Abstract:
A method of generating a spread spectrum signal is disclosed. The method includes selecting a first pseudorandom slope for a modulation curve. A current frequency on the modulation curve is selected. An oscillating signal is produced at the current frequency for a respective time. The current frequency is set to a next frequency on the modulation curve. The steps of producing an oscillating frequency and setting the current frequency to a next frequency are repeated until the current frequency is a final frequency on the modulation curve.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to spread spectrum solutions for electromagnetic interference (EMI) in switched mode power supplies by utilization of spread spectrum switching frequencies. 
         [0002]    Switched mode power supplies, due to the nature of their switching behavior, introduce spectral spurs at their fundamental switching frequency and corresponding harmonics. These spurs are referred to as electromagnetic interference (EMI) and are regulated by the CISPR, FCC and other standards.  FIG. 1B  illustrates an EMI spur for a 2.2 MHz fundamental frequency at −24.2 dB. This is for a square wave with no spread spectrum switching.  FIG. 1A  illustrates a corresponding low frequency spur at −79.7 dB associated with harmonics of the 2.2 MHz fundamental frequency. Board level solutions to such interference typically utilize a combination of shielding or filter techniques to suppress EMI spurs in order to comply with regulations and design specifications. However, board level methods to mitigate EMI through layout techniques fail to address the source of noise generation. Best practice layout techniques can only mitigate the introduction of additional EMI noise by minimizing current conducting loop area, filtering, shielding, and use of ground planes. Furthermore, these methods increase system cost as well as solution size. 
         [0003]    Spread spectrum switching is a control technique to dither or change the switching frequency over a predetermined bandwidth. This reduces the EMI spur at the fundamental frequency by spreading the spectral energy over adjacent frequencies. There are two broad categories for spread spectrum algorithms. In the first category of fixed pattern dither algorithms, Apps Team, “A Solution for Peak EMI Reduction with Spread Spectrum Clock Generators,” ON Semiconductor Application Note AND9015, (July 2011) disclose triangular (FIG. 1) and Hershey Kiss (FIG. 2) spread spectrum profiles. Kumar et al., “Reducing EMI in Digital Systems Through Spread Spectrum Clock Generators,” Cypress Semiconductor Application Note published in EE Times Design, 1, 16 (February 2011) also compare triangular (FIG. 5a) and Hershey Kiss (FIG. 5b) spread spectrum profiles. Hardin et al., U.S. Pat. No. 5,488,627 discuss various fixed pattern, spread spectrum profiles. Details of the foregoing references are incorporated by reference herein in their entirety. Fixed pattern dither algorithms provide the best reduction of fundamental frequency spurs at the cost of introducing large spurs at the modulation frequency of their fixed patterns. This additional spectral noise is further exacerbated when optimizing for the CISPR/FCC specifications and results in modulation spurs being placed in the audio band around 9 kHz. This may cause an undesirable hum in switching power supplies operating in the MHz range. 
         [0004]      FIG. 3B  illustrates the spectral energy of a fixed pattern, triangular modulation curve of the prior art with a 2.2 MHz center frequency. The spectral energy is spread between 2.0 MHz and 2.4 MHz with a maximum of −36.6 dB.  FIG. 3A  illustrates a corresponding low frequency spectrum having a dominant EMI spur of −76.6 dB at 9.2 kHz. By way of comparison,  FIG. 4B  illustrates the spectral energy of a fixed pattern, Hershey Kiss modulation curve of the prior art with a 2.2 MHz center frequency. The spectral energy is spread between 2.0 MHz and 2.4 MHz with a maximum of −29.2 dB.  FIG. 4A  illustrates a corresponding low frequency spectrum having a dominant EMI spur of −77.9 dB at 1.0 kHz. Both triangular and Hershey Kiss modulation curves reduce EMI with spread spectrum switching. However, both produce corresponding low frequency EMI spurs in the audio band due to their respective modulation frequencies. 
         [0005]    In the second category of spread spectrum algorithms, Lin et al., “Reduction of Power Supply EMI Emission by Switching Frequency Modulation,” IEEE Trans. on Power Electronics, Vol. 9, No. 1, 132, 137 (January 1994) disclose a pseudorandom dither algorithm of spread spectrum switching. Details of the foregoing reference are incorporated by reference herein in their entirety. Pseudorandom variation of the fundamental frequency, however, provides inferior fundamental spur reduction but decreases other spectral content. This is illustrated at  FIG. 2B  where fundamental frequency spectral energy is spread between 1.8 MHz and 2.6 MHz. A large spur of −27.3 dB still exists at the 2.2 MHz center frequency. However, corresponding low frequency spurs of  FIG. 2A  have a maximum noise floor of −84.9 dB. 
         [0006]    The foregoing spread spectrum algorithms reduce EMI at the source through spread spectrum techniques. However, the present inventors have realized a need to further reduce EMI in switching power supplies. Accordingly, the preferred embodiments described below are directed toward improving upon the prior art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In a preferred embodiment of the present invention, a method of generating a spread spectrum signal is disclosed. The method includes selecting a first pseudorandom slope for a modulation curve and selecting a current frequency on the modulation curve. The method further includes producing an oscillating signal at the current frequency for a respective time and setting the current frequency to a next frequency on the modulation curve. The steps of producing the oscillating signal and setting the current frequency to a next frequency are repeated until the current frequency is a final frequency on the modulation curve. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0008]      FIGS. 1A and 1B  are spectral energy diagrams of respective low frequency and fundamental frequency energy distribution for a square wave with no spread spectrum switching; 
           [0009]      FIGS. 2A and 2B  are spectral energy diagrams of respective low frequency and fundamental frequency energy distribution for pseudorandom fundamental frequency spread spectrum switching; 
           [0010]      FIGS. 3A and 3B  are spectral energy diagrams of respective low frequency and fundamental frequency energy distribution for fixed pattern, triangular modulation with fundamental frequency spread spectrum switching; 
           [0011]      FIGS. 4A and 4B  are spectral energy diagrams of respective low frequency and fundamental frequency energy distribution for fixed pattern, Hershey Kiss modulation with fundamental frequency spread spectrum switching; 
           [0012]      FIG. 5  is a diagram illustrating a modulation curve having spread spectrum fundamental frequencies and a modulation frequency; 
           [0013]      FIG. 6A  is a simplified circuit diagram of a switched mode power supply according to the present invention; 
           [0014]      FIG. 6B  is a timing diagram showing operation of the switched mode power supply of  FIG. 6A ; 
           [0015]      FIG. 7A  is a diagram illustrating a simplified modulation curve according to the present invention; 
           [0016]      FIG. 7B  is a flow diagram according to one embodiment of the present invention that may be used to produce the modulation curve of  FIG. 7A ; 
           [0017]      FIG. 7C  is a flow diagram according to another embodiment of the present invention that may be used to produce the modulation curve of  FIG. 7A ; 
           [0018]      FIG. 8A  is a diagram illustrating a simplified modulation curve according to another embodiment of the present invention; 
           [0019]      FIG. 8B  is a flow diagram according to yet another embodiment of the present invention that may be used to produce the modulation curve of  FIG. 8A ; 
           [0020]      FIGS. 9A and 9B  are spectral energy diagrams of respective low frequency and fundamental frequency energy distribution for a triangular modulation curve with a pseudorandom modulation frequency according to the present invention; and 
           [0021]      FIG. 10  is a diagram of dither profiles showing fundamental frequency variation of four modulation curves as a percent of the center frequency with a pseudorandom modulation frequency. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The preferred embodiments of the present invention provide significant advantages over the prior art in EMI reduction of switching power supplies as will become evident from the following detailed description. 
         [0023]    Referring to  FIG. 5 , there is a diagram illustrating a Modulation Curve having spread spectrum Fundamental Frequencies and a Modulation Frequency. Terms defined in this diagram will be used in the following discussion to explain embodiments of the present invention. The diagram shows frequency variation or dithering as a function of time for one modulation cycle. The modulation cycle has a period tm and modulation frequency of 1/tm. The modulation curve illustrates fundamental frequency variations between minimum (fmin) and maximum (fmax) frequencies distributed about center frequency fc. A positive slope  500  of the modulation curve is defined by an incremental frequency increase (df) and a corresponding incremental time increase (dt) Likewise, a negative slope  502  of the modulation curve is defined by an incremental frequency decrease (−df) and a corresponding incremental time increase (dt). As will become evident in the following discussion, both positive and negative slopes are selected in a pseudorandom manner so that the modulation frequency is not constant. Incremental values of df and dt may vary along a single slope and with respect to different slopes. Moreover, although a triangular modulation curve is illustrated by way of example, other modulation curves may be employed according to various embodiments of the present invention. 
         [0024]    Turning now to  FIG. 6A , there is a simplified circuit diagram of a switched mode power supply according to the present invention. The circuit includes an error integrator circuit  600  to compare an output voltage Vout to a reference voltage Vref and produce control voltage Vcontrol. The circuit of  FIG. 6A  also includes a sawtooth ramp generator circuit  604  to produce a ramp voltage Vramp. Control voltage Vcontrol is applied to a positive input terminal of comparator  602 . Ramp voltage Vramp is applied to a negative input terminal of comparator  602 . Comparator  602  produces a pulse width modulation (PWM) compare signal that is applied to PWM generation control circuit  606 . A system clock signal (SYS_CLK) is also applied to PWM generation control circuit  606 . The system clock signal is divided by frequency divider  608  in response to dither modulation circuit  610  to produce various fundamental frequencies of a modulation curve. The PWM compare signal and the selected frequency are applied to PWM generation circuit  624  to control respective falling and rising edges of a PWM gate drive signal. The PWM gate drive signal is applied to gate drive circuit  612 . Gate drive circuit  612  drives p-channel transistor  614  to produce a high level signal (Vout) at output circuit  616 . Various operational functions of the switched mode power supply may be realized in hardware or in software by optional system processor  630 . 
         [0025]    In operation, error integrator circuit  600  receives reference voltage Vref, which is set to a desired output voltage. Output voltage Vout is also applied to error integrator circuit  600  and compared with Vref to produce control voltage Vcontrol. Vcontrol is an error voltage that corresponds to a difference between Vout and Vref. Sawtooth ramp generator circuit  604  operates synchronously with PWM generation control circuit  606  and produces a sawtooth ramp signal Vramp by charging programmable capacitor array  622  with programmable current source  620 . Programmable current source  620  and programmable capacitor array  622  control the rising edge of Vramp so that the duty cycle of the PWM compare signal is held constant for each variation of its period. This advantageously reduces output voltage ripple during spread spectrum operation. Reset circuit  618  periodically discharges capacitor  622  to produce the falling edge of Vramp. Comparator  602  compares control signal Vcontrol with ramp signal Vramp to produce a PWM compare signal that is applied to PWM generation control circuit  606 . 
         [0026]    Referring now to  FIG. 6B , operation of PWM generation control circuit  606  will be explained in detail. PWM generation control circuit  606  receives system clock signal SYS_CLK. Frequency divider circuit  608  divides SYS_CLK by N, where N is a positive integer, in response to control signals from dither modulation circuit  610  to produce a fundamental frequency signal. For example, if SYS_CLK is a 64 MHz signal, N may vary from 27 to 32 to produce fundamental frequencies of 2.37 MHz to 2.0 MHz, respectively. PWM generation circuit  624  receives the PWM compare signal from comparator  602  and the fundamental frequency signal from frequency divider circuit  608 . At time t 0 , the fundamental frequency signal goes high and causes the PWM gate drive signal to go high. Gate drive circuit  612  inverts the PWM gate drive signal to turn on p-channel transistor  614 . Output voltage Vout is correspondingly switched high and Vramp signal rises until reaching the Vcontrol signal causing the PWM compare signal goes high at time t 1 . The high level of PWM compare causes the PWM gate drive signal to fall, thereby turning off p-channel transistor  614  via gate drive circuit  612 . P-channel transistor  614  remains off until time t 2  when the fundamental frequency signal again goes high. The high level of the fundamental frequency signal causes the PWM gate drive signal to go high and turn on p-channel transistor  614 . Here output voltage Vout is switched high and the Vramp signal rises until reaching the Vcontrol signal where the PWM compare signal again goes high at time t 3 . The high level of the PWM compare signal causes the PWM gate drive signal to fall, thereby turning off p-channel transistor  614 . P-channel transistor  614  remains off until the next positive transition of the fundamental frequency signal. In this manner, the on time (t 0  to t 1 ) and off time (t 1  to t 2 ) are modulated to keep Vout approximately equal to Vref under variable load conditions. 
         [0027]    Referring now to  FIG. 7A , there is a diagram illustrating a simplified triangular modulation curve according to the present invention. The modulation curve shows a range of discrete fundamental frequencies as a function of time. There are four discrete frequencies from fmin to fmax with a center frequency fc. The modulation curve begins at frequency fmin at time t 0 , where the frequency increases by df to a second frequency  700 . The oscillation frequency  700  continues for duration dt 1  until time t 1 . The frequency again increases by df to a third frequency  702 . The oscillation frequency  702  continues for duration dt 1  until time t 2 . The frequency finally increases by df to a fourth frequency fmax  704 . The stepwise increase of the modulation curve from fmin to fmax has a positive slope  712  of df/dt 1 . The positive slope is determined by pseudorandom selection and, therefore, determines dt 1  for the modulation curve. At time t 3 , the frequency decreases by df to the third frequency  706 . The oscillation frequency  706  continues for duration dt 2  until time t 4 . The frequency again decreases by df to the second frequency  708 . The oscillation frequency  708  continues for duration dt 2  until time t 5 . The frequency finally decreases by df to the first frequency fmin  710  to complete the modulation curve having period  716 . The stepwise decrease of the modulation curve from fmax to fmin has a negative slope  714  of −df/dt 2 . The negative slope is also determined by pseudorandom selection separately from the positive slope and, therefore, determines dt 2  for the modulation curve. 
         [0028]    Referring next to  FIG. 7B , there is a flow diagram according to an embodiment of the present invention that may be used to produce the modulation curve of  FIG. 7A . The diagram begins at step  720  with inputs of minimum frequency fmin, maximum frequency fmax, and differential frequency df. At step  722  a pseudorandom slope (SLOPE) is selected. SLOPE preferably has a value between a minimum and maximum value. Then duration dt is set equal to df/SLOPE, and the current frequency is set to fmin. A cycle count variable (CYCLES) is set to dt*f at step  724 . This is a number of cycles of frequency f corresponding to duration dt. The PWM circuit ( FIG. 6A ) then oscillates for time dt at frequency f. At step  726 , the current frequency f is increased by df, corresponding to the increase from fmin to frequency  700  ( FIG. 7A ). Test  728  determines if the current frequency f is greater than fmax. If not, control transfers to step  724 . At step  724  a new cycle count (CYCLES) is calculated, since more cycles at frequency  700  correspond to time dt. The PWM circuit again oscillates for time dt at frequency f. At step  726 , the current frequency f is again increased by df, corresponding to the increase from frequency  700  to frequency  702  ( FIG. 7A ). Test  728  again determines if the current frequency f is greater than fmax. If not, the process repeats until test  728  determines the current frequency f is greater than fmax. Then control transfers to step  730  where another pseudorandom slope (SLOPE) is selected. Then duration dt is set equal to df/SLOPE, and the current frequency is set to fmax. A cycle count variable (CYCLES) is set to dt*f at step  732 . This is a number of cycles of frequency f corresponding to duration dt. The PWM circuit ( FIG. 6A ) then oscillates for time dt at frequency f. At step  734 , the current frequency f is decreased by df, corresponding to the decrease from fmax to frequency  706  ( FIG. 7A ). Test  736  determines if the current frequency f is less than fmin. If not, control transfers to step  732 . At step  732  a new cycle count (CYCLES) is calculated, since fewer cycles at frequency  706  correspond to time dt. The PWM circuit again oscillates for time dt at frequency f. At step  734 , the current frequency f is again decreased by df, corresponding to the decrease from frequency  706  to frequency  708  ( FIG. 7A ). Test  736  again determines if the current frequency f is less than fmin. If not, the process repeats until test  736  determines the current frequency f is less than fmin. Then control transfers to step  722  to repeat another pseudorandom positive slope of the modulation curve. This embodiment of the present invention recalculates the number of cycles at each step of the modulation curve for a transition between fmin and fmax. This advantageously produces a triangular modulation curve with a linear slope. The recalculation, however, requires additional computation. 
         [0029]    Computation such as pseudorandom slope and differential time determination at steps  722 - 724  and  730 - 732  may be accomplished by the optional system processor  630  ( FIG. 6A ). Fundamental frequency cycle counts may be accomplished in hardware by either PWM generation circuit  624  or dither modulation circuit  610  or in software by the system processor  630 . Other embodiments will be apparent to those of ordinary skill in the art having access to the instant specification. 
         [0030]      FIG. 7C  is a flow diagram according to another embodiment of the present invention that may be used to produce the modulation curve of  FIG. 7A . The diagram of  FIG. 7C  is the same as the diagram of  FIG. 7B  except that step  724  is replaced by step  725  and step  732  is replaced by step  733 . The embodiment of  FIG. 7C , therefore, does not require a cycle count of current frequency f to determine duration dt. Duration dt may be determined by a timed interrupt, a system clock, a sawtooth ramp generator, or any other suitable digital or analog method. Moreover, dt values may be preselected in a pseudorandom manner to produce nonsymmetrical rising and falling transitions of the modulation curve. 
         [0031]    Referring now to  FIG. 8A , there is a diagram illustrating a simplified triangular modulation curve according to the present invention. The modulation curve shows a range of discrete fundamental frequencies as a function of modulation pulse index, n. The modulation pulse index refers to the index of a specific pulse in the sequence of PWM pulses in a modulation curve. There are four discrete frequencies from fmin to fmax with a center frequency fc. The modulation curve begins at frequency fmin at modulation pulse index n 0 , where the frequency increases by df to a second frequency  800 . The oscillation frequency  800  continues for number of pulses dn 1  until modulation pulse index n 1 . In other words, dn 1  indicates the number of times oscillation frequency  800  is repeated before increasing by df to a third frequency  802 . The corresponding time duration spent oscillating at frequency  800 =t 1 =dn 1 *1/f_ 800 , where f_ 800  indicates the oscillation frequency  800 . The oscillation frequency  802  continues for number of pulses dn 1  until modulation pulse index n 2 . Then the frequency finally increases by df to a fourth frequency fmax  804 . The stepwise increase of the modulation curve from fmin to fmax has a positive slope  812  of df/dn 1 . The positive slope is determined by pseudorandom selection and, therefore, determines dn 1  for the modulation curve. At modulation pulse index n 3 , the frequency decreases by df to the third frequency  806 . The oscillation frequency  806  continues for a number of pulses dn 2  until modulation pulse index n 4 . The frequency again decreases by df to the second frequency  808 . The oscillation frequency  808  continues for a number of pulses dn 2  until modulation pulse index n 5 . At n 5  the frequency finally decreases by df to the first frequency fmin  810  to complete the modulation curve having total number of modulation pulses  816 . The stepwise decrease of the modulation curve from fmax to fmin has a negative slope  814  of -df/dn 2 . The negative slope is also determined by pseudorandom selection separately from the positive slope and, therefore, determines dn 2  for the modulation curve. 
         [0032]    Referring next to  FIG. 8B , there is a flow diagram according to yet another embodiment of the present invention that may be used to produce the modulation curve of  FIG. 8A . The diagram begins at step  820  with inputs of minimum frequency fmin, maximum frequency fmax, and differential frequency df. At step  822  a pseudorandom slope (SLOPE) is selected. SLOPE preferably has a value between a minimum and maximum value. Then number of modulation pulses dn is set equal to df/SLOPE, and the current frequency is set to fmin. A cycle count variable (CYCLES) is reset to 1 at step  824 . This is a number of integer oscillation cycles of frequency f corresponding to duration dt=dn*1/f=dn/f. The PWM circuit ( FIG. 6A ) then oscillates for dn oscillations at frequency f. At step  826 , the current frequency f is increased by df, corresponding to the increase from fmin to frequency  800  ( FIG. 8A ). Test  828  determines if the current frequency f is greater than fmax. If not, control transfers to step  824 . At step  824  cycle count variable (CYCLES) is reset to 1. The PWM circuit again oscillates for time dt=dn*1/f=dn/f at the updated frequency f. At step  826 , the current frequency f is again increased by df, corresponding to the increase from frequency  800  to frequency  802  ( FIG. 8A ). Test  828  again determines if the current frequency f is greater than fmax. If not, the process repeats until test  828  determines the current frequency f is greater than fmax. Then control transfers to step  830  where another pseudorandom slope (SLOPE) is selected. The number of modulation pulses dn is set equal to df/SLOPE, and the current frequency is set to fmax. A cycle count variable (CYCLES) is reset to 1 at step  832 . This is a number of integer oscillation cycles of frequency f corresponding to duration dt=dn*1/f=dn/f. The PWM circuit ( FIG. 6A ) then oscillates for time dt at frequency f. At step  834 , the current frequency f is decreased by df, corresponding to the decrease from fmax to frequency  806  ( FIG. 8A ). Test  836  determines if the current frequency f is less than fmin. If not, control transfers to step  832 . At step  832  cycle count variable (CYCLES) is reset to 1. The PWM circuit again oscillates for time dt=dn*1/f=dn/f at the updated frequency f. At step  834 , the current frequency f is again decreased by df, corresponding to the decrease from frequency  806  to frequency  808  ( FIG. 8A ). Test  836  again determines if the current frequency f is less than fmin. If not, the process repeats until test  836  determines the current frequency f is less than fmin. Then control transfers to step  822  to repeat another pseudorandom positive slope of the modulation curve. The embodiment of  FIG. 8B  is similar to the embodiments of  FIGS. 7B and 7C  except that the number of cycles at each step of the modulation curve for a transition between fmin and fmax remains constant. This advantageously eliminates the need to recalculate do for each fundamental frequency change. As a result, however, the slope of each transition between fmin and fmax is nonlinear. 
         [0033]    The foregoing embodiments of the present invention significantly reduce EMI by reducing low frequency noise spurs due to the modulation frequency while maintaining low EMI at the fundamental operating frequencies. This improvement is illustrated by comparing the diagrams of prior art  FIGS. 3A and 3B  with diagrams of respective  FIGS. 9A and 9B  according to the present invention.  FIG. 3A  illustrates a low frequency spectrum having a dominant EMI spur of −76.6 dB at 9.2 kHz. By way of comparison,  FIG. 9A  illustrates a low frequency spectrum having a dominant EMI spur of −83.2 dB at 7.4 kHz. This is a significant reduction of 6.6 dB of the peak low frequency noise spur.  FIG. 3B  illustrates the spectral energy of a fixed pattern, triangular modulation curve of the prior art with a 2.2 MHz center frequency. The spectral energy is spread between 2.0 MHz and 2.4 MHz with a maximum of −36.6 dB.  FIG. 9B  of the present invention illustrates a triangular modulation curve with pseudorandom modulation frequency. The maximum EMI spur is −34.0 dB at 2.3 MHz. The present invention, therefore, advantageously reduces the maximum low frequency spur by 6.6 dB. The corresponding increase of the maximum fundamental spur by 2.6 dB is less significant than the reduced EMI spur in the low frequency audio band. Moreover, this improvement is accomplished without the benefit of expensive solutions such as filtering and shielding. 
         [0034]      FIG. 10  is a diagram of a dither profile showing fundamental frequency variation of four modulation curves as a percent of the center frequency with a pseudorandom modulation frequency. Each positive or negative slope of each modulation curve is determined by pseudorandom selection between minimum and maximum slope values. Thus, low frequency spectral energy due to modulation frequency is advantageously distributed over a wider bandwidth with lower maximum energy. 
         [0035]    Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling within the inventive scope as defined by the following claims. For example, pseudorandom slope selection is not limited to triangular modulation curves and may be advantageously applied to any modulation curve to reduce the maximum magnitude of low frequency energy spurs. Moreover, pseudorandom slope selection may be accomplished by preselecting different values for dt ( FIGS. 7A-7C ) or for do ( FIGS. 8A-8B ) that provide adjacent nonsymmetrical rising and falling transitions of a modulation curve. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.