SMPS; and
scaling said average current signal using said discharge time to estimate said SMPS output current; and
wherein said low-pass filtering includes:
filtering using a combination of a series resistor and a parallel capacitor; and
implementing said series resistor using a switched capacitor resistor, wherein said implementing comprises including at least one resistive component in said switched capacitor resistor and time-slicing said resistive component using a time-slicing signal having a duty cycle of less than 100% on time, to increase an effective value of said resistive component.
8. The method of claim 7 , wherein time-slicing includes:
controlling a first controllable switch coupled between a first terminal and an intermediate terminal;
controlling a second controllable switch coupled between a second terminal and said intermediate terminal; and
controlling the 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.