Patent Publication Number: US-2007096706-A1

Title: Power supply control method and power supply apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-317694, filed Oct. 31, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      One embodiment of the invention relates to a power supply control method and power supply apparatus using pulse width modulation (PWM) control.  
      2. Description of the Related Art  
      It is disclosed by, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-289754 to improve the output accuracy of a switching power supply using PWM control. In recent years, various so-called digital power supply apparatuses are also developed, each of which PWM-controls a DC/DC converter by using a digital signal processor (DSP).  
      In order to obtain a desirable output voltage from an input voltage, the DC/DC converter of the switching power supply using PWM control has a feedback loop (control loop) for monitoring and controlling a voltage and current. However, the detected current accuracy for PWM control in the control loop influences the output accuracy, thus posing a problem.  
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
      A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
       FIG. 1  is a diagram showing the structure of a power supply apparatus to which a power supply control method is applied according to a first embodiment of the invention;  
       FIG. 2  is a flowchart showing a processing procedure executed in a switching period (T) according to the first embodiment;  
       FIG. 3A  is a chart showing the waveform of a current flowing through an inductor for explaining the operation according to the first embodiment;  
       FIG. 3B  is a chart showing a PWM signal waveform for explaining an operation according to the first embodiment;  
       FIG. 4  is a chart showing a signal waveform of a PWM control mode according to the first embodiment; and  
       FIG. 5  is a chart showing a waveform obtained by enlarging a portion of the waveform shown in  FIG. 4  according to the first embodiment.  
    
    
     DETAILED DESCRIPTION  
      Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, there is provided a power supply control method for a switching power supply using pulse width modulation (PWM) control, comprising detecting an input voltage to be supplied to the switching power supply, detecting an output voltage from the switching power supply, calculating a timing for sampling a current for the PWM control, based on a ratio between the detected input voltage and the detected output voltage, and detecting the current based on the calculated timing.  
      According to another embodiment of the invention, there is provided a power supply apparatus comprising a first detection unit which detects an input voltage to be supplied to a switching power supply using PWM control, a second detection unit which detects an output voltage from the switching power supply, a calculation unit which calculates, based on a ratio between the input voltage detected by the first detection unit and the output voltage detected by the second detection unit, a timing for sampling a current for the PWM control, and a third detection unit which detects the current based on the calculated timing.  
      According to still another embodiment of the invention, there is provided a power supply apparatus comprising a DC/DC conversion unit which includes an inductor, and a switching element controlled in accordance with a PWM signal, a first detection unit which detects an input voltage to be supplied to the DC/DC conversion unit, a second detection unit which detects an output voltage from the DC/DC conversion unit, a calculation unit which calculates, based on a ratio between the input voltage detected by the first detection unit and the output voltage detected by the second detection unit, a timing for sampling a current flowing through the inductor, a third detection unit which detects, at the timing calculated by the calculation unit, the current flowing through the inductor, and a PWM control unit which controls, by using a value of the current detected by the third detection unit and a value of the output voltage detected by the third detection unit, a pulse width of a PWM signal to be supplied to the switching element.  
      According to an embodiment,  FIG. 1  shows the arrangement of a power supply apparatus to which a power supply control method is applied according to the first embodiment of the invention.  
      According to the first embodiment of the invention, the power supply apparatus to which the power supply control method is applied includes a DC/DC converter  1  and control apparatus  10 . The DC/DC converter  1  implements a switching power supply. The DC/DC converter  1  includes a switching element  2  (using, e.g., a switching FET), rectifier  3 , smoothing inductor  4 , and capacitor  5 .  
      The DC/DC converter  1  also includes a current detection unit  6 , voltage detection unit  7 , and output voltage terminal  8 . The current detection unit  6  detects a current (IL) flowing through the inductor  4  in the DC/DC converter  1 . The voltage detection unit  7  detects an output voltage (V OUT ) of the DC/DC converter  1 . Additionally, a main power supply  9  is connected as an input power supply to the DC/DC converter  1 .  
      The control apparatus  10  serves as a DSP, and has a PWM control function. The control apparatus  10  includes a V IN  input unit  11 , V OUT  input unit  12 , IL input unit  13 , PWM output unit  14 , sampling timing calculation unit  15 , PWM control unit  16 , and the like.  
      The V IN  input unit  11  analog-to-digital-converts l ) the input voltage (V IN ) from the main power supply  9 , and inputs the converted input voltage. That is, the V IN  input unit  11  functions as a detection unit for detecting the input voltage (V IN ) from the main power supply  9 . The V OUT  input unit  12  analog-to-digital-converts the output voltage (V OUT ) detected by the voltage detection unit  7 , and inputs the converted output voltage.  
      The IL input unit  13  analog-to-digital-converts, based on a sampling timing calculated by the sampling timing calculation unit  15 , the current (IL) which flows through the inductor  4  and is detected by the current detection unit  6 . Then, the IL input unit  13  inputs an average current (IL_Avg).  
      The PWM output unit  14  digital-to-analog-converts the PWM signal for performing switching control of the switching element  2 , and outputs the converted PWM signal.  
      The sampling timing calculation unit  15  calculates, based on the ratio between the input voltage (V IN ) and the output voltage (V OUT ), the sampling timing for analog-to-digital-converting the current (IL) flowing through the inductor  4  to input the average current (IL_Avg) to the IL input unit  13 .  
      In accordance with, e.g., the average current (IL_Avg), the output voltage (V OUT ), and a reference voltage (not shown), the PWM control unit  16  outputs the PWM signal whose pulse width (ON period) is controlled. The average current (IL_Avg) is obtained by analog-to-digital-converting the current based on the sampling timing calculated by the sampling timing calculation unit  15 .  
       FIG. 2  shows a processing procedure executed in a switching period (T) in the control apparatus  10 . In this processing, the sampling timing calculation unit  15  detects the input voltage (V IN ) from the V IN  input unit  11 , and the output voltage (V OUT ) from the V OUT  input unit  12  (step S 1 ). Based on the ratio between the input voltage V IN  and the output voltage V OUT , the sampling timing calculation unit  15  calculates the sampling timing of the current (IL) flowing through the inductor  4  (step S 2 ). A sampling timing calculation process in the sampling timing calculation unit  15  will be described later with reference to FIGS.  3  to  5 .  
      The IL input unit  13  analog-to-digital-converts the current (IL) which flows through the inductor  4  and is detected by the current detection unit  6 , based on the sampling timing (detection timing) calculated (determined) by the sampling timing calculation unit  15 . Then, the IL input unit  13  inputs the converted current (step S 3 ).  
      The PWM control unit  16  controls the pulse width (ON period) of the PWM signal in accordance with the average current (IL_Avg) output from the IL input unit  13 , the output voltage (V OUT ), the reference voltage (not shown), and the like. The PWM control unit  16  then outputs the controlled PWM signal to the PWM output unit  14  (step S 4 ).  
      The PWM output unit  14  digital-to-analog-converts the PWM signal output from the PWM control unit  16 , and outputs the converted PWM signal to the DC/DC converter  1 . The switching element  2  of the DC/DC converter  1  executes switching control in accordance with the PWM signal output from the PWM output unit  14 , and then executes power supply output control in accordance with the ON duty of the PWM signal.  
      The sampling timing calculation process in the sampling timing calculation unit  15  will be described with reference to FIGS.  3  to  5 .  
      The ON duty (D) of the PWM signal output from the PWM output unit  14  can be approximately given by the ratio between the input voltage (V IN ) and the output voltage (V OUT ) as follows, or by the ratio between the switching period (T) and the ON period (T on ):  
                   D   =       (     output   ⁢           ⁢   voltage     )     ÷     (     input   ⁢           ⁢   voltage     )                   =       (     ON   ⁢           ⁢   period     )     ÷     (     switching   ⁢           ⁢   period     )                     (   1   )             
 
       FIG. 3A  shows the waveform of the current (IL) flowing through the inductor  4 , and  FIG. 3B  shows the waveform of the PWM signal output from the PWM output unit  14 .  
      In the ON period (T on ) of the PWM signal output from the PWM output unit  14 , the current flowing through the inductor  4  increases to the maximum amplitude value (IL_H).  
      In the OFF period (T off ) of the PWM signal output from the PWM output unit  14 , the current flowing through the inductor  4  decreases to the minimum amplitude value (IL_L).  
      Accordingly, the average current [IL_Avg] is given by:
 
IL_Avg=(IL_H+IL_L)÷2  (2)
 
      A timing  t  when the current flowing through the inductor  4  decreases to the average value (IL_Avg) in the OFF period (T off ) is given by:  
                   t   =       (     T   +     T   on       )     ÷   2                 =       (     T   ÷   2     )     ×     {     1   +       (     output   ⁢           ⁢   voltage     )     ÷     (     input   ⁢           ⁢   voltage     )         }                     (   3   )             
 
      As described above, since the current flowing through the inductor  4  is supplied from the IL input unit  13  at the timing  t , the control apparatus  10  always measures the average current [IL_Avg].  
      Thus, PWM control can be accurately performed by using average current mode control to accurately detect the average current [IL_Avg].  
       FIGS. 4 and 5  show the waveforms (inductor current waveforms) of the current (IL) flowing through the inductor  4  for PWM control.  FIG. 5  shows the waveforms obtained by enlarging portions of the waveforms in an area EL shown in  FIG. 4 . In  FIG. 4  or  5 , a partial waveform A indicated by the solid line is a normal inductor current waveform, and a partial waveform B indicated by the dashed line is an inductor current waveform when the ON duty (D) changes. Reference symbol TF denotes a fixed sampling timing.  
      Generally, in this type of switching power supply, overvoltage protection (OVP), overcurrent protection (OCP), over-temperature protection (OTP), and the like are performed. A question is raised about overcurrent protection (OCP) of these operations.  
      When the average current (IL_Avg) exceeds a peak switch current (IL_SW (Peak)), the operation stops due to an overcurrent protection function. Assume that the sampling timing at which the average current (IL_Avg) is supplied to the IL input unit  13  is fixed (the fixed sampling timing TF), and that the normal inductor current waveform shown as the waveform A instantaneously changes to the inductor current waveform shown as the waveform B in changing the ON duty, and then returns to the waveform A immediately after that. In this case, the average current (IL_Avg) actually detected in changing the ON duty exceeds the peak switch current (IL_SW (Peak)), and the operation stops due to the overcurrent protection function. Hence, even if the current waveform returns to the normal inductor current waveform shown as the waveform A within a short period of time, and the average current (IL_Avg) decreases below the peak switch current (IL_SW (peak)), the operation may be kept stopped. To cope with this problem, the sampling timing is calculated based on the ratio between the input voltage and the output voltage. Accordingly, the average current can always be measured, and PWM control can be accurately performed by average current mode control.  
      As described above, in the power supply system for PWM control, the current flowing through the coil is controlled based on the ratio between the input voltage and the output voltage. Accordingly, the average current [IL_Avg] can always be measured, and PWM control can be stably performed.  
      While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.