Abstract:
A modulated power supply comprises a power switching stage having at least one power switching device for generating a power signal in response to an input modulating signal. A current source is positioned in parallel with the power switching stage and continuously generates an output current. An output stage combines the power signal and the output current to form an output power supply signal. The current source supplies some, or all, of the required current at any given time. The switching device in the power switching stage either supplies the remaining required current or sinks any excess current. This has an advantage of reducing the average and peak currents flowing through the switching device, and hence the average power dissipation in the device. The output current can be set at an average (e.g. RMS) value of the current in the output power supply signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to modulated power supplies and to a method of generating a power supply signal. 
     BACKGROUND TO THE INVENTION 
     Modulated power supplies, such as Pulse Width Modulated (PWM) power supplies, are widely used in a variety of applications. In a PWM power supply a power switching device, such as a power transistor, is turned on and off at a high frequency, with the width of the ‘on’ periods varying in sympathy with the amplitude of a modulating input signal. The resulting train of output pulses from the switching device is smoothed by a low pass filter to deliver a supply voltage which varies in sympathy with the modulating input signal. 
     A PWM power supply can have a single phase or multiple phases, with the contributions of individual phases summing to provide an overall output. Multi-phase PWM power supplies have an advantage over single phase PWM supplies in that they can deliver better resolution in the time domain and increased current. 
     One known application of a modulated power supply is in supplying power for a linear RF power amplifier. An envelope of the RF signal which is to be amplified is used as a modulating signal for the power supply and the resulting, modulated, power supply signal is fed to the power amplifier. In this way, the power supply signal follows the envelope of the signal to be amplified and the efficiency of the linear power amplifier can be improved. 
     For high frequency (e.g. RF) power supply applications small, fast, switching devices are required in each phase, such as Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistors. These devices have a small junction, which results in them having a relatively high resistive loss during the periods that they are switched on. This high resistive loss incurs power losses and generates heat which must be dissipated to prevent device failure. 
     Accordingly, the present invention seeks to improve the performance of a modulating power supply particularly, but not limited to, situations where the modulating signal has a wide bandwidth. 
     SUMMARY OF THE INVENTION 
     A first aspect of the present invention provides a modulated power supply comprising: 
     a power switching stage having at least one power switching device for generating a power signal in response to an input modulating signal; 
     a current source which is operable to continuously generate an output current, the current source being positioned in parallel with the power switching stage; and, 
     an output stage which combines the power signal and the output current to form an output power supply signal. 
     In this arrangement the current source supplies some, or all, of the required current at any given time. The switching device in the power switching stage either supplies the remaining required current or sinks any excess current. This has an advantage of reducing the average and peak currents flowing through the switching device, and hence the average power dissipation in the device. Ideally, the average current in the power switching device should tend to zero. Operating in this manner also has an advantage of reducing the operating temperature of the switching device which leads to improved reliability and simplified heat sinking requirements. There are cost savings arising from the reduced power consumption, simplified heat sinking requirements and improved reliability. Operating switching devices at a lower junction temperature also lowers the on-resistance. The reduction in peak current can result in a more linear transfer function or can allow the use of smaller switching devices with lower capacitance and therefore lower capacitive loss. The power supply can use several switching devices per power switching stage, and there can be a plurality of power switching stages (phases) in parallel with one another. 
     The current source continuously generates an output current as long as it is efficient to do so. In a power supply which is required to operate over a range of output power levels it has been found that it can be undesirable to use the current source at the lowest power levels as it may require the switching device in the power switching stage to sink an undesirably large amount of current. 
     Preferably, the output current of the current source is set at a value which achieves best overall system power efficiency. This can be a current which is at, or close to, the average value of the current in the output power signal, such as the root mean square (RMS) value of the current in the output power signal. Preferably, the current source is controllable such that it tracks the average value of the current in the output power signal. 
     The current source can be implemented as a power converter which has at least one power switching device. It is preferable that the power switching device used within the power converter has a lower resistive loss than the power switching device used within the power switching stage. This is possible because the power converter will operate at a lower switching frequency than the main power switching stage. 
     The power supply can be used in a wide range of applications. It is particularly well-suited to wireless telecommunications base stations where power amplifiers in the transmit chains are required to amplify a signal having a wide bandwidth. This is particularly true in third generation Universal Mobile Telecommunications System (UMTS) base stations. The input modulating signal to the power supply can be an envelope of a signal to be transmitted and the output of the power supply can form the power supply to a power amplifier, so that the power supply tracks the envelope of the input signal. Power costs are one of the most significant operating costs of a base station and thus and reduction in these can yield considerable savings. The invention is not limited to communication systems. Any application requiring modulation of voltage or current that has a significant DC component in the signal will benefit. The invention can improve overall power efficiency, linearity, modulator size, cost and reliability. 
     The power supply can include a single power switching stage or multiple power switching stages which are operated in parallel with one another as a group of phases. Each power switching stage can be operated in a pulse width modulated (PWM) manner or alternatively as a pulse density modulated (PDM) or a Sigma Delta Modulated (SDM) manner. 
     Further aspects of the invention provide a power amplifier which includes such a modulated power supply, a wireless base station comprising the power amplifier, a method of generating a power supply signal and a power supply signal resulting from this method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  shows a modulated power supply for use with a power amplifier; 
         FIG. 2  shows operation of the arrangement of  FIG. 1 ; 
         FIG. 3  shows an embodiment of a modulated power supply; 
         FIG. 4  shows current flow through the supply of  FIG. 3  over a period of time; 
         FIG. 5  shows a graph which compares the efficiency of the modified power supply with a conventional power supply; 
         FIG. 6  shows an example constant current source; 
         FIG. 7  shows a wireless communications base station incorporating the modulated power supply. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Before describing the invention in detail,  FIGS. 1 and 2  illustrate an application of the invention in order to put the invention into context.  FIG. 1  shows a power amplifier arrangement comprising a power amplifier  100  and a modulated power supply  110 . An input signal Vin, which is to be amplified by the power amplifier  100 , is also applied to an envelope detector  105 . A signal, Vmod, representing the envelope of the input signal is applied to an input  103  of the modulating power supply  110 . A control circuit  230  within the modulating power supply  110  receives the signal Vmod and determines appropriate control signals which cause the power supply  110  to generate a supply voltage Vsupply which substantially tracks Vmod. An amplified output signal Vout is taken from an output  102  of the power amplifier  100 .  FIG. 2  shows the operation of the power supply over a period of time, showing the envelope of the input signal Vmod and the envelope of the dynamically modulated power supply voltage Vsupply. It can be seen that the power supply voltage tracks the signal envelope, including peaks  125 . As a comparison, the power supply voltage of a fixed supply is shown by line  120 . 
     In the following embodiments the modulating power supply  110  is a pulse width modulated (PWM) converter, and preferably a multi-phase PWM converter.  FIG. 3  shows the main blocks within a multi-phase PWM converter  110  with N phases. Phase  1   200  is shown in detail and other phases  201 ,  205  have the same layout. Each phase includes two power switching devices  210 ,  211  which can be a Field Effect Transistor (FET), Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistor or any other suitable switching device. A first switching device  210  is connected between a positive supply rail +Vs and an output node  213 . A second switching device  211  is connected between the output node  213  and ground. Each phase also includes a drive circuit  212 . The control signal output by PWM controller  230 , which is typically implemented as a FPGA, is at a low level which is unsuitable for directly driving the switching devices  210 ,  211 . Therefore, drive circuit  212  converts the control signal to a suitable level for driving the switching devices  210 ,  211 . It will be appreciated that this topology of switching devices is only shown as an example and variants will be well known to a skilled person. 
     PWM controller  230  receives a signal Vmod indicative of the required output voltage/current and generates a set of control signals CTRL_ 1 , CTRL_ 2 , CTRL_N which are applied to the switching devices in each of the phases  200 ,  201 ,  205 . In a known manner, each control signal has pulses with an ‘on’ time related to the required output signal. In a multi-phase supply, each of the N phases receives a control signal in which the pulses are offset in time from the pulses applied to other phases. The resulting output of each phase is a stream of pulses which vary in width, the average level of the pulse stream representing a desired output level. 
     The respective outputs I 1 , I 2 , I N  of each phase  200 ,  201 ,  205  are summed and low-pass filtered in an output stage  250 . Each phase  200 ,  201 ,  205  is connected in series with an inductor L 1 , L 2 , L 3  and the remote ends of the inductors L 1 , L 2 , L 3  are commonly connected to a summing node  251 . A capacitor C is shunted across the output. The combination of inductors L 1 , L 2 , L 3  and capacitor C have the effect of low-pass filtering the outputs of the phases, turning the pulsed outputs of individual phases into a summed, smoothed, output signal Vsupply having the form shown in  FIG. 2 . So far, the arrangement of  FIG. 3  is conventional. According to an embodiment of the invention, a constant current source  220  is placed in parallel with the phases  200 ,  201 ,  205 . The constant current source (CCS)  220  generates a current at a value Iccs. The value Iccs can be permanently fixed, or can be varied as described below. It should be noted that the value of Iccs does not vary in sympathy with the modulating signal, and any variation is controlled at a much slower rate than the power switching stages  200 ,  201 ,  205 . In the same manner as the phases  200 ,  201 ,  205 , the constant current source  220  is connected in series with an inductor L 4  and the output of the inductor L 4  is connected to summing node  251 . To preserve the filter characteristic it can be beneficial to use L 1 =L 2 =L 3 =L 4  and also provide some small capacitance, similar to that of a pair of VMOD FETs  210 ,  211 , in series with L 4 . An additional large inductor (&gt;100 μH) should be used to de-couple the current source from the filter and provide quasi-DC conditions at the current source. 
     PWM controller  230  supplies a control signal CTRL_CCS which sets the value of Iccs. In an ideal implementation where the power switching stage(s)  200 ,  201 ,  205  track the signal envelope (Vmod,  FIG. 2 ) perfectly, the current source would need very little control, and would simply maintain an output current at a constant value. In a preferred embodiment where the static power output level is variable (e.g. to satisfy the different power demands of the RF amplifier  100  at different transmit power levels) it is necessary to provide a slow control loop to adjust the output current of the current source so as to maintain best overall power efficiency. 
     At the lowest output power levels it may be more efficient to turn the current source off. The current level used at each power level is preferably pre-determined, based on what is known to be required, but the actual value can be adjusted as necessary during operation based on monitoring the output current, shown as feedback loop  261  in  FIG. 3 . The provision of the current source  220  does not require any significant changes to the control functions of the power switching stages  200 ,  201 ,  205 . The switching devices  210 ,  211  in the power switching stages  200 ,  201 ,  205  are non-ideal devices and have impairments such as I 2 R loss and pulse rise &amp; fall times. The level of these impairments may be slightly changed by the addition of the current source to the power supply and it is desirable to control these impairments by feeding back a sample of the output voltage Vsupply, shown as feedback loop  262  in  FIG. 3 . 
     One particularly advantageous value of Iccs is the root mean square (RMS) value of the output signal Isupply, although the invention is not limited to this value. The effect of operating in this way will now be illustrated with reference to  FIG. 4 , which shows overall output current Isupply of the power supply over a period of time. During this time period Isupply  300  varies about a rms value Irms. It is assumed that the constant current source generates a current Iccs which is set to this rms value. The combination of the individual currents (I 1 , I 2 , I N ) generated by each modulated phase  200 ,  201 ,  205  together generate a current Imod. During the periods when the total required output current is greater than the rms value, shown as +ve in  FIG. 4 , the total output current is:
 
 I   supply   =I   ccs   +I   mod  
 
i.e. the modulated phase(s) only supply a current which is the difference between the output of the constant current supply and the required value.
 
Similarly, during the periods when the total output current is less than the rms value, shown as −ve in  FIG. 4 , the total required output current is:
 
 I   supply   =I   ccs   −I   mod  
 
i.e. the excess current, amounting to the difference between the output of the constant current source and the required output value is sunk by the modulated phase(s) and is returned to the supply. The primary source of power losses in the switching devices  211 ,  212  of the modulator phases  200 ,  201 ,  205  are resistive power losses between the drain and source of the devices during the time that the devices are switched on (R ds-on ). The resistive power losses are governed by the relationship I 2 .R ds     —     on . Since a smaller current is now passing through the switching devices in the phases  200 ,  201 ,  205  the overall power dissipation in the power switching devices is significantly reduced. This has also been found to reduce the operating temperature of the devices, which further reduces their operating resistance (R ds     —     on ) and the associated power dissipation.  FIG. 5  shows a graph which compares the efficiency of a conventional power supply having only modulated phases, with a power supply having a constant current source in the manner just described. This graph does not take into account the effects of temperature, which would further improve the efficiency of the power supply using a constant current source. The trace “mod+100% eff CCS” is a baseline efficiency for a system with an ideal (but not feasible) 100% efficient current source and the trace “mod+95% eff CCS” is an efficiency contour for a system with a feasible 95% efficient current source.
 
     One way of achieving a current source is by using a switched mode power supply.  FIG. 6  shows an example form of switched mode power supply (SMPS) which is suitable for use as a current source. In a similar manner to one of the phases  200 , the current source  220  includes a power switching device  221  such as a power switching FET which is placed in series with a rectifier diode  223  or a synchronous rectifier switch between ground and a supply rail Vsupply. The FET  221  is driven by a drive circuit  222  which receives a control signal CTRL_CCS from a PWM control unit  228 . Although shown as part of the current source  220 , the control unit  228  can form part of the overall controller  230  of the power supply. A current sensing loop comprises a current sensing resistor  224  placed in series with the output and a differential amplifier  226  which senses the voltage across the sensing resistor  224 . The sensed voltage at  226  is fed to the PWM control unit  228 . The PWM control unit  228  adjusts the width of the PWM control signal CTRL_CCS according to the sensed current so as to maintain the output current at a desired value. 
     The switching device(s)  221  used in the SMPS usually operate in the frequency range of 10-100 kHz which allows the use of switching devices having an on resistance in the range 1-10 mΩ. In contrast, in wideband RF applications the switching devices in the modulator phases  200 ,  201 ,  205  are commonly required to operate at a switching speed of &gt;10 MHz, which requires specialised low capacitance switching devices having an on resistance of around 1Ω. It can be seen that the use of a SMPS with switching devices having a lower resistance is more efficient than operating modulated phases with higher resistance switching devices. The reduced peak current flowing in the switching devices of the modulated phase allows those devices to operate in a region where their transfer function is more linear. Also, the reduction in junction temperature reduces the value of R ds     —     on , which further reduces resistive losses. It has been found that a reduction in junction temperature of 50° C. can reduce R ds     —     on  by 20%. The current source shown in  FIG. 6  is a switching current source, i.e. it uses a switching device. An alternative form of current source is a linear type although linear current sources are generally only efficient over a narrow range of current. For an application where the output power can take a range of possible values (e.g. a range of 21 dB in the case of a power amplifier for a base station) a switch-mode current source offers superior power efficiency. 
     It will be well understood that the functions of the control stage  230  can be implemented by software which is executed by a processor, by hardware such as a FPGA or dedicated integrated circuit, or a combination of these. 
     The techniques described herein are applicable to the control of modulated power supplies used in a wide range of applications. One particularly suitable application is a base station of a wireless communications system which processes wideband signals such as CDMA, wideband CDMA (W-CDMA) and Orthogonal Frequency Division Multiplexed (OFDM). 
       FIG. 7  schematically shows a base station for a wireless communications system, in which the invention can be applied. The baseband section of the base station BTS includes a core switch CCM  70 , an interface  71  to the operator&#39;s network  73  and a plurality of signal processing units CEM 1 , CEM 2 , CEM 3 . Signals in Packet Data Format including user messages and control signals may be provided on a connection  72  between the network  73  and the BTS, the signals being received at the interface  71  and passed from there to the core switch CCM  70 . The core switch  70  is responsible for controlling the complete operation of the transmission and reception of signals to and from the antennas  78  and to and from the signal processing units CEM 1 , CEM 2 , CEM 3  and the interface  71 . The signal processing units undertake baseband signal processing. The core switch CCM  70  is connected  74  to a transceiver unit TRM  75 . Transceiver unit TRM  75  performs digital to analog conversion and up-conversion to RF for signals to be transmitted, and performs down-conversion from RF and analog-to-digital conversion on received signals. The arrangement shown has three sectors: α, β and γ. In a typical arrangement, different signals will be transmitted in each sector α, β, γ, e.g. in sector α a signal from a transmit unit in TRM  75  is amplified by power amplifier  100 , passed through duplexer  77 - 2  and transmitted from antenna  78 - 2 . As previously described with respect to  FIG. 1 , an envelope detector  105  receives the signal which is to be transmitted and detects the envelope of it. The envelope signal forms a modulating signal for the modulated power supply  110 . The resulting output from the modulated power supply forms the power supply to the power amplifier  100  such that the power supply tracks the envelope of the signal which is to be transmitted. 
     The invention is not limited to the embodiments described herein, which may be modified or varied without departing from the scope of the invention.