JP3051675B2 - Self-excited switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit technology of a self-excited oscillation type switching power supply capable of obtaining a stable and low output voltage with a simple structure.

[0002]

2. Description of the Related Art The driving system of a switching power supply is roughly classified into a self-excited oscillation system and a separately excited oscillation system. The former self-excited oscillation system is applied to a power supply device having a relatively small capacity. As a self-excited oscillation type (hereinafter, referred to as self-excited type) switching power supply device, there is one having a circuit configured as shown in FIG. In FIG. 4, 1 indicates an input terminal on the high potential side, 2 indicates an output terminal on the opposite potential side, and both the input terminal and the output terminal on the reference potential side are connected to the ground. And the illustration is omitted.

A first switching transistor Q1 and a choke coil L1 are connected in series between the input terminal 1 and the ground, and a connection point between the first switching transistor Q1 and the choke coil L1 (hereinafter referred to as a connection point (a)). ) And the output terminal 2 are connected with a rectifier diode.
A smoothing capacitor C3 is connected between the output terminal 2 and the ground. The first switching transistor Q1, the choke coil L1, the rectifier diode D1, and the smoothing capacitor C3 form a power supply device generally known as a polarity inversion converter. doing. This circuit is further provided with the following configuration for self-excited oscillation.

A second switching transistor Q2 and a resistor R1 are connected in series between the input terminal 1 and the ground.
The collector of the second switching transistor Q2 is connected to the base of the first switching transistor Q1,
The base of the switching transistor Q2 is connected to the emitter of the first switching transistor Q1 via the trigger circuit 4, and the base of the second switching transistor Q2 is further connected to the base of the first switching transistor Q1 via the resistor R2. With this connection configuration, the first switching transistor Q1 and the second switching transistor Q2 form a multivibrator.
Note that the trigger circuit 4 in FIG. 4 is configured by a series circuit of a resistor R3 and a capacitor C2.

A series circuit of a resistor R6 and a resistor R7 and a series circuit of a constant voltage diode D2 and a resistor R5 are connected in parallel between the output terminal 2 and the ground.
A transistor Q3 having a base connected to a connection point between the resistors R6 and R7 and an emitter connected to a connection point between the constant voltage diode D2 and the resistor R5 is provided. The transistor Q3 is connected to the resistors R5, R6, R7 and the constant voltage diode D2. A control circuit 3 is formed. The collector of the transistor Q3 is connected as an output terminal of the control circuit 3 to the base of the second switching transistor Q2 via the resistor R4. The outline of the self-excited oscillation operation of the circuit having the above configuration is as follows.

When an input voltage is applied to the input terminal 1, the first
A current flows through the path of the base of the switching transistor Q1, the resistor R1, and the ground, and the first switching transistor Q1 is turned on. At this time, the base between the base and the emitter of the second switching transistor Q2 does not receive a forward bias necessary to turn on, and is turned off.
When the first switching transistor Q1 is turned on, a current that increases linearly starts flowing through the choke coil L1. At the same time, the capacitor C2 of the trigger circuit 4 is charged with the connection point (a) at a high potential. Done.

Here, the first switching transistor Q
Although the upper limit value of the collector current of 1 (hereinafter referred to as the collector saturation current) changes depending on the value of the current flowing through the base at that time, it is set substantially constant by the resistor R1 in the circuit having the configuration of FIG. When the value of the current flowing through the choke coil L1 reaches the collector saturation current of the first switching transistor Q1, the current flowing through the choke coil L1 becomes constant.
No potential difference is present between the terminals. At this time, the capacitor C2 of the trigger circuit 4 starts discharging, guides the base current of the second switching transistor Q2, and makes the second switching transistor Q2 conductive. Then, the base current of the first switching transistor Q1 decreases, and the first switching transistor Q1 generates a flyback voltage in the choke coil L1 in order to reduce the flow rate of the collector current.

[0008] The flyback voltage generated in the choke coil L1 is applied to the base of the second switching transistor Q2 via the trigger circuit 4, and the second switching transistor Q2 is turned on. When the second switching transistor Q2 is turned on, the base potential of the first switching transistor Q1 increases, the forward bias state is released, and the first switching transistor Q1 is turned off. While the second switching transistor Q2 is on, the trigger circuit 4 and the choke coil L are connected to the base of the second switching transistor Q2.
1 flows, and the capacitor C of the trigger circuit 4
2 is charged with the resistor R3 side at a high potential.

As the charging of the capacitor C2 progresses, the base current of the second switching transistor Q2 decreases, and the second switching transistor Q2 shifts to the off state soon. When the second switching transistor Q2 is turned off, a current flows again from the base to the base of the first switching transistor Q1 through the resistor R1 and the ground, and the first switching transistor Q1 is turned off.
Shifts to the ON state. The self-excited oscillation operation is performed by repeating the above operation.

The constant voltage control operation of the circuit shown in FIG. 4 is performed as follows. The choke coil L1 is turned on and off by the first switching transistor Q1.
, An induced voltage and a flyback voltage appear alternately at both ends.
This flyback voltage is rectified and smoothed by the rectifier diode D1 and the smoothing capacitor C3. As a result, a DC voltage appearing at both ends of the smoothing capacitor C3 is supplied to the outside from the output terminal 2 as an output voltage. Here, the output voltage is divided by the resistors R6 and R7, and the divided voltage is applied to the base of the transistor Q3. Transistor Q3
Changes the current flowing into its collector in accordance with the difference between the divided voltage applied to the base and the reference voltage supplied to the emitter by the constant voltage diode D2.

When the collector current of the transistor Q3 changes, the second switching transistor Q2
Changes, and as a result, the on-duty of the second switching transistor Q2 changes. Since the first switching transistor Q1 and the second switching transistor Q2 form a multivibrator, and the operations of the two transistors are complementary, the change in the on-duty of the second switching transistor Q2 is the on-duty of the first switching transistor Q1. Bring change.

For example, if the output voltage (absolute value) is going to become larger than the set value, the divided voltage inputted to the base of the transistor Q3 becomes larger than the reference voltage provided to the emitter, and the transistor Q3 becomes more Conducts more current to the collector. Then, since the base current of the second switching transistor Q2 increases, the ON period of the second switching transistor Q2 increases. An increase in the on-period of the second switching transistor Q2 means an increase in the off-period of the first switching transistor Q1.
The on-duty of 1 is reduced, and as a result, an increase in output voltage is suppressed. If the output voltage is going to be lower than the set value, an operation opposite to the above-mentioned operation occurs, and a decrease in the output voltage is prevented. The constant voltage control of the output voltage is performed by such an operation.

[0013]

In the circuit shown in FIG. 4, the output voltage is equal to the reference voltage value of the constant voltage diode D2,
It is determined by the voltage appearing at the connection point between the resistors R6 and R7. When considering the range of the output voltage obtained by the circuit now Figure 4, the voltage value of the lower limit, based Zener voltage V _Z2 and transistor Q3 of the constant voltage diode D2, the voltage of the combined emitter voltage V _BE3 (V _Z2 + _VBE3 ).
By the way, the constant voltage diode element that exists on the market is usually
The Zener voltage becomes about 3 [V] or more. Therefore, the lower limit value of the output voltage of the circuit shown in FIG.
[V] is the limit, and it is difficult to obtain a voltage lower than [V].

Although there are rarely products having a Zener voltage of about 2 [V], such a constant voltage diode element has a drawback that the temperature dependence of its characteristics is greater than that of a high Zener voltage element. Therefore, when a constant voltage diode element having a Zener voltage of about 2 [V] is incorporated in the circuit of FIG.
While it is possible to obtain an output voltage as low as [V], there is a problem that the output voltage greatly varies depending on the ambient temperature. Therefore, an object of the present invention is to provide a self-excited switching power supply device which can obtain a stable low output voltage with a simple configuration.

[0015]

According to the present invention, a first switching transistor and a choke coil are connected in series between an input terminal and a reference potential point, and the choke coil is connected when a current is interrupted by the first switching transistor. A self-excited switching power supply of the polarity inversion type, which rectifies and smoothes the voltage generated at the input terminal and obtains an output voltage having a polarity opposite to that of the input voltage. Terminals are connected to each other, and the base thereof is connected to the first terminal via the first resistor.
Connected to the base of the switching transistor of
A second switching transistor forming a multivibrator together with the switching transistor of claim 1, a trigger circuit connected between the base of the second switching transistor and the collector of the first switching transistor,
A series circuit of a second resistor and a constant voltage generator connected between an input terminal and an output terminal of the power supply device, a base is connected to a connection point of the second resistor and the constant voltage generator, and an emitter is connected to a reference potential point. A control transistor connected to the base of the first switching transistor via a third resistor.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A second switching transistor constituting a multivibrator together with a first switching transistor is provided, and a trigger circuit is connected between a base of the second switching transistor and a collector of the first switching transistor. . A series circuit of a bias resistor and a constant voltage generator is connected between the input terminal of the power supply device and the output terminal on the polarity inversion side. Connecting a series circuit of a current limiting resistor and a control transistor between the collector of the second switching transistor and a reference potential point;
The base of the control transistor is connected to a connection point between the biasing resistor and the constant voltage generating means. Here, the constant voltage generating means comprises a constant voltage diode as a first example, and a shunt regulator and a plurality of resistors respectively connected between a cathode, a reference, an anode and a reference of the shunt regulator as a second example. As a third example, a shunt regulator in which the cathode and the reference are short-circuited is used.

[0017]

FIG. 1 shows a circuit of a self-excited switching power supply according to the present invention capable of obtaining a low and stable output voltage.
In FIG. 1, a first switching transistor Q1, a choke coil L1, a rectifier diode D1, and a smoothing capacitor C3 form a polarity inversion type converter circuit, and a second switching transistor Q2 for self-excited oscillation, a trigger circuit 4, and a resistor R2 are provided. It has the same connection configuration as the conventional circuit of FIG.

The configuration of the circuit of FIG. 1 differs from that of FIG. 4 in the control function of the power supply device, and has the following connection configuration. That is, a resistor R8 and a constant voltage generating means 5a are connected in series between the input terminal 1 and the output terminal 2, and a control transistor Q4 of an NPN transistor is provided. The base of the control transistor Q4 is connected to the resistor R8 and the constant voltage. The emitter is connected to the ground, the collector is connected to the base of the first switching transistor Q1 and the collector of the second switching transistor Q2 via a resistor R1. Here, the constant voltage generating means 5a connects the cathode to the resistor R8.
, And a constant voltage diode D3 having an anode connected to the output terminal 2.

In the circuit of FIG. 1 having such a configuration, the self-excited oscillation operation is substantially the same as that of the circuit of FIG.
The constant voltage control operation of the output voltage is as follows. The output voltage appearing at the output terminal 2 is a negative voltage value with respect to the ground, and a detection voltage higher than the output voltage by a predetermined voltage in the positive direction is obtained at the connection point between the constant voltage generating means 5a and the resistor R8. . Here, in the circuit of FIG. 1, the predetermined voltage is the Zener voltage of the constant voltage diode D2. This detection voltage is applied to the base of the control transistor Q4, and the collector current of the control transistor Q4 has a value corresponding to the detection voltage. The collector current of the control transistor Q4 determines the magnitude of the base current of the first switching transistor Q1, and causes the first switching transistor Q1 to perform an on / off operation at an on-duty according to the output voltage.

If the output voltage (absolute value) is going to become larger than the set value, the detection voltage drops, so that the current flowing to the base of the control transistor Q4 decreases, and the control transistor Q4 conducts the current guided to its collector. Is limited to small. Then, the base current of the first switching transistor Q1 decreases, and the collector saturation current of the first switching transistor Q1 decreases. Therefore, the time required for the current flowing through the choke coil L1 to reach the collector saturation current of the first switching transistor Q1 is shortened. Become. This means that the ON period of the first switching transistor Q1 is shortened, and the ON duty of the first switching transistor Q1 is reduced. As a result, the amount of energy supplied from the input side to the output side decreases, and an increase in output voltage is suppressed. If the output voltage is going to be lower than the set value, an operation opposite to the above operation will occur, and a decrease in the output voltage will be prevented. The constant voltage control of the output voltage is performed by such an operation.

In the circuit of FIG. 1, when the zener voltage of the constant voltage diode D3 constituting the constant voltage generating means 5a is V _Z3 and the voltage between the base and the emitter of the control transistor Q4 is V _BE4 , FIG. The set value V _OS of the output voltage of the circuit shown is (V _Z3 −V _BE4 ). As can be seen, the circuit of FIG. 1 can obtain an output voltage lower than that of the conventional circuit of FIG. 4 by the voltage between the base and the emitter of the control transistor Q4. In order to obtain a predetermined low output voltage, for example, a constant voltage diode element having good temperature characteristics can be used even if the Zener voltage is one rank higher, so that the output voltage of the power supply greatly varies depending on the ambient temperature. Can be prevented, and the stability of the output voltage can be improved.

The control transistor Q4 is provided with a minimum necessary first switching transistor Q4 according to the output voltage.
Since the base current of 1 flows, the current flowing from the second switching transistor Q2 to the resistor R1 when the second switching transistor Q2 is in the on state is suppressed to a small value.
There is also an advantage that the loss generated in the resistor R1 is reduced as compared with the conventional circuit. In FIG. 1, the trigger circuit 4 is constituted by a series circuit of a resistor R3 and a capacitor C2, but the resistor R3 may be omitted from the circuit in some cases. Although the constant voltage generating means 5a in FIG. 1 is constituted by the constant voltage diode D3, the constant voltage generating means 5b and 5c shown in FIG. 2 or FIG. 3 may be replaced with the constant voltage generating means 5a in FIG. .

The constant voltage generating means 5b shown in FIG.
It comprises a shunt regulator SR having a cathode on the 8 side and an anode on the output terminal 2 side, and resistors R9 and R10 connected between the cathode and the reference and between the anode and the reference, respectively. According to this configuration, the detection voltage higher than the output voltage by the voltage set by the shunt regulator SR is applied to the base of the control transistor Q4. Naturally, the value of the output voltage at this time is obtained by subtracting the voltage between the base and the emitter of the control transistor Q4 from the set voltage value of the shunt regulator SR. The set voltage value of the shunt regulator SR can be changed by adjusting the resistance ratio of the resistors R9 and R10, and the output voltage of the power supply device can be made variable.

The constant voltage generating means 5c shown in FIG. 3 is a shunt regulator S having a short circuit between the cathode and the reference.
R. This is understood as the case where the resistance value of the resistor R9 in FIG. 2 is set to zero and the resistance value of the resistor R10 is set to infinity. In this case, the set voltage value of the shunt regulator SR is the reference voltage source built in the shunt regulator SR. Becomes the same value as the reference voltage. Since the reference voltage of this shunt regulator SR has very low temperature dependency,
The output voltage of the power supply device can be stabilized with respect to the ambient temperature. Further, since the reference voltage of the shunt regulator SR is generally 2.5 [V] or 1.25 [V], there is an advantage that a very low output voltage can be obtained.

[0025]

As described above, according to the present invention, the resistor R is connected between the input terminal 1 of the power supply device and the output terminal 2 on the polarity inversion side.
8 and a series circuit of the constant voltage generating means 5a, and a series circuit of a resistor R1 and a control transistor Q4 is connected between the collector of the second switching transistor Q2 and a reference potential point (earth). Is connected to the connection point between the resistor R8 and the constant voltage generating means 5a. With this configuration, the output voltage of the power supply becomes the value obtained by subtracting the voltage between the base and the emitter of the control transistor from the set voltage value of the constant voltage generating means, and it is possible to obtain an output voltage lower than the conventional one.

In order to obtain a predetermined low output voltage,
Even if the Zener voltage increases by one rank, a constant voltage diode element as a constant voltage generating means having good temperature characteristics can be used, and the stability of the output voltage of the power supply device can be improved. Further, the current flowing through the resistor (R1) by the operation of the control transistor has a minimum value required for the operation of the first switching transistor, so that the loss generated in the resistor (R1) is reduced, and the efficiency of the power supply device is improved. I do. Therefore, it is possible to provide a self-excited switching power supply device that can obtain a stable low output voltage and has high power supply efficiency.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a self-excited switching power supply according to the present invention.

FIG. 2 is a diagram showing a second configuration example of the constant voltage generation means.

FIG. 3 is a diagram showing a third configuration example of the constant voltage generation means.

FIG. 4 is a circuit diagram of a conventional self-excited switching power supply.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal (polarity inversion side) 3 Control circuit 4 Trigger circuit 5a Constant voltage generating means 5b, 5c Constant voltage generating means using a shunt regulator C2 Trigger capacitor C3 Smoothing capacitor D1 Rectifying diode D2 Constant voltage diode D3 Constant voltage diode L1 Choke coil Q1 First switching transistor Q2 Second switching transistor Q4 Control transistor SR Shunt regulator

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/155

Claims (3)

(57) [Claims] 1. A first switching transistor and a choke coil are connected in series between an input terminal and a reference potential point, and a voltage generated in the choke coil when current is interrupted by the first switching transistor is rectified.・
In a polarity-reversal type self-excited switching power supply device for smoothing and obtaining an output voltage having a polarity opposite to an input voltage, an emitter and a collector terminal are connected to an emitter and a base terminal of the first switching transistor, respectively. A second switching transistor connected to the base of the first switching transistor via a first resistor to form a multivibrator with the first switching transistor; a base of the second switching transistor; A trigger circuit connected between the collector of the first switching transistor, a series circuit of a second resistor and a constant voltage generating means connected between an input terminal and an output terminal of the power supply device, the second resistor and the constant A base is connected to a connection point of the voltage generation means, an emitter is connected to the reference potential point, and a first Switch ing transistor base and self-excited switching power supply apparatus characterized by comprising a control transistor, which is connected to the collector through a third resistor to the connection point of the second switching transistor collector. 2. A shunt regulator having a cathode connected to the second resistor and an anode connected to the output terminal, wherein the constant voltage generator is connected between a cathode and a reference and between the anode and the reference of the shunt regulator. Done fourth
The self-excited switching power supply device according to claim 1, further comprising a fifth resistor and a fifth resistor. 3. The constant voltage generating means includes a shunt regulator having a cathode connected to the second resistor, an anode connected to the output terminal, and a short circuit between the cathode and the reference. The self-excited switching power supply device according to claim 1.