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Modulated supply stage with feedback to switched supply

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20140203653 patent thumbnailZoom

Modulated supply stage with feedback to switched supply


There is disclosed a voltage supply stage comprising: a selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; a combining means for combining the selected power supply voltage with a correction signal to generate an adjusted power supply voltage; and an adjusting means adapted to generate the correction signal in dependence on the reference signal and the adjusted power supply voltage, wherein the selection means is arranged to select the one of the plurality of supply voltages further in dependence on a signal derived from one of the inputs to the combining means.
Related Terms: Modulate

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USPTO Applicaton #: #20140203653 - Class: 307 80 (USPTO) -


Inventors: Martin Wilson

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The Patent Description & Claims data below is from USPTO Patent Application 20140203653, Modulated supply stage with feedback to switched supply.

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CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application is a Continuation of U.S. patent application Ser. No. 12/991,695, with a 371(c) date of Jun. 13, 2011, and is incorporated herein by reference.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The present invention relates to a modulated supply stage, and particularly to such a stage in which a feedback loop is connected to provide an input to control selection of a low frequency switched supply. The feedback may be provided from the output of the low frequency switched supply stage or from the output of a high frequency correction stage.

2. Description of the Related Prior Art

It is known by those skilled in the art that envelope tracking (ET) and envelope elimination and restoration (EER) can give large improvements in efficiencies of power amplifier operation, particularly with signals such as orthogonal frequency-division multiplexing (OFDM) which have large crest factors. However, it is also known that the application of these techniques presents considerable difficulties due to the large powers and bandwidths involved. These difficulties become particularly formidable when applied to portable wireless terminals where the number of discrete components must be minimised and large dimensioned magnetics must be avoided.

An apparently simple solution would be to make the modulator a fast responding linear regulator. However, this would simply change power wasted in the power amplifier with that wasted in the linear regulator, resulting in no net gain in efficiency.

In order to gain some efficiency, some prior art implementations have been known to follow the switched mode supply with a low drop-out (LDO) fast responding linear regulator. This removes the errors inherent in the switched mode operation. However a problem arises in that there must be sufficient range in the linear regulator to allow for the peaks in the switched mode error, which can be considerably larger than the root mean square (RMS) error. This results in a large standing dissipation in the LDO.

A significant improvement on this is provided by techniques disclosed in GB2398648. This implementation is shown in FIG. 1. FIG. 1 shows a diagram of a typical switched mode power supply used as an efficient power conversion means. It must be noted that this is given as an example; the invention is not restricted to topologies of this type.

A coarse DC-DC switched supply 102 provides an approximation to the required waveform, provided as a reference waveform on input line 118, after filtering with filter network 104. The filter comprises an inductor 106 for storage of magnetic energy, and a capacitor 108 for storage of electric energy. A transformer 110 is used which can give true summation, e.g. signals can be added and subtracted, so the mean correction from a correction amplifier 114 can be set to zero, eliminating large standing dissipation. The output of the transformer provides an output to a load 112. The output of the transformer 110 is fedback to provide an input to the correction amplifier 114, which receives as a further input a reference signal on line 116 (which may be the same as, or derived from the same source as, the reference signal on line 118). The transformer combines the switched supply voltage with the output of the correction amplifier to provide a corrected output voltage.

A potential problem with the architecture of FIG. 1 is that the transformer 110 has to have a high self-inductance to prevent shunting of the correction current through the unwanted inductance of the transformer. This means that large ferrite cores must typically be used. Whilst this is acceptable for wireless infrastructure implementations, this presents particular difficulties for portable handset implementations or any implementation where size restrictions may apply.

The supply stage of FIG. 1 is capable of very efficient operation, but the circuit can only be switched between two levels: intermediate levels can only be obtained by the filtering action of the energy storage elements 106 and 108. For low frequency outputs (frequencies much less than the switching frequencies), this arrangement will be able to perform tracking, but the circuit may provide poor tracking at high frequency. There will also be substantial breakthrough of switching related products at high frequencies. When the said power conversion circuit is used as a modulator, the energy storage elements form a parallel resonant tank which will present a high impedance to the load at some frequencies.

The effect of this can be seen in FIG. 2. The reaction of the energy storage elements to the rapidly changing current demand produces a waveform 204 at the power amplifier. This shows severe mistracking when compared with the wanted waveform 202. Also, the high output impedance may result in instability of the load.

Examples of prior art switched mode modulators can be found in U.S. Pat. Nos. 5,905,407, 6,054,914, 6,198,374, 6,300,826, 6,583,664, 6,661,210, 6,661,217, 6,710,646, 6,792,252, and in US Patent application No. 2002/0008574.

It is an aim of the invention to provide an improved modulated power supply stage.

SUMMARY

OF THE INVENTION

In one aspect the invention provides a voltage supply stage comprising: a selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; a combining means for combining the selected power supply voltage with a correction signal to generate an adjusted power supply voltage; and an adjusting means adapted to generate the correction signal in dependence on the reference signal and the adjusted power supply voltage, wherein the selection means is arranged to select the one of the plurality of supply voltages further in dependence on a signal derived from one of the inputs to the combining means.

The inputs to the combining means are the signals to be combined: the selected power supply voltage and the correction signal. The selected power supply voltage is the output of the selection means, which is preferably a switched voltage supply. The correction signal is the output of the adjustment means.

The signal derived from one of the inputs to the combining means may be the output of the selection means.

The signal derived from one of the inputs to the combining means may be the output of the adjusting means.

A feedback control stage may provide the signal derived from one of the inputs to the combining means to the selection means. The feedback control stage may be adapted to receive as a first input the reference signal and as a second input one of the inputs to the combining means, and further adapted to adjust the reference signal in dependence on the one of the inputs to the combining means to provide an adjusted reference signal for the selection means.

In an embodiment the feedback control stage may comprise: a subtractor for subtracting the output of the selection means from the reference signal; a proportional-integral, PI, controller for receiving the subtracted signal and generating a modified output, and a summer for adding the modified output of the PI controller to the reference signal, to form the output of the feedback control stage being the adjusted reference signal.

In an alternate embodiment the feedback control stage may comprise: a proportional-integral, PI, controller for receiving the output of the adjusting means and generating a modified output, and a summer for adding the modified output of the PI controller to the reference signal, to form the output of the feedback control stage being the adjusted reference signal.

The combining means preferably comprises an inductor, and the adjustment means preferably comprises a voltage-to-current converter, wherein a current representing the correction signal is injected at the second terminal of the inductor to adjust the current flowing in the inductor provided by the selected power supply voltage, a thus adjusted current flowing in a load connected to the second terminal of the inductor to thereby develop the adjusted supply voltage across said load.

The voltage supply stage preferably comprises a reference adjustment stage for adjusting the reference signal to provide a modified reference signal. The selection means may be adapted to select in dependence on the modified reference signal. The feedback control means may be adapted to provide the signal derive from one of the inputs to the combining means in dependence on the modified reference signal.

The reference adjustment stage may comprise a means for adjusting the amplitude of the reference signal in dependence upon a difference between the amplitude of the reference signal and the amplitude of the selected supply voltage. The means for adjusting the amplitude of the reference signal may include: a correlator for determining the amplitude error between the reference signal and the selected supply voltage; and an amplitude adjustment block for modifying the reference signal in dependence on said error. The reference adjustment stage may comprise a means for controlling a current flow in the combining means to maximize current slow in the combining means and thereby minimize current flow in the adjustment means. The means for controlling the current flow may include: a correlator for determining the current flow in the inductor and for providing a control signal to modify coefficients of a differentiator in dependence thereon, the differentiator being arranged to receive the reference signal and generate a differentiated version thereof. The differentiator may be arranged to receive as an input the amplitude adjusted reference signal generate a differentiated amplitude adjusted reference signal, the reference adjustment stage further comprising a summer for summing the amplitude adjusted reference signal with the differentiated amplitude adjusted reference signal to form the modified reference signal.

A tracking modulated power supply stage for a mobile wireless device preferably includes a voltage supply stage as defined.

In this aspect the invention also provide a method for generating a supply voltage comprising: selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; combining the selected power supply voltage with a correction signal to generate an adjusted power supply voltage; generating the correction signal in dependence on the reference signal and the adjusted power supply voltage; and providing as a feedback signal one of the input signals to the combining step, wherein the selecting step is further arranged to select the one of the plurality of supply voltages in dependence on the feedback signal.

The providing step may provide the output of the selection means as the feedback signal. The providing step may provide the output of the adjusting means as the feedback signal.

The method may further comprise the step of controlling the feedback signal for providing the signal derived from one of the inputs to the combining means to the selection means.

The step of controlling the feedback may comprise receiving as a first input the reference signal and as a second input one of the inputs to the combining step, and adjusting the reference signal in dependence on the one of the inputs to the combining step to provide an adjusted reference signal for the selecting step.

The step of controlling the feedback may comprise: subtracting the output of the selection means from the reference signal; receiving the subtracted signal and generating a proportional-integral, PI, modified output, and adding the modified output to the reference signal, to form the adjusted reference signal.

The step of controlling the feedback may comprise: receiving the output of the adjusting means and generating a proportional-integral, PI, modified output, and adding the modified output to the reference signal, to form the adjusted reference signal.

The combining means may comprise an inductor, and the adjustment means may comprise a voltage-to-current converter, the method further may comprise injecting a current representing the correction signal at the second terminal of the inductor to adjust the current flowing in the inductor provided by the selected power supply voltage, a thus adjusted current flowing in a load connected to the second terminal of the inductor to thereby develop the adjusted supply voltage across said load.

The method may further comprise the step of adjusting the reference signal to provide a modified reference signal.

The selecting step may be adapted to select in dependence on the modified reference signal.

The feedback control step may be adapted to provide the signal derived from one of the inputs to the combining step in dependence on the modified reference signal.

The step of adjusting may include adjusting the amplitude of the reference signal in dependence upon a difference between the amplitude of the reference signal and the amplitude of the selected supply voltage.

The step of adjusting the amplitude of the reference signal may include: determining the amplitude error between the reference signal and the selected supply voltage; and modifying the reference signal in dependence on said error.

The step of adjusting may comprise controlling a current flow in the combining means to maximize current flow in the combining means and thereby minimize current flow in the adjustment means.

The step of controlling the current flow may include: determining the current flow in the inductor and for providing a control signal to modify coefficients of a differentiator in dependence thereon, the differentiator being arranged to receive the reference signal and generate a differentiated version thereof.

The method may further comprise the steps of: receiving at the differentiator as an input the amplitude adjusted reference signal; and generating a differentiated amplitude adjusted reference signal, the adjustment step further comprising summing the amplitude adjusted reference signal with the differentiated amplitude adjusted reference signal to form the modified reference signal.

In another aspect the invention provides a combiner for combining a first voltage signal with a second voltage signal to provide a combined voltage signal, comprising: an inductor having a first terminal connected to the first voltage signal; a load connected to the second voltage terminal; and a conversion means for receiving at an input the second voltage signal and generating at an output a current representing the second voltage signal, the output of the conversion means being connected to the second terminal of the inductor, wherein a current is generated in the load representing the combined first and second voltages, the combined voltage signal thus being developed across the load.

The combiner may further comprise a capacitor connected at the second terminal of the inductor, wherein in combination the inductor and the capacitor form an L-C filter for the combined signal.

The conversion means may be a voltage-to-current converter.

The load may be a power amplifier, and the combined voltage is a supply voltage for the power amplifier.

A modulated voltage supply may comprise a combiner as defined, and may further comprise: a selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal, the selected supply being the first voltage signal, the conversion means being an adjusting means for generating a correction signal comprising the second voltage signal in dependence on the reference signal and the combined voltage signal.

In this aspect the invention also provides a method for combining a first voltage signal with a second voltage signal to provide a combined voltage signal, comprising: connected to the first voltage signal to a first terminal of an inductor; connecting a load to the second terminal of the inductor; converting the second voltage signal into a current representing the second voltage signal; providing the current representing the second voltage signal at the second terminal of the inductor, wherein a current is generated in the load representing the combined first and second voltages, the combined voltage signal thus being developed across the load.

The step of providing the current representing the second voltage signal at the second terminal of the inductor may comprise injecting current into the second terminal of the inductor.

In a further aspect the invention provides a voltage supply stage comprising: a selection means for selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; a combining means for combining the selected power supply voltage with a correction signal to generate an adjusted power supply voltage; a correction means adapted to generate the correction signal in dependence on the reference signal and the adjusted power supply voltage; an adjustment means for adjusting the amplitude of the reference signal in dependence on a difference between the amplitude of the reference signal and the amplitude of the selected supply voltage; and differentiation means for controlling the current in the combining means to maximize the current flowing in the combining means and thereby minimize the current required to flow in the correction means.

The adjustment means may include: a correlator for determining an amplitude error between the reference signal and the selected supply voltage; and an amplitude adjustment block for modifying the reference signal in dependence on said error.

The means for controlling the current in the combining means to maximize the current flowing in the combining means and thereby minimize the current flowing in the correction means may include a correlator for determining the current flow in the inductor and for modifying coefficients of a differentiator in dependence thereon.

The amplitude adjustment block may receive the reference signal and generates the amplitude adjusted reference signal, the differentiator receives the amplitude adjusted reference signal and generates a differentiated version thereof at its output, and a summer sums the amplitude adjusted reference signal and the modified differentiated reference signal to provide the reference signal for use by the modulated power supply stage.

In this further aspect the invention also provides a method for a generating a modulated supply voltage, comprising: selecting one of a plurality of power supply voltages in dependence on a reference signal representing a desired power supply voltage; combining the selected power supply voltage with a correction signal to generate an adjusted power supply voltage; generating the correction signal in dependence on the reference signal and the adjusted power supply voltage; adjusting the amplitude of the reference signal in dependence on a difference between the amplitude of the reference signal and the amplitude of the selected supply voltage; and controlling the current in the combining means by differentiation to maximize the current flowing in the combining means and thereby minimize the current required to flow in the correction means.

The adjustment step may include: determining an amplitude error between the reference signal and the selected supply voltage; and modifying the reference signal in dependence on said error.

The controlling the current in the combining means to maximize the current flowing in the combining means and thereby minimize the current flowing in the correction means may include determining the current flow in the inductor and modifying coefficients of a differentiator in dependence thereon.

The amplitude adjustment block may receive the reference signal and generate the amplitude adjusted reference signal, the differentiator may receives the amplitude adjusted reference signal and generates a differentiated version thereof at its output, and a summer sums the amplitude adjusted reference signal and the modified differentiated reference signal to provide the reference signal for use by the modulated power supply stage.

All aspects and feature of the invention as defined or as discussed in the following description may be implemented individually or in any combination.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described by way of example with reference to the accompanying figures in which:

FIG. 1 illustrates a modulated power supply stage including a low frequency switched supply and a high frequency error correction in accordance with the prior art;

FIG. 2 illustrates a problem associated with a prior art arrangement such as FIG. 1;

FIG. 3 illustrates an improvement in a modulated power supply stage in accordance with a first exemplary embedment of the invention;

FIG. 4 illustrates a modification to the preferred implementation of the first embodiment;

FIG. 5 illustrates a further modification to the preferred implementation of the first embodiment;

FIG. 6 illustrates an exemplary implementation of a second embodiment of the invention; and

FIG. 7 illustrates an exemplary implementation of embodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described by way of example with reference to its application in various embodiments. One skilled in the art will appreciate that the invention is not limited in its scope to the specifics of implementation details of any particular embodiment.

The broad principle in accordance with the invention is to provide an additional feedback path. The feedback path provides an input to the switched supply.

The provision of the feedback path is in accordance with one of two broad embodiments. In a first broad embodiment the feedback path originates from the output of the switched supply, i.e. the output of a coarse path. In a second broad embodiment the feedback path originates from the output of the correction path. Thus the switched supply stage is provided an input derived from an input to the combiner stage for combining the switched supply with the correction signal. This feedback reduces errors at low frequencies, and allows the bandwidth of the combiner stage to be reduced.

A first arrangement for the implementation of the first broad embodiment is now described with reference to FIG. 3. Like reference numerals are used in the following figures where any element corresponds to an element shown in another figure.

The modulated supply stage of FIG. 3, generally denoted by reference numeral 300, comprises a switched supply stage 302, a switched supply controller 304, a correction amplifier 310, a combiner stage 308, a feedback control stage 306, a capacitor 312 and a load 314.

Modulated supply stage 300 of FIG. 3 provides a modulated supply on an output line 318 to the load 314 in dependence on a reference signal provided on input line 316. The load 314 may be a power amplifier.

The switched supply controller 304 receives an input signal from the feedback control stage 306. In dependence upon the signal from the feedback control stage 306, the switched supply controller 304 controls the switched supply 302 to provide a switched supply output on line 320. The switched supply output on line 320 provides a first input to the combiner stage 308.

The feedback control stage 306 receives two inputs: a first input is provided on line 322 from the output of the switched supply stage 302 on line 320, and a second input is provided by the reference signal on the input line 316. The feedback control stage 306 operates to adjust the received reference signal in dependence upon the feedback signal to provide a modified input to the switched supply controller 304.

A second input to the combiner stage 308 is provided by the output of the correction amplifier 310. The correction amplifier 310 receives as a first input the reference signal on line 316, and receives as a second input a feedback signal on line 324 comprising the output of the combiner stage 308 on line 318.

An optional capacitor 312 is connected between the output line 318 and ground.

In the example arrangement of FIG. 3, in accordance with the first embodiment, the combiner stage 308 is implemented as a transformer. The transformer has a first winding 340 and a second winding 342. A first tap of the first winding 340 is connected to the output of the switched supply stage 302 on line 320. A second tap of the first winding 340 provides the output signal on line 318. A first tap of the second winding 342 is connected to receive the output of the correction amplifier 310. A second tap of the second winding 342 is connected to ground. In this way, the transformer combines the output of the switched supply with the output of the correction amplifier to generate a corrected switched supply at its output.

The feedback control stage 306 operates to utilise the feedback on line 322 from the output of the switched supply to provide an improved version of the reference signal on line 316 to the input of the switched supply controller 304. The feedback control stage 306 includes a subtractor 326, a summer 330, and a PI control block 328. The subtractor receives the reference signal on line 316 as one input and the feedback signal on 322 as another input. The feedback signal on line 322 is subtracted from the reference signal on line 316 to provide an input to the PI control block 328. The implementation of a PI (proportional-interval) controller is well-known in the art. The output of the PI control block 328 forms a first input to the summer 330, the second input to the summer 330 being provided by the reference signal on line 316. The summer adds the output of the PI control block 328 to the reference signal 316, to generate a modified reference signal for the switched supply controller 304 as the output of the feedback control stage 306. The summer 330 adds a feedforward element to the feedback control, which is needed as large amplitude signals are being handled.

The feedback control stage 306 operates by sensing differences in level using subtracting means 326. The output level from the subtractor 326 is sensed by the PI control block 328 and used to provide a slow adjustment trim to the input level to the switched supply controller 304 so that both levels are as close as possible.

The feedback provided in the switched supply stage path on line 322 removes low frequency errors in the switched supply output on line 320 such that the combiner stage 308 may be implemented as a smaller device than would otherwise be possible.

The switched supply controller 304 controls the switched supply to select the appropriate supply voltage in accordance with techniques known in the art. The switched supply controller controls the switched supply 302 in accordance with the signal on its input line, which in the illustrated arrangement is provided by the output of the feedback control stage 306.

In a modified arrangement, the combiner stage 308 is implemented as an inductor rather than as a transformer. This modified implementation is shown in FIG. 4.

As illustrated in FIG. 4, the combiner stage 308 is provided with an inductor 402 having a first terminal connected to the output line 320 of a switched supply stage 302. A second terminal of the inductor 402 provides the output signal on line 318.

Implementing the combiner stage 308 as an inductor, an additional modification is provided to achieve the combining function. The correction amplifier 310 of FIG. 3 is replaced by the correction amplifier 410 of FIG. 4. The correction amplifier 410 provides a current output on line 412, which injects current at the terminal of the inductor 402 which is connected to the output line 318. This provides the function of combining the correction signal with the switched supply signal to obtain a modified modulated supply voltage on line 318.

The inductor 402 of the exemplary arrangement of FIG. 4 has two functions. Firstly, the inductor combines the switched supply signal with the correction (or adjustment) signal. Secondly, the inductor 402 may combine with the capacitor 312 to provide the L-C filter provided by inductor 106 and capacitor 108 in FIG. 1.

In comparison to the architecture of GB2398648, the magnetising or self inductance of the inductor 402 is used as part of the circuit function, rather than being an unwanted but necessary additional means, as in the transformer arrangement of FIG. 3. Where the L-C arrangement as shown in FIG. 1 is provided, the inductor 106 may implement the inductor 402, and the capacitor 108 may implement the capacitor 312. Thus the combiner is implemented using existing circuitry. This suggests that the bandwidth requirement of the output combining circuit is much reduced.

Another significant different between the architecture of FIG. 3 and that of FIG. 4 is that most of the output current is shunted through the inductor 402 by the switched supply stage rather than being provided by the correction amplifier.



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stats Patent Info
Application #
US 20140203653 A1
Publish Date
07/24/2014
Document #
14187743
File Date
02/24/2014
USPTO Class
307 80
Other USPTO Classes
International Class
02J4/00
Drawings
7


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