Abstract:
Aspects of the invention provide a switching power supply in which frequency reduction control in a light load condition both in a power factor correction converter and a DC-DC converter restrains energy loss and achieves optimum efficiency. A switching power supply can include a power factor correction converter and a DC-DC converter. The DC-DC converter can include a load condition detecting means for detecting a condition of the load, and a frequency reducing means for reducing a switching frequency in the DC-DC converter when a light load condition is detected by the load condition detecting means. The power factor correction converter can include a frequency reducing means for reducing a switching frequency in the power factor correction converter corresponding to the load condition detected by the load condition detecting means of the DC-DC converter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on, and claims priority to, Japanese Patent Application No. 2012-289265, filed on Dec. 30, 2012, the contents of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the invention relate to switching power supplies exhibiting improved efficiency in a light load condition. 
     2. Description of the Related Art 
     To ensure stability and safety of commercial power systems, power factor correction is obligated to switching power supplies with a power consumption larger than 75 W. Accordingly, proposed recently are switching power supplies composed of a power factor correction converter (PFC) with a small sized and high efficiency and a DC-DC converter that converts a DC voltage obtained using the power factor correction converter to a DC output voltage corresponding to a specification of a load. Japanese Unexamined Patent Application Publication No. 2007-288855 (also referred to herein as “Patent Document 1”), for example, discloses this type of switching power supply. Most of such DC-DC converters, with a rated load power of about 100 W, employ a quasi-resonance (QR) converter, which impose relatively little burden on a secondary side rectifying diode. 
       FIG. 8  shows a schematic construction of a switching power supply  1  comprising a power factor correction converter  2  and a DC-DC converter  3 , which is a quasi-resonance converter. A rectifying circuit  4  rectifies an AC power supplied from a commercial power supply  5  through a noise filter  6  and delivers to the power factor correction converter  2 . 
     The power factor correction converter  2  is basically composed of an inductor L 1  connected to the rectifying circuit  4  and a switching element Q 1  to form a current path through the inductor L 1  in an ON period of the switching element Q 1 . The power factor correction circuit  2  also comprises a diode D 1  to form a current path between the inductor L 1  and an output capacitor C 2  in an OFF period of the switching element Q 1 . The control circuit IC 1  ON/OFF-drives the switching element Q 1  and controls the current through the inductor L 1  to obtain a stabilized DC voltage Vb. 
     Resistors R 1  and R 2  divide the DC voltage Vb obtained across the output capacitor C 2  to detect the voltage Vb, and feeds back the detected voltage to the control circuit IC 1 . A shunt resistor R 3  detects the current flowing through the inductor L 1 . Japanese Unexamined Patent Application Publication No. 2010-220330 (also referred to herein as “Patent Document 2”), for example, discloses operation and effect of such a power factor correction converter  2  in detail. 
     The DC-DC converter  3 , which is a quasi-resonance converter, is basically provided with a switching element Q 2  connected in series to a primary winding P 1  of an isolation transformer T, the primary winding P 1  receiving the output, the DC voltage Vb, of the power factor conversion converter  2 . The DC-DC converter  3  is also provided with a resonance capacitor C 4  in parallel with the switching element Q 2  and an output capacitor C 5  connected through a rectifying diode D 2  to the secondary winding S 1  of the isolation transformer. A control circuit IC 2  ON/OFF-drives the switching element Q 2  to generate a quasi-resonant oscillation in a resonance circuit composed of a leakage inductance of the isolation transformer T and the resonance capacitor C 4 , thereby generating a specified DC output voltage Vo. 
     Resistors R 4  and R 5  divides the DC output voltage Vo obtained across the output capacitor C 5  to detect the output voltage Vo and feeds back the divided voltage to the control circuit IC 2  through a feedback circuit FB. A shunt resistor R 6  detects the current flowing in the switching element Q 2 . The DC-DC converter  3  detects a ZCD voltage developed across an auxiliary winding P 2  of the isolation transformer T and controls the turning ON timing of the switching element Q 2 . Japanese Unexamined Patent Application Publication No. 2011-015570 (also referred to herein is “Patent document 3”), for example, discloses details about operation and effect of such a DC-DC converter  3 , which is a quasi-resonance converter. 
     The switching power supply  1  significantly improves power factor thereof owing to the power factor correction converter  2  provided on the preliminary stage of the DC-DC converter  3 . The power factor correction converter  2  however, also generates energy loss inevitably. Especially in a light load condition, a switching frequency becomes high in both the power factor correction converter  2  and the DC-DCC converter  3 . Therefore, switching loss increases in the switching elements Q 1  and Q 2  deteriorating the efficiency of the switching power supply  1 . 
     In order to reduce the switching loss in the switching elements Q 1  and Q 2 , International Patent Application Publication No. WO2004/023634 (also referred to herein as “Patent Document 4”), for example, discloses a control method of so-called bottom skip which uses a timing at which a resonant oscillation current that arises after turning OFF of the switching elements Q 1  and Q 2  becomes zero. This bottom skip control delays a turning ON timing of the switching elements Q 1  and Q 2  in a light load condition to reduce a switching frequency, thereby restraining a loss. The number of bottom skips in the bottom skip control is set at [0] in a normal condition, or heavy load condition, and set at gradually larger values as the load becomes lighter. 
     The bottom skip control is conducted in the power factor correction converter  2  and DC-DC converter  3  by detecting a load condition, a magnitude of the load, with a load detecting means provided in the control circuits IC 1  and IC 2 . Conducting such a bottom skip control, however, does not necessarily improve efficiency. In a heavy load condition, for example, conduction loss is generally dominant over switching loss. As a result, switching frequency reduction in a heavy load condition increases the conduction loss, rather deteriorating the efficacy. 
     Consequently, a load condition, or a magnitude of the load, needs to be detected precisely in order to obtain optimum efficiency. A load condition detecting means in a DC-DC converter  3  generally carries out load condition detection based on information about the DC output voltage Vo obtained through the feedback circuit FB. Here, an input voltage to the DC-DC converter  3  is stabilized by the power factor correction converter  2 . Consequently, the load condition is detected precisely in the DC-DC converter  3 . 
     On the other hand, a load a condition detecting means in the power factor correction converter  2  detects a load condition from an information about the load current detected through the shunt resistor R 3 . Here in the power factor correction converter  2 , a magnitude of an inductor current is controlled corresponding to the phase angle of the input AC voltage Vac. As a consequence, the detection precision of the load condition in the power factor correction converter  2  changes inevitably depending on the phase angle of the input AC voltage Vac. It is therefore difficult to detect the load condition with high precision in the overall input voltage range of the input AC voltage Vac. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention have been made in view of the foregoing and embodiments of the present invention provide a switching power supply that exhibits optimized efficiency allowing minimum loss by conducting frequency reduction control in both the power factor correction converter and the DC-DC converter in the light load condition through detection of a load condition independently of the input AC voltage 
     Embodiments of the invention include a switching power supply of the present invention comprises: a power factor correction converter that switches an input AC voltage and delivers a DC voltage; and a DC-DC converter that switches the output voltage of the power factor correction converter and delivers a specified DC output voltage to a load. The DC-DC converter comprises a load condition detecting means for detecting a condition of the load, and a frequency reducing means for reducing a switching frequency in the DC-DC converter when a light load condition is detected by the load condition detecting means. The power factor correction converter comprises a frequency reducing means for reducing a switching frequency in the power factor correction converter corresponding to the load condition detected by the load condition detecting means of the DC-DC converter. 
     Embodiments of the invention include a power factor correction converter and a DC-DC converter is characterized in that the information on the load condition detected in the DC-DC converter for frequency reduction control of the DC-DC converter is utilized as information for frequency reduction control of the power factor correction converter. 
     The DC-DC converter can include a quasi-resonance converter. 
     The frequency reducing means of the power factor correction converter and the frequency reducing means of the DC-DC converter can include a bottom skip control means that delays a turning ON timing of a respective switching element provided in the power factor correction converter and in the DC-DC converter, and the load condition detecting means of the DC-DC converter comprises a load information delivering means for delivering bottom skip control information corresponding to the load condition to the power factor correction converter. 
     Embodiments of the invention detect the load dividing a magnitude of the load into n steps, where n is a natural number of two or larger, determines a number of bottoms to regulate the turning ON timing of the switching element provided in the DC-DC converter, and delivers the determined number of bottoms as the bottom skip control information to the power factor correction converter. 
     Embodiments of the bottom skip control means of the power factor correction converter can control the turning ON timing of the switching element provided in the power factor correction converter with a number of bottoms different from the number of bottoms in the DC-DC converter. 
     Embodiments of the invention include a switching power supply having a construction as stated above, the input voltage to the DC-DC converter is stabilized by the power factor correction converter. As a result, the load condition can be detected with a high precision by the load condition detecting means provided in the DC-DC converter. Under these circumstances, frequency reduction control, which is a bottom skip control, is conducted in the DC-DC converter and the power factor correction converter based on the detected load condition i.e., load detection information. 
     Consequently, frequency reduction control, which is bottom skip control, in the power factor correction converter is performed appropriately without depending on the input AC voltage. As a result, energy losses in the DC-DC converter and the power factor correction converter are restrained to optimize conversion efficiency. Here, it is only needed to inform the load detecting information, which is bottom skip control information, from the DC-DC converter to the power factor correction converter, and thus the overall construction is very simple. Therefore, a great advantage is obtained in practical application of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic construction of a switching power supply according to an embodiment of the present invention; 
         FIG. 2  shows an example of construction of a bottom skip control circuit in a DC-DC converter; 
         FIG. 3  shows an example of output signals in the bottom skip control corresponding to the load condition; 
         FIG. 4  shows an example of schematic construction of a control circuit in the power factor correction converter; 
         FIG. 5  shows an example of delay circuit in the control circuit shown in  FIG. 4 ; 
         FIG. 6  shows an example of conversion circuit for the control signals; 
         FIG. 7  shows a schematic construction of a switching power supply according to another embodiment of the present invention; and 
         FIG. 8  shows a schematic construction of a conventionally common switching power supply provided with a power factor correction converter and a DC-DC converter. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes in detail a switching power supply according to a certain embodiments of the invention with reference to accompanying drawings. 
       FIG. 1  shows a schematic construction of a switching power supply according to an embodiment of the present invention. The switching power supply  1 , like the conventional switching power supply  1  shown in  FIG. 8 , comprises a power factor correction converter  2  (PFC  2 ) that switches an input AC voltage Vac and generates a DC voltage Vb, and a DC-DC converter (QR)  3  that switches the DC voltage Vb and generates a DC output voltage Vo for supplying to a load. The same components are given the same symbols as in the switching power supply  1  shown in  FIG. 8  and description thereon is omitted. 
     This switching power supply  1  is characterized in that the DC-DC converter  3  detects load detection information, which is information indicating load condition, for frequency reduction control, which is a bottom skip control, and give the load detection information to the power factor correction converter  2 ; and the power factor converter  2  conducts frequency reduction control, which is a bottom skip control, according to the load detection information delivered by the DC-DC converter  3 . 
     A control circuit IC 2  of the DC-DC converter  3  comprises a load condition detecting means  7  for detecting a magnitude of the load from the information on the DC output voltage Vo, which is an FB signal, fed back through the feedback circuit FB. The control circuit IC 2  of the DC-DC converter  3  also comprises a frequency reducing means  8  that controls to delay a turning ON timing of the switching element Q 2  according to the load condition detected with the load condition detecting means  7  and reduces the switching frequency of the switching element Q 2  in the light load condition. The control circuit IC 1  of the power factor correction converter  2  likewise comprises a frequency reducing means  9  that controls to delay a turning ON timing of the switching element Q 1  in a light load condition and reduces the switching frequency of the switching element Q 1 . The frequency reducing means  8  and  9  specifically composed of respective bottom skip control means. 
     In the switching power supply  1  according to the embodiment of the invention, the load condition detecting means  7  provided in the DC-DC converter  3  detects load detection information to use in control of the frequency reducing means  8 , which is a bottom skip control means, and delivers the load detecting information also to the power factor correction converter  2 . The frequency reducing means  9  of the power factor correction converter  2  is operated according to the load detection information delivered by the DC-DC converter  3 . 
     Now, a description is made here about the frequency reduction control, i.e., bottom skip control, in the DC-DC converter  3 . The bottom skip control in the DC-DC converter  3  is conducted by detecting a load condition based on the tendency that the ON width of the switching element Q 2  becomes longer as the load becomes heavier, which means larger output power.  FIG. 2  shows an example of construction of a bottom skip control circuit  10  of the DC-DC converter  3 . This bottom skip control circuit  10  includes the load condition detecting means  7  and the bottom skip control means  8 , which is a frequency reducing means. The bottom skip control circuit  10  generates an output signal bot-out that regulates a turning ON timing of the switching element Q 2 . 
     The load condition detecting means  7  of the bottom skip control circuit  10  determines the magnitude of the load using the fact that an L level period of a driving signal ‘drv’ for the switching element Q 2  corresponds to an ON width ‘ts’ of the switching element Q 2 . The load condition detecting means  7  determines a load condition, or a magnitude of the load, by comparing the ON-width ts of the switching element Q 2  with reference ON widths ts_ref 1 , ts_ref 2 , and ts_ref 3  generated in a reference ON width generating circuit  11 . The reference ON width generating circuit  11  generates the reference ON widths ts_ref 1 , ts_ref 2 , and ts_ref 3  with different pulse width according to a setting signal ‘set’ and a bottom detecting signal ‘bot’, which are issued upon turning OFF of the switching element Q 2 . 
     The bottom detecting signal ‘bot’ is detected when a ZCD voltage developed on an auxiliary winding P 2  of the isolation transformer T decreased below a predetermined threshold value recognizing a zero value of a quasi-resonant oscillation current after turning OFF of the switching element Q 2 . The reference ON width generating circuit  11  generates the reference ON widths ts_ref 1 , ts_ref 2 , and ts_ref 3  with a pulse width from a common reference timing of turning OFF of the switching element Q 2  to a first, second, or third input timing of the bottom detecting signal ‘bot’, respectively. These reference ON widths ts_ref 1 , ts_ref 2 , and ts_ref 3  are in an inequality relationship: ts_ref 1 &gt;ts_ref 2 &gt;ts_ref 3 . 
     More specifically, the load condition detecting means  7  is provided with two reset preference type flip-flops  7   a  and  7   b . The flip-flop  7   a  is reset by a logical output through a NOR circuit  7   d  of the driving signal ‘dry’ with an ON width ts inverted through a NOT circuit  7   c  and the reference ON width ts_ref 1 . The flip-flop  7   a  is set by a logical output of the driving signal ‘dry’ and the reference ON width ts_ref 2 , the logical output being executed in an AND circuit  7   e . On the other hand, the flip-flop  7   b  is reset by a logical output through a NOR circuit  7   f  of the driving signal ‘dry’ inverted through a NOT circuit  7   c  and the reference ON width ts_ref 2 . The flip-flop  7   b  is set by a logical output of the driving signal ‘dry’ and the reference ON width ts_ref 3 , the logical output being executed in an AND circuit  7   g.    
     As a consequence, the flip-flop  7   a  is set under a relationship: ts_ref 1 &gt;ts&gt;ts_ref 2 ; and the flip-flop  7   b  is set under a relationship: ts_ref 2 &gt;ts&gt;ts_ref 3 . The output signals of the flip-flops  7   a  and  7   b  are used for bottom skip control after passing through a NOT circuit  7   h , a NOT circuit  7   i , and an AND circuit  7   j . More specifically, the output signal of the flip-flop  7   a , which is a first selection control signal sel 1  is given to an AND circuit  8   g  after inversion through the NOT circuit  7   i . Consequently, the AND circuit  8   g  is active only when the flip-flop  7   a  is reset. 
     The output of the flip-flop  7   a  and the output of the flip-flop  7   b  that is inverted through the NOT circuit  7   h  are given to the AND circuit  7   j  and logically processed there. The output of the AND circuit  7   j  is given to an AND circuit  8   h , which will be described later. Consequently, the AND circuit  8   h  is active only when the flip-flop  7   a  is set and the flip-flop  7   b  is reset. 
     The bottom skip control means  8 , which is a frequency reducing means, is provided with a delay circuit  8   a  for delaying the bottom detecting signal ‘bot’ and a delay circuit  8   b  for delaying the setting signal ‘set’. The delay circuits  8   a  and  8   b  give a delay time of half the pulse width, 200 ns, for example, of the bottom detecting signal ‘bot’, to the bottom detecting signal ‘bot’ and to the setting signal ‘set’ to regulate operation timing in the bottom skip control. The bottom detecting signal ‘bot’ delayed through the delay circuit  8   a  is used for generating the output signal bot-out and simultaneously used as a clock signal for setting operation of series-connected two stages of D flip-flops  8   c  and  8   d.    
     The first stage D flip-flop  8   c  is reset by the setting signal ‘set’ delayed through the delay circuit  8   b  and is set receiving a power supply voltage VDD with the clock signal. The second stage D flip-flop  8   d  is reset by the setting signal ‘set’ delayed through the delay circuit  8   b  and is set receiving the output of the first stage D flip-flop  8   c.    
     Consequently, the first stage D flip-flop  8   c  is set to an H level at the timing delayed from the input of the first bottom detecting signal ‘bot’ by half the pulse width of the bottom detecting signal ‘bot’. The second stage D flip-flop  8   d  is set to an H level at the timing delayed from the input of the second bottom detecting signal ‘bot’ by half the pulse width of the bottom detecting signal ‘bot’. 
     The set output signal from the first stage D flip-flop  8   c  is delivered to an AND circuit  8   e , and an output signal bot_out 2  is delivered at the timing of the second input of the bottom detecting signal ‘bot’ in synchronism with the input timing of the bottom detecting signal ‘bot’. The set output signal from the second stage D flip-flop  8   d  is delivered to an AND circuit  8   f , and an output signal bot_out 3  is delivered at the timing of the third input of the bottom detecting signal ‘bot’ in synchronism with the input timing of the bottom detecting signal ‘bot’. This output signal bot_out 3  is delivered through an OR circuit  8   i  as the output signal bot-out for regulating turning ON timing of the switching element Q 2 . 
     The bottom detecting signal ‘bot’ delayed through the delay circuit  8   a  is given to the AND circuit  8   g , which is controlled to be active when the flip-flop  7   a  is reset and the first selection control signal sel 1  is not delivered, i.e., in the normal load condition or in a heavy load condition. Thus, the AND circuit  8   g  delivers the output signal bot_out 1  at the timing of input of the first bottom detecting signal ‘bot’ under a heavy load condition in synchronism with an input timing of the bottom detecting signal ‘bot’. The output signal bot_out 1  is delivered through the OR circuit  8   i  as the output signal bot-out for regulating the timing of turning ON of the switching element Q 2 . The output signal bot_out 1  is of course delivered before the output signals bot_out 2  and bot_out 3 . 
     The output signal bot_out 2  that is generated at the timing of input of the second bottom detecting signal ‘bot’ in the AND circuit  8   e  is delivered to the AND circuit  8   h . This AND circuit  8   h  is controlled to be active, as described earlier, when the flip-flop  7   a  is set and the flip-flop  7   b  is reset, i.e., in a middle load condition. Thus, the AND circuit  8   e  delivers the output signal ‘bot_out 2 ’ at the timing of the second input of the bottom detecting signal ‘bot’ in synchronism with the input timing of the bottom detecting signal ‘bot’ under the middle load condition. The output signal bot_out 2  is delivered through the OR circuit  8   i  as the output signal ‘bot-out’ for regulating the timing of turning ON of the switching element Q 2 . 
     The output signal ‘bot_out 3 ’ that is generated at the timing of third input of the bottom detecting signal ‘bot’ in the AND circuit  8   f  is controlled to be active when the flip-flop  7   a  is set and the flip-flop  7   b  is set, i.e., under a light load condition. The output signal bot_out 3  is delivered through the OR circuit  8   i  as the output signal ‘bot-out’ for regulating the timing of turning ON of the switching element Q 2 . 
     Thus, the output signal ‘bot-out’ is delivered, as shown in  FIG. 3 , corresponding to the load condition: at the timing of the first detection of the bottom detecting signal ‘bot’ in a heavy load condition, at the timing of the second detection of the bottom detecting signal ‘bot’ in a middle load condition, and at the timing of the third detection of the bottom detecting signal ‘bot’ in a light load condition. The timing of turning ON of the switching element Q 2  is regulated by the output signal ‘bot-out’ that is delay-controlled at three steps corresponding to the load condition. Thus, the switching frequency is reduced in the middle load condition and the light load condition. 
     The bottom skip control circuit  10  for controlling bottom skip operation having the construction described above is further provided with a load information delivering means that delivers load detection information indicated by the first and second selection control signals sel 1  and sel 2  to the power factor correction converter  2 . The load information delivering means is composed, for example, of an encoder  12  that generates control signals Qb 1 , Qb 2 , and Qb 3  for determining the number of bottoms to regulate the timing of turning ON of the switching element Q 1  from the first and second selection control signals sel 1  and sel 2 . 
     More specifically, the encoder  12  is provided with a function for generating the control signals Qb 1  and Qb 2  by logical processing similar to the logical processing function of the NOT circuits  7   h  and  7   i , and the AND circuit  7   j . The encoder circuit is also provided with a logical processing function that generates the control signal Qb 3  at an H level only when the flip-flop  7   a  is set and the flip-flop  7   b  is set. Consequently, the control signal Qb 1 , Qb 2 , and Qb 3  delivered from the encoder  12  in parallel are [100 or HLL] in the heavy load condition, [010 or LHL] in the middle load condition, and [001 or LLH] in the light load condition. The control signal Qb 1  is a signal indicating the first bottom detection at the number of skips of [0]; the control signal Qb 2  is a signal indicating the second bottom detection at the number of skips of [1]; and the control signal Qb 3  is a signal indicating the third bottom detection at the number of skips of [2]. 
     The control circuit IC 1  in the power factor correction converter  2  conducts frequency reduction control, which is a bottom skipping control, receiving the control signals Qb 1 , Qb 2 , and Qb 3  from the DC-DC converter  3  and constructed, for example, as shown in  FIG. 4 . The control circuit IC 1  is provided as a main component with a flip-flop  43  that is set upon detecting, by a zero current detector  41 , a timing of zero value of the resonant oscillation voltage after turning OFF of the switching element Q 1 , and reset by the output of the ON width generating circuit  42 , as shown by the schematic construction of  FIG. 4 . The output of the flip-flop  43  drives an output driver circuit  44  to generate an output signal PFC-OUT for ON/OFF-driving the switching element Q 1 . 
     The ON width generating circuit  42  generates a reset signal with a pulse width regulating the ON width of the switching element Q 1  corresponding to the output of an error amplifier  45  for detecting a feedback voltage PFC-FB, which is a divided voltage of the output voltage Vb. More specifically, the ON width generating circuit  42  generates a signal with a wide pulse width, i.e., a wide ON width, when the output voltage of the error amplifier  45  is high, and the ON width generating circuit  42  generates a signal with a narrow pulse width as the output voltage of the error amplifier  45  decreases. 
     The setting signal ‘set’, which sets the flip-flop  43  and triggers the ON width generating circuit  42 , is generated through a delay circuit  46  that delay-controls an output signal Vzcd of the zero current detector  41  corresponding to the control signals Qb 1 , Qb 2 , and Qb 3  delivered by the DC-DC converter  3 . The delay circuit  46  comprises as shown in  FIG. 5 , for example, a NOT circuit  51  inverting the output signal Vzcd, a semiconductor switch  52 , which can be a MOSFET, ON/OFF-driven by the NOT circuit  51 , and a capacitor  53  parallel connected to the semiconductor switch  52 . 
     The capacitor  53  is charged in the OFF period of the semiconductor switch  52  with selected current of I 1 , I 2  and I 3  delivered by the constant current sources  54   a ,  54   b , and  54   c , respectively. The capacitor  53  is discharged in the ON period of the semiconductor switch  52 . A comparator  55  generates a setting signal ‘set’ for setting the flip-flop  43  indicated in  FIG. 4  when the charged voltage of the capacitor  53  exceeds a predetermined specified reference voltage Vref. 
     The control signals Qb 1 , Qb 2 , and Qb 3  are used for controlling charging of the capacitor  53  by the constant current sources  54   a ,  54   b , and  54   c . More specifically, the constant current source  54   a ,  54   b , and  54   c  are driven by the power supply voltage VDD through switches  56   a ,  56   b , and  56   c  to deliver the constant current I 1 , I 2 , and I 3 . The switch  56   a  turns ON receiving the control signal Qb 1  and drives the constant current source  54   a . The switch  56   b  turns ON receiving the control signal Qb 1  or the control signal Qb 2  through an OR circuit  57  to drive the constant current source  54   b . The switch  56   c  turns ON receiving any one of the control signals Qb 1 , Qb 2 , and Qb 3  through an OR circuit  58  to drive the constant current source  54   c.    
     Consequently, the capacitor  53  is charged rapidly, when the control signal Qb 1  is given, with the current I 1 +I 2 +I 3  delivered by the constant current sources  54   a ,  54   b , and  54   c . When the Qb 2  is given, the capacitor  53  is charged with the current I 2 +I 3  delivered by the constant current sources  54   b  and  54   c . When the Qb 3  is given, the capacitor  53  is charged slowly with the current I 3  delivered by the constant current sources  54   c.    
     As a result, the period of time for the terminal voltage of the capacitor  53  to be charged up to the reference voltage Vref set for the comparator  55  decreases as the charging current increases. Thus, the comparator  55  reverses the output thereof after passing the charging time on the capacitor  53  determined corresponding to the control signals Qb 1 , Qb 2 , and Qb 3  from the input timing of the output signal Vzcd. In other words, the comparator  55  delivers the setting signal ‘set’ after passing delay times Td 1 , Td 2 , and Td 3  corresponding to the control signals Qb 1 , Qb 2 , and Qb 3 , wherein Td 1 &lt;Td 2 &lt;Td 3 . 
     The setting signal ‘set’ delivered by the comparator  55  with the delay time control as described above sets the flip-flop  43  and at the same time triggers the ON width generating circuit  42 . Accordingly, the turning ON timing of the switching element Q 1  is controlled through the delay times Td 1 , Td 2 , and Td 3  corresponding to the control signals Qb 1 , Qb 2 , and Qb 3  that indicate the load condition, thereby conducting the frequency reduction control in the light load condition. 
     In the switching power supply  1  of an embodiment of the invention having the construction described above, the power factor correction converter  2  performs frequency reduction control corresponding to the load condition that is detected by the DC-DC converter  3 . Therefore, the power factor correction converter  2  is not affected by the variation of the input AC voltage Vac, which is the case in power factor correction converters having a conventionally common construction. The DC-DC converter  3  in the switching power supply  1  conducts switching operation receiving a DC voltage Vb stabilized through the power factor converter  2  and generates a DC output voltage Vo for supplying the load. Thus, the DC-DC converter  3  that detects the load condition from the ON width of the switching element Q 2  detects the load condition, i.e., a magnitude of the load, with high precision. 
     Therefore, the DC-DC converter  3  performs frequency reduction control, which is a bottom skip control, in the light load condition corresponding to the load condition that is detected with high precision. The power factor correction converter  2  also performs frequency reduction control in the light load condition corresponding to the load condition that is detected in the DC-DC converter  3  with high precision. Consequently, frequency reduction control is performed appropriately in both the power factor correction converter  2  and the DC-DC converter  3  to restrain energy losses in the switching elements Q 1  and Q 2 , thereby effectively improving the power factor. 
     Moreover, the power factor correction converter  2  effectively uses the load condition just as detected in the DC-DC converter  3  with a high precision for conducting frequency reduction control in the power factor correction converter  2 . Consequently, the frequency reduction control in the power factor correction converter  2  is performed in a simple construction with sufficiently high precision. It is therefore a great advantage in practical application that energy losses are restrained in a simple overall construction of a switching power supply  1  to improve a power factor. 
     Although the description thus far is made about the bottom skip control in three steps as an example, the number of steps of bottom skip control is not limited to a special number. The load condition, i.e., a magnitude of the load, can be detected by dividing into n steps, where n is a natural number of two or larger, and frequency reduction control, which is a bottom skip control, is conducted corresponding to these load conditions. 
     The control precision of the frequency reduction control, which is a bottom skip control, in the power factor correction  2  does not necessarily equal to the control precision of the frequency reduction control, which is a bottom skipping control, in the DC-DC converter  3 . For example, bottom skipping control in the DC-DC converter  3  can be conducted with five steps, while the bottom skipping control in the power factor correction converter  2  is conducted with three steps. In such a case, bottom skip control information indicating the load condition can be converted using a conversion circuit  60  as shown in  FIG. 6  and given to the power factor correction converter  2 . 
     The conversion circuit  60  executes logical processing to converts control signals Qb 1 , Qb 2 , Qb 3 , Qb 4  and Qb 5  indicating five steps of bottom numbers to control signals Qb 1 ′, Qb 2 ′, and Qb 3 ′ indicating three steps of bottom numbers, in which the control signals Qb 1  and Qb 2  are logically processed through an OR circuit  61  and the control signals Qb 4  and Qb 5  are logically processed through an OR circuit  62 . Use of the conversion circuit  60  allows the power factor correction converter  2  maintaining a bottom detecting number of [1] even when the bottom detecting number in the DC-DC converter  3  is changed from [1] to [2]. Even in the case the bottom detecting number in the DC-DC converter  3  is a large number of [4] or [5], the bottom detecting number in the power factor correction converter  2  can be restrained to [3]. Therefore, appropriate effects of the frequency reduction control, which is a bottom skip control, can be readily achieved in the power factor correction converter  2  and the DC-DC converter  3  corresponding to the load condition. 
     The present invention is not limited to the embodiment described above. The present invention can be applied to a power factor correction converter  2  as shown in  FIG. 7 , for example, which uses a control circuit IC 1  that performs bottom skip control based on the voltage developing through an auxiliary winding of the inductor L 1 . The present invention can be applied to power factor correction converter  2  with an average current control method as well as the fixed ON width control method as described above. In such a case, the output of an error amplifier used for average current control can be corrected corresponding to the control signals Qb 1 , Qb 2 , and Qb 3 . 
     Whereas the load condition is detected according to the fact that the ON width of the switching element is proportional to the magnitude of the load in the DC-DC converter  3  in the above description, the load condition can be detected based on the ON-OFF width of the switching element, where the ON-OFF width means the period of time of a switching period of the switching element subtracted by a resonant oscillation period. The function of the conversion circuit  60  described previously can be alternatively provided in the side of the power factor correction converter  2 . The present invention can be applied with various modifications within the spirit and scope of the invention. 
     Examples of specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the above description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. Further, portions of different embodiments and/or drawings can be combined, as would be understood by one of skill in the art.