JP2000197346A - Ripple filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ripple filter, capable of removing ripples from the output voltage through the instantaneous return to the action in the active region even when the load is rapidly increased, the input voltage is dropped, the ripple voltage exceeding the voltage drop by the resistance is superposed on the input voltage, and a first transistor is in an unstable action of the saturation region. SOLUTION: When a first transistor Q1 is operating in the saturation region, the action is detected by a second transistor Q2. A current IR1 of a resistor R1 of a low-pass filter is increased by a current generating circuit 4, in response to the current of the second transistor Q2 to drop the reference voltage at a point P of connection. By causing the reference voltage to be compared with the output voltage to be dropped, the output of a comparator drops the output voltage and increases the voltage between a collector and an emitter of the first transistor to restore the transistor to the action of the stable active region.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an instantaneous stable activation even when a power transistor for adjusting an output voltage enters an operation in a saturation region due to an abrupt change in an input voltage or a ripple voltage superimposed on the input voltage. The present invention relates to a ripple filter that can return to an operation in a region.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram of a conventional ripple filter. A first transistor Q1 serving as a power transistor is connected in series between the input terminal 1 and the output terminal 2, and the output voltage V _OUT is adjusted. The non-inverting input terminal of the comparator 3 is connected to the input terminal 1 via the resistor R1,
Grounded via capacitor C1. The resistor R1 and the capacitor C1 form a low-pass filter for the input voltage V _IN applied to the input terminal 1. Further, a constant current source S1 is connected to a connection point P1 between the resistor R1 and the capacitor C1. The inverting input terminal of the comparator 3 is connected to the output terminal 1 to which the output voltage V _OUT is applied.

The output side of the comparator 3 is connected to the base of the transistor Q1. In the ripple filter configured as described above, the reference voltage obtained by dropping the input voltage V _IN by the resistor R1 of the low-pass filter and the output voltage V _OUT are compared by the comparator 3, and the output of the comparator 3 causes the transistor Q1 to operate. Controlled by the active area. The output voltage V _OUT has the same value as the reference voltage at the connection point P1, and is expressed by equation (1) where the current of the resistor R1 by the constant current source S1 is I _R1 . _{V OUT = (V IN -I R1} · R1) (1) Incidentally, V _OUT in (1), V _IN, I _{R1, R1} is the output voltage V _OUT,
The values of the input voltage V _IN , the current I _R1 , and the resistance R1 are shown. Since the ripple voltage superimposed on the input voltage V _IN is removed by the low-pass filter, it is not superimposed on the output voltage V _OUT . The removed ripple voltage is substantially equal to the resistance R
1 within the voltage drop due to a voltage value within words (I _R1 · _R1).

Such a ripple filter removes a ripple voltage superimposed on an input voltage obtained from a battery or the like, and obtains an output voltage lower by a predetermined value than the input voltage. However, when the output current increases due to the sudden increase in load and the input voltage V _IN decreases, the reference voltage at the connection point P1 may not follow the low-pass filter because it has a time constant. This also occurs when a large ripple voltage exceeding the voltage drop due to the resistor R1 is superimposed on the input voltage V _IN . In such a case, when the input voltage V _IN becomes lower than the reference voltage, the voltage between the collector and the emitter of the transistor Q1 decreases, and the transistor Q1 tends to enter an unstable operation in the saturation region. In particular, it is likely to occur when the use time of a power supply such as a battery for supplying the input voltage V _IN is prolonged and the internal resistance is increased. In such a state, the ripple voltage is not removed because the ripple filter does not operate normally.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the input voltage due to a sudden increase in the load, or to superimpose a ripple voltage exceeding a voltage drop due to a resistor on the input voltage, thereby causing the first transistor to operate. It is an object of the present invention to provide a ripple filter capable of instantaneously returning to the operation in the active region even when unstable operation in the saturation region is started and removing ripples from the output voltage.

[0006]

A ripple filter according to the present invention comprises a first transistor connected in series between an input terminal and an output terminal, and a reference obtained by dropping an output voltage and an input voltage by a resistance of a low-pass filter. A second transistor connected to an output terminal for detecting a saturation state of the first transistor and conducting, and a low-pass filter in response to a current of the second transistor; A current generating circuit for increasing the current of the resistor and increasing the voltage drop.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The ripple filter of the present invention increases the current of a resistor for obtaining a reference voltage by dropping an input voltage when a first transistor serving as a power transistor enters an operation in a saturation region. I do. This increase in current
Performed by a current generating circuit responsive to the current of the second transistor that detects saturation, the output of the comparator decreases the output voltage of the first transistor by reducing this reference voltage, which is compared with the output voltage. By raising the collector-emitter voltage, the transistor is restored to a stable active region operation.

[0008]

FIG. 1 is a circuit diagram showing an embodiment of a ripple filter according to the present invention. Note that FIG.
The same reference numerals are given to the same parts. In FIG.
A first transistor Q1 of PNP type is connected in series between the input terminal 1 and the output terminal 2, and the emitter of the transistor Q1 is connected to the input terminal 1,
The collector is connected to the output terminal 2. Comparator 3
Is connected to a resistor R1 and a capacitor C1 forming a low-pass filter, and one end of the resistor R1 is connected to the input terminal 1. Further, the constant current source S1 is connected, and the current I _{R1 from the} constant current source S1 flows through the resistor R1 (1).
A reference voltage having the same value as the output voltage V _OUT shown in the equation is obtained. The inverting input terminal of the comparator 3 is connected to the output terminal 2 to which the output voltage V _OUT is applied. The output side of the comparator 3 is connected to the base of the transistor Q1.

The output terminal 2 is connected to the emitter of a PNP-type second transistor Q2, whose base is connected to the base of the transistor Q1. Transistor Q
2 is grounded via a resistor R2 and NP
It is connected to the base of an N-type third transistor Q3.
The collector of the transistor Q3 is connected to the connection point P1 between the resistor R1 of the low-pass filter and the capacitor C1, and the emitter is grounded. The bias resistor R2 and the transistor Q2 form a current generating circuit 4. The normal operation of the thus configured ripple filter of the present invention, that is, the case where the transistor Q1 is controlled in the active region, is shown in FIG.
The circuit is the same as that of FIG. In the circuit of FIG. 1, the input voltage V _IN suddenly decreases, or a large ripple voltage exceeding the voltage drop due to the resistor R1 is applied to the input voltage V _IN.
When the transistor Q1 enters a saturation state by being superimposed on _IN , the state is detected by the transistor Q2.

That is, when the transistor Q1 is saturated, the transistor Q2 conducts by detecting its state by the base-emitter voltage. Since the collector current of the transistor Q2 flows as the base current of the transistor Q3, a collector current of the transistor Q3 is generated. Since the collector current of the transistor Q3 flows through the resistor R1 of the low-pass filter, the current I _{R1 of the} resistor R1 increases and the voltage drop increases. This voltage drop is stabilized by the smoothing action of the capacitor C1. As a result, the reference voltage at the connection point P1 can be reliably reduced below the input voltage V _IN . Then, by lowering the output voltage V _OUT , the voltage between the collector and the emitter of the transistor Q1 can be increased to return to the operation of the active region. In the active region, a base-emitter voltage for turning on the transistor Q2 cannot be obtained, so that the transistor Q2 is turned off. Then, the normal operation of the ripple filter is performed.

As described above, in the ripple filter of the present invention, even when the transistor Q1 temporarily enters the operation in the saturation region, the reference voltage at the node P1 instantaneously becomes lower than that in the normal state, and the transistor Q1 becomes active region. It returns to the stable operation in. FIG. 2 is a circuit diagram showing another embodiment of the ripple filter according to the present invention. The current generating circuit 4 is formed by a current mirror circuit including an NPN type fourth transistor Q4 and a fifth transistor Q5. . The collector of the diode-connected transistor Q4 on the input side is connected to the collector of the transistor Q2, and the collector of the transistor Q5 on the output side is connected to the connection point P1. The emitters of the transistors Q4 and Q5 are grounded. The configuration of the current generation circuit 4 has various modifications as described above, and there is no particular limitation. Further, the first transistor may be formed of a plurality of Darlington-connected transistors.

[0012]

As described above, the ripple filter according to the present invention is provided with a resistor for obtaining a reference voltage by dropping an input voltage when a first transistor serving as a power transistor enters an operation in a saturation region. Increase the current. This current increase is performed by a current generation circuit that responds to the detection of the saturation state, and the reference voltage, which is compared with the output voltage, automatically falls below the normal state. The voltage between the collector and the emitter of one of the transistors is raised to return the transistor to a stable operation in the active region. As a result, the load suddenly becomes heavy and the input voltage drops, and the ripple voltage exceeding the voltage drop due to the resistance is superimposed on the input voltage, so that the first transistor enters an unstable operation in a saturation region. Even in such a case, the operation can be instantaneously returned to the operation in the active region, and the ripple can be removed from the output voltage. Such a ripple filter of the present invention which can cope with a wide change in input voltage including a ripple voltage is extremely practical and has a wide range of application.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a ripple filter of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the ripple filter of the present invention.

FIG. 3 is a circuit diagram of a conventional ripple filter.

[Explanation of symbols]

 Q1 First transistor Q2 Second transistor 3 Comparator 4 Current generation circuit

Claims (3)

[Claims] A first transistor connected in series between an input terminal and an output terminal, comparing an output voltage with a reference voltage obtained by dropping the input voltage by a resistance of a low-pass filter, and A comparator connected to the control, a second transistor connected to the output terminal to detect the saturation state of the first transistor and conduct, responding to the current of the second transistor, increase the current of the resistance of the low-pass filter and reduce the voltage drop. A ripple filter having a current generating circuit for increasing the size. 2. The ripple generating circuit according to claim 1, wherein the current generating circuit is formed by a third transistor having a collector connected to a connection point between a resistor and a capacitor of a low-pass filter, with the current of the second transistor serving as a base current. Filter. 3. The ripple filter according to claim 1, wherein the current generating circuit is formed by a current mirror circuit having an input side connected to the second transistor and an output side connected to a connection point between the resistor and the capacitor of the low-pass filter.