Switch mode power supply controllers

Abstract

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

FIG. 1 shows an example of an SMPS incorporating primary side sensing.

FIG. 2 shows a switch mode power supply (SMPS) including an SMPS controller according to an embodiment of the invention;

FIG. 3 shows details of the voltage sensing block of the controller of FIG. 2;

FIG. 4 shows an example decaying peak detector for the voltage sensing block of FIG. 3;

FIG. 5 shows an example sample/hold module for the voltage sensing block of FIG. 3;

FIG. 6 shows an example error amplifier for the voltage sensing block of FIG. 3;

FIG. 7 shows waveforms illustrating the principle of operation of an SMPS controller according to an embodiment of the invention; and

FIG. 8 shows example waveforms illustrating the operation of the SMPS controller of FIG. 3.

DETAILED DESCRIPTION

Broadly speaking we will describe an apparatus and a related method for measuring an output voltage from a primary side of a power converter. A winding on the power transformer, such as a primary or auxiliary winding, provides a waveform to a peak detector with defined decay characteristic. The peak detector voltage thus forms a tangent to a selected portion of the auxiliary winding waveform. A status signal from the peak detector indicates the time(s) when the tangent coincides with (and departs from) the auxiliary winding waveform, thus in DCM/CRM providing an estimated instant when the transformer secondary winding current has dropped to zero. The status signal controls a sample/hold circuit, which at that instant captures a voltage reflecting a secondary voltage of the transformer, such as a voltage from the primary or an auxiliary winding of the transformer. In CCM essentially the same technique may be employed to determine when the (primary side) power switching device has turned on.

In embodiments an error amplifier compares the captured voltage against a reference to determine an error signal, preferably an analogue error signal, which may be used to regulate the power converter output voltage. The use of an analogue error signal allows the loop gain to be predicted accurately, facilitating loop compensation. Further analogue embodiments of the technique facilitate implementation of a controller with a low power consumption.

One difficulty in primary-side sensing, in particular when operating in DCM/CRM modes, is deciding exactly when to sample the reflected secondary voltage. Ideally this voltage should be sampled at the point at which the current in the secondary winding just falls to zero, as it is at this point that the sampled voltage most accurately represents the output voltage. This is because when the secondary current has just dropped to zero, there is no voltage drop across the secondary rectifier diode or its and the transformer's series resistance, and thus the voltage across the secondary winding is equal to the output voltage. The voltage across, say, the auxiliary winding is equal to the voltage across the secondary winding multiplied by the (known) turns ratio between the two windings, and the secondary voltage can thus be inferred by measuring the voltage across (say) the auxiliary winding at this point.

In CCM mode the secondary voltage can be sensed via a primary or auxiliary winding in a similar way to DCM mode except that the secondary voltage is sampled at a non-zero secondary side current. This non-zero (although sometimes small) current introduces a non-zero voltage drop across the secondary side components, which may comprise for example a diode and some output resistance. Thus preferably in CCM mode some compensation is made for the voltage drop from the secondary side winding to the SMPS output across these components. This compensation can be made, for example, based upon an approximate knowledge of the secondary side current, which can be inferred from the current in the primary side switch.

Referring now to FIG. 2, this shows a block diagram of a flyback single-switch SMPS 200 incorporating an embodiment of an SMPS controller according to the invention. As illustrated the controller is operating in the context of a flyback SMPS converter, but the skilled person will understand that the techniques we describe are also applicable to other forms of SMPS converters.

A DC source 100 is connected to the primary winding of a transformer in series with a primary side switch 106. The secondary winding of the transformer is connected to an output diode 101 in series with a capacitor 102. A load, represented by a resistor 103 is connected across the output capacitor 102. One end of an auxiliary winding on the transformer 104 is connected between the negative terminal of the DC supply 100 and the other end “VAUX” is connected to an Oscillator and Timing Block 105 and to a Voltage Sense Block 107.

The Voltage Sense Block 107 generates a signal (or value) VCTL representing the required level of output power, from signals VAUX and T1. The VCTL signal is fed back to the Oscillator and Timing Block which generates a DRIVE pulse for switch 106 at an appropriate frequency and duration.

In embodiments the timing signal T1 is derived from the VAUX signal, providing the timing control for the Voltage Sense Block 107. Typically T1 is driven active shortly after VAUX goes positive (allowing time for the initial overshoot waveform artefacts to decay), for example based on a comparison of VAUX with zero or on the DRIVE signal. T1 may be driven inactive when VAUX goes negative again. For example, a comparator may be employed to identify a negative-going zero-crossing of VAUX to drive T1 inactive. Timing signal T1 may be generated either by oscillator block 105 or within voltage sensing block 107.

As previously mentioned, the Oscillator and Timing Block 105 uses the input VCTL to control the frequency and pulse duration applied to the DRIVE output, which controls the main primary switch 106. As the skilled person will understand, the Oscillator and Timing Block 105 may be implemented in many different ways; examples of some particularly advantageous techniques are described in the Applicant's patent applications U.S. 60/698,808 (0513772.4) and PCT/GB2005/050244, hereby incorporated by reference.

Before describing details of the voltage sensing module 107 we first refer to FIG. 7 to describe the tangent-based method of output voltage sensing. The aim of the tangent method of output voltage sensing is to accurately detect the voltage in the transformer auxiliary winding at the knee point, that is the point at which the transformer secondary current drops to substantially zero, as shown in FIG. 7.

The VAUX (sensing) signal from the primary or auxiliary winding of the power transformer typically appears as shown. This is a transform of the secondary winding, generally with superimposed artefacts generated by winding leakage inductance, stray capacitance, and the like. Broadly, the tangent method works by fitting a tangent with a negative slope to the flyback portion of the VAUX waveform. The tangent slope is chosen to optimise the accuracy of identifying the knee point and to ensure that the waveform artefacts have minimal influence. The VAUX signal is then sampled at the knee point and compared to a voltage reference to determine the output error voltage. A preferred practical implementation, as described below.

Referring now to FIG. 3, this shows the main functional blocks of the Voltage Sensing circuit 107, which together comprise a decaying peak detector block 109, a sample/hold block 110 and an error amplifier block 111, generating the output signal VCTL (output voltage control). Typical waveforms are shown in FIG. 8. The output VPD (voltage peak detect) from the decaying peak detector block 109 is not used in some embodiments; in others it may be used to sense or sample a value of VAUX since it approximately tracks VAUX during its approximately linearly decaying portion and is substantially equal to VAUX at the knee point.

FIG. 4 shows an implementation of the decaying peak detector (DPD) block 109 of FIG. 3.

Referring to FIG. 4, the VAUX is fed into the input (IN) of the DPD block as shown. When timing signal T1 is inactive (low in FIG. 8) the DPD is reset, forcing the output voltage VPD to 0 volts. As shown, T1 is active, and therefore switch S1 is closed and switch S2 is open so that the DPD is not reset. When T1 is active, the circuit works as a peak detector, providing output VPD which decays at a predetermined rate.

Alternatively the peak detector may be free-running, in which case the EN signal may be gated by T1. As shown in FIG. 8, VPD follows the VAUX waveform except when the slope of VAUX exceeds a certain (negative) value, at which point the VAUX and VPD waveforms separate from one another. The STATUS signal from the DPD is active when the DPD is updating (increasing) the VPD signal.

An example implementation for the decaying peak detector 109, shown as a behavioural model, is illustrated in FIG. 4. A diode D1 and a capacitor C1 together comprise a peak detector; this is enabled when switch S1 is closed and S2 is open. A current sink I1 discharges the voltage on C1, thus defining the slope of the tangent. A comparator COMP1 compares the tangent approximating voltage on C1 with the VAUX input. Preferably a voltage source V1 adds a small DC offset compensating for the forward voltage drop of D1. Thus comparator COMP1 will issue a STATUS active if VAUX is greater than or equal to the (decaying) voltage on C1. Thus the DPD effectively detects when VAUX has greater than a threshold downwards or negative slope. The peak detector is re-initialised by the RST signal, closing switch S2 and opening switch S1, thereby discharging the voltage on capacitor C1. The rate of discharge of C1 is set by I1, which is chosen according to the implementation so that, in embodiments, the voltage on C1 follows the approximately linear descent of VAUX, that is so that it follows an approximate tangent to VAUX prior to its oscillatory or resonant portion.

An example implementation for the sample/hold module 110 is illustrated in FIG. 5. Buffer BUF1, capacitor C2 and switch S3 together comprise a sample/hold circuit, which samples the VAUX input when EN is active and holds the sampled value when EN is inactive. Thus, the voltage output VSENSE holds the instantaneous value of VAUX when STATUS is driven inactive (at various points in the flyback phase and finally at the knee point), as shown in FIG. 8.

An example implementation for the error amplifier module 111 is illustrated in FIG. 6. Amplifier OP1, capacitor C1 and resistor R1 form a simple integrator, enabled by switch S4. While input EN is active, switch S4 is closed, enabling the amplifier OP1 to integrate the difference between VSENSE and VREF. The time constant is preferably at least several cycles of oscillator 105, for example around 10 cycles. In this way the accumulated error over many switching cycles may be used by the Oscillator and Timing Block to modify the delivered power and thereby regulate the output voltage. Those skilled in the art will appreciate that the resistor and capacitor shown may be replaced by a variety of different impedance networks, for example in order to compensate the control loop using, say, pole-cancellation techniques.

Referring back once more to FIG. 7, it will be appreciated that it is desirable that the waveform to which the tangent-detection technique we have described is applied is relatively clean, and thus a modicum of filtering may be applied. Additionally or alternatively the waveform may be “qualified” to disable the operation of the tangent detection except in the vicinity of the knee point, for example by disabling the peak detector until a point close to the knee point is reached. This may be implemented, for example, by modelling the flux in the transformer by integrating the voltage on a primary or auxiliary winding of the transformer, more particularly by integrating the sensing signal, from a point of known zero transformer flux to determine a next point of zero transformer flux. This latter point corresponds to the knee on the primary or auxiliary winding sensing signal and hence the timing of this point may be used to define a window within which the tangent method should look at the sensing signal waveform, for example by enabling the peak detector over this time window. Points of known zero-transformer flux correspond to peaks and troughs on the oscillatory portion of the sensing signal waveform and thus, for example, the integrator may be reset at each of these peaks and troughs so that it is always reset at a point of known zero flux before the power switching device is switched on and the switching cycle begins. The peaks and troughs may conveniently be detected using a peak detector, which may take the form of, for example, a differentiator circuit or a diode capacitor circuit. Preferably the circuit which defines a time window, for example, the aforementioned integrator together with a comparator to determine when the integrator once again reaches its reset value, is arranged so that the window is “opened” just before when the knee point is expected. This can be arranged, for example, by comparing the output of the integrator to its reset value, say zero, modified by a small offset.

Broadly, we have described a method and system for identifying the knee point by fitting a tangent to a portion of the power transformer voltage waveform, and sampling the VAUX 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.

The techniques we have described provide a low cost method of accurately estimating the output voltage of a switched-mode power supply which achieves better output regulation, reduced audio noise and lower implementation cost than other primary-side sensing techniques.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

All documents, patents, and other references listed above are hereby incorporated by reference for any purpose.

Claims

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; an 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 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; anda 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 any preceding claim 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 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.

14. An SMPS controller as claimed in claim 13 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; andwherein 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.

15. An SMPS controller as claimed in claim 13 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.

16. An SMPS controller as claimed in claim 15 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.

17. An SMPS controller as claimed in claim 13 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. 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.

21. 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.

22. An SMPS controller as claimed in claim 13 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 13 wherein said magnetic device comprises a transformer and wherein said sensing signal is from a primary winding of said transformer.

24. An SMPS including an SMPS controller as claimed in claim 10.

25. An SMPS including an SMPS controller as claimed in claim 13.

26. 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; andusing a value of said sensing signal at said knee point to sense said SMPS output voltage; andwherein said identifying of said knee point comprises fitting an approximate tangent to said decaying portion of said sensing signal waveform; andidentifying departure of said sensing signal waveform from said approximate tangent to identify said knee point.

27. A method as claimed in claim 26 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.

28. A method as claimed in claim 27 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.

29. A method as claimed in claim 28 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.

30. A method as claimed in claim 29 wherein said sampling comprises integrating said sensing signal waveform from said knee point.

31. A method as claimed in claim 29 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.

32. A method as claimed in claim 31 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.

33. A method as claimed in claim 31 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.

34. A method as claimed in claim 31 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.

35. 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; andusing a value of said sensing signal at said knee point to sense said SMPS output voltage; andwherein said identifying of said knee point comprises detecting a point of greater than a threshold negative slope in said sensing signal waveform.

36. A method as claimed in claim 35 wherein said knee point identifying further comprises selecting a portion of said sensing signal waveform for said detecting, to exclude at least a part of said second portion of said waveform.

37. A method as claimed in claim 35 wherein said detecting comprises detecting a plurality of said points of greater than said threshold negative slope and selecting a said point immediately prior to said second portion of said waveform to identify said knee point.

38. A method as claimed in claim 26 further comprising regulating said output voltage of said SMPS responsive to said SMPS output voltage sensing.

39. A method as claimed in claim 35 further comprising regulating said output voltage of said SMPS responsive to said SMPS output voltage sensing.

40. A method as claimed in any one of claims 26 wherein said magnetic device comprises a transformer, and wherein said inputting of said sensing signal waveform comprises inputting said sensing signal from an auxiliary winding of said transformer.

41. A method as claimed in claim 35 wherein said magnetic device comprises a transformer, and wherein said inputting of said sensing signal waveform comprises inputting said sensing signal from an auxiliary winding of said transformer.

42. A method as claimed in claim 26 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.

43. A method as claimed in claim 35 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.

44. 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; andmeans for using a value of said sensing signal at said knee point to sense said SMPS output voltage; andwherein 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; andmeans for identifying departure of said sensing signal waveform from said approximate tangent to identify said knee point.

45. 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; andmeans for using a value of said sensing signal at said knee point to sense said SMPS output voltage; andwherein said means for said identifying of said knee point comprises:means for detecting a point of greater than a threshold negative slope in said sensing signal waveform.