JPS6122756A - Dc power source circuit - Google Patents

Abstract

PURPOSE:To increase the margin degree of designing a circuit by connecting an inductance between an input terminal and an output terminal, and controlling the first transistor connected with the output side of an inductance element by the second and third transistors. CONSTITUTION:An inductance element L is connected between an input terminal 22 and an output terminal 24, and the output of the element L is switched by the first transistor Q3. The base current of the first transistor Q3 is controlled by the second and third transistors Q1, Q2 connected between the input side and the ground side of the element L. The second transistor Q1 is driven by the output of a pulse width modulator O controlled in response to the compared result of the output DC voltage with a reference voltage, and a bias current is supplied from the input side of the element L to the base of the third transistor Q2.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a DC power supply circuit, and particularly to a DC power supply circuit that converts an input DC voltage into a predetermined output DC voltage.

1. Description of the Related Art Conventionally, there is a forward type DC power supply circuit that uses a transformer to convert an input DC voltage that can take a wide range of values into a predetermined output DC voltage. This involves connecting a transistor in series with the primary winding of the transformer, and adjusting this transistor depending on the output voltage of the circuit, for example using pulse width modulation (PWM).
), thereby obtaining a predetermined DC output voltage on the secondary winding side regardless of the input DC voltage. This has the advantage that both input voltage step-up and step-down functions can be realized with a single, relatively simple circuit. However, on the other hand, there are problems such as magnetic flux leakage from the transformer causing noise interference in surrounding electronic circuits, and low power conversion efficiency between input and output.

To solve this problem, DC power supply circuits that use choke coils instead of transformers are often used. This connects a transistor in series with a choke coil,
This transistor is driven to PW according to the output voltage, thereby stepping up or stepping down the input voltage. This utilizes the flyback characteristics of the choke coil, but has the advantages of high power conversion efficiency and low leakage magnetic flux.

A conventional DC power supply circuit using such a choke coil is shown in FIG. 2, for example. This is Pw
In response to the Pw signal generated at the output 12 of the circuit 10, the transistors Q1 and Q2 are turned on and off alternately, thereby causing the current flowing through the choke coil L to be interrupted by the transistor Q3, thereby changing the flyback characteristic of the coil L. Adjust the output voltage using ? This is to perform a pull-up. The output voltage of the circuit is monitored by comparator 14 via resistors R6 and R7. The comparator 14 generates an output 18 corresponding to the difference from the reference voltage, thereby controlling the pulse width of the PWM signal 12 generated by the PWM circuit 10. For example, there are devices that obtain an output voltage of about 9 volts from an input voltage of about 4 to 8.5 volts.

However, alternately turning on and off the transistors Q1 and Q2 in this way has the disadvantage that the tolerance range of the resistors R1, R2 and R5 for biasing their base currents is very narrow. For example, in order to prevent the transistor Q1 from conducting when the PIIIM signal on the signal line 12 is at a high level, the resistor R1 may be made small. However, PWM
When the signal is at a low level, the same transistor Q1
In order to make R2 also conductive, the resistance R2 must also be made small.

On the other hand, losses due to switching of these transistors appear as heat generation in the circuit elements.

Therefore, for example, when the transistor Q1 transitions from a conductive state to a cutoff state, it is undesirable for its collector current to trail off sluggishly. Therefore, in order to cause the collector current to fall sharply, the resistor R2 must be selected to have a sufficiently larger value than the resistor R5.

Selecting the value of each resistor so as to satisfy these contradictory conditions will narrow the design margin. Moreover, in an actual circuit device, highly accurate elements must be used so that variations in these elements can be compensated for. This situation is particularly severe when the input terminal varies over a wide range relative to the output voltage, such as from 4 to 12 volts in the example above; It becomes difficult to use a circuit with

OBJECTIVE OF THE INVENTION It is an object of the present invention to eliminate the drawbacks of the prior art and provide a DC power supply circuit with a simple configuration that has a wide margin of circuit design and can therefore use circuit elements with a wide tolerance for accuracy. do.

According to the present invention, a DC power supply circuit that converts an input DC voltage applied to an input terminal into a predetermined output DC voltage and outputs it to an output terminal is connected between an input terminal and an output terminal. an inductance element, and a first element connected to the output side of the inductance element to switch the output of the inductance element.
transistor and the input side of the inductance element.
second and third transistors connected between a reference potential of and controlling the base current of the first transistor, the collector-emitter paths of the second and third transistors being connected in series with each other; The circuit further includes driving means for comparing the output DC voltage with a second reference potential and driving the base of the second transistor according to the comparison result;
71477 means for supplying a bias current to the base of the third transistor from the input side of the inductance element.

Next, embodiments of the DC power supply circuit according to the present invention will be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, an input terminal 2 is connected to a DC power source 20, such as a dry cell battery, which supplies an input voltage vIN.
2. Output voltage V to other circuits used such as resistor RL. ,1
In this example, a choke coil L and a diode D are connected in series with the output terminal 24, which provides the output voltage, with the polarities shown. The connection point 26 between the choke coil L and the diode D is connected to the emitter of the NPN transistor Q3.
Grounded via the collector path.

Input terminal 22 is also grounded as shown through resistor R4, the emitter-collector path of PNP) transistor Q1, and the emitter-collector path of NPN) transistor Q2. The emitter of transistor Q1 is connected through a resistor R5 to its base, which is connected to the output terminal 12 of a pulse width modulation (PWM) circuit 10.

In this embodiment, the emitter of transistor Q1 is connected to resistor R2.
and is connected to the base of transistor Q2 through a parallel connection of capacitor 011. This base also
It is grounded through resistor R3.

The pulse width modulation circuit 10 outputs at output 12 a rectangular pulse with a predetermined repetition period, the pulse width of which depends on the voltage applied to the control input terminal 16. In other words, control input 1
If the voltage at 6 is positive, the pulse width at output 12 becomes wider.

If it is negative, it becomes narrower.

The control terminal 16 is connected to the output of the comparator 14, and the inverting input (-) of this comparator '14 is connected to the output terminal 24 of the device.
The midpoint 28 of a voltage divider 40 consisting of resistors R6 and R7 connected to is connected. Further, a reference voltage vREF is supplied to the non-inverting input (+) of the comparator 14.

Comparator 14 compares the voltages of these two input terminals,
When the voltage at the inverting input (-) is lower than the voltage at the non-inverting input (i.e., when the voltage at the midpoint 28 of the voltage divider 40 connected to the output terminal 24 of the device is lower than the reference voltage vREF, this implementation In the example, a positive voltage is output to its output 18. Furthermore, when the voltage at the midpoint 28 is higher than the reference voltage VREF, a negative voltage is output to its output !6. Therefore, from the output 12 of the modulator 10 The width of the output rectangular pulse will be expanded in the former case, and will be decreased in the latter case.

In this embodiment, when in use, for example, a dry battery with a rated output voltage of 4 ports can be connected to the input terminal as the power source 20, and in that case, the output terminal 24 is connected to a device that requires a rated power supply voltage of 9 ports. Connected.

As can be seen from a comparison with the conventional circuit configuration shown in FIG. 2, in this embodiment, the base current of transistor Q2 (7) flows from the emitter side of transistor
is supplied independently from the base current.

That is, from the emitter 30 of transistor Q1 to resistor R2
The current supplied through resistor R3 and transistor Q
The current is divided between the two bases. Therefore, the bias current at the base of transistor Q2 is the same as that of resistors R2 and R
By selecting a value of 3, it can be determined independently of the resistor R5. Furthermore, the base current bias of the transistor Ql can be determined independently of other factors by selecting the resistor R5.

For example, in order to sufficiently conduct the transistor Q1 when the output 12 of the modulation circuit 10 is at a low level, the value of the resistor R5 may be increased. In order to ensure sufficient conduction of transistor Q2 when output 12 is at a high level, it is advantageous to set the value of resistor R2 low compared to resistor R3. In this embodiment, the settings of these resistance values can be determined independently for transistors Q1 and Q2.

As mentioned above, losses due to switching of these transistors appear as heat generation in the circuit elements, but it is advantageous to sharpen the fall of the collector current when the transistor Ql transitions from a conductive state to a cut-off state, for example. It is. For this purpose, the value of the resistor R2 is set to be sufficiently larger than the resistor R5. However, in this embodiment, unlike the conventional circuit shown in FIG. 2 as described above, the base current of the transistor Q2 can be determined independently by the resistors R2 and R3. The condition of setting the value sufficiently large can be satisfied with a sufficient tolerance range.

In operation, when the output 12 of the modulation circuit IO goes low, the transistor Ql conducts. This supplies current to the base of transistor Q3, making transistor Q3 conductive and lowering the emitter potential of transistor Q1. This drop in emitter potential reduces the base current of transistor Q2, causing transistor Q
20 pace bias current decreases. Therefore, transistor Q2 becomes non-conductive.

When the output 12 of the modulation circuit 10 goes high, the transistor Q2 shuts off. This cuts off the base current of transistor Q3, making transistor Q3 non-conductive and increasing the emitter potential of transistor Q1. This rise in emitter potential increases the base current of transistor Q2, and a sufficient bias current is supplied to the base of I transistor Q2. Therefore, transistor Q2 becomes conductive.

In this way, the transistors Q1 and Q2 are alternately turned on, and in response, the transistor Q3 is repeatedly turned on and off, and the input voltage is stepped up by the choke coil L.

Note that in the embodiment described in -1-, the transistors Q1 and Q
2 was driven by pulse width modulation (PWM), but the present invention is not necessarily limited to this, and for example, pulse number modulation or frequency modulation can also be advantageously applied.

Since the DC power supply circuit according to the present invention does not use a transformer, the surrounding circuit 10 will not be damaged by leakage magnetic flux. Furthermore, there is a wide margin in circuit design, so circuit elements with a wide tolerance range of accuracy can be used, and the configuration is simple.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a DC power supply circuit according to the present invention, and FIG. 2 is a circuit diagram showing an example of a DC power supply circuit according to the prior art. 2nd Sergeant)'s Theory of Batting Tools 100. Pulse width modulation circuit +4. ,, Comparator,, Choke coil 01~Q3. Transistors R2-R7, resistors

Claims (1)

[Claims] A DC power supply circuit that converts an input DC voltage applied to an input terminal into a predetermined output DC voltage and outputs it to an output terminal, the circuit comprising: a DC power supply circuit connected between the input terminal and the output terminal. an inductance element; a first transistor connected to the output side of the inductance element and switching the output of the inductance element; a first transistor connected between the input side of the inductance element and a first reference potential; second and third transistors controlling the base current of the transistors, the collector-emitter paths of the second and third transistors being connected in series with each other, the circuit further comprising: controlling the output DC voltage to a second transistor; drive means for driving the base of the second transistor according to the comparison result, and bias means for supplying a bias current from the input side of the inductance element to the base of the third transistor. A DC power supply circuit characterized by: