Abstract:
The peak current flowing to the switching device drops in a no-load state with PWM control, but because the number of switching operations is constant regardless of the load, further reducing power consumption is difficult. The switching power supply device has a PWM signal generator for generating a PWM signal, a switching device Q 1  for switching the first supply voltage VIN based on the PWM signal, converters for outputting the difference between a second supply voltage and a reference voltage as a difference signal, and an intermittent oscillation control circuit for stopping the switching operation of the first switching device when the difference signal is less than a predetermined first threshold value (Vp 1 ). The PWM signal generator changes the pulse width based on the difference signal to generate the PWM signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Technology 
   The present invention relates to a switching power supply device, and relates more particularly to an apparatus that enables power conservation during standby mode operation and high efficiency during operation at a rated load. 
   2. Description of Related Art 
     FIG. 9 ,  FIG. 10 , and  FIG. 11  show examples of a step-down voltage type switching power supply. Step-down voltage switching power supplies include power supply devices with a bootstrap circuit to assure control power for the high side block as shown in  FIG. 9  and  FIG. 10 , and power supply devices that do not have a bootstrap circuit and control the switching power using only the high side block as shown in  FIG. 11 . 
     FIG. 9  shows a first example of the prior art described in Japanese Unexamined Patent Appl. Pub. 2000-350440. Operation of the control circuit  108  in the first example of the prior art shown in  FIG. 9  is referenced to the ground terminal  106 , and derives control power for the high side switching device  107  when the switching device  107  is ON from both ends of the capacitor  110  in the bootstrap circuit  111 , to which current is supplied from input power supply  101  through diode  109  when the high side switching device  107  is OFF. 
     FIG. 10  shows a second example of the prior art described in Japanese Unexamined Patent Appl. Pub. 2001-112241. The power supply device shown in  FIG. 10  is a synchronous rectifier type power supply device having a bootstrap circuit similarly to the first prior art example shown in  FIG. 9 . The main difference between the device shown in  FIG. 10  and the first example shown in  FIG. 9  is that the diode  103  shown in  FIG. 9  is replaced by a switching device  202 . A level shifter  209  is also provided so that switching device  201  and switching device  202  are not on at the same time, and the switching devices are controlled so that when one is on the other is off. By using a switching device instead of a diode on the low side, the power supply efficiency is improved by lowering the voltage drop occurring at the ends of diode  103  in  FIG. 9  when the high side switching device is off to the voltage drop caused by the on resistance of the switching device  202 . 
   The simultaneous rectifier, step-down voltage type switching power supply shown in  FIG. 10  has the following two main features. 
   (1) Generally when current flow to the high side switching device  107  rises in the prior art example shown in  FIG. 9 , current flow to the diode  103  also rises when the high-side switching device  107  is off, the forward voltage therefore also rises, and power loss from the diode  103  also rises. Power loss is therefore reduced by connecting a low on-resistance switching device parallel to the diode  103  (or a switching device  202  is used instead of the diode as shown in  FIG. 10 ) so that the voltage produced when the circuit is energized is lower than the forward voltage of the diode  103 . 
   (2) The switching devices connected to the high and low sides are switched on and off by PWM control so that both switching devices are not on at the same time. 
     FIG. 11  shows a third example of the prior art as taught in Japanese Unexamined Patent Appl. Pub. H10-191625. In  FIG. 11  VOUT is the output node voltage, IOUT is the output node current, VDS is the voltage between the drain and source of switching device  302 , IDS is the drain current flowing to the switching device  302 , and VCC is the voltage at the CONTROL node in  FIG. 11 . The device described in  FIG. 11  comprises an input capacitor  301 , switching device  302 , a control circuit  303  for the switching device  302 , a capacitor  304  for the control circuit reference voltage, a conversion circuit  305 , output voltage detection circuit  309 , and protection device  310 . 
   When the input terminal voltage VIN (a DC voltage or the voltage from a commercial AC power source rectified by a diode bridge or other rectifier and smoothed by input capacitor  301 ) is applied to the drain of switching device  302 , the internal circuit current supply circuit  311  of the control circuit  303  supplies current through switch  312  to the capacitor  304  connected to the control node, VCC thus rises, and the control circuit  303  starts on/off control of the switching device  302 . On/off switching of the switching device  302  is controlled by the comparator  316  comparing the sawtooth wave output signal from the internal oscillator  313  with the voltage-divided VCC output by the two resistances  314  and  315 . 
   Once on/off control of the switching device  302  begins, power is supplied to the conversion circuit  305  comprising diode  306 , coil  307 , and output capacitor  308 , and VOUT rises. VOUT is detected by output voltage detection circuit  309 . When VOUT rises to or above a predetermined level, current flows from the OUT node to the CONTROL node of the control circuit  303  when switching device  302  is OFF. As a result, as a result of VCC rising and the ON duty of the output signal from comparator  316  decreasing, the ON duty of the switching device  302  is also short, and the switching device  302  is controlled with PWM control. 
   PWM control thus seeks to stabilize the output voltage and conserve energy by gradually reducing the ON duty ratio (ultimately lowering the peak of current IDS flow to the switching device) of the switching device as the output load decreases. 
     FIG. 12  shows a fourth example of the prior art as taught in Japanese Unexamined Patent Appl. Pub. 2003-189632 corresponding to United States Patent Appl. Pub. US 2003/0112040 A1. More particularly,  FIG. 12  shows a bridge circuit using HVIC (high voltage driver IC) circuits  450 ,  451 , and  452  for inverter control of a motor, and more particularly shows a three-phase motor drive circuit having three half-bridge circuits parallel connected with the output terminals connected to a motor. Power switching devices  417 ,  418 ,  419 ,  420 ,  421 ,  422  in a three-phase (U, V, W) bridge circuit are connected between the high and low potential sides of a main DC power source for inverter drive  423 , and diodes  431 ,  432 ,  433 ,  434 ,  435 ,  436  are parallel connected to the power switching devices. 
   HVIC circuit  450  is a single-chip circuit device comprising input signal processing circuit  402 , power device drive/protection circuit  412 , and a level shifter  437  having a photocoupler and electrical isolation function. A similarly arranged single-chip HVIC circuit  450 ,  451 ,  452  is rendered separately for each phase, U, V, and W, but devices having a separate HVIC circuit for phases U, V, and W rendered on a single chip are also known. 
   The reference potential nodes of the three HVIC circuits and the emitter of each low potential power switching device are connected to the U, V, W phases. The emitters of the high potential side power switching devices and the second reference potential nodes of the high potential side drive circuits connected to the level shifters of the HVIC circuits are respectively connected to the U, V, W phases. The output drive signal nodes of the HVIC circuits are connected to the gate of each power switching device. 
   The input signal processing circuit of the HVIC circuit is connected to the output port of the microcomputer or other device that generates the control signals for driving the power switching devices, and power for controlling and driving the HVIC circuits is supplied from external power source  430 . The power for driving the high potential power switching devices of the HVIC circuit is supplied from a bootstrap power circuit comprising external power source  430 , high voltage diodes  440 ,  441 ,  442  and capacitors  443 ,  444 ,  445  connected in series to the external power source  430  for each U, V, W phase, and the ends of the capacitors  443 ,  444 ,  445  connected to both sides of the drive circuits for the high potential power switching devices. 
   When inverter drive is used with an actual motor, control signals are passed to the U, V, W phase HVIC circuits from the inverter drive control signal generating circuit of the microcomputer, and the power switching devices on the high and low potential sides of the U, V, W phase bridge circuit are switched according to the drive signal to supply AC power between the output nodes and control the motor. 
   A bootstrap power supply circuit drives the high side power switching circuits in this bridge drive circuit. This bootstrap power supply circuit operates so that when the main DC power source is applied to the bridge circuit, the microcomputer drive signal that drives the low-side power switching device is passed to the HVIC circuit, and the low-side power switching device turns on. Because current flows in this state from the external power source to the high voltage diode, to the capacitor, to the low-potential power switching device, and to the reference node of the external power source, both sides of the capacitor are charged by voltage Vcap as defined in equation (1).
 
 V cap= V (external power source voltage)− Vf−Vc ( V )  (1)
 
where Vf is the forward voltage drop of the high voltage diode, and Vc is the collector potential of the low potential power switching device.
 
   Operation of the drive circuit that drives the high side power switching devices of the HVIC circuit is maintained by the power accumulated in the capacitor. Therefore, when the main DC power source is applied to the inverter drive circuit, a charge is not accumulated in the capacitors  443 ,  444 ,  445 , and the high-potential side drive circuit is therefore unable to operate. 
   After the main DC power source is applied, a drive signal causing the low-potential power switching device for each phase to stay on for a predetermined time is passed from the microcomputer to the HVIC circuit in order to charge capacitors  443 ,  444 ,  445 . The motor is then controlled by passing the motor drive control signal from the microcomputer to each HVIC circuit. 
   If the voltage at both sides of the capacitors  443 ,  444 ,  445  is not regularly recharged, the charge stored in the capacitors will drop below the level required to the drive the power switching devices after a certain amount of time due to natural dissipation of the stored charge. A signal causing the low-potential power switching devices to turn on is applied to the HVIC circuits within a maximum time determined by the constant of the inverter drive circuit during motor drive, thereby controlling motor operation with a control signal that causes the capacitors  443 ,  444 ,  445  to charge. 
   The following three problems are present with the simultaneous rectifier method shown in  FIG. 10 . 
   (1) The input supply voltage is typically approximately 20 V. This is because signal transmission is required between the high side switching device control circuit unit and the low side switching device control circuit unit in order to achieve PWM control. 
   (2) If the input supply voltage is greater than 20 V, a bootstrap circuit (bootstrap circuit  111  in  FIG. 9  and bootstrap circuit  203  in  FIG. 10 ) for supplying power to the high side switching device control circuit, and a level shifting circuit (level shifter  209  in  FIG. 10 ) for signal transmission are needed. Supplying power to the high-side switching device control circuit unit through the bootstrap circuit and signal transmission by the level shifting circuit from the low side to the high side are limited to periods when the high side switching device is off (that is, when the low-side switching device is on). As a result, on-time control of the high-side switching device is extremely difficult when the high-side switching device is on because the supply voltage of the high-side switching device control circuit discharges naturally and gradually drops. More specifically, this renders operation of the high-side switching device control circuit unstable. This is also the case with (3) described next. 
   (3) When used with an even high input supply voltage of 60 V or more, for example, the bootstrap circuit for supplying power to the high-side switching device control circuit comprises a diode  440  and capacitor  443 , and is connected to a power source  430  that is separate from the main power source  423  connected to the high potential node of the high-side switching device  417 . Two or more input power sources are thus required as shown in the inverter drive bridge circuit for a motor in the fourth example of the prior art shown in  FIG. 12 . 
   Reducing the size and improving the power supply efficiency of the switching power supply device, and further reducing the power consumption during standby states, and particularly in a no-load state, therefore cannot be expected using a step-down voltage switching power supply according to the prior art. 
   (1) The peak level of current flowing to the switching device drops in the no-load state with PWM control of the step-down voltage switching power supply taught in the third example of the prior art above, but further reduction in power consumption is difficult because the number of switching operations is constant irrespective of the load. 
   (2) The bootstrap circuit must be externally attached to the voltage step-down switching power supply described in the first and second examples of the prior art, and this prevents reducing the size of the power supply device. 
   (3) With the voltage step-down power supply device described in the first and second examples of the prior art, the high-side switching devices are switched on/off by the voltage at both ends of the capacitors in the bootstrap circuit, and a loss of control precision caused by the drop in capacitor voltage and fluctuation in the drain current caused by the fluctuating gate voltage of the switching device both occur easily. 
   (4) The input voltage range of the voltage step-down power supply device described in the first and second examples of the prior art is limited because use only at a relatively low voltage is difficult. 
   (5) The low potential side of the voltage step-down power supply device described in the third example of the prior art is composed of diodes as described in the first prior art example. This increases power loss in the diodes during steady state operation, and thus prevents further improvement in power efficiency. 
   (6) Separate power supplies are required for the control circuit and switching devices when using a high voltage input supply as described in the fourth prior art example. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to solving these problems, and an object of the invention is to further increase efficiency and reduce power consumption and device size in a switching power supply. 
   To achieve this object, a switching power supply device according to the present invention comprises a first source voltage supply arrangement for supplying a first supply voltage; a PWM signal generator operable to generate a PWM signal; a first switching arrangement operable to switch the first supply voltage based on the PWM signal; a converter operable to convert the switched first supply voltage to a second supply voltage; a difference signal detector operable to output the difference between the second supply voltage and a predetermined reference voltage as a difference signal; and an intermittent controller operable to stop the switching operation of the first switching arrangement when the difference signal is less than or equal to a first threshold value. The PWM signal generator varies the pulse width based on the difference signal and generates the PWM signal. 
   A switching power supply device according to another aspect of the invention comprises a first source voltage supply arrangement operable to supply a first supply voltage; a PWM signal generator operable to generate a PWM signal; a first switching arrangement operable to switch the first supply voltage based on the PWM signal; a converter operable to convert the switched first supply voltage to a second supply voltage; a difference signal detector operable to output the difference between the second supply voltage and a predetermined reference voltage as a difference signal; a current converter operable to convert the difference signal to current; a voltage converter operable to convert the current-converted difference signal to voltage; a first regulator operable to convert the first supply voltage to a predetermined third supply voltage, and to supply the third supply voltage to the PWM signal generator and voltage converter; and a second regulator operable to convert the second supply voltage to a predetermined fourth supply voltage, and to supply the fourth supply voltage to the difference signal detector and current converter. The PWM signal generator generates the PWM signal based on the voltage-converted difference signal. 
   A switching power supply device according to the present invention can reduce power consumption in the standby state, and particularly in a no-load state, over a wide input voltage range, and can thus provide a high efficiency power supply in steady-state operation. 
   Furthermore, because a bootstrap circuit is not needed and the gate drive voltage precision of the first switching device is improved, power supply to the output node from the first switching device can be stabilized. 
   Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a switching power supply according to a first embodiment of the invention. 
       FIG. 2  describes the operation of a switching power supply according to the first embodiment of the invention. 
       FIG. 3  is a block diagram of a switching power supply according to a second embodiment of the invention. 
       FIG. 4  is a block diagram of a switching power supply according to a third embodiment of the invention. 
       FIG. 5  is a block diagram of a switching power supply according to a fourth embodiment of the invention. 
       FIG. 6  is a block diagram of a switching power supply according to a fifth embodiment of the invention. 
       FIG. 7  is a block diagram of a switching power supply according to a sixth embodiment of the invention. 
       FIG. 8  is a block diagram of a switching power supply according to a seventh embodiment of the invention. 
       FIG. 9  is a block diagram of a switching power supply according to a first example of the prior art. 
       FIG. 10  is a block diagram of a switching power supply according to a second example of the prior art. 
       FIG. 11  is a block diagram of a switching power supply according to a third example of the prior art. 
       FIG. 12  is a block diagram of a three-phase motor drive circuit according to a fourth example of the prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention are described below with reference to the accompanying figures. 
   Embodiment 1 
     FIG. 1  shows a switching power supply device according to a first embodiment of the invention, and  FIG. 2  shows the operating waves when the output load of the switching power supply in this first embodiment of the invention changes from a heavy load to a light load. In  FIG. 1  and  FIG. 2  the first supply voltage VIN is the voltage input to the input terminal IN from the first power supply voltage supply device where the ground potential of the ground terminal GND is the lowest potential; the second supply voltage VOUT is the voltage output from output terminal OUT where the lowest potential is the ground potential; IOUT is the current level at output terminal OUT; VDS 1  is the DRAIN 1  voltage of switching device Q 1 ; IFB is the FB 1  node current (=FB 2  node current); IDS 1  is the DRAIN 1  current flowing through switching device Q 1 . 
   The switching power supply device according to this first embodiment of the invention comprises input capacitor  1 , switching device Q 1  and switching device Q 1  control circuit  3 , first capacitor for the control circuit reference voltage  4 , a conversion circuit comprising third diode  5 , coil  6 , and output capacitor  7 , switching device Q 2  and a control circuit  9  for on/off control detecting the output voltage of the switching device Q 2 , a second reference voltage capacitor  10  for the control circuit  9 , and two resistances R 1  and R 2 . The switching device Q 1  is a MOS transistor or high voltage transistor. 
   As shown in  FIG. 1 , the control circuit  3  has a first regulator  11  that is connected to node VIN 1  and produces and maintains a constant third supply voltage at node BY 1  for supplying power to the other elements of the control circuit  3 ; a start/stop circuit  12  for starting the control circuit  3  when the third supply voltage equals or exceeds a predetermined level, and stopping the control circuit  3  when the third supply voltage is less than the predetermined level; a I-V conversion circuit  13  which takes the third supply voltage supplied from node BY 1  and outputs the current flowing out from the control circuit  3  from node FB 1  as output voltage signal VL; an oscillator  16  that outputs the MAX-DUTY signal  14  controlling the maximum on-duty ratio of the switching device Q 1  and CLOCK signal  15 , which is an internal reference signal; an overcurrent detection circuit  17  for detecting the DRAIN 1  current flowing to the switching device Q 1  using the output voltage signal VL from the I-V conversion circuit as the reference voltage, and turning the switching device Q 1  off; an intermittent oscillation control circuit  18  for pausing or stopping on/off control of the switching device Q 1  when the output voltage signal VL from the I-V conversion circuit becomes less than first threshold level Vp 1  (at which time the threshold level of the inverted input terminal changes from first threshold level Vp 1  to second threshold level Vp 2 ), and resuming on/off control of the switching device Q 1  when the output voltage signal VL from the I-V conversion circuit rises above second threshold level Vp 2 ; an AND circuit  19  for outputting to the set node of flip-flop  21  based on the output from the intermittent oscillation control circuit  18  and the CLOCK signal  15  from oscillator  16 ; an OR circuit  20  for outputting tot he reset node of the flip-flop  21  based on the inverted MAX-DUTY signal  14  from the oscillator  16  and the output signal from the overcurrent detection circuit  17 ; a flip-flop  21 ; and an AND circuit  22  for controlling the GATE1 node of switching device Q 1  based on the output signal from start/stop circuit  12 , the MAX-DUTY signal  14  from oscillator  16 , and the output signal from the flip-flop  21 . A capacitor  23  and second diode  24  are connected between node FB 1  of I-V conversion circuit  13  and SOURCE 1 . 
   The control circuit  9 , which provides on/off control and detects the output voltage of the switching device Q 2 , comprises a second regulator  25  that generates and maintains a constant fourth supply voltage at node BY 2  for supplying power to the other elements of the control circuit  9  from second supply voltage VOUT; a start/stop circuit  26  for starting control circuit  9  when the node BY 2  voltage is greater than or equal to a predetermined level, and stops the control circuit  9  when the node BY 2  voltage is less than the predetermined level; a differential amplifier  27  which takes as inputs the fourth supply voltage supplied from node BY 2  as the power supply voltage and the voltage of second supply voltage VOUT divided by the two resistances R 1  and R 2 , and amplifies and outputs as a difference signal the potential difference between the voltage-divided VOUT and a reference voltage input to the non-inverted input node; a V-I conversion circuit  28  having the fourth supply voltage supplied from node BY 2  as the power supply voltage for converting the difference signal to current IFB at node FB 2 ; a Q 1  OFF state detection circuit  29  for detecting if switching device Q 1  is off from the voltage at node FB 2 ; and an AND circuit  30  for controlling the GATE 2  node of switching device Q 2  based on the outputs from Q 1  OFF state detection circuit  29  when the output signal from start/stop circuit  26  is HIGH. 
   When first supply voltage VIN (a DC voltage or voltage from a commercial AC power supply rectified by a diode bridge or other rectifier and then smoothed by input capacitor  1 ) is applied to input terminal IN, the first regulator  11  of control circuit  3  supplies current to the first capacitor  4  for the control circuit reference voltage connected to node BY 1 . This causes the voltage at node BY 1  to rise, the start/stop circuit  12  to start control circuit  3  operation, and on/off control of the switching device Q 1  to start. When on/off control of the switching device Q 1  starts, power is supplied to the conversion circuit comprising third diode  5 , coil  6 , and output capacitor  7 , and the second supply voltage VOUT rises at output terminal OUT. 
   When second supply voltage VOUT rises, the second regulator  25  operates and the voltage at reference voltage node BY 2  of control circuit  9  rises. When the voltage at reference voltage node BY 2  is greater than or equal to the predetermined level used by the start/stop circuit  26 , control circuit  9  starts operating and starts detecting the voltage at the output terminal OUT of differential amplifier  27 . The second supply voltage VOUT is detected by the two resistances R 1  and R 2  and differential amplifier  27 . When second supply voltage VOUT is greater than or equal to a desired voltage (more precisely, when the VO 1  node voltage is greater than or equal to the predetermined reference voltage input to the non-inverted input terminal of the differential amplifier  27 ) the difference between the voltage at VO 1  and the reference voltage of the differential amplifier  27  is amplified and passed as the difference signal to V-I conversion circuit  28 . 
   When second supply voltage VOUT is greater than or equal to the desired voltage and second supply voltage VOUT rises, the difference signal decreases linearly and is converted by the V-I conversion circuit  28  so that the current level at node FB 2  rises and the output voltage signal VL from I-V conversion circuit  13  drops. 
   VL is the reference voltage of the overcurrent detection circuit  17 . When VL decreases, the peak current at DRAIN 1  flowing to the switching device Q 1  decreases. As a result, as shown in  FIG. 2 , current IDS 1  at node DRAIN 1  is PWM controlled in a current mode, and the DRAIN 1  node voltage VDS 1  is switched by PWM control. The oscillator  16 , overcurrent detection circuit  17 , OR circuit  20 , and flip-flop  21  thus constitute a PWM signal generator  42 , the gate voltage at the GATE 1  node of switching device Q 1  is controlled based on the PWM signal generated by the PWM signal generator  42 , and PWM switching of switching device Q 1  is achieved. The third supply voltage is supplied to PWM signal generator  42  from node BY 1  as the supply voltage. 
   When the second supply voltage VOUT rises to or above a desired voltage (a low output load state) and the output voltage signal VL of I-V conversion circuit  13  is less than or equal to first threshold level Vp 1  of the intermittent oscillation control circuit  18 , the output load state is determined to be a low load state, and the intermittent oscillation control circuit  18  pauses or stops switching device Q 1  operation. Stopping on/off control of the switching device Q 1  stops power supply to the output, and second supply voltage VOUT gradually decreases. As second supply voltage VOUT drops, output voltage signal VL gradually rises. When output voltage signal VL becomes equal to or greater than second threshold level Vp 2  of intermittent oscillation control circuit  18 , on/off control of the switching device Q 1  resumes and power is supplied to the output. As a result, second supply voltage VOUT rises again and on/off control of switching device Q 1  stops. This intermittent control thus continues in a low output load state. 
   Note that second threshold level Vp 2  is normally set higher than first threshold level Vp 1 . 
   During PWM control and intermittent control of the switching device Q 1  by control circuit  3 , the Q 1  OFF state detection circuit  29  monitors the node FB 2  voltage to detect the OFF state of the switching device Q 1  so that the switching device Q 2  is controlled by the AND circuit  30  to be ON only when switching device Q 1  is OFF. The voltage between DRAIN 2  [NOT LABELLED IN THE FIGURE] and SOURCE 2  when switching device Q 1  is ON (=IDS 2 *Ron (Q 2 ) denoted by VDS 2  in  FIG. 2 ) is set lower than the forward voltage Vf of the third diode  5 . Third diode  5  is connected parallel to the switching device Q 2  to improve the turn-on time of switching device Q 2 . 
   When a switching power supply device according to this first embodiment of the invention is used, the following effects are achieved over a wide input range. 
   (1) As the output load decreases, the peak of the current flow to the switching device Q 1  decreases and is PWM controlled in a low load state, and when the output load then approaches a no-load state, intermittent control is applied, thereby achieving even greater power conservation in the standby state. 
   (2) Because switching device Q 2  goes ON when switching device Q 1  is OF, the forward voltage of the third diode  5  can be further reduced, and a high efficiency power supply can be achieved in steady-state operation. 
   (3) A low-side V-I conversion circuit  28  and a high-side I-V conversion circuit  13  are disposed to use a new signal transmission method using current signals for signal transmission between the high-side control circuit  3  and the low-side control circuit  9 . A level shifting circuit is therefore not needed even when using a high voltage power supply, and circuit design is thus simplified. 
   (4) Because a bootstrap circuit and a level shifting circuit that are necessary when a high input supply voltage is used are not needed, a simultaneous rectifier switching power supply device can be provided using a single input supply voltage regardless of the range of the input power supply voltage. 
   (5) The supply voltage of the high-side control circuit  3  and the supply voltage of the low-side control circuit  9  are maintained at constant level by the first regulator  11  and second regulator  25 , respectively, and the supply voltage does not drop as a result of natural voltage discharge. This simplifies ON time control of the high-side switching device Q 1 . 
   Switching device Q 1  and control circuit  3  are preferably integrated on the same semiconductor substrate, in which case the DRAIN 1 , SOURCE 1 , BY 1 , and FB 1  nodes are rendered as external connection pins. By incorporating these devices in a package with at least four pins, the parts count can be greatly reduced, part dimensions can be reduced, and a small, low price power supply device can be rendered. 
   Switching device Q 2  and control circuit  9  are also preferably integrated on the same semiconductor substrate, in which case the DRAIN 2 , SOURCE 2 , BY 2 , and FB 2  nodes are rendered as external connection pins. By incorporating these devices in a package with at least four pins, the parts count can be greatly reduced, part dimensions can be reduced, and a small, low price power supply device can be rendered. 
   Furthermore, by integrating switching device Q 1  and control circuit  3  on the same semiconductor substrate, integrating switching device Q 2  and control circuit  9  on the same semiconductor substrate, and assembling both semiconductor substrates in a single package with at least 7 pins, the parts count can be greatly reduced, part dimensions can be reduced, and a small, low price power supply device can be rendered. 
   Furthermore, by integrating switching device Q 1  and control circuit  3  on the same semiconductor substrate, integrating switching device Q 2  and control circuit  9  on the same semiconductor substrate, and assembling both semiconductor substrates with the first capacitor  4  for the reference voltage of the control circuit  3 , second capacitor  10  for the reference voltage of the control circuit  9 , capacitor  23 , third diode  5 , output capacitor  7 , and resistances R 1  and R 2  in a single package with at least the four pins DRAIN 1 , SOURCE 1 , SOURCE 2 , and OUT, the parts count can be greatly reduced, part dimensions can be reduced, and a small, low price power supply device can be rendered. 
   Note, further, that switching device Q 1  is also referred to as a first switching device, switching device Q 2  as a second switching device, intermittent oscillation control circuit  18  an intermittent controller, differential amplifier  27  as a difference signal detector, I-V conversion circuit  13  as a voltage converter, V-I conversion circuit  28  as a current converter, and Q 1  OFF state detection circuit  29  as an inversion signal generator. The circuit including the overcurrent detection circuit  17  is also called an overcurrent protection circuit. 
   Second Embodiment 
     FIG. 3  shows a switching power supply device according to a second embodiment of the invention. This embodiment of the invention renders a negative output power supply by connecting the negative terminal of the input capacitor  1  connected to the anode of the third diode  5  in  FIG. 1  to the positive terminal of the output capacitor  7 . As a result, the minimum potential of the second supply voltage VOUT is equal to the minimum potential of the first supply voltage VIN minus second supply voltage VOUT. 
   More specifically, the polarity of the second supply voltage is the same as the polarity of the first supply voltage in the first embodiment of the invention shown in  FIG. 1 , but in the second embodiment of the invention shown in  FIG. 3  the polarity of the second supply voltage is the opposite of the polarity of the first supply voltage. The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   In addition to the effects afforded by the first embodiment of the invention, using a switching power supply device according to this second embodiment of the invention enables easily changing the polarity of the output voltage. 
   Third Embodiment 
     FIG. 4  shows a switching power supply device according to a third embodiment of the invention. In this embodiment of the invention power is supplied to the second regulator  25  of the control circuit  9  not from second supply voltage VOUT but instead directly from first supply voltage VIN. The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   In addition to the effects afforded by the first embodiment of the invention, using a switching power supply device according to this third embodiment of the invention enables easily lowering the output voltage. 
   Fourth Embodiment 
     FIG. 5  shows a switching power supply device according to a fourth embodiment of the invention. This fourth embodiment of the invention adds an overheating protection circuit  33  to the control circuit  3  shown in  FIG. 1 , and additionally connects a restart trigger circuit  34  for canceling the interrupt imposed by the overheating protection circuit  33  to the input to AND circuit  22 . The overheating protection circuit  33  renders a protection function that unconditionally stops on/off control of the switching device Q 1  when the junction temperature of the switching device Q 1  rises to or above a predetermined threshold temperature. 
   The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   In addition to the effects afforded by the first embodiment of the invention, using a switching power supply device according to this fourth embodiment of the invention enables protecting the switching device and assuring the safety of the switching power supply device. 
   Fifth Embodiment 
     FIG. 6  shows a switching power supply device according to a fifth embodiment of the invention. This switching power supply device adds a junction field effect transistor JFET 1  connected to node VIN 1  in the first regulator  11  of the control circuit  3  shown in  FIG. 1 , and a junction field effect transistor JFET 2  connected to node FB 2  of the Q 1  OFF state detection circuit  29  in the control circuit  9  shown in  FIG. 1 . The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   Using a switching power supply device according to this fifth embodiment of the invention affords the same effects as the first embodiment of the invention even when the input voltage is high. 
   Sixth Embodiment 
     FIG. 7  shows a switching power supply device according to a sixth embodiment of the invention. As shown in  FIG. 7 , this embodiment of the invention adds a first diode  35  and a zener diode  36  between the reference voltage node BY 1  and output terminal OUT of the control circuit  3  in the first embodiment. The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   When a switching power supply device according to this sixth embodiment of the invention is used the power supply to the reference voltage node BY 1  of the control circuit  3  is from output terminal OUT instead of first regulator  11 . As a result, the power conservation effect in the standby mode is even greater than with the first embodiment of the invention. 
   Seventh Embodiment 
     FIG. 8  shows a switching power supply device according to a seventh embodiment of the invention. The overcurrent detection circuit  17  shown in  FIG. 1  is rendered by a sense MOS transistor  37 , sense resistor  38 , and comparator in this embodiment of the invention. The power supply operation is the same as in the switching power supply device of the first embodiment of the invention. 
   A switching power supply device according to this seventh embodiment of the invention affords the same effect as the first embodiment of the invention. 
   The present invention is described herein using a step-down type switching power supply device by way of example, but the invention is not limited to a step-down type power supply device and can be used with all types of switching power supply devices, including both step-down and step-up types. The embodiments herein are also described by way of example only, and the invention is not limited to these embodiments. 
   The present invention can be used in a switching power supply. 
   The invention being thus described, it will be obvious that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.