GB2490542A

GB2490542A - Sensing arrangement for estimating the output voltage of an isolated flyback converter

Info

Publication number: GB2490542A
Application number: GB1107558.7A
Authority: GB
Inventors: Bernhard Keogh; Hagen Wegner
Original assignee: Texas Instruments Cork Ltd
Current assignee: Texas Instruments Ireland Trading Ltd
Priority date: 2011-05-06
Filing date: 2011-05-06
Publication date: 2012-11-07
Also published as: GB201107558D0

Abstract

A sense winding 19 is used to provide a voltage which represents the voltage appearing across an in-circuit magnetic component. In a flyback phase, when the component is supplying the output, that voltage represents the output voltage more accurately towards the end of the flyback period. This arrangement provides a solution to the problem of taking a measurement late in the period without risk of sampling so late that the flyback has collapsed and the sample is inaccurate. Samples are taken earlier in the flyback period, t1 and t2, and used to extrapolate an estimate of the output Vout_est at the predicted end of the period, t3. A means for the prediction of the length of the period t3 and other variables needed for the output estimate to be made is also disclosed. The invention is suitable for use in a transformer based flyback power converter since it enables an output value to be returned to the transformer primary side control circuit in a fully isolated manner. In an embodiment shown in figure 1, an isolated flyback converter comprises a sense winding (19), a sample and hold circuit (100), an analogue to digital converter (ADC 102), which inputs to a sample capture and controller device (101). A PWM circuit (18) controls the switch on and off times of a power switch (10), connected to one end of the primary side transformer 14. The secondary side of the transformer 14 drives a load via a series diode, and a freewheeling output capacitor provides power to the load during the non-conductive period. Another embodiment, figure 3b has the sense winding derived from an output inductor in the output circuit.

Description

Improvements in or relating to Sensing Arrangements This invention relates to the sensing of a variable in an electrical circuit by means of a sense winding interacting with a magnetic component in the circuit. In such a circuit the magnetic component may be storing energy supplied from the circuit or, when supply to the component is removed, itself supplying previously stored energy to the circuit. During the supply phase, when the component is driven, the voltage output of the sense winding is determined by the supplied drive and the sense winding may be used to measure the drive. During the so-called flyback phase, when stored energy is returned to the circuit, the magnetising current in the component will fall as the energy transfer occurs. The rate at which this occurs depends upon the circuit loading and so during this time, the output of sense winding may be used to measure circuit behaviour. At some point, however the stored energy will be exhausted and the magnetising current fall to zero and a post conduction resonance occurs, whereupon the sense winding output is no longer valid for that purpose.

When sensing the winding output voltage in the flyback phase, there may be advantage in sampling the output as late as possible in the flyback conduction interval so that errors due to parasitic resistance and inductance are reduced, as will be the case as the magnetising current falls. Unfortunately, unless the precise flyback conduction interval is known, as will be the case should the component not be re-supplied before its magnetising current falls to zero, there is the danger of an invalid measurement if the sampling is too late. The present invention addresses the problem of obtaining an output value from a sense winding at a sample point close to the end of the flyback conduction period.

One field in which use of a magnetic sensing winding is attractive is in power converters. Power converter designs are often based on magnetic coupling between a primary drive circuit fed from a mains derived unregulated do supply and a secondary output circuit which supplies the load because of the electrical isolation between the primary side and the secondary side. The attraction of the sense winding is that it allows the output on the secondary side to be sensed (for example for the purpose of regulation) without compromising isolation. Indeed, sense windings are already commonly used in power converters, especially for output over-voltage detection. Such a winding is almost always needed for bias generation in any event. As stated, there are challenges in extracting information from this winding, choosing the optimum point at which to sample the voltage, avoiding noise and ringing in the waveform, accounting for effects of transformer parasitics such as winding resistance and leakage inductance. In particular, the resistance of the power secondary winding will cause a droop in the sense waveform across the bias winding, due to the typically high secondary current in the secondary of flyback transformers, and the resultant I*R drop that actually increases the sensed voltage above the actual voltage.

This results in additional output regulation error as line voltage or load current are varied again encouraging the designer to leave sampling as late as possible in the flyback conduction interval.

Unfortunately, there potential are difficulties in taking late samples, since once the current falls to zero a period of post conduction ringing occurs in which the voltage changes rapidly. Indeed a late sample taken just after the end of the conduction will likely encounter a negative going ring and as such be a potential gross underestimate of the output voltage. This can cause instability in the control system since it will appear that supply should be curtailed (the output is apparently high), when it should not. The overestimate causes the switch erroneously to be off for even longer than it takes the actual flyback current to decay; what is worse, it causes the sample to be taken still later in the cycle resulting in a yet further erroneous sample. Such a string of samples may result in loss of regulation and potential loss of control all together. Hence, although desirable to sample as late as possible, the risk associated with late sampling is too great and sampling earlier in the cycle is the practice, despite the error.

A robust technique for allowing later sampling is desirable and in this regard, United States Patent US 6,958,920 shows a sense winding used to derive a feedback signal representative of converter output for the purpose of output regulation. The arrangement includes circuitry for placing the sampling point for the feedback signal as late as is prudent in the flyback interval which is based on an integrator to generate a voltage which ramps during the flyback conduction interval and a differentiator to detect voltage reversal in the primary which occurs during the first cycle of post conduction resonance. At the time, the integrator voltage is at the same level as it was at the end of flyback conduction and that value (plus a margin for safety) is used to set the sampling point for the next supply cycle.

There is always a need for alternatives for this technique that may be simpler or advantageous in a particular application. In particular, techniques which are better adapted to a digital or programmed applications since the integration and differentiation calculations and the multiple samplings that would be required to implement the prior art technique described above are unlikely to be compatible with the level of computation resources available in a digital power converter.

The industry trend is towards higher performance (i.e. those which themselves consume lower power) power converters and this means better regulation since the better the regulation the more the power consuming supply phase of the regulator may be minimised. Unfortunately the inevitable error which is present in the sample value of the output voltage provided by a sense coil is ruling it out in many applications and more accurate secondary side sensing schemes that provide a direct measurement of output voltage are being used. Such schemes do not have the isolation advantage of the sense winding and signals are typically fed back to the primary side controller through opto-oouplers which arrangements add significantly to the cost of the converter.

The present invention has been made as a result of studying whether such additional components could be eliminated from converter design in favour of a sense winding.

The present invention provides apparatus and method as set forth in the claims. In particular, the invention provides a sampling point for a sense winding output that is reliably towards the end of a flyback conduction interval.

Moreover, according to the present invention a sensing arrangement for a circuit comprising a magnetic component arranged such that the component is driven during a supply period and provides an output during a flyback period when the component is not supplied, the arrangement includes: a sense winding coupled to the magnetic component; a sample and hold circuit arranged to sample the voltage across the sense winding; a sample capture and control circuit adapted to trigger said sample and hold oirouit to sample a first value of the voltage aoross the sense winding at a first time; and trigger said sample and hold oirouit to sample a seoond value of the voltage aoross the sense winding at a seoond time; the sample oapture and oontrol oirouit being further arranged to provide an estimate of the voltage aoross the sense winding at a third time using the values of said samples taken at said first time and said seoond time.

Advantageously, the flybaok period inoludes a period of flybaok oonduotion and a period of post oonduotion resonanoe; and the third time substantially oorresponds to the end of said flybaok oonduotion. Preferably, the oirouit is oyolioally driven and the third time is derived from the time of onset of post oonduotion resonanoe in a previous oyole. The arrangement may inolude a deteotion oirouit for deteoting said onset.

Alternatively, the oirouit may be oyolioally driven and the third time is derived from volt-seoond balanoe of the magnetio oomponent between the supply period and the flybaok period.

In a preferred arrangement, the sense winding is used to measure drive voltage during the supply period. Moreover, said estimate is derived by fitting said values to a straight line or other predetermined funotion, whioh may itself be matohed to magnetising ourrent deoay profile during flybaok oonduotion.

In embodiments of the present invention, the sample oapture and oontrol oirouit may be adapted to trigger said sample and hold oirouit to sample a plurality of values of the voltage aoross the sense winding at a plurality of times before said third time and the sample oapture and oontrol oirouit may be further arranged to provide said estimate of the voltage aoross the sense winding at said third time using said plurality of values taken at said plurality of times.

1011 The sensing arrangement of any preoeding olaim wherein the estimate of the voltage aoross the sense winding at a third time is soaled by the ooupling ooeffioient between the sense winding and the magnetio oomponent.

In an embodiment of the invention, the magnetio oomponent may be a transformer driven on a primary side and providing said output on a seoondary side. Moreover, the sense winding may be a winding on said transformer.

Alternatively, the sense winding may be an in-oirouit induotor on the seoondary side, suoh as a seoondary winding on an in-oirouit transformer on the seoondary side.

The present invention may be adapted for use in a power converter.

In order that features and advantages of the present invention may be further appreciated some embodiments will be described by way of example only and with reference to the accompanying diagrammatic drawings, of which: Fig 1 is a power converter incorporating a sense winding in accordance with the principles of the present invention; Fig 2 represents a waveform during a cycle of operation; and Fig 3 depicts alternative embodiments.

A basic flyback converter set up is shown in Fig.1 built around a transformer 11. When the switch 10 is on, energy is supplied to the transformer by primary winding 12 from the supply at dc which causes an increasing magnetising current in the primary. There is no current through secondary winding 14 since diode 15 is reverse biased. The load 16 is supplied from bulk capacitor, having been charged on a previous cycle. When the switch is turned off, the magnetising current transfers to the secondary and the polarity reverses. Diode 15 now conducts and the energy stored in the transformer supplies the load and replenishes the capacitor 15.

It is normally desirable to keep the output voltage of the power converter, i.e. the voltage across the load 16, substantially constant and, as is well known in the art applicable to flyback conversion, such regulation may be achieved by controlling switch 10. A Pulse Width Modulation (PWM) scheme may be used to implement regulation on a cyclical basis wherein, if the output voltage is low during a cycle, the switch 10 is kept closed for a relatively longer time to supply more energy to the transformer for transfer to the output and, if the output voltage is low during a cycle, the switch 10 is kept closed for a relatively shorter time (or even not at all) to supply less energy to the transformer.

Such a scheme is implemented in the arrangement of Fig 1, wherein a PWM control signal is supplied to switch by a driver circuit 17. The PWM control signal is provided by PWM controller 18, which functions to generate pulse widths that are appropriate to the regulation required. Clearly to achieve this, some feedback of the actual value of the output of the power converter is required and this is derived from a sense winding 19 wound on the core of transformer 11. The present invention provides a means of deriving suoh a value.

In Fig. 2 there is depioted a traoe 20 whioh represents the voltage aoross sense winding 19 during a PWM oyole. The traoe oomprises portions 21 where the switoh 10 is driven on, followed by flybaok when energy is supplied from the transformer as desoribed above.

There is a flybaok oonduotion period 22 during whioh the voltage linearly deoreases as ourrent is provided by the seoondary winding and a post oonduotion resonanoe is 000urring at 23. As mentioned above, a point 24 represents the ideal time to sample the waveform 20 in order to obtain a sample whose value oorresponds to the output voltage of the oonverter sinoe at point 24, the flybaok ourrent will have fallen to zero. A prinoipal oause of error is the voltage drop aoross the resistanoe of the transformer seoondary whilst flybaok ourrent is flowing, whioh will be refleoted through the seoondary to sense winding turns ration in the same way as the voltage to be measured. The later the measurement, the lower the ourrent and therefore the lower the oonsequent error, however attempting to perform a later sampling runs the risk of gross error if the sampling is even slightly late sinoe onoe post oonduotanoe resonanoe ooours, the voltage ohanges rapidly. By oontrast, the present embodiment obtains two samples muoh earlier in the oyole.

Returning to Fig. 1, the samples are oaptured by Sample and Hold oirouit 100 triggered by sample and oapture oontroller 101. The output of the sample and hold oirouit 100 is oonverter to a digital representation by Analogue to Digital Converter 102 and reoeived by the oontroller 101. It is envisaged that oontroller 18 and 101 are realised as a programmed mioro-oontroller. The operation is as follows.

At time tl (Fig2) oontroller 101 triggers S/H oirouit 100 to sample the voltage aoross sense winding 19. The moment of sampling is not important exoept that is should be after any osoillation oaused by leakage has deoayed. The sample value is stored in digital form in oontroller 10 via ADC 102. Likewise, a seoond sample is taken at time t2. Again the exaot instant of sampling is not important exoept that it ooours earlier in the oyole than any possibility that the magnetising ourrent oould be approaohing zero even under the most adverse of oiroumstanoes (a suddenly applied high load, for

example)

Following t2, the two sample values (Si at ti and S2 at t2) allowing a differenoe txv to be oomputed), together with ti and t2 themselves may be stored in the oontroller, whioh is programmed to perform a simple linear extrapolation to obtain an estimated value (vout_est) of the voltage aoross the sense winding 19 (and thereby the output voltage of the oonverter) at time t3, whioh oorresponds to ideal sampling point 24: Voutest = 7s2+s1 + Av(t3 + t2 + tl)/(t2 -ti) Finally, to be used as an aotual output voltage value, vout_est is soaled by the ooupling ooeffioient between the sense winding 19 and the output, in this oase the turns ratio between the sense winding 19 and the seoondary winding 14.

It will be appreoiated that the only value whioh is not available by measurement or oaloulation is t3 and for that an estimate is required. This may be obtained from a previous oyole in the way desoribed in United States Patent US 6,958,920 or alternatively oomputed as follows.

Given that the switoh on-time (to) is known in advanoe, for example by measurement of aotual on-time from the previous oyole, and assumption of slowly varying on-time from oyole-to-oyole or by direot oaloulation of required on-time as is the oase with the digital PWM used in this embodiment (the on-time is shown fed baok from PWM oontroller 103 to Sample Controller 101 in Fig 1) and that the line input voltage (Vdo) is known again by measurement from a previous oyole or preferably the ourrent on-time by triggering the SH oirouit 100 to make a sampling during the on-time, then the applied transformer primary volt-seoonds produot is known. Under steady-state oonditions, the volt-seoonds aoross the transformer primary during the on-time must balanoe the volt-seoonds aoross the seoondary during flybaok. Thus, an estimate of the seoondary volt-seoonds is known in advanoe of the oommenoement of flybaok. If output voltage (vo) is estimated based on measurement during a previous oyole, the length of the flybaok oonduotion interval t3 may be estimated: t3 = Vdo.to/vo It follows that onoe this oomputation has taken plaoe, voutest may itself be oaloulated and oommunioated to PWM oontroller 18 whioh then serves to oompute the on-time required for the next oyole for the output voltage to be brought into or maintained in regulation.

Given that the implementation of the embodiment described may be realised with a programmed micro-controller, there may be scope for reducing power consumption of the converter itself by allowing the microcontroller to enter low power or sleep modes during those parts for the cycle during which no control events are expected. To this end, the present invention provides an advantage in that vout_est is available early in the cycle the controller may enter a low mode mode likewise; ther is no need to wake the processor towards the end of flyback in order to take a sample since the estimate already exists. Thus the low power mode may be maintained for longer.

Thus far, the present embodiment has been described operating in a discontinuous conduction mode, that is to say that the flyback current has fallen to zero before the drive phase of the next cycle occurs. It will be appreciated that the embodiment may also be controlled to operate in a continuous conduction mode, wherein the flyback current is still flowing when the next drive phase begins. Clearly, in such a circumstance the value of the output voltage at zero current can never be measured directly since the system never reaches that operating point. As a consequence, systems which rely on direct measurement have an in-built error which cannot be avoided. The present invention does not suffer from this drawback.

In Fig 2(b) there is depicted a waveform representing the embodiment operating in a continuous conduction mode. Where parts of or points on the waveform are equivalent to those of Fig 2(a), common reference numerals have been used. It will be noted that at a point 25, being the onset of a drive phase, flyback current is flowing, indicating continuous conduction mode. However, voutest may be computed as before, being the voltage at a point 24' where the flyback conduction would come to an end if it had been allowed to continue.

This remains a viable point at which to estimate the output voltage even though it is an operating point that is not reached in practice. The computation may be made by means of for example the volt-second balance mentioned above, which is still valid even though actual flyback current never falls to zero. As an alternative, an estimate of the voltage at point 25 may be derived. This would be useful in a system where compensation for the error caused by operating in continuous conduction mode were incorporated and still represents a improvement over direct measurement since no margin of safety is required to avoid the risk of sampling at or after the onset of drive.

It will be appreoiated that for a oontroller whioh is operating in heavily oontinuos oonduotion, it may be several oyoles before the zero ourrent point would be reaohed. This situation is likewise aooommodated by the present invention and even for this oase, the vout_est value is available early in the initial supply oyole.

Having desoribed one embodiment of the invention, some alternative embodiments will be oonsidered. In a seoond embodiment, a sample is taken at time t3, the expeoted end of the flybaok oonduotion. This sample will be used unless it differs by more than a reasonable predetermined amount, whioh indioates an unreliable sample taken during post oonduotion resonanoe under whioh oiroumstanoe voutest is used instead. In other embodiments, more samples may be taken to give a better linear estimate or estimates based on funotions other than a straight line. In suoh latter embodiments, the ohosen funotion maybe matohed to the magnetising ourrent deoay profile during flyback oonduotion. Further regulation improvement oan be aohieved by inoorporating a oorreotion to the voltage estimate based on other measured parameters suoh as switohing frequenoy (if that is variable) or using temperature measurement from a built-in temp sensor in the oontroller 101, to adjust for the inoreasing winding resistanoe at higher temperature.

Given that oopper is almost exolusively used for transformer winding, the temperature ooeffioient of Cu resistance is well known and predictable; a predefined look up table could be used to provide such a correction.

Further robustness can be introduced by comparing the two samples taken, and disregarding the second if it deviates grossly from the first, indicating that possibly the timing of the second sample has drifted beyond the winding current point of collapse. As long as a sufficient primary side minimum switch on-time is enforced, then there will thus also always be a sufficient secondary-side flyback conduction interval during which at least the first sample can be taken with assurance. Moreover, under lighter load conditions, the double-sampling scheme may be intentionally suspended due to insufficient flyback conduction interval. However, in such oases, peak currents and conseguential I*R drop on the winding will be less significant so that the deleterious effect of the greater error in the sample value is ameliorated.

Even though the embodiments described above are based on a programmed digital implementation, an entirely analogue realisation should be within the competence of those skilled in the art.

A great advantage of the present invention is that it can provide a sample value of output voltage which may be on a par with that provided by more expensive isolated secondary side sensing scheme, allowing a higher performance converter to be realised at less cost.

Indeed, in the embodiment of Fig 1, winding 19, which is used as a sense winding, would have been present even if a secondary side scheme had been used to feed the sampler since it provides a circuit start up supply at terminal 104 via a bias generator 103. In view of the programmed nature of the controller which is required for PWM control in any case, the sensing scheme is realised without component cost.

Although it may not be the ideal choice for a low cost higher performance power converter, the sensing scheme of the present invention may itself be associated with the secondary side. Suitable configurations are shown in the secondary output circuits depicted in Fig. 3. In Fig 3(a) sense coil 30 is incorporated as an inductor. In Fig 3(b) sense coil 31 is incorporated as a transformer having a primary 32 and provides the benefit of isolated sensing.

Placing the sense winding in the secondary circuit has the effect that a supply voltage value Vdc cannot be obtained from the sense winding itself. This way of connecting the coil will also be appropriate for circuits in which no transformer is present, such as a buck power converter.

Claims

CLAIMSWhat I claim is: 1. A sensing arrangement for a circuit comprising a magnetic component arranged such that the component is driven during a supply period and provides an output during a flyback period when the component is not supplied, the arrangement including: a sense winding coupled to the magnetic component; a sample and hold circuit arranged to sample the voltage across the sense winding; a sample capture and control circuit adapted to trigger said sample and hold circuit to sample a first value of the voltage across the sense winding at a first time; and trigger said sample and hold circuit to sample a second value of the voltage across the sense winding at a second time; the sample capture and control circuit being further arranged to provide an estimate of the voltage across the sense winding at a third time using the values of said samples taken at said first time and said second time.
2. The sensing arrangement of claim 1 wherein the flyback period includes a period of flyback conduction and a period of post conduction resonance; and wherein the third time substantially corresponds to the end of said flyback conduction.
3. The sensing arrangement of claim 1 or claim 2 and wherein the circuit is cyclically driven and wherein the third time is derived from the time of onset of post conduction resonance in a previous cycle.
4. The sensing arrangement of claim 3 and including a detection circuit for detecting said onset.
5. The sensing arrangement of claim 1 or claim 2 and wherein the circuit is cyclically driven and wherein the third time is derived from volt-second balance of the magnetic component between the supply period and the flyback period.
6. The sensing arrangement of any preceding claim wherein the sense winding is used to measure drive voltage during the supply period.
7. The sensing arrangement of any preceding wherein said estimate is derived by fitting said values to a straight line.
8. The sensing arrangement of any preceding claim wherein the sample capture and control circuit is adapted to trigger said sample and hold circuit to sample a plurality of values of the voltage across the sense winding at a plurality of times before said third time; and wherein the sample capture and control circuit is further arranged to provide said estimate of the voltage across the sense winding at said third time using said plurality of values taken at said plurality of times.
9. The sensing arrangement of claim 8 wherein said estimate is derived by fitting said values to a predetermined function.
10. The sensing arrangement of claim 9 wherein said predetermined function is matched to magnetising current decay profile during flyback conduction.
11 The sensing arrangement of any preceding claim wherein the estimate of the voltage across the sense winding at a third time is scaled by the coupling coefficient between the sense winding and the magnetic component.
12 The sensing arrangement of any preceding claim wherein the magnetic component is a transformer driven on a primary side and providing said output on a secondary side.
13. The sensing arrangement of claim 12 wherein the sense winding is a winding on said transformer.
14. The sensing arrangement of claim 12 wherein the sense winding is an in-circuit inductor on the secondary side.
15. The sensing arrangement of claim 12 wherein the sense winding is a secondary winding on an in-circuit transformer on the secondary side.
16. A method of sensing for a circuit comprising a magnetic component arranged such that the component is driven during a supply period and provides an output during a flybaok period when the component is not supplied, the method including the steps of: providing a sense winding coupled to the magnetic component; sampling a first value of the voltage across the sense winding at a first time; and sampling a second value of the voltage across the sense winding at a second time;and providing an estimate of the voltage across the sense winding at a third time using the values of said samples taken at said first time and said second time.
17. The method of claim 16 wherein the flyback period includes a period of flyback conduction and a period of post conduction resonance; and wherein the third time substantially corresponds to the end of said flyback conduction.
18. The method of claim 16 or claim 17 including driving the circuit cyclically and wherein the third time is derived from the time of onset of post conduction resonance in a previous cycle.
19. The method of claim 18 and including the step of detecting said onset.
20. The method of claim 16 or claim 17 including driving the circuit cyclically and deriving the third time from volt-second balance of the magnetic component between the supply period and the flyback period.
21. The method of any of claims 16 to 20 including using the sense winding to measure drive voltage during the supply period.
22. The method of any of claims 16 to 21 including deriving said estimate by fitting said values to a straight line.
23. The method of any of claims 16 to 22 including sampling a plurality of values of the voltage across the sense winding at a plurality of times before said third time; and providing said estimate of the voltage across the sense winding at said third time using said plurality of values taken at said plurality of times.
24. The method of claim 23 including deriving said estimate by fitting said values to a predetermined function.
25. The method of claim 24 including matching said predetermined function to magnetising current decay profile during flyback conduction.
26 The method of any of claims 16 to 25 including scaling the estimate of the voltage across the sense winding at a third time by a coupling coefficient between the sense winding and the magnetic component.
27 The method of any of claims 16 to 26 including providing the magnetic component as a transformer and driving said transformer on a primary side to provide said output on a secondary side.
28. The method of claim 27 including providing the sense winding as a winding on said transformer.
29. The method of claim 27 including providing the sense winding as an in-circuit inductor on the secondary side.
30. The method of claim 27 including providing the sense winding as a secondary winding on an in-circuit transformer on the secondary side.
31. The method of any of claim 16 to 30 wherein a subsequent sample is not used if is differs greatly from a previous sample.
32 The sensing arrangement of any of claims 1 to 15 or the method of any of claim 16 to 31 and wherein the third time represents a time beyond the end of the flyback period at which flyback current would have fallen to zero had flyback conduction been allowed to continue.
33 A power converter including the sensing arrangement of any of claims 1 to 15 or claim 32 or embodying the method of any of claim 16 to 31 or claim 32.
34. A power converter as claimed in claim 33 and wherein a single or reduced number of samples are taken under light converter load conditions.
35. A sensing arrangement or method or power converter substantially as herein described with reference to the drawings.