Abstract:
In accordance with the invention there is also provided a voltage combiner comprising: a transformer having a first and second winding each having a first and second tap; and an inductor connected between the first and second taps of the second winding, wherein: the first tap of the first winding is adapted for connection to a first voltage, the first tap of the second winding is adapted for connection to a second voltage, and the second tap of the second winding is adapted to provide an output being the first and second voltages combined, and further wherein: the inductor is adapted to provide a bypass path for the current associated with the second voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a combiner for combining signals, preferably for combining an AC signal with a DC signal. The invention is concerned particularly, but not exclusively, with modulatable power supplies including a combiner implemented as a transformer. 
     BACKGROUND TO THE INVENTION 
     Transistor amplifiers have a peak efficiency for a particular input power that is a function of geometry (i.e. circuit components and layout), load and supply voltage. In conventional radio frequency (RF) power amplification these characteristics are fixed based on the peak input level expected. For amplifiers presented with an input signal having a wide dynamic range, the input signal infrequently achieves peak levels and frequently operates below peak levels. As such, the amplifier may exhibit low overall efficiency. 
     Various techniques are known in the art for enhancing amplifier efficiency based on the supply voltage. One broad classification of solution is envelope tracking. 
     In a prior art envelope tracking technique, a switched mode pulse width variable modulator may be combined with a linear amplifier such that the efficient switched mode supply provides the low frequency components of the output signal that contains a majority of the required power, and the linear amplifier provides a high bandwidth signal to provide the high frequency components of the output signal and correct errors in the switched supply output. A power supply with high bandwidth and good efficiency is thereby provided. 
     GB 2409115 describes an example of such a prior art envelope tracking power supply. A schematic representation of the described power supply is illustrated in  FIG. 1 . In general, the outputs from a switch mode supply and a linear amplifier are summed in a transformer in order to provide the required voltage to a load. The power supply, generally designated by reference numeral  100 , comprises a switchable main voltage source  102  coupled to a first tap  103  of a secondary winding  110  of a transformer  104 . A second tap  105  of the secondary winding  110  is coupled to a load  101   a . A reference voltage source  114  is provided which is coupled to a first input of a subtractor  112 , the subtractor  112  having a second input coupled to the second tap  105  of the secondary winding  110 . An output of the subtractor  112  is coupled to an input of a correction amplifier, or driver,  106 . The output of the correction amplifier  106  is coupled to a first tap  107  of a primary winding  108  of the transformer  104 . A second tap  109  of the primary winding  108  is coupled to ground. 
     In operation, the switchable main voltage source  102  provides a coarse voltage signal for delivery to the load. Typically this is provided by a selection of one of a plurality of voltage supplies in dependence on a reference voltage source derived from the same origin as the reference voltage source  114 . 
     Subtractor  112  determines an error value between the required voltage, as provided by the reference voltage source  114 , and the actual output voltage being provided to the load  101   a . The correction amplifier  106  receives this error value and provides an error correction signal at its output which is applied to the primary winding  108  to be summed with the output of the switchable main voltage source  102  passing through the secondary winding  104 . 
     The output voltage provided at tap  105  of the secondary winding is therefore the coarse voltage provided by the switchable main voltage source  102  combined with an error voltage provided by the subtractor/driver  112 / 106 . The transformer  104  performs the combining. The use of a transformer to perform the combining is advantageous and desirable. 
     However, the coarse voltage signal output by the switchable main voltage source  102  may include a significant DC component which will flow through the secondary winding  110  of the transformer  104 . DC current flowing in the transformer windings may result in saturation of the core, with a consequent loss of magnetising inductance, which is clearly undesirable. 
     One possible way to avoid saturation of the core due to a DC current present in one of the transformer windings is to increase the size of the core, and thereby increase the amount of magnetic flux in the core before it becomes saturated. However, increasing core size increases the transformer leakage inductance and reduces the high frequency response of the transformer and in turn, reduces the high frequency response of the power supply. 
     An alternative technique for avoiding saturation is to introduce an airgap in the core, but this increases the number of turns required to achieve a given low frequency performance and indirectly increases the leakage inductance, and again reduces the high frequency response of the supply. 
     A further method of avoiding core saturation is described in GB 2409115. The solution proposed is to inject a DC current into the primary winding  108  of the transformer  104  in such a way as to cancel the flux due to the secondary current. However, this requires control systems to monitor the currents in both the secondary and primary windings of the transformer, and is complex and difficult to control. 
     It is an aim of the present invention to provide an improved scheme which addresses one or more of the above-stated problems in particular it is an aim of the invention to provide a scheme for reducing DC current in a transformer used as a combiner to combine a DC and an AC signal. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided an apparatus comprising: a transformer comprising at least one winding, wherein said at least one winding has a first tap and a second tap; and an inductor having a first terminal coupled to said first tap, and a second terminal coupled to said second tap. 
     The at least one winding is one of a plurality of transformer windings. Preferably the at least one winding is one of two transformer windings. 
     The apparatus may further comprise means for redirecting a DC current to flow through said inductor, said means for directing DC current being arranged between said first tap and said first terminal of the inductor. 
     Said means for redirecting a DC current may comprise a resistor. The resistor may have a resistance value greater than the resistance value of the inductor. The means for redirecting a DC current may further comprise a capacitor arranged in parallel with said resistor. The capacitor allows AC current to bypass said resistor. This is achieved by the low impedance path offered by the capacitor. 
     Said transformer may comprise at least a primary winding and a secondary winding, and wherein said transformer is configured as a voltage combiner. A first voltage may be connected between first and second taps of said primary winding. A second voltage may be connected to a first tap of said secondary winding, said second voltage comprising a DC component, and wherein said at least one winding comprises said secondary winding. An output voltage being the combined first and second voltages may be provided on a second tap of said secondary winding. 
     The second voltage may be a coarse voltage signal. The first voltage may be a fine voltage signal representative of an error in the coarse voltage signal. 
     A power supply may include the stated apparatus. Said power supply may be configured to provide a modulated supply voltage to a power amplifier. 
     In accordance with the invention there is also provided a voltage combiner comprising: a transformer having a first and second winding each having a first and second tap; and an inductor connected between the first and second taps of the second winding, wherein: the first tap of the first winding is adapted for connection to a first voltage, the first tap of the second winding is adapted for connection to a second voltage, and the second tap of the second winding is adapted to provide an output being the first and second voltages combined, and further wherein: the inductor is adapted to provide a bypass path for the current associated with the second voltage. 
     The voltage combiner may further include means for directing the DC current to the bypass path. Said means may be a resistor connected between the first tap and the inductor. Said means may further include a capacitor in parallel with the resistor to provide a low resistance path for AC current in the second voltage. 
     The ratio of the resistance value of the resistor and the resistance value of the inductor may determine the DC current flow in the inductor. The ratio may be very much greater than 1. 
     The first voltage may be provided by a switchable voltage source and the second voltage is provided by subtracting the output voltage from a reference voltage. 
     In accordance with the invention there is also provided a method of reducing DC current in a winding of a transformer, the method comprising: applying a voltage including a DC component to a first tap of the winding; and providing an inductor coupled between the first tap and a second tap of the winding; wherein a proportion of the DC component of the voltage is bypassed around the transformer through said inductor. 
     The method may further comprise controlling the proportion of the DC component that is bypassed around the transformer by providing a resistive element coupled between the first tap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention in now described by way of example with reference to the accompanying Figures, in which:— 
         FIG. 1  illustrates a prior art transformer based voltage supply of GB 2409115; 
         FIGS. 2   a  and  2   b  illustrate a transformer based voltage supply with bypass inductor in accordance with an embodiment of the invention; 
         FIG. 3  illustrates a transformer based voltage supply with a DC steering resistor in accordance with an embodiment of the invention; 
         FIG. 4  illustrates output voltage versus time characteristics for a transformer based supply embodying the principles of the present invention; and 
         FIG. 5  illustrates output power versus time characteristics for a transformer based supply embodying the principles of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is described herein by way of particular examples and specifically with reference to a preferred embodiment. 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 the provision of a power supply for an RF amplification stage. However more generally the invention may apply to any arrangement where it is necessary to reduce DC current in a transformer winding, and particularly in a transformer in which an AC signal and a DC signal are combined. 
     It should be noted that where the same reference numerals are used in different Figures, they refer to the same elements. 
     Referring to  FIG. 2   a , there is illustrated a transformer based power supply  200  which may be used for modulating the power supplied to a power amplifier in an envelope tracking system, in accordance with the general principles of the present invention. The power supply  200  comprises a switchable main voltage source  102 , a transformer  104  having primary  108  and secondary  110  windings, a correction amplifier  106 , a subtractor  112 , a reference voltage source  114 , and a bypass inductor  120 . 
     The switchable main voltage source  102  is coupled a first tap  103  of the secondary winding  110  of transformer  104 . A second tap of the secondary winding  110  is coupled to the load  101   a . Bypass inductor  120  has first and second terminals, the first terminal being coupled to the first tap  103  of the secondary winding  110 , and the second terminal coupled to the second tap  105  of the secondary winding  110 . The reference voltage source  114  is coupled to the first input of the subtractor  112 , the subtractor  112  having a second input coupled to the second tap  105  of the secondary winding  110 . The output of the subtractor  112  is coupled to the input of the correction amplifier  106 . The output of correction amplifier  106  is coupled to the first tap  107  of the primary winding  108  of the transformer  104 , and the second tap  109  of the primary winding  108  is coupled to ground. The load  101  is coupled between the second tap  105  of the secondary winding  110  and ground. 
     The transformer  104  of  FIG. 2   a  is shown as an ideal transformer plus an n inductor  130  which represents the magnetising inductance of the transformer. This inductance is present in parallel with the secondary winding  110 . The magnetising inductance stores energy in the core of the transformer. 
     Power supply  200  may optionally comprise filter  132  coupled in series between the switchable main voltage source  102  and the first tap  103  of the secondary winding  110 , such that the main voltage is delivered to the transformer via the filter  132 . 
     The power supply stage  200  of  FIG. 2   a  provides a modulated output voltage supply to the load  101   a . The load  101  may, for example, comprise a RF power amplifier  101   b  as shown in  FIG. 2   b . The switchable main voltage source provides a coarse power supply signal that comprises the DC and low frequency components of the required voltage to be delivered to the load  101   a . The output of the voltage source  102  may be low pass filtered by filter  132  to remove unwanted high frequency components from the DC and low frequency voltage signal that would otherwise need to be corrected. The output of the voltage source  102  is then applied to the first tap  103  of the secondary winding and the first terminal of the bypass inductor  120 . The subtractor  112  receives the reference voltage value and subtracts the value of the voltage present at the second tap  105  of the secondary winding  110  to produce a voltage error signal. This voltage error signal is then amplified in the correction amplifier  108  and applied to the first tap  107  of the primary winding  108 . The outputs of the voltage source  102  and the correction amplifier  106  are then combined by the transformer to provide a corrected voltage output at the second tap  105  of the secondary winding  110 . 
     The contribution of the voltage source  102  to the output voltage of power supply  200  is illustrated in  FIG. 4 . The voltage output by voltage source  102 , V sw , is relatively constant for the time period shown in  FIG. 4 , as would be expected for the DC and low frequency components. In contrast, the corrected output voltage of power supply  200 , V out , is seen to vary significantly at a much higher frequency. The difference between V sw  and V out  is provided by the correction amplifier  106  as described above. In the prior art power supply  100 , the coarse voltage signal, V sw , is applied to the secondary winding  110  of the transformer, and may cause a significant DC current to flow in the secondary winding. This DC current may generate a significant magnetic flux in the transformer core, and may lead to magnetic saturation of the core. 
     In order to address this problem, the voltage supply stage  200  of  FIG. 2   a  is provided with the bypass inductor  120 . The bypass inductor  120  preferably comprises a high power inductor and therefore presents a high impedance to high frequency signals, but a very low impedance to DC current and low frequency signals. Therefore, the bypass inductor presents a low impedance DC current bypass path around the transformer, and a proportion of the DC current output by voltage source  102  will flow through the bypass inductor and not through the transformer. The flux in the transformer core due to the DC current flowing through the transformer windings will be reduced as less DC current flows through the secondary winding  110 . Thus, the susceptibility of the core to magnetic saturation is reduced. This allows a physically smaller transformer, compared with the transformer that would be required in the prior art power supply  100  of  FIG. 1 . This is advantageous as a smaller transformer may have an improved high frequency response. 
     Referring to  FIG. 3 , a further embodiment of the present invention is illustrated. The transformer based supply  300  shown in  FIG. 3  is similar to that shown in  FIG. 2 , but further comprises a resistor  118  and a capacitor  116 . 
     In the embodiment of  FIG. 3 , the voltage source  102  is coupled to the first terminal of the bypass inductor  120 . Resistor  118  is coupled between the first tap  103  of the secondary winding  110  and the first terminal of the bypass inductor  120 . The capacitor  116  is arranged in parallel with the resistor  118 , between the bypass inductor  120  and the secondary winding  110 . The remaining components and connections are identical to those described in conjunction with  FIG. 2 . 
     Bypass inductor  120  has an associated resistance value, R L , due to the length of wire in the inductor coil. This resistance value is represented in  FIG. 3  by resistor  140  connected between the second terminal of the inductor  120  and the load  101   a . This resistance value R L  is normally small. The resistor  118  has a resistance value R trans  that is greater than R L , preferably much greater. Therefore, DC current output by the voltage source  102  will flow preferentially through the low resistance path provided by bypass inductor  120 , with its inherent low resistance, R L . The ratio of R trans  to R L  determines the reduction in DC current flowing in the transformer secondary  110  and hence the reduction in the DC magnetic flux in the transformer  104 . 
     The resistor  118  thus provides a means for directing current through the bypass path rather than through the transformer. 
     The capacitor  116  provides a low impedance path for AC current flow through the secondary winding  110  of the transformer  104 . By providing a low impedance path for AC currents, excessive dissipation of high frequency signals in resistor  118  may be avoided. 
     Thus, in the exemplary embodiment described in relation to  FIG. 3 , it is possible to control the proportion of the DC current that is bypassed around the transformer  104  via the bypass inductor  120 . By increasing R trans  in relation to R L  a greater proportion of the DC current is directed through the bypass inductor  120 , further reducing the magnetic flux and stored energy in the transformer as compared to the embodiment shown in  FIG. 2   a . As the flux in the transformer is further reduced, the transformer itself may be further reduced in size whilst avoiding magnetic saturation of the core. 
     The presence of bypass inductor  120  in the power supply stage  200  or  300  of  FIGS. 2   a  and  3  may provide further benefits as will now be discussed in relation to  FIG. 5 . 
     Assuming a lossless transformer, the average power delivered to the load  101   a  is the combination of the average power delivered by the switchable main voltage source  102  and the correction amplifier  106 . However, the instantaneous power delivered to the load  101   a  may not be equal to the sum of the instantaneous power delivered by the switchable main voltage source  102  and the correction amplifier  106 . This is because energy is stored in the bypass inductor  120  and in the transformer magnetising inductance  105  and this energy may be released during periods of high instantaneous output power. 
       FIG. 5  shows as waveform  502  the total output power (P TOT ) of the transformer based power supply  200  or  300  delivered to the load.  FIG. 5  also shows: as waveform  504  the power (P SW ) delivered to the load by the switched main voltage source  102 ; as waveform  506  the power (P CORR ) delivered by the correction amplifier  106 ; and as waveform  508  the power (P IND ) delivered by the bypass inductor  120 . It will be understood that P TOT =P IND +P SW +P CORR . 
     As can be seen from  FIG. 5 , the majority of the power being delivered to the load at any one point in time is provided by the switchable main voltage source  102 . However, during peaks in output power, a significant amount of power is provided by the bypass inductor  120 . 
     The average power output of the (ideal) bypass inductor must be zero, in order for energy to be conserved. As can be seen in  FIG. 5 , the inductor  120  is ‘charged’ during periods of low instantaneous output power, and releases the stored-energy during periods of high instantaneous output power. Advantageously, this significantly reduces the peak power requirement of the correction amplifier  106 . 
     In order to improve the high frequency response of the power supply  200 , it is preferable to use a small transformer, as smaller transformers are known to have better high frequency responses. 
     The provision of bypass inductor  120  enables the high frequency performance of a small transformer to be enjoyed without experiencing transformer saturation due to DC current flowing through the transformer secondary winding. 
     Thus, embodiments of the present invention may allow the DC and low frequency components supplied by the switchable main voltage source  102  to be diverted to flow predominantly through the bypass inductor  120 , and thereby reduce the DC current flowing through the transformer. By reducing the DC current flowing through the transformer  104 , the magnetic flux in the core is reduced, allowing a smaller transformer to be used without allowing the core to become saturated, and thereby improving the high-frequency performance of the power supply  200 . 
     Energy which would otherwise be stored in the magnetising inductance  105  of the transformer  104  is instead stored in a physically separate inductor  120 , which unlike the transformer  104 , does not need to be optimised for high frequency operation. As the inductor  120  does not need to be optimised for high frequency operation a large inductance may be preferred. The stored energy may then be delivered to the load  101  during periods of high instantaneous output power, thereby reducing the peak power requirement of the correction amplifier  106 . 
     The present invention has been described herein by way of reference to particular preferred embodiments. However, the invention is not limited to such embodiments. The present invention has particular application in relation to RF amplifiers, but is not limited to such implementation. The invention can be advantageously utilised in any environment in which a transformer winding is required to carry a DC current that could lead to magnetic saturation of the transformer core. 
     The described preferred embodiments utilising an RF amplifier are not limited to any particular load being driven by such RF amplifier. However it is envisaged that such an RF amplifier will typically drive an antenna. As such, the present invention has particular advantageous uses in the field of communications, including the field of mobile communications.