BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to an improved switching power supply which saves power under a small load to a no load condition. 
     2. Description of the Prior Art 
     A conventional switching power supply is shown in FIG.  1  and comprises a transformer TR including a primary winding L 1  and a secondary winding L 2 ; a magnetic flux detector  10 ; a current loop circuit  40  for controlling a DC current applied to the primary winding L 1 ; a feedback circuit  30  for detecting and feeding an output voltage supplied to a load Z, on the secondary winding L 2  side, back to the current loop circuit  40 ; and a secondary circuit  20  for supplying a voltage from the secondary winding L 2  to the load Z. 
     The magnetic flux detector  10  includes a first comparator TRCMP, wherein a magnetic flux energy accumulated in the transformer TR is detected as a voltage signal by means of a current I 2  flowing through the secondary winding L 2  and a resistor R 1 , and to which the voltage signal and a reference voltage Vt 1  are applied; and a first flip flop circuit FF 1 , wherein the output signal V 6  from the first comparator TRCMP is inputted to the set terminal S; a gate signal V 2  for a switching device SW is inputted to the reset input terminal R; and an output signal V 7  from the output terminal Q of the first flip flop circuit FFL is connected to the set input terminal S of the second flip flop circuit FF 2 , discussed hereinafter. 
     The secondary circuit  20  includes the secondary winding L 2  of transformer TR; a rectifying diode D connected in series to the secondary winding L 2 ; a capacitor C connected in parallel to the secondary winding L 2 ; and a load Z connected in parallel to the capacitor C. The feedback circuit  30  is located on the output load side, where a voltage applied to the load Z and a reference voltage Vt 2  are inputted to the feedback circuit  30  in order to negatively feed back a current control signal V 4  from an error amplifier EA, which outputs the current control signal V 4 , to a second comparator CSCMP so that a given output voltage is maintained. 
     The current loop circuit  40  includes the second comparator CSCMP wherein a current L 1  flowing through the primary winding L 1  is detected by means of a resistor R 2  and the resulting voltage signal V 3  is inputted to the non-inverting input terminal and the current control signal V 4  from the feedback circuit  30  is inputted to the inverting input terminal; the second flip flop circuit FF 2  wherein the output signal V 5  of the second comparator CSCMP is inputted to the reset input terminal R; the output signal V 7  of the first flip flop circuit FF 1  is inputted to the set input terminal S and the output signal V 2  of the output terminal Q is inputted to the gate of the switching device SW; and the switching device SW is turned ON and OFF by means of the output signal V 2  from the second flip flop circuit FF 2 . The switching device SW is connected in series to the primary winding L 1  of the transformer TR to control the current L 1  flowing through the primary winding L 1 . 
     The switching power supply of FIG. 1 is operated as follows, with reference to the timing chart of FIG. 2, wherein the current L 1  (voltage signal V 3 ) flowing through the switching device SW, for applying a voltage to the primary winding L 1 , reaches the current control signal V 4 ; the output signal V 5  of the second comparator CSCMP changes to a high state and the output V 2  of the second flip flop circuit FF 2  is changed from a high state to a low state to turn OFF the switching device SW. That is, the switching device SW remains turned ON until the current flowing through the switching device SW reaches the current control signal V 4 , so that magnetic flux energy is accumulated in transformer TR. 
     When the switching device SW is turned OFF, the magnetic flux energy accumulated in transformer TR is supplied to the load Z through the secondary winding L 2 , rectifying diode D, etc, as a load current. 
     When the magnetic flux energy in the transformer TR becomes depleted, as time lapses, the voltage of the secondary winding L 2  drops rapidly to fall below the reference voltage Vt 1 . Thus, the output signal V 6  of the first comparator TRCMP goes low and the output signal V 7  of the first flip flop circuit FFL goes high, thereby causing the output signal V 2  of the second flip flop circuit FF 2  to be changed to a higher state. This in turn causes the switching device SW to be turned ON. If the switching device SW is turned ON, the current I 1  flowing through the switching device SW (i.e. voltage signal V 3 ) continues to rise until the current again reaches the current control signal V 4  level. In this manner, the switching power supply repeats the above operation to sustain self excited oscillation. 
     This means that the self excited oscillation in the conventional switching power supply is based on the mechanism wherein energy accumulated in the transformer TR is controlled by switching ON and OFF, the switching device SW to the difference between the voltage detected on the secondary winding L 2  side and the reference voltage Vt 1   
     In the described switching power supply, the current supplied to the load is, in principle, inversely proportional to the oscillation frequency. This is because the energy exchanged with the transformer TR at each cycle is also reduced when the amount of current supplied to the load is decreased, thereby resulting in a shorter time interval at which the switching device SW is turned ON and OFF. When the ON-OFF time interval of the switching device SW is shortened, the frequency of self excited oscillation is increased. This could cause such problems as power loss in the switching device SW, core loss in the transformer TR, increase in noise, and failure in oscillation. An excess increase in the oscillation frequency thus must be avoided. For this purpose, the minimum load is fixed using a bleeder resistor in some cases. This could also cause a problem, namely, that the power consumption then is increased even when the load is small or there is no load at all, since increases in the above discussed losses and other losses result from the frequency increases. Consequently, in the convention apparatus, it is difficult to reduce power consumption. 
     Moreover, in the art, the unresolved problem is how to prevent the ON-OFF time interval of the switching device SW from becoming shortened more or less under a small load to no load condition, and thereby avoid any excess increase in the frequency of self-excited oscillation. 
     An unsatisfactory attempt to resolve the above problem is shown in FIG. 3, wherein attempt was made to prevent an increase in switching frequency by providing the switching power supply with an oscillator for outputting a fixed frequency pulse signal, that is high state and low state pulses, as ON-OFF signals, rather than using a magnetic flux detector  10  of FIG.  1 . Further power saving is required, however, even when the switching power supply is under a small load condition or a no load condition. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to overcome the aforementioned and other deficiencies and problems of the prior art. 
     The foregoing and other objects are attained in the switching power supply of the invention, wherein delay circuit means are provide for prolonging the time at which the switching device makes the OFF to ON transition. In an illustrative embodiment, the delay circuit comprises a comparator having a hysteresis characteristic and a gate circuit for determining priority of signals applied to a set input terminal and a reset input terminal of a flip flop so as to suitably control the switching device. By such control, the switching power supply of the invention saves power when the load is small or no load condition exists. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram depicting a conventional switching power supply. 
     FIG. 2 is a timing chart depicting behavior of components of the apparatus of FIG.  1 . 
     FIG. 3 is a diagram depicting another conventional switching power supply. 
     FIG. 4 is a diagram depicting an illustrative embodiment of the invention. 
     FIG. 5 is a graph depicting the hysteresis characteristics of the comparator used in the embodiment of FIG.  4 . 
     FIG. 6 is a timing chart depicting behavior of components of the embodiment of FIG.  4 . 
     FIG. 7 is a graph depicting the difference between comparators having hysteresis characteristics and those not having hysteresis characteristics. 
     FIG. 8 is a graph depicting the relationship between a current control signal from an error amplifier and a current in the primary winding of the transformer. 
     FIG. 9 is a diagram depicting another illustrative embodiment of the invention. 
     FIG. 10 is a timing chart depicting behavior of components of the embodiment of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the embodiment shown in FIG. 4, elements which are the same as those in FIGS. 1-3 have the same reference symbols and are not described hereat for sake of clarity of description. FIG. 4 shows a switching power supply provided with a gate G for receiving the output signal V 7  from the first flip flop circuit FFL, to allow an input signal to be applied to the set input terminal S of FF 2 , to precede an input signal to be applied to the reset input terminal R of the second flip flop circuit FF 2 , and with a comparator CSCMP*, which has a hysteresis characteristic, as an alternaative to the second comparator CSCMP located in the current loop circuit  40  of FIG. 1, and in FIG. 4, shown in current loop circuit  40 A. 
     With the use of the gate G in FIG. 4, as compared to lack of such gate in the conventional apparatus of FIG. 1, the reset input terminal R of the second flip flop circuit FF 2  takes precedence whenever the set input terminal conflicts with the reset input terminal. Thus, the output Q of the second flip flop circuit FF 2  always becomes low whenever there is a set-reset conflict. 
     FIG. 5 shows the hysteresis characteristic of the comparator CSCMP*, wherein, the comparator CSCMP* operates in such a manner that: (a) The threshold follows the path (width=Vhys) on the right side of the figure when a differential voltage V 3 −V 4  , resulting from the difference between input V 3  to the non-inverting input terminal (+) and an input V 4  to an inverting input terminal (−), is caused to rise; and, on the other hand (b) The threshold follows the path (width=Vhys) on the left side of the figure when the differential voltage V 3 −V 4  falls. 
     That is to say, the characteristics of the switching power supply is such that the comparator CSCMP* is provided a certain time lag by virtue of the hysteresis characteristics thereof, so that the output signal V 5 , being in a high state, is not immediately changed to a low state. Hence, the interval at which the switching device SW is turned ON and OFF is prolonged so as to suppress the oscillation frequency of the switching power supply. 
     Returning to FIG. 4, the switching power supply is designed to operate in two modes, one of which is normal operation and the other is burst operation with a small load or no load condition. The normal mode is no different from that of the conventional switching power supply described with reference to FIGS. 1 and 2. Thus, such a mode is excluded from the following, and only the “burst operation mode” is discussed hereat with reference to a small load condition and/or no load condition. 
     As shown in FIG. 6, the “burst operation” of the invention is similar to the conventional apparatus, only insofar as the transformer TR is energized by turning ON the switching device SW and then turned OFF to induce energy into the secondary circuit  20 . On the other hand, the switching power supply of the invention differs from the conventional apparatus in that in the prior art, the ON stage period of the switching device SW begins immediately after the magnetic flux accumulated in the transformer TR becomes depleted. In contrast, in the instant invention, such as shown in FIG. 4, the time interval of the OFF to ON transition of the switching device SW is prolonged. 
     The reason for the prolongation of the transition period is as follows. The depletion of the magnetic flux in the transformer TR is detected and memorized by the magnetic flux detector  10 , in an attempt to immediately turn ON the switching device SW by setting the second flip flop circuit FF 2  through activation of the first flip flop circuit FF 1 . However, since the comparator CSCMP*, which has a hysteresis characteristic, continues to hold the output thereof high as long as the time required for the current control signal V 4  to rise by as much as determined by the hysteresis width Vhys, the reset state of the second flip flop circuit FF 2  is not cancelled for the reason of the action take by gate C. Hence, the output signal V 2  of the second flip flop circuit FF 2  remains high for a certain amount of time. 
     This means the threshold voltage V 4  of the comparator CSCMP* is at a point A during the on state period of the switching device SW, as shown in FIG.  7 . Hence, the reset signal V 5  becomes high in the event that signal V 3  becomes greater than the signal V 4  (i.e. point A). Once the output signal V 2  of the second flip flop circuit FF 2  becomes low when the reset signal V 5  changes the state, the switching device SW is turned OFF and the signal V 3  is dropped rapidly. Thus, the reset signal V 5  immediately becomes low, when the comparator CSCMP* has no hysteresis characteristic. On the other hand, when the comparator CSCMP* has a hysteresis characteristic, such as provided in the instant invention, the signal V 5  does not return to a low state until signal V 3  is less the signal V 4  minus the signal Vhys, that is V 3 &lt;(V 4 −Vhys) holds true, at point B. If the current control signal V 4  is smaller than (V 3 +Vhys), the signal V 5  remains high. Thus, the output signal V 2  of the second flip flop circuit FF 2  remains low. If this condition continues for a period of time, the voltage on the load side will fall, and the error amplifier EA will raise the level of the output signal thereof V 4 . Thus, the relationship between the input signals supplied to the comparator CSCMP* satisfies the condition V 3 &lt;(V 3 +Vhys),at point C. This in turn causes cancellation of the reset signal V 5  and allows the output from the second flip flop circuit FF 2  to become low. However, since the set signal V 7  of the first flip flop circuit FF 1  is continuously applied even after the depletion of the magnetic flux in the transformer is detected, the output signal V 2  of the second flip flop circuit FF 2  is changed to a high state, thus bringing one cycle to an end. 
     The switching power supply of the invention is operated in a burst mode in which there is a short pause after the depletion of the magnetic flux in transformer TR. Hence, the operating frequency at which the switching device SW is turned ON and OFF is reduced. Also, there is no pulse signal produced for the levels of the current control signal that satisfy the condition V 4 &lt;Vhys. This means continuous oscillation occurs only when the load is equal to or greater than the value that satisfies the condition V 4 =Vhys. Hence, an increase in the oscillation frequency under a small load condition and/or no load condition, is limited to values equal to or smaller than the value of the above load conditions. 
     FIG. 8 shows the relationship between the current control signal of the current loop circuit  40  and the current output per pulse. It is reasonable to conclude that a limit has been placed on the minimum value of the current output as a result of using the comparator CSCMP* having the hysteresis characteristic. This means the switching power supply of the invention is operated in such a manner that a current supplied to the load Z is regulated by thinning the density of the pulses, such as by means of negative feedback action under load conditions corresponding to values smaller than the minimum values, that is under a small load condition and/or no load condition. Also, continuous oscillation occurs only under a load condition corresponding to values greater than the minimum values. This means that limit has also been placed on the frequency increase. 
     Moreover, in the embodiment of FIG. 9, wherein an oscillator OSC for producing a fixed frequency pulse signal (signal of high state pulses and low state pulses, or ON-OFF signals) is used in place of the magnetic flux detector  10  in FIG. 4, it is possible to obtain the same effects as obtained by the embodiment of FIG.  4 . The embodiment of FIG. 9 is operated in the following manner, with reference to FIG.  10 . 
     FIG. 10 is a timing chart illustrating the behavior of components shown in FIG.  9 . The behavior is basically similar to that shown in the timing chart of FIG.  6 . At a point A or B in FIG. 7, the voltage signal V 5  is increased and the switching power supply is placed in a burst operation mode, thereby prohibiting the signal V 2  from going high, that is provide an ON state. 
     The invention enjoys many advantageous features. For example, utilizing the comparator having the hysteresis characteristics in the current loop circuit, the amount of current that can be turned ON through the switching device is for values greater than the minimum value determined by the hysteresis. Hence, the maximum operating frequency is readily controllable. Another advantage is that by use of the feedback loop action to maintain equilibrium between the load current and the current transferred by the transformer, the switching power supply of the invention changes to the burst operation mode when there is only a small load condition and/or no load condition. Thus, with the instant invention, it is possible to readily decrease the frequency at which the switching device is turned ON and OFF. 
     Moreover, power losses in the components as a result of increase in oscillation frequency at which the switching device is turned ON and OFF is effectively reduced. The burst frequency, maximum oscillation frequency and points at which a transition is made to the burst operation mode is determined by the hysteresis characteristics. The power supply design can be simplified while maintaining design freedom. 
     The foregoing description is illustrative of the principles of the invention. Numerous modifications and extensions thereof would be apparent to the worker skilled in the art. All such modifications and extensions are to be considered to be within the spirit and scope of the invention.
An improved switching power supply which saves power when under a small to no load, wherein a transformer is provided comprising a primary winding to which a DC current is supplied by turning ON and OFF a switching device and a secondary winding for supplying an output signal to the load; an ON state control circuit turns ON the switching device by using the output signal induced at the secondary winding; an OFF state control circuit turns OFF the switching device using a current control signal obtained from the output signal from the secondary winding and a reference signal; and wherein a delay circuit prolongs the time at which the switching device makes the OFF to ON transition.