JP2968670B2 - DC-DC converter with protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter with a protection circuit used for a switching regulator such as a DC-DC converter.

[0002]

2. Description of the Related Art A conventional overcurrent protection circuit used for a switching regulator such as a DC-DC converter is shown in FIG.
Shown in First, the DC-DC converter is provided with a DC power supply 1 for a DC voltage to be inputted, and a current from the DC power supply 1 on the primary side of a transformer 5 and a current transformer 4 for detecting a current change.
A switching transistor 3 for generating an intermittent current by a chopper voltage to be described later through the primary side, diodes 6 and 7 for rectifying an AC voltage induced on the secondary side of the transformer 5, and a choke coil for smoothing the rectified voltage. 8 and a capacitor 9, a sawtooth wave generating circuit 21 for supplying the above-mentioned chopper voltage, and a PWM modulation circuit 20 which forms a part of a protection circuit described later. Next, the overcurrent protection circuit smoothes the AC voltage induced on the secondary side of the current transformer 4 by the diode 18 and the capacitor 19 to generate a DC voltage corresponding to the change, and compares the DC voltage with the reference voltage of the reference power supply 13. Error amplifier 15 for amplifying voltage
And a change in the DC voltage of the other output to detect the reference power supply 1
An error amplifier 14 that compares the difference voltage with the reference voltage and amplifies the difference voltage, and combines the voltages of the error amplifiers 14 and 15 to control the pulse width of the chopper voltage corresponding to the change. And a circuit 20.
With such a circuit configuration, the pulse width of the intermittent voltage flowing through the primary side of the transformer 5 is controlled, so that an excessive current on the primary side and an excessive voltage on the output side are suppressed to constant voltages and protected. FIG. 3 shows a circuit configuration in which two such DC-DC converters with a protection circuit are connected in series. That is, the DC power supply 1, the switch 33, and the DC-D
In the case where the DC-DC converter 32 is connected in series with the C converter 31 and includes a load 35, the DC-DC converter 32 in the latter stage outputs the output power when the input current decreases from a constant output power state. The input current increases from It to Ip as shown in FIG. On the other hand, as shown in FIG. 4B, when the switch 33 is turned on, the output voltage and current characteristics of the DC-DC converter 31 at the preceding stage are such that the maximum value Iomax of the output current is
Maximum input current value Ip of the subsequent DC-DC converter 32
Otherwise, both power supplies will not start normally.
That is, Ip needs to satisfy Iomax> Ip.

[0003]

In this conventional DC-DC converter with a protection circuit, the maximum input current value Ip is two to three times the current value It at the normal input voltage.
The output current value of the DC-DC converter in the preceding stage needs to have a capacity of two to three times the output current in a normal state. For this reason, the shape of the DC-DC converter in the former stage becomes large, and there is a drawback that the space and the cost of this type of power supply system increase.

[0004]

DISCLOSURE OF THE INVENTION According to the present invention, a D with a protection circuit is provided.
The C-DC converter includes a transformer that inputs an AC current obtained by intermittently applying a DC input voltage by a switching element at a certain cycle and converts the AC current into an AC output voltage; a current transformer connected in series to a primary side of the transformer; A first error amplifier for amplifying a difference voltage between a DC voltage obtained by rectifying the output of the current transformer and a first reference voltage supplied through a first resistor; A second error for amplifying a difference voltage between a comparison DC voltage obtained by dividing a voltage between a DC output voltage of the DC-DC converter obtained by rectifying an AC voltage and the reference voltage and a second reference voltage. An amplifier, and a DC-DC converter with a protection circuit that combines output voltages of the first and second error amplifiers and supplies the output voltage to a pulse width modulation circuit that varies a control pulse width of the switching element.
The DC converter includes a photodiode that inputs a shunt current from the DC output voltage, and a phototransistor that is connected in parallel with the first resistor and receives light from the photodiode and changes resistance.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of one embodiment of the present invention.

In FIG. 1, the same reference numerals as those in the conventional example of FIG. 5 have the same configuration and function. That is, in this embodiment, D
Photocoupler 23 for light emission at C-DC converter output
A photocoupler 23-2 for receiving light is added in parallel with -1 and the resistor 16 (R16).

Next, in the operation of the present embodiment, in FIG. 1, when the output voltage (Vout) is a normal voltage, the photocoupler 23-2, which is a phototransistor, is on, so that the resistor 16 (R16) is short-circuited. , The voltage Vr at the inverting input terminal of the error amplifier 15 for overcurrent protection is
Is given by VR, the expression (1) is obtained.

[0009]

Now, when the output current increases, the detection voltage Vi of the current transformer 4 also increases. When Vi exceeds Vr, the output voltage of the error amplifier 15 of the overcurrent protection circuit increases, and the error for output voltage control increases. Since the output voltage becomes higher than the output voltage of the amplifier 14, the input voltage of the PWM modulation circuit 20 increases, and the pulse width decreases. As a result, the output voltage decreases. When the output voltage decreases, in equation (1),
Since Vout decreases, (Vout-VR) decreases and Vr increases. When the output voltage further decreases, the current flowing through the photodiode of the photocoupler 23-1 also decreases, and the current flowing through the collector of the photocoupler 23-2 also decreases. If the resistance value of the phototransistor is RF, Vr is given by equation (2).

[0011]

When the value of RF increases, R2 in equation (2)
The value of 4+ (R16.RI / R16 + RI) increases, and as a result, Vr decreases. When the voltage further decreases, the photocoupler turns off and the RF becomes infinite,
Vr is given by equation (3) and is a constant value.

[0013]

FIG. 2A, which is a characteristic diagram of the above description, shows that the reference voltage of the error amplifier in the overcurrent protection circuit with respect to the change of the output voltage changes according to the change of the output voltage Vout.

Therefore, the drooping characteristic of the overcurrent protection circuit is as shown in FIG. When the overcurrent protection circuit operates and the output voltage decreases, the reference voltage increases as shown in FIG. 2A, so that the output current also increases. When the output voltage further decreases, the reference voltage decreases, so that the output current decreases again. When the output voltage becomes close to 0 volt, the reference voltage becomes constant again, so that the output current also becomes a constant value.

Therefore, even when such a DC-DC converter is connected in series, the DC-DC
It is possible to flow a current two to three times the current flowing when the converter starts up, and it is possible to reduce the output current at a normal voltage.

[0017]

As described above, according to the present invention, by adding a photocoupler circuit, even when a power supply is connected in series, it is possible to increase the number of times by two to three times the normal flow that occurs when the subsequent DC-DC converter rises. DC-D
It is possible to flow to the C converter. Further, since the output current can be reduced to a small value at a normal voltage, the output power of the DC-DC converter in the preceding stage may be small, so that such a power supply system can be reduced in size and cost. There is an effect that becomes possible.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

FIG. 2 is a characteristic explanatory diagram of the embodiment of FIG.

FIG. 3 is a block diagram when the DC-DC converters of the embodiment of FIG. 1 are connected in series.

FIG. 4 is an explanatory diagram of characteristics when a conventional DC-DC converter is connected in series.

FIG. 5 is a circuit diagram of a conventional DC-DC converter with a protection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2,9,19 Capacitor 3 Switching transistor 4 Current transformer 5 Transformer 6,7,18 Diode 8 Choke coil 10,11,16,17,22,24,25 Resistance 12,13 Reference power supply 14,15 Protection amplifier Reference Signs List 20 PWM modulation circuit 21 Saw wave generation circuit 23-1, 23-2 Photocoupler

Claims (2)

(57) [Claims] 1. A transformer for inputting an alternating current obtained by intermittently switching a DC input voltage by a switching element at a certain period and converting the AC current into an AC output voltage, a current transformer connected in series to a primary side of the transformer, A first error amplifier for amplifying a difference voltage between a DC voltage obtained by rectifying an output of the current transformer and a first reference voltage supplied via a first resistor; and a secondary side of the transformer. Amplifying a difference voltage between a comparison DC voltage obtained by dividing a voltage between the DC output voltage of the DC-DC converter obtained by rectifying the AC voltage and the reference voltage, and a second reference voltage. An error amplifier, and a DC-DC with a protection circuit that combines the output voltages of the first and second error amplifiers and supplies the combined output voltage to a pulse width modulation circuit that varies a control pulse width of the switching element.
The converter includes a photodiode that inputs a shunt current from the DC output voltage, and a phototransistor that is connected in parallel with the first resistor and receives light from the photodiode and changes resistance. DC-DC converter with protection circuit. 2. A first error amplifier supplied to the first error amplifier.
The DC-DC converter with a protection circuit according to claim 1, wherein the reference voltage is generated by the first resistor and a second plurality of resistors that divides the DC output voltage.