JP2797545B2 - Switching power supply circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a switching power supply circuit.

[Conventional technology]

As shown in FIG. 3, the conventional switching power supply circuit has a capacitor 11 connected to the output of the error amplifier 8, and when the input DC power supply 1 is turned on, the output of the comparator 6 gradually increases the conduction time of the semiconductor switch 2. Soft start (slow start) circuit. In the figure, 3 is a choke coil, 4 and 5 are diodes and capacitors for rectification, 7 is a triangular wave oscillator, 9 is a reference power supply,
14, 15 are output power terminals.

[Problems to be Solved by the Invention] In the conventional power supply circuit described above, since a capacitor is connected to the output of the error amplifier, the characteristics of the control circuit of the switching power supply fluctuate depending on the capacitance of the capacitor. Therefore, it was necessary to determine the capacity of the capacitor in consideration of both the time constant of the soft start and the control characteristics, but since an optimum value does not always exist, it is necessary to sacrifice either of the characteristics. I can't escape.

[Means for solving the problem]

The switching power supply circuit of the present invention is configured such that a switching element and a choke coil are connected in series to a DC power supply, and a series circuit of a first diode and a first capacitor for rectification is connected in parallel to the choke coil. Connecting the first and second terminals of the capacitor as first and second output power terminals, connecting the first output power terminal to a first input of an error amplifier via a reference power source, Terminal is connected to the second input of the error amplifier, and the first
A second input of a comparator to which an input is connected, is connected to an output of the error amplifier, an output of the comparator is connected to a control input of the switching element, and a series circuit of a resistor and a second capacitor is connected to the choke coil. Connected to a common connection point of the first diode and the first diode and the first output power terminal;
A second diode is connected to a common connection point between the resistor and the second capacitor and an output of the error amplifier.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

Referring to FIG. 1 and FIG. 2 showing an embodiment of the present invention, in the chopper type switching power supply circuit shown here, when the semiconductor switching 2 connected to the input DC power supply 1 becomes conductive, a current flows through the choke coil 3. When energy is stored and the switch 2 is turned off, the current energy stored in the choke coil 3 flows into the rectifying capacitor 5 via the rectifying diode 4 and becomes a DC voltage. Both terminals of the capacitor 5 become output power terminals 14 and 15 (15 is a ground terminal) so that an output current can be taken out. To keep the terminal voltages of the output power terminals 14 and 15, that is, the output voltage constant, the difference between the voltage of the terminal 14 and the voltage of the reference power supply 9 is detected by an error amplifier 8. The comparator 6 operates so that the switch 2 is turned on when the output voltage of the comparator 6 is exceeded. FIG. 2 shows a time relationship in which the switch 2 is turned on in accordance with the output of the amplifier 8.

The operation of the resistor 10, the capacitor 11, and the diode 12 is as follows when the input DC power supply 1 is applied. First,
A current flows into the capacitor 11 via the resistor 10, but when the voltage between the terminals of the capacitor 11 is low, the output of the error amplifier 8 is connected to the output power supply terminal 15 via the diode 12, so that the conduction time of the switch 2 ton is very short. The voltage (V ₁₁ ) between the terminals of the capacitor 11 rises exponentially due to the resistance 10, and the conduction period ton of the switch 2 increases. As the voltage (V ₁₁ ) increases, the voltage (V ₅ ) between the terminals of the capacitor 5 increases. When the voltage (V ₅ ) corresponds to the voltage of the reference power supply 9, the output of the comparator 6 turns on the switch 2. Decrease time. In this manner, the voltage (V ₅ ), that is, the output voltage Vout starts to be controlled to be constant, but thereafter, the capacitor 11 is charged by the input DC power supply 1 via the resistor 10, so that the diode 12 is turned off. . At this point, the series circuit of the resistor 10 and the capacitor 11 is electrically separated from the output of the amplifier 8, so that the capacitance of the capacitor 11 is independent of the control circuit constant.

〔The invention's effect〕

As described above, according to the present invention, a series circuit including a resistor and a capacitor is provided between an input DC power supply and an output power supply terminal, and a common connection point between the resistor and the capacitor and the output of the error amplifier are connected to a diode. By connecting, the soft start circuit section can be separated from the control circuit section after the input DC power supply starts to rise,
The soft start circuit constant becomes independent of the constant of the control circuit, so that the circuit constant can be easily determined and the constant selection range becomes very wide. Therefore, it is possible to easily stabilize the power supply circuit and to easily determine the characteristic that follows a sudden change in load.

[Brief description of the drawings]

1 and 2 are views showing an embodiment of the present invention, and FIG. 3 is a block diagram showing a conventional example. DESCRIPTION OF SYMBOLS 1 ... Input DC power supply, 2 ... Semiconductor switch, 3 ... Choke coil, 4 ... Rectifier diode, 5 ... Rectifier capacitor, 6 ... Comparator, 7 ... Triangular wave oscillator, 8 ... Error amplifier, 9 Reference power supply, 10 Resistance, 11 Capacitor, 12 Diode, 14, 15 Output power supply terminal.

Claims (1)

(57) [Claims] 1. A switching element and a choke coil are connected in series to a DC power supply, and a series circuit of a first diode and a first capacitor for rectification is connected in parallel to the choke coil. The first and second terminals are first and second output power terminals, the first output power terminal is connected to a first input of an error amplifier via a reference power source, and the second output power terminal is A second input of a comparator connected to a second input of the error amplifier, a first input connected to the triangular wave oscillator, a second input of the comparator connected to an output of the error amplifier, and an output of the comparator connected to a control input of the switching element. ,
A series circuit of a resistor and a second capacitor is connected to a common connection point of the choke coil and the first diode and to the first output power terminal, and a second diode is connected to the resistor and the second diode. A switching power supply circuit connected to a common connection point with the capacitor and an output of the error amplifier.