Patent Publication Number: US-2009237050-A1

Title: Switching power supply

Description:
FIELD OF THE INVENTION 
     The invention relates to a switching power supply that operates intermittently under a light load and stops the operation of the control circuit during a period during which switching is stopped for reducing the power consumption therein. 
     BACKGROUND 
     It has been required for the switching power supply apparatuses to exhibit a high conversion efficiency, to cause less noise, to be small in size, to be manufactured with less manufacturing costs, and to be very reliable. To meet these requirements, various circuit configurations have been proposed. 
       FIG. 7  is a block diagram of a conventional switching power supply. The switching power supply shown in  FIG. 7  is equivalent to the switching power supply described in FIG. 4 in Japanese Unexamined Patent Application Publication No. 2005-006386 (FIG. 4 and the description with reference to FIG. 4). 
     Referring now to  FIG. 7 , switching power supply  1  generates, from the DC output of input power supply  2 , a DC output different from the DC output of input power supply  2  and feeds the generated DC output to load  3 . Switching power supply  1  is a so-called isolated DC-DC converter. 
     The positive side terminal of input power supply  2  is connected to the first end of primary winding Np 1  in transformer  121 . The second end of primary winding Np 1  is connected to the drain of MOSFET  110 . The source of MOSFET  110  is connected to the negative side terminal of input power supply  2 . Both ends of secondary winding Ns in transformer  121  are connected to the input of secondary-side main circuit  122 . The output from secondary-side main circuit  122  is connected to load  3  and error amplifier  123 . 
     Error amplifier  123  houses therein a circuit that generates a preset voltage set therein in advance. Error amplifier  123  amplifies the error between the output voltage from secondary-side main circuit  122  (hereinafter referred to simply as the “output voltage”) and the preset voltage and outputs the amplified error as a feedback signal. 
     Switching power supply  1  employs photocoupler  108  for a signal transmitter. The feedback signal outputted from error amplifier  123  is isolated by photocoupler  108  and transmitted to pulse width modulation control circuit (hereinafter referred to as “PWM control circuit”)  102  as feedback signal Vfb. 
     PWM control circuit  102  houses therein soft start control circuit  112 . As operation power is fed, or supplied, to PWM control circuit  102 , soft start control circuit  112  conducts soft start control. 
     PWM control circuit  102  determines the gate pulse width for driving MOSFET  110  based on feedback signal Vfb and feeds a gate pulse signal to driver circuit  101 . In response to the gate pulse signal fed from PWM control circuit  102 , driver circuit  101  feeds a gate pulse to MOSFET  110  for driving MOSFET  110 . 
     Switching power supply  1  further includes first comparator circuit  105  and first reference voltage supply  106 . First reference voltage supply  106  outputs a first reference voltage value (hereinafter referred to sometimes as a “burst threshold value”) Vth 1 . First comparator circuit  105  compares feedback signal Vfb and burst threshold value Vth 1  and feeds the result of the comparison to PWM control circuit  102  and first power supply circuit  103 . As feedback signal Vfb exceeds burst threshold value Vth 1  to the smaller side, PWM control circuit  102  stops the switching operation that feeds the gate pulse signal to driver circuit  101  and first power supply circuit  103  stops feeding the operation power to PWM control circuit  102 . 
     Now the operations conducted under a light load by the circuit shown in  FIG. 7  will be described with reference to  FIG. 8 . 
     In  FIG. 8 , feedback signal Vfb, that is a voltage signal, fed from error amplifier  123  to PWM control circuit  102 , operation power supply voltage Vref fed from first power supply circuit  103  to PWM control circuit  102 , and gate pulse Vgs fed from driver circuit  101  to the gate of MOSFET  110  are described. 
     Under a light load and in a period T 1 , for which feedback signal Vfb is higher than burst threshold value Vth 1 , PWM control circuit  102  conducts switching operation by the operation power fed thereto from first power supply circuit  103 . Under a light load and in a period T 2 , for which feedback signal Vfb is lower than burst threshold value Vth 1 , PWM control circuit  102  stops the switching operation and, therefore, the operation power feed from first power supply circuit  103  to PWM control circuit  102  is also stopped. 
     As described above, switching power supply  1  conducts burst mode of operations under a light load. In the period, for which switching power supply  1  stops the switching operation, switching power supply  1  stops the operation power feed from first power supply circuit  103  to PWM control circuit  102  for greatly reducing the total electric power consumption in switching power supply  1 . 
     However, it is impossible for the PWM control circuit in  FIG. 7  to resume (start) the switching operation immediately after the operation power is fed thereto. Therefore, if the load current increases rapidly in a period, for which the operation power feed to the PWM control circuit is stopped, the output voltage will fall before the PWM control circuit resumes the switching operation. (Hereinafter, the period, for which the operation power feed to the PWM control circuit is stopped, will be referred to as the “OFF-period”.) Moreover, the output voltage will cause ringing due to the reasons described later after the PWM control circuit resumes the switching operation. As a result, the voltage fed to the load changes greatly. The greatly changing voltage affects the IC and such electronic component parts in the load adversely, causing malfunctions of the IC and such electronic component parts in the load. 
     Now the problems described above will be described more in detail below with reference to  FIG. 9 .  FIG. 9  is a wave chart describing the operations of the circuit shown in  FIG. 7  when the PWM control circuit is in the OFF-state and load current I O  increases rapidly. 
     In  FIG. 9 , load current I O  and output voltage V O  are described. Load I O  flows from secondary-side main circuit  122  to load  3 . Output voltage V O  is outputted (fed) from secondary-side main circuit  122 . In  FIG. 9 , feedback signal Vfb fed from error amplifier  123  to PWM control circuit  102 , operation power supply voltage Vref fed from first power supply circuit  103  to PWM control circuit  102 , and gate pulse Vgs fed from driver circuit  101  to the gate of MOSFET  110  are also described in the same manner as in  FIG. 8 . 
     As load current I O  increases at a time t 1  in  FIG. 9 , output voltage V O  starts falling. In response to the fall of output voltage V O , feedback signal Vfb starts increasing. As feedback signal Vfb exceeds burst threshold value Vth 1  to the higher side at a time t 2 , first power supply circuit  103  starts feeding operation power to PWM control circuit  102 . However, operation power supply voltage Vref is not stabilized so soon at a certain voltage. After operation power supply voltage Vref is stabilized, PWM control circuit  102  initializes the internal logic circuit and finally resumes the switching operation at a time t 3 . (Hereinafter the operation between the start of the operation power feed and the logic circuit initialization will be referred to as the “restart”.) However, output voltage V O  falls greatly during the restart of PWM control circuit  102 . 
     Soft start control circuit  112  in PWM control circuit  102  operates linking with operation power supply voltage Vref that first power supply circuit  103  feeds. Even if operation power supply voltage Vref rises and PWM control circuit  102  resumes the switching operation, the control that does not widen the gate pulse width will be conducted for a while by soft start control circuit  112 . Since a sufficient current is not fed to the secondary side due to the above-described control conducted by soft start control circuit  112 , output voltage V O  further falls. 
     To make matters worse, as the soft start control conducted by soft start control circuit  112  ends, PWM control circuit  102  raises output voltage V O  rapidly for controlling output voltage V O  at the set value in a hurry. Since feedback signal Vfb delays reacting to the rapid rise of output voltage V O , output voltage V O  exceeds the set voltage to the higher side (hereinafter referred to as “overshooting”). 
     Feedback signal Vfb delays reacting to the overshooting described above, falling rapidly. As feedback signal Vfb exceeds burst threshold value Vth 1  to the lower side, PWM control circuit  102  is brought into the OFF-state thereof (at a time t 4 ). Even if output voltage V O  falls later and feedback signal Vfb exceeds burst threshold value Vth 1  to the higher side again (at a time t 5 ), it will be impossible to resume the switching operation until the restart of PWM control circuit  102  is finished. Therefore, output voltage V O  falls again during the restart. 
     Although PWM control circuit  102  resumes the switching operation later at a time t 6 , the soft start control is conducted again, making output voltage V O  further fall. After the soft start control is finished, PWM control circuit  102  tries to set output voltage V O  at the reference value as soon as possible, causing overshooting (at a time t 7 ). 
     As described above, output voltage V O  causes ringing, in which output voltage V O  repeats overshooting and falling and slowly converses to the preset voltage value. 
     In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a switching power supply that facilitates preventing the electric power consumption under a light load from increasing, suppressing the output voltage fall, caused when the load current increases rapidly from the light load state, as much as possible, and reducing the output voltage ringing. 
     SUMMARY OF THE INVENTION 
     According to a first objective, there is provided a switching power supply of an isolated type, the switching power supply generating a stabilized DC output from a DC output of a DC power supply, the switching power supply including: an error amplifier for amplifying the error between the voltage of the stabilized DC output and a preset voltage;a signal transmitter for isolating the output signal from the error amplifier for forming a feedback signal, the signal transmitter transmitting the feedback signal to a control circuit; and a control power supply for supplying power to a control circuit, the control circuit including a main switching device, a control section for controlling the ON and OFF switching of the main switching device in response to the feedback signal, a driver section for driving the main switching device in response to the output from the control section and stopping the driving when the first comparator section detects the feedback signal decreasing below the first reference voltage value, a first comparator section for comparing the feedback signal with a first reference voltage value, a second comparator section for comparing the feedback signal with a second reference voltage value lower than the first reference voltage value, and a first power supply section supplied with operation power from the control power supply, the first power supply section feeding operation power to a constituent element in the control section and stopping the feeding the operation power when the second comparator section detects the feedback signal decreasing below the second reference voltage value. 
     According to a second objective, there is provided a switching power supply of a non-isolated_type, the switching power supply generating a stabilized DC output from a DC output of a DC power supply, the switching power supply including an error amplifier for amplifying an error between the voltage of the stabilized DC output and a preset voltage, and outputting the amplified error to a control circuit as a feedback signal, and a control power supply for supplying power to the control circuit, the control circuit including a main switching device, a control section for controlling ON and OFF switching of the main switching device in response to the feedback signal, a driver section for driving the main switching device in response to the output from the control section and stopping the driving when the first comparator section detects the feedback signal decreasing below the first reference voltage value, a first comparator section for comparing the feedback signal with a first reference voltage value, a second comparator section for comparing the feedback signal with a second reference voltage value lower than the first reference voltage value, a first power supply section supplied with operation power from the control power supply, the first power supply section feeding operation power to a constituent element in the control section and stopping the feeding the operation power when the second comparator section detects the feedback signal decreasing below the second reference voltage value. 
     Advantageously, the switching power supply further includes a second power supply section that feeds operation power to the signal transmitter. 
     Advantageously, the first power supply section feeds operation power to the first comparator section. Advantageously, the control section includes a soft start control circuit fed with operation power from the control power supply, and the soft start control circuit is made to work only when DC power is fed at or before startup to the switching power supply. 
     According to the invention, the first reference voltage value for stopping the switching operation and the second reference voltage value for stopping the electric power feed to the PWM control circuit are set individually. According to the invention, the second reference voltage value is set to be lower than the first reference voltage value. The reference voltage setting described above makes the PWM control circuit resume the switching operation thereof quickly even when the load current increases rapidly in the OFF-state of the PWM control circuit. Therefore, the output voltage is prevented from falling. Further, the output voltage ringing caused in the conventional switching power supply in association with the rapid increase of the load current is reduced by the switching power supply according to the invention. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a switching power supply according to a first embodiment of the invention. 
         FIG. 2  is a wave chart describing the operations of the switching power supply shown in  FIG. 1  under a light load. 
         FIG. 3  is a wave chart describing the operations of the switching power supply shown in  FIG. 1  in a state in which the load current increases rapidly. 
         FIG. 4  is a block diagram of a switching power supply according to a second embodiment of the invention. 
         FIG. 5  is a block diagram of a switching power supply according to a third embodiment of the invention. 
         FIG. 6  is a block diagram of a switching power supply according to a fourth embodiment of the invention. 
         FIG. 7  is a block diagram of a conventional switching power supply. 
         FIG. 8  is a wave chart describing the operations of the circuit shown in  FIG. 7  under a light load. 
         FIG. 9  is a wave chart describing the operations of the circuit shown in  FIG. 7  when the PWM control circuit is in the OFF-state and the load current increases rapidly. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the invention will be described in detail hereinafter with reference to the accompanied drawings which illustrate the preferred embodiments of the invention. 
       FIG. 1  is a block diagram of a switching power supply according to a first embodiment of the invention. The same reference numerals as used in  FIG. 7  are used to designate the same constituent elements and their duplicated descriptions are omitted for the sake of simplicity. 
     The switching power supply shown in  FIG. 1  includes second power supply circuit  109  and second reference voltage supply  107  added to the constituent elements of the switching power supply shown in  FIG. 7 . In the switching power supply shown in  FIG. 1 , a path, through which a result of comparison is transferred from second comparator  104  to first power supply circuit  103 , is added. The switching power supply shown in  FIG. 1  is different from the conventional switching power supply shown in  FIG. 7  also in that a current is fed from second power supply circuit  109  to photocoupler  108 . 
     The switching power supply shown in  FIG. 1  is configured such that the operation power of soft start control circuit  112  in PWM control circuit  102  is fed from rectifier circuit  111 . Due to the configuration described above, operation power will be fed to soft start control circuit  112 , even if PWM control circuit  102  is in the OFF-state thereof. Since it is not necessary for PWM control circuit  102  to initialize the internal logic circuit thereof at the restart, a soft start control is prevented from causing after the restart of PWM control circuit  102  and the soft start control is conducted only when input power supply  2  is connected at or before startup to the switching power supply. 
     First comparator  105  compares burst threshold value Vth 1  fed from first reference voltage supply  106  and feedback signal Vfb and feeds the comparison result to PWM control circuit  102 . PWM control circuit  102  stops the switching operation thereof as feedback signal Vfb exceeds burst threshold value Vth 1  to the lower side. 
     Second comparator  104  compares second reference voltage value (hereinafter referred to as “power-supply-interruption threshold value”) Vth 2  fed from second reference voltage supply  107  and feedback signal Vfb and feeds the comparison result to first power supply circuit  103 . As feedback signal Vfb exceeds power-supply-interruption threshold value Vth 2  to the lower side, first power supply circuit  103  stops feeding the operation power to PWM control circuit  102  to bring PWM control circuit  102  into the OFF-state thereof. 
     Now the operations of the switching power supply shown in  FIG. 1  under a light load will be described with reference to  FIG. 2 . The same reference symbols as used in  FIG. 8  are used to designate the same signals in  FIG. 2  and their duplicated descriptions will not be made for the sake of simplicity. 
     As feedback signal Vfb lowers under a light load and exceeds burst threshold value Vth 1  to the lower side, PWM control circuit  102  stops the switching operation thereof. At this stage, however, first power supply circuit  103  keeps feeding operation power to PWM control circuit  102 . 
     As feedback signal Vfb reduces further and exceeds power-supply-interruption threshold value Vth 2  to the lower side, PWM control circuit  102  is brought into the OFF-state thereof. 
     As feedback signal Vfb turns to rising later and exceeds power-supply-interruption threshold value Vth 2  to the higher side, PWM control circuit  102  restarts. As feedback signal Vfb further rises and exceeds burst threshold value Vth 1  to the higher side, PWM control circuit  102  resumes the switching operation thereof. 
     Now the operations of the switching power supply shown in  FIG. 1  in the state, in which the current of the load that has been light increases rapidly, will be described with reference to  FIG. 3 . The same reference symbols as used in  FIG. 2  are used to designate the same signals in  FIG. 3  and their duplicated descriptions will not be made for the sake of simplicity. 
     As load current I O  increases at a time t 1  in  FIG. 3 , output voltage V O  starts falling. In response to the fall of output voltage V O , feedback signal Vfb increases. As feedback signal Vfb exceeds power-supply-interruption threshold value Vth 2  to the higher side (at a time t 2 ), operation power is fed from first power supply circuit  103  to PWM control circuit  102  and PWM control circuit  102  restarts. 
     Feedback signal Vfb further increases and exceeds burst threshold value Vth 1  to the higher side at a time t 3 . Since PWM control circuit  102  has already finished the restart thereof, PWM control circuit  102  resumes the switching operation thereof immediately after the time t 3 . Since the soft start control inside PWM control circuit  102  is prevented from working, PWM control circuit  102  can widen the gate pulse width quickly after resuming the switching operation thereof. 
     Through the operations described above, the switching power supply according to the first embodiment resumes the switching operations thereof more quickly than the conventional switching power supply and facilitates widening the gate pulse width for minimizing the fall of output voltage V O . 
     If output voltage V O  overshoots at a time t 4 , the switching operation will be stopped as soon as feedback signal Vfb exceeds burst threshold value Vth 1  to the lower side. However, since feedback signal Vfb is still higher than power-supply-interruption threshold value Vth 2 , PWM control circuit  102  is not brought into the OFF-state thereof. Therefore, as output voltage V O  starts lowering, PWM control circuit  102  resumes the switching operation thereof soon (at a time t 5 ) and stabilizes output voltage V O  at the set value thereof. 
     Through the operations described above, the output voltage ringing caused in the conventional switching power supply is reduced. Although the operation power of soft start control circuit  112  is fed from rectifier circuit  111  in  FIG. 1 , the operation power of soft start control circuit  112  may be fed from second power supply circuit  109  with no problem. A pulse amplitude modulation (PAM) control circuit or a pulse frequency modulation (PFM) control circuit may be employed in substitution for PWM control circuit  102  in  FIG. 1  with no problem. Due to the reasons described below, photocoupler  108  is driven by second power supply circuit  109 . 
     If the operation power of photocoupler  108  is fed from rectifier circuit  111 , feedback signal Vfb will vary by the fed voltage variations and output voltage ringing will be caused more badly. Thus, the intended effects will not be obtained. In detail, as the drive pulse output is resumed, the output voltage from rectifier circuit  111  rises and, in association with this, feedback signal Vfb also rises. As a result, PWM control circuit  102  further widens the gate pulse width, causing overshooting in the output voltage. The overshooting further causes ringing in the same manner as described with reference to  FIG. 9 . For preventing the overshooting from causing and further for preventing the ringing from causing, a constant voltage is fed from second power supply circuit  109  to photocoupler  108 . 
       FIG. 4  is a block diagram of a switching power supply according to a second embodiment of the invention. The non-isolated_switching power supply shown in  FIG. 4  realizes the same operations same with the operations conducted by the switching power supply shown in  FIG. 1 . The same reference numerals as used in  FIG. 1  are used to designate the same constituent elements in  FIG. 4  and their duplicated descriptions are omitted for the sake of simplicity. 
     Photocoupler  108 , second power supply circuit  109 , rectifier circuit  111 , transformer  121 , and secondary-side main circuit  122  shown in  FIG. 1  are not included in switching power supply  4  shown in  FIG. 4 . Switching power supply  4  shown in  FIG. 4  includes a main circuit formed of diode  201 , inductor  202  and capacitor  203  added thereto. Switching power supply  4  according to the second embodiment forms a non-isolated_DC-DC converter. 
     Although operation power is fed from input power supply  2  to second comparator  104 , first power supply circuit  103  and driver circuit  101  in  FIG. 4 , the operation power may be fed from the other power supply with no problem. 
     As described above, the switching power supply according to the invention facilitates reducing the electric power consumption under a light load, reducing the output voltage variations caused by the load current increasing rapidly from a light load state as much as possible, and reducing the output voltage ringing. 
     The circuit configuration of a switching power supply according to the invention is not always limited to those shown in  FIGS. 1 and 4 . For example, burst threshold value Vth 1 _and power-supply-interruption threshold value Vth 2  may be provided with hysteresis characteristics with no problem. A bipolar transistor and an insulated gate bipolar transistor (IGBT) may be used for MOSFET  110 . DC power obtained by rectifying and smoothing AC power may be used in substitution for the DC power fed from input power supply  2 . A series regulator may be used in substitution for rectifier circuit  111 . 
     Now the hysteresis characteristics preferable for burst threshold value Vth 1 _and power-supply-interruption threshold value Vth 2  will be described below. 
     The switching is resumed from the stopped state thereof at the time point, at which rising feedback signal Vfb exceeds burst threshold value Vth 1  to the higher side. Therefore, by lowering burst threshold value Vth 1  a little bit, the resumed switching is prevented from stopping soon. Then, as the output voltage rises due to the switching operation, feedback signal Vfb lowers. As lowering feedback signal Vfb exceeds the a-little-bit-lowered burst threshold value Vth 1  to the lower side, the switching is stopped. As the switching is stopped, burst threshold value Vth 1  is raised a little bit so that the switching may not be resumed soon. 
     If feedback signal Vfb causes small variations around burst threshold value Vth 1 , the switching will repeat stopping and restarting in a short period, causing an oscillation. For preventing the oscillation from causing, first reference voltage supply  106  and second reference voltage supply  107  are provide with the hysteresis characteristics as described above. The hysteresis characteristics prevent audible sounds from causing from the transformer and widen the burst period for reducing the standby energy. Since feedback signal Vfb is accompanied usually by noises, the hysteresis characteristics also work for preventing malfunctions caused by the noises from occurring. The same holds for Vth 2 . Having the hysteresis characteristics, the control power operate alternating between on and off, and that wasted power is reduced. 
     Moreover, power supply circuit  103  in  FIGS. 1 and 4  may be configured by a simple switching circuit with no problem. The switching circuit may be opened by the output from second comparator circuit  104  with no problem, as feedback signal Vfb exceeds power-supply-interruption threshold voltage Vth 2  to the lower side. The switching circuit may be closed by the output from second comparator circuit  104  with no problem, as feedback signal Vfb exceeds power-supply-interruption threshold value Vth 2  to the higher side. By obtaining the voltages necessary for operating the constituent elements of the switching power supply directly from input power supply  2 , rectifier circuit  111  may be omitted. 
       FIG. 5  is a block diagram of a switching power supply according to a third embodiment of the invention. The switching power supply shown in  FIG. 5  is a modification of the switching power supply shown in  FIG. 1 . 
     As shown in  FIG. 5 , rectifier circuit  111  shown in  FIG. 1  is omitted and DC power supply  141  is employed in substitution for rectifier circuit  111 . DC power supply  141  is employed for a control power supply. The configuration described above prevents the problems as described in the paragraph [0019] from causing and facilitates omitting second power supply circuit  109 . 
     When the voltage of input power supply  2  is as high as to be employable for the control power without boosting nor bucking, rectifier circuit  111  and second power supply circuit  109  may be omitted by employing the structure, in which the power from input power supply  2  is used directly for control power.