DC-DC converter capable of soft starting function by slowly raising reference voltage

A DC-DC converter includes an oscillation circuit for oscillating at a predetermined frequency and generating a pulse with a predetermined pulse width, a reference voltage generating circuit for generating a reference voltage, means mounted in the reference voltage generating circuit for slowly rising the reference voltage, a switching circuit for chopping the current inputted from a DC primary electric power source in accordance with the pulse, and feedback control means for stopping/restarting the oscillation of the oscillation circuit on the basis of the voltage that the output of the switching circuit is smoothed and including the reference voltage with a detection voltage upon stopping control, so that the DC-DC converter having a soft starting function is provided with a simple structure and a compact size without utilizing a PWM circuit and a VCO.

BACKGROUND OF THE INVENTION
 The present invention relates to a DC-DC converter of the chopper type, and
 more particularly to a DC-DC converter provided with soft starting
 function suitable for minimizing its size and reducing its manufacturing
 cost. The DC-DC converter of this type is suitable to be used for an
 electric power source or the like of portable devices adapted to be
 operated by a battery, and further it is suitable to drive an
 electromagnetic relay, a compact electric motor, an EL
 (Electro-Luminescence) or the like through a battery.
 Hitherto, there are known a DC-DC converter provided with a soft starting
 circuit for the purpose of a restriction or the like for a rush current
 (JP-A-5-76167), and a DC-DC converter provided with not only a soft
 starting circuit but also an over current detection circuit
 (JP-A-7-298614). A PWM (Pulse Width Modulation) system for varying the
 width of a pulse for controlling a switching operation and a PFM (Pulse
 Frequency Modulation) system for varying the switching frequency are a
 precondition for those DC-DC converters.
 Moreover, there is known a reference voltage generating circuit suitable to
 be easily structured by IC elements, which utilizes the band gap voltage
 power source with a precise voltage output but an insufficient output
 current. Such reference voltage generating circuit 5, as shown in FIG. 4,
 is structured so that the reference voltage Vref generated by a band gap
 voltage power source 5a is amplified for current by a voltage follower 5b
 or the like in order to supply it to necessary circuits.
 In contrast, since there is a boundary of minimizing the size and
 decreasing the manufacturing cost for the conventional DC-DC converters in
 which PWM and PFM are made precondition because of complexities of its
 circuit structure and its adjusting operation, there is presented by the
 same applicant as the present application, Japanese Application No.
 10-322720 of the type (referred to as an oscillation stopping/restarting
 type hereinafter) in which the oscillation of an oscillation circuit for
 generating a fixed frequency is stopped suitably. According to the type,
 it is only sufficient to adapt an operation property of the circuit to a
 switching condition utilizing a predetermined frequency thereby enabling
 PWM and VCO unnecessary.
 Namely, as shown in FIG. 3, for the purpose of boosting or decreasing an
 input voltage Vin from a DC primary electric power source 1 to a desired
 output voltage Vo to be supplied to a load 4 connected to a smoothing
 circuit 3, the DC-DC converter 2 of the oscillation stopping/restating
 type is composed of an oscillation circuit 2c for oscillating a
 predetermined frequency and generating a pulse signal A with a
 predetermined pulse width, a switching circuit 2b for chopping the inertia
 current I flowing into an inductance element section 2a on the basis of
 the input voltage Vin from the DC primary electric power source 1, in
 accordance with the pulse signal A, and a feedback control circuit 2d for
 generating a feedback control signal B on the basis of the output voltage
 Vo obtained by smoothing the output from the switching circuit 2b, and
 feeding it to the oscillation circuit 2c thereby stopping/restarting the
 oscillation.
 However, in the case where it is tried to provide a soft starting function
 for the DC-DC converter having the above-mentioned structure, it is
 difficult to adopt the conventional measure as it were and further it is
 not easy to provide the soft starting function therefor, because a
 switching control type as a premise is different form each other,
 Therefore, it is a technical problem as to how the function for preventing
 and controlling the rush current and the over current is provided for the
 DC-DC converter of the oscillation stopping/restarting type.
 SUMMARY OF THE INVENTION
 The object of the present invention is to solve the technical problem as
 mentioned above, and to present a DC-DC converter having a simple
 structure and enabling a soft starting operation.
 The DC-DC converter invented as a first solving means, which is made for
 the purpose of solving the conventional technical problem is composed of
 an oscillation circuit for oscillating a predetermined frequency and
 sequentially generating a pulse having a predetermined pulse width with
 the predetermined frequency if it is in a free running state, a switching
 circuit for chopping the current inputted from the Dc primary electric
 power source in accordance with the pulse, and feedback control means for
 stopping/restarting the oscillation of the oscillation circuit on the
 basis of the voltage that the output of the switching circuit is smoothed
 by a smoothing circuit which is installed within the system or the
 outside, wherein the operation of the oscillation circuit is controlled to
 stop so that the feedback control circuit compares the detected voltage
 with the reference voltage, and further means for slowly rising the
 reference voltage is provided within the reference voltage generating
 circuit for generating the reference voltage.
 According to such DC-DC converter, the oscillation circuit for sequentially
 oscillating the pulse having a predetermined pulse width and with a
 predetermined frequency in the independent state in which no feedback
 signal is applied thereto, a feedback signal is applied to the oscillation
 circuit and the feedback signal is utilized for the switching circuit
 without modulating the oscillation signal and further the feedback to the
 oscillation circuit is performed by the voltage that the output is
 smoothed thereby enabling the oscillation of the oscillation circuit to be
 stopped and restarted, and therefore the frequency of the switching
 circuit becomes similar to that of PFM even if PWM and VCO are not used,
 thereby maintaining a suitable output.
 Moreover, since the current inputted from the DC primary electric power
 source is chopped by the switching circuit and the switching control is
 performed in accordance with the pulse having a predetermined pulse width
 and a predetermined frequency from the oscillation circuit. Therefore,
 such switching operation is restored to the original state at a constant
 period corresponding to the pulse width in a normal condition, the
 switching condition is fixed substantially even if the switching system is
 similar to the PFM system.
 As a result, the switching condition suitable to the operation property of
 the switching circuit or the like is established completely prior to a
 decision of the load condition, and therefore it becomes easy to design
 the system. Moreover, since PWM and VCO are unnecessary, the circuit
 construction becomes to be simple and the operation of the circuit becomes
 to be stable.
 Moreover, when the output voltage is detected by the feedback control
 means, the detected voltage is compared with the reference voltage and
 further the stopping control of the oscillation circuit is performed on
 the basis of the result of the comparison. Therefore, since the reference
 voltage is adapted to be risen slowly, the feedback operation is achieved
 such that the output voltage is also increased slowly as the reference
 voltage rises slowly. Therefore, during some period from energization of
 the electric power source, namely during the transitional period until the
 reference voltage reaches a constant value, the stopping control for the
 oscillation circuit is performed more frequently than usual state, and
 therefore the chopped current is restricted from abrupt increment thereby
 slowly increasing to a normal state. The slow rise of the reference
 voltage is thus achieved, and its slow rise is achieved by a simple and
 local modification of the circuit such that a capacitance element or the
 like is added into the reference voltage generating circuit thereby
 restricting complexity of circuit to minimize extent.
 As a result, there is presented a DC-DC converter provided with a soft
 starting function with maintaining the property that the PWM and VCO
 circuits are not necessary and it is easy to make the converter with a
 simple and compact structure.
 Therefore, according to the present invention there is presented a DC-DC
 converter provided with a soft starting function with a simple and compact
 structure.
 The DC-DC converter invented as a second solving means, which is made for
 the purpose of solving the conventional technical problem is composed of,
 in a DC-DC converter invented as a first solving means, an over-current
 detection circuit provided for the switching circuit for detecting as to
 whether the current flowing into the switching circuit exceeds over a
 predetermined threshold value which is corresponding to the over-current
 or not, and the means provided between the oscillation circuit and the
 feedback control circuit & the over-current detection circuit, for
 stopping/restarting the oscillation of the oscillation circuit in
 accordance with the detection output of the over-current detection circuit
 in addition to the control of the feedback control means.
 According to such DC-DC converter, by means of the over-current detection
 circuit is installed within the switching circuit or attached thereto, the
 detection as to whether the switching current flowing into the switching
 circuit exceeds over a predetermined threshold value or not is performed,
 namely it is detected as to whether the switching circuit is likely to a
 over-current state or not. Then, if the state of the switching circuit is
 likely to a over-current was detected, the stop control of the oscillation
 circuit is performed through the stopping/restarting means, as is similar
 to the stopping/restarting of the oscillation performed by the control of
 the feedback control means. Moreover, such the stopping/restarting means
 may be realized by utilizing a simple logic circuit or the like. As
 mentioned above, the detection for the over-current can be directly
 realized on the basis of the switching current merely by attaching a
 simple circuit, and further the control for stopping the oscillation
 circuit is performed in accordance with the detection result of the
 over-current.
 As a result, such over-current is surely avoided even in the case where a
 variation or the like of load which is exceed over assumption causes
 during not only the starting time but also its operation. Moreover, in
 this case there is not formed a coalition between the condition of the
 soft starting and the condition of the over-current detection, and
 therefore it is easily achieved to faithfully change a design in
 accordance with different specifications even when required specifications
 for voltage and current given or to be supplied are different from each
 other for each of applications.
 Therefore, according to the present invention there is presented a DC-DC
 converter with a simple structure in which a soft starting and a
 prevention of over-current during its operation are possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to Figures a first and a second preferable embodiments of a
 DC-DC converter of the present invention will be specifically described
 hereinafter.
 The above-mentioned first solving means is realized by the first embodiment
 as shown in FIGS. 1A, 1B, and 1C, and the second solving means is realized
 by the second embodiment as shown in FIG. 2.
 The structure of the DC-DC converter as the first embodiment of the present
 invention will be specifically described with reference to figures
 hereinafter. FIG. 1A shows an entire circuit, FIG. 1B shows an oscillation
 circuit, and FIG. 1C shows a reference voltage generating circuit.
 As shown in FIG. 1A, the circuit composed of a DC primary electric power
 source 1 for supplying the electric power of, for example, about 3 volts,
 which may be a disposal cell, a chargeable cell or the like, a DC-DC
 converter 20 of the chopper type for booting an input voltage Vin to the
 output voltage Vo, for example 15 volts, a smoothing circuit 3 for
 stabling the output voltage Vo of the DC-DC converter 20, and a load 4 for
 being operated by the output voltage, for the purpose of operating the
 load 4 by converting the voltage of the electric power source into a
 suitable voltage even if the load 4 does not operate by the voltage of the
 cell.
 The load 4 is different in accordance with applications, and a typical one
 of them is a motor driving circuit for driving, for example, an electric
 motor which provides a power transistor or the like for controlling a
 motor driving current, and the motor driving current is supplied from the
 output voltage Vo of the DC-DC converter 20.
 The smoothing circuit 3 is composed of a condenser C1 connected to the
 output line of the DC-DC converter 20 to be connected also to the load 4,
 and there is used for the condenser C1 an electrolytic condenser or the
 like as an independent element, which has a large capacity enough for
 stabling the output voltage Vo.
 The DC-DC converter 20 is formed as a hybrid module in which an inductance
 element section 2a and an one tip IC are installed on a common substrate.
 There is used for the inductance element section 2a a coil L1 with, for
 example, 47.mu.H connected in series with the line of the input voltage
 Vin. Moreover, the one chip IC includes other integrated circuits on one
 silicon chip, specifically a switching circuit 2b, an oscillation circuit
 21 and a feedback control circuit 22.
 The switching circuit 2b is composed of a transistor Trl and a diode D1,
 and mounted on the series line of electric current supply from the DC
 primary electric power source 1 to the load 4 through the inductance
 element section 2a and the smoothing circuit 3. The anode of the diode D1
 is connected to the line of the input voltage Vin, and the cathode of the
 diode D1 is connected to the line of the output voltage Vo so that the
 inertia current I supplied from the inductance element section 2a to the
 smoothing circuit 3 does not flow in reverse. Schottky diode having less
 voltage loss is positively used for the diode D1. Moreover, MOS transistor
 or the like which can drive several hundred milliampere, for example, 200
 mA is used for the transistor Tr1, and the source and the drain of the
 transistor Tr1 are connected to the anode side of the diode D1 and the
 ground or a reference line respectively and the gate thereof is connected
 to the line of the pulse signal A.
 In case of OFF state of the transistor Tr1, the transistor does not
 influence the line of the inertial current I, and therefore the inertia
 current I passed through the coil L1 from the DC primary electric power
 source 1 flows into the smoothing circuit 3 through the diode D1 and the
 line of the output voltage Vo. In contrast, if the pulse signal A becomes
 effective, namely when the pulse signal A is received by the gate, the
 transistor Tr1 is switched from the OFF state to the ON state thereby
 bypassing the inertia current I to the ground line. Upon no pulse, the
 transistor Tr1 is restored to the OFF state. Thus, the switching circuit
 2b performs the chopping operation for chopping the current inputted from
 the DC primary electric power source 1 in accordance with the pulse of the
 pulse signal A.
 The oscillation circuit may be structured by the circuit (as disclosed in
 the publication No. JP-A-10-322720) which is composed of a plurality of
 NOT gates and buffers connected in series, however the oscillation circuit
 is composed of, as shown in FIG. 1B, a condenser C2 used for charge and
 discharge, a constant current circuit for effecting the charge thereof
 with a constant current I1, a constant current circuit for effecting the
 discharge thereof with a constant current I2, and a binary circuit
 composed of a buffer with hysteresis, a comparator or the like, for
 cording the voltage of the condenser C2 changed by the charge and
 discharge thereof with a predetermined threshold value, so that the design
 of the charge and discharge time constant for defining the oscillation
 frequency and the pulse width becomes easy. The binary signal is outputted
 as the pulse signal A and used for controlling switch circuits inserted
 into the charge and discharge line of the constant current I1 and I2. By
 this, the oscillation circuit 21 oscillates a predetermined frequency and
 sequentially generates the predetermined frequency with a predetermined
 pulse width, if it is in a free running state.
 Moreover, the oscillation of the oscillation circuit 21 is adapted to be
 stopped and restarted in accordance with the feedback control signal B in
 such a manner that both constant current circuits receive the feedback
 control signal B as a control signal to stop and restart the outputs of
 the constant currents I1 and I2 in accordance with the condition that the
 feedback control signal B is effective or not. Moreover, in order to
 achieve a quick response to the request of oscillation stop, there is
 provided a switch circuit in parallel with the condenser C2 thereby
 instantaneously discharging the capacity stored in the condenser C2. The
 switch circuit is also controlled through the feedback control signal B.
 The feedback control circuit 22 is provided for the purpose of generating
 the feedback control signal B for stopping/restarting the oscillation of
 the oscillation circuit 21, and composed of a resistive potential divider
 circuit composed of a series of resistors R1 and R2 inserted between the
 line of the output voltage Vo and the reference line of the ground or the
 like, for generating the detection voltage C by dividing the output
 voltage Vo through the resistivity thereof, a reference voltage generating
 circuit 50 which will be described hereinafter, for generating a
 predetermined reference voltage D, and a comparing circuit 23 composed of
 a comparator or the like for generating the feedback control signal B by
 inputting the detection voltage C and the reference voltage D and
 comparing the detection voltage C with the reference voltage D so that the
 generation of the feedback control signal B is performed on the basis of
 the output voltage Vo, namely on the basis of the output voltage Vo that
 the output of the switching circuit 2b is smoothed by the smoothing
 circuit 3 attached to the DC-DC converter 20. When the detection voltage C
 exceeds over the reference voltage D, the oscillation of the oscillation
 circuit 21 is stopped, on the other hand in the reverse case the
 oscillation is restarted thereby generating the feedback control signal B.
 The reference voltage generating circuit 50, as shown in FIG. 1c, includes
 the band gap voltage source 5a and the voltage follower 5b in order to
 generate the reference voltage D with a constant value in a normal state
 as is similar to the conventional reference voltage generating circuit 5
 as shown in FIG. 4, and further there is provided a condenser C3 one end
 of which is connected to the connection line between the band gap voltage
 source 5a and the voltage follower 5b, and the other end of which is
 connected to the reference line of the ground or the like, for the purpose
 of achieving a slow raise of the reference voltage D in the transitional
 period immediately after the energization of the electric power source. In
 addition to the condenser C3, there are provided for the purpose of the
 charge and discharge of the condenser C3, a constant current circuit 53
 for discharging the capacity of the condenser C3 with a constant current
 I4, a constant current circuit 52 for charging it with the larger current
 than that of the constant current circuit 53, and a difference amplifier
 circuit 51 for driving a transistor Tr2 in accordance with the difference
 between the voltage of the condenser C3 and the output voltage Vref of the
 band gap voltage source 5a.
 According to the circuit 50, the output of the constant current circuit 52
 is restricted or released by the transistor Tr2 inserted into the charging
 line from the constant current circuit 52 to the condenser C3, and
 therefore its charging is performed with the difference voltage (I3-I4)
 when the voltage of the condenser C3 is below the voltage Vref, and when
 the voltage of the condenser C3 is balanced with the voltage Vref at which
 the charge thereof is balanced with the discharge thereof, the stable
 condition that the voltage of the condenser C3 is consistent with the
 voltage Vref, is maintained. According to the circuit thus structured, not
 only the inclination and the shaping time or the like with respect to the
 risen waveform of the reference voltage D can be clearly decided on the
 basis of the difference voltage (I3-I4) and the capacity of the condenser
 C3, but also it is achieved to enable the reference voltage D more stable
 than the conventional case by the combination of the condenser C3 having
 smoothing function and the transistor Tr2 for normally maintaining the
 active state by the idling current 14.
 The first embodiment of the DC-DC converter will be described with respect
 to its utilization and its operation hereinafter. In this case, if the
 electric power supply from the Dc electric power source 1 to the DC-DC
 converter 20 is started by the actuation of an electric power switch which
 is not shown, at first a soft starting thereof is effected and then it
 becomes its stable state as follows.
 When the input voltage Vin raises, the inertia current I through the coil
 L1 is gradually increased. Moreover, the oscillation circuit 21 oscillates
 a constant frequency and the pulse signal A becomes the signal including a
 constant period and a constant pulse width. Upon existence of the pulse,
 the inertia current I is bypassed by the transistor Tr1 to the ground side
 to energizing it. On the other hand, upon absence of the pulse, the
 inertia current I loses the bypass through the transistor Tr1 thereby
 boosting it, and if its voltage is increased over the output voltage Vo,
 the current flows into the condenser C1 through the diode D1.
 Then, by repeating such operations the output voltage Vo is try to increase
 toward a predetermined target value (D.times.(R1+R2)/R2) but the reference
 voltage D hardly rises initially thereby enabling the feedback control
 signal B effective frequently and therefore interrupting the oscillation
 of the oscillation circuit 21 frequently. Due to each interruption of the
 oscillation, the pulse of the pulse signal A is missed or becomes one with
 narrow width, whereby the inertia current I hardly energized and therefore
 both of the inertia current I and the output voltage Vo are slowly risen
 in accordance with the slow rise of the reference voltage D.
 As mentioned above the soft starting function is achieved, if the reference
 voltage D reaches the output voltage Vref of the band gap voltage source
 5a, a normal state thereof is obtained. In this normal state, if the
 maximum current to be supplied is almost consumed because of a heavy load
 4, the condition in which the detection voltage C of the output voltage Vo
 exceeds over the reference voltage D is almost never caused, whereby it is
 a rare chance that the feedback control signal B stops the oscillation of
 the oscillation circuit 21. In contrast, if the electric consumption is
 reduced because of a light load 4, though the output voltage Vo has a
 slight pulsation, the output voltage Vo is gradually increased, whereby
 the detection voltage C exceeds over the reference voltage D, the feedback
 control signal B becomes effective, and the oscillation of the oscillation
 circuit 21 is stopped.
 Then, if the inertia current I decreases and the output voltage Vo returns
 to the target value, the feedback control signal B becomes ineffective
 thereby restarting the oscillation of the oscillation circuit 21.
 As mentioned above, in the normal state the stopping/restarting of the
 oscillation circuit 21 is repeated with the frequency in accordance with
 the condition of the load 4 thereby suitably skipping the pulses of the
 pulse signal A and therefore maintaining the output voltage Vo to a
 constant value (Vref.times.(R1+R2)/R2)) substantially.
 Moreover, in the case where the switching property on the basis of the
 transistor Tr1 and the coil L1 is different because of the difference of
 the load 4, it may be possible to reset the oscillation frequency and the
 pulse width of the oscillation circuit 21, and further in the of different
 output voltage Vo, it may be possible to reset the values of the resistors
 R1 and R2 or the reference voltage D. Moreover, in the case where it is
 desired to change the condition of the soft start function, it may be
 possible to modify the constant current I3 and I4 and the capacity of the
 condenser C3, whereby it is easily achieved to adapt the parts to
 different conditions of operation.
 Referring now figures the specific structure of a second embodiment of the
 DC-DC converter of the present invention will be described hereinafter.
 FIG. 2 is a view showing an entire circuit diagram corresponding to FIG.
 1A.
 In the DC-DC converter 20 as shown in FIG. 2, the difference from that in
 the above FIG. 1A is the point that an over current detection circuit 60
 and a logic circuit 70 are added thereinto.
 The over current detection circuit 60 is composed of a resistor R3
 connected in series with the bypass line of the inertia current I due to
 the transistor Tr1, for generating the voltage corresponding to the
 inertia current I, and a comparator 61 for generating an over current
 detection signal E by comparing the predetermined threshold value
 generated by a threshold voltage generating circuit 62 and the detection
 voltage due to the resistor R3, whereby the over current detection signal
 E is effective when the detection voltage exceeds over the threshold
 value. Namely, the over current detection circuit is mounted with respect
 to the switching circuit 2b for detecting as to whether the over current I
 exceeds over the predetermined threshold value corresponding to the over
 current or not.
 The logic circuit 70 is an OR gate inputting the feedback control signal B
 and the over current detection signal E, and its output is applied to the
 oscillation circuit 21 in place of the feedback control signal B in FIG.
 1A. By this, if at least one of the feedback control signal B and the over
 current detection signal E becomes effective, the oscillation of the
 oscillation circuit 21 is stopped. Such logic circuit 70 which is made
 simple and is mounted between the oscillation circuit 21 and a feedback
 control means 22 & the over current detection circuit 60 for the purpose
 of stopping/restarting the oscillation of the oscillation circuit 21 in
 response to the detection signal E of the over current detection circuit
 60 in addition to the feedback control signal B of the feedback control
 means 22.
 The utilization and its operation of the second embodiment of the DC-DC
 converter will be described hereinafter.
 The DC-DC converter 20 can operate as is similar to the above mentioned
 case in spite of the transitional state and the normal state, because the
 over current detection signal E does not become effective until the
 inertia current I increases to the over current.
 On the other hand, if the inertia current reaches the over current state,
 the voltage generated at both ends of the resister R3 exceeds over the
 threshold voltage from the threshold voltage generating circuit 62 in
 spite of the transitional state and the normal state, and therefore the
 over current detection signal E becomes effective by the comparator 61
 thereby stopping the oscillation of the oscillation circuit 21. Then, if
 the inertia current I is restored at the normal state, the over current
 detection signal E is also restored thereby restarting the oscillation of
 the oscillation circuit 21.
 As mentioned above, the over current can be detected thereby surely
 preventing the over current, in spite of a transitional state or normal
 state.
 In the case where it is desired to change a detection reference by which
 the over current state is discriminated from the normal state in
 accordance with the properties or the like the transistor Tr1 and the coil
 L1, it may be sufficient to change the value of resistance the resistor 3
 and/or the threshold value of the threshold value voltage generating
 circuit 62. In the case, there is no any trouble such that the condition
 enabling the soft starting function is influenced thereby being deviated
 due to the setting of the feedback control circuit 22 side. Moreover,
 there is no any trouble such that the condition enabling the detection of
 the over current detection circuit 60 side is influenced thereby being
 deviated due to the change of the feedback control circuit 22 side.
 Therefore, it is easily achieved to adapt the circuit to different
 conditions.
 By the way, according to the above-mentioned embodiments there is used MOS
 transistor as the transistor Tr1, however the present invention should not
 be limited to it, it may be a voluntary type of transistor such as bipolar
 transistor, transistors connected in parallel with each other, or
 transistors with Darlington connection. Moreover, the pulse signal A for
 controlling the switching operation is directly inputted into the gate or
 the like of the transistor Tr1 in the above-mentioned embodiments, however
 the pulse signal A may be inputted into the gate through a logically
 inverting element or a level shift circuit or the like, in accordance with
 the property of transistor such as PMOS, NMOS, an enhancement type, or a
 depression type, and further in accordance with that the output of the
 oscillation circuit 21 is a positive logic or a negative logic.
 Moreover, since the output voltage Vo is decided on the basis of the
 property of the load 4, it should not be limited to 15 volts, and it may
 be 12V, 18V or more higher or lower values than them.
 Moreover, the smoothing circuit 3 should not be the type to be attached to
 the converter, it may be integrally installed within a hybrid module of
 the DC-DC converter 20.
 In brief, as mentioned above according to the DC-DC converter as a first
 solving means of the present invention, the oscillation circuit for
 oscillating a fixed frequency is controlled to stop in accordance with the
 result of comparing the detection voltage with the reference voltage, and
 further the reference voltage is adapted to be risen slowly, and therefore
 the slow starting function is performed without disturbing such
 characteristics that the DC-DC converter is provided with a simple
 structure and a compact size without utilizing PWM circuit and VCO.
 Moreover, according to the DC-DC converter as a second solving means of the
 present invention, the over current detection is directly performed on the
 basis of the switching current, and further the oscillation circuit can be
 controlled to be stopped on the basis of the result of detection result,
 and therefore the over current can be surely avoided even if unexpected
 variation of load or the like is caused not only at the starting time but
 also during operation, and further it is easily achieved to modify the
 design for each application so that there is no coalition between the
 condition for enabling the soft starting and the condition for detecting
 the over current.