JP2554736Y2 - Switching regulator - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator having a constant off time width and a variable on time width of a switching element.

[0002]

2. Description of the Related Art As a switching regulator, a self-excited type having no control oscillator such as a ringing choke converter and a separately-excited type having a PWM (pulse width modulation) circuit are known.

[0003]

However, the conventional self-excited switching regulator has a drawback that when the load fluctuates, the on / off frequency of the switching element greatly changes. On the other hand, the on / off frequency of the switching element is constant, but the PWM pulse is formed based on the comparison between the triangular wave and the error output. It has the disadvantage of being complicated.
That is, the amplitude of the triangular wave is relatively low, usually about 0.5 to 1.0 V, and this is compared with the error output.

Accordingly, an object of the present invention is to provide a switching regulator which has a relatively small variation in on / off frequency of a switching element and a small malfunction due to noise.

[0005]

According to the present invention for achieving the above object, a voltage controlled by turning on / off a DC voltage by a switching element 7 will be described with reference to the reference numerals in the drawings showing the embodiments. In the switching regulator for obtaining the following, the on / off control circuit 14 of the switching element 7 includes a comparator 24 and first and second capacitors C
1, C2 and the first, second and third resistors R1, R2, R
3, a reference voltage source 27, first, second, and third control elements Q1, Q2, Q3, a diode D2, and a charge control element 29, and the first capacitor C1 is connected to the first capacitor C1.
The first resistor R1 is connected in parallel with the first capacitor C1, and the control terminal of the first control element Q1 is connected to the third terminal.
And one input terminal of the comparator 24 is connected to the reference voltage source 27 through the third resistor R3, and is connected to the reference voltage source 27 via the third resistor R3.
The other input terminal is connected to the first capacitor C1, and the second resistor R2 is connected between one input terminal and the output terminal of the comparator 24 via the diode D2. The second control element Q2 is connected between one input terminal of the comparator 24 and the ground, the second capacitor C2 is connected between the control element of the second control element Q2 and the ground, The charge control element 29 is connected between a charge power supply and the second capacitor C2,
The third control element Q3 is connected in parallel with the second capacitor C2, and the control terminal of the third control element Q3 has an output for turning off the switching element.
4, the third control element Q3 is connected to the output terminal of the comparator 24 so that the third control element Q3 is turned on, and the charge control element 29 charges the second capacitor C2. The present invention relates to a switching regulator characterized in that an on-time width of the switching element 7 is changed by changing a constant.

[0006]

The comparator outputs a control pulse. The voltage of the reference voltage source 27 is input to one input terminal of the comparator 24. When the second control element Q2 is turned on in response to the second capacitor C2 having been charged to a predetermined amount or more, the voltage at one input terminal of the comparator 24 is lower than the voltage at the other input terminal. Thus, the output of the comparator 24 changes. In response, the discharge of the first capacitor C1 starts, the voltage of the other input terminal of the comparator 24 decreases, becomes lower than the voltage of the one input terminal again, and the output of the comparator 24 becomes Return. When the resistance value of the charge control element 29 changes, the charging speed of the second capacitor C2 changes, and the ON time width changes.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a switching regulator according to an embodiment of the present invention will be described with reference to FIGS.

In FIG. 1, an output terminal of a DC power supply 2 comprising a rectifying and smoothing circuit connected to an AC power supply terminal 1, that is, between a DC power supply terminal 3 and a ground terminal 4, a primary winding 6 of a transformer 5 and A series circuit with a switching element 7 composed of an FET is connected.

A load 13 is connected to the secondary winding 8 of the transformer 5 via an output rectifying / smoothing circuit 11 comprising a diode 9 and a capacitor 10 and an output terminal 12.

The on / off control circuit 14 of the switching element 7 includes a power supply terminal 15 and a ground (common) terminal 16.
And a control pulse output terminal 17 and a voltage detection terminal 18. The power terminal 15 is connected to the control power circuit 19,
The ground terminal 16 is connected to the ground terminal 4 of the DC power supply 2, the control pulse output terminal 17 is connected to the control terminal (gate) of the switching element 7, and the voltage detection terminal 18 is connected to the output terminal 12.

The control power supply circuit 19 includes a tertiary winding 20 provided in the transformer 5, a rectifier diode 21 connected to the tertiary winding 20, a smoothing capacitor 22, and a starting resistor 23. Since this switching regulator is an on / off type (reverse type) in which the output rectifier diode 9 is turned on while the switching element 7 is off, a substantially constant voltage is applied to the tertiary winding 19 during the off period of the switching element 7. Is obtained, whereby the smoothing capacitor 22 is charged.

The control circuit 14 shown in FIG. 1 is configured as shown in FIG. The control circuit 14 is roughly divided into a voltage comparator 24, an OFF width control circuit 25, and an ON width control circuit 2.
6 and a reference voltage source 27, and shown in FIG.
Is supplied to the switching element 7 via the output terminal 17. FIG. 3 shows the state of each part under heavy load, and FIG. 4 shows the state of each part under light load.

The off-width control circuit 25 includes a first capacitor C1, and first, second and third resistors R1, R2, R3.
And a transistor Q1 as a first control element, and two diodes D1 and D2. The first capacitor C1 is connected between the power supply terminal 15 and the ground via a first transistor Q1 and a diode D1.
The first resistor R1 is connected in parallel with the first capacitor C1 to form a discharge circuit. The upper end of the first capacitor C1 is connected to the negative input terminal of the comparator 24. The base (control terminal) of the first transistor Q1 is connected to a reference voltage source 27 via a resistor R3, and a capacitor C comprising a resistor R2 and a diode D2.
It is connected to the output terminal of the comparator 24 via the first charge stop control circuit. The positive input terminal of the comparator 24 is connected to a reference voltage source 27 via a resistor R3.
2 and the output terminal of the comparator 24 via the diode D2.

The ON width control circuit 26 includes a second capacitor C2, a transistor Q2 as a second control element,
A transistor Q3 as a third control element, a NOT circuit (inverter) 28, a phototransistor 29 as a charge control element, a light emitting diode 30, an error amplifier 31, a reference voltage source 32, a voltage dividing resistor 33, 34.

The upper end of the second capacitor C2 is connected to the power supply terminal 15 via the phototransistor 29, and the lower end is connected to the ground. In order to relate the voltage Vc2 of the second capacitor C2 to the comparator 24,
Of the second transistor Q between the positive input terminal of
2 is connected, and the base (control terminal) of the second transistor Q2 is connected to the upper end of the second capacitor C2. Also, the discharge of the second capacitor C2 is
4 as a discharge control element in parallel with the second capacitor C2 in order to relate to the output of the third transistor Q2.
3 is connected, and the output terminal of the comparator 24 is
Through the base of the third transistor Q3 (control terminal)
It is connected to the.

In order to change the ON width of the switching element 7 in inverse proportion to the value of the output voltage, the voltage detection terminal 18
Are connected to one input terminal of the error amplifier 31, a reference voltage source 32 is connected to the other input terminal, and the light emitting diode 30 is connected to this output terminal. Have been. Since the light emitting diode 30 is optically coupled to the phototransistor 29, the light input corresponding to the output of the error amplifier 31 is
9 given. The reference voltage source 32 is formed by a resistor and a Zener diode connected to the voltage detection terminal 18, but is shown in principle in FIG. The reference voltage source 27 is also formed by a combination of a resistor and a Zener diode, but is shown in principle in FIG.

[0017]

Next, the operation of the circuits shown in FIGS. 1 and 2 will be described with reference to the waveform chart of FIG. When the power supply is started from the DC power supply 2 in FIG. 1, the capacitor 22 is charged via the starting resistor 23, and when this voltage reaches a predetermined value, the reference voltage Vr is supplied from the reference voltage source 27 composed of a Zener diode or the like.
(About 6.3 V), which is supplied to the positive input terminal of the comparator 24. At the same time, the first transistor Q1 is also turned on, and the voltage of the power supply terminal 15 + V causes the first capacitor C1 to turn on.
1 is charged. The first capacitor C1 receives a sum ( Vbe ± V) of the base-emitter voltage Vbe of the transistor Q1 and the voltage Vf of the diode D1 from the reference voltage Vr.
The battery is charged in a short rise time (0.5 μs) to a value (about 5 V) from which f ) is subtracted. On the other hand, the voltage Vr (6.3 V) of the reference voltage source 27 is applied to the positive input terminal of the comparator 24. Since the voltage V1 at the positive input terminal of the comparator 24 shown by the solid line in FIG. 3A is about 6.3V and the voltage V2 at the negative input terminal shown by the broken line is about 5V, the output of the comparator 24 is High level (H). In addition, the two-person power voltage V1,
Since the difference between V2 is about 1.3 V, malfunction due to noise can be sufficiently prevented.

On the other hand, since the charging time constant of the second capacitor C2 is set sufficiently larger than the charging time constant of the first capacitor C1, this voltage Vc2 is turned on as shown in FIG. It rises slowly with a slope in the period Ton. During the ON period Ton, the output of the comparator 24 is at a high level, the output of the NOT circuit 28 is at a low level, and the third transistor Q3 is off, so that the discharging circuit of the second capacitor C2 is not formed. The voltage V of the second capacitor C2
When c2 becomes about 0.7 to 0.8 V, the second transistor Q2 is turned on, and the voltage V at the positive input terminal of the comparator 24 is turned on.
1 drops below the voltage V2 at the negative input terminal, and the output of the comparator 24 changes to a low level (L). As a result, the second diode D2 is turned on, and the off period Tof
For f, the voltage V1 at the positive input terminal of the comparator 24 becomes {(Vr-Vf) (R2) / (R2 + R3)} + Vf, and is fixed at about 3V. Vf indicates the forward voltage of each of the diodes D1 and D2. NO
When the output of the T circuit 28 is at a high level, the third transistor Q3
Is turned on, the electric charge of the second capacitor C2 is rapidly discharged, and the voltage Vc2 drops rapidly.

During the off period Toff, the voltage V of the positive input terminal is
1 is lower than the voltage V2 at the negative input terminal, the first transistor Q1 and the first diode D1 are in a reverse bias state, the charging of the first capacitor C1 is stopped, and the charge of the first capacitor C1 is changed to C1. R1 is discharged with a discharge time constant, and this voltage, that is, the voltage V2 at the negative input terminal, decreases with a slope. Voltage V1 of positive input terminal during off period
Is fixed and the discharge time constant of the first capacitor C1 is constant, so that the time width until the voltage V2 at the negative input terminal crosses the voltage V1 at the positive input terminal, that is, the width of the off period Toff is constant.

When the voltage V2 at the negative input terminal of the comparator 24 becomes lower than the voltage V1 at the positive input terminal, the output of the comparator 24 goes high again, and the same operation is repeated. Since the output Vout of the comparator 24 is given to the switching element 7, the switching element 7 outputs the output Vou shown in FIG.
Turns on / off in response to t.

When the load on the output terminal 12 increases due to a light load, the resistance R2 of the phototransistor 29 decreases, and the charging time constant C2 R2 of the second capacitor C2 decreases. As a result, as shown in FIG. 4B, the charging speed of the voltage Vc2 of the second capacitor C2 is increased, and the second transistor is turned on within a short period from the time when the output Vout of the comparator 24 is changed to a high level. Q2 is turned on, and the on-period Ton is shortened. Since the off-period Toff is kept substantially constant, the voltage at the output terminal 12 is eventually returned to the predetermined voltage value.

The switching regulator of this embodiment has the following effects. (1) Since the off period Toff is constant even when the load 13 changes, the fluctuation of the on / off frequency of the switching element 7 is smaller than that of the RCC switching regulator. (2) The difference between the voltage V1 at one input terminal of the comparator 24 and the voltage V2 at the other input terminal can be made relatively large, so that malfunction due to noise is small. (3) A switching regulator that performs an intermediate operation between a conventional self-excited switching regulator and a separately excited switching regulator can be provided with a relatively simple circuit configuration.

[0023]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible.

Between the phototransistor 29 and the power supply terminal 15, a constant voltage circuit or a voltage control circuit composed of a Zener diode or the like can be interposed. The second capacitor C2 serves as a part or all of the reference voltage source 27.
Can be constituted.

A resistor can be connected in series and / or in parallel to the phototransistor 29 to form a charging circuit for the second capacitor C2.

The transistors Q1, Q2 and Q3 as the first, second and third control elements can be replaced with FETs or the like. Further, the phototransistor serving as the charge control element can be replaced with a voltage-controlled transistor so as to be directly controlled by the output of the error amplifier 31.

Two NOT circuits (inverters) are sequentially connected to the output stage of the comparator 24, the output of the first-stage NOT circuit is supplied to the base of the third transistor Q3,
The cathode of the diode D2 can be connected to the output terminal of the T circuit.

A well-known totem-pole type driving circuit or the like can be connected between the output terminal 17 and the switching element 7.

The power supply terminal 15 of the control circuit 14 is connected to the tertiary winding 2
It can be connected to an independent power supply circuit without connecting to the power supply circuit 19 including 0.

The present invention is applicable to various types of switching regulators including one or more switching elements.

[0031]

As is apparent from the above description, according to the present invention, it is possible to provide a switching regulator in which the fluctuation of the switching frequency due to the load fluctuation is relatively small and the malfunction due to noise is small.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a switching regulator according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a control circuit of FIG. 1 in detail.

FIG. 3 is a waveform diagram showing a state of each part of the control circuit under a heavy load.

FIG. 4 is a waveform diagram showing a state of each part of the control circuit at a light load.

[Explanation of symbols]

 7 Switching element 24 Comparator 27 Reference voltage source 29 Phototransistor C1, C2 First and second capacitors Q1, Q2, Q3 First, second and third transistors R1, R2, R3 First, second and third 3 resistors D1, D2 first and second diodes

Claims (1)

(57) [Scope of request for utility model registration] 1. A switching regulator for obtaining a controlled voltage by turning on / off a DC voltage by a switching element (7), wherein an on / off control circuit (14) of the switching element (7) comprises a comparator (24), the first and second capacitors (C1, C2), the first and second capacitors
And third resistors (R1, R2, R3), a reference voltage source (27), and first, second, and third control elements (Q1, Q2).
2, Q3), a diode (D2), and a charge control element (29), and the first capacitor (C1) is connected to a power supply terminal (15) via the first control element (Q1). The first resistor (R1) is connected in parallel with the first capacitor (C1), and the control terminal of the first control element (Q1) is connected to the control terminal of the first control element (Q1) via the third resistor (R3). The comparator (2) is connected to a reference voltage source (27).
One input terminal of the comparator (2) is connected to the reference voltage source (27) via the third resistor (R3).
The other input terminal of 4) is the first capacitor (C1).
The second resistor (R2) is connected between one input terminal and the output terminal of the comparator (24) via the diode (D2), and the second control element (Q2
2) is connected between one input terminal of the comparator (24) and ground, and the second capacitor (C2) is connected between the control element of the second control element (Q2) and ground. Connected, the charge control element (29) is connected between a charging power supply and the second capacitor (C2), and the third control element (Q3) is connected to the second capacitor (C2).
And the control terminal of the third control element (Q3) is connected to the third control element (Q3) during the period when the output for turning off the switching element is generated from the comparator (24). Is connected to the output terminal of the comparator (24) so that the charge control element (2) is turned on.
9) A switching regulator characterized in that the switching time constant of the switching element (7) is changed by changing the charging time constant of the second capacitor (C2).