Switch mode power supply controllers
This invention relates to SMPS controllers employing primary side sensing. We describe a system for identifying a knee point in a sensing waveform, at which the output voltage of the SMPS may be sampled accurately on the primary side. The system identifies the knee point by fitting a tangent to a portion of a power transformer voltage waveform, and samples the voltage waveform at the knee point to determine the SMPS output voltage. In preferred embodiments this technique is implemented using a decaying peak detector, providing a timing signal indicating detection of the knee point. Sample/hold and error amplifier circuits may be employed to achieve output voltage regulation.
1. A method of sensing an output voltage of a switch mode power supply (SMPS), the SMPS including a switched magnetic energy storage device for conveying power from an input to an output of said SMPS, said magnetic device having at least one winding, the method comprising:
inputting a sensing signal from said at least one winding of said magnetic device, said sensing signal having a waveform with a first, decaying portion during which power is supplied by said magnetic device to said SMPS output and a second portion during which substantially no power is supplied by said magnetic device to said SMPS output;
identifying a knee point on said sensing signal waveform between said decaying portion and said second portion of said waveform; and
using a value of said sensing signal at said knee point to sense said SMPS output voltage; and
wherein said identifying of said knee point comprises fitting an approximate tangent to said decaying portion of said sensing signal waveform; and
identifying departure of said sensing signal waveform from said approximate tangent to identify said knee point, wherein said departure identifying comprises comparing said tangent approximating signal and said sensing signal waveform to identify a time when said sensing signal waveform departs from said tangent approximating signal to identify said knee point.
2. A method of sensing an output voltage of a switch mode power supply (SMPS), the SMPS including a switched magnetic energy storage device for conveying power from an input to an output of said SMPS, said magnetic device having at least one winding, the method comprising:
inputting a sensing signal from said at least one winding of said magnetic device, said sensing signal having a waveform with a first, decaying portion during which power is supplied by said magnetic device to said SMPS output and a second portion during which substantially no power is supplied by said magnetic device to said SMPS output;
identifying a knee point on said sensing signal waveform between said decaying portion and said second portion of said waveform;
using a value of said sensing signal at said knee point to sense said SMPS output voltage;
wherein said identifying of said knee point comprises fitting an approximate tangent to said decaying portion of said sensing signal waveform;
identifying departure of said sensing signal waveform from said approximate tangent to identify said knee point; and
wherein said tangent fitting comprises detecting one or more peaks of said sensing signal waveform, storing a peak level response to said detecting, and decaying said stored peak level to generate a tangent approximating signal.
3. A method as claimed in claim 2 wherein said departure identifying comprises comparing said tangent approximating signal and said sensing signal waveform to identify a time when said sensing signal waveform departs from said tangent approximating signal to identify said knee point.
4. A method as claimed in claim 3 further comprising sampling said sensing signal at said time when said sensing signal waveform departs from said tangent approximating signal to provide said value of said sensing signal at said knee point.
5. A method as claimed in claim 4 wherein said sampling comprises integrating said sensing signal waveform from said knee point.
6. A method as claimed in claim 4 wherein said departure identifying identifies a plurality of said times of departure of said sensing signal waveform from said tangent approximating signal, the method further comprising identifying a time when substantially no power is supplied by said magnetic device to said SMPS output and using a most recent said sample of said sensing signal prior to said identified time to provide said value of said sensing signal at said knee point.
7. A method as claimed in claim 6 wherein said identifying of a time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a crossing through a reference level of said second portion of said sensing signal waveform.
8. A method as claimed in claim 6 wherein said identifying of a time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a crossing, through a reference level, of a time integral of said sensing signal waveform, said time integral being from a point of known substantially zero energy storage in said magnetic device.
9. A method as claimed in claim 6 wherein said identifying of time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a time with a fixed time delay from a crossing through a reference level of said sensing signal waveform.
10. A method as claimed in claim 1 wherein said magnetic device comprises a transformer, and wherein said inputting of said sensing signal waveform comprises inputting said sensing signal from a primary winding of said transformer.
11. A system for sensing an output voltage of an SMPS, the SMPS including a switched magnetic energy storage device for conveying power from an input to an output of said SMPS, said magnetic device having at least one winding, the system comprising:
means for inputting a sensing signal from said at least one winding of said magnetic device, said sensing signal having a waveform with a first, decaying portion during which power is supplied by said magnetic device to said SMPS output and a second portion during which substantially no power is supplied by said magnetic device to said SMPS output;
means for identifying a knee point on said sensing signal waveform between said decaying portion and said second portion of said waveform; and
means for using a value of said sensing signal at said knee point to sense said SMPS output voltage; and
wherein said means for said identifying of said knee point comprises:
means for fitting an approximate tangent to said decaying portion of said sensing signal waveform; and
means for identifying departure of said sensing signal waveform from said approximate tangent to identify said knee point, wherein said means for identifying includes means for comparing said tangent approximating signal and said sensing signal waveform to identify a time when said sensing signal waveform departs from said tangent approximating signal to identify said knee point.
12. The method of claim 1 , comprising sampling said sensing signal at said time when said sensing signal waveform departs from said tangent approximating signal to provide said value of said sensing signal at said knee point.
13. The method of claim 12 , wherein said sampling comprises integrating said sensing signal waveform from said knee point.
14. The method of claim 12 , wherein said departure identifying identifies a plurality of said times of departure of said sensing signal waveform from said tangent approximating signal, the method further comprising identifying a time when substantially no power is supplied by said magnetic device to said SMPS output and using a most recent said sample of said sensing signal prior to said identified time to provide said value of said sensing signal at said knee point.
15. The method of claim 14 , wherein said identifying of a time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a crossing through a reference level of said second portion of said sensing signal waveform.
16. The method of claim 14 , wherein said identifying of a time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a crossing, through a reference level, of a time integral of said sensing signal waveform, said time integral being from a point of known substantially zero energy storage in said magnetic device.
17. The method of claim 14 , wherein said identifying of time when substantially no power is supplied by said magnetic device to said SMPS output comprises identifying a time with a fixed time delay from a crossing through a reference level of said sensing signal waveform.