Switch mode power supply controller with feedback signal decay sensing
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 system for 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 system comprising;
a sense input to receive 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;
a signal follower coupled to said sense input to generate a decay signal approximating said decaying portion of said sensing signal waveform;
a comparator to compare said decay signal with said sensing signal waveform to identify when said sensing signal waveform decays faster than said decay signal; and
a sampler to sample said sensing signal responsive to said comparator to provide an output signal sensing said output voltage of said SMPS.
2. A system as claimed in claim 1 further comprising an enable input to receive an enable signal, to disable the operation of said sampler during said second portion of said sensing signal.
3. A system as claimed in claim 2 wherein said signal follower has a reset input coupled to said enable input to reset said decay signal responsive to said enable signal.
4. A system as claimed in claim 1 wherein said signal follower comprises a decaying peak detector to detect and hold with a decaying characteristic peaks of said sensing signal, said decaying peak detector having an output to provide said decay signal.
5. A system as claimed in claim 4 wherein said decaying peak detector comprises a rectifier coupled to a capacitor and a discharge circuit coupled across said capacitor, and wherein said decaying peak detector output is provided from said capacitor.
6. A system as claimed in claim 1 wherein said sampler comprises a sample-hold circuit coupled to said comparator to sample and hold said sensing signal when said sensing signal waveform decays faster than said decay signal to provide said output signal.
7. A system as claimed in any claim 1 wherein said sampler comprises an integrator to sample said sensing signal by integrating said sensing signal waveform from when said sensing signal waveform decays faster than said decay signal to a later point on said sensing signal waveform.
8. A system as claimed in claim 1 for controlling said SMPS to operate in a discontinuous or critical conduction mode, wherein said second portion of said sensing signal waveform comprises an oscillatory portion of said waveform.
9. A system as claimed in claim 1 for controlling said SMPS to operate in a continuous conduction mode, wherein said second portion of said sensing signal waveform comprises a portion of said waveform during which input power is switched to said magnetic energy storage device.
10. An SMPS controller including the sensing system of claim 1 and a comparator to compare said output signal with a reference to provide a control output for controlling an SMPS responsive to said comparison.
11. An SMPS controller as claimed in claim 10 further comprising a timing signal generator to generate said enable signal.
12. An SMPS controller as claimed in claim 11 wherein said timing signal generator is responsive to said sensing signal to detect a negative going zero-crossing of said sensing signal and to control said enable signal to disable operation of said sampler responsive to said detection.
13. An SMPS including an SMPS controller as claimed in claim 10 .
14. A system as claimed in claim 1 wherein said magnetic device has at least two windings, including an auxiliary winding, and wherein said sensing signal is from said auxiliary winding.
15. A system as claimed in claim 1 wherein said magnetic device comprises a transformer and wherein said sensing signal is from a primary winding of said transformer.
16. An SMPS controller for controlling the output 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 controller comprising:
a sense input to receive a sense signal waveform from said magnetic device;
a decaying peak detector coupled to said sense input to detect when said sense signal waveform has a falling slope of greater than a threshold value and to generate a first timing signal:
an output to provide an SMPS control signal responsive to a value of said sense signal waveform at a time indicated by said first timing signal.
17. An SMPS controller as claimed in claim 16 wherein said decaying peak detector comprises a peak detector coupled to said sense input and including a signal level memory element to store a peak level of said sense signal waveform and a decay element coupled to said memory element to reduce said stored peak level over time.
18. An SMPS controller as claimed in claim 17 wherein said signal level memory element comprises a capacitor, and wherein said decay element comprises a current generator.
19. An SMPS controller as claimed in claim 17 wherein said decaying peak detector further comprises a comparator to compare said stored peak level with said sense signal waveform to provide said first timing signal.
20. An SMPS controller as claimed in claim 16 further comprising a sample-hold module to sample and hold said sense signal waveform responsive to said first timing signal and having an output for providing said SMPS control signal.
21. An SMPS controller as claimed in claim 20 further comprising a reference level input to receive an output voltage reference level signal, and an error detector coupled to said sample-hold module output and to said reference level input and having an output coupled to said SMPS controller output to provide said SMPS control signal responsive to a difference between said sampled sense signal waveform and said reference level signal.
22. An SMPS controller as claimed in claim 16 wherein said magnetic device has at least two windings, including an auxiliary winding, and wherein said sensing signal is from said auxiliary winding.
23. An SMPS controller as claimed in claim 16 wherein said magnetic device comprises a transformer and wherein said sensing signal is from a primary winding of said transformer.
24. An SMPS controller as claimed in claim 16 further comprising:
a timing signal input to receive a second timing signal indicating a time when substantially no power is being supplied by said SMPS; and
wherein said SMPS control signal output is responsive to said second timing signal such that said control signal is substantially non-responsive to a value of said sense signal waveform at a time indicated by said first timing signal when said second timing signal indicates that substantially no power is being supplied by said SMPS.
25. An SMPS including an SMPS controller as claimed in claim 16 .