Abstract:
A switch-mode power circuit comprises a controllable element and a control unit. The controllable element is configured to control a current in response to a control signal supplied to the controllable element. The control unit is connected to the controllable element and provides the control signal. The control unit comprises a first signal processing unit, a second signal processing unit, and a combiner unit. The first signal processing unit has an output and is supplied with a first carrier signal and an input signal. The second signal processing unit has an output and is supplied with a second carrier signal and the input signal. The combiner unit is connected to the first and second signal processing units combining the outputs of the first and the second signal processing units to form a signal representative of the control signal.

Description:
CLAIM OF PRIORITY 
       [0001]    This patent application claims priority to European Patent Application serial number 07 021 329.3 filed on Oct. 31, 2007. 
       FIELD OF THE INVENTION  
       [0002]    This invention relates to control circuits, and in particular to switch-mode power circuits. 
       RELATED ART  
       [0003]    Switch-mode power circuits generate a broad band spectrum due to their high speed signals. Such broad band spectral components can disturb other electronic equipment like receivers used in communication systems. Approaches to lower high frequency signal contents in switch-mode power circuits lead to extensive filter components and shielding. The filter components are bulky, expensive and dissipative due to the need to be designed for high voltage and current levels. The implementation of shielding is difficult as the mechanical efforts need to be combined with electrical isolation due to different electrical potentials on various conductive elements of power components. Consequently, compromises are made between good thermal design and good electrical design. 
         [0004]    There is a general need to improve switching power circuits for the reasons outlined above. 
       SUMMARY OF THE INVENTION 
       [0005]    A switch-mode power circuit comprises a controllable element and a control unit. The controllable element controls a current in response to a control signal provided by the control unit. The control unit comprises a first signal processing unit, a second signal processing unit, and a combiner unit. The first signal processing unit has an output and is supplied with a first carrier signal and an input signal. The second signal processing unit has an output and is supplied with a second carrier signal and the input signal. The combiner unit is connected to the first and second signal processing units combining the outputs of the first and the second signal processing units to form a signal representative of the control signal. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings: 
           [0007]      FIG. 1  is a block diagram illustration of a switch-mode power circuit; 
           [0008]      FIG. 2  is a diagram showing a comparison between a single carrier modulated signal and a multi carrier modulated signal; 
           [0009]      FIG. 3  is a block diagram illustration of an ORed multi carrier modulator arrangement; 
           [0010]      FIG. 4  is a block diagram illustration of an ANDed multi carrier modulator arrangement; 
           [0011]      FIG. 5  shows simulation results of ORed MCM with two carriers; 
           [0012]      FIG. 6  shows simulation results of ORed MCM with three carriers; 
           [0013]      FIG. 7  shows the statistical distribution of ones and zeros in OR and AND operations; 
           [0014]      FIGS. 8A-8F  show waveforms and spectra of a known single carrier modulation; 
           [0015]      FIG. 9  shows waveforms and spectra of an ORed multi carrier modulation; 
           [0016]      FIG. 10  shows waveforms and spectra of an ANDed multi carrier modulation; 
           [0017]      FIG. 11  is a block diagram illustration of a circuit arrangement that switches after each output pulse between ORed modulation and ANDed modulation; 
           [0018]      FIG. 12  shows waveforms and spectra of the two-carrier modulation arrangement of  FIG. 11 ; 
           [0019]      FIG. 13  is a block diagram illustration of a circuit arrangement that switches dependant on a master carrier; and 
           [0020]      FIG. 14  shows waveforms and spectra of the two-carrier modulation arrangement of  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  is a block diagram illustration of an example of a switch-mode power circuit comprising, as a controllable element, a controllable switch  1  that switches a current through a load  2  dependant on a control signal p supplied to the switch  1  by a control unit  3  connected to the switch  1 . The switch  1  may be, for example, a transistor such as a bipolar transistor, a MOS field effect transistor or other suitable transistors. In other examples, the control signal and an inverted control signal may control an array of switches  1 ,  100 , for example, configured in a half bridge or a full bridge arrangement. The load  2 , for example, may be a coil of a switched power supply, a voice coil of a loudspeaker, an ohmic load provided by a heating element, or other suitable switch-mode power circuits. The control unit  3 , for example, comprises, as a signal processing unit, a modulator arrangement. In one example, the modulator arrangement is configured as a frequency or pulse width modulator arrangement providing a signal p whose frequency or pulse width is dependant on a modulation signal r. The modulation signal r, supplied to the control unit  3 , forms an input signal illustrated in equation 1: 
         [0000]        r=a ( y−x ) 
         [0000]    where x is a modulation signal and y is a rectangular signal that is supplied to the load  2  and forms a carrier signal clocking the switch-mode power circuit. In other examples, the signal p or the inverted signal p is also supplied to the at least one additional switch  100  switching a load  200 . In a further example, where the switches  1 , 100  are configured in a bridge circuit, the switch  100  may also switch the load  2 . 
         [0022]    Conventional switch power circuits comprise modulator arrangements that use no carrier or only one carrier signal to be, for example, frequency modulated or pulse width modulated (PWM). These conventional switch power circuits create high frequency (HF) bands at multiples of the carrier frequency and their sidebands. Whereas, the disclosed arrangements use at least two carriers with different frequencies resulting in multiple HF bands, which in some examples may overlap. Such modulator arrangements are referred to as multi-carrier modulators (MCM) in the following description. The carriers are fed into different modulators and combined with a reference signal r. To drive a power switch, for example a transistor, the outputs of the several modulators are combined into a resulting bit stream. Examples of combinations of two or more modulators and their attributes are described below, whereby comparators serve as modulators. In examples having more than two modulators, the AND or OR gates may have more than two inputs. 
         [0023]    A simple decision rule may be an OR operation or an AND operation of the comparator outputs. In one example, the rules that determine whether the output of the modulator arrangement is low or high in the time domain are illustrated in equation 2 as follows: 
         [0000]        p= 1 if  r &lt;car1 or  r &lt;car2 
         [0000]      p=0 else 
         [0000]        p= 1 if  r &lt;car1 &amp;  r &lt;car2 
         [0000]      p=0 else 
         [0024]    where p is the bit-stream output, r is the reference signal and car 1  and car 2  are the two carrier signals. However, the arrangement is not limited to two carriers. In other examples, the directions of the inequality can be changed, resulting in a phase shift of 180°. 
         [0025]      FIG. 2  is a diagram illustrating a comparison between a single carrier modulated signal and a multi carrier modulated signal combined with an OR. The modulation signal, for example at a frequency f of 2 Hz, maintains its full amplitude A while the carriers and its sidebands have an amplitude half of A when using multi-carrier modulation (MCM) such as an ORed (i.e., combined by an OR operation) multi carrier modulation instead of a single-carrier modulation such as, for example, a known natural sampled, double side modulated pulse width modulation (NADD). In this example, the ORed MCM has twice as many peaks as the NADD. The intermodulation between the two carriers and their side bands define an additional spectral component. In some examples, the intermodulation is beneficial because energy may be put into a band where communication systems available in the market do not operate, for example, basebands of cellular phones have cut-off frequencies below this frequency range. The utilization of this out-of-band intermodulation (OIM) band can be adjusted by the distance of the two carriers in frequency and/or the bandwidth around them. 
         [0026]      FIG. 3  is a block diagram illustration of an example of an ORed MCM arrangement using comparators  4 ,  5  as modulators and standard diodes  6 ,  7  operated as wired OR gates. The comparators  4 ,  5  each have two inputs and an output, where the inputs of each comparator are supplied with the reference signal r and one of the carrier signals car 1 , car 2 , respectively. The signal r may have any kind of waveform and signals car 1 , car 2  may be, for example, sinusoidal signals. The outputs are OR wired by diodes  6 ,  7  such that the cathodes of the diodes  6 ,  7  are connected together forming an output for the control signal p. The example in  FIG. 4  differs from the example in  FIG. 3  in that the diodes  6 ,  7  have opposite polarities (i.e., the anodes are connected together forming an AND gate) and that the carrier signals car 1 , car 2  have a triangular waveform. Where, in the examples in  FIGS. 3 and 4 , the voltage drop across the diodes  6 ,  7  is disadvantageous, the signal can be retriggered with a driver  8  (e.g., a Schmitt trigger, inverter, comparator, amplifier, etc.) connected downstream of the OR gate or the AND gate established by diodes  6  and  7 . Pull-up or pull-down resistors, after the diode logic, may be applied dependent on the input attributes of the following stage. 
         [0027]      FIG. 5  illustrates the results of a simulation using models of the particular physical components which match the calculation results illustrated in  FIG. 2 . The signal was retriggered in this simulation as shown in  FIGS. 3 and 4 . The ORed and the ANDed MCMs may be extended to N carriers, where N is an integer number. For each additional carrier, the peaks of the fundamentals are lowered by a=1/N, while the number of peaks increase by N. 
         [0028]      FIG. 6  illustrates the behaviour of an example of a switch-mode power circuit when increasing the number of carriers to three. Both ANDed and ORed MCMs generate a direct current (DC) offset. The offset results from the statistical distribution of highs and lows in the two logical operations shown in the truth tables in  FIG. 7 . In the present example, a XOR logic operation has a 50% occurrence resulting in no DC offset. However, the XOR has 50% redundancy in its truth table due to its symmetry, where the symmetry results in a doubling of the frequency of the modulation signal at the output. This is for some applications less desirable. Both, ANDing and ORing MCMs have an adequate amount of second order total harmonic distortion (THD2nd). These modulator topologies are adequate, due to low complexity, for power supplies with low line and load regulation demands. Amplifiers with low distortion demands and high EMI requirements may also benefit from these modulation schemes. 
         [0029]      FIGS. 8A to 8F  illustrate examples of waveforms and spectra in a single carrier modulation.  FIG. 8A  illustrates the waveforms of signals r, p and y for a sinusoidal stimulation r.  FIG. 8C  illustrates the spectrum of a waveform of signals p as illustrated in  FIG. 8A  on a linear frequency scale.  FIG. 8E  illustrates the spectrum of a waveform of signals p as illustrated in  FIG. 8A  on a logarithmic scale.  FIG. 8B  illustrates the waveforms of signals r, p and y for two superimposed sinusoidal stimulations of r.  FIG. 8D  illustrates the spectrum of a waveform p as illustrated in  FIG. 8B  on a linear frequency scale.  FIG. 8F  illustrates the spectrum of a waveform of signals p as illustrated in  FIG. 8B  on a logarithmic scale. In some examples, it may be desirable that no spectral component occur other than the reference signal r supplied to the modulator in the audio band, for example between 20 Hz and 20 kHz. 
         [0030]      FIGS. 9 and 10  illustrate waveforms and spectra for ANDed and ORed multi carrier modulations, respectively. In the spectrum of the signal p that controls a transistor  19 , the frequency of the reference signal r has the same amplitude as in a single carrier system. However, the amplitude of higher frequency components is reduced by half. 
         [0031]    Another approach to cancel the undesired DC effects of ORing and ANDing, as described above, is to switch after each resulting pulse between the ORed PWM result and the ANDed PWM result. An example of a circuit arrangement implementing the aforementioned technique is illustrated in  FIG. 11 . An MCM arrangement comprises comparators  10 ,  11  as modulators and standard AND gates  12 ,  13 ,  14  and OR gates  15 ,  16 . The comparators  10 ,  11  each include two inputs and an output where the inputs of each comparator are supplied with the reference signal r and one of the carrier signals car 1 , car 2 , respectively. The outputs are fed into an OR gate and into an AND gate by gates  12  and  15 . 
         [0032]    The example in  FIG. 11  further comprises a sampling element  9 , having a clock input &gt;, a data input D, an output Q 1  and an inverted output Q 2 . The data input D of the sample element  9  is supplied with a signal from the inverted output Q 2  of the sample element  9 . The inputs of AND gate  13  are connected to the outputs of the sampling element  9  and the AND gate  12 . The inputs of AND gate  14  are connected to the outputs of the sample element  9  and the OR gate  15 . The inputs of OR gate  16  that provides the load signal y are connected to the outputs of the AND gates  13  and  14 . 
         [0033]    The sampling element  9 , for example, a D-type flip-flop, alternately allows only one of the ORed PWM and the ANDed PWM to pass. For further lowering of high frequency peaks according to equation 2, ORing and ANDing PWMs can have multiple inputs coming from multiple comparators, where each input has its own carrier. In some examples, applying an OR-ANDed MCM causes the DC as well as the second order harmonic distortion THD2nd to vanish. However the OR-ANDed MCM may increase the third order harmonic distortion THD3rd. Additionally, the gain of such arrangement is higher than 1. 
         [0034]      FIG. 12  illustrates examples of waveforms and spectra of the two-carrier modulation arrangement in  FIG. 11 . The high frequency peaks are suppressed and the intermodulation frequency is reduced to zero. The side bands have a distance from the carrier that is equal to the frequency of the reference signal r. OR-ANDed MCM can be used for switch-mode power supplies as well as amplifiers or any other kind of PWM driven application because there is no DC component in the spectrum. The intermodulations in the audio band (e.g., 20 Hz-20 kHz) resulting from, for example, two sinusoidal carrier signals are reduced so that this type of switch-power circuit can also be used in frequency converter applications, for example, to convert a 400 Hz aircraft power supply to a 50 Hz or 60 Hz in audio applications. 
         [0035]      FIG. 13  illustrates an example of a Master-Slave (MS) MCM that includes a master carrier and one or more slave carriers where the slope of the master carrier determines whether the ORed MCM or the ANDed MCM is passed into the single bit stream forming the signal p. The arrangement in  FIG. 13  differs from the arrangement illustrated in  FIG. 11  in that the sample element is substituted by a differentiating element  17  that is supplied with carrier car 1  (master carrier) and provides a differentiated signal to AND gate  13  and an inverted (by means of an inverter  18 ), differentiated signal to AND gate  14 , via the inverter  18 . In other examples, instead of differentiating the carrier signal car 1 , a differentiated carrier signal may be used that is supplied to the comparator  10  via an integrator. This arrangement allows for a very high linearity while still lowering the complete out-of-band spectrum as illustrated in  FIG. 14 . This arrangement is capable of driving any kind of PWM system, including high-class switch-mode, for example audio, power amplifiers and power supplies. 
         [0036]    The multi-carrier modulation lowers the amplitude of the whole spectrum of a switch-mode modulator while keeping the in-band performance, uses more frequencies and is relatively easy to implement and therefore it saves cost, space, and complexity in switch-mode power circuits. 
         [0037]    Although various exemplary embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. It will be obvious to those reasonably skilled in the art that other components performing the same functions may be suitably substituted. Instead of comparators, for example, multipliers or differential amplifiers may be used as modulators. Further, the methods of the invention may be achieved in either all software implementations, using the appropriate processor instructions, or in hybrid implementations that utilize a combination of hardware logic and software logic to achieve the same results. Such modifications to the inventive concept are intended to be covered by the appended claims.