Controllable circuit

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.

CLAIM OF PRIORITY

This patent application claims priority to European Patent Application serial number 07 021 329.3 filed on Oct. 31, 2007.

FIELD OF THE INVENTION

This invention relates to control circuits, and in particular to switch-mode power circuits.

RELATED ART

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.

There is a general need to improve switching power circuits for the reasons outlined above.

SUMMARY OF THE INVENTION

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.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustration of an example of a switch-mode power circuit comprising, as a controllable element, a controllable switch1that switches a current through a load2dependant on a control signal p supplied to the switch1by a control unit3connected to the switch1. The switch1may 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 switches1,100, for example, configured in a half bridge or a full bridge arrangement. The load2, 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 unit3, 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 unit3, forms an input signal illustrated in equation 1:
r=a(y−x)
where x is a modulation signal and y is a rectangular signal that is supplied to the load2and 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 switch100switching a load200. In a further example, where the switches1,100are configured in a bridge circuit, the switch100may also switch the load2.

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.

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:
p=1 ifr<car1 orr<car2
p=0 else
p=1 ifr<car1 &r<car2
p=0 else
where p is the bit-stream output, r is the reference signal and car1and car2are 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°.

FIG. 2is 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.

FIG. 3is a block diagram illustration of an example of an ORed MCM arrangement using comparators4,5as modulators and standard diodes6,7operated as wired OR gates. The comparators4,5each 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 car1, car2, respectively. The signal r may have any kind of waveform and signals car1, car2may be, for example, sinusoidal signals. The outputs are OR wired by diodes6,7such that the cathodes of the diodes6,7are connected together forming an output for the control signal p. The example inFIG. 4differs from the example inFIG. 3in that the diodes6,7have opposite polarities (i.e., the anodes are connected together forming an AND gate) and that the carrier signals car1, car2have a triangular waveform. Where, in the examples inFIGS. 3 and 4, the voltage drop across the diodes6,7is disadvantageous, the signal can be retriggered with a driver8(e.g., a Schmitt trigger, inverter, comparator, amplifier, etc.) connected downstream of the OR gate or the AND gate established by diodes6and7. Pull-up or pull-down resistors, after the diode logic, may be applied dependent on the input attributes of the following stage.

FIG. 5illustrates the results of a simulation using models of the particular physical components which match the calculation results illustrated inFIG. 2. The signal was retriggered in this simulation as shown inFIGS. 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.

FIG. 6illustrates 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 inFIG. 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.

FIGS. 8A to 8Fillustrate examples of waveforms and spectra in a single carrier modulation.FIG. 8Aillustrates the waveforms of signals r, p and y for a sinusoidal stimulation r.FIG. 8Cillustrates the spectrum of a waveform of signals p as illustrated inFIG. 8Aon a linear frequency scale.FIG. 8Eillustrates the spectrum of a waveform of signals p as illustrated inFIG. 8Aon a logarithmic scale.FIG. 8Billustrates the waveforms of signals r, p and y for two superimposed sinusoidal stimulations of r.FIG. 8Dillustrates the spectrum of a waveform p as illustrated inFIG. 8Bon a linear frequency scale.FIG. 8Fillustrates the spectrum of a waveform of signals p as illustrated inFIG. 8Bon 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.

FIGS. 9 and 10illustrate waveforms and spectra for ANDed and ORed multi carrier modulations, respectively. In the spectrum of the signal p that controls a transistor19, 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.

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 inFIG. 11. An MCM arrangement comprises comparators10,11as modulators and standard AND gates12,13,14and OR gates15,16. The comparators10,11each 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 car1, car2, respectively. The outputs are fed into an OR gate and into an AND gate by gates12and15.

The example inFIG. 11further comprises a sampling element9, having a clock input >, a data input D, an output Q1and an inverted output Q2. The data input D of the sample element9is supplied with a signal from the inverted output Q2of the sample element9. The inputs of AND gate13are connected to the outputs of the sampling element9and the AND gate12. The inputs of AND gate14are connected to the outputs of the sample element9and the OR gate15. The inputs of OR gate16that provides the load signal y are connected to the outputs of the AND gates13and14.

The sampling element9, 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.

FIG. 12illustrates examples of waveforms and spectra of the two-carrier modulation arrangement inFIG. 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.

FIG. 13illustrates 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 inFIG. 13differs from the arrangement illustrated inFIG. 11in that the sample element is substituted by a differentiating element17that is supplied with carrier car1(master carrier) and provides a differentiated signal to AND gate13and an inverted (by means of an inverter18), differentiated signal to AND gate14, via the inverter18. In other examples, instead of differentiating the carrier signal car1, a differentiated carrier signal may be used that is supplied to the comparator10via an integrator. This arrangement allows for a very high linearity while still lowering the complete out-of-band spectrum as illustrated inFIG. 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.

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.