JPH07261862A - Stabilized power circuit - Google Patents

Abstract

PURPOSE:To provide a stabilized power source which uses a simple protection circuit and can stably operates even in a half-short state or at power source startup time. CONSTITUTION:A transistor(TR) Q3 serves as a protection TR, a constant voltage circuit 1 generates a detection voltage, and a Zener diode ZD1 and a resistance R1 generate a reference voltage. A TR Q2 operates as a comparison part and a TR Q1 functions as a control part. If a load-side voltage Vo drops owing to the short circuit, etc., of a load, the collector current of the TR Qs begins to flow. Consequently, the base current IB2 of the TR Q2 decreases and the collector current, i.e., the base current IB1 of the TR Qu also decreases. Therefore, a load current It decreases in a fold-back shape as the base current IB2 of the TR Q2 decreases, so that the protecting operation of the TR Q1 is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called stabilized power supply circuit, and more particularly to improvement of a protection circuit.

[0002]

2. Description of the Related Art Stabilized power supply circuits are important circuits used in all kinds of electronic equipment. However, these stabilized power supply circuits are equipped with a protection circuit so as to be able to cope with load short circuits and application of excessive voltage. It is usually provided.

As a protection circuit, it is easiest to provide a fuse on the load side. However, a protection circuit is often provided in the power supply circuit itself because of the drawback that it must be replaced when the fuse is blown.

As a conventional protection circuit, a protection circuit having a so-called fold-back characteristic as a good protection circuit characteristic is used. FIG. 3 shows a stabilized power supply circuit with a foldback characteristic current limiter circuit. As shown in FIG. 3, a protection circuit is provided separately from the stabilized power supply circuit 10.

If the voltage between the collector terminal and the emitter terminal of the transistor Q ₅ is expressed as V _{CE5 and} the voltage between the base terminal and the emitter terminal of the transistor Q ₆ is expressed as V _BE6 , then in the circuit of FIG. In operation, R ₆ · I _C5 + V _CE5 · R ₇ / (R ₇ + R ₈ ) <V _BE6 , and the transistor Q ₆ is in the cutoff state.

When an overcurrent flows due to a load short circuit,
The left side of the above formula is the transistor Q₆V _BE6Bigger
Transistor Q₆Enters the active area and the transistor
Q_FiveIt works to raise the base potential of.

When the load becomes heavier, the transistor Q ₅
V _CE5 becomes larger and I _C5 decreases. At this time, the output voltage / load current characteristic shows a "folded" input / output characteristic as shown in FIG. In the figure, I _MAX represents the load current when the protection circuit starts to work.

In this circuit, even if the load side is not completely short-circuited (referred to as a half-short state), the load current is reduced, so that the power loss of the transistor Q ₅ is small and there is no fear of destroying the transistor.

If the resistor R ₈ is not present, the circuit of FIG. 3 exhibits the current control characteristic shown in FIG. 2 '. As another conventional protection circuit, an example of a stabilized power supply circuit with a short circuit protection circuit is shown in FIG. As shown in FIG. 4, in this circuit, the resistors R ₁₄ and R ₁₅ are set so that the transistor Q ₉ is turned on in the normal operation. Transistor Q ₉ V
_{BE 9} is V _o · R ₁₅ / (R ₁₄ + R ₁₅ )> V _BE9 .

When the load current I _L becomes large and the regulated voltage cannot be maintained, the Zener diode Z
Assuming that the Zener voltage of D ₃ is V _ZD3 , R ₁₁ ‖ R ₁₂ is the parallel resistance value of both resistors, and h _fe7 is the current amplification factor of the transistor Q ₇ , then {(V _ZD3 −V _BE8 ) / (R ₁₁ ‖R ₁₂ )} ・ h _fe7 <I
_When it becomes _L , the output voltage V _o starts to fall, and when V _o · R ₁₅ / (R ₁₄ + R ₁₅ ) <V _BE9 , the transistor Q ₉ is turned off. As a result, the current flowing through R ₁₂ is cut off and the transistor Q ₈
By reducing the emitter current (base current of the transistor Q ₇ ) of, the current during overload is {(V _ZD3 −V
_BE8 ) / R ₁₁ } ・ h _fe7 .

The load characteristic of this protection circuit has a characteristic as shown in FIG.

[0012]

However, in the protection circuit having the current limiter characteristic of the foldback characteristic shown in FIG. 3, since the current flows to the load side through the resistor R ₈ , stabilization in the true sense is achieved. Not a power source, separately stabilized power supply circuit 1
0 is required.

Further, in the stabilized power supply circuit with the short-circuit protection circuit shown in FIG. 4, in the half-short state, V _{CE7 of the} transistor Q ₇ becomes large and the load current I _L (=
Since the collector current of transistor Q ₇ ) is maximum,
There is a problem that the power loss due to the transistor Q ₇ increases. Further, at the rising of the input voltage V _i (power-on state or the like), the transistor Q ₉ is in the cutoff state, so that the base current of the transistor Q ₇ is only the current flowing through the resistor R ₁₁ . At this time, if the load current I _L is large, the output voltage V _o cannot rise immediately until the transistor Q ₉ can be turned on, and the output voltage V _o becomes unstable. Therefore, normal operation may not be performed in some cases, and load conditions may be restricted.

Therefore, an object of the present invention is to provide a stabilized power supply using a protection circuit capable of performing stable operation even in a half-short state or power supply startup.

[0015]

In order to achieve the above object, the invention according to claim 1 inputs a reference voltage generated from a power supply source to a control input terminal and detects a reference voltage and a reference voltage corresponding to an output voltage. In a stabilized power supply circuit having a control unit that supplies a load current from the output terminal based on the result of comparison with the voltage and holds the output voltage constant, it is connected between the control input terminal and the output terminal and It is configured to include a switch unit that becomes conductive when the comparison result with the output voltage is a certain voltage or more.

According to a second aspect of the invention, in the stabilized power supply circuit according to the first aspect, the switch means inputs the detection voltage to the base terminal, the collector terminal is connected to the control input terminal, and the emitter terminal is connected to the output terminal. It is constituted by a protection transistor connected.

[0017]

FIG. 1 shows a stabilized power supply circuit of the present invention. In FIG. 1, the transistor Q ₃ is a protection transistor,
The constant voltage circuit 1 detects the detection voltage. Further, the Zener diode ZD ₁ and the resistor R ₁ generate the reference voltage V _ref . The transistor Q ₂ compares the detection voltage with the reference voltage V _ref and generates a drive current. The transistor Q ₁ produces a load current by this drive current.

According to the present invention, when the load side voltage V _o drops due to a load short circuit or the like, and V _ZD1 (= V _ref ) −V _BE2 −V _BE3 ≧ V _o , the collector current of the transistor Q ₃ changes. It begins to flow. As a result, the base current I _B2 of the transistor Q ₂ decreases.

[0019] When the base current I _B2 of the transistor Q ₂ is reduced, the collector current of the transistor Q _2, i.e.,
The base current I _B1 of the transistor Q ₁ decreases. Therefore, if the current amplification factor of the transistor Q ₁ is h _fe1 ,
The collector current I _C1 of the transistor Q ₁ is expressed by the relationship of I _C1 = I _B1 × h _fe1 .

Therefore, as the base current of the transistor Q ₂ decreases, the load current decreases in a fold-back shape, and the protection operation of the transistor Q ₁ is performed. The constant voltage applied to the switch means according to claim 1 is the transistor Q.
Equivalent to V _BE2 in ₂ .

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the stabilized power supply circuit of the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of the first embodiment of the present invention. As shown in FIG. 1 (A), stabilized power supply circuit of this embodiment, an emitter terminal connected to a power source side (V _i side), the collector terminal is connected to the load side (V _o side), the base The control transistor Q ₁ whose terminal is connected to the collector terminal of the drive transistor Q ₂ , the Zener diode ZD ₁ whose cathode terminal is connected to the input power source side through the resistor R _{1 and} whose anode terminal is grounded, and the base terminal connected but are connected to the cathode terminal of the Zener diode ZD _1, a collector terminal connected to the base terminal of the control transistor Q _1, a driving transistor Q ₂ to which the emitter terminal is grounded through a resistor R _2, the resistance R ₂ The stabilized power supply circuit as in the related art is configured by the generated constant voltage circuit 1.

Further, the base terminal is connected to the emitter terminal of the drive transistor Q ₂ , and the Zener diode ZD ₁
The stabilized power supply circuit of the present embodiment is configured by including a current cut-off transistor Q ₃ whose collector terminal is connected to the cathode terminal of and the emitter terminal is connected to the collector terminal of the control transistor Q ₁ .

Next, the operation will be described. The constant voltage circuit 1 functions to keep the voltage across the constant voltage constant in a steady state. The constant voltage circuit 1 and the resistor R ₂ form a detection unit that generates a detection voltage. Further, the reference voltage V _ref is generated by the resistor R ₁ and the Zener diode ZD ₁ .

When the load fluctuates in the normal operation state, for example, in the direction in which the output voltage V _o on the load side is lowered, the potential at the point c in FIG. 1 (A) also works in the lowering direction. Then, V _{BE 2} of the drive transistor Q ₂ having the reference voltage V _ref applied to the base terminal works in the direction of increasing, so that the base current I _B2 of the drive transistor Q ₂ increases. Therefore, the collector current of the drive transistor Q ₂ also increases.

The collector current of the driving transistor Q ₂ is namely because the driving current I _B1 of the control transistor Q _1, the control transistor Q ₁ is the collector current I _C1 (= I _L)
To increase. That is, the drive transistor Q _{2 functions} as a comparison section.

The [0026] above, even if load fluctuations occur, the current is constantly supplied output voltage V _o is a constant potential (reference voltage V
_ref ). However, when the load is too heavy, the drive transistor Q ₂ is saturated and the drive current I _B1 that can be supplied to the control transistor Q ₁ is also saturated. As a result, the load current I _L (I _MAX ) in the saturated state.
The output voltage V _{o begins} to drop while being fixed at. This state is the state of step S1 shown in FIG.

Next, the voltage is further reduced, and the output voltage V _o
Is reduced until V _ref −V _BE2 −V _BE3 ≧ V _o , the protection transistor Q ₃ becomes conductive. Since the collector terminal of the protection transistor Q ₃ is fixed to the reference voltage V _ref , the output voltage V _o
The conduction current increases with the decrease of.

As a result, the transistor Q ₃ acts in the direction of decreasing the base current I _B2 of the drive transistor Q ₂ , so that the base current I _B1 of the control transistor Q ₁ decreases as the output voltage V _o further decreases. . Therefore, the output current I _L also starts to decrease (FIG. 2: step S2). This means that the load current I _L decreases due to the current-voltage characteristic similar to that of the protection circuit with the foldback characteristic.

The voltage V _C from the steady voltage until the output current starts to fall is in the range of about 0.6 to 3 [V] as shown in FIG. 2 depending on the set voltage of the constant voltage circuit 1 which is the detecting portion. fluctuate. This is because the operating point of the transistor Q ₂ changes depending on the setting of the constant voltage circuit 1. As the constant voltage circuit 1, each constant voltage circuit such as a diode, a Zener diode, and a transistor can be applied as shown in FIG.

When the load side is completely short-circuited (step S
3) The load current I _L is fixed at the output current value I _f defined by the driving current I _B1 at that time. According to the above embodiment, only by adding one transistor Q _3, can constitute a protection circuit which shaped current control of off works if more than a certain value the output voltage is lowered, the control transistor Q ₁ current breakdown Can be protected from. Other Modifications The present invention is not limited to the above-described embodiments, and various modifications can be made.

That is, the present invention can be applied to any stabilized power supply that drives the control transistor by detecting the load voltage and comparing it with the reference voltage, regardless of the circuit shown in FIG.

Further, in the present embodiment, the NPN type transistor is adopted as the transistor Q ₃ as the protection circuit on the positive power source side, but the same operation is achieved even if the PNP type transistor is adopted as the protection circuit on the negative power source side. It is possible.

Further, not only the transistor but also an active element such as FET and a switching element can be applied instead of the transistor Q ₃ .

[0034]

As described above, according to the present invention, the current limiter having the fold-back characteristic for the stabilized power supply function is provided by the simple circuit configuration in which only one transistor element is added to the conventional stabilized power supply circuit. The protection operation can be realized, the power loss of the control transistor can be reduced, and the destruction thereof can be prevented.

Furthermore, the circuit of the present invention can perform good operation even in a half-short state or a power-on state.

[Brief description of drawings]

FIG. 1 is a stabilized power supply circuit according to an embodiment of the present invention.
(A) is an embodiment and (B) is a mode of a constant voltage circuit.

FIG. 2 is an explanatory diagram showing an IV characteristic by the stabilized power supply circuit of the present invention.

FIG. 3 is a conventional stabilized power supply circuit with a foldback characteristic current limiter circuit.

FIG. 4 is a conventional stabilized power supply circuit with a short-circuit protection circuit.

[Explanation of symbols]

1 ... constant voltage circuit 10 ... stabilized power supply circuit Q ₁ to Q ₉ ... transistor R ₁ to R ₁₅ ... resistor ZD ₁ ~ZD ₃ ... Zener diode D ... Diode V _i ... Input voltage V _o ... Output Voltage

Claims (2)

[Claims] 1. A reference voltage generated from a power supply source is input to a control input terminal, and a load current is supplied from the output terminal based on a comparison result between a detection voltage corresponding to the output voltage and the reference voltage. In a stabilized power supply circuit having a control unit that holds the output voltage constant, connected between the control input terminal and the output terminal, and conducts when the comparison result of the detection voltage and the output voltage is a certain voltage or more. A stabilized power supply circuit, characterized in that it is provided with a switch means for setting a state. 2. The stabilized power supply circuit according to claim 1, wherein the switch means inputs the detection voltage to a base terminal, a collector terminal is connected to the control input terminal, and an emitter terminal is connected to the output terminal. Stabilized power supply circuit characterized by comprising a protective transistor.