JP2001016846A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent variations in overcurrent detecting point for overcurrent protection. SOLUTION: An FFT 34 and a primary winding of a transformer 36 are connected in series with a DC power supply 30. Induced AC voltage of a secondary winding of the transformer 36 is rectified by diodes 33a and 33b, smoothed by a capacitor 40, and fed to positive and negative output terminals 42P and 42N. A diode 44 and a capacitor 46 are connected between one end of the secondary winding of the transformer 36 and the capacitor 40. A voltage across both ends of the capacitor 46 is changed through a Zener diode 50 into a stabilized voltage and fed as an operational voltage for an error amplifier 54. An input terminal of the error amplifier 54 is connected to a path joined with a positive output terminal 52P. A detection resistor 56 is interposed between one input terminal of the error amplifier 54 and the positive output terminal 52P. A resistance serial circuit is connected in series with the detection resistor 56. The resistance serial circuit includes voltage-dividing resistors 58 and 60 connected in series on the output side of the Zener diode 50. The connection point of the resistors 58 and 60 is connected to the other input terminal of the error amplifier 54. The output signal of the error amplifier 54 is fed to an overcurrent protective circuit 62.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly to a switching power supply provided with an overcurrent protection means.

[0002]

2. Description of the Related Art Conventionally, as the above-mentioned switching power supply device, for example, there is one as shown in FIG. In this switching power supply device, a series circuit of a primary winding 4 a of a transformer 4 and a switching element 6 is connected to both ends of the DC power supply 2. When the switching element 6 is turned on, an AC voltage is induced in the secondary winding 4b of the transformer 4, rectified by the rectifying diode 7a, and a current flows through the reactor 8 and the smoothing capacitor 10. When the switching element 6 is turned off, a back electromotive force is generated in the reactor 8 and flows through the smoothing capacitor 10 and the rectifying diode 7b. Hereinafter, this is repeated. Thus, a DC voltage is generated between the output terminals 12P and 12N. Positive electrode side of smoothing capacitor 10 and output terminal 1
An overcurrent detection resistor 14 is connected between the resistor 14 and 2P. Voltage dividing resistors 16 and 18 are connected in series between one end of the overcurrent detecting resistor 14 and the negative output terminal 12N. Similarly, voltage dividing resistors 20 and 22 are connected between the other end of the overcurrent detecting resistor 14 and the negative output terminal 12N. The voltage between both ends of the voltage dividing resistor 18 and the voltage between both ends of the voltage dividing resistor 22 are supplied to the error amplifier 24. The operating voltage of the error amplifier 24 is supplied from the positive and negative output terminals 12P and 12N. When the current flowing through the overcurrent detection resistor 14 increases, the error amplifier 24 supplies an output signal to an overcurrent protection circuit 26, and the overcurrent protection circuit 26 supplies an overcurrent detection signal to a control circuit 28. , The control circuit 28 changes the ON / OFF state of the switching element 6.

[0003]

In such a switching power supply, in order to reduce the influence of the voltage drop generated by the overcurrent detection resistor 14 on the voltage between the positive and negative output terminals 12P and 12N, Overcurrent detection resistor 14
Use a resistor with a small resistance value. However, the voltage drop at the overcurrent detection resistor 14 is given to the error amplifier 24 via the voltage dividing resistors 16, 18, 20, and 22. Therefore, the voltage dividing resistors 16, 18, 20, 2
Under the influence of the resistance value deviation of 2, the fluctuation of the overcurrent protection detection point increases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply having almost no change in an overcurrent protection detection point.

[0005]

The switching power supply of one embodiment of the present invention has a DC power supply. As the DC power supply, for example, a DC power supply obtained by converting a commercial AC power supply into a DC power by a DC converter including a rectifier and a smoother can be used. DC-DC conversion means is connected to this DC power supply. The DC-DC converter has a DC-AC converter and a first AC-DC converter, and flows a current from a positive output terminal to a negative output terminal via a load.
The first AC-DC converter includes, for example, a rectifier that rectifies the AC voltage from the DC-AC converter, and a smoother that smoothes an output signal of the rectifier. The reference potential side of the second AC-DC converter is connected to a line connected to the output side of the DC-DC converter and the positive output terminal, and the second AC-DC converter is a DC-AC converter. Is converted to a DC voltage. Second exchange-
Constant voltage converting means is provided between the output side of the DC converting means and the line connected to the positive output terminal, and converts the DC voltage from the output side of the second AC-DC converting means into a constant voltage. The DC voltage from the constant voltage means is supplied to the error amplifier as an operating voltage, and one input terminal of the error amplifier is connected to a line connected to a positive output terminal. A detection resistor is interposed in the line between one input side of the error amplifier and the positive output terminal. A resistor series circuit is connected in series with the detection resistor. This resistor series circuit
It includes a plurality of voltage dividing resistors connected in series connected to the output side of the constant voltage generating means, and a connection point between the voltage dividing resistors is connected to the other input terminal of the error amplifying means. An output signal of the error amplification means is supplied to the overcurrent protection means.

[0006] According to this switching power supply, the voltage that has been made constant by the constant voltage means is supplied to the error amplifying means, and the operating state of the error amplifying means is not affected by the load. Further, since the detection resistor is provided between the positive output terminal and the output side of the first AC-DC conversion means, that is, on the reference potential side of the error amplification means, the detection resistor is provided on the reference potential side of the error amplification means. There is no need to provide a voltage-dividing resistor, and there is no influence from the deviation of the resistance value of the voltage-dividing resistor.

A switching power supply according to another aspect of the present invention has a DC power supply. This DC power supply device has rectifying means connected to a commercial AC power supply, and smoothing means for smoothing the output of the rectifying means. The switching means switches the DC voltage of the DC power supply. As the switching means, a single semiconductor switching element that conducts while the control signal is being supplied to the control terminal can be used, or one in which these semiconductor switching elements are connected to a bridge circuit can be used. The switched voltage is supplied to the primary winding of the transformer. The first rectifying and smoothing means rectifies and smoothes the switching voltage induced in the secondary winding of the transformer, and supplies the rectified and smoothed voltage to the load from a positive output terminal. The second rectifying / smoothing means rectifies and smoothes the switching voltage induced in the secondary winding. The second rectifying / smoothing means uses a line between the positive output terminal and the first rectifying / smoothing means as a common line. The reference potential point of the constant voltage generating means is connected to the common line, and the constant voltage converting means converts the DC voltage from the second rectifying / smoothing means into a constant voltage. The DC voltage of the constant voltage generator is supplied to the error amplifier as an operating voltage, and the error amplifier has two input terminals, one of which is connected to the common line.
A detection resistor is interposed in the common line between one input terminal of the error amplification means and the positive output terminal. A plurality of voltage-dividing resistors are connected in series between the positive output terminal and the output side of the constant-voltage generating means, and a connection point between these voltage-dividing resistors is connected to the other input terminal of the error amplifying means. It is connected. An output signal of the error amplifying means is supplied to an overcurrent protection means.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A switching power supply according to one embodiment of the present invention has a DC power supply 30. This DC power supply rectifies and smoothes an AC voltage from, for example, a commercial AC power supply. This DC power is supplied to DC-DC conversion means.

The DC-DC converter 32 includes DC-AC converter. The DC-AC conversion means is a semiconductor switching element, for example, a power FET 34, in which the conductive path conducts while the control signal is supplied to the control electrode.
have. The conduction path of the power FET 34 is DC-
The serial circuit is connected in series with the primary winding 36a of the transformer 36 of the AC conversion means, and this series circuit is connected between both ends of the DC power supply 30.

The DC-DC converter 32 further has an AC-DC converter. The AC-DC converter has a rectifier having an anode connected to one end of the secondary winding 36 of the transformer 36, for example, a rectifier diode 33a. When the power FET 34 is turned on, the transformer 36
The AC voltage induced in the secondary winding 36b of the diode 3
The current is rectified by 3a and charges the smoothing capacitor 40 via a smoothing means, for example, a smoothing reactor 38. A diode 33b having an anode connected to a connection point between the other end of the secondary winding of the transformer 36 and one end of the smoothing capacitor 40;
Is connected to the cathode of the diode 33a. When the power FET 34 is turned off, the smoothing reactor 3
8, a back electromotive force is generated, and the resulting current flows through the smoothing capacitor 40 and the diode 33b. Thus, a DC voltage is generated between the positive and negative output terminals 42P and 42N. That is, the DC-DC converter is of the on-on control type.

At one end of the secondary winding 36b of the transformer 36,
The anode of a diode 44 of another rectifier is connected,
The cathode is provided with a smoothing means, for example, a smoothing capacitor 4.
6 is connected to one end. The other end of the capacitor 46 is connected to the positive output terminal 42P. That is, the rectifying / smoothing means including the diode 44 and the smoothing capacitor 46 uses the potential of the positive output terminal 42P as the reference potential.

A series circuit of a resistor 48 and a constant voltage element, for example, a Zener diode 50 is connected between both ends of the smoothing capacitor 46, and a voltage of a predetermined value is generated between both ends of the Zener diode 50. I do. That is, the zener diode 50 operates as a constant voltage means. The Zener diode 50 has a cathode connected to the resistor 48 and an anode connected to the positive output terminal 42P. A smoothing capacitor 52 is connected in parallel with the Zener diode 50.

A stabilized DC voltage between both ends of the smoothing capacitor 52 is supplied as an operating voltage to an error amplifier, for example, an error amplifier. Therefore, even if the load connected between the positive and negative output terminals 42P and 42N changes and the voltage between the positive and negative output terminals 42P and 42N changes, the operating voltage of the error amplifier 54 does not change. This error amplifier 5
4 has two input terminals, one of which is connected to a connection point between the smoothing reactor 46 and the Zener diode 50. An overcurrent detection resistor 56 is connected between this connection point and the positive output terminal 42P. The resistance of the resistor 56 is smaller than that of the load, and the voltage drop generated between both ends of the resistor 56 changes due to the fluctuation of the load. A voltage dividing resistor 5 is connected between the cathode of the Zener diode 50 and the positive output terminal 42P.
8 and 60 are connected, and these voltage dividing resistors 50 and 50 are connected.
Are connected to the other input terminal of the error amplifier 54. Therefore, the error amplifier 54 includes the resistor 5
A voltage between 6 and 60 is provided. Further, the series circuit of the voltage dividing resistors 58 and 60 and the detecting resistor 56 includes:
The constant voltage is supplied by the Zener diode 50. Therefore, the current flowing through the detection resistor changes with the load change, and this change is caused by the change in the resistance of the resistor 60,
It appears as a voltage change in the series circuit of the detecting resistor 56, and this is supplied to the error amplifier 54.

The error amplifier 54 supplies the error output signal to the overcurrent protection circuit 62. The overcurrent protection circuit 62
When the error output signal becomes larger than a predetermined value, an overcurrent control signal is supplied to the control circuit 64. When the control circuit 64 receives the overcurrent control signal, the control circuit 64
By shortening the ON period of 34, the current flowing to the load is reduced, or the power FET 34 is turned off to prevent the current from flowing to the load.

In the switching power supply configured as described above, a constant voltage is supplied as the operating voltage of the error amplifier 54.
The operation state of the error amplifier 54 does not change. The voltage input to the error amplifier 54 is connected to a detection resistor 56.
And the voltage between both ends of the voltage dividing resistor 60, unlike the conventional one, and it is not necessary to separately provide a voltage dividing resistor. This is because the detection resistor 60 is connected to the reference potential side of the error amplifier 54.
And applying a voltage to a series circuit of the voltage dividing resistors 58 and 60. With such a configuration, the voltage representing the overcurrent is affected only by the deviation of the resistance values of the voltage dividing resistors 58 and 60, and the two voltage dividing series circuits are used as in the related art. Compared to the case where there is little influence of the deviation of the resistance value.

In the above embodiment, the constant voltage circuit is
Although the configuration using the resistor 48 and the Zener diode 50 is shown, the configuration is not limited to this, and a configuration using, for example, a Zener diode and a transistor can also be used.

[0017]

As described above, according to the switching power supply of the present invention, the detection point of the overcurrent protection hardly fluctuates without being affected by the deviation of the resistance value of the voltage-dividing resistor, and the overcurrent protection is accurately performed. Current detection can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a switching power supply according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional switching power supply device.

[Explanation of symbols]

 Reference Signs List 30 DC power supply 32 DC-DC conversion means 34 Power FET (switching means) 33a, 33b Diode (first rectification smoothing means) 36 Transformer 38 Smoothing reactor (first rectification smoothing means) 40 Smoothing capacitor (first No. 1 rectifying / smoothing means) 44 Diode (second rectifying / smoothing means) 46 Smoothing capacitor (second rectifying / smoothing means) 50 Zener diode (stabilizing means) 54 Error amplifier (error amplifying means) 56 Detection resistor 58 , 60 voltage divider resistor (resistor series circuit) 62 overcurrent protection circuit (overcurrent protection means)

Claims (2)

[Claims] 1. A DC power supply; DC-AC conversion means connected to the DC power supply; and first AC-DC conversion means, wherein a positive output terminal is connected to a negative output terminal via a load. DC-DC converter for flowing a current, a reference potential side is connected to a line connected to the output side of the DC-DC converter and the positive output terminal, and converts the AC voltage from the DC-AC converter into a DC voltage. A second AC-DC converter for conversion; and a second AC-DC converter provided between the output side of the second AC-DC converter and the line connected to the positive output terminal, and provided from the output side of the second AC-DC converter. Constant voltage converting means for converting a DC voltage to a constant voltage, and an error amplifying means in which the DC voltage from the constant voltage converting means is supplied as an operating voltage and one input terminal is connected to a line connected to the positive output terminal. , One of the error amplification means A detection resistor interposed in the line between the positive side and the positive output terminal; and a plurality of components connected in series with the detection resistor and connected in series to the output side of the constant voltage means. A resistor series circuit including a piezoresistor, and a connection point between the voltage dividing resistors is connected to the other input terminal of the error amplifying means; and an overcurrent protection means to which an output signal of the error amplifying means is supplied. A switching power supply device comprising: 2. A DC power supply, switching means for switching a DC voltage of the DC power supply, a transformer for supplying the switched voltage to a primary winding, and a voltage induced in a secondary winding of the transformer. First rectifying and smoothing means for rectifying and smoothing the switching voltage thus obtained and supplying the load from a positive output terminal, and rectifying and smoothing the switching voltage induced in the secondary winding, A second rectifying / smoothing means using a line between the first rectifying / smoothing means as a common line, and a constant voltage for connecting a reference potential point to the common line and converting a DC voltage from the second rectifying / smoothing means into a constant voltage Voltage amplifying means; an error amplifying means having a DC voltage of the constant voltage providing means supplied as an operating voltage, having two input terminals, one of which is connected to the common line; One input terminal of A detection resistor interposed in the common line between the positive output terminal, and a plurality of voltage-dividing resistors connected in series between the positive output terminal and the output side of the constant-voltage generator. A switching circuit comprising: a series circuit in which a connection point between these voltage dividing resistors is connected to the other input terminal of the error amplifying means; and an overcurrent protection means to which an output signal of the error amplifying means is supplied. Power supply.