Abstract:
In a digital pulse width modulation generator unit, a phase register is coupled to the clocked counter providing the generator unit time base. In response to a control signal, the contents of the phase register over-write the present counter, thereby changing the phase of pulse width modulated generator output signal. When a plurality of pulse width modulated generator units, the phases of the units can be controlled relative to a reference generator. The contents of the phase register can be altered by hardware or by software.

Description:
This application claims priority under 35 USC §119(e)(1) of Provisional Application No. 60/555,891 (TI-38163PS) filed Mar. 24, 2004. 

   BACKGROUND OF THE INVENTION  
   1. Field of the Invention 
   This invention relates generally to data processing systems and, more particularly, to the regulation of power in integrated circuits using pulse width modulation (PWM) techniques. 
   2. Background of the Invention 
   Pulse width modulation techniques are an essential part of the control of many of the power related systems found in both commercial and industrial equipment. The main application areas include digital motor control, digital switch mode power supply control, uninterruptible (UPS) power supplies, and other forms of power conversion. 
   Although duty cycle control adjustment is one of the most commonly used PWM control methods, phase adjustment between a plurality of PWM channels is important in many power supply applications. The PWM phase relationship is important in two power supply categories, multi-phase power applications and resonant switch full bridge applications. 
   In the multi-phase power applications, a constant phase relationship is established between PWM channels prior to operation of the power stage. Power delivery is then controlled by duty cycle adjustment. The ability to be able to dynamically reconfigure a number of phases (depending on load and other parameters) becomes increasingly important. The phase relationship must be re-configured on-the-fly depending on system conditions. 
   With respect to the resonant switched full bridge applications the duty cycle remains essentially constant, while the power delivery is controlled entirely by the phase relationship between PWM channels. The phase adjustment occurs at very high up-date rates which in many cases can equal the PWM switching frequency. 
   Referring to  FIG. 1 , a PWM generator according to the prior art is shown. Period register  11  applies an output signal to a first input terminal of comparator  14 , while counter  12  applies output signals to a second input terminal of comparator  14  and to a first input terminal of comparator  15 . The compare register  13  applies output signals to a second input terminal of comparator  15 . The output signal of comparator  14  is applied to the clear terminal of Q flip-flop  16  and to the reset terminal of counter  12 , while the output terminal of comparator  15  is applied to the set terminal of Q flip-flop  16 . The system clock signal is applied to the clock terminal of counter  12 . The Q terminal of Q flip-flop  16  provides the PWM signal, i.e., the signal controlling the activation of the power components. 
   The operation of  FIG. 1  can be understood by reference to  FIG. 2 , the waveforms for circuit shown in  FIG. 1 . The counter  12  (providing the system time base) is activated by the system clock, typically at 20–100 MHz, and counts upward. The count value is transmitted over a 16 bit bus (CTR[0–15]) to the comparators. Comparator  14  provides a comparison to period register and determines the PWM period, i.e., the frequency of the operation. The output signal of comparator  14  reset the counter when the COUNTER VALUE equals the PERIOD VALUE. Comparator  15  provides a comparison of the counter  12  value with the compare register  13 . The output signal of comparator  15  sets the duty cycle when COUNTER VALUE equals COMPARE REGISTER VALUE. These two events drive CLEAR/SET logic to generate the PWM waveform shown in  FIG. 2 . The ramp waveform is a virtual one and only represents the upward counting vales of counter  12  over time. The Y-axis represents the counter  12  value while the x-axis represents time. 
   A need has therefore been felt for apparatus and an associated method having the feature that the phase adjustment in a PWM system is accomplished under software control. It would be yet another feature of the apparatus and associated method to provide a phase control register, the phase value (lag or lead) being written to the phase control register. It would be a still further feature of the present invention to incur relatively little software overhead when performing the phase adjustment. It would be a still further feature of the apparatus and associated method to update the phase at the end of each PWM cycle, thereby avoiding transitory glitches. It would be yet another feature of the apparatus and associated method to provide non-software dependent, reliable synchronizing mechanism that “re-locks” PWM channels to the programmed phase control register value every PWM cycle. It would be yet a further feature of the apparatus and associated method to support a friendly programmer interface/model with a microprocessor or DSP. It would be still further feature of the apparatus and associated method to support both up-count (asymmetrical) PWM applications and up/down count (symmetrical) PWM applications. 
   SUMMARY OF THE INVENTION  
   The foregoing and other features are accomplished, according the present invention, by coupling a phase register to the time base counter (i.e., counting system clock signals) in all the PWM generator units except for the generator unit providing the reference time base. The phase register is loaded periodically with a count that over-writes the count in the counter. In this manner, the phases of all the non-reference PWM generator output signals can be adjusted with respect to the reference PWM generator output signal. By locking the loading of the counter by the phase register to a selected point of the reference PWM generator output signal, the phase relationship can be reinforced. The present invention can be applied to multi-phase interleaved power stage and to zero voltage switched full bridge power stage. The contents of the phase register can be under hardware or software control. 
   Other features and advantages of present invention will be more clearly understood upon reading of the following description and the accompanying drawings and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is block diagram of a PWM generator according to the prior art. 
       FIG. 2  illustrates the waveforms provided by the PWM generator shown in  FIG. 1 . 
       FIG. 3  is a block diagram of a PWM generator with phase support according to the present invention. 
       FIG. 4  illustrates the waveforms of the PWM generator illustrated in  FIG. 3 . 
       FIG. 5A–B  illustrates a multi (3)-phase PWM generator with phase control according to the present invention. 
       FIG. 6  illustrates the timing diagrams and PWM waveforms of the PWM generator as shown in  FIG. 5 . 
       FIG. 7  illustrates the application of the three-phase PWM generator system in a three-phase interleaved power stage. 
       FIG. 8  illustrates the application of PWM generator circuits to a zero voltage switched full bridge system. 
       FIG. 9  is a two channel PWM generator with phase control capabilities according to the present invention. 
       FIG. 10  illustrates a timing diagram for PWM control of a zero voltage switched full bridge circuit according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT  
   1. Detailed Description of the Figures 
     FIGS. 1 and 2  have been described with respect to the related art. 
   Digital phase control can be provided to the PWM generator shown in  FIG. 1  by adding a mechanism to change asynchronously the contents of the counter  12  to a predetermined value. The contents of counter  12  can be changed by clocking (i.e., asynchronously loading) the contents of a register with a preset value (i.e., a phase value) thereby over-writing the counter  12  current value. Referring now to  FIG. 3 , a block diagram of a PWM generator with phase control support is shown.  FIG. 3  is similar to  FIG. 1 . However, to components illustrated in  FIG. 1  a phase register  21  applies signals to counter  12 . In addition, a PHASE LOAD signal can be applied to a ld phase terminal of counter  12 . 
   Referring to  FIG. 4 , the effect of a LOAD PHASE signal on the waveforms of the PWM generator of  FIG. 3  is shown. Of particular interest is that by overwriting the number in the counter  12 , the phase of the resulting PWM signal will be altered. The result arises from the arrival at the set point in advance of the unperturbed periodic signal generation. 
   Referring to  FIG. 5 , a 3-phase PWM generator system is shown according to the present invention. Three PWM generators,  10 ,  10 ′ and  10 ″, as configured as in  FIG. 3 , form the generator system. PWM generator  10  does not have a phase register associated therewith. PWM generator  10 ′ has phase register  21 ′ coupled to counter  12 ′. PWM generator  10 ″ has a phase register  21 ″ coupled to comparator  12 ″. The output signal of comparator  10  (PWM generator  10 ) is applied to the reset terminal of counter  12  (PWM generator  10 ), to the load phase terminal of counter  12 ′ (PWM generator  10 ′), and to the load phase terminal of counter  12 ″ (PWM generator  10 ″). 
   The operation of PWM generator system shown in  FIG. 5  can be understood by reference to  FIG. 6 . PWM generator  10  generates the reference phase signal PWM 1 . Therefore, no phase adjustment is needed. With respect to the PWM2 signal of PWM generator  10 ′, the phase of this signal is 240° out of phase with the PWM1 signal. With respect to the PWM3 signal, this signal is 120° out of phase with the PWM1 signal. Therefore, the phase register is loaded with a value that causes the count in the counter to be switched to a point on the PWM generator  10 ″ counter  12 ′ output signal that is 240° out of phase with PWM generator  10  counter  12  output signal of PWM generator. Similarly, the PWM generator  10 ″ counter  12 ″ output signal to be switched to a point that is 120° out of phase with the counter  10  output signal. Consequently, the resulting PWM2 signal will be 240° out of phase with the PWM1 signal and the PWM3 signal will be 120° out of phase with the PWM1 signal. Notice that the periodic load signal will reinforce the phase relationship each cycle. 
   Referring to  FIG. 7 , an implementation of a 3-phase interleaved power stage using the techniques of the present invention is shown. The complementary dead-band logic units  71 ,  71 ′ and  71 ″ receive PWM1 signals, PWM2 signals, and the PWM3 signals, respectively. The complementary dead-zone logic units  71 ,  71 ′ and  71 ″ apply signals to the gate terminals of FET transistors  72  and  73 , to the gate terminals of FET transistors  72 ′ and  73 ′, and to the gate terminals of FET transistors  72 ″ and  73 ″, respectively. Each FET transistor pair  72  and  73 ,  72 ′ and  73 ′, and  72 ″ and  73 ″ are coupled in series between a common terminal and a ground terminal. The common terminal of the  72 – 73  transistor pair, of the  72 ′– 73 ′ transistor pair and of the  72 ″– 73 ″ transistor pair are coupled through inductors  74 ,  74 ′ and  74 ″, respectively to the Vout terminal. The Vout terminal is coupled through capacitor  75  to the ground potential. 
   Referring to  FIG. 8 , a block diagram of a power stage for a zero voltage switched full bridge system (hereinafter referred to as a ZVSFB system) is illustrated. A V DC-bus  conductor is coupled through capacitor  81  to a common conductor, through FET transistor  82  to a first input terminal of transformer  86 , and through FET transistor  84  to a second input terminal of transform  86 . The first input terminal of transformer  86  is coupled through FET transistor  83  to the common conductor, while the second input terminal of transformer  86  is coupled through FET transistor  85  to the common conductor. A Vout terminal is coupled through capacitor  89  to a second common terminal and through inductor  89 ′ to the cathode of diode  88  and to the cathode of diode  87 . The anode terminal of diode  87  is coupled to a first output terminal of transformer  86  and the cathode terminal of diode  87 ′. The anode terminal of diode  88  is coupled to second output terminal of transformer  86  and to a cathode terminal of diode  88 ′. The anode terminal of diode  87 ′ and the anode terminal of diode  88 ′ are coupled to the second common conductor. The PWM1A signal is applied to the gate circuit of FET transistor  82 , while the PWM1B signal is applied to the FET transistor  83 . The PWM2A signal is applied to the gate of transistor  84 , while the PWM2B signal is applied to FET transistor  85 . 
   Referring to  FIG. 9 , the apparatus for generating the PWM1A signals, the PWM1B signals the PWM2A signals and the PWM2Bsignals. The circuit is similar to the circuit in  FIG. 5 , except that two instead of three PWM generators are used. In addition, the signals from the Q-gates  16  and  16 ′ are applied to complementary dead-band logic units  91  and  91 ′. From the output signal of Q-gate  16 , the complementary dead-band logic unit  91  generates the PWM1A signal and the PWM1B signal. From the output signal of the Q-gate  16 ′, the complementary dead-band logic unit provides the PWM2A signal and the PWM2B signals. 
   Referring to  FIG. 10 , the timing diagrams for the waveforms generated by PWM generator system of  FIG. 9  are illustrated. The saw-tooth waveforms illustrate the time bases for the two PWM generators provided by the counters associated with each PWM generator. These waveforms are out of phase (i.e. Φ 2  in the Figure) by an amount determined by the count in the phase register of the second PWM generator. The dead-band delays before the generation of the PWM signals is provided by the complementary dead-band logic units providing the output PWM signals. The complementary dead-band logic units are standard logic blocks which provide some gap time separation between PWMs controlling upper and lower power switches within a phase or leg. These logic units are commonly available in MOSFET driver chips. The first two power phases are illustrated in  FIG. 10 . 
   2. Operation of the Preferred Embodiment 
   The PWM generator system shown in  FIG. 4  was developed to be fabricated on a silicon chip together with a microprocessor or DSP core and any associated resources (memory, I/O, communications. etc.) needed to support the application. Consequently, all of the PWM generator&#39;s operating resources (period, duty cycle, phase Off-set, etc. can be made read/write addressable registers that are available to the microprocessor or DSP core under software control. 
   The typical 3-phase power system shown in  FIG. 5  can be extended to multi-phase power systems. This system can be made to change the number of PWM generator units/channels. In addition, the phase “offset” between each reference generator output signals and the other generator output signals must be altered when the number of generators is changed. This capability is useful in scalable power stage wherein additional generators can be brought on-line to respond to increasing power demands. Each time an additional generator is brought on-line. The phase relationship must be re-calculated and the phase registers updated. Subsequently, the phase loading apparatus will ensure all generators are correctly synchronized. 
   While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, the scope of the invention being defined by the following claims.