fig1 is a block diagram showing an example of a switching power supply device of the invention , and fig2 is a block diagram showing the switching power supply device of embodiment 1 , comprising a correction circuit employing a resistor . first , the switching power supply device comprising a correction circuit 2 employing only a resistor r 0 , as shown in fig2 , is explained . here , the correction circuit 2 is connected as an external circuit to the integrated circuit ic . as shown in fig2 , the integrated circuit ic comprising the power supply control circuit ( power supply control means ) has a current comparator 17 , which compares a feedback signal vfb from a feedback circuit 25 with a voltage signal from a current detection input terminal is ; a voltage controlled oscillator 14 , which , when the load is judged to be a light load based on the magnitude of the feedback signal vfb , lowers the operating frequency such that the switching intervals of the switching element q 1 are lengthened ; and , a flip - flop circuit 19 , which generates control signals having a frequency and pulse width corresponding to the operating frequency of the voltage controlled oscillator 14 and the comparison result of the current comparator 17 , to drive the switching element q 1 . the power supply control circuit 10 also has a signal inversion circuit 23 and a level shift circuit 24 . these are the same as those shown in fig1 , and so a detailed explanation is omitted . the resistor r 0 , serving as the correction circuit 2 , is connected between the output terminal out of the power supply control circuit 10 and the signal input terminal for current detection is . similarly to that shown in fig1 , the switching power supply device of fig1 is a negative - detection type switching power supply device . on the other hand , the switching power supply device shown in fig1 differs in that a correction circuit 1 is connected such that a positive offset voltage is applied to the current detection signal vis output from the sense resistor rs , which is the current detection means , and is configured so as to act only during on intervals of the switching element q 1 . otherwise the configuration is as explained for fig1 ; hereinafter , corresponding portions are assigned the same reference numbers , and explanations are omitted . as the power supply control circuit 10 itself in fig1 , an integrated circuit ic configured as in the prior art is used . in the case of the switching power supply device of embodiment 1 shown in fig2 , current flows in the correction resistor r 0 only during on intervals of the switching element q 1 , that is , only during intervals in which the switching signal output from the output terminal out is high , and no current flows during off intervals . ( strictly speaking , because the potential at the connection point between the resistors rs and r 0 during off intervals is the result of division of the output voltage from the auxiliary windings lb by the resistors rb , ra , rs , a small current flows , although it is smaller than the current during on intervals .) consequently , losses can be reduced compared with the prior art . in particular , the lighter the loading , the smaller is the turn - on time ratio , so that power losses due to current flowing in the correction resistor r 0 can be further decreased . fig3 ( a ) and 3 ( b ) show operating waveforms of input signals to the current comparator 17 in the switching power supply device of fig2 . in fig3 ( a ) , a current detection signal ( voltage signal ) during heavy loading is shown . this current detection signal is a voltage signal of polarity which declines with respect to a reference potential ( in this case , ground potential ) of the switching power supply device as the current flowing in the switching element q 1 increases , and upon reaching the judgment reference voltage vfb 3 of the current comparator 17 ( the same as the hypothetical signal vfb 3 in the explanation of fig1 ( a )- 16 ( c )), and causes the switching element q 1 to be turned off . during the instant in which a switching signal to turn on the switching element q 1 has been output but the switching element q 1 has not yet turned on , correction of the current detection signal by the resistor r 0 begins and the current detection signal takes on a positive value , but when the switching element turns on and a current detection signal with large absolute value corresponding to the heavy load appears , the corrected current detection signal vis becomes a negative signal in the initial stage . in fig3 ( b ) , a current detection signal during light loading is shown . similarly to fig3 ( a ) , at the instant the switching element q 1 is turned on , correction of the current detection signal by the resistor r 0 starts , and the current detection signal vis becomes positive . because the absolute value of the current detection signal is not large for a light load , the interval during the time wherein the corrected current detection signal vis is positive is a comparatively longer fraction of the cycle . here , not only is the absolute value of the judgment reference voltage vfb 3 small , but the switching interval t 2 during light loading is long compared with the switching interval t 1 for heavy loading . during intervals in which the switching element q 1 is off , there is no correction by the resistor r 0 , so that the current detection signal vis is at ground potential ( 0 v ). the resistance value of the resistor rb is higher than the values of the other resistors r 0 , ra , rs , and the effect on current values and power consumption are smaller than for other resistors , and so to simplify the explanation , the effect of the resistor rb is ignored ( similarly in fig4 and fig6 below ). the correction circuit 1 need not be an external circuit , but may be incorporated into the integrated circuit ic . fig4 is a block diagram showing the switching power supply device of embodiment 2 , comprising a correction circuit employing a resistor and a capacitor ; fig5 ( a ) and 5 ( b ) show operating waveforms of input signals to the current comparator 17 of the switching power supply device of fig4 . here , the correction circuit 3 comprises the series circuit of the resistor r 0 and capacitor c 0 . in this case , the time constant determined by the resistor r 0 and capacitor c 0 is made shorter than the switching period t 1 set in the power supply control circuit 10 . in fig5 ( a ) , the current detection signal ( voltage signal ) during heavy loading is shown ; in fig5 ( b ) , the current detection signal during light loading is shown . when the switching element q 1 is turned on , the switching signal changes from low to high , current flows through the capacitor c 0 and resistor r 0 of the correction circuit 3 , and an offset occurs in the current detection signal . thereafter , charging of the capacitor c 0 ends , and current no longer flows in the correction circuit 3 . in this way , because of the fact that only a current to charge the capacitor c 0 flows in the correction circuit 3 ( or from another perspective , because the correction circuit 3 forms a differentiating circuit or a high - pass filter ), power losses in the correction circuit 3 occurring during on intervals of the control signal can be further reduced . overcurrent detection is performed by comparing the current detection signal vis corrected by the correction circuit 3 with the reference voltage serving as the overload protection ( olp ) judgment reference ( judgment reference voltage vth ); however , because during the correction interval tc of the correction circuit 3 the correction circuit 3 operates as a differentiating circuit immediately after turn - on of the switching element q 1 , there is no effect on the overcurrent limiting action . fig6 is a block diagram showing the switching power supply device of embodiment 3 , in which the correction circuit in fig1 employs a capacitor . when the correction circuit 4 comprises only a capacitor c 0 , the operation is substantially the same as the correction circuit 3 , in which a resistor r 0 and capacitor c 0 are connected in series . this is because in the actual correction circuit 4 , parasitic resistance components in the circuit , as well as the sense resistance rs , or the resistors ra and rb connected in series , and other resistance components are equivalent in operation to a resistance component . in the switching power supply devices of embodiment 2 and embodiment 3 explained above , the on time is lengthened during heavy loading in which overcurrents may be a problem , so that by means of a configuration comprising a capacitor c 0 as in the correction circuits 3 and 4 shown in fig4 and fig6 , by setting the time constant to be shorter than the on time , the effect of the correction circuit 3 or 4 can be eliminated before the current detection signal reaches the judgment reference voltage vth . as a result , there is no need to consider the effect of the correction circuit 3 or 4 relative to the overcurrent detection level , and only the resistors rs , ra , rb need be considered , so that circuit constant adjustment is simplified . however , the case of a correction circuit 2 comprising only a resistor r 0 ( fig2 ), such as in embodiment 1 , is separate . fig7 is a block diagram showing the switching power supply device of embodiment 4 , comprising a correction circuit employing a resistor , a capacitor , and a zener diode . here , the correction circuit 5 is formed from the resistor r 0 , capacitor c 0 , and zener diode zd 0 . in this case , it is desirable that the time constant determined by the resistor r 0 and capacitor c 0 be set to approximately the same as the switching interval t 1 set in the power supply control circuit 10 , or to a length not more than t 1 ; but the time constant may be longer than the switching interval t 1 . the zener voltage vzd of the zener diode zd 0 is set lower than the high level of the driving signal q 1 output from the output terminal out . fig8 ( a )- 8 ( d ) show operating waveforms of different portions of the switching power supply device of fig7 . as shown in fig8 ( a ) , when the switching element q 1 is turned on , the voltage of the switching signal from the output terminal out of the power supply control circuit 10 changes from low to high ( time t 1 ). then , similarly to the case of the above - described correction circuit 3 ( see fig4 ), a current ic 0 flows through resistor r 0 to the capacitor c 0 of the correction circuit 5 , and this current ic 0 is injected from midway in the series circuit comprising the resistors rs , ra , rb , so that an offset occurs in the current detection signal vis . then , the capacitor c 0 is charged , and the voltage at point a in fig7 rises ( see fig8 ( b ) ). thereafter , when the voltage at point a reaches the zener voltage vzd of the zener diode zd 0 , the current flowing in the resistor r 0 is shifted from the capacitor c 0 to the zener diode zd 0 , and as shown in fig8 ( c ) , the current no longer flows into the capacitor c 0 . as a result , as shown in fig8 ( d ) , after time t 2 at which the current flowing in resistor r 0 shifts to the zener diode zd 0 , the offset which had been present in the current detection signal vis due to the correction circuit 5 no longer occurs . thus in embodiment 4 , by adjusting the zener voltage vzd of the zener diode zd 0 , the time at which an offset occurs in the current detection signal vis due to the correction - circuit 5 can be freely adjusted . hence even when the time constant determined by the resistor r 0 and capacitor c 0 is set to be longer than the switching interval t 1 , by selecting the zener voltage vzd appropriately , the interval in which an offset occurs due to the correction circuit 5 can be set to immediately after turn - on of the switching element q 1 , and as a result there is no effect on the overcurrent limiting action . hence compared with the correction circuit 3 of fig4 , the resistor r 0 and capacitor c 0 can be selected from a broader range of resistance values or capacitance values , and adjustment is made easier . the disclosures of japanese patent applications no . 2007 - 307743 filed on nov . 28 , 2007 and no . 2008 - 114705 filed on apr . 25 , 2008 are incorporated as a reference . while the invention has been explained with reference to the specific embodiments of the invention , the explanation is illustrative and the invention is limited only by the appended claims .