JP2795232B2 - DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a highly stable DC
A DC converter;

[0002]

2. Description of the Related Art FIG. 2 shows an example of a conventional general DC-DC converter. This circuit is a forward type switching power supply. In FIG. 2, MOSFET3
Is a pulse width modulation circuit (hereinafter referred to as a PWM circuit) 23
ON-OFF is repeated by the square wave output from the. From DC power supply 1, ON-OFF of MOSFET3
As a result, the current I11 is intermittently supplied to the primary side 4a of the transformer 4, and a voltage corresponding to the transformation ratio of the transformer 4 is generated.
b.

When the MOSFET 3 is ON, the transformer 4
A current flows through a route from the secondary side 4b, the diode 5, the coil 7, the load 9, and the secondary side 4b. MOSFET3
Is OFF, a current I20 flows through the route of the coil 7, the load 9, the diode 6, and the coil 7. Further, the diode 1 connected to another secondary side auxiliary winding 4c of the transformer 4
1. DC power generated by an auxiliary power supply circuit including a constant voltage circuit 12 and smoothing capacitors 13 and 14 is supplied to an error detection circuit 10 and the like.

In the error detection circuit 10, a secondary output voltage generated at both ends of the load 9 is applied as a detection voltage from a connection point between the resistors 16 and 17 to a control input terminal of a shunt regulator 21 constituting a part of the error detector 10. Added to This shunt regulator 21 is commercially available, for example, model; TL
431 is used. The operation of the error detection circuit 10 is described in detail in Japanese Patent Application Laid-Open No. 5-260743.

A resistor 15 is inserted between a connection point between the resistors 16 and 17 and an output terminal of the constant voltage circuit 12, and the output voltage is input to a shunt regulator 21. In the shunt regulator 21, the detection voltage of the output voltage and the reference voltage Vref (approximately 2.5
V). Shunt regulator 2
The cathode side of 1 is connected to a connection point of the resistors 16 and 17 via a series circuit of the resistor 18 and the capacitor 20,
The anode side is connected to the ground side of the output terminal.

Here, the case where the input voltage increases will be described. As the input voltage increases, the conventional circuit operates to stabilize the output voltage by the following feedback operation. That is, an increase in the input voltage V11, an increase in the output voltage, an increase in the output detection voltage, a decrease in the cathode voltage of the shunt regulator, an increase in the light emitting diode current of the photocoupler,
The operation is performed in the order of increasing the phototransistor current of the photocoupler, reducing the ON width of the switching FET, and decreasing the output voltage. On the other hand, when the input voltage decreases, the above operations are performed in opposite directions.

[0007]

However, in the above-described conventional DC-DC converter, for example, when the fluctuation width of the input voltage V11 is large, the phase margin of the feedback circuit is reduced, oscillation is easily performed, and phase adjustment is difficult. With the problem that

SUMMARY OF THE INVENTION It is an object of the present invention to provide a DC-DC converter having high output voltage stability with respect to input voltage fluctuation.

[0009]

In order to achieve the above object, a DC-DC converter according to the present invention detects a secondary voltage obtained by converting a primary voltage and converts the detected voltage to a predetermined reference voltage. An error detection circuit for comparing and outputting the compared error signal, a feedback circuit using a photocoupler having a light emitting diode, a transistor, and a photodiode and having a base connected to an anode of the photodiode and an anode of the photodiode; The feedback circuit is formed by connecting the error signal as an input signal, the transistor constituting an output for controlling the conversion operation, and connecting the cathode of the photodiode to a point generating a voltage corresponding to the primary voltage. The primary side voltage is directly fed back to the photodiode of the feedback circuit.

It is preferable to use a voltage dividing resistor for generating a voltage corresponding to the primary side voltage and to detect the secondary side voltage, and to use an NPN transistor as a transistor.

[0011]

Therefore, according to the DC-DC converter of the present invention, the secondary voltage converted from the primary voltage is detected,
The detected voltage is compared with a predetermined reference voltage, and a comparison error signal is output. Further, a feedback circuit is formed using a photocoupler configured by connecting the base of the transistor and the anode of the photodiode. This feedback circuit is formed by using an error signal as an input signal, a transistor constituting an output for controlling a conversion operation, and connecting a cathode of a photodiode to a point where a voltage corresponding to a primary side voltage is generated. Therefore, the primary voltage is directly fed back to the photodiode of the feedback circuit.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An embodiment of the DC converter will be described in detail. FIG. 1 shows an embodiment of a DC-DC converter according to the present invention. In the components constituting the present embodiment, those having substantially the same functions as the components constituting the conventional example of FIG.
The same numbers are given.

In FIG. 1, a DC-DC converter according to the present embodiment includes a primary-side input circuit, a secondary-side output circuit and an auxiliary power supply circuit, and an error detection circuit 1 for outputting an error signal.
0, a feedback circuit for feeding back the error signal on the secondary side to the primary side by the photocoupler 22, and a pulse width modulation circuit (hereinafter referred to as a PWM circuit) 23 for controlling the switching operation.

The input circuit of each of the above constituent circuits includes a DC power supply 1, a primary side 4a of a transformer 4, and a MOSFET 3 connected in series, and a resistor 2 and a resistor 24, 25 connected in series with a capacitor 2 at both ends of the DC power supply 1. Both ends are connected to form a primary circuit of the DC-DC converter.

The secondary side output circuit includes a secondary side 4b of the transformer 4, a diode 5, a coil 7, and a load 9, which are connected in series, and an output side of the diode 5 and a ground line (GN).
D), and a capacitor 8 is connected between the output side of the coil 7 and the ground line.

The auxiliary power supply circuit includes a secondary auxiliary winding 4c of the transformer 4, a diode 11, and a constant voltage circuit 12 connected in series. A capacitor 13 is provided between the output side of the diode 11 and the COM line. 12 output terminal side and CO
Capacitors 14 are respectively connected between the M lines.

The error detection circuit 10 includes a shunt regulator 21 having a cathode connected to a resistor 16 and a resistor 1 forming a voltage dividing resistor via a series circuit of a resistor 18 and a capacitor 20.
7 are connected. The anode side is connected to the ground side of the output terminal. The control input terminal is connected to a connection point between the resistors 15 and 17 forming a voltage dividing resistor, and this connection point is connected via a resistor 16 to the active side of the output terminal.

The feedback circuit includes a photocoupler 22. This photocoupler 22 includes a light emitting diode 22
a, an NPN transistor 22b and a photodiode 22c, wherein the anode of the photodiode 22c is connected to the base of the NPN transistor. The anode terminal of the light emitting diode 22a is connected to the resistor 15 terminal of the error detection circuit 10, and the cathode is connected to the cathode terminal of the shunt regulator 21 constituting the error detection circuit 10 via the resistor 19. The emitter terminal and the collector terminal of the NPN transistor 22b on the light receiving side are connected to a PWM circuit, and the cathode terminal of the photodiode 22c is connected to a connection point of series-connected resistors 24 and 25 constituting an input circuit.

The PWM circuit 23 has an input signal terminal connected to the emitter terminal and the collector terminal of the NPN transistor 22b of the photocoupler 22, and an output signal terminal connected to the MOSFET3.
Are connected to the respective gate terminals.

In the DC-DC converter of the present embodiment configured as described above, the MOSFET 3 is connected to the PWM circuit 23.
ON-OFF is repeated by the square wave. The current I1 is intermittently supplied from the DC power supply 1 to the primary coil 4a of the transformer 4 by turning on and off the MOSFET 3, and a voltage corresponding to the transformer ratio of the transformer 4 is applied to the secondary coil 4a.
b.

When the MOSFET 3 is ON, the transformer 4
The current I0 flows through a route from the secondary side 4b to the diode 5 to the coil 7 to the load 9 to the secondary side 4b. Also, MOSFE
When T3 is OFF, coil 7 to load 9 to diode 6
The current I0 flows through the route of the coil.

Also, another secondary side auxiliary winding 4 of the transformer 4
DC power generated by an auxiliary power supply circuit including a diode 11, a constant voltage circuit 12, and smoothing capacitors 13 and 14 connected to the power supply c is supplied to the error detection circuit 10 and the like.

The secondary output voltage generated at both ends of the load 9 is
A shunt regulator 21 constituting a part of the error detector 10 as a detection voltage from a connection point of the resistors 16 and 17
To the control input terminal.

A resistor 15 is inserted between the connection point of the resistors 16 and 17 and the output terminal of the constant voltage circuit 12 and is input to a shunt regulator 21. The shunt regulator 21 outputs an output voltage detection voltage and a shunt voltage. It is compared with a reference voltage Vref built in the regulator 21.
The internal reference voltage Vref of this embodiment is 2.5V.

Here, the connection point between the resistors 24 and 25 becomes a voltage proportional to the voltage V1 of the DC power supply 1,
The cathode of the photodiode 22c constituting the second element 2 is connected to this point. Therefore, a reverse current according to the input voltage V1 flows through the photodiode 22c.

The control procedure is as follows: increase of the input voltage V1, increase of the voltage between the resistors 24 and 25, increase of the phototransistor current, reduction of the ON width of the switching FET, and decrease of the output voltage. When this procedure is compared with the control procedure of the conventional example shown in the column of "Prior Art", it is understood that the procedure is shortened. The above-mentioned reverse current occurs when the increase in the input voltage V1 directly increases the reverse current in the light-receiving photodiode of the photocoupler. The increased reverse current increases the phototransistor current of the photocoupler, and there is almost no time delay due to feedback. Therefore, unstable factors such as oscillation caused by insufficient phase margin are eliminated, and a stable operation can be obtained. Shortened control action is DC
Causing a high stability of the DC converter;

Although the above embodiment is one preferred embodiment of the present invention, the present invention is not limited to this embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0028]

As is apparent from the above description, the DC-DC converter of the present invention detects the secondary voltage obtained by converting the primary voltage and compares the detected voltage with a predetermined reference voltage. , And outputs the compared error signal. Further, a feedback circuit is formed using a photocoupler configured by connecting the base of the transistor and the anode of the photodiode. This feedback circuit is formed by using an error signal as an input signal, a transistor constituting an output for controlling a conversion operation, and connecting a cathode of a photodiode to a point where a voltage corresponding to a primary side voltage is generated. Therefore, the primary voltage is directly fed back to the photodiode of the feedback circuit.

According to this configuration, the fluctuation of the input voltage directly becomes the reverse current of the light receiving photodiode of the photocoupler, and controls the current of the phototransistor of the photocoupler. This control has almost no time delay due to feedback. Therefore, unstable factors such as oscillation caused by insufficient phase margin are eliminated, and a stable output operation can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a DC-DC converter of the present invention.

FIG. 2 is a circuit diagram showing a configuration example of a conventional DC-DC converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2, 8, 13, 14, 20 Capacitor 3 MOSFE 4 Transformer 5, 6, 11 Diode 7 Coil 9 Load 10 Error detection circuit 12 Constant voltage circuit 15-19, 24, 25 Resistance 21 Shunt regulator 22 Photocoupler 23 Pulse width modulation circuit

Claims (4)

(57) [Claims] 1. An error detection circuit for detecting a secondary voltage converted from a primary voltage, comparing the detected voltage with a predetermined reference voltage, and outputting an error signal obtained by the comparison. A feedback circuit using a photocoupler configured by connecting a base of the transistor and an anode of the photodiode, the feedback circuit including the error signal as an input signal, the transistor Constitutes an output for controlling the conversion operation, and is formed by connecting a cathode of the photodiode to a point where a voltage corresponding to the primary side voltage is generated, and connecting the primary side voltage to a photo of the feedback circuit. A DC-DC converter characterized by directly returning to a diode. 2. The method according to claim 1, wherein the generation of the voltage corresponding to the primary side voltage includes:
2. The DC-DC converter according to claim 1, wherein the DC-DC converter is performed using a voltage dividing resistor. 3. The DC according to claim 1, wherein the detection of the secondary voltage is performed using a voltage dividing resistor.
-DC converter. 4. The DC-DC converter according to claim 1, wherein the transistor is an NPN transistor.