Abstract:
A voltage conversion circuit and a switching power supply device achieving both a good response and a low loss of a power supply voltage, wherein a main voltage conversion portion (circuit block) for conversion an AC voltage to a DC voltage, an auxiliary voltage conversion portion for the same conversion but in transit period up to shifting to a stationary state, a voltage limiting portion (Zener diode) for limiting the DC voltage output from the auxiliary voltage conversion portion to a constant limit-voltage, and an output control switch (transistor) connected in the output pass and for switching the pass to conductive or nonconductive possible to apply a higher voltage in between a voltage at the output node and the limit-voltage based on their magnitude relation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to technology for converting an AC voltage to a DC voltage, more particularly relates to technology built into a switching power supply device and generating a desired power supply voltage based on an electric signal excited in a secondary winding of a transformer.  
         [0003]     2. Description of the Related Art  
         [0004]     In a switching power supply device, sometimes a drive circuit for operating the switching element and an auxiliary power supply circuit for supplying electric power to the control circuit are provided. An example of this auxiliary power supply circuit will be described related to  FIG. 1 .  FIG. 1  is a diagram showing a circuit configuration of a forward type switching power supply device in related art. In  FIG. 1 , in addition to a basic configuration of the forward type switching power supply device, an auxiliary power supply circuit generating a power supply voltage supplied to a control circuit node (not shown) for driving a rectification circuit on the secondary side of a transformer T 1  is shown.  
         [0005]     The auxiliary power supply circuit shown in  FIG. 1  includes a diode D 100 , capacitors C 200  and C 300 , a transistor Q 100 , and a resistor R 100  and generates a targeted power supply voltage Vcc from an emitter of the transistor Q 100 . In the auxiliary power supply circuit of  FIG. 1 , a pulse width modulation signal using an input voltage V 1  as a peak voltage is given to the transformer T 1  under the control of a switch element M 300 . Usually, in order to reduce the stress of a load at the time of activation of the power source, the switching power supply device performs a soft start gradually raising an output voltage until a prescribed value is reached. In this soft start, the PWM signal given to the transformer T 1  gradually prolongs a conductive time of the switch element M 300 , that is, gradually enlarges a duty ratio.  
         [0006]     In the auxiliary power supply circuit of  FIG. 1 , when a voltage Vs of one end of the secondary winding of the transformer T 1  is positive, a base current is supplied to the transistor Q 100  through the diode D 100 , the transistor Q 100  turns on, and the desired power supply voltage Vcc is generated on both ends of the capacitor C 300 .  
         [0007]     In the auxiliary power supply circuit shown in  FIG. 1 , however, when the voltage Vs of one end of the secondary winding of the transformer T 1  becomes positive, it quickly turns on the transistor Q 100 , therefore the response of the power supply voltage Vcc is fast, so overshoot of an output voltage VO due to a delay of the power supply voltage Vcc can be reduced. However, after the power supply voltage reaches the prescribed value, there is a disadvantage of a large loss of the electric power by the transistor Q 100 . Therefore, an auxiliary power supply circuit with a high efficiency cannot be configured in the switching power supply device.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore desirable in the present invention to provide a voltage conversion circuit and a switching power supply device achieving both a good response so as to achieve a DC output in the transit period up to be stable in input and low loss of the power supply voltage.  
         [0009]     According to the present invention, there is provided a voltage conversion circuit comprising a main voltage conversion portion for conversion an input AC voltage to a DC voltage and outputting from an output terminal; an auxiliary voltage conversion portion for inputting the AC voltage, converting the AC voltage to a DC voltage in transit period of the AC voltage up to shifting to a stationary state and capable of outputting to the output terminal; a voltage limiting portion for limiting the DC voltage output from the auxiliary voltage conversion portion to a constant limit-voltage; and an output control switch connected in a pass between an output of the auxiliary voltage conversion portion and the output terminal and for switching the pass to conductive or nonconductive possible to apply a higher voltage in between a voltage at the output terminal and the limit-voltage based on their magnitude relation.  
         [0010]     According to a second aspect of the present invention, there is provided a switching power supply device comprising: a switching circuit for switching an input voltage and generating a pulse width modulation signal; a transformer having a secondary winding and receiving the pulse width modulation signal; a rectification circuit including a plurality of switch elements for rectifying a voltage excited in the secondary winding of the transformer; a control circuit for switching conductive states of the plurality of rectifiers based on an output voltage of the rectification circuit; and a power supply voltage generation circuit for generation a power supply voltage to be supplied to the control circuit, wherein the power supply voltage generation circuit is comprises a main voltage conversion portion for conversion an AC voltage exited on a secondary side of the transformer to a DC voltage and outputting the DC voltage to an output terminal connected with the control circuit, an auxiliary voltage conversion portion for inputting the AC voltage, converting the AC voltage to a DC voltage in transit period of the AC voltage up to shifting to a stationary state and capable of outputting the DC voltage to the output terminal, a voltage limiting portion for limiting the DC voltage output from the auxiliary voltage conversion portion to a constant limit-voltage, and an output control switch connected in a pass between an output of the auxiliary voltage conversion portion and the output terminal and for switching the pass to conductive or nonconductive possible to apply a higher voltage in between a voltage at the output terminal and the limit-voltage based on their magnitude relation.  
         [0011]     In the above present invention, wherein the switching circuit executes a soft start control in which duty ratio of the pulse width modulation signal linearly increases up to a stable state having a constant duty ratio.  
         [0012]     Note that, in the present invention, the “AC voltage excited on the secondary side of the transformer” is can be input via not only one terminal of the secondary winding, but also for example one end of a control winding, an auxiliary winding, or the like provided on the secondary side.  
         [0013]     According to the present invention, it becomes possible to achieve both a good response and a low loss of the input AC voltage or the power supply voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:  
         [0015]      FIG. 1  is a diagram showing the circuit configuration of a forward type switching power supply device of the related art;  
         [0016]      FIG. 2  is a diagram of the system configuration of a switching power supply device according to an embodiment of the present invention;  
         [0017]      FIG. 3  is a diagram showing an example of the circuit configuration of an auxiliary power supply circuit;  
         [0018]      FIGS. 4A  to  4 D are timing charts showing an operation at the time of activation of the auxiliary power supply circuit;  
         [0019]      FIG. 5  is a diagram showing the circuit configuration of a synchronized rectification circuit of the switching power supply device according to a second embodiment of the present invention;  
         [0020]      FIGS. 6A  to  6 E are timing charts showing the operation of the synchronized rectification circuit; and  
         [0021]      FIG. 7  is a diagram showing a modification of the switching power supply device according to the second embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Below, a switching power supply device provided with a voltage conversion circuit according to an embodiment of the present invention is described with reference to the attached drawings.  
         [0023]     First Embodiment  FIG. 2  is a diagram of the system configuration of a switching power supply device  1  according to an embodiment of the present invention. In the present embodiment, as an example, a forward type switching power supply device  1  is described below. In the forward type switching power supply device  1 , an input voltage VI is given to a primary side of the transformer T 1 . The transformer T 1  is given a pulse width modulation (PWM) signal using the input voltage VI as a peak voltage by the switch operation of an NMOS transistor M 3 . The PWM signal is transmitted to the secondary side with the same polarity by the transformer T 1 .  
         [0024]     On the secondary side of the transformer T 1 , a coil L 2  is connected between one end of the secondary winding and a node  120  (output terminal), and a capacitor C 3  is connected between the node  120  (output terminal) and a node  121  (ground terminal) to thereby configure a smoothing circuit of choke input type. Further, the rectification circuit is configured by an NMOS transistor M 2  rectifying the current when ON and an NMOS transistor M 1  for carrying the energy released from the choke (coil L 2 ) when the NMOS transistor M 2  is OFF.  
         [0025]     The secondary side control circuit  30  monitors the output voltage VO and controls the conductive states of the NMOS transistors M 1  and M 2  by control signals CTRL 1  and CTRL 2  so that the output voltage VO becomes a desired value. The output voltage VO is insulated by for example a photocoupler via the secondary side control circuit  30  and transmitted to the primary side control circuit  20 . The primary side control circuit  20  controls the conductive state of the NMOS transistor M 3  by a control signal CTRL 3  so that the output voltage VO becomes the desired value. Namely, the duty ratio of the PWM signal given to the transformer T 1  is controlled.  
         [0026]     The transformer T 1  is provided with an auxiliary winding AW. The power of the PWM signal generated on the primary side of the transformer T 1  is transmitted via this auxiliary winding AW by the auxiliary power supply circuit  10 . The auxiliary power supply circuit  10  generates the power supply voltage Vcc supplied to the secondary side control circuit  30  based on this power. This auxiliary power supply circuit  10  corresponds to the voltage conversion circuit of the power supply voltage generation circuit of the present invention.  
         [0027]     Next, the specific configuration of the auxiliary power supply circuit  10  is described below with reference to  FIG. 3 .  FIG. 3  is a diagram showing an example of the circuit configuration of the auxiliary power supply circuit  10 .  
         [0028]     Usually, the switching power supply device reduces the stress of the load at the time of activation of the power source by gradually raising the output voltage until the prescribed value is reached in a “soft start”. In this soft start, the PWM signal given to the transformer T 1  gradually increases the conductive time of the switch element M 3 , that is, gradually enlarges the duty ratio. In the present embodiment, in the auxiliary power supply circuit  10 , as shown in  FIG. 3 , a circuit block  11  and a circuit block  12  are connected in parallel between a node  100  (one end of the auxiliary winding AW) and a node  108  (output terminal of the auxiliary power supply circuit  10 ). The circuit block  11  is a linear mode use circuit having a high response in a transit period until the power supply voltage reaches the prescribed value Vcc, while the circuit block  12  becomes a switching mode use circuit having a high efficiency in a period after the power supply voltage reaches the prescribed value Vcc.  
         [0029]     Below, the circuit configurations of the circuit blocks  11  and  12  are described.  
         [0030]     The circuit block  11  includes a transistor Q 1 , diodes D 1  and D 2 , a resistor R 1 , and a capacitor C 1 . The diode D 1  is connected at the anode to the node  100  and connected at the cathode to a node  101 . The diode D 1  turns on when a predetermined positive voltage is generated in the voltage Vs of the node  100  as one end of the auxiliary winding AW and transmits the voltage Vs to the capacitor C 1  and the collector etc. of the transistor Q 1 . The capacitor C 1  is connected between the node  101  and a node  102  (ground terminal). The capacitor C 1  is charged when the voltage Vs is the positive voltage and holds its charged voltage when the voltage Vs is 0. The resistor R 1  is connected between a node  103  and a node  104  and supplies the base current to the transistor Q 1 . The diode D 2  is connected between the node  104  and a node  105  (ground terminal). The diode D 2  is a Zener diode (constant voltage diode) provided for clamping the output voltage of the circuit block  11 , that is, the emitter voltage of the transistor Q 1 , to a predetermined level. Note that a breakdown voltage of the diode D 2  is set to a value a little smaller than (target power supply voltage V TAR −forward direction voltage V BE  of transistor Q 1 ). Due to the above configuration, the circuit block  11  linearly generates the output voltage with respect to the input voltage Vs, therefore functions in a linear mode.  
         [0031]     The circuit block  12  includes a coil L 1 , a capacitor C 2 , and diodes D 3  and D 4 . The diode D 3  is connected at the anode to the node  100  (one end of the auxiliary winding AW) and connected at the cathode to a node  106 . The diode D 3  turns on when the voltage Vs is a positive voltage and rectifies the voltage generated in the auxiliary winding AW. The coil L 1  is connected between the node  106  and the node  108  (power supply voltage output terminal), and the capacitor C 2  is connected between the node  108  and a node  109  (ground terminal). The coil L 1  and the capacitor C 2  configure a smoothing circuit. Due to this, the ripple of the voltage rectified by the diode D 3  and the current is reduced, and the power supply voltage output is generated at the node  108 . Note that the output response with respect to the voltage Vs is delayed when compared with the circuit block  11 . The diode D 4  is connected between the node  106  and a node  107  (ground terminal) and functions as a return diode. Namely, during the period when the voltage Vs is 0, it releases the energy stored in the coil L 1 . Due to the above configuration, the circuit block  12  operates in response to the switching on the primary side of the transformer T 1 , therefore functions as a switching mode.  
         [0032]     In the auxiliary power supply circuit  10  shown in  FIG. 3 , the circuit block  11  and the circuit block  12  are connected in parallel between the node  100  (one end of the auxiliary winding AW) and the node  108  (power supply voltage output terminal), but at the time of the start of operation of the auxiliary power supply circuit  10 , first, the voltage generated by the circuit block  11  is output, then, during the period until the output voltage Vcc reaches the target power supply voltage V TAR , the conductive state of the output route of the circuit block  11  is switched so that the voltage generated by the circuit block  12  is output. Namely, at the time of the start of the operation, it operates in the linear mode first, then switches to the switching mode. This switching operation is described below.  
         [0033]     In a soft start, the duty ratio immediately after the PWM signal generated on the primary side of the transformer T 1  at the time of the activation of the power source is started is small. Due to the delay operation of the coil L 1  and the capacitor C 2 , the rise of the output is delayed in the circuit block  12 . On the other hand, in the circuit block  11 , irrespective of the small duty ratio, due to the first rise of the voltage Vs generated in accordance with the peak voltage of the PWM signal, the diode D 1  and the transistor Q 1  quickly turn on, and the rise of the output is fast. Accordingly, immediately after activation by a soft start, the power supply voltage output Vcc observed at the node  108  is generated by the circuits block  11 .  
         [0034]     Thereafter, the duty ratio increases, so the output generated by the circuit block  12  (output of the coil L 1 ) gradually rises. On the other hand, the output of the circuit block  11  (emitter voltage of the transistor Q 1 ) cannot reach the target power supply voltage V TAR  since the breakdown voltage of the Zener diode D 2  is set at a value a little smaller than (target power supply voltage V TAR −forward direction voltage V VE  of transistor Q 1 ). Then, the transistor Q 1  turns off since V BE  becomes smaller than 0.7V before the output generated by the circuit block  12  reaches the target power supply voltage V TAR . Thereafter, the circuit block  11  cannot output. Accordingly, the conductive state of output route is switched before the target power supply voltage V TAR  is observed in the node  108 . Then, after the target power supply voltage V TAR  is generated in the node  108 , the power supply voltage will be generated mainly by the circuit block  12 . In this way, the linear mode is switched to the switching mode.  
         [0035]     Next, the operation at the time of activation of the auxiliary power supply circuit  10  is described below with reference to the timing charts of  FIGS. 4A  to  4 D.  FIG. 4A  shows a waveform of the voltage Vs of the auxiliary winding AW,  FIG. 4B  shows a waveform of a charge voltage V C1 , of the capacitor C 1  of the circuit block  11 ,  FIG. 4C  shows a waveform of an output voltage V sw  of the circuit block  12  when assuming that the circuit block  11  does not exist, and  FIG. 4D  shows a waveform of the output voltage Vcc of the auxiliary power supply circuit  10 .  
         [0036]     As shown in  FIG. 4A , at the soft start, the duty ratio immediately after the PWM signal generated on the primary side of the transformer T 1  at the time of the activation of the power source is started is small. The period where the voltage Vs is a peak voltage V peak  gradually increases. In the circuit block  11 , the first pulse of the voltage Vs passes through the diode D 1  and quickly charges the capacitor C 1 . As shown in  FIG. 4B , the charge voltage V C1  of the capacitor C 1  becomes (V peak −V F ) (V F : forward direction voltage of the diode D 1 ) soon. Further, due to the first pulse of the voltage Vs, the base current is supplied via the resistor R 1  to the transistor Q 1  and the transistor Q 1  quickly turns on. As shown in  FIG. 4B , the output voltage Vcc in the node  108  becomes (V Z −V BE ) (V Z : breakdown voltage of the diode D 2 ). In this way, immediately after the activation of the power source, the output becomes output in the linear mode.  
         [0037]     Immediately after the commencement of the soft start, when assuming that the circuit block  11  does not exist, the rise of the output of the circuit block  12  becomes very slow as shown in  FIG. 4C  due to the delay operation of the coil L 1  and the capacitor C 2 .  
         [0038]     Next, as the duty ratio of the voltage Vs increases, the output generated by the circuit block  12  (output of the coil L 1 ) gradually rises. On the other hand, the output of the circuit block  11  (emitter voltage of the transistor Q 1 ) cannot reach the target power supply voltage V TAR  since the breakdown voltage Vz of the Zener diode D 2  is set at a value slightly smaller than the (target power supply voltage V TAR −forward direction voltage V BE  of the transistor Q 1 ). Next, at a time t 1  of  FIGS. 4A  to  4 D, the output voltage generated by the circuit block  12  coincides with (V Z −V BE ). The transistor Q 1  turns off at the time t 1  (V BE =0.7V). After the time t 1 , the output becomes output in the switching mode of the circuit block  12 . Namely, after the time t 1 , the waveforms shown in  FIGS. 4C and 4D  coincide. The time t 1  which becomes the switching timing from the linear mode to the switching mode is set so as to become earlier than a rising time t 2  of the auxiliary power supply circuit  10 .  
         [0039]     As described above, according to the auxiliary power supply circuit  10  according to the present embodiment, the circuit block  11  operating in the linear mode and the circuit block  12  operating in the switching mode are connected in parallel between the node  100  of one end of the auxiliary winding AW of the transformer T 1  and the power supply output terminal (node  108 ). Immediately after the activation of the power source, the conductive state of output route is switched so that the power supply voltage Vcc is generated by the circuit block  11 , and the power supply voltage Vcc is generated by the circuit block  12  before the target power supply voltage is reached. Accordingly, the following effects are obtained.  
         [0040]     Namely, immediately after the activation of the power source, the circuit block  11  operates and the output voltage Vcc quickly rises up to (V Z −V BE ) (value very near the target power supply voltage V TAR ), therefore the secondary side control circuit  30  can start normal operation soon. Accordingly, in the switching power supply device  1  according to the present embodiment, overshoot of the output voltage VO etc. which may occur since the rectifiers constituted by the NMOS transistors M 1  and M 2  are not correctly controlled immediately after activation do not occur.  
         [0041]     Further, in the circuit block  11 , the loss due to the transistor Q 1  is large although the response speed of the output is fast, but after the output voltage Vcc of the auxiliary power supply circuit  10  reaches the target power supply voltage V TAR  (more accurately, (V Z −V BE )), the output voltage Vcc is generated mainly through the circuit block  11 , therefore there is almost no power loss, and the efficiency is very high. In this way, in the auxiliary power supply circuit  10 , by switching between the linear mode by the circuit block  11  and the switching mode by the circuit block  12  immediately after activation, a good response and a low loss (high efficiency) of the power supply voltage can be achieved.  
         [0042]     Note that, in the explanation of the embodiment mentioned above, the case of the soft start was described, but even in a case where the soft start is not carried out, the response delay by the circuit block  12  occurs, therefore the same effects are obtained. Where the soft start is carried out, the duty ratio immediately after the activation is very small, and a quick output response by the circuit block  12  can not be expected, therefore it can be the that effects of the present invention are particularly big. Namely, the responsibility and low loss of the output by the auxiliary power supply circuit  10  can be made consistent while considering the stress of the load of the switching power supply device  1 .  
         [0043]     Note that the correspondence between the embodiment described above and the claims will be described below. The transformer T 1  corresponds to the “transformer” of the claims of the present invention. The transistor Q 1  corresponds to the “output control switch” of the claims of the present invention. The capacitors C 1  and C 2  correspond to the “first and second capacitors” of the claims of the present invention. The diodes D 1 , D 2 , D 3 , and D 4  correspond to the “first, second, third, and fourth diodes” of the claims of the present invention. Further, the diode also corresponds to the “voltage limiting portion”. The coil L 1  corresponds to the “inductor” of the claims of the present invention. The circuit blocks  12  correspond to the “main voltage conversion portion” of the claims of the present invention. The diode D 1  and the capacitor C 1  correspond to the “auxiliary voltage conversion portion” of the claims of the present invention.  
         [0044]     Second Embodiment  
         [0045]     Next, a second embodiment of the present invention is described. In the present embodiment, there is described below the mode of assembling the power supply voltage generation circuit of the present invention in a synchronized rectification circuit of a current doubler type switching power supply device.  FIG. 5  is a diagram showing the circuit configuration of a synchronized rectification circuit  50  on the secondary side of the transformer T 1  in a switching power supply device  2  according to the present embodiment.  
         [0046]     In the switching power supply device  2 , under the control of a not shown primary side, the transformer T 1  is controlled so as to alternately output a plus voltage and a minus voltage, turn off a rectifier constituted by the NMOS transistor M 10  when outputting the plus voltage, and turn off a rectifier constituted by the NMOS transistor M 20  when outputting the minus voltage. Note that when there is no output from the transformer T 1 , both of the rectification use NMOS transistor M 10  and the SW 2  become ON, and a commutation state where the energy stored in an inductor L 10  or L 20  is released is exhibited.  
         [0047]     A synchronized rectification circuit  50  of  FIG. 5  is configured by two systems of drive circuits performing reverse operations to each other in one cycle in order to control the NMOS transistors M 10  and M 20 . Namely, the synchronized rectification circuit  50  has a drive circuit  51  for the rectification use NMOS transistor M 10  and a drive circuit  52  for the rectification use NMOS transistor M 20 . The drive circuits  51  and  52  are symmetric about the ground line.  FIG. 5  shows the circuit configuration of only the drive circuit  51  as a representative case. Below, the configuration of the drive circuit  51  will be described.  
         [0048]     In  FIG. 5 , the drive circuit  51  receives a trigger signal Vt 1  having a narrow bandwidth from a node  199  and supplies it to an NMOS transistor M 30 . Note that, a trigger signal Vt 2  (not shown) fetched by the drive circuit  52  is a signal obtained by inversion of the phase from the trigger signal Vt 1 . The time when the drive circuit  51  receives the trigger signal Vt 1  is set so as to become slightly earlier than the time when the voltage generated on the secondary side of the transformer T 1  becomes plus. Due to this, before the V ds  of the NMOS transistor M 10  rises, the NMOS transistor M 10  is turned off. Accordingly, at the time of the start of rectification of the NMOS transistor M 20 , a penetration current is prevented from flowing between the NMOS transistors M 10  and M 20 .  
         [0049]     The N channel transistor M 30  is a control transistor for controlling the potential level of a node  201 . The N channel transistor M 30  is connected at the gate to the node  199 , connected at the source to the ground terminal, and connected at the drain to bases of the transistors Q 20  and Q 30 . Accordingly, it turns on in accordance with the time when the trigger signal Vt rises and makes the node  201  the ground potential.  
         [0050]     The transistor Q 30  is a control transistor for controlling the NMOS transistor M 10 . An emitter of the transistor Q 30  is connected to a gate of the NMOS transistor M 10 , and a collector is connected to a ground terminal. A base of the transistor Q 30  is connected via the node  201  to the drain of an N channel transistor M 40 . Accordingly, the transistor Q 30  turns on when the potential level of the node  201  becomes the ground potential, drains the gate charges of the NMOS transistor M 10 , and turns off the NMOS transistor M 10 .  
         [0051]     The transistor Q 20  is a control transistor for controlling the NMOS transistor M 10 . An emitter of the transistor Q 20  is connected to a gate of the NMOS transistor M 10 , and a collector is connected to a node  202 . A base of the transistor Q 20  is connected via the node  201  to the drain of the N channel transistor M 40 . In the state where the transistor Q 20  becomes ON, the discharged current of a coil L 30  charges the gate of the NMOS transistor M 10  in the route from base to emitter of the transistor Q 20 . At the same time, the charge voltage of the capacitor C 30  charges the gate of the NMOS transistor M 10  by the route from the collector to the emitter.  
         [0052]     The N channel transistor M 40  is a control transistor for controlling the potential level of the node  201 . The trigger signal Vt 1  fetched from the node  199  returns to 0V soon in a shorter time than the time during which the voltage on the secondary side of the transformer T 1  holds the H level, therefore, during the period where the Vs holds the H level (positive voltage) after the trigger signal Vt 1  becomes 0V, the node  201  is brought to the ground potential by the N channel transistor M 40  turning on. A gate of the N channel transistor M 40  is connected to a node  203 , a drain is connected to the node  201 , and a source is connected to the ground terminal.  
         [0053]     A resistor R 20  and a diode D 60  are connected between a node  200  and the ground terminal, and the node between the resistor R 20  and the diode D 60 , that is, the node  203 , is connected to the gate of the N channel transistor M 40 . The diode D 60  and the resistor R 20  configure a protection circuit for enabling adjustment of the gate potential level of the N channel transistor M 40  and protecting it.  
         [0054]     The coil L 30  and a diode D 40  are connected in series between the node  200  and a node  204 . A diode D 30  is connected between the node  204  and the node  202 . The node  204  and the node  201  are connected. The node  201  is connected to bases of the transistors Q 20  and Q 30  for controlling the NMOS transistor M 10 . Due to this, when the voltage generated on the secondary side of the transformer T 1  is at the H level (positive voltage), that is, when the node  200  is at the H level (positive voltage), the energy is stored by a current I L30  of the coil L 30 , while when the voltage generated on the secondary side of the transformer T 1  is at the L level (0V), that is, when the node  200  is at the L level (0V), the stored energy is released. By this released energy, the gate of the NMOS transistor M 10  is charged, the NMOS transistor M 10  is quickly turned on, and, at the same time, the excess of the released energy is stored in the capacitor C 30 .  
         [0055]     The capacitor C 30  is connected between a node  205  and the ground terminal. The capacitor C 30  clamps the gate-source voltage V gs  of the NMOS transistor M 10  by its charge voltage via the transistor Q 20 . Further, when the voltage generated on the secondary side of the transformer T 1  becomes the L level, the capacitor C 30  quickly charges the gate of the NMOS transistor M 10  via the collector→emitter of the transistor Q 20  and turns on it.  
         [0056]     The configuration of the drive circuit  51  was mainly described above, but the same is also true for the drive circuit  52 . In this way, in the synchronized rectification circuit  50 , the NMOS transistors M 10  and M 20  alternately perform a rectification operation in accordance with the polarity of the voltage generated on the secondary side of the transformer T 1 .  
         [0057]     As described above, the drive circuit  51  drives the NMOS transistor M 10  based on a signal obtained by combining the trigger signal Vt 1  advanced in the rising timing with respect to the output of the transformer T 1  and the drain voltage of the N channel transistor M 40 . At that time, the energy of the coil L 10  is controlled and the gate of the NMOS transistor M 10  is charged or discharged for the drive, therefore the time for turning on the parasitic diode of the NMOS transistor M 10  is very short. Further, in the synchronized rectification circuit  50 , no penetration current is generated in the NMOS transistors M 10  and M 20 . The NMOS transistor M 10  is always ON even at the time of commutation. Therefore, a circuit having an extremely high efficiency at the time of synchronized rectification is obtained.  
         [0058]     In this synchronized rectification circuit  50 , the drive circuit  51  includes a circuit corresponding to the circuit block  12  performing the switching mode operation described in the first embodiment. Namely, the coil L 30  corresponds to the coil L 1  in  FIG. 3 . The diodes D 30  and D 40  correspond to the diode D 3  in  FIG. 3 . The capacitor C 30  corresponds to the capacitor C 2  in  FIG. 3 . The NMOS transistor M 10  corresponds to the return diode D 4  in  FIG. 3 . The synchronized rectification circuit  50 , as shown in  FIG. 5 , is provided with a circuit block  11   a  corresponding to the circuit block  11  performing the linear mode operation described in the first embodiment. Note that the configuration of the circuit block  11   a  is the same as that of the circuit block  11 , so the explanation is omitted here.  
         [0059]     Immediately after the start of activation of the switching power supply device  2 , the positive voltage generated in the secondary winding of the transformer T 1  is supplied to the diode D 10  of the circuit block  11   a , the base current is supplied to the transistor Q 10  via the resistor R 10 , and the transistor Q 10  quickly turns on, therefore the voltage of the node  205  becomes (V Z −V BE ) (note that, V z : breakdown voltage of the diode D 20 , V BE : forward direction voltage between the base and the emitter of the transistor Q 10 ). Thereafter, by the positive voltage generated in the secondary winding of the transformer T 1 , when the output by the switching mode increases through a route of node  200 →node  204 →node  202 →node  205 , the operation switches from the linear mode to the switching mode, and the voltage of the node  205  (output terminal of the voltage Vcc) is determined mainly by the drive circuit  51 .  
         [0060]      FIGS. 6A  to  6 E are timing charts showing the operation of the synchronized rectification circuit  50 , in which  FIG. 6A  shows a waveform of V ds  of the NMOS transistor M 10 ,  FIG. 6B  shows a waveform of V ds  of the NMOS transistor M 20 ,  FIG. 6C  shows a waveform of a charge voltage V C20  of the capacitor C 20  of the circuit block  11   a ,  FIG. 6D  shows a waveform of the output voltage V SW  of the node  205  when assuming that the circuit block  11   a  does not exist, and  FIG. 6E  shows a waveform of the actual output voltage Vcc of the node  205 .  
         [0061]     In the synchronized rectification circuit  50 , as shown in  FIGS. 6A and 6B , V ds  of the NMOS transistors M 10  and M 20  gradually increase in duty ratio by the soft start. At the time of normal operation, their phases of them are offset by 180 degrees. Here, the peak voltage of V ds  is V peak . The circuit block  11   a  is provided in only the drive circuit  51 . As shown in  FIG. 6C , the capacitor C 20  is quickly charged to (V peak −V F  (D 10 )) in response to the first pulse generated in V ds  of the NMOS transistor M 10 . In the same way as the explanation with reference to  FIGS. 4A  to  4 D in the first embodiment, the operation switches from the linear mode to the switching mode at the time t 1 . Thereafter, the voltage of the node  205  is generated by the drive circuit  51 . Since, as described above, the synchronized rectification circuit  50  according to the present embodiment includes a circuit block operating in the linear mode and a circuit block operating in the switching mode, effects the same as those of the auxiliary power supply circuit  10  described in the first embodiment are obtained. Namely, power supply voltage achieving both a good response and low loss can be extracted from the node  205 .  
         [0062]     Note that it is possible to suitably modify the circuit configuration described in the above embodiment. For example,  FIG. 7  is a modification of the switching power supply device  2  shown in  FIG. 5  to a center tap synchronized rectification type, but the operation is the same as that of the switching power supply device  2 . Further, the circuit configuration shown in  FIG. 5  and  FIG. 7  can be widely applied to a push-pull type, half bridge type, or full bridge type switching power supply device.  
         [0063]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.