Abstract:
A method for generating a synchronous clock signal and a circuit ( 10 ) for implementing the method described. To generate the positive-going transition of the clock signal, the method generates a synchronization pulse train using a synchronization signal input. The method generates a second pulse train, having pulses offset in time from and later than those of the synchronization pulse train, to generate the negative-going transition of the clock signal. Because there is little loss in duty cycle, when the synchronous clock signal is input to a power factor correction (“PFC”) and pulse width modulation (“PWM”) controller circuit, the PFC and PWM controller is able to operate normally.

Description:
This application claims priority under 35 USC 119(e)(1) of provisional application No. 60/140,394 filed Jun. 22, 1999. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to the field of electronic clock generation circuits and methods. More specifically, this invention relates to the use of a method for generating a synchronous clock signal having an improved duty cycle. Such a clock signal may be used in a power factor correction and pulse width modulation controller used in computer display monitors. 
     BACKGROUND OF THE INVENTION 
     A PC video display monitor may have several resolutions because of various program requirements. An external synchronization signal is typically used to synchronize these resolutions. Each resolution typically has its own frequency, and the external synchronization signal pulse width may vary depending on its frequency. The frequency range is typically from 30 kHz to 95 kHz. 
     Power supplies in these display monitors typically use a technique called switched-mode pulse width modulation (“PWM”). Switched-mode PWM synchronizes the switching frequency of the power supply to the display monitor horizontal frequency in order to prevent picture interrupt and to address electromagnetic compatibility/electromagnetic interference (EMC/EMI) issues. Because the horizontal frequency is at least 30 kHz, the switching frequency of the power supply must be set to a frequency somewhat lower than 30 kHz. 
     With such a variety of frequencies and resolutions, it is difficult for one designing a power supply to use the external synchronization signal to synchronize the PWM controller power supply to the display monitor. One method of synchronizing these devices is to use the turns ratio method to extract from the yoke extra flyback voltage to generate a synchronous signal having an amplitude of about 30V. However, this signal requires a resistor/capacitor network to reduce the voltage level and improve its shape. 
     Another synchronization method requires a synchronization signal having a clock width of approximately 1 μs or more and an amplitude exceeding a certain voltage. Such a method is used by SGS-Thomson Microelectronics in its L4981A Power Factor Corrector (“PFC”) integrated circuit. The L4981A requires the external synchronization pulse width to be greater than 800 ns and the signal voltage to be greater than 3.5V. In that scenario, as shown in the timing diagram in FIG. 1, a ramp waveform used by the circuit to generate an output clock signal must be kept low until the external synchronization pulse ends, reducing the duty cycle of the output clock. FIG.  1 ( a ) shows the external synchronization signal, SYNC. FIG.  1 ( b ) shows a ramp voltage waveform V T  which has a slope controlled by an RC time constant. FIG.  1 ( c ) shows the output that is used in the PFC circuit. When SYNC is low (“asynchronous operation”), V T  ramps up. When V T  is greater than a preset low threshold voltage  118 , the output is high. As soon as V T  crosses a preset high threshold voltage  112 , the output goes low, causing V T  to ramp downward. When V T  crosses low threshold voltage  118 , the output goes high which reverses the ramp waveform and the cycle repeats. As shown in FIG. 1, the output has a duty cycle of approximately 87% between times t 0  and t 1 . 
     When the SYNC signal begins cycling (“synchronous operation”), a high SYNC signal causes V T  to ramp downward and the output to go low. However, unlike before when V T  crossed low threshold voltage  118  and caused the output to go high, when SYNC is high, the output remains low. This prior art synchronization method requires the SYNC signal to be high for at least time interval  124 , between times t 2  and t 4 , and V T  must be kept low during time interval  130 , from time t 3  to time t 4 , until the SYNC pulse ends. The L4981A requires that time interval  124  be at least 800 ns, and it is typically more. Because of this restriction, the duty cycle of the output may be reduced significantly, to, for example, approximately 60% between times t 4  and t 5 . 
     SUMMARY OF THE INVENTION 
     Therefore, a need has arisen for an improved synchronous clock generation circuit and method which allow a synchronization signal to be used without sacrificing duty cycle. 
     In accordance with the present invention, a circuit for generating a synchronous clock signal includes a synchronization circuit, a ramp and pulse generator, and a clock generator. A synchronization signal and the fed-back synchronous clock signal are inputs to the synchronization circuit which generates a first pulse train. The synchronous clock signal is also fed back to the ramp and pulse generator to generate a second pulse train which includes pulses which are offset in time from the first pulse train pulses. The first and second pulse trains are provided to the clock generator to generate the synchronous clock signal which has a duty cycle that is nearly equal to the duty cycle of an asynchronous clock signal generated in the absence of a synchronization signal. 
     Preferably, each pulse of the first pulse train generates a positive-going transition for the synchronous clock signal, and each pulse of the second pulse train generates a negative-going transition for the synchronous clock signal. 
     More specifically, in accordance with one embodiment of the present invention, the synchronization circuit may include an inverter, an S-R latch, and a NOR gate. The synchronization signal is an input to the inverter, and the inverter&#39;s output is connected to the R input of the latch. The synchronous clock signal is fed back to the S input of the latch, and the non-inverting output of the latch is connected to one input of the NOR gate. The other input of the NOR gate is the output of the inverter. The output of the NOR gate provides the synchronization circuit pulse train. 
     The ramp and pulse generator of this embodiment includes a ramp generator and a pulse generator. The ramp generator includes a bipolar junction transistor whose base is connected to the fed-back synchronous clock signal, whose emitter is grounded, and whose collector is connected to the pulse generator. A discharge capacitor is connected between the collector and ground, and a pull-up resistor is connected between the collector and a supply voltage. The pulse generator includes a voltage comparator, the inverting input of which is connected to the collector of the transistor of the ramp generator. The non-inverting input of the voltage comparator is coupled to a reference voltage. The voltage comparator generates the second pulse train. Preferably, the voltage across the capacitor does not have to remain at zero volts until a pulse from the synchronization signal ends. 
     The clock generator of this embodiment includes an S-R latch. The first pulse train is coupled to the S input of the latch, and the second pulse train is coupled to the R input. The output of the S-R latch is the synchronous clock signal. 
     In a preferred embodiment of the present invention, an external reference voltage may be input to the circuit. 
     The synchronous clock signal is preferably used in a second circuit that performs power factor correction and controls pulse width modulation. 
     Also in accordance with the present invention, a circuit for generating a clock signal capable of having synchronous and asynchronous portions includes a synchronization circuit, a ramp and pulse generator, and a clock generator. A synchronization signal and the fed-back clock signal are inputs to the synchronization circuit which generates a first pulse train. The clock signal is also fed back to the ramp and pulse generator to generate a second pulse train, which includes pulses which are offset in time from the first pulse train pulses, and a third pulse train, which includes pulses earlier in time than those of the second pulse train. The first, second, and third pulse trains are provided to the clock generator to generate the clock signal, as follows. Each pulse of the first pulse train generates a positive-going transition for the synchronous portion of the clock signal, each pulse of the third pulse train generates a positive-going transition for the asynchronous portion of the clock signal, and each pulse of the second pulse train generates a negative-going transition for both the synchronous and asynchronous clock signal portions. The clock signal has a duty cycle that is nearly equal to the duty cycle of the asynchronous clock signal portion generated in the absence of the synchronization signal. 
     In a preferred embodiment of the method of the present invention, the method for generating the synchronous clock signal includes generating a first pulse train using the synchronous clock signal and a synchronization signal, in which each pulse of the first pulse train generates a positive-going transition for the synchronous clock signal, and generating a second pulse train using the synchronous clock signal, in which the pulses of the second pulse train are offset in time from and later than pulses of the first pulse train, each pulse of the second pulse train generates a negative-going transition for the synchronous clock signal, and the duty cycle of the synchronous clock signal is nearly equal to the duty cycle of an asynchronous clock signal generated in the absence of the synchronization signal. 
     Preferably, the method also includes generating a ramp signal which is used to generate the second pulse train. Preferably, a portion of each period of the ramp signal does not have to remain at zero volts until an associated pulse from the first pulse train ends. More specifically, a reference voltage is used to generate the second pulse train. 
     Also in accordance with the present invention, a method for generating a clock signal capable of having synchronous and asynchronous portions includes generating a first pulse train using the clock signal and a synchronization signal, generating second and third pulse trains using the clock signal, and generating a ramp signal which is used to generate the third pulse train. Each pulse of the first pulse train generates a positive-going transition for the synchronous portion of the clock signal and each pulse of the second pulse train generates a positive-going transition for the asynchronous portion of the clock signal. The pulses of the third pulse train are offset in time from and later than pulses of the first and second pulse trains, and each pulse of the third pulse train generates a negative-going transition for the synchronous and asynchronous portions of the clock signal. In this method, a portion of each period of the ramp signal does not have to remain at zero volts until an associated pulse from the first pulse train ends. 
     Preferably, a reference voltage is used to generate the second pulse train. 
     The present invention provides various technical advantages. As used in a PFC/PWM controller circuit, one technical advantage is that a synchronous clock signal is generated that is not affected by the width of the synchronization signal. The duty cycle of the clock signal is not significantly decreased as compared to the asynchronous clock signal. Such an advantage retains the PWM turn-on duty cycle in the power supply applications. 
     Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: 
     FIG. 1 shows a timing diagram of a prior art clock generation circuit; 
     FIG. 2A is a block diagram of a clock generation circuit in accordance with one embodiment of the present invention; 
     FIG. 2B is a block diagram of the ramp and pulse generator of FIG. 2A; 
     FIG. 2C is a circuit diagram of the block diagram of FIG. 2A; 
     FIG. 3 illustrates a timing diagram of the circuit illustrated in FIGS. 2A and 2C; 
     FIG. 4 is a magnified view of the timing relationships occurring in the circuit illustrated in FIGS. 2A and 2C when no synchronization signal is present; and 
     FIG. 5 is a magnified view of the timing relationships occurring in the circuit illustrated in FIGS. 2A and 2C when a synchronization signal is present. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2A is a block diagram illustrating a clock generation circuit in accordance with one embodiment of the present invention. The circuit may have three inputs, a synchronization signal, SYNC, a high threshold voltage, and a low threshold voltage. The circuit generates a clock signal which is preferably inverted before being provided to a power factor correction/pulse width modulation controller. The clock signal is “asynchronous” when there is no synchronization signal, and “synchronous” when the clock signal is generated in response to a synchronization signal. 
     In a power supply application for a video display monitor, the synchronization signal varies based on the resolution and the frequency of the monitor. The typical frequency range of the synchronization signal is from 30 kHz to 100 kHz. The PFC/PWM controller synchronizes the switching frequency of the power supply to the display monitor horizontal frequency. The duty cycle of the synchronous clock signal generated using the present invention is not substantially less than that of an asynchronous clock signal, yet having a synchronous clock signal properly synchronizes the power supply to the monitor frequency. 
     The clock generation circuit  10  shown in FIG. 2A includes three main parts: a synchronization circuit  210 , a ramp and pulse generator  220 , and a clock generator  230 . The inputs to synchronization circuit  210  are the synchronization signal, SYNC, and the output clock signal, the latter being fed back from clock generator  230 . Synchronization circuit  210  generates a train of pulses  218  having the same frequency as the synchronization signal. As will be seen shortly, synchronization circuit  210  acts as a “one-shot” generator producing pulses typically having widths on the order of less than 100 ns. Each pulse of the synchronization pulse train provides the trigger for the rising edge of the synchronous output clock signal. 
     The inputs to ramp and pulse generator  220  are high threshold voltage V H , low threshold voltage V L , and the output clock signal, the latter again being fed back from clock generator  230 . As shown in FIG. 2B, ramp and pulse generator  220  includes ramp generator  221  and pulse generator  223 . Ramp generator  221  generates ramp waveform V T  and provides it to pulse generator  223  which generates two pulse trains  226  and  228 . First pulse train  228  is generated regardless of whether there is a synchronization signal. Pulse train  228  has the same frequency as the synchronization signal. Each pulse of pulse train  228  provides the trigger for the falling edge of the output clock signal. Second pulse train  226  is generated only when the synchronization signal is absent. Similar to synchronization circuit pulse train  218 , each pulse of pulse train  226  provides the trigger for the rising edge of the asynchronous output clock signal. 
     Clock generator  230  generates the output clock signal by using either synchronization circuit pulse train  218  or pulse train  226  as the trigger for the rising edge of the clock, depending on whether the synchronization signal is present or absent, respectively, and by using pulse train  228  as the trigger for the falling edge of the clock. When the synchronization signal is present, the frequency of the output clock signal is the same as that of the synchronization signal. 
     FIG. 2C is a circuit diagram of the block diagram of FIG.  2 A and illustrates the electronic elements used in the this embodiment of the present invention. Synchronization circuit  210  may include inverter  212 , S-R latch  214 , and NOR gate  216 . The synchronization signal is inverted by inverter  212  and the inverted signal is provided to the R 2  input of S-R latch  214  and to an input of NOR gate  216 . The output clock signal that is fed back to synchronization circuit  210  is provided to the S 2  input of S-R latch  214 . The output Q 2  of S-R latch  214  is provided to the second NOR gate input, and the NOR gate output provides synchronization pulse train  218  to clock generator  230 . 
     Ramp and pulse generator  220  includes bipolar transistor Q T,  discharge capacitor C T , pull-up resistor R to supply voltage VCC, high threshold voltage comparator  222 , and low threshold voltage comparator  224 . The output clock signal is fed back to the base of Q T , whose emitter is grounded and whose collector is connected to ground via C T . The collector, whose voltage is V T , threshold voltage comparator  222  and to the inverting input of low threshold voltage comparator  224 . V CC  can be between 12 and 20V and is typically 15V. As shown in FIG. 2C, V H  equals 6V and is provided to the inverting input of high threshold voltage comparator  222 . V L  equals 1.3V and is provided to the non-inverting input of low threshold voltage comparator  224 . These threshold values are based on the integrated circuit technology used. It is not necessary for the high and low threshold voltages to be separate inputs. Instead, a resistive divider can be designed having a resistor between V CC  and V H , one between V H  and V L , and one between V L  and ground. By appropriately choosing the three resistors, the high and low threshold voltages can be generated. High threshold voltage comparator  222  generates pulse train  226  and provides it to clock generator  230 , and low threshold voltage comparator  224  generates pulse train  228  and provides it to clock generator  230 . In an alternative embodiment (not shown), instead of using a common-emitter configuration using pull-up resistor R, Q T  can be set up in an emitter-follower configuration. In such a scenario, C T  and R are connected in parallel between the emitter and ground, and the collector is connected directly to V CC . 
     Clock generator  230  includes OR gate  232  and S-R latch  234 . Synchronization circuit pulse train  218  and pulse train  226  are provided to the inputs of OR gate  232  whose output is provided to the S 1  input of S-R latch  234 . Pulse train  228  is provided to the R 2  input of S-R latch  234 . The output Q 1  of S-R latch  234  is the desired output clock signal. 
     With the help of a general timing diagram in FIG.  3  and magnified timing diagrams in FIGS. 4 and 5 (not to scale), the operation of clock generation circuit  10  will now be explained. In this example, V H =6V and V L =1.3V. When the SYNC signal is low, {overscore (SYNC)} goes high forcing the output of the NOR gate low, effectively taking synchronization circuit  210  out of clock generation circuit  10  and causing the circuit to generate an asynchronous clock. Input S 1  of S-R latch  234  now follows the output of high voltage comparator  222 . With SYNC low, and starting with the output Q 1  going low, bipolar transistor Q T  is turned off and collector voltage V T  tries to immediately go high. However, discharge capacitor C T  prevents V T  from instantaneously changing, and while C T  charges, V T  ramps up. Although not explicitly shown in FIGS. 3-5, the slope of the ramp is exponential based on the values of R and C T . If the emitter-follower configuration were used instead of the common-emitter configuration, the resulting operation of the circuit would be similar, except that the V T  waveform would be inverted (i.e., V T  would slope downward when Q 1  is low). V T  is compared to V L  in low threshold comparator  224 . The output of low threshold voltage comparator  224  is low when V T  is greater than V L  and high when V T  is less than V L . V T  is also compared to V H  in high threshold voltage comparator  222 . The output of high threshold voltage comparator  222  is low when V T  is less than V H  and high when V T  is greater than V H . Thus, when V T  is between V H  and V L , both comparator outputs are low. This keeps S 1  and R 1  low, and Q 1  does not change. 
     When V T  crosses above V H , the output of high threshold comparator  222  goes high forcing S 1  high (step  401 ). This forces Q 1  high (step  402 ), which turns on Q T  and causes V T  to move down toward its saturation voltage (typically ˜0.2 V) (step  403 ). However, C T  prevents V T  from instantaneously changing, and while C T  discharges, V T  ramps down. As soon as V T  goes below V H , S 1  again goes low (step  404 ) and Q 1  remains high. V T  continues to ramp down, and when V T  crosses below V L , low voltage threshold comparator  224  goes high, forcing R 1  high (step  405 ) which forces Q 1  low (step  406 ). This causes Q T  to turn off which causes V T  to move toward V CC  and ramp up (step  407 ). When V T  rises above V L , R 1  goes low (step  408 ) and Q 1  remains low. 
     It can be seen that the rising edge of each pulse of pulse train  226  from high voltage comparator  222  causes S 1  to go high (step  401 ) generating a positive-going transition for the output clock signal Q 1 . Each pulse of pulse train  226  is high for as long as V T  is greater than V H . Similarly, it can be seen that the rising edge of each pulse of pulse train  228  from low voltage comparator  224  causes Q 1  to go low (step  406 ) generating a negative-going transition for the output clock signal Q 1 . Each pulse of pulse train  228  is high for as long as V T  is less than V L . 
     This cycle continues as V T  ramps up and down. Thus, Q 1  is low when V T  ramps up and is high when V T  ramps down. The output clock signal Q 1  is inverted and amplified (as illustrated in FIG.  3 ( h )) and provided to the PFC/PWM controller. The duty cycle of the PFC/PWM control signal is based on the time constants for the ramp-up and the ramp-down of V T . If the ramp-up time is 20 μs and the ramp-down time is 5 μs, the duty cycle is 20/25=80%. As shown in FIG. 3, the asynchronous portion of the PFC/PWM control signal has a duty cycle of approximately 83%. 
     When the SYNC signal is high, the synchronization circuit  210  takes an active role in generating the output clock signal Q 1 . When SYNC goes high, {overscore (SYNC)} and R 2  go low, and NOR, gate  216  generates the inverse of Q 2  (i.e. {overscore (Q 2 )}). Q 2  and Q 1  are initially low. With Q 1 /S 2  low and R 2  low, Q 2  does not change and remains low causing the output of NOR gate  216  (i.e. the output of synchronization circuit  210 ) high. This causes S 1  to go high (step  501 ). As with the asynchronous operation, S 1  going high (while R 1  is low) causes Q 1  to go high (step  502 ). This turns on Q T  and causes V T  to ramp down (step  503   a ). In addition, when Q 1  goes high, S 2  goes high which, because R 2  is low, forces Q 2  high (step  503   b ). Because NOR gate  216  acts as an inverter, the output  218  goes low forcing S 1  low (step  504 ) because V T  is below V H.  As with asynchronous operation, when S 1  goes low, Q 1  remains the same until V T  crosses below V L,  forcing R 1  high (step  505 ) which forces Q 1  low (step  506 ). Q 2  still remains high because both R 2  and S 2  are low. When Q 1  goes low, Q T  turns off, causing V T  to ramp up (step  507 ). When V T  crosses above V L , Ri goes low (step  508 ). Finally, SYNC goes low forcing {overscore (SYNC)} and R 2  high which forces Q 2  low (step  509 ). The inputs to S-R latch  214  remain the same until either SYNC goes high or V T  crosses V H . Therefore, in order to control the output clock signal using the synchronization signal, the frequency of the SYNC signal must be greater than that of the ramp waveform during asynchronous operation. 
     In synchronous operation, the frequency of the output clock signal increases to that of the SYNC signal. Each time {overscore (SYNC)} goes low, a pulse is formed in pulse train  218  at the output of synchronization circuit  210  which causes S 1  to go high (step  501 ), generating a positive-going transition for the output clock signal Q 1 . As with asynchronous operation, the rising edge of each pulse of pulse train  228  from low voltage comparator  224  causes Q 1  to go low (step  506 ) generating a negative-going transition for the output clock signal Q 1 . As before, each pulse of pulse train  228  is high for as long as V T  is less than V L . 
     Because the times during which V T  ramps up and down are not as long during synchronous operation, the output clock signal frequency is greater and the positive pulses in Q 1  do not last as long. This is beneficial for the PFC/PWM control signal which is the inverted and amplified Q 1 , because the turn-off time of the PFC/PWM signal is reduced. Thus, the duty cycle of the PFC/PWM control signal remains nearly the same as compared to what it was under asynchronous operation. As shown in FIG. 3, the synchronous portion of the PFC/PWM control signal still has a duty cycle of approximately 83%. 
     The duty cycle of this synchronous output can be compared to that of the prior art previously described. The prior art required that the SYNC signal remain high for at least time interval  124  (in FIG. 1) and that V T  remain low during time interval  130  until the SYNC pulse ends. Thus, according to the prior art, V T  would appear as shown by trace  330 , causing the on-time of Q 1  to increase as shown by trace  332 , which in turn decreases the turn-on time for the PFC/PWM control signal as shown in trace  334 . As shown in FIG.  3 ( h ), the duty cycle of trace  334  is less than 60%, as compared with 83% for the synchronous PFC/PWM control signal according to the present invention. 
     The use of this invention is not limited to power supplies for PC display monitors. Such a circuit and method could be used for control of power supplies for other applications. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.