JP4106594B2 - Switching power supply - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply device having an external control input circuit that performs setting input of a DC output voltage.
[0002]
[Prior art]
Various configurations of switching power supply devices are already known. For example, in the case of a flyback converter configuration, the switching power supply device has a basic configuration as shown in FIG. In the figure, Vin is a DC input voltage, Vout is a DC output voltage, C1 and C2 are capacitors, T is a transformer, N1 is a primary winding, N2 is a secondary winding, D is a rectifying diode, and Q is A switching transistor such as a field effect transistor (FET), PWM indicates a pulse width control circuit, and CIN indicates an external control input circuit.
[0003]
For example, the DC input voltage Vin is generally a DC voltage obtained by rectifying an AC voltage of a commercial AC power supply of 100 V or 200 V by a rectifier circuit, and this DC input voltage Vin is switched with the primary winding N1 of the transformer T. Applied to a series circuit with the transistor Q, the current flowing through the primary winding N1 of the transformer T is turned on and off by the switching transistor Q. The voltage induced in the secondary winding N2 of the transformer T is rectified by the diode D, smoothed by the capacitor C2, and is applied as a DC output voltage Vout to a load not shown.
[0004]
The pulse width control circuit PWM detects the DC output voltage Vout and outputs an ON control signal for the switching transistor Q having a pulse width corresponding to the difference from the set reference voltage. For example, when the DC output voltage Vout becomes higher than the reference voltage, the pulse width is narrowed to shorten the ON period of the switching transistor Q. Conversely, when the DC output voltage Vout becomes lower than the reference voltage, the pulse width is widened. The ON period of the switching transistor Q is lengthened. As a result, the DC output voltage Vout can be stabilized to a value according to the reference voltage. The reference voltage in this case is input from the external control input circuit CIN to the pulse width control circuit PWM.
[0005]
FIG. 4 is an explanatory diagram of a conventional external control input circuit, SW is a changeover switch, OP1 and OP2 are operational amplifiers, R11 to R15 and R21 to R24 are resistors, CI1 and CI2 are constant current sources, and VF is a voltage follower ( (Voltage Follower) circuit, E is a voltage source, VR is a variable resistor, PT1 and PT2 are connection ports, + V and -V are power supply voltages, and G is ground. (PWM) indicates connection to the pulse width control circuit PWM.
[0006]
The connection port PT1 is for connecting the voltage follower circuit VF, and the connection port PT2 is for connecting the variable resistor VR. That is, two connection ports PT1 and PT2 are provided so that setting input means by the voltage follower circuit VF or the variable resistor VR when the switching power supply device is used can be selected. The changeover switch SW switches to the state shown in the figure when the voltage follower circuit VF is connected to the connection port PT1, and switches to the operational amplifier OP2 side when the variable resistor VR is connected to the connection port PT2.
[0007]
In the operational amplifiers OP1 and OP2, feedback resistors R11 and R21 are connected between the inverting input terminal (−) and the output terminal, and ground resistors R12 and R22 are connected between the non-inverting input terminal (+) and the ground G. The resistors R13, R14, R23, and R24 are connected between the inverting input terminal (−) and the non-inverting input terminal (+) and one terminal and the other terminal of the connection ports PT1 and PT2, respectively.
[0008]
Resistors R15 and R16 are connected to one terminal and the other terminal of the connection port PT1 to which the voltage follower circuit VF is connected, respectively, and constant currents are respectively applied to one terminal and the other terminal of the connection port PT2 to which the variable resistor VR is connected. The sources CI1 and CI2 are connected.
[0009]
When the voltage follower circuit VF is connected to the connection port PT1, the switch circuit SW is switched to the state shown in the figure as described above, and the set input voltage according to the voltage from the voltage source E is switched from the operational amplifier OP1 to the switch SW. To the pulse width control circuit PWM. When the variable resistor VR is connected to the connection port PT2, the switch circuit SW is switched to the operational amplifier OP2 side, and the variable resistor VR is adjusted, whereby a current from the constant current sources CI1 and CI2 flows. A set input voltage according to the voltage at both ends is input from the operational amplifier OP2 to the pulse width control circuit PWM via the changeover switch SW.
[0010]
[Problems to be solved by the invention]
In general, an integrated circuit including the pulse width control circuit PWM and the external control input circuit CIN is integrated. In order to reduce the size, it is necessary to reduce the number of pins. However, in the conventional example, as shown in FIG. 4, two connection ports PT1 and PT2 are required in order to be able to cope with the setting input means of the voltage follower circuit VF and the variable resistor VR. It was a thing. Furthermore, it is necessary to provide a change-over switch SW that switches in accordance with the setting input means. Therefore, since the number of pins increases, there has been a problem that miniaturization is not easy.
[0011]
An object of the present invention is to reduce the number of pins of an external control input circuit that applies a set input voltage to a pulse width control circuit of a switching power supply device, thereby enabling miniaturization.
[0012]
[Means for Solving the Problems]
The switching power supply device of the present invention will be described with reference to FIG. 1. A pulse width control circuit that detects a DC output voltage and performs switching control so as to become a value according to a set input, and the pulse width control circuit A switching power supply apparatus including an external control input circuit for inputting and setting a DC output voltage, wherein the external control input circuit is between an operational amplifier and an inverting input terminal (−) of the operational amplifier and an output terminal. A feedback resistor R1 connected to the ground, a ground resistor R2 connected between the non-inverting input terminal (+) and the ground G, and between the inverting input terminal (−) and the non-inverting input terminal (+) and the connection port PT. Are connected to resistors R3 and R4 respectively, and a constant current source CIa or resistor is connected to one terminal of the connection port PT, and a resistor Ra or constant current source is connected to the other terminal of the connection port PT. That.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is an explanatory diagram of a main part of the first embodiment of the present invention, showing an external control input circuit of the switching power supply device shown in FIG. 3, OP being an operational amplifier, R1 to R4 and Ra being resistors, and VR being A variable resistor, VF is a voltage follower circuit, CIa is a constant current source, and PT is a connection port.
[0014]
Calculation is performed by inputting a voltage drop from the voltage follower circuit VF when the voltage follower circuit VF is connected to the connection port PT or a voltage drop due to a current from the constant current source CIa when the variable resistor VR is connected to the connection port PT. A setting input to the pulse width control circuit PWM can be made from the amplifier OP. That is, the voltage follower circuit VF or the variable resistor VR can be used as the setting input means by using one connection port PT. Further, the conventional changeover switch SW can be dispensed with.
[0015]
FIG. 2 is an explanatory view of the main part of the second embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts, CIb denotes a constant current source, and Rb denotes a resistor. 1 corresponds to the case where the connection position of the constant current source and the resistor is reversed, and the voltage from the voltage follower circuit VF when the voltage follower circuit VF is connected to the connection port PT, or By inputting a voltage drop due to the current from the constant current source CIb when the variable resistor VR is connected to the connection port PT, it is possible to make a setting input to the pulse width control circuit PWM from the operational amplifier OP.
[0016]
【The invention's effect】
As described above, the present invention detects a DC output voltage and performs a switching control so as to obtain a value according to the setting input, and a DC output voltage setting input to the pulse width control circuit. A switching power supply apparatus including an external control input circuit for performing the operation, wherein the external control input circuit includes an operational amplifier OP and a feedback resistor connected between the inverting input terminal (−) and the output terminal of the operational amplifier OP. R1, a ground resistor R2 connected between the non-inverting input terminal (+) and the ground G, and a resistor connected between the inverting input terminal (−) and the non-inverting input terminal (+) and the connection port PT, respectively. The voltage follower circuit V has a configuration in which a constant current source CIa or a resistor is connected to one terminal of the connection port PT and a resistor Ra or a constant current source is connected to the other terminal of the connection port PT. Or it is possible to perform setting input connected to the same connection port PT for any of the variable resistor VR. That is, the conventional one requiring two connection ports is only one, and there is an advantage that downsizing is possible.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a first embodiment of the present invention.
FIG. 2 is an explanatory diagram of a second embodiment of the present invention.
FIG. 3 is an explanatory diagram of a switching power supply device.
FIG. 4 is an explanatory diagram of a conventional external control input circuit.
[Explanation of symbols]
OP operational amplifier R1 feedback resistor R2 ground resistor R3, R4, Ra, Rb resistor CIa, CIb constant current source PT connection port VF voltage follower circuit VR variable resistor

Claims (1)

A pulse width control circuit that detects a DC output voltage and performs switching control so as to be a value according to a setting input; and an external control input circuit that performs a setting input of the DC output voltage to the pulse width control circuit. In the switching power supply including
The external control input circuit includes an operational amplifier, a feedback resistor connected between the inverting input terminal and the output terminal of the operational amplifier, a ground resistor connected between the non-inverting input terminal and the ground, and the inverting input. A resistor connected between the terminal and the non-inverting input terminal and the connection port, a constant current source or resistor connected to one terminal of the connection port, and a resistor or constant current source connected to the other terminal of the connection port A switching power supply device characterized by having a configuration in which are connected.

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