JP2005304269A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply that can dispense with a secondary output current detecting resistor and a constant current control circuit and that can achieve fully accurate constant-current drooping characteristics at low cost and with minimum number of components. <P>SOLUTION: An overcurrent protection reference voltage varying circuit 7 reduces an overcurrent protection reference voltage VR, that determines a primary overcurrent protection level ILIMIT following the drop of an auxiliary power source voltage VCC proportional to an output voltage VO at the drooping of the output voltage VO of the secondary side. The reduction ratio of the overcurrent protection reference voltage VR to the auxiliary power source voltage VCC is made larger step by step, each time the overcurrent protection reference voltage VR becomes lower than one or more preset voltages. Thus, the output voltage VO is configured to droop, in a range where the secondary output current IO becomes substantially a constant value. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device.

  Conventionally, a switching power supply having a constant current drooping characteristic has been required as a power supply for a charger. That is, conventionally, in order to charge a battery or the like with a constant current using the constant current drooping characteristic, for example, a detection resistor for detecting an output current and a current flowing through the detection resistor are provided on the secondary side of the switching power supply device. A constant current drooping characteristic has been realized by providing a constant current control circuit for controlling the current to a constant value.

  However, although the conventional method of providing the detection resistor and the constant current control circuit can realize a highly accurate constant current drooping characteristic, the constant current control circuit is expensive and has a large number of parts. This hindered cost reduction. In addition, since power loss due to the detection resistor and the constant current control circuit also occurs, it has been an obstacle to energy saving and high efficiency.

  On the other hand, when the output voltage on the secondary side of the switching power supply device drops to one or more predetermined output voltages, the output current at the time of overload such as a short circuit is reduced by reducing the overcurrent protection level on the primary side. A technique for improving the drooping characteristic so as not to be excessive has been proposed (see, for example, Patent Document 1).

However, the above-described conventional technique for improving the drooping characteristic is intended to protect the switching power supply device from a severe overload such as a short circuit by limiting the peak output current at the time of overload. Since the output current at the time cannot be controlled to the extent that the constant current drooping characteristic is realized, the switching power supply device with the drooping characteristic improved by this method cannot be used as a charger.
JP-A-6-149396

  In view of the above problems, the present invention reduces the overcurrent protection reference voltage that determines the overcurrent protection level on the primary side as the auxiliary power supply voltage that is proportional to the secondary output voltage decreases, Each time the voltage becomes lower than one or more preset voltages, the rate of decrease of the overcurrent protection reference voltage with respect to the auxiliary power supply voltage is increased step by step, and the secondary output voltage becomes the secondary output current. The output current detection resistor on the secondary side and the constant current control circuit can be made unnecessary by drooping within a range in which the current becomes substantially constant, and the constant current droop with sufficient accuracy can be achieved with low cost and the minimum number of parts. An object of the present invention is to provide a switching power supply device capable of realizing the characteristics.

  According to a first aspect of the present invention, there is provided a switching power supply comprising: a transformer having a first winding, a secondary winding, and an auxiliary winding; and a DC input voltage input to the first winding by switching control. A switching element for generating a secondary AC voltage in the secondary winding and an auxiliary AC voltage in the auxiliary winding, and rectifying and smoothing the secondary AC voltage to generate an output voltage and an output current A first rectifying / smoothing circuit that detects the output voltage and generates a control signal for stabilizing the output voltage at a constant value; and rectifies and smoothes the auxiliary AC voltage to assist A second rectifying / smoothing circuit for generating a power supply voltage, and controlling a switching operation of the switching element in accordance with the control signal, and before a current flowing through the switching element reaches an overcurrent protection level. And a control circuit that stops the switching operation by turning off the switching element, wherein the output current becomes a substantially constant value when the output voltage droops, and the control circuit includes the input voltage Based on any one of these auxiliary power supply voltages, a regulator that generates an internal circuit current for setting the voltage of the internal circuit power supply to a constant value, and an element that detects the current flowing through the switching element An element current detection signal generation circuit that generates a current detection signal, and compares the voltage value of the element current detection signal with an overcurrent protection reference voltage, and based on the comparison result, the current flowing through the switching element is the overcurrent protection. An overcurrent that generates a signal to turn off the switching element and stop the switching operation upon detecting that the level has been reached The overcurrent protection reference voltage is lowered so that the overcurrent protection level is lowered as the auxiliary power supply voltage is lowered, and the overcurrent protection reference voltage is more than one or more preset voltages. Each time the voltage decreases, the rate of decrease of the overcurrent protection reference voltage with respect to the auxiliary power supply voltage is increased stepwise so that the output voltage droops in a range where the output current becomes a substantially constant value. And a protection reference voltage variable circuit.

  Moreover, the switching power supply device according to claim 2 of the present invention is the switching power supply device according to claim 1, wherein the regulator is configured to perform the operation based on the input voltage while the auxiliary power supply voltage is below a certain value. An internal circuit current is generated, and the internal circuit current is generated based on the auxiliary power supply voltage while the auxiliary power supply voltage is a predetermined value or more.

  According to a third aspect of the present invention, there is provided the switching power supply device according to the first or second aspect, wherein the overcurrent protection reference voltage variable circuit supplies the overcurrent protection reference voltage. The minimum value is variable so as to be about 10% of the maximum value.

  A switching power supply device according to claim 4 of the present invention is the switching power supply device according to any one of claims 1 to 3, wherein the control circuit has the overcurrent protection reference voltage lower than a predetermined value. An oscillation frequency lowering circuit that generates a signal for lowering the oscillation frequency of the switching element when lowering, the oscillation frequency lowering circuit of the switching element when the overcurrent protection reference voltage becomes lower than a predetermined value The oscillation frequency is lowered.

  Further, the switching power supply device according to claim 5 of the present invention is the switching power supply device according to claim 4, wherein the oscillation frequency reducing circuit reduces the oscillation frequency of the switching element to about 20% of the normal time. It is characterized by that.

  A switching power supply device according to claim 6 of the present invention is the switching power supply device according to any one of claims 1 to 5, wherein the switching element and the control circuit are formed on the same semiconductor substrate, Two connection terminals between the input voltage and the switching element; a connection terminal for the control circuit and the auxiliary power supply voltage; an input terminal for inputting a signal for transmitting the control signal to the control circuit; A semiconductor device having an input terminal for inputting a signal corresponding to the auxiliary power supply voltage to the current protection reference voltage variable circuit.

  According to the present invention, a secondary output current detection resistor and a constant current control circuit can be dispensed with, and a sufficiently accurate constant current drooping characteristic can be realized with a low cost and a minimum number of parts. Accordingly, a switching power supply for a charger with sufficient accuracy can be configured with a small number of parts, and the switching power supply for a charger can be reduced in cost and size.

  In addition, since the minimum value of the overcurrent protection reference voltage is variable so as to be about 10% of the maximum value, the output current can be sufficiently reduced during an overload such as when the load is short-circuited and flows to the switching element. A highly safe protection function can be realized by sufficiently limiting the current.

  Moreover, since the oscillation frequency can be reduced and the output current can be sufficiently reduced during an overload such as a short circuit, the current flowing through the switching element can be sufficiently limited, and a highly safe protection function can be realized.

  In this way, the overcurrent protection function always works in the region of a substantially constant current when the output voltage is drooping, and in the case of an overload such as a short circuit, the output current is made sufficiently small by limiting the overcurrent protection level and the oscillation frequency to a small value. Therefore, a highly safe protection function can be realized.

  In addition, the switching element and the control circuit can be provided in the same semiconductor and can be easily unified. Thus, when the main circuit components are provided in the single semiconductor, the circuit is configured. The number of parts can be reduced, and the power supply device can be easily reduced in size and weight and further reduced in cost.

Hereinafter, a switching power supply device according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration example of a switching power supply device according to the present embodiment.

  In FIG. 1, a semiconductor device 20 is a semiconductor device for controlling a switching power supply device, and includes a switching element 1 and its control circuit. In addition, the semiconductor device 20 includes, as external input terminals, an input terminal (DRAIN terminal) of the switching element 1, an auxiliary power supply voltage input terminal (VCC terminal), a feedback signal input terminal (FB terminal), an overcurrent protection level variable terminal (CL Terminal) and 5 terminals of the GND terminal (GND terminal) of the control circuit which is also the output terminal of the switching element 1.

  The regulator 2 receives the input voltage VIN applied to the DRAIN terminal via the primary winding 40A of the transformer 40 and the auxiliary power supply voltage VCC applied to the VCC terminal, and based on one of them. Then, an internal circuit current for maintaining the voltage VDD of the internal circuit power supply of the semiconductor device 20 at a constant value is generated and supplied to the internal circuit power supply.

  Specifically, the regulator 2 supplies an internal circuit current based on the input voltage VIN to the internal circuit power supply while the auxiliary power supply voltage VCC is lower than a set value (a constant value) at the time of startup or the like. Is equal to or greater than the set value, the internal circuit current based on the auxiliary power supply voltage VCC is supplied to the internal circuit power supply to keep the voltage VDD of the internal circuit power supply at a constant value.

  In addition, the regulator 2 supplies a current (internal circuit current) based on the input voltage VIN applied to the DRAIN terminal to the internal circuit power supply at the time of start-up to raise the voltage VDD of the internal circuit power supply and The capacitor 32 is charged by supplying current to the capacitor 32 via the capacitor. After the voltage VDD of the internal circuit power supply reaches a certain value, the supply of the internal circuit current from the VCC terminal to the internal circuit power supply is stopped while the auxiliary power supply voltage VCC falls below the set value.

  When the voltage VDD of the internal circuit power supply becomes a constant value by the regulator 2, the switching signal control circuit 3 outputs a switching control signal for controlling the switching operation (repeat of on / off operation) of the switching element 1 to the gate drive circuit 4. The switching element 1 performs a switching operation according to an output signal from the gate drive circuit 4 and performs switching control of the input voltage VIN input to the primary winding 40A of the transformer 40.

  The overcurrent protection reference voltage variable circuit 7 detects overcurrent a voltage signal that becomes a voltage (overcurrent protection reference voltage VR) corresponding to the current value of the current (CL terminal current ICL) flowing from the CL terminal through the P-type MOSFET 10. The overcurrent protection level ILIMIT of the switching element 1 is determined by this overcurrent protection reference voltage VR. That is, the overcurrent protection reference voltage variable circuit 7 increases or decreases the overcurrent protection reference voltage VR in accordance with the increase or decrease of the current value of the CL terminal current ICL, and increases or decreases the overcurrent protection level ILIMIT of the switching element 1.

  As will be described later, since the CL terminal current ICL has a current value corresponding to the auxiliary power supply voltage VCC, the overcurrent protection reference voltage VR and the overcurrent protection level ILIMIT change according to the auxiliary power supply voltage VCC.

  Further, the overcurrent protection reference voltage variable circuit 7 outputs a signal to the oscillation frequency lowering circuit 8 to lower the oscillation frequency of the switching element 1 when the current value of the CL terminal current ICL becomes a certain value or less.

  The oscillation frequency lowering circuit 8 generates a signal for lowering the oscillation frequency of the switching element 1 according to the signal from the overcurrent protection reference voltage variable circuit 7. Therefore, when the current value of the CL terminal current ICL becomes a certain value or less, that is, when the overcurrent protection reference voltage VR becomes smaller than a predetermined value, the oscillation frequency of the switching element 1 is lowered.

  The P-type MOSFET 10 is an element for passing a current from the CL terminal to the overcurrent protection reference voltage variable circuit 7 and fixing the voltage at the CL terminal to a constant value, and its drain is connected to the overcurrent protection reference voltage variable circuit 7. The gate is connected to the reference voltage source, and the source is connected to the CL terminal.

  The switching element current detection circuit (element current detection signal generation circuit) 6 detects the drain current IDS flowing through the switching element 1 and generates a voltage signal (element current detection signal) that becomes a voltage corresponding to the current value to generate an overcurrent. Output to the detection circuit 5.

  The overcurrent detection circuit 5 compares the voltage of the voltage signal from the switching element current detection circuit 6 with the voltage of the voltage signal from the overcurrent protection reference voltage variable circuit 7 (overcurrent protection voltage VR), and based on the comparison result. Thus, it is determined whether or not the drain current IDS has reached the overcurrent protection level ILIMIT. When it is detected that the drain current IDS has reached the overcurrent protection level ILIIT, a signal for turning off the switching element 1 is output to the switching signal control circuit 3.

  The feedback signal control circuit 9 performs switching signal control of a feedback signal for determining the duty cycle of the switching element 1 or the current value of the drain current IDS flowing through the switching element 1 according to the current value of the current input to the FB terminal. Output to circuit 3.

  The switching signal control circuit 3 generates a switching control signal based on the feedback signal from the feedback signal control circuit 9 and outputs it to the gate drive circuit 4. When the switching signal control circuit 3 receives the signal from the overcurrent detection circuit 5, the switching signal control circuit 3 generates a switching control signal for turning off the switching element 1 and stopping the switching operation, and outputs the switching control signal to the gate drive circuit 4.

  The transformer (transformer) 40 includes a primary winding 40A, a secondary winding 40B, and an auxiliary winding 40C. When the DC input voltage VIN input to the primary winding 40A is subjected to switching control by the switching element 1, an AC voltage is generated in the secondary winding 40B and the auxiliary winding 40C.

  A rectifying / smoothing circuit (second rectifying / smoothing circuit) including a diode 31 and a capacitor 32 is connected to the auxiliary winding 40 </ b> C, and this rectifying / smoothing circuit serves as an auxiliary power supply unit of the semiconductor device 20. Be utilized. That is, the rectifying / smoothing circuit rectifies and smoothes the AC voltage (auxiliary AC voltage) generated in the auxiliary winding 40C to generate the auxiliary power supply voltage VCC and inputs it to the VCC terminal.

A resistor 33 is connected between the VCC terminal and the CL terminal, and a current corresponding to the voltage value of the auxiliary power supply voltage VCC flows into the CL terminal.
A rectifying / smoothing circuit (first rectifying / smoothing circuit) composed of a diode 35 and a capacitor 36 is connected to the secondary winding 40B, and a load 37 is connected to the rectifying / smoothing circuit. The rectifying / smoothing circuit rectifies and smoothes the AC voltage (secondary AC voltage) generated in the secondary winding 40B, generates the output voltage VO and the output current IO, and supplies the output voltage VO and the output current IO to the load 37.

  The constant voltage control circuit 38 is constituted by, for example, a Zener diode or the like, and detects a secondary output voltage VO and generates a control signal for stabilizing the output voltage VO at a constant value. Specifically, when the output voltage VO reaches a preset value, the constant voltage control circuit 38 increases the current flowing through the photodiode 34B, thereby stabilizing the output voltage VO at a constant value.

  The control signal transmission circuit 34 includes a phototransistor 34A and a photodiode 34B, and transmits the control signal generated by the constant voltage control circuit 38 from the secondary side to the primary side. The collector of the phototransistor 34A is connected to the VCC terminal, and the emitter of the phototransistor 34A is connected to the FB terminal. That is, when the output voltage VO becomes a preset value, the current flowing through the photodiode 34B increases, and the current flowing from the phototransistor 34A into the FB terminal increases.

  Therefore, a current signal for transmitting a control signal generated by the constant voltage control circuit 38 is input to the control circuit (feedback signal control circuit 9) at the FB terminal. The feedback signal control circuit 9 receives the current of the current signal. A feedback signal for determining the duty cycle of the switching element 1 or the current value of the drain current IDS flowing through the switching element 1 according to the value is generated.

  As described above, the control circuit of the switching element 1 controls the switching operation of the switching element 1 according to the control signal generated by the constant voltage control circuit 38, and the drain current IDS flowing through the switching element 1 is at the overcurrent protection level. When ILIMIT is reached, the switching element 1 is turned off to stop the switching operation.

  As described above, the switching element 1 and its control circuit include two connection terminals (DRAIN terminal and GND terminal) between the input voltage VIN and the switching element 1, and a connection terminal (VCC) of the control circuit and the auxiliary power supply voltage VCC. Terminal), an input terminal (FB terminal) for inputting a signal for transmitting the control signal generated by the constant voltage control circuit 38 to the control circuit, and the overcurrent protection reference voltage variable circuit 7 according to the auxiliary power supply voltage VCC Formed on the same semiconductor substrate (semiconductor device 20) having an input terminal (CL terminal) for inputting the received signal. Thus, by forming the switching element 1 and its control circuit on the same semiconductor substrate, the number of parts for constituting the circuit can be reduced, and the size and weight can be easily reduced.

The operation of the switching power supply device configured as described above will be described below with reference to the drawings.
A DC input voltage VIN obtained by rectifying and smoothing a commercial AC power supply, for example, is input to the input terminal of the switching power supply apparatus. The input voltage VIN is applied to the DRAIN terminal of the semiconductor device 20 via the primary winding 40A of the transformer 40.

  When starting up, the regulator 2 supplies a current (internal circuit current) based on the input voltage VIN applied to the DRAIN terminal to the internal circuit power supply to increase the voltage VDD of the internal circuit power supply, and via the VCC terminal The capacitor 32 is charged by supplying current also to the capacitor 32. After the voltage VDD of the internal circuit power supply reaches a certain value, the supply of the internal circuit current from the VCC terminal to the internal circuit power supply is stopped while the auxiliary power supply voltage VCC falls below the set value.

  When the voltage VDD of the internal circuit power supply becomes a constant value by the regulator 2, the switching operation of the switching element 1 is started. When the switching operation is started, energy is supplied to each winding of the transformer 40, and a current flows through the secondary winding 40B and the auxiliary winding 40C.

  Even when the output voltage VO decreases and the auxiliary power supply voltage VCC falls below the set value, the current based on the input voltage VIN is supplied to the internal circuit power supply by the regulator 2, so that the operation can be continued stably. .

  The AC power generated in the secondary winding 40B is rectified and smoothed by the diode 35 and the capacitor 36 to become DC power, and is supplied to the load 37. By repeating the switching operation, the output voltage VO gradually rises, and when the voltage set by the constant voltage control circuit 38 is reached, the current flowing from the constant voltage control circuit 38 to the photodiode 34B increases.

  As a result, the current flowing through the phototransistor 34A increases and the current flowing into the FB terminal also increases. When the current flowing into the FB terminal increases, a feedback signal for reducing the duty cycle of the switching element 1 or the current value of the drain current IDS flowing through the switching element 1 is output from the feedback signal control circuit 9 to the switching signal control circuit 3, The drain current IDS flowing through the switching element 1 is reduced. As a result of negative feedback, the output voltage VO becomes a constant value.

  The AC power generated in the auxiliary winding 40C is rectified and smoothed by the diode 31 and the capacitor 32, and is supplied to the VCC terminal as the auxiliary power supply voltage VCC of the semiconductor device 20. When the oscillation operation (switching operation) of the switching element 1 is started and the auxiliary power supply voltage VCC rises and reaches the set voltage, a current based on the auxiliary power supply voltage VCC is supplied to the internal circuit power supply. The polarity of the auxiliary winding 40C is the same as that of the secondary winding 40B, and the auxiliary power supply voltage VCC is a voltage proportional to the output voltage VO. When the auxiliary power supply voltage VCC falls below the set voltage, a current based on the input voltage VIN applied to the DRAIN terminal is supplied to the internal circuit power supply.

  When the output current IO flowing through the load 37 is increased after the output voltage VO is stabilized, the output voltage VO decreases, the current flowing through the photodiode 34B decreases, and the current flowing from the phototransistor 34A into the FB terminal also increases. Since it decreases, the drain current IDS flowing through the switching element 1 increases and the output voltage VO increases. By applying such negative feedback, the output voltage VO is stabilized.

  Further, when the output current IO flowing through the load 37 is increased and the current value of the drain current IDS flowing through the switching element 1 reaches the overcurrent protection level ILIMIT, the power supplied to the secondary side via the transformer 40 is limited. Therefore, when the output current IO is further increased, the output voltage VO decreases while maintaining the following relationship.

VO × IO = 1/2 × L × ILIMIT ² × fosc × η (1)
Here, “L” is the inductance of the primary winding 40A of the transformer 40, “ILIMIT” is the overcurrent protection level of the switching element 1, “fosc” is the oscillation frequency of the switching element 1, and “η” is the efficiency.

  FIG. 6 shows the characteristics of the output voltage VO versus the output current IO determined by the above equation (1). The solid line in FIG. 6 shows the characteristic of the output voltage VO versus the output current IO when the overcurrent protection level ILIMIT is fixed to a constant value. Also, the dotted line in FIG. 6 shows the characteristics of the output voltage VO versus the output current IO when the overcurrent protection reference voltage VR, that is, the overcurrent protection level ILIMIT is lowered as the output voltage VO is lowered.

  As shown in FIG. 6, when the overcurrent protection level ILIMIT is lowered in accordance with the reduction in the output voltage VO, the width of the drooping region of the output current IO is increased as compared with the case where the overcurrent protection level ILIMIT is fixed to a constant value. Can be small.

  On the other hand, as shown in FIG. 3, the overcurrent protection reference voltage variable circuit 7 has an overcurrent protection reference voltage VR of the switching element 1, that is, an overcurrent protection level, according to the current value of the CL terminal current ICL flowing into the CL terminal. Determine the size of ILIMIT.

  That is, when the CL terminal current ICL is larger than “ICL1”, the overcurrent protection level ILIMIT is fixed at the maximum value, that is, “ILIMMAX”. When the CL terminal current ICL becomes smaller than “ICL1”, the overcurrent protection level ILIMIT decreases linearly. Further, when the CL terminal current ICL becomes smaller than “ICL2”, the slope of the drop line increases, and the overcurrent protection level ILIMIT decreases. Further, when the CL terminal current ICL becomes smaller than “ICL3”, the slope of the decreasing straight line further increases, the decrease of the overcurrent protection level ILIMIT increases, and the overcurrent protection level ILIMIT decreases to the minimum value, that is, “ILIMTIMIN”. To do.

  As described above, in the present embodiment, the overcurrent protection reference voltage VR is reduced so that the overcurrent protection level ILIMIT decreases as the CL terminal current ICL, that is, the auxiliary power supply voltage VCC decreases, and the overcurrent protection reference voltage VR is reduced. Each time becomes smaller than one or more preset voltages (here, three), the rate of decrease of the overcurrent protection reference voltage VR with respect to the CL terminal current ICL (auxiliary power supply voltage VCC) is increased stepwise.

  Therefore, from the characteristic shown in FIG. 3 and the characteristic shown by the dotted line in FIG. 6, the characteristic of the output voltage VO versus the output current IO in the present embodiment is as shown in FIG. In this range, the output voltage VO droops.

  By setting the minimum value ILIMITMIN of the overcurrent protection level ILIMIT to about 10% of the maximum value ILIMITMAX, the output current IO at the time of overload such as when a load is short-circuited can be sufficiently reduced, and thus flows to the switching element. The current can be sufficiently limited, and a highly safe protection function can be realized.

  Further, as shown in FIG. 4, when the CL terminal current ICL becomes smaller than “ICL4”, the overcurrent protection reference voltage variable circuit 7 outputs a signal for reducing the oscillation frequency fosc of the switching element 1 to the oscillation frequency reduction circuit 8. Output to. As a result, the oscillation frequency reduction circuit 8 generates a signal for reducing the oscillation frequency fosc of the switching element 1 and inputs it to the switching signal control circuit 3, and the oscillation frequency fosc of the switching element 1 is reduced by the switching signal control circuit 3. Therefore, since the output current IO at the time of overload such as when the load is short-circuited can be sufficiently reduced, the current flowing through the switching element can be sufficiently limited, and a highly safe protection function can be realized.

  Further, by setting the oscillation frequency fosc when the CL terminal current ICL is smaller than “ICL4” to about 20% of the normal operation, as shown in FIG. 5, the overcurrent protection level and the oscillation frequency fosc are limited. As a result, the drooping characteristic of the output voltage VO becomes a “F” character characteristic, so that the output current IO can be sufficiently limited, and a highly safe protection function can be realized.

  In the present embodiment, due to the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency lowering circuit 8, the characteristics of the output voltage VO to the output current IO of the switching power supply device are as shown in FIG. That is, after the output voltage VO is stabilized, the output current IO flowing through the load 37 is increased, and when the current value of the drain current IDS flowing through the switching element 1 reaches a region limited by the overcurrent protection level ILIMITMAX, As the voltage VO decreases, the auxiliary power supply voltage VCC decreases, and the current flowing into the CL terminal through the resistor 33 (CL terminal current ICL) also decreases (ICL <ICL1). Therefore, the overcurrent protection level ILIMIT decreases and the output Suppresses the growth of current IO.

  Then, from the relationship among the output voltage VO, the output current IO, and the overcurrent protection level ILIMIT obtained by the equation (1), the decrease in the overcurrent protection level ILIMIT is increased stepwise, as shown in FIG. VO can be drooped almost linearly. That is, the output voltage VO droops within a range where the output current IO becomes a substantially constant value.

  Therefore, since the output current IO becomes a substantially constant value when the output voltage VO droops, a sufficiently accurate constant current drooping characteristic can be realized, and can be used as a switching power supply device for a charger.

  When the output voltage VO further decreases, the overcurrent protection level ILIMIT becomes about 10% of the maximum value ILIMITMAX, and the oscillation frequency fosc becomes about 20% during normal operation. The output current at the time can be kept small, and so-called “f” character protection characteristics can be realized.

  As described above, when the CL terminal current ICL becomes smaller than “ICL4”, that is, when the overcurrent protection reference voltage VR becomes lower than a predetermined voltage value, the output voltage VO droops in a so-called “F” character characteristic. .

  As described above, in the drooping region of the output voltage VO (region of substantially constant current when the output voltage droops), the overcurrent protection function always works according to the overcurrent protection level ILIMIT, and the peak value of the output current is limited to a certain value or less. Is done. Further, in the case of an overload such as a short circuit, the overcurrent protection level ILIMIT is about 10% of the maximum value ILIMITMAX, and the oscillation frequency fosc is limited to a small value, so that the output current can be reduced. Can be improved.

  Next, details of the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency reduction circuit 8 in the present embodiment will be described. FIG. 2 is a diagram for explaining an example of the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency lowering circuit 8. However, the same members as those described with reference to FIG.

  The overcurrent protection reference voltage variable circuit 7 includes N-type MOSFETs 7A, 7B, 7D, 7E, 7G, 7H, 71B, 71D, 71E, 72B, 72D, 72E, P-type MOSFETs 7I, 7J, and constant current sources 7C, 7F. 71C, 72C, and a resistor 7K, and the oscillation frequency lowering circuit 8 includes an N-type MOSFET 8B, a constant current source 8C, and an inverter 8A. As shown in FIG. The element is connected.

  In FIG. 2, the constant current values I1 (ICL1), I2, I3 (ICL2), I4 (ICL3), and I5 (ICL4) of the constant current sources 7C, 7F, 71C, 72C, and 8C satisfy the following relationship: It is configured.

I2>I1>I3>I4> I5 (2)
I2> I1 + I3 + I4 (3)
In addition, a voltage generated by the P-type MOSFET 7J and the resistor 7K is output to the overcurrent detection circuit 5 as the overcurrent protection reference voltage VR.

  A signal generated by the constant current source 8C, the N-type MOSFET 8B, and the inverter 8A is output to the switching signal control circuit 3 as an oscillation frequency lowering signal. Here, it is assumed that when the signal level of the oscillation frequency lowering signal becomes “L” level, the oscillation frequency of the switching element 1 decreases.

The operations of the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency reduction circuit 8 configured as described above will be described below.
As shown in FIG. 2, the N-type MOSFETs 7A, 7B, 71B, 72B, and 8B constitute a current mirror circuit with the N-type MOSFET 7A as a reference, and the same current as the CL terminal current ICL is the N-type MOSFETs 7A, 7B, It flows to 71B, 72B, and 8B, respectively.

  When the CL terminal current ICL is ICL> I1 (ICL1), all of the current I1 from the constant current source 7C flows to the N-type MOSFET 7B from the relationship of the equation (2), so that current flows to the N-type MOSFETs 7D and 7E. Absent. Similarly, since all of the current I3 from the constant current source 71C flows to the N-type MOSFET 71B from the relationship of the expression (2), no current flows to the N-type MOSFETs 71D and 71E, and the constant current source 72C flows to the N-type MOSFET 72B. Since all of the current I4 from the current flows through the N-type MOSFETs 72D and 72E, and no current flows through the constant current source 8C through the N-type MOSFET 8B, no current flows through the inverter 8A. The signal level of the 8A output signal is “H” level.

  Accordingly, since all of the current I2 from the constant current source 7F flows to the N-type MOSFETs 7G and 7H, the current IR for determining the overcurrent protection reference voltage VR becomes IR = I2, which is the maximum value of the overcurrent protection reference voltage VR. At this time, the overcurrent protection level ILIMIT is “ILIMITMAX”.

  When the CL terminal current ICL decreases and I1 (ICL1)> ICL> I3 (ICL2), the current “I1-ICL” flows through the N-type MOSFETs 7D and 7E. Note that no current flows through the N-type MOSFETs 71D, 71E, 72D, 72E and the inverter 8A from the relationship of the expression (2). Therefore, the current flowing through the N-type MOSFETs 7G and 7H is “I2- (I1-ICL)”, and the current IR decreases as the CL terminal current ICL decreases. As a result, the overcurrent protection level ILIMIT is smaller than “ILIMITMAX”. Note that the CL terminal current ICL when the overcurrent protection level ILIMIT starts to decrease is determined by the current value I1 of the constant current source 7C.

  When the CL terminal current ICL further decreases and I3 (ICL2)> ICL> I4 (ICL3), the current “I1-ICL” flows in the N-type MOSFETs 7D and 7E, and “I3−3” flows in the N-type MOSFETs 71D and 71E. ICL "current flows. Note that no current flows through the N-type MOSFETs 72D and 72E and the inverter 8A from the relationship of the expression (2). Therefore, the current flowing through the N-type MOSFETs 7G and 7H is “I2- (I1-ICL) − (I3-ICL)”, and the rate of decrease of the current IR with respect to the decrease of the CL terminal current ICL is increased. The rate of decline also increases. The first change point of the decrease rate of the overcurrent protection level ILIMIT is determined by the current value I3 of the constant current source 71C.

  When the CL terminal current ICL further decreases and I4 (ICL3)> ICL, the current “I1-ICL” flows through the N-type MOSFETs 7D and 7E, and the current “I3-ICL” flows through the N-type MOSFETs 71D and 71E. The current “I4-ICL” flows through the N-type MOSFETs 72D and 72E. Therefore, the current flowing through the N-type MOSFETs 7G and 7H becomes “I2- (I1-ICL) − (I3-ICL) − (I4-ICL)”, and the rate of decrease of the current IR with respect to the decrease of the CL terminal current ICL is further increased. Further, the decrease rate of the overcurrent protection level ILIMIT is further increased. The second change point of the decrease rate of the overcurrent protection level ILIMIT is determined by the current value I4 of the constant current source 72C.

  Further, when the CL terminal current ICL becomes smaller and the CL terminal current ICL does not flow completely, the current flowing through the N-type MOSFETs 7G and 7H becomes “I2-I1-I3-I4”, and the overcurrent protection level ILIMIT at this time is The minimum value, that is, “ILIMTIMIN”.

  If the overcurrent protection reference voltage variable circuit 7 is configured as shown in FIG. 2, the rate of change of the overcurrent protection level ILIMIT can be changed stepwise with respect to the change of the CL terminal current ICL. The characteristics shown can be realized.

  On the other hand, when the CL terminal current ICL becomes ICL <I5 (ICL4), the signal level of the input signal to the inverter 8A is inverted from the “L” level to the “H” level. 8 is inverted from “H” level to “L” level, and the oscillation frequency of the switching element 1 is lowered.

  If the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency lowering circuit 8 are configured as shown in FIG. 2, the oscillation frequency fosc can be decreased when the current value of the CL terminal current ICL becomes smaller than a certain value. Thus, the characteristics as shown in FIG. 4 can be realized.

  Therefore, if the overcurrent protection reference voltage variable circuit 7 and the oscillation frequency lowering circuit 8 are configured as shown in FIG. 2, the characteristics of the output voltage VO versus the output current IO shown in FIG. Characteristics can be realized, and a switching power supply device for a charger can be realized.

  In the present embodiment, the overcurrent protection level ILIMIT is changed in three stages. However, there is no need for the three stages, and the constant current drooping characteristics can be further increased by increasing the four stages and the five stages. Accuracy can be improved.

  As described above, according to the present embodiment, the overcurrent protection level ILIMIT changes with a decrease in the auxiliary power supply voltage VCC, that is, the output voltage VO, and the output voltage VO is maintained while the output current IO remains substantially constant. Can be drooped.

  In addition, secondary output current detection resistors and constant current control circuits can be eliminated, and constant current drooping characteristics with sufficient accuracy can be realized with low cost and minimum number of parts. The switching power supply for the charger with high accuracy can be configured, and the switching power supply for the charger can be reduced in cost and size.

  According to the switching power supply device according to the present invention, a constant current drooping characteristic with sufficient accuracy can be realized with a low cost and a minimum number of parts, so that a charger for portable devices such as a mobile phone and a digital still camera, etc. Useful for.

The block diagram which shows the example of 1 structure of the switching power supply device in embodiment of this invention FIG. 3 is a block diagram showing a configuration example of an overcurrent protection reference voltage variable circuit and an oscillation frequency reduction circuit in the switching power supply device according to the embodiment. The characteristic view which shows an example of the characteristic of the overcurrent protection reference voltage variable circuit in the switching power supply of the embodiment The characteristic view which shows an example of the characteristic of the oscillation frequency reduction circuit in the switching power supply device of the embodiment The characteristic view which shows an example of the characteristic of the output voltage versus output current in the switching power supply device of the same embodiment Characteristic diagram for explaining general characteristics of output voltage vs. output current of switching power supply

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching element 2 Regulator 3 Switching signal control circuit 4 Gate drive circuit 5 Overcurrent detection circuit 6 Switching element current detection circuit 7 Overcurrent protection reference voltage variable circuit 7A, 7B, 7D, 7E, 7G, 7H, 71B, 71D, 71E 72B,
72D, 72E N-type MOSFET
7C, 7F, 71C, 72C Constant current source 7I, 7JP P-type MOSFET
7K Resistance 8 Oscillation frequency reduction circuit 8A Inverter 8B N-type MOSFET
8C constant current source 9 feedback signal control circuit 10 P-type MOSFET
20 Semiconductor device 31, 35 Diode 32, 36 Capacitor 33 Resistance 34 Control signal transmission circuit 34A Phototransistor 34B Photodiode 37 Load 38 Constant voltage control circuit 40 Transformer 40A Primary winding 40B Secondary winding 40C Auxiliary winding

Claims (6)

A transformer having a first winding, a secondary winding, and an auxiliary winding;
A switching element for controlling the DC input voltage input to the first winding to generate a secondary AC voltage in the secondary winding and generating an auxiliary AC voltage in the auxiliary winding;
A first rectifying / smoothing circuit that rectifies and smoothes the secondary AC voltage to generate an output voltage and an output current;
A constant current control circuit for detecting the output voltage and generating a control signal for stabilizing the output voltage at a constant value;
A second rectifying / smoothing circuit for rectifying and smoothing the auxiliary AC voltage to generate an auxiliary power supply voltage;
A control circuit for controlling the switching operation of the switching element according to the control signal, and for turning off the switching element and stopping the switching operation when a current flowing through the switching element reaches an overcurrent protection level;
A switching power supply device in which the output current becomes a substantially constant value when the output voltage drops.
The control circuit includes:
A regulator for generating an internal circuit current for setting the voltage of the internal circuit power supply to a constant value based on any one of the input voltage and the auxiliary power supply voltage; and
An element current detection signal generation circuit that detects an electric current flowing through the switching element and generates an element current detection signal;
The voltage value of the element current detection signal is compared with an overcurrent protection reference voltage, and when the current flowing through the switching element reaches the overcurrent protection level based on the comparison result, the switching element is turned off. And an overcurrent detection circuit that generates a signal for stopping the switching operation,
The overcurrent protection reference voltage is lowered so that the overcurrent protection level is lowered as the auxiliary power supply voltage is lowered, and each time the overcurrent protection reference voltage becomes lower than one or more preset voltages. The overcurrent protection reference voltage is variable so that the output voltage droops in a range where the output current becomes a substantially constant value by gradually increasing the decrease rate of the overcurrent protection reference voltage with respect to the auxiliary power supply voltage. Circuit,
A switching power supply device comprising:   The regulator generates the internal circuit current based on the input voltage while the auxiliary power supply voltage is below a certain value, and based on the auxiliary power supply voltage while the auxiliary power supply voltage is above a certain value. 2. The switching power supply device according to claim 1, wherein an internal circuit current is generated.   3. The overcurrent protection reference voltage variable circuit varies the overcurrent protection reference voltage so that a minimum value thereof is about 10% of a maximum value. Switching power supply.   4. The switching power supply device according to claim 1, wherein the control circuit is configured to reduce the oscillation frequency of the switching element when the overcurrent protection reference voltage is lower than a predetermined value. 5. An oscillation frequency reduction circuit for generating the switching power supply device, wherein the oscillation frequency reduction circuit reduces the oscillation frequency of the switching element when the overcurrent protection reference voltage becomes lower than a predetermined value.   5. The switching power supply device according to claim 4, wherein the oscillation frequency lowering circuit reduces the oscillation frequency of the switching element to about 20% of a normal time.   The switching element and the control circuit are formed on the same semiconductor substrate, the connection terminal between the input voltage and the switching element, the connection terminal of the control circuit and the auxiliary power supply voltage, and the control circuit A semiconductor device having an input terminal for inputting a signal for transmitting a control signal and an input terminal for inputting a signal corresponding to the auxiliary power supply voltage to the overcurrent protection reference voltage variable circuit. The switching power supply device according to any one of claims 1 to 5.

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