JP2010130826A - Step-up type switching power supply unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a step-up type switching power supply unit reducing rush current into an output capacitor when activated. <P>SOLUTION: This step-up type switching power supply unit has a current limit transistor M5 having an ON resistance voltage larger than that of a synchronous rectification transistor M1 between an external terminal T1 and an external terminal T2. In the power supply unit, when shifting from a standby state to an active state, a control section 1 turns on the current limit transistor M5 only for a predetermined period before starting switching control of the synchronous rectification transistor M1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a step-up switching power supply device (chopper type power supply device) that boosts an input voltage to generate an output voltage.

  5A to 5C are circuit diagrams showing first to third conventional examples of a step-up switching power supply device, respectively. As an example of the prior art related to the step-up switching power supply, Patent Document 1 by the applicant of the present application can be cited.

JP 2006-304500 A

  Certainly, in the conventional step-up switching power supply, the output voltage of the output transistor M2 is increased by boosting the input voltage (the power supply voltage Vcc in FIGS. 5A to 5C). It is possible to obtain

  However, in the step-up switching power supply device shown in FIG. 5A, there is a current leakage path from the input terminal of the power supply voltage Vcc to the output terminal of the output voltage Vout via the parasitic diode D1 associated with the synchronous rectification transistor M1. It was. Therefore, when the power supply voltage Vcc is turned on while the boosting operation is stopped and the output voltage Vout is lower than the power supply voltage Vcc, even if the synchronous rectification transistor M1 is turned off, There is a possibility that a large inrush current flows into the output capacitor C1. At this time, if the current supply capability of a power supply (for example, a battery) that supplies the power supply voltage Vcc is small, the power supply voltage Vcc is lowered due to the inrush current, and thus the power supply voltage Vcc is supplied for driving. There is a risk of adversely affecting other ICs and devices (not shown in FIGS. 5A to 5C). Further, in the step-up switching power supply device shown in FIG. 5B, when the output voltage Vout is lower than the power supply voltage Vcc, the synchronous rectifier diode D2 is in a conductive state, so that the same event as described above may occur.

  In the step-up switching power supply device shown in FIG. 5C, when stopping the step-up operation, the synchronous rectification transistor M1 is turned off, and the transistor M3 connected between the back gate and the source of the synchronous rectification transistor M1. Since the current leakage path through the parasitic diode D1 can be cut off by turning off the power supply, the generation of the inrush current and the decrease in the power supply voltage Vcc during the stop of the boosting operation should be avoided in advance. Is possible. However, in the step-up switching power supply device having this configuration, the output capacitor C1 is not charged at all even when the power supply voltage Vcc is turned on while the step-up operation is stopped. Since the output voltage Vout is lower than the power supply voltage Vcc and a large inrush current flows into the output capacitor C1, the power supply voltage Vcc may be lowered as described above.

  Further, in the step-up switching power supply device shown in FIGS. 5A and 5C, if the output terminal is grounded during the step-up operation (short circuit to the ground terminal or a low potential terminal corresponding thereto), an overcurrent is generated in the synchronous rectification transistor M1. There was a risk of flow and destruction. Further, even in the step-up switching power supply device shown in FIG. 5B, there is a possibility that the synchronous rectifier diode D2 may be destroyed due to a ground fault.

  In view of the above problems, an object of the present invention is to provide a step-up switching power supply device that can reduce an inrush current to an output capacitor at startup.

  In order to achieve the above object, a step-up switching power supply according to the present invention includes an inductor having one end connected to an input end of an input voltage and an output connected between the other end of the inductor and a ground end. A transistor, a synchronous rectification transistor connected between the other end of the inductor and an output terminal of the output voltage, an output capacitor connected between an output terminal of the output voltage and a ground terminal, and the output transistor A control unit that performs switching control of the synchronous rectification transistor, and is a step-up switching power supply device that includes the other end of the inductor and the output end of the output voltage, or the input end of the input voltage and the A current limiting transistor having an on-resistance value larger than that of the synchronous rectification transistor is provided between the output terminal of the output voltage and the control unit includes a stamper. When shifting from the state to the active state, prior to starting the switching control of the synchronous rectification transistor is only turned on to configuration (first configuration) a predetermined period said current limit transistor.

  The step-up switching power supply device having the first configuration includes: a first back gate control transistor connected between a back gate of the synchronous rectification transistor and an output terminal of the output voltage; and the synchronous rectification transistor. And a second back gate control transistor connected between the back gate of the synchronous rectification transistor and the input terminal of the input voltage. A configuration (second configuration) is preferable.

  In the step-up switching power supply device having the second configuration, in the standby state, the control unit turns on a second back gate control transistor, the output transistor, the synchronous rectification transistor, the current limiting transistor, While the first back gate control transistor is turned off, in the active state, the first back gate control transistor is turned on, and the second back gate control transistor and the current limiting transistor are both turned off. The output transistor and the synchronous rectification transistor are controlled in a complementary manner, and when the transition from the standby state to the active state is performed, in the first start state of the first half, the second back gate control transistor and The current limiting current All of the transistors are turned on, and the output transistor, the synchronous rectification transistor, and the first back gate control transistor are all turned off, while in the second activation state in the latter half, the first back gate control transistor and the synchronous rectification A configuration in which all the transistors are turned on and all of the output transistor, the current limiting transistor, and the second back gate control transistor are turned off (third configuration) may be employed.

  Further, in the step-up switching power supply device having any one of the first to third configurations, the control unit, when turning on the current limiting transistor, gradually increases the conductivity of the current limiting transistor. A structure for controlling the gate voltage of the transistor (fourth structure) may be employed.

  The step-up switching power supply device having any one of the first to fourth configurations may have a configuration (fifth configuration) including a current limiting resistor connected in series with the current limiting transistor.

  Further, the step-up switching power supply device having any one of the first to fifth configurations includes a ground fault detection unit that detects whether or not the output terminal of the output voltage has a ground fault. The unit may be configured to turn off the synchronous rectification transistor and turn on the current limiting transistor when a ground fault is detected (sixth configuration).

  Further, the step-up switching power supply device having any one of the first to sixth configurations generates an error voltage by amplifying a difference between a feedback voltage varying according to the output voltage and a predetermined reference voltage. And an oscillator that generates a predetermined triangular wave voltage, and a PWM comparator that compares the error voltage and the triangular wave voltage to generate a PWM signal, and the control unit is configured to generate the PWM signal in the active state. Based on the signal, the output transistor and the synchronous rectification transistor may be configured to perform complementary switching control (seventh configuration).

With the step-up switching power supply device according to the present invention, it is possible to reduce the inrush current to the output capacitor during startup.
It is possible to

  FIG. 1 is a circuit block diagram showing an embodiment of a step-up switching power supply device according to the present invention. As shown in FIG. 1, the step-up switching power supply according to the present embodiment includes a control unit 1, an error amplifier 2, an oscillator 3, a PWM [Pulse Width Modulation] comparator 4, a ground fault detection unit 5, and synchronous rectification. A transistor M1, an output transistor M2, a first back gate control transistor M3, a second back gate control transistor M4, a current limiting transistor M5, an inductor L1, an output capacitor C1, a resistor R1, and a resistor R2. Have.

  Of the above components, components other than the inductor L1, the output capacitor C1, and the resistors R1 and R2 may be integrated in the semiconductor device 100. At that time, other circuit blocks (such as a low voltage lockout circuit and a temperature protection circuit) may be appropriately incorporated in the semiconductor device 100 in addition to the above-described components.

  The transistor M1 is a P-channel MOS [Metal-Oxide-Semiconductor] field effect transistor, and its drain is connected to an external terminal T1 (switch terminal). The source of the transistor M1 is connected to the external terminal T2 (output terminal). The gate of the transistor M1 is connected to the first gate signal output terminal of the control unit 1. Although not depicted in FIG. 1, a parasitic diode is attached between the drain and back gate of the transistor M1 in such a manner that the anode is connected to the drain and the cathode is connected to the back gate.

  The transistor M2 is an N-channel MOS field effect transistor, and its drain is connected to the external terminal T1. The source and back gate of the transistor M2 are connected to the external terminal T3 (ground terminal). The gate of the transistor M2 is connected to the second gate signal output terminal of the control unit 1.

  The transistor M3 is a P-channel MOS field effect transistor, and its drain is connected to the external terminal T2. The source and back gate of the transistor M3 are connected to the back gate of the transistor M1. The gate of the transistor M3 is connected to the third gate signal output terminal of the control unit 1.

  The transistor M4 is a P-channel MOS field effect transistor, and its drain is connected to the external terminal T1. The source and back gate of the transistor M4 are connected to the back gate of the transistor M1. The gate of the transistor M4 is connected to the fourth gate signal output terminal of the control unit 1. Note that the drain of the transistor M4 may be connected to the external terminal T0 (power supply terminal).

  The transistor M5 is a P-channel MOS field effect transistor having a larger on-resistance value than the transistor M1, and its drain is connected to the external terminal T1. The source of the transistor M5 is connected to the external terminal T2. The gate of the transistor M5 is connected to the fifth gate signal output terminal of the control unit 1. The back gate of the transistor M5 is connected to the back gate of the transistor M1. Note that the drain of the transistor M5 may be connected to the external terminal T0.

  The inverting input terminal (−) of the error amplifier 2 is connected to the external terminal T4 (feedback terminal). The non-inverting input terminal (+) of the error amplifier 2 is connected to the application terminal for the reference voltage Vref. The non-inverting input terminal (+) of the PWM comparator 4 is connected to the output terminal of the error amplifier 2. The inverting input terminal (−) of the PWM comparator 4 is connected to the output terminal of the oscillator 3. The output terminal of the PWM comparator 4 is connected to the PWM signal input terminal of the control unit 1.

  The ground fault detection unit 5 is a means for detecting whether or not the external terminal T2 is grounded (short circuit to a grounding end or a low potential end corresponding thereto). In FIG. A comparator in which the inverting input terminal (−) is connected to the external terminal T2, the non-inverting terminal (+) is connected to the application terminal of the threshold voltage Vth, and the output terminal is connected to the ground fault detection signal input terminal of the control unit 1. It is used.

  Outside the semiconductor device 100, one end of the external terminal T0 and the inductor L1 is connected to the input end of the input voltage (power supply voltage Vcc in FIG. 1). The power supply voltage Vcc applied to the external terminal T0 is a circuit block integrated in the semiconductor device 100 (in FIG. 1, the control unit 1, the error amplifier 2, the oscillator 3, the PWM comparator 4, and the ground). It is used to drive the envelope detector 5). The external terminal T1 is connected to the other end of the inductor L1. The external terminal T2 is connected to a load (not shown), and is connected to the ground terminal via a path via the output capacitor C1 and a path via a voltage dividing circuit including the resistors R1 and R2. The external terminal T3 is connected to the ground terminal. The external terminal T4 is connected to a connection node between the resistor R1 and the resistor R2. The external terminal T5 is connected to an enable signal output terminal of a host (not shown) (such as a set side CPU [Central Processing Unit]). Note that the enable signal XSHDN given to the external terminal T5 is inputted to the control unit 1 and used for enable / disable control of the boosting operation.

  First, the basic operation (step-up operation) of the step-up switching power supply device configured as described above will be described.

  The transistor M2 is an output transistor that is switching controlled (opening / closing control) according to the second gate voltage from the control unit 1, and the transistor M1 is switching controlled (opening / closing control) according to the first gate voltage from the control unit 1. ) Synchronous rectification transistor.

  When the control unit 1 boosts the power supply voltage Vcc to obtain the output voltage Vout, the transistor M3 is turned on, both the transistor M4 and the transistor M5 are turned off, and the transistors M1 and M2 are switched in a complementary manner. To do.

  Note that the term “complementary” used in this specification refers to the case where the transistors M1 and M2 are turned on / off in addition to the case where the on / off of the transistors M1 and M2 are completely reversed. The case where a predetermined delay is given to the off transition timing is also included.

  When the transistor M2 is turned on, the inductor current IL flows through the inductor L1 toward the ground terminal via the transistor M2, and the electrical energy is stored. Note that if the charge has already been accumulated in the output capacitor C1 during the ON period of the transistor M2, the current from the output capacitor C1 flows through the load (not shown) connected to the external terminal T2. At this time, the transistor M1, which is a synchronous rectifier, is turned off in a complementary manner to the on state of the transistor M2, so that no current flows from the output capacitor C1 toward the transistor M2.

  On the other hand, when the transistor M2 is turned off, the electric energy stored therein is released by the back electromotive voltage generated in the inductor L1. At this time, since the transistor M1 is turned on complementarily to the off state of the transistor M2, the current flowing from the external terminal T1 through the transistor M1 flows into the load from the external terminal T2 and the output capacitor C1. Then, it also flows into the ground terminal and charges the output capacitor C1. By repeating the above operation, the output voltage Vout smoothed by the output capacitor C1 is supplied to the load.

  As described above, the step-up switching power supply according to this embodiment boosts the input voltage Vin and generates the output voltage Vout by switching control of the transistors M1 and M2.

  Next, feedback control of the step-up switching power supply device having the above configuration will be described.

  The error amplifier 2 includes an output feedback voltage Vfb (corresponding to an actual value of the output voltage Vout) drawn from a connection node between the resistors R1 and R2, and a predetermined reference voltage Vref (corresponding to a target value of the output voltage Vout). The difference is amplified to generate an error voltage Verr. That is, the voltage level of the error voltage Verr becomes higher as the output voltage Vout is lower than the target value. On the other hand, the oscillator 3 generates a triangular wave voltage (sawtooth voltage) Vsaw having a predetermined frequency.

  The PWM comparator 4 compares the error voltage Verr and the triangular wave voltage Vsaw to generate a PWM signal S1. That is, the on-duty (ratio of the on-period of the transistor M2 in the unit period) of the PWM signal S1 sequentially varies according to the relative level of the error voltage Verr and the triangular wave voltage Vsaw. More specifically, as the output voltage Vout is lower than its target value, the on-duty of the PWM signal S1 increases. As the output voltage Vout approaches the target value, the on-duty of the PWM signal S1 decreases.

  When boosting the power supply voltage Vcc to obtain the output voltage Vout, the control unit 1 performs complementary switching control on the transistors M1 and M2 in accordance with the PWM signal S1. More specifically, the control unit 1 turns on the transistor M2 and turns off the transistor M1 during the high level period of the PWM signal S1, while turning off the transistor M2 during the low level period of the PWM signal S1. The transistor M1 is turned on.

  As described above, the step-up switching power supply according to the present embodiment can adjust the output voltage Vout to the target value by the output feedback control based on the error voltage Verr.

  Next, the start-up operation and the ground fault protection operation of the step-up switching power supply device configured as described above will be described in detail with reference to FIGS. FIG. 2 is a timing chart for explaining the start-up operation of the step-up switching power supply apparatus, and FIG. 3 is a correlation diagram between the operation state of the step-up switching power supply apparatus and the on / off states of the transistors M1 to M5. . In FIG. 3, in order from the top, the power supply voltage Vcc, the output voltage Vout, the enable signal XSHDN, the inductor current IL, the switch voltage Vsw (voltage appearing at the switch terminal T1), and the gate voltages (first voltages) of the transistors M1 to M5. 1 to 5 gate signals) are depicted.

  From time t1 when the power supply voltage Vcc is input to the step-up switching power supply device until time t2 when the enable signal XSHDN is raised from low level to high level, State (standby state of the boosting operation).

  In the standby state, the control unit 1 turns on only the transistor M4 and turns off the remaining transistors M1, M2, M3, and M5. That is, the control unit 1 sets the gate voltages of the transistors M2 and M4 to low level (GND), and sets the gate voltages of the transistors M1, M3, and M5 to high level (Vcc).

  By such gate voltage control, it is possible to reliably cut off all current leak paths from the external terminal T1 to the external terminal T2, including the current leak path via the parasitic diode associated with the transistor M1.

  Further, by turning on the transistor M4, the back gates of the transistors M1, M3, M4, and M5 can be connected to the highest potential point (switch voltage Vsw (= power supply voltage Vcc)) in the standby state. , M3, and M5 can be more surely turned off.

  From the time when the enable signal XSHDN is raised from the low level to the high level at time t2, until the predetermined period X elapses (time t2 to time t3), the step-up switching power supply device is in the first activation state ( A precharge state of the output capacitor C1).

  In the first activation state, the control unit 1 turns on both the transistors M4 and M5 and turns off the transistors M1 to M3. That is, the control unit 1 sets the gate voltages of the transistors M2, M4, and M5 to low level (GND), and sets the gate voltages of the transistors M1 and M3 to high level (Vcc).

  With such gate voltage control, when the switching from the standby state to the active state is performed, the transistor M5 having a larger on-resistance value than the transistor M1 is turned on for a predetermined period X before starting the switching control of the transistor M1. Since the output capacitor C1 can be slowly charged through the current path through the transistor M5, it is possible to prevent an excessive inrush current from flowing into the output capacitor C1 and prevent the power supply voltage Vcc from decreasing. Is possible.

  When the power supply voltage Vcc is not turned on when the enable signal XSHDN is raised to a high level, or when the power supply voltage Vcc has not risen to a predetermined voltage level, the step-up switching power supply device After waiting for the voltage Vcc to reach a predetermined voltage level, the state shifts to the first activation state.

  In the first activation state, the control unit 1 changes the gate voltage of the transistor M5 from the high level (Vcc) to the low level (GND) so as to gradually increase the conductivity when the transistor M5 is turned on. ) May be configured to perform sweep control. With such a configuration, the current flowing into the output capacitor C1 via the transistor M5 can be gradually increased, so that the inrush current can be further suppressed.

  From time t3 when the first activation state is expired until a predetermined period Y elapses (time t3 to time t4), the step-up switching power supply device is in the second activation state (to the active state). Ready for transition).

  In the second activation state, the control unit 1 turns on the transistors M1 and M3 and turns off the transistors M2, M4, and M5. That is, the control unit 1 sets the gate voltages of the transistors M1 to M3 to low level (GND), and sets the gate voltages of the transistors M4 and M5 to high level (Vcc).

  By such gate voltage control, when the transistor M5 is turned off prior to the transition to the active state, the transistor M1 is turned on at the same time, so that the precharge of the output capacitor C1 can be continued. It becomes possible.

  The means for timing the predetermined periods X and Y may be configured to use a timer (counter) that starts a count operation simultaneously with the cancellation of the standby state, or the voltage level is adjusted simultaneously with the cancellation of the standby state. The soft start voltage Vss that starts to rise slowly is monitored, and based on the comparison result between the soft start voltage Vss and the threshold voltages Vx and Vy (corresponding to predetermined periods X and Y, respectively), the first start state to the second start state It is good also as a structure which performs the transfer process to 2nd and the transfer process from a 2nd starting state to an active state.

  The soft start voltage Vss may be used as the reference voltage Vref input to the non-inverting input terminal (+) of the error amplifier 2 or may be used with respect to the non-inverting input terminal (+) of the PWM comparator 4. Thus, the error voltage Verr and the soft start voltage Vss may be input in parallel, and the PWM comparator 4 may compare the lower one of the error voltage Verr and the soft star voltage Vss with the triangular wave voltage Vsaw.

  When the second activation state is completed at time t4, the step-up switching power supply device enters an active state (step-up operation state).

  In the above active state, as described above, the control unit 1 turns on the transistor M3, turns off both the transistor M4 and the transistor M5, and performs complementary switching control on the transistor M1 and the transistor M2. That is, the control unit 1 fixes the gate voltage of the transistor M3 to the low level (GND), and fixes the gate voltages of the transistors M4 and M5 to the high level (Vout), and then the transistors M1 and M2 The gate voltage is pulse-driven between a low level (GND) and a high level (Vout).

  By such gate voltage control, complementary switching control of the transistors M1 and M2 can be performed, and the input voltage Vin can be boosted to generate the output voltage Vout.

  The control unit 1 monitors the ground fault detection signal S2 from the ground fault detection unit 5 and determines that a ground fault has occurred at the external terminal T2 (that is, immediately after the ground fault detection signal S2 is activated). The transistor M4 and the transistor M5 are both turned on, and the transistors M1 to M3 are all turned off. That is, at the time of detecting the ground fault, the control unit 1 sets the gate voltages of the transistors M2, M4, and M5 to low level (GND), and sets the gate voltages of the transistors M1 and M3 to high level (Vcc). .

  With such gate voltage control, when a ground fault is detected, a current can flow through the transistor M5 having a larger on-resistance value than that of the transistor M1, so that the peak value of the overcurrent can be suppressed.

  The configuration of the present invention can be variously modified within the scope of the present invention in addition to the above embodiment.

  For example, in the above-described embodiment, the configuration in which only the transistor M5 is provided as an example of the current limiting element for suppressing the current that flows at the time of start-up or output ground fault is described, but the configuration of the present invention is limited to this. For example, as shown in FIG. 4, it may be configured to have a current limiting resistor R3 connected in series with the transistor M5.

  With such a configuration, it is not necessary to design the on-resistance value of the transistor M5 to be extremely large, so that the insufficient withstand voltage of the transistor M5 due to the reduction in the element size can be solved. Further, in the configuration using the current limiting resistor R3, the resistance value of the current limiting resistor R3 can be arbitrarily and easily adjusted by laser trimming. Therefore, the current limitation according to the allowable loss of the semiconductor device 100 is performed. It becomes possible to do.

  4 illustrates the configuration in which the current limiting resistor R3 is inserted between the source of the transistor M5 and the external terminal T2, the insertion position is not limited to this, and the drain of the transistor M5 It may be inserted between the external terminal T1 (or the external terminal T0) or may be inserted in both the above positions.

  The present invention is a technique useful for increasing the reliability of a step-up switching power supply device, and is used for all electronic devices that require an output voltage higher than an input voltage (for example, an optical disc drive such as a Blu-ray disc drive, a digital still camera, etc. / Digital video camera / portable equipment such as a mobile phone).

These are the circuit block diagrams which show one Embodiment of the pressure | voltage rise type switching power supply device based on this invention. FIG. 5 is a correlation diagram between the operating state of the step-up switching power supply device and the on / off states of the transistors M1 to M5. These are timing charts for explaining the starting operation of the step-up switching power supply device. These are the circuit block diagrams which show the modification of the pressure | voltage rise type switching power supply device which concerns on this invention. These are the circuit diagrams which show the 1st prior art example of a step-up type switching power supply device. These are circuit diagrams which show the 2nd prior art example of a step-up type switching power supply device. These are circuit diagrams which show the 3rd prior art example of a step-up type switching power supply device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Control part 2 Error amplifier 3 Oscillator 4 PWM comparator 5 Ground fault detection part (comparator)
100 Semiconductor Device M1 Synchronous Rectification Transistor (P-Channel MOS Transistor)
M2 output transistor (N-channel MOS transistor)
M3 First back gate control transistor (P-channel MOS transistor)
M4 Second back gate control transistor (P-channel MOS transistor)
M5 Current limiting transistor (P-channel MOS transistor)
L1 Inductor C1 Output capacitor R1 to R2 Resistor R3 Current limiting resistor T1 to T5 External terminal

Claims (7)

One end of the inductor connected to the input end of the input voltage, the output transistor connected between the other end of the inductor and the ground end, and the other end of the inductor connected to the output end of the output voltage. A step-up switching power supply device comprising: a synchronous rectification transistor; an output capacitor connected between an output terminal of the output voltage and a ground terminal; and a control unit that performs switching control of the output transistor and the synchronous rectification transistor. Because
A current limiting transistor having a larger on-resistance value than the synchronous rectification transistor between the other end of the inductor and the output end of the output voltage or between the input end of the input voltage and the output end of the output voltage Comprising
The step-up switching power supply device wherein the control unit turns on the current limiting transistor for a predetermined period before starting the switching control of the synchronous rectification transistor when shifting from the standby state to the active state .   A first back gate control transistor connected between a back gate of the synchronous rectification transistor and an output terminal of the output voltage; between the back gate of the synchronous rectification transistor and the other end of the inductor, or the synchronization 2. The step-up switching power supply device according to claim 1, further comprising: a second back gate control transistor connected between a back gate of the rectifying transistor and an input terminal of the input voltage. The controller is
In the standby state, the second back gate control transistor is turned on, and the output transistor, the synchronous rectification transistor, the current limiting transistor, and the first back gate control transistor are all turned off, while in the active state, The first back gate control transistor is turned on, the second back gate control transistor and the current limiting transistor are both turned off, and the output transistor and the synchronous rectification transistor are complementarily switched. further,
When shifting from the standby state to the active state, in the first activation state in the first half, both the second back gate control transistor and the current limiting transistor are turned on, the output transistor, the synchronous rectification transistor, and the first While all the back gate control transistors are turned off, in the second activation state in the second half, both the first back gate control transistor and the synchronous rectification transistor are turned on, and the output transistor, the current limiting transistor, and the second 3. The step-up switching power supply device according to claim 2, wherein all of the back gate control transistors are turned off.   The said control part controls the gate voltage of the said current limiting transistor so that the conductivity may be gradually raised when turning on the said current limiting transistor, The any one of Claims 1-3 characterized by the above-mentioned. A step-up switching power supply device according to claim 1.   5. The step-up switching power supply device according to claim 1, further comprising a current limiting resistor connected in series with the current limiting transistor. It comprises a ground fault detector for detecting whether or not the output terminal of the output voltage has a ground fault,
The step-up type according to claim 1, wherein the control unit turns off the synchronous rectification transistor and turns on the current limiting transistor when a ground fault is detected. Switching power supply. An error amplifier that generates an error voltage by amplifying a difference between a feedback voltage that varies according to the output voltage and a predetermined reference voltage, an oscillator that generates a predetermined triangular wave voltage, and the error voltage and the triangular wave voltage are compared. And a PWM comparator for generating a PWM signal,
The said control part performs switching control of the said output transistor and the said synchronous rectification transistor complementarily based on the said PWM signal in the said active state, The Claim 1 characterized by the above-mentioned. Boost switching power supply.

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