Abstract:
Using a combination of frequency dithering of a PWM counter and a variable time delay circuit yields improved PWM frequency resolution with realizable circuit components and clock operating frequencies. A controllable time delay circuit lengthens a PWM signal during the first PWM cycle. During the second PWM cycle, the PWM period is increased beyond the desired amount, but the delay is reduced during this second PWM cycle to achieve the correct (desired) PWM signal period. The dithering of the PWM signal period enables the time delay circuit to be “reset” so that an infinite delay circuit is not required. The time delay circuit provides short term (one cycle) frequency adjustment so that the resulting PWM cycle is not dithered and has a period at the desired frequency resolution.

Description:
RELATED PATENT APPLICATION 
     This application is continuation application of U.S. patent application Ser. No. 12/045,759 filed Mar. 11, 2008 now U.S. Pat. No. 7,714,626, which claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 60/946,810 filed Jun. 28, 2007, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to digital devices having digital pulse width modulation (PWM) capabilities, and more particularly, to digital devices having improved PWM frequency resolution. 
     BACKGROUND 
     Power conversion techniques using pulse width modulation (PWM) controllers typically vary the duty cycle, the phase, or the frequency of the PWM signal to control the flow of power in a power supply. Analog or digital PWM controllers may be used for PWM signal generation. Regardless of the PWM signal generation technique used, the general goal is to provide as much resolution as possible to minimize the output voltage ripple in the power supply. 
     For analog PWM signal generation, it is difficult to generate stable and consistent frequency PWM signals over temperature, process and when subjected to a range of ambient noise conditions. 
     The digital generation of PWM signals with high frequency resolution is very difficult. PWM signal generators typically use a digital counter with an associated comparator and PWM period register. The frequency resolution of this typical PWM circuit is determined by the frequency of the clock signal that drives the counter. To obtain one (1) nanosecond of frequency resolution, the counter must be clocked with a 1 GHz (Billion Hertz) signal. In many technologies, this high a frequency of operation is impossible to obtain, the required power consumption is too high, and/or the cost is prohibitive. 
     Most common power conversion controllers are implemented with analog circuitry that do not require high frequency clocks to achieve frequency resolution. It is know in the art of power supply design to increase the clock rate of the PWM generator counter to improve frequency resolution. This approach works well at lower frequencies for applications such as motor control but becomes impractical for higher frequency power conversion circuits. Very few companies have attempted to provide microcontrollers and/or digital signal processors with high resolution digital PWM generators to serve the power conversion market. Historically, the PWM parameter that was considered important was the duty cycle (PWM pulse width). Only recently, have “resonant mode” power conversion circuits become “fashionable” because of the desire to improve the power conversion efficiency. Resonant mode power conversion uses the PWM frequency to control the flow of power, instead of the PWM duty cycle. 
     SUMMARY 
     Therefore there is a need for higher resolution of frequency in PWM systems. According to the teachings of this disclosure, a circuit is used to control variation of both the PWM period and the PWM phase in a coordinated manner so as to achieve higher resolution of PWM frequency. 
     In PWM systems, frequency dithering is when a PWM generator operates at two distinct frequencies in an alternating manner. The resultant effect is the arithmetic average of the two chosen frequencies. However, use of frequency dithering in a resonant mode power conversion circuit will generate ripple currents and voltages in a process called “limit cycling.” This not desirable behavior in a PWM power system. 
     Phase shifting of a periodic signal involves changing the signal delay of a circuit as the signal is propagated through the circuit. If the delay is varied as the signal passes through the circuit, the signal will be distorted, e.g., stretched or compressed in time. Phase shifting a signal is similar to shifting a signal&#39;s frequency on a “short time” basis. However, to change a signal&#39;s frequency continuously requires an infinite delay circuit, an impossibility. 
     Therefore according to teachings of this disclosure, use of a combination of frequency dithering of a PWM counter and a variable delay circuit yields improved PWM frequency resolution with realizable circuit components and clock operating frequencies. For example, a controllable delay lengthens a PWM period during the first PWM cycle. During the second PWM cycle, the PWM period is increased beyond the desired amount, but the delay is reduced during this second PWM cycle to achieve the correct (desired) PWM signal period. Period is the reciprocal of frequency and both will be used herein interchangeably. The dithering of the PWM signal period enables the delay circuit to be “reset” so that an infinite delay circuit is not required. The delay element provides short term (one cycle) frequency adjustment so that the resulting PWM cycle is not dithered and has a period at the desired frequency resolution. 
     Another way to view this frequency/period resolution optimization, having a finer frequency control then what would normally be possible at a given clock frequency, is to consider that the least significant bits (LSBs) of the period value represent a stretching of the PWM cycle. A programmable delay circuit can add delay during a PWM cycle thus effectively lengthening the PWM cycle. But on succeeding PWM cycles, the delay circuit would need to continually add more delays thereto. This requirement becomes an impossibility after a few PWM cycles. An infinite delay element cannot be built. 
     After a number of stretched (delayed) PWM cycles, the PWM counter period value can be temporarily increased so that the PWM period is lengthened beyond what has been specified. To compensate for the overly long period, the delay circuit&#39;s delay can be reduced to adjust the PWM period to the specified value. This process allows the delay element to be “unwound” after it was previously “wound up.” 
     According to a specific example embodiment of this disclosure, an apparatus having improved pulse width modulation (PWM) frequency resolution comprises: a period register ( 302 ) for storing a pulse width modulation (PWM) period value; control logic ( 316 ) coupled to a least significant bit (LSB) of the period register ( 302 ); a plus one effective period adder ( 306 ) coupled to the period register ( 302 ) and having a carry-in input coupled to the control logic ( 316 ), wherein when the carry-in input is at a first logic level, one (1) is added to the PWM period value; a period comparator ( 308 ) coupled to the plus one effective period adder ( 306 ) and receiving the PWM period value when the carry-in input is at a second logic level and the PWM period value plus one when the carry-in input is at the first logic level; a clock counter ( 310 ) for counting clock pulses from a clock source, the clock counter ( 310 ) being coupled to the period comparator ( 308 ), wherein the period comparator ( 308 ) compares a clock count from the clock counter ( 310 ) with the PWM period value or the PWM period value plus one from the plus one effective period adder ( 306 ), whereby the period comparator ( 308 ) resets the clock counter ( 310 ) when the clock count is greater than or equal to the PWM period value or the PWM period value plus one; a duty cycle register ( 314 ) for storing a PWM duty cycle; a duty cycle comparator ( 312 ) coupled to the clock counter ( 310 ) and the duty cycle register ( 314 ), wherein the duty cycle comparator ( 312 ) compares the PWM duty cycle to the clock count and whenever the clock count is less than or equal to the PWM duty cycle a first logic level is generated from an output of the duty cycle comparator ( 312 ), otherwise a second logic level is generated therefrom, thereby generating a first PWM signal; a delay element ( 320 ) coupled to the output of the duty cycle comparator ( 312 ), wherein the delay element ( 320 ) delays the first PWM signal from the duty cycle comparator ( 312 ); a multiplexer ( 322 ) coupled to the delay element and the output of the duty cycle comparator, wherein either a delayed first PWM signal or non-delayed first PWM signal is selected to produce a second PWM signal from an output of the multiplexer ( 322 ); and a delay flip-flop ( 318 ) coupled to the control logic ( 316 ) and the multiplexer ( 322 ), wherein the delay flip-flop ( 318 ) states are controlled by the control logic ( 316 ) depending on whether the delayed first PWM signal or the non-delayed first PWM signal was last selected, and the delay flip-flop ( 316 ) generates a multiplexer control signal for selection of either the delayed first PWM signal or the non-delayed first PWM signal, whereby the second PWM signal has a PWM period resolution substantially equal to a time delay of the delay element ( 320 ). 
     According to another specific example embodiment of this disclosure, an apparatus having improved pulse width modulation (PWM) frequency resolution comprises: a period register ( 402 ) for storing a pulse width modulation (PWM) period value, the period register having m bits of which n bits are least significant; a delay adder ( 430 ) coupled to the n least significant bits of the period register ( 402   b ); a delay register ( 432 ) coupled to the delay adder ( 430 ), wherein a value from the delay adder ( 430 ) is stored in the delay register ( 432 ) each time the delay register ( 432 ) is clocked; a plus one effective period adder ( 306 ) coupled to the period register ( 402   a ) and a carry-out from the delay adder ( 430 ), wherein when the carry-out from the delay adder ( 430 ) is at a first logic level, one (1) is added to the PWM period value from the period register ( 402   a ); a period comparator ( 308 ) coupled to the plus one effective period adder ( 306 ) and receiving the PWM period value when the carry-out from the delay adder ( 430 ) is at a second logic level and the PWM period value plus one when the carry-out from the delay adder ( 430 ) is at the first logic level; a clock counter ( 310 ) for counting clock pulses from a clock source, the counter ( 310 ) being coupled to the period comparator ( 308 ), wherein the period comparator ( 308 ) compares a clock count from the clock counter ( 310 ) with the PWM period value or the PWM period value plus one from the plus one effective period adder ( 306 ), whereby the period comparator ( 308 ) resets the clock counter ( 310 ) and clocks the delay register ( 432 ) when the clock count is greater than or equal to the PWM period value or the PWM period value plus one; a duty cycle register ( 314 ) for storing a PWM duty cycle; a duty cycle comparator ( 312 ) coupled to the clock counter ( 310 ) and the duty cycle register ( 314 ), wherein the duty cycle comparator ( 312 ) compares the PWM duty cycle to the clock count and whenever the clock count is less than or equal to the PWM duty cycle a first logic level is generated from an output of the duty cycle comparator ( 312 ), otherwise a second logic level is generated therefrom, thereby generating a first PWM signal; a plurality of multi-tap delay elements ( 420 ), the plurality of multi-tap delay elements ( 420 ) are coupled in series to produce a plurality of time delays of the first PWM signal from the duty cycle comparator ( 312 ), a first one of the plurality of multi-tap delay elements ( 420 ) being coupled to the output of the duty cycle comparator ( 312 ); and a multiplexer ( 322 ) coupled to the plurality of multi-tap delay elements ( 420 ), the output of the duty cycle comparator ( 312 ) and an output of the delay adder ( 430 ), wherein the output of the delay adder ( 430 ) controls selection of which one of the plurality of multi-tap delay elements ( 420 ) is coupled to an output of the multiplexer ( 322 ) to produce a second PWM signal, whereby the second PWM signal has a PWM period resolution substantially equal to a time delay of a single one of the plurality of multi-tap delay elements ( 420 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a schematic block diagram of an electronic system powered by a switch mode power supply; 
         FIG. 2  illustrates a schematic block diagram of a switch mode power supply; 
         FIG. 3  illustrates a schematic block diagram of a digital PWM generator having a single bit delay, according to a specific example embodiment of this disclosure; 
         FIG. 4  illustrates a schematic block diagram of a digital PWM generator having a selectable multi-bit delay, according to another specific example embodiment of this disclosure; and 
         FIG. 5  illustrates schematic timing diagrams of course and fine frequency resolutions of PWM waveforms, according to teachings of this disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring to  FIG. 1 , depicted is a schematic block diagram of an electronic system powered by a switch mode power supply. An electronic system, generally represented by the numeral  100 , may comprise a power supply, e.g., switch mode power supply  102  that may supply operating voltages and currents to electronic circuits  104  of the electronic system  100 . The switch mode power supply  102  may convert a power source voltage  106 , e.g., 120 VAC, 48 VDC, etc., to all required operating voltages used by the electronic circuits  104 . 
     Referring to  FIG. 2 , depicted is a schematic block diagram of a switch mode power supply. The switch mode power supply  102  may comprise a PWM control circuit  208  and a power charging circuit  210 . The power charging circuit  210  is controlled by the PWM control circuit  208 . The PWM control circuit  208  has clock circuits that control frequency of the PWM signal(s). The power charging circuit  210  comprises at least one power switching element (not shown), e.g., power transistor, power field effect transistor, etc. 
     Referring to  FIG. 3 , depicted is a schematic block diagram of a digital PWM generator having a single bit delay, according to a specific example embodiment of this disclosure. The terms “period” and “frequency” are used interchangeably herein since period and frequency are the inverse of each other, if the period resolution is improved then the frequency resolution is similarly improved. The digital PWM generator  208  may comprise a period register  302 , a plus one (+1) effective period adder  306 , a period comparator  308 , a counter  310 , a duty cycle comparator  312 , a duty cycle register  314 , control logic  316 , a delay flip-flop  318 , a delay element  320  and a multiplexer  322 . 
     According to teachings of this disclosure, the new, novel and nonobvious components of the aforementioned digital PWM generator  208  may include the plus one (+1) effective period adder  306 , the control logic  316 , the delay element  320  with its associated multiplexer  322 , and the delay flip-flop  318 . The frequency/period resolution is improved without increasing clock frequency by an “LSB” bit in a portion of the period register  302   b . If the LSB bit of the period register  302   b  is nonzero, the control logic  316  sets the delay flip-flop  318  to select the PWM path that includes the delay element  320  following deassertion of an active PWM pulse (duty cycle). This action effectively stretches the PWM signal  324 . On the next PWM cycle that has the LSB bit of the period register  302   b  set, the plus one (+1) effective period adder  306  will increase the PWM period specified by the PWM counter  310  by one count. This action will stretch the PWM signal too much for the desired PWM period, but then the PWM signal period is compensated by selecting the PWM output path via the multiplexer  322  that does not include the delay element  320 . The action of adding one more count to the value of the counter  310  by increasing the value of the period register by one, allows the delay element  320  to be removed from the signal path, thus allowing the process to be continuously repeated for the following PWM cycles. 
     The programmed PWM period may be constantly changing. Therefore, the PWM period/frequency resolution enhancement circuit, according to teachings of this disclosure, has to remember previous logic states to insure that the delay element  320  does not “saturate” and that the period adjustment logic is appropriately applied. The following table indicates what occurs in the operation of the circuit disclosed in  FIG. 3 : 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Current 
                   
                 Next 
                   
               
             
          
           
               
                   
                 DLY 
                 LSB 
                 DLY 
                 ADD1 
                 State 
               
               
                   
                   
               
               
                   
                 0 
                 0 
                 0 
                 0 
                 Initial state 
               
               
                   
                 0 
                 1 
                 1 
                 0 
                 First state 
               
               
                   
                 1 
                 0 
                 1 
                 0 
                 Hold state 
               
               
                   
                 1 
                 1 
                 0 
                 1 
                 Second state 
               
               
                   
                   
               
               
                   
                 Where DLY is the delay state, i.e., 1 = delay and 0 = no delay. 
               
               
                   
                 The LSB is the least significant bit of the desired period stored in the period register 302 that may change at any time. 
               
               
                   
                 When ADD1 = 1 the effective counter period is increased by one (1) increment determined by the delay element 320. 
               
             
          
         
       
     
     Referring to  FIG. 4 , depicted is a schematic block diagram of a digital PWM generator having a selectable multi-bit delay, according to another specific example embodiment of this disclosure. The digital PWM generator  208   a  may comprise a period register  402 , a plus one (+1) effective period adder  306 , a period comparator  308 , a counter  310 , a duty cycle comparator  312 , a duty cycle register  314 , a multi-bit adder  430 , a delay register  432 , a plurality of multi-tap delay elements  420  and a multiplexer  322 . 
     According to teachings of this disclosure, the new, novel and nonobvious components of the aforementioned digital PWM generator  208   a  may include the plus one (+1) effective period adder  306 , the multi-bit adder  430 , the delay register  432 , and the plurality of multi-tap delay elements  420  with its associated multiplexer  322 . An additional n-bits of frequency/period resolution are in a least significant n-bit portion of the period register  402   b . When a nonzero value is in the least significant n-bit portion of the period register  402   b , the plurality of multi-tap delay elements  420  are used to introduce from one to n−1 time units of delay (selectable) to the PWM signal period following deassertion of an active PWM pulse (duty cycle). This action effectively stretches the PWM signal  324  by the selected one to n−1 time units. 
     For illustrative purposes without creating any limitation thereby, examples of the operation of the digital PWM generator  208   a  for n=3 bits is described hereinbelow. One of ordinary skill in the digital arts and having the benefit of this disclosure could ready understand the operation of the PWM generator  208   a  for any value of n. The delay register  432  is, for this example, a three bit register (N=3). Associated with the delay register  432  is, for this example, a three bit adder  430 . At the start of every PWM cycle, the adder  430  adds the contents of the delay register  432  with the least significant three bits of the period register  402   b . The carry-out signal  440  from the adder  430  is coupled to the carry-in input of the plus one (+1) effective period adder  306 . 
     The plus one (+1) effective period adder  306  does not permanently alter the contents of the period register  402 . The output of the plus one (+1) effective period adder  306  provides the period value used by the time base (not shown) of the PWM generator  208   a . The process of the adder  430  generating carry-out signals  440  (co) to the carry-in input (ci) of the plus one (+1) effective period adder  306  is what dithers the PWM cycle period. The least significant three bits of the period register  402   b  determines the rate at which period dither occurs. Thus if the least significant three bits of the period register  402   b  are all zeros, the PWM period will not be dithered. If the least significant three bits of the period register  402   b  are equal to 001 then the dither operation will occur once every eight PWM cycles. And if the least significant three bits of the period register  402   b  are equal to 100 then the dither operation will occur once every two PWM cycles. 
     While the carry-out (co) of the adder  430  controls the PWM period dithering, the three bit contents (signal bus  438 ) of the delay register  432  controls the delay selected from the plurality of multi-tap delay elements  420 . However, updating the selection of the plurality of multi-tap delay elements  420  (through the multiplexer  322 ) preferably occurs during the end of the PWM cycle so as to minimize distortion of the PWM signal  324 . 
     Control of the plus one (+1) effective period adder  306  via the carry-out (co) of the adder  430  may occur substantially at any time during the PWM cycle because the change to the PWM period is effective at the end of the PWM cycle via the period comparison circuitry (period comparator  308 ). 
     It is desirable to have the period register  402  and the duty cycle register  314  updated at the end of the PWM cycle so as to permit the PWM application, e.g., user, to update the PWM frequency (period) during the PWM cycle and the circuit logic of the PWM generator  208   a  respond as fast as possible to minimize control loop latency. 
     The delay register  432  is clocked at substantially the same time the associated time base counter  310  rolls over to zero. This occurs when the period comparator  308  resets the time base counter  310  at a cycle end. 
     An example of the operation of the circuit shown in  FIG. 4 , where m=15 and n=3, the period register  402  is 16-bits wide with the least significant 3 bits of the period register  402   b  is to be “processed” to yield the improved frequency resolution, according to the teachings of this disclosure. The required number of taps of the plurality of multi-tap delay elements  420  is equal to 2 n −1=7 delay taps. If the multi-tap delay element  420  is designed for a 1 nanosecond resolution, then the seven taps provide 1, 2, 3, 4, 5, 6 or 7 nanosecond delays that are selectable through inputs 1-7 of the multiplexer  322 . The multiplexer  322  also has an input (0) coupled to no delay tap, e.g., the input of the first one of the plurality of multi-tap delay elements  420  that receives the PWM signal from the output of the duty cycle comparator  312 . 
     The adder  430  and its associated delay register  432  are also n-bits wide and add and store numbers with values from 0 to 2 n −1. Whenever the addition of the lower n-bits of the period register  402   b  and the delay register  432  yields a carry-out signal  440  (overflow) of the adder  430 , the main dither operation is performed via the plus one (+1) effective period adder  306 . The “resetting” of the multi-tap delay element  420  tap selection is automatic in the summation process of the multi-bit adder  430 . 
     For a specific first example, assume that the value in the lower n-bits of the period register  402   b  is 001 (decimal 1). The delay register  432  will contain the following decimal values: 0, 1, 2, 3, 4, 5, 6, 7. These same values represent the selected delay element tap. On the eighth cycle, the value in the delay register  432  is 0, and a carry out signal  440  is generated by the adder  430 . Notice that the tap selection is now back to zero. 
     For a specific second example, assume that the value in the lower n-bits of the period register  402   b  is 010 (decimal 2). The delay register  432  will contain the following decimal values: 0, 2, 4, 6. These same values represent the selected delay element tap. On the fourth cycle, the value in the delay register  432  is 0, and a carry out signal  440  is generated by the adder  430 . Notice that the tap selection is now back to zero. 
     For a specific third example, assume that the value in the lower n-bits of the period register  402   b  is 011 (decimal 3). The delay register  432  will contain the following decimal values: 0, 3, 6. These same values represent the selected delay element tap. On the third cycle, the value in the delay register  432  is (3×3−8=1), and a carry out signal  440  is generated by the adder  430 . Notice that the tap selection is now back to “1.” Every time the adder  430  generates a carry-out signal  440  which performs the frequency “dither”, the summed value in the delay register  432  is reduced by 2 n  or “8” in this example. This is a natural property of performing “modulo” arithmetic. 
     Referring to  FIG. 5 , depicted are schematic timing diagrams of course and fine frequency resolutions of PWM waveforms, according to teachings of this disclosure. The waveforms of  FIG. 5(   a ) show how phase shifting affects PWM cycle frequency. When the waveform  550  has a phase delay of 2 time units, e.g., nanoseconds, added thereto, the resultant waveform  552  is shifted by 2 time units. Conversely, when phase delay of the waveform  550  is decreased by 2 time units, the resultant waveform  554  results. The waveforms of  FIG. 5(   b ) show a PWM counter period adjustable by 4 time units. Waveform  558  can only be changed from the waveform  556  by increments of 4 time units which is the frequency/period resolution of the clock to the counter  310  (frequency dithering). 
     The waveforms of  FIG. 5(   c ) show a PWM counter period adjustable by 2 time units, according to the teachings of this disclosure, by frequency dithering, and appropriate addition and removal of time delay to the PWM output from the duty cycle comparator  312  (e.g., by the delay element  320  and the multiplexer  322 ). During a first period, a 2 time unit delay is added to waveform  560  to produce the first cycle of the PWM output waveform  562 . During a second period the 2 time unit delay is removed and the waveform  560  is dithered by +4 time units to produce the second cycle of the PWM output waveform  562 . Since the clock frequency resolution is only 4 time units, by removing the 2 time unit delay an effective period/frequency shift of 2 time units is obtained. 
     This sequence of adding delay to a first PWM period value, then dithering to a longer time period second PWM period value without delay, then repeating this sequence over and over again allows finer frequency resolution of the PWM signal output  324  with a lower clock frequency. The n-bit embodiment of  FIG. 4  allows even finer frequency resolution that is only limited by the number of least significant bits (n) of the period register portion  402   b  and the time unit resolution of the multi-tap delay element  420 . 
     While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.