Patent Publication Number: US-8970790-B2

Title: Switching power supply device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on the following Japanese Patent Applications, and the contents of which are hereby incorporated by reference: 
     (1) Japanese Patent Application No. 2012-233778 (the filing date: Oct. 23, 2012) 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a switching power supply device that uses a non-linear control system and to an electronic apparatus (e.g., television) that uses the switching power supply device. 
     2. Description of Related Art 
       FIG. 14A  to  FIG. 14C  are each a circuit block diagram and an operation waveform view that show a conventional example of a switching power supply device which uses a non-linear control method, of which  FIG. 14A  illustrates a switching power supply device that employs a hysteresis window system,  FIG. 14B  illustrates a switching power supply device that employs a bottom detection on-period fixing system, and  FIG. 14C  illustrates a switching power supply device that employs an upper detection off-period fixing system. In the meantime, any of the switching power supply devices illustrated in  FIG. 14A  to  FIG. 14C  is a voltage step-down type DC/DC converter that steps down an input voltage Vin to generate a desired output voltage Vout. 
     In the meantime, as an example of the conventional art related to the above description, there is JP-A-2012-115047. 
     The switching power supply device using a non-linear control method easily achieves high efficiency during a light load period compared with a switching power supply device using a linear control method (e.g., voltage mode control method and electric current mode control method). However, in recent years, as energy saving attracts more attention, power loss (power consumption during a standby period and the like) of an electronic apparatus during a light load period is becoming unable to be negligible, and also the switching power supply device using a non-linear control method is required to further increase efficiency during a light load period. 
     SUMMARY OF THE INVENTION 
     In light of the above problems found by the inventor of the present application, it is an object of the present invention to provide a switching power supply device capable of increasing efficiency during a light load period and an electronic apparatus that uses the switching power supply device. 
     To achieve the above object, a switching power supply device according to the present invention includes a structure which has: a switching control portion that generates an output voltage from an input voltage by performing on/off control of a switch device using a non-linear control method in accordance with a comparison signal and a timer signal; a main comparator that compares a feedback voltage corresponding to the output voltage and a predetermined reference voltage with each other to generate the comparison signal; a timer portion that performs a one-shot output of the timer signal at a time point when a predetermined fixed period elapses after the on/off of the switch device is switched; and a backward flow detection portion that detects a backward flow current for the switch device to forcibly turn off the switch device; wherein the timer portion and the backward flow detection portion are turned on at a time point when a pulse edge of the comparison signal occurs and turned off at a time point when operation of each of the timer portion and backward flow detection portion is completed. 
     In the meantime, other features, elements, steps, advantages and characteristics of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] is a block diagram showing a first embodiment of a switching power supply device. 
       [ FIG. 2 ] is a time chart showing a sleep operation of the first embodiment. 
       [ FIG. 3 ] is a time chart showing trouble due to a restart delay of a backward flow detection portion  15 . 
       [ FIG. 4 ] is a block diagram showing a second embodiment of a switching power supply device. 
       [ FIG. 5 ] is a time chart showing a sleep operation of the second embodiment. 
       [ FIG. 6 ] is a time chart showing a mask operation of a comparison signal S 1 . 
       [ FIG. 7 ] is a block diagram showing a structural example of the backward flow detection portion  15 . 
       [ FIG. 8 ] is a block diagram showing a third embodiment of a switching power supply device. 
       [ FIG. 9 ] is a time chart showing a sleep operation of the third embodiment. 
       [ FIG. 10 ] is a block diagram showing a fourth embodiment of a switching power supply device. 
       [ FIG. 11 ] is a time chart showing a sleep operation of the fourth embodiment. 
       [ FIG. 12 ] is a block diagram showing a structural example of a television that incorporates a switching power supply device A. 
       [ FIG. 13A ] is a front view of a television that incorporates the switching power supply device A. 
       [ FIG. 13B ] is a side view of a television that incorporates the switching power supply device A. 
       [ FIG. 13C ] is a rear view of a television that incorporates the switching power supply device A. 
       [ FIG. 14A ] is a circuit block diagram and operation waveform view showing a first conventional example (hysteresis window method) of a switching power supply device that employs a non-linear control method. 
       [ FIG. 14B ] is a circuit block diagram and operation waveform view showing a second conventional example (bottom detection on-period fixing method) of a switching power supply device that employs a non-linear control method. 
       [ FIG. 14C ] is a circuit block diagram and operation waveform view showing a third conventional example (upper detection off-period fixing method) of a switching power supply device that employs a non-linear control method. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     &lt;First Embodiment&gt; 
       FIG. 1  is a block diagram showing a first embodiment of a switching power supply device. A switching power supply device A according to the first embodiment is a voltage step-down type DC/DC converter that generates an output voltage Vout from an input voltage Vin by means of a non-linear control method (here, bottom detection on-period fixing method). The switching power supply device A has: a semiconductor apparatus  1 ; and various discrete components (inductor L 1 , capacitor C 1 , resistors R 1  and R 2 ) externally connected to the semiconductor apparatus  1 . 
     The semiconductor apparatus  1  has at least external terminals T 1  to T 5  to secure electric connection with outside. Outside the semiconductor apparatus  1 , the external terminal (power supply terminal) T 1  is connected to an application terminal for the input voltage V 1 . The external terminal (switch terminal) T 2  is connected to a first terminal of the inductor L 1 . A second terminal of the inductor L 1 , a first terminal of the capacitor C 1 , and a first terminal of the resistor R 1  are all connected to an application terminal for the output voltage Vout. A second terminal of the capacitor C 1  is connected to a ground terminal. A second terminal of the resistor R 1  and a first terminal of the resistor R 2  both are connected to the external terminal (feedback terminal) T 4  of the semiconductor apparatus  1 . A second terminal of the resistor R 2  is connected to the ground terminal. The resistors R 1  and R 2  function as a feedback voltage generation portion that outputs a feedback voltage Vfb, which is obtained by dividing the output voltage Vout, from a connection node between them. The external terminal (ground terminal) T 3  of the semiconductor apparatus  1  is connected to the ground tell final. The external terminal (sleep terminal) T 5  of the semiconductor apparatus  1  is connected to an application terminal for a sleep signal SLEEP. 
     The semiconductor apparatus  1  is a monolithic semiconductor integrated circuit apparatus (so-called switching power supply IC) that integrates: MOS field effect transistors  11  and  12  of N channel type; a main comparator  13 ; a timer portion  14 ; a backward flow detection portion  15 ; a latch portion  16 ; and a switching control portion  17 . 
     The transistor  11  is a switch device (output transistor) that is connected between the external terminal T 1  and the external terminal T 2 , and undergoes on/off control in accordance with a gate signal G 1  input from the switching control portion  17 . Describing a connection form, a drain of the transistor  11  is connected to the external terminal T 1 . A source of the transistor  11  is connected to the external terminal T 2 . A gate of the transistor  11  is connected to an application terminal for the gate signal G 1 . 
     The transistor  12  is a switch device (synchronization rectification transistor) that is connected between the external terminal T 2  and the ground terminal, and undergoes on/off control in accordance with a gate signal G 2  input from the switching control portion  17 . Describing a connection form, a drain of the transistor  12  is connected to the external terminal T 2 . A source of the transistor  12  is connected to the ground terminal. A gate of the transistor  12  is connected to an application terminal for the gate signal G 2 . 
     The main comparator  13  compares the feedback voltage Vfb (divided voltage of the output voltage Vout) applied to the inverting input terminal (−) via the external terminal T 4  and a predetermined reference voltage Vref applied to the non-inverting input terminal (+) with each other to generate a comparison signal S 1 . The comparison signal S 1  goes to a low level when the feedback voltage Vfb is higher than the reference voltage Vref, and goes to a high level when the feedback voltage Vfb is lower than the reference voltage Vref. In the meantime, it is desirable that a constant voltage (band gap voltage or the like), which does not depend on the input voltage Vin and an ambient temperature, is used as the reference voltage Vref. 
     The timer portion  14  performs a one-shot output of a timer signal S 2  at a time point when a predetermined on-period ton elapses after the transistor  11  is turned on. In the meantime, to know an on-timing of the transistor  11 , it is useful to monitor an internal signal (e.g., a drive signal for a driver that generates the gate signal G 1 ) of the switching control portion  17 . 
     During an on-period of the transistor  12 , the backward flow detection portion  15  compares a switch voltage Vsw appearing at the external terminal T 2  and a ground voltage GND with each other to generate a zero-cross detection signal S 3 . The zero-cross detection signal S 3  goes to a low level when the switch voltage Vsw is lower than the ground voltage GND, and goes to a high level when the switch voltage Vsw is higher than the ground voltage GND. In other words, the zero-cross signal S 3  goes to the low level when an inductor current IL is flowing from the ground terminal to the inductor L 1  via the transistor  12 , and goes to the high level when the inductor current IL is flowing backward from the inductor L 1  to the ground terminal via the transistor  12 . 
     The latch portion  16  sets a skip signal S 2  to a high level at a rising edge of the zero-cross detection signal S 3 , and resets the skip signal S 2  to a low level at a rising edge of the comparison signal S 1 . In other words, the skip signal S 2  is latched to the high level when a backward flow current for the transistor  12  is detected, and reset to the low level immediately before the transistor  11  is turned on next. 
     The switching control portion  17  includes an SR flip-flop and a driver, and performs on/off control (process of generating the gate signals G 1  and G 2 ) of the transistors  11  and  12  using the non-linear control method in accordance with the comparison signal S 1  and the timer signal S 2 , thereby generating the output voltage Vout from the input voltage Vin. Besides, the switching control portion  17  includes a function (switching stop function) that forcibly turns off the transistor  12  during a period when the skip signal S 4  is kept at the high level. By including such a function, it becomes possible to shut down the backward flow current for the transistor  12  to increase efficiency during a light load period. 
     Besides, the sleep signal SLEEP, which is used to switch an operation mode (sleep mode/nom-sleep mode) of the switching power supply apparatus A, is input in the semiconductor apparatus  1 . For example, when the sleep signal SLEEP is at a low level, the switching power supply device A is brought to the non-sleep mode, while when the sleep signal SLEEP is at a high level, the switching power supply apparatus A is brought to the sleep mode (operation mode in which the power consumption of the switching power supply device A is reduced by performing power supply to only the smallest possible number of necessary circuit blocks). 
     especially, the sleep signal SLEEP is input in both timer portion  14  and backward flow detection portion  15 ; in the case where the switching power supply device A is kept in the non-sleep mode, the timer portion  14  and the backward flow detection portion  15  are kept in a normally turned-on state to prioritize increase in stability of output feedback control. On the other hand, in the case where the switching power supply device A is kept in the sleep mode, the timer portion  14  and the backward flow detection portion  15  undergo on/off control when necessary to prioritize increase in efficiency during the light load period. 
       FIG. 2  is a time chart showing a sleep operation (operation in the case where the sleep signal SLEEP is at the high level) of the first embodiment, and illustrates, from top in order, the feedback voltage Vfb; the reference voltage Vref; the comparison signal S 1 ; the timer signal S 2 ; the gate signals G 1  and G 2 ; the inductor current IL; the switch voltage Vsw; the zero-cross detection signal S 3 ; the skip signal S 4 ; and on/off states of the timer portion  14  and backward flow detection portion  15 . 
     At a time t 11 , if the feedback voltage Vfb becomes lower than the reference voltage Vref and the comparison signal S 1  is raised to the high level, the gate signal G 1  is raised to the high level and the transistor  11  is turned on. On the other hand, during a period of t 11  to t 12 , the gate signal G 2  is kept at the low level and the transistor  12  is kept in the off-state. As a result of this, during the period of t 11  to t 12 , the switch voltage Vsw rises to substantially the input voltage Vin and the inductor current IL increases. 
     Besides, at the time t 11 , if the comparison signal S 1  is raised to the high level, the rising edge is used as a trigger to turn on the timer portion and the backward flow detection portion  15 . In the meantime; the transistor  11  is turned on at the time point when the comparison signal S 1  is raised to the high level; accordingly, the timer portion  14  starts to count the on-period ton immediately after the turning-on of the transistor  11  at the time t 11 . 
     At the time t 12 , if the counting of the on-period ton by the timer portion  14  is completed and a trigger pulse is generated in the timer signal S 2 , the gate signal G 1  is dropped to the low level and the gate signal G 2  is raised to the high level. As a result of this, the transistor  11  is turned off and the transistor  12  is turned on. At this time, an induced electromotive force occurs in the inductor L 1  to continue flowing the inductor current IL in the same direction as until now; accordingly, the inductor current IL flows from the ground terminal into the inductor L 1  via the transistor  12 . Therefore, the switch voltage Vsw declines to a negative voltage value that is lower than the ground voltage GND by a drop voltage across the transistor  12 . 
     In the meantime, in  FIG. 2 , the on/off transition timings of the transistors  11  and  12  completely coincide with each other; however, from a viewpoint of preventing a through-current, a concurrent off-period of the transistors  11  and  12  may be set by giving a delay to the on/off transition timings of the transistors  11  and  12 . 
     Besides, the timer portion  14  is turned off with no delay at the time point when the counting of the on-period ton is completed. Describing more specifically, after performing the one-shot output of the timer signal S 2 , the timer portion  14  shuts down a power supply route for itself. By performing such on/off control, it becomes possible to reduce power consumption of the timer portion  14  and achieve the efficiency increase during the light load period. 
     Here, during a heavy load period when an output current Iout flowing in a load is sufficiently large, energy stored in the inductor L 1  is large; accordingly, the inductor current IL continues flowing to the load without becoming smaller than a zero value until a time t 14  when the gate signal G 1  is raised again to the high level, and the switch voltage Vsw is kept at the negative voltage value. On the other hand, during the period when the output current Iout flowing in the load is small, the energy stored in the inductor L 1  is small; accordingly, at a time t 13 , the inductor current IL becomes smaller than the zero value and a backward flow current for the transistor  12  occurs, whereby polarity of the switch voltage Vsw is switched from negative to positive. In such a state, electric charges stored in the capacitor C 1  are discarded to the ground terminal, which causes an efficiency decline during the light load period. 
     Because of this, a structure is employed, in which the switching power supply device A uses the backward flow detection portion  15  to generate the zero-cross detection signal S 3  in accordance with presence/non-presence of a backward flow current (polarity reversal of the switch voltage Vsw) and forcibly turns off the transistor  12  during a high level period (times t 13  to t 14 ) of the skip signal S 4  that is latched to the high level at a rising edge of the zero-cross detection signal S 3 . By employing such a structure, it is possible to quickly shut down the backward flow current for the transistor  12 ; accordingly, it becomes possible to solve the efficiency decline during the light load period. 
     In the meantime, the backward flow detection portion  15  is turned off with no delay at a time point when the backward flow detection operation is completed. Describing more specifically, the backward flow detection portion  15  raises the zero-cross detection signal S 3  to the high level, thereafter, shuts down a power supply route for itself. By performing such on/off control, it becomes possible to reduce power consumption of the backward flow detection portion  15  and to achieve the efficiency increase during the light load period. 
     Also after a time t 15 , like in the above description, the switching stop process at the time of detecting the backward flow and the on/off control of the timer portion  14  and backward flow detection portion  15  are repeated. In other words, during a period when the output voltage Vout is larger than the reference voltage Vref, the switching power supply device A in the sleep mode stops the switching operation of the transistors  11  and  12  and turns off the circuit blocks other than the main comparator  13 , thereby reducing the self-consumption of electric current as much as possible. After that, if a decline in the output voltage Vout is detected by the main comparator  13 , the circuit blocks kept in the off-state restart to resume the switching operation of the transistors  11  and  12 . By employing such a structure, it is possible to pull down an average electric-current consumption of the switching power supply device A; accordingly, it becomes possible to achieve the efficiency increase during the high load period. 
     In the meantime, in the above first embodiment, the timer portion  14  and the backward flow detection portion  15  are completely kept in the off-state until the comparison signal S 1  rises to the high level; accordingly, it takes a relatively long time to restart various operations after the comparison signal S 1  rises to the high level. On the other hand, at the time point when the comparison signal S 1  rises to the high level, the switching control portion  17  turns on the transistor  11  with no delay. Because of this, in the above first embodiment, a restart delay of the timer portion  13  and backward flow detection portion  15  can become a problem. 
     For example, the timer portion  14  generally uses a constant current to charge the capacitor and counts a period as the on-period ton required to reach a predetermined threshold value after the charge start. At this time, the restart period of the timer portion  14  (period required for the constant current to reach a predetermined target value (constant value) after the timer portion  14  is turned on) must be sufficiently shorter than a target value of the on-period ton. However, the restart period of the timer portion  14  completely kept in the off-state does not have a large difference from the target value of the on-period ton; accordingly, the on-period ton of the transistor  11  becomes unnecessarily long, and there is a risk that an overshoot of the output voltage Vout occurs. 
     Besides, if the restart of the backward flow detection portion  15  delays, it becomes impossible to turn off the transistor  12  with no delay at the time point when the backward flow for the transistor  12  occurs; accordingly, the electric charges stored in the capacitor C 1  are discarded to the ground terminal, which incurs the efficiency decline. Especially, in an application (application in which the output voltage Vout is low) that has a small duty, the period allowed for the restart of the backward flow detection portion  15  becomes short; accordingly, the above trouble easily occurs. 
       FIG. 3  is a time chart showing the trouble due to the restart delay of the backward flow detection portion  15 , and illustrates, from top in order, the feedback voltage Vfb; the reference voltage Vref; the comparison signal S 1 ; the timer signal S 2 ; the gate signals G 1  and G 2 ; the inductor current IL; the switch voltage Vsw; the zero-cross detection signal S 3 ; the skip signal S 4 ; and on/off states of the timer portion  14  and backward flow detection portion  15 . 
     In essence, after the backward flow detection portion  15  is turned on at a time t 21  when the comparison signal S 1  is raised to the high level, the backward flow detection portion  15  must complete the restart by a time t 23  when the backward flow current for the transistor  12  occurs. In a case where the restart of the backward flow detection portion  15  is not completed at the time t 23 , as shown in  FIG. 3 , it is impossible to raise the zero-cross detection signal S 3  to the high level at the time t 23 ; accordingly, the backward flow current continues flowing in the transistor  12 . 
     Besides, after the time t 23 , in the state where the backward flow current is flowing in the transistor  12 , the output voltage Vout sharply declines to become smaller than the reference voltage Vref; accordingly, the comparison signal S 1  is raised to the high level at a timing (time t 24 ) earlier than usual and the on-timing of the transistor  11  becomes earlier than usual. 
     In the meantime, the example of  FIG. 3  shows the operation in which the restart of the backward flow detection portion  15  is completed by a time t 26  until when a second backward flow current flows in the transistor  12  and the backward flow current for the transistor  12  is shut down at the time t 26 ; however, in a case where the restart of the backward flow detection portion  15  is not completed at the time t 26 , the on-timing of the transistor  11  becomes earlier as described above. 
     As described above, if the restart delay of the backward flow detection portion  15  occurs, the switching stop function and sleep function during the light load period do not sufficiently work; accordingly, the efficiency decline is incurred. 
     Hereinafter, a switching power supply device, which is further improved considering that it takes a long time for the timer portion  14  and the backward flow detection portion  15  to restart, is described in detail using an example. 
     &lt;Second Embodiment&gt; 
       FIG. 4  is a block diagram showing a second embodiment of the switching power supply device. The second embodiment has substantially the same structure as the above first embodiment, and has a feature in that a delay portion  18  is added between the main comparator  13  and the switching control portion  17 . Because of this, the same components as the first embodiment are indicated by the same reference numbers as in  FIG. 1  to skip double description, and hereinafter, description is performed focusing on the feature portion of the second embodiment. 
     The delay portion  18  performs a one-shot output of a delay comparison signal S 1   d  at a time point when a predetermined delay period td elapses after a rising edge of the comparison signal S 1  occurs. In the meantime, the delay period td may be set at the restart period of the timer portion  14 , for example. 
     The switching control portion  17  receives an input of the delay comparison signal S 1   d  instead of the comparison signal S 1 , and performs the on/off control of the transistors  11  and  12  using the non-linear control method in accordance with the delay comparison signal S 1   d  and the timer signal S 2 . 
       FIG. 5  is a time chart showing a sleep operation (operation in a case where the sleep signal SLEEP is at the high level) in the second embodiment, and illustrates, from top in order, the feedback voltage Vfb; the reference voltage Vref; the comparison signal S 1 ; the delay comparison signal S 1   d ; the timer signal S 2 ; the gate signals G 1  and G 2 ; the inductor current IL; the switch voltage Vsw; the zero-cross detection signal S 3 ; the skip signal S 4 ; and on/off states of the timer portion  14  and backward flow detection portion  15 . 
     At a time t 31 , if the feedback voltage Vfb becomes lower than the reference voltage Vref and the comparison signal S 1  is raised to the high level, the delay portion  18  uses the rising edge as a trigger to start the counting of the delay period td. 
     Besides, at the time t 31 , the rising edge of the comparison signal S 1  is used as a trigger to turn on the timer portion  14  and the backward flow detection portion  15 . In the meantime, at the time t 31 , the transistors  11  and  12  both are still kept in the off-state and the timer portion  14  does not start the counting (operation of charging the capacitor using the constant current) of the on-period ton. 
     At a time t 32 , if the delay portion  18  completes the counting of the delay period td and a trigger pulse is generated in the delay comparison signal S 1   d , the gate signal G 1  is raised to the high level to turn on the transistor  11 . On the other hand, during a period of t 32  to t 33 , the gate signal G 2  is kept at the low level and the transistor  12  is still kept in the off-state. As a result of this, during the period of t 32  to t 33 , the switch voltage Vsw rises to substantially the input voltage Vin and the inductor current IL increases. 
     Besides, at the time t 32 , the transistor  11  is turned on, thereafter, the timer portion  14  starts the counting of the on-period ton. As described above, according to the switching power supply device A in the second embodiment, it is sufficient if the restart of the timer portion  14  is completed not by the time t 31  when the comparison signal S 1  is raised to the high level but by the time t 32  when the trigger pulse is generated in the delay comparison signal S 1   d . Therefore, it is possible to add a margin of the delay period td to the restart period of the timer portion  14  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the timer portion  14  and to prevent the overshoot of the output voltage Vout. 
     In the meantime, because the on-timing of the transistor  11  is delayed by the delay period td, during the period of t 31  to t 32 , the output voltage Vout (and the feedback voltage Vfb) declines to become lower than a target value. However, the output current Iout flowing in the load is small during the light load period; accordingly, also the decline in the output voltage Vout becomes small. For example, in a case where the delay period td is 1 μs; the output current Iout is 1 mA; and the capacitor C 1  has a capacitance value of 22 μF, the output voltage Vout declines by only 45 μV (=Iout×td/C 1 ). Accordingly, even in the case where the structure in the second embodiment is employed, it is conceivable that trouble with the load is unlikely to occur. 
     At the time t 33 , if the timer portion  14  completes the counting of the on-period ton and a trigger pulse is generated in the timer signal S 2 , the gate signal G 1  is dropped to the low level and the gate signal G 2  is raised to the high level. As a result of this, the transistor  11  is turned off and the transistor  12  is turned on. At this time, an induced electromotive force occurs in the inductor L 1  to continue flowing the inductor current IL in the same direction as until now; accordingly, the inductor current IL flows from the ground terminal into the inductor L 1  via the transistor  12 . Therefore, the switch voltage Vsw declines to the negative voltage value that is lower than the ground voltage GND by the drop voltage across the transistor  12 . In the meantime, like in the first embodiment, the timer portion  14  is turned off with no delay at the time point when the counting of the on-period ton is completed. 
     At a time t 34 , if the inductor current IL becomes lower than the zero value; a backward flow current for the transistor  12  occurs and the polarity of the switch voltage Vsw is switched from negative to positive, the zero-cross detection signal S 3  is raised to the high level, and further, the skip signal S 4  is raised to the high level. As a result of this, the transistor  12  is forcibly turned off. By employing such a structure, it is possible to quickly shut down the backward flow current for the transistor  12 ; accordingly, it becomes possible to solve the efficiency decline during the light load period. Here, according to the switching power supply device A in the second embodiment, it is possible to add a margin of the delay period td to the restart period of the backward flow detection portion  15  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the backward flow detection portion  15  and to quickly shut down the backward flow current for the transistor  12 . In the meantime, like in the first embodiment, the backward flow detection portion  15  is turned off with no delay at the time point when the backward flow detection operation is completed. 
     Also after a time t 35 , like in the above description, the switching stop process at the time of detecting the backward flow and the on/off control of the timer portion  14  and backward flow detection portion  15  are repeated. 
     As described above, in the switching power supply device A according to the second embodiment, at the time point when the feedback voltage Vfb becomes lower than the reference voltage Vref, first, only the restart of the timer portion  14  and backward flow detection portion  15  is performed, and further at the time point when the predetermined delay period td elapses, the switching operation of the transistors  11  and  12  is resumed. By employing such a structure, it becomes possible to solve the restart delay of the timer portion  14  and backward flow detection portion  15  and to achieve the efficiency increase during the light load period. 
     In the meantime, in the case where the sleep signal SLEEP is input in the delay portion  18  and the switching power supply device A is kept in the sleep mode, the above one-shot output of the delay comparison signal S 1   d  is performed. On the other hand, in the case where the switching power supply device A is kept in the non-sleep mode, a through-output of the comparison signal S 1  as the delay comparison signal S 1   d  is performed. In other words, in the case where the switching power supply device A is kept in the non-sleep mode, the comparison signal S 1  bypasses the delay portion  18  to be directly input into the switching control portion  17 . 
     By employing such a structure, in the case where the output current tout for the load becomes large and the switching power supply device A is brought to the non-sleep mode, the above delay period td becomes 0; accordingly, it becomes possible to curb a decline in the output voltage Vout. In the meantime, in the case where the switching power supply device A is kept in the non-sleep mode, the timer portion  14  and the backward flow detection portion  15  both are kept in the normally turned-on state; accordingly, their restart delay does not become a problem. 
       FIG. 6  is a time chart showing a mask operation of the comparison signal S 1  in the delay portion  18 , and illustrates, from top in order, the feedback voltage Vfb; the reference voltage Vref; the comparison signal S 1 ; the delay comparison signal S 1   d ; and the timer signal S 2 . 
     As described above, in the switching power supply device A according to the second embodiment, because the on-timing of the transistor  11  is delayed by the delay period td, during a period of t 41  to t 43 , the feedback voltage Vfb becomes lower than the reference voltage Vref and the comparison signal S 1  is kept at the high level. Here, if the comparison signal S 1  drops to the low level by a time t 44  when the counting of the on-period ton is completed, a problem does not occur; however, in a case where the comparison signal S 1  is kept at the high level even after the time t 44  because of an internal delay of the main comparator  13  and the like, there is a risk that the delay portion  18  receiving the input of the comparison signal S 1  generates an unexpected trigger pulse in the delay comparison signal S 1   d  and the transistor  11  is turned on unnecessarily. 
     Because of this, in the switching power supply device A according to the second embodiment, a structure is employed, in which alter performing the one-shot output of the delay comparison signal S 1   d , the delay portion  18  neglects the comparison signal S 1  for a predetermined mask period tmask. By employing such a structure, the transistor  11  is prevented from unnecessarily being turned on a plurality of times; accordingly, it becomes possible to achieve the efficiency increase during the light load period. 
       FIG. 7  is a block diagram showing a structural example of the backward flow detection portion  15 . The backward flow detection portion  15  in the present structural example includes: a comparator  150 ; electric current sources  151  and  152 ; switches  153  to  155 ; a resistor  156 ; and a logic portion  157 . 
     A non-inverting input terminal (+) of the comparator  150  is connected to a first terminal of the switch  155  and to a first terminal of the resistor  156 . A second terminal of the switch  155  is connected to an application terminal (external terminal T 2 ) for the switch voltage Vsw. An inverting input terminal (−) of the comparator  150  and a second terminal of the resistor  156  both are connected to an application terminal (external terminal T 3 ) for the ground voltage GND. 
     In the meantime, the switch  155  is turned on/off in synchronization with the transistor  12 . Accordingly, when the transistor  12  is kept in the on-state, the non-inverting input terminal (+) of the comparator  150  is electrically connected to the application terminal for the switch voltage Vsw via the switch  155 , while when the transistor  12  is kept in the off-state, the non-inverting input terminal (+) of the comparator  150  is electrically pulled down to the application terminal for the ground voltage GND via the resistor  156 . 
     First terminals of the electric current sources  151  and  152  both are connected to the power supply line. A second terminal (output terminal for a drive current I 1 ) of the electric current source  151  is connected to a first terminal of the switch  153 . A second terminal (output terminal for drive current I 2 ) of the electric current source  152  is connected to a first terminal of the switch  154 . A second terminal of the electric current source  154  is connected to the first terminal of the switch  153 . A second terminal of the switch  153  is connected to a power supply terminal of the comparator  150 . 
     The logic portion  157  receives inputs of the comparison signal S 1 , zero-cross detection signal S 3  and sleep signal SLEEP to perform on/off control of the switches  153  and  154 . Describing more specifically, in the ease where the sleep signal SLEEP is at the low level, the logic portion  157  puts the switch  153  into a normally turned-on state and the switch  154  into a normally turned-off state irrespective of the comparison signal S 1  and the zero-cross detection signal S 3 . In other words, in the case where the switching power supply device A is kept in the non-sleep mode, as a drive current I 0  for the comparator  150 , the drive current I 1  only is normally supplied. 
     On the other hand, in the case where the sleep signal SLEEP is at the high level, the logic portion  157  puts the switch  154  into a normally turned-on state, uses a rising edge of the comparison signal S 1  as a trigger to turn on the switch  153 , and uses a rising edge of the zero-cross detection signal S 3  as a trigger to turn off the switch  153 . In other words, in the case where the switching power supply device A is kept in the sleep mode, as the drive current I 0  for the comparator  150 , a sum current of the drive currents I 1  and I 2  is intermittently supplied. As described above, the backward flow detection portion  15  in the present structural example operates on the drive current I 0  (=I 1 ÷I 2 ) larger than usual in the sleep mode and operates on the usual drive current I 0  (=I 1 ) in the non-sleep mode; accordingly, it becomes possible to shorten the restart period of the backward flow detection portion  15  in the sleep mode. 
     &lt;Third Embodiment&gt; 
       FIG. 8  is a block diagram showing a third embodiment of the switching power supply device. The third embodiment has substantially the same structure as the above first embodiment, and has a feature in that a startup comparator  19  is added in parallel with the main comparator  13 . Because of this, the same components as the first embodiment are indicated by the same reference numbers as in  FIG. 1  to skip double description, and hereinafter, description is performed focusing on the feature portion of the third embodiment. The startup comparator  19  compares the feedback voltage Vfb applied to the inverting input terminal (−) and a threshold value voltage Vref 0  (&gt;Vref) applied to the non-inverting input terminal (+) with each other to generate a startup signal S 0 . The startup signal S 0  goes to a low level when the feedback voltage Vfb is higher than the threshold value voltage Vref 0 , and goes to a high level when the feedback voltage Vfb is lower than the threshold value voltage Vref 0 . The threshold value voltage Vref 0  may be set such that a rising edge of the comparison signal S 1  occurs at a time point when a predetermined preparation period tpre elapses after a rising edge of the startup signal S 0  occurs. In the meantime, the preparation period tpre may be set at the restart period of the timer portion  14 , for example. Besides, it is desirable that a constant voltage (band gap voltage or the like), which does not depend on the input voltage Vin and the ambient temperature like the reference voltage Vref, is used as the reference voltage Vref. 
     The timer portion  14  and the backward flow detection portion  15  receive an input of the startup signal S 0  instead of the comparison signal S 1 , and are turned on at a time point when the rising edge of the startup signal S 0  occurs before the rising edge of the comparison signal S 1  occurs. 
       FIG. 9  is a time chart showing a sleep operation (operation in the case where the sleep signal SLEEP is at the high level) of the third embodiment, and illustrates, from top in order, the feedback voltage Vfb; the threshold value voltage Vref 0 ; the reference voltage Vref; the startup signal S 0 ; the comparison signal S 1 ; the timer signal S 2 ; the gate signals G 1  and G 2 ; the inductor current IL; the switch voltage Vsw; the zero-cross detection signal  53 ; the skip signal S 4 ; and on/off states of the timer portion  14  and backward flow detection portion  15 . 
     At a time t 51 , if the feedback voltage Vfb becomes lower than the threshold value voltage Vref 0  and the startup signal S 0  is raised to the high level, the rising edge is used as a trigger to turn on the timer portion  14  and the backward flow detection portion  15 . In the meantime, at the time t 51 , the transistors  11  and  12  both are still kept in the off-state, and the timer portion  14  does not start the operation of counting (operation of charging the capacitor using the constant current) the on-period ton. 
     At a time t 52 , if the feedback voltage Vfb becomes lower than the reference voltage Vref and the comparison signal S 1  is raised to the high level, the gate signal G 1  is raised to the high level and the transistor  11  is turned on. On the other hand, during a period of times t 52  to t 53 , the gate signal G 2  is kept at the low level and the transistor  12  is still kept in the off-state. As a result of this, during the period of times t 52  to t 53 , the switch voltage Vsw rises to substantially the input voltage Vin and the inductor current IL increases. 
     Besides, at the time t 52 , the transistor  11  is turned on, thereafter, the tinier portion  14  starts the counting of the on-period ton. As described above, according to the switching power supply device A in the third embodiment, a pre-restart of the timer portion  14  is started at the time t 51  that is earlier than the time t 52  when to start the counting of the on-period ton by the preparation period tpre. Therefore, it is possible to add a margin of the preparation period tpre to the restart period of the timer portion  14  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the timer portion  14  and to prevent the overshoot of the output voltage Vout. 
     At the time t 53 , if the timer portion  14  completes the counting of the on-period ton and a trigger pulse is generated in the timer signal  52 , the gate signal G 1  is dropped to the low level and the gate signal G 2  is raised to the high level. As a result of this, the transistor  11  is turned off and the transistor  12  is turned on. At this time, an induced electromotive force occurs in the inductor L 1  to continue flowing the inductor current IL in the same direction as until now; accordingly, the inductor current IL flows from the ground terminal into the inductor L 1  via the transistor  12 . Therefore, the switch voltage Vsw declines to a negative voltage value that is lower than the ground voltage GND by a drop voltage across the transistor  12 . In the meantime, like in the first embodiment, the timer portion  14  is turned off with no delay at the time point when the counting of the on-period ton is completed. 
     At a time t 54 , if the inductor current IL becomes lower than the zero value; a backward flow current for the transistor  12  occurs and the polarity of the switch voltage Vsw is switched from negative to positive, the zero-cross detection signal S 3  is raised to the high level, and further, the skip signal S 4  is raised to the high level. As a result of this, the transistor  12  is forcibly turned off. By employing such a structure, it is possible to quickly shut down the backward flow current for the transistor  12 ; accordingly, it becomes possible to solve the efficiency decline during the light load period. Here, according to the switching power supply device A in the third embodiment, it is possible to add a margin of the preparation period tpre to the restart period of the backward flow detection portion  15  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the backward flow detection portion  15  and to quickly shut down the backward flow current for the transistor  12 . In the meantime, like in the first embodiment, the backward flow detection portion  15  is turned off with no delay at the time point when the backward flow detection operation is completed. 
     Also after a time t 55 , like in the above description, the switching stop process at the time of detecting the backward flow and the on/off control of the timer portion  14  and backward flow detection portion  15  are repeated. 
     As described above, in the switching power supply device A according to the third embodiment, at the time point when the feedback voltage Vfb becomes lower than the threshold value voltage Vref 0  (&gt;Vref), first, only the restart of the timer portion  14  and backward flow detection portion  15  is performed, and further at the time point when the feedback voltage Vfb declines to become lower than the reference voltage Vref, the switching operation of the transistors  11  and  12  is resumed. By employing such a structure, it becomes possible to solve the restart delay of the timer portion  14  and backward flow detection portion  15  and to achieve the efficiency increase during the light load period. Besides, according to the switching power supply device A in the third embodiment, unlike the second embodiment, the on-timing of the transistor  11  is not delayed; accordingly, the decline in the output voltage Vout is not incurred. 
     &lt;Fourth Embodiment&gt; 
       FIG. 10  is a block diagram showing a fourth embodiment of the switching power supply device. The fourth embodiment has substantially the same structure as the above first embodiment, and has a feature in that the reference voltage Vref is a variable value and a pulse distribution portion  20  is added between the main comparator  13  and the switching control portion  17 . Because of this, the same components as the first embodiment are indicated by the same reference numbers as in  FIG. 1  to skip double description, and hereinafter, description is performed focusing on the feature portion of the fourth embodiment. 
     The pulse distribution portion  20  distributes two pulses (described in detail later), which occur in the comparison signal S 1  thanks to variable control of the reference voltage Vref, to a first comparison signal S 1   a  and a second comparison signal S 1   b.    
     The timer portion  14  and the backward flow detection portion  15  receive an input of the first comparison signal S 1   a  instead of the comparison signal S 1 , and are turned on at a time point when a rising edge of the first comparison signal S 1   a  occurs before a rising edge of the second comparison signal S 1   b  occurs. 
     The switching control portion  17  receives an input of the second comparison signal S 1   b  instead of the comparison signal S 1 , and performs the on/off control of the transistors  11  and  12  using the non-linear control method in accordance with the second comparison signal S 1   b  and the timer signal S 2 . 
       FIG. 11  is a time chart showing a sleep operation (operation in the case where the sleep signal SLEEP is at the high level) of the fourth embodiment, and illustrates, from top in order, the feedback voltage Vfb; the reference voltage Vref; the comparison signal S 1 ; the first comparison signal S 1   a ; the second comparison signal S 1   b ; the timer signal S 2 ; the gate signals G 1  and G 2 ; the inductor current IL; the switch voltage Vsw; the zero-cross detection signal S 3 ; the skip signal S 4 ; and on/off states of the timer portion  14  and backward flow detection portion  15 . 
     The reference voltage Vref is a variable value that is switched to voltages Va and Vb (where Va&gt;Vb) in two steps. The voltage Va is a voltage to decide the restart timing of the timer portion  14  and backward flow detection portion  15 , and may be set such that the rising edge of the second comparison signal S 1   b  occurs at a time point when the predetermined preparation period tpre elapses after the rising edge of the first comparison signal S 1   a  occurs. In the meantime, the preparation period tpre may be set at the restart period of the timer portion  14 , for example. Besides, the voltage Vb is a voltage to decide the target value of the output voltage Vout. 
     The reference voltage Vref is triggered by the rising edge of the first comparison signal S 1   a  to be pulled down from the voltage Va to the voltage Vb, on the other hand, is triggered by the rising edge of the timer signal S 2  to be pulled up from the voltage Vb to the voltage Va. However, the variable timing of the reference voltage Vref is not limited to this, but for example, the rising edge of the second comparison signal S 1   b  or the rising edge of the zero-cross detection signal S 3  may be used as a trigger to pull up the reference voltage Vref from the voltage Vb to the voltage Va, or a structure may be employed, in which the reference voltage Vref may be alternately switched between the voltage Va and the voltage Vb at every rising edge of the comparison signal S 1 . 
     At a time t 61 , if the feedback voltage Vfb becomes lower than the reference voltage Vref (=Va) and the first pulse occurring in the comparison signal S 1  is distributed as the first comparison signal S 1   a , the rising edge of the first comparison signal S 1   a  is used as a trigger to turn on the timer portion  14  and the backward flow detection portion  15 . In the meantime, at the time t 61 , the transistors  11  and  12  both are still kept in the off-state, and the timer portion  14  does not start the operation of counting (operation of charging the capacitor using the constant current) the on-period ton. 
     At a time t 62 , if the feedback voltage Vfb becomes lower than the reference voltage Vref (=Vb) and the second pulse occurring in the comparison signal S 1  is distributed as the second comparison signal S 1   b , the gate signal G 1  is raised to the high level and the transistor  11  is turned on. On the other hand, during a period of times t 62  to t 63 , the gate signal G 2  is kept at the low level and the transistor  12  is still kept in the off-state. As a result of this, during the period of times t 62  to t 63 , the switch voltage Vsw rises to substantially the input voltage Vin and the inductor current IL increases. 
     Besides, at the time t 62 , the transistor  11  is turned on, thereafter, the timer portion  14  starts the counting of the on-period ton. As described above, according to the switching power supply device A in the fourth embodiment, the pre-restart of the timer portion  14  is started at the time t 61  that is earlier than the time t 62  when to start the counting of the on-period ton by the preparation period tpre. Therefore, it is possible to add a margin of the preparation period tpre to the restart period of the timer portion  14  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the timer portion  14  and to prevent the overshoot of the output voltage Vout. 
     At the time t 63 , if the timer portion  14  completes the counting of the on-period ton and a trigger pulse is generated in the timer signal S 2 , the gate signal G 1  is dropped to the low level and the gate signal G 2  is raised to the high level. As a result of this, the transistor  11  is turned off and the transistor  12  is turned on. At this time, an induced electromotive force occurs in the inductor L 1  to continue flowing the inductor current IL in the same direction as until now; accordingly, the inductor current IL flows from the ground terminal into the inductor L 1  via the transistor  12 . Therefore, the switch voltage Vsw declines to a negative voltage value that is lower than the ground voltage GND by a drop voltage across the transistor  12 . In the meantime, like in the first embodiment, the timer portion  14  is turned off with no delay at the time point when the counting of the on-period ton is completed. 
     At a time t 64 , if the inductor current IL becomes lower than the zero value; a backward flow current for the transistor  12  occurs and the polarity of the switch voltage Vsw is switched from negative to positive, the zero-cross detection signal S 3  is raised to the high level, and further, the skip signal S 4  is raised to the high level. As a result of this, the transistor  12  is forcibly turned off. By employing such a structure, it is possible to quickly shut down the backward flow current for the transistor  12 ; accordingly, it becomes possible to solve the efficiency decline during the light load period. Here, according to the switching power supply device A in the fourth embodiment, it is possible to add a margin of the preparation period tpre to the restart period of the backward flow detection portion  15  compared with the first embodiment; accordingly, it becomes possible to solve the restart delay of the backward flow detection portion  15  and to quickly shut down the backward flow current for the transistor  12 . In the meantime, like in the first embodiment, the backward flow detection portion  15  is turned off with no delay at the time point when the backward flow detection operation is completed. 
     Also after a time t 65 , like in the above description, the switching stop process at the time of detecting the backward flow and the on/off control of the timer portion  14  and backward flow detection portion  15  are repeated. 
     As described above, in the switching power supply device A according to the fourth embodiment, the reference voltage Vref is the variable value that is switched to the voltages Va and Vb in the two steps, and at the time point when the feedback voltage Vfb becomes lower than the voltage Va, first, only the restart of the timer portion  14  and backward flow detection portion  15  is performed, and further at the time point when the feedback voltage Vfb declines to become lower than the voltage Vb, the switching operation of the transistors  11  and  12  is resumed. By employing such a structure, it becomes possible to solve the restart delay of the timer portion  14  and backward flow detection portion  15  and to achieve the efficiency increase during the light load period. Besides, according to the switching power supply device A in the fourth embodiment, unlike the third embodiment, it is not necessary to dispose another startup comparator  19 ; accordingly, the circuit scale does not increase unnecessarily. 
     &lt;Application to Television&gt; 
       FIG. 12  is a block diagram showing a structural example of a television that incorporates the switching power supply device A. Besides,  FIG. 13A to 13C  are respectively a front view, side view, and rear view of the television that incorporates the switching power supply device A. The television X in the present structural example has: a tuner portion X 1 ; decoder portion X 2 ; a display portion X 3 ; a speaker portion X 4 ; an operation portion X 5 ; an interface portion X 6 ; a control portion X 7 ; and a power supply portion X 8 . 
     The tuner portion X 1  selects a broadcast signal for a desired channel from a reception signal received by an antenna X 0  that is externally connected to the television X. 
     The decoder portion X 2  generates an image signal and a voice signal from the broadcast signal selected by the tuner X 1 . Besides, the decoder portion X 2  includes a function as well to generate an image signal and a voice signal based on an external input signal from the interface portion X 6 . 
     The display portion X 3  outputs the image signal, which is generated by the decoder portion X 2 , as an image. 
     The speaker portion X 4  outputs the voice signal, which is generated by the decoder portion, as a voice. 
     The operation portion X 5  is a human interface that accepts a user operation. As the operation portion X 5 , it is possible to use a button, a switch, a remote controller and the like. 
     The interface portion X 6  is a front end that accepts an external input signal from external devices (optical disc player, hard disc drive and the like). 
     The control portion X 7  comprehensively controls the operation of each of the portions X 1  to X 6 . As the control portion X 7 , it is possible to use a CPU [central processing unit] and the like. 
     The power supply portion X 8  performs power supply to each of the portions X 1  to X 7 . As the power supply portion X 8 , it is possible to preferably use the above switching power supply device A. 
     &lt;Other Modifications&gt; 
     Besides, the structure of the present invention is able to be modified in various ways without departing from the spirit of the present invention besides the above embodiments. For example, in the above embodiments, the switching power supply device employing the bottom detection on-period fixing system is described as an example; however, the present invention is also applicable to a switching power supply device that employs an upper detection off-period fixing system. 
     As described above, it should be considered that the above embodiments are examples in all respects and are not limiting, and the technological scope of the present invention is not indicated by the above description of the embodiments but by the claims, and all modifications within the scope of the claims and the meaning equivalent to the claims are covered. 
     INDUSTRIAL APPLICABILITY 
     The switching power supply device according to the present invention is preferably usable for personal computers, liquid crystal televisions, DVD recorders and the like. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1  semiconductor apparatus (switching power supply IC) 
               11  MOS field effect transistor of N channel type (output transistor) 
               12  MOS field effect transistor of N channel type (synchronization rectification transistor) 
               13  main comparator 
               14  timer portion 
               15  backward flow detection portion 
               150  comparator 
               151 ,  152  electric current sources 
               153  to  155  switches 
               156  resistor 
               157  logic portion 
               16  latch portion 
               17  switching control portion 
               18  delay portion 
               19  startup comparator 
               20  pulse distribution portion 
             L 1  inductor 
             R 1 , R 2  resistors 
             C 1  capacitor 
             T 1  to T 5  external terminals 
             A switching power supply device 
             X television 
             X 0  antenna 
             X 1  tuner portion 
             X 2  decoder portion 
             X 3  display portion 
             X 4  speaker portion 
             X 5  operation portion 
             X 6  interface portion 
             X 7  control portion 
             X 8  power supply portion