Filter for switched mode power supply

Abstract

There is provided a filter for receiving a rectangular or stepped source voltage to be filtered and for providing an output voltage, the filter including means arranged to determine the output voltage in dependence on the frequency components of the source voltage within the filter passband, and independent of output current drawn.

Claims

1. A modulated power supply including a filter for receiving a source voltage to be filtered and for providing an output voltage, the filter including an inductor connected in parallel with a resistive element, wherein an output impedance of the filter with the resistive element is reduced across a filter transition band compared to the output impedance of the filter without the resistive element across the filter transition band.

2. The modulated power supply of claim 1, wherein the filter is configured to determine the output voltage in dependence on frequency components of the source voltage within a filter passband.

3. The modulated power supply of claim 1, wherein the filter is configured to reduce the output impedance without adversely affecting the input impedance of the filter.

4. The modulated power supply of claim 3, wherein the filter is configured to include a second resistive element connected in parallel across part of an input inductor of the filter.

5. The modulated power supply of claim 1, wherein the resistive element comprises a resistor.

6. The modulated power supply of claim 1, wherein the filter is configured to reduce the output impedance of an input inductor and an input capacitor.

7. The modulated power supply of claim 6, wherein the filter comprises a j.sup.th order filter, wherein the filter is configured to reduce the output impedance of the input inductor and the input capacitor in one or more orders of the filter.

8. The modulated power supply of claim 6, wherein the inductance of the input inductor is divided by a value n and the capacitance of the input capacitor is multiplied by the value n to reduce the output impedance.

9. The modulated power supply of claim 1, wherein the filter comprises one or more series resonant output traps at an output of the filter, each output trap having a low Q factor.

10. The modulated power supply of claim 9, wherein at least one of the series resonant output traps includes a resonant inductor and a capacitor connected in series.

11. An RF amplification stage including the modulated power supply according to claim 1.

12. The modulated power supply of claim 1, the filter further comprising a first stage having a second inductor and a second resistive element, wherein the second resistive element is connected in parallel across at least part of the second inductor of the first stage of the filter.

13. The modulated power supply of claim 12, wherein the second inductor of the first stage of the filter is split into a first part and a second part, and wherein the second resistive element is connected across the second part.

14. The modulated power supply according to claim 1, wherein the filter comprises a j.sup.th order filter, and wherein a second resistive element is placed across a second inductor of at least one order of the filter.

15. The modulated power supply of claim 1, wherein the filter is configured such that part of an input inductor therein is split into a series of parallel resonant circuits.

16. A modulated power supply including a filter for receiving a source voltage to be filtered and for providing an output voltage, the filter comprising a resistive element and an inductor, wherein the resistive element is connected in parallel across at least part of the inductor, and wherein the resistance of the resistive element reduces an output impedance of the filter at a passband, a transition band, and a stop band of the filter as compared to the output impedance of the filter without the resistance.

17. The modulated power supply of claim 16, wherein the filter is configured to reduce the output impedance without adversely affecting an input impedance of the filter.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The present invention in now described by way of example with reference to the accompanying Figures, in which:

(2) FIG. 1 illustrates a block diagram of an RF amplification stage embodying the concept of the present invention;

(3) FIG. 2 illustrates a conventional filter arrangement;

(4) FIG. 3 illustrates a conventional multi-stage filter arrangement;

(5) FIG. 4 illustrates an improved filter arrangement according to a first embodiment of the invention;

(6) FIG. 5 illustrates an improved filter arrangement according to the first and a second embodiment of the invention;

(7) FIG. 6 illustrates an improved filter arrangement according to the first and a third embodiment of the invention;

(8) FIG. 7 illustrates a modification to the filter arrangement of FIG. 6; and

(9) FIG. 8 illustrates a preferred filter implementation.

DESCRIPTION OF PREFERRED EMBODIMENTS

(10) The present invention is described herein by way of particular examples and specifically with reference to preferred embodiments. It will be understood by one skilled in the art that the invention is not limited to the details of the specific embodiments given herein. In particular the invention is described herein by way of reference to an RF amplification stage including a switched mode voltage supply. However more generally the invention may apply to any arrangement where it is necessary to filter a rectangular or stepped drive signal.

(11) Referring to FIG. 1, there is illustrated an RF amplification stage 100 in accordance with an exemplary application for describing the present invention. The RF amplification stage 100 includes an RF amplifier 102, a supply voltage selection block 106, an envelope detector 104, and a filter 108.

(12) In the illustrated example of FIG. 1, the supply voltage selection block 106 receives four supply voltages V.sub.1-V.sub.4 on respective input lines 132.sub.1-132.sub.4. In general, however, a supply voltage selection block may select between any number of levels, four being a non-limiting example. The selected supply voltage is output from the supply voltage selection block 106 on line 120. The RF amplification stage 100 receives an RF input signal RFT.sub.IN on line 110. The envelope detector 104 has an input 114 coupled to line 110 to thereby detect the RF input signal. The envelope detector provides an output on line 118 to the supply voltage selection block 106 to provide the necessary information for the supply voltage selection to take place. The filter 108 receives the output of the supply voltage selection block on line 120. The filter 108 provides a filtered supply voltage on line 122 for the RF amplifier 102. The RF amplifier 102 provides on line 112 the RF output signal RF.sub.OUT.

(13) The example arrangement of FIG. 1 is illustrative, and the invention is not limited to any details shown. For example elements of the illustrative RF amplification stage of FIG. 1, specifically the envelope detector 104, the supply voltage selection block 106 or the filter 108, may be implemented in the digital domain in an alternative arrangement.

(14) In general, given a selection of the desired supply voltage for the RF input signal to be amplified, the supply voltage selection block 106 connects the selected supply voltage to its output on line 120. The filter 108 functions to filter the supply voltage on line 120 to the RF amplifier 102.

(15) FIG. 2 illustrates an equivalent circuit for the supply voltage selection block 106 and a conventional arrangement for the filter 108. The filter 108 receives a rectangular drive voltage, as represented by the voltage waveform 210, which is provided by voltage source 202 in the equivalent circuit arrangement of FIG. 2. The rectangular drive voltage is provided by semiconductor switches with low on resistance, represented by resistance R.sub.SW in FIG. 2 and denoted by reference numeral 204. The filter circuitry is provided by an inductor 206.sub.1, having an inductance value L1 and a capacitor 208.sub.1 having a capacitance value C1. The filter substantially removes frequency components at the switching frequency and the associated harmonics, leaving only the DC components of the input waveform. The output DC voltage provided on output line 212 is then determined by the duty cycle of the input switching waveform.

(16) Dynamic modulation of the output voltage provided on the output line 212 may be obtained by varying the duty cycle of the input waveform. The duty cycle of the input waveform may be varied by varying the pulse width of the input waveform, the repetition rate of the pulse, or both. The modulation bandwidth and switching frequency residual ripple are both determined by the design of the output filter 108.

(17) The maximum tracking bandwidth for a given switching frequency and output ripple may be increased by adding additional sections to the filter, as shown in FIG. 3. As shown in FIG. 3, additional inductor-capacitor pair arrangements are added to the filter arrangement of FIG. 2, in order to provide a higher order filter. As shown in FIG. 3 a second stage or section comprising an inductor 206.sub.2 having an inductance value L2 and a capacitor 208.sub.2 having a capacitance value C2 are added, and in general a j.sup.th stage is added by an inductor 206.sub.j having an inductance value Lj and a capacitor 208.sub.j having a capacitance value Cj.

(18) The input switching waveform may in general be regarded as a m-level quantised representation of the desired output waveform. High order quantisation results in reduced quantisation noise and hence reduced filtering requirements.

(19) The efficiency of the supply voltage selection stage 106 is determined by losses in the switching devices within the selection stage 106 and losses in the output filter 108, as set out in the background section above. The losses within the switching devices may further be classified into static and dynamic or switching losses. The static losses occur as a result of a filter input current being drawn through the on resistance of the switching devices. The input current comprises an unavoidable DC term due to the output load and a ripple current determined by the filter input impedance. The ripple current is determined by the filter input impedance at the switching frequency and its odd harmonics. Hence for high efficiency the filter should present high impedance at these frequencies.

(20) Ideally, it is desired for the voltage provided at the filter output to be determined solely by the source voltage and to be independent of the output current drawn. To approach this ideal, in accordance with embodiments of the present invention, a filter arrangement is provided in which the output impedance is low across the filter pass band, transition band, and stop band.

(21) Achieving low output impedance at the transition band is more difficult than in the pass band and stop band. Typically the transition band shows large impedance peaks due to resonances within the filter. If the spectrum of the load current is a white noise spectrum, then large errors in output voltage will occur at the frequencies of resonance.

(22) There is now described three embodiments for implementing the present invention. Each embodiment, on its own, offers a solution to reduce the output impedance of the filter in the transition band, and thereby make the output voltage of the filter less dependent on the output current drawn. The embodiments may be utilised individually or in any combination.

(23) The first embodiment of the invention is shown in FIG. 4. In this first embodiment the magnitude of the impedance peaks is reduced by introducing at least one lossy resistive element into the filter. The lossy resistive elements are chosen so as to introduce loss at the resonance peaks without significantly increasing the passband loss of the filter, or the loss at the switching frequency and its harmonics.

(24) A resistor is preferably provided for each inductor in each order of the filter.

(25) Whilst the filter of FIG. 4 is adapted to achieve a reduced output impedance, it is important to ensure that the input impedance of the filter is not adversely affected, and particularly that the input impedance is not reduced. A reduction in the filter input impedance increases the static losses in the switching devices, which is undesirable.

(26) To ensure the input impedance is not reduced, for the first section of a j.sup.th filter, or in a first order filter, the inductor is split such that the resistor is connected in parallel across only a part of the inductor. Thus as shown in FIG. 4 the inductor 206.sub.1 of FIG. 3 is split into a first part 206.sub.1a having an inductance value L1a and a second part 206.sub.1b having an inductance value L1b. A lossy resistor 502.sub.1 having a value R1 is connected in parallel across the inductor 206.sub.1b. The inductor 206.sub.1a ensures that the input impedance of the filter, Zin, remains high at the switching frequency and its harmonics.

(27) As also shown in FIG. 4 for a j.sup.th order filter each inductor of each filter stage, other than the inductor of the first stage, has a resistor connected in parallel across it. The inductor 206.sub.2 is thus shown to have a resistor 502.sub.2 having a resistance value R2 connected across it, and the inductor 206.sub.j is shown to have a resistor R.sub.j 502.sub.j having a resistance value R.sub.j connected across it.

(28) In this first embodiment, when applied to a j.sup.th order filter, advantages are obtained by connecting a lossy resistor across the inductor of one or more stages. It is not essential to connect a lossy resistor across all stages.

(29) Using the exemplary technique of FIG. 4, the output impedance is maintained low across the passband, transition band and stopband of the filter, i.e. across the full frequency range.

(30) A second embodiment is described with reference to FIG. 5. The embodiment of FIG. 5 is shown by way of additional modification to the embodiment of FIG. 4. It should be understood, however, that the embodiment of FIG. 5 does not require to be implemented in combination with the embodiment of FIG. 4. The principles of the embodiment of FIG. 5 offer an improvement in themselves when implemented without the features of the first embodiment.

(31) In accordance with the second embodiment, the impedance of all elements within the filter is reduced by a factor n, to further reduce the output impedance of the filter stage. This is illustrated in FIG. 5 by the notation of the values of all the inductors shown therein being divided by n, and similarly the values of the lossy resistors 502 in a multiple-order arrangement being divided by n. The capacitance values are multiplied by n.

(32) This modification to the filter, however, whilst reducing the output impedance also reduces the filter input impedance.

(33) This effect may be counteracted, in a preferred modification, by splitting the input inductor into several sections to create parallel resonance circuits at the switching frequency and its odd harmonics. This may be achieved in the preferred arrangement of FIG. 5 by splitting the input inductor 206.sub.1b into k sections. As shown in FIG. 5 each of the k sections includes a parallel arrangement of an inductor 502, a capacitor 504 and a resistor 506.

(34) The inductors 502.sub.1, 502.sub.2, 502.sub.k in total have an inductance value equivalent to the value of the inductor 206.sub.1b.

(35) This second embodiment is shown as an arrangement in combination with features of the first embodiment, where only a portion of the input inductance is modified.

(36) Where the arrangement to implement counteraction of static losses is desired, i.e. to avoid a reduction of input impedance, and the arrangement of the first embodiment is not implemented, the input inductance 206.sub.1b of FIG. 5 may still be split up into parallel resonance circuits as shown for the inductance 206.sub.1b of FIG. 5.

(37) Using the exemplary technique of FIG. 5, the output impedance is maintained low across the passband, transition band and stopband of the filter, i.e. across the full frequency range.

(38) A third embodiment is illustrated with reference to FIG. 6. The principles of this third embodiment are again illustrated in combination with, the principles of the first embodiment described hereinabove, but they need riot be implemented in combination with the first embodiment.

(39) In the third embodiment as illustrated by FIG. 6, a plurality p of output traps are utilised, each output trap including an inductor and capacitor connected in series to ground. Thus there is shown a first output trap comprising an inductor 502.sub.1 and capacitor 504.sub.1 connected in series; a second output trap comprising an inductor 502.sub.2 and a capacitor 504.sub.2 connected in series; and a p.sup.th output trap comprising an inductor 502.sub.p and capacitor 504.sub.p connected in series.

(40) The output traps each have a low Q factor. The Q factor of each inductor 502 in the output traps may be deliberately reduced through use of series and parallel resistors as shown in FIG. 7. Thus, for example, with reference to FIG. 7 the inductor 502.sub.1 may be implemented by an inductor 510 and resistor 512 in series, with a further resistor 514 connected across in parallel.

(41) The output traps reduce the output impedance of the filter. The number of output traps, p, provided is dependent upon the number of frequency regions over which traps are required: each trap lowers the output impedance for a given frequency region.

(42) In the above there is described a first embodiment with reference to FIG. 4, a second embodiment described in combination with the first embodiment with reference to FIG. 5, and a third embodiment described in combination with the first embodiment with reference to FIG. 6. Each embodiment may be utilised on its own or with any combination of the other embodiments. For completeness, a particularly preferred arrangement in which all three embodiments are combined is illustrated in FIG. 8.

(43) This preferred arrangement of FIG. 8 offers a particularly advantageous reduced output impedance. It should be noted that in the arrangement of FIG. 8 the principle of the second embodiment, in which the impedance values of the elements in the Figure are divided by a factor n, is only illustrated as implemented in the input stage of the filter, and not in subsequent orders of the filter. Thus each of the inductors 506.sub.1, 506.sub.2, 506.sub.k combine to provide an inductance value which is an n.sup.th of the value of the inductor 206.sub.1b of FIG. 4.

(44) There is thus described three embodiments, exemplified by FIGS. 4, 5 and 6 respectively. The second embodiment is described with reference to FIG. 5, in combination with the first embodiment. Each embodiment may be implemented independently or in combination with any other embodiment.

(45) However, whilst advantages in accordance with the invention can be achieved by implementing only the techniques of the second embodiment, it is preferable to implement the second embodiment in combination with either the first or third embodiment, The first and third embodiments have in common the provision of at least one lossy resistor. In the first embodiment the lossy resistor is provided in combination with the inductor of each order of the filter. In the second embodiment the lossy resistor is provided by one or more output traps. Thus in the preferred embodiment at least one lossy resistor is provided.

(46) The present invention has been described herein by way of reference to particular preferred embodiments, and Particularly by way of reference to an application in a modulated voltage supply. This description is, however, only illustrative of examples. In particular the invention may be implemented more broadly.