JP3406585B2 - 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 technique for improving the conversion efficiency of a self-oscillation type switching power supply device.

[0002]

2. Description of the Related Art The drive 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 this self-oscillation type (hereinafter referred to as self-excited type) switching power supply device, there is one in which the circuit is configured as shown in FIG. In FIG. 2, reference numerals 1 and 2 denote high-potential-side input terminals and output terminals, and both low-potential-side input terminals and output terminals are considered to be the same as ground because they are both connected to ground and are not shown. is there. Here, the earth is not necessarily grounded, but indicates the reference potential point of the circuit.

A transformer T is provided between the input terminal 1 and the ground.
The primary winding N1 and the first switching transistor Q1 are connected in series, and the secondary winding N2 of the transformer T has one end connected to the output terminal 2 via the rectifying diode D1 and the other end connected to the ground. ing. A filter capacitor C1 is connected between the input terminal 1 and ground, and a smoothing capacitor C2 is connected between the output terminal 2 and ground. The first switching transistor Q1, transformer T, rectifying diode D1, With the smoothing capacitor C2,
It constitutes a circuit of a general power supply device. The following components are further connected to this circuit for self-excited oscillation.

Between the input terminal 1 and the ground, a resistor R1 and
2 The switching transistor Q2 is connected in series.
The collector of the second switching transistor Q2 is connected to the base of the first switching transistor Q1, and the base is connected to the collector of the first switching transistor Q1 via a series circuit of a resistor R2 and a capacitor C3. The base of the second switching transistor Q2 and the first
The resistors R3 and R4 are connected in series between the base of the switching transistor Q1 and the resistors R3 and R4.
Is connected to the ground and the output terminal 2 through a series circuit of a capacitor C4, a diode D2 and a constant voltage diode DZ1, respectively. The outline of the self-oscillation operation of the circuit configured as described above is as follows.

When an input voltage is applied to the input terminal 1, a current flows from the input terminal 1 through the resistor R1 into the base of the first switching transistor Q1, and the first switching transistor Q1 is forward biased to turn on. It becomes a state. At this time, the forward bias required for turning on the second switching transistor Q2 between the base and the emitter cannot be received and the second switching transistor Q2 is turned off. When the first switching transistor Q1 is turned on, a linearly increasing current begins to flow in the primary winding N1 of the transformer T, and at the same time, the capacitor C3 is charged with the resistance R2 side at a high potential. .

Here, the first switching transistor Q
The upper limit value of the collector current of 1 (hereinafter referred to as collector saturation current) changes depending on the value of the current flowing through the base at that time, and is set to be substantially constant by the resistor R1 in the circuit of the configuration of FIG. When the value of the current flowing through the primary winding N1 reaches the collector saturation current of the first switching transistor Q1, the current flowing through the primary winding N1 becomes constant, and at that moment, a potential difference occurs between the terminals of the primary winding N1. Disappears. At this time, the capacitor C3 starts discharging and supplies the base current to the second switching transistor Q2 to make the second switching transistor Q2 conductive. Then, the base current of the first switching transistor Q1 decreases, and the first switching transistor Q1 suppresses the flow rate of its collector current to a small value, so that a flyback voltage is generated in each winding of the transformer T.

The flyback voltage generated in the primary winding N1 is applied to the base of the second switching transistor Q2 via the capacitor C3 and the resistor R2, and the second switching transistor Q2 is turned on. When the second switching transistor Q2 is turned on, the first switching transistor Q2 is turned on.
The base potential of the switching transistor Q1 drops,
The forward bias state is released and the first switching transistor Q1 is turned off. While the second switching transistor Q2 is in the ON state, a current flows through the base of the second switching transistor Q2 through the primary winding N1, the capacitor C3 and the resistor R2, and the capacitor C3.
Includes a primary winding N1 and a first switching transistor Q1
Charging is performed with the connection point side (hereinafter referred to as connection point (a)) side having a high potential.

As the charging of the capacitor C3 progresses, the base current of the second switching transistor Q2 decreases, and eventually the second switching transistor Q2 shifts to the off state. When the second switching transistor Q2 is turned off, current again flows into the base of the first switching transistor Q1 via the resistor R1, and the first switching transistor Q1 is turned on. The self-oscillation operation is performed by repeating the above operation. The series circuit of the diode D2 and the constant voltage diode DZ1 outputs a signal corresponding to the output voltage to the resistor R3.
Is applied to the base of the second switching transistor Q2 via the, to change the on-duty of the second switching transistor Q2. As a result, the on-duty of the first switching transistor Q1 changes and acts so as to keep the output voltage constant.

[0009]

In the circuit shown in FIG. 2, the current passing through the resistor R1 is the first switching transistor Q1 when the first switching transistor Q1 is in the on state and the second switching transistor Q2 is in the off state. Contributes to turning on. However, when the first switching transistor Q1 is off and the second switching transistor Q2 is on, the current passing through the resistor R1 flows into the collector of the second switching transistor Q2 and does not contribute to anything. Since loss occurs when a current flows through the resistance element, in the circuit shown in FIG. 2, unnecessary loss occurs in the resistor R1 during the period when the first switching transistor Q1 is in the off state and the second switching transistor Q2 is in the on state. . This has been a cause of increasing the loss generated in the circuit and reducing the conversion efficiency of the power supply device. Therefore, an object of the present invention is to provide a self-excited switching power supply device that can obtain high conversion efficiency by controlling the generation of unnecessary loss that occurs in a circuit.

[0010]

According to the present invention, a first switching transistor and a choke coil are connected in series between an input end and a reference potential, and a second switching transistor and an impedance element are connected between the input end and the reference potential. Are connected in series and supply a signal from between the first switching transistor and the choke coil to the control terminal of the second switching transistor, and between the second switching transistor and the impedance element and the control terminal of the first switching transistor. In the self-excited switching power supply device in which the two switching transistors perform complementary operation and self-oscillate to obtain an output voltage from the first switching transistor and the choke coil, the second element is connected in series with the impedance element. Complementary operation of the switching transistor of Wherein the switch is connected.

[0011]

A second switching transistor is provided in addition to the first switching transistor, and a series circuit of a resistor and a capacitor is connected between the base of the second switching transistor and the collector of the first switching transistor. , By connecting the collector of the second switching transistor and the base of the first switching transistor, the first switching transistor and the second switching transistor perform complementary operations. Between the base of the first switching transistor and the reference potential of the power supply device, a series circuit of an auxiliary switch that operates complementary to the impedance element for bias and the second switching transistor is connected. Here, as an example of the auxiliary switch, its base is connected to a connection point between the first switching transistor and the choke coil via a capacitor, and the base is further connected to an input terminal or a resistor via a resistor for setting a base current. Is composed of a transistor connected to the reference potential point.

[0012]

FIG. 1 shows an embodiment of a self-excited switching power supply device according to the present invention, which makes it possible to suppress the generation of unnecessary loss. The components in FIG. 1 having the same functions as the components shown in the circuit of FIG. 2 are designated by the same reference numerals. In FIG. 1, 1 indicates an input terminal on the high potential side, 3 indicates an output terminal on the potential side having a polarity opposite to that of the input terminal 1, and both the input and output terminals on the reference potential side are connected to the ground. Therefore, it is not shown in the drawing. A first switching transistor Q1 and a choke coil L1 which are PNP transistors connected in series between the input terminal 1 and ground, and a connection point (b) between the first switching transistor Q1 and the choke coil L1 and the output terminal 3. The connected rectifier diode D1 and the smoothing capacitor C2 connected between the output terminal 3 and the ground constitute a power supply circuit generally known as a polarity inversion converter.

The first switching transistor Q
A second switching transistor Q2, which is a PNP type transistor, is provided, the emitter of which is connected to the emitter of 1 and the collector of which is connected to the base of the first switching transistor Q1. A series circuit of a resistor R2 and a capacitor C3 is connected between the base of the second switching transistor Q2 and the collector of the first switching transistor Q1, and a resistor R3 is connected between the base of the second switching transistor Q2 and the base of the first switching transistor Q1. Connect. A resistor R1 and a control transistor Q4 are connected in series between a connection point between the collector of the second switching transistor Q2 and the base of the first switching transistor Q1 and ground, and the base of the control transistor Q4 is connected via a capacitor C6. Connect to connection point (b). A series circuit of a resistor R5 and a constant voltage diode DZ2 is connected between the input terminal 1 and the output terminal 3, and a connection point of the resistor R5 and the constant voltage diode DZ2 is connected to a control transistor Q4.
Connect to the base of.

The self-oscillation operation of the circuit of FIG. 1 having such a configuration is as follows. When the input voltage is applied to the input terminal 1, the transistor Q4 in which the current path of the base current is formed by the resistor R5 is biased in the forward direction and is turned on. Then, a current flows into the base of the first switching transistor Q1 through the transistor Q4 and the resistor R1, and the first switching transistor Q1 is forward biased and turned on. At this time, the forward bias necessary for turning on the second switching transistor Q2 cannot be received between the base and the emitter, and the second switching transistor Q2 is turned off. When the first switching transistor Q1 is turned on, a linearly increasing current starts to flow in the choke coil L1 and at the same time, the resistor R2 flows in the capacitor C3.
Charging is performed with the side at a low potential.

Here, the first switching transistor Q
The collector saturation current of 1 is determined by the magnitude of the current flowing through the base at that time, and in the circuit of FIG. 1 is set to be substantially constant by the resistor R1 when the transistor Q4 is in the ON state. 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, and at that moment there is no potential difference between the terminals of the choke coil L1. At this time, the capacitor C3 starts discharging and supplies the base current to the second switching transistor Q2 to make the second switching transistor Q2 conductive. Then, the base current of the first switching transistor Q1 decreases and its collector current decreases, so that a flyback voltage is generated in the choke coil L1.

The flyback voltage generated in the choke coil L1 is applied to the base of the second switching transistor Q2 via the capacitor C3 and the resistor R2, and the second
The switching transistor Q2 is turned on. At the same time, the flyback voltage is applied to the base of the transistor Q4 via the capacitor C6,
4 is turned off. Since the second switching transistor Q2 is turned on and the transistor Q4 is turned off, the base potential of the first switching transistor Q1 is lowered, and at the same time, the first switching transistor Q1 is turned on.
Since the base current of No. 1 is cut off, the first switching transistor Q1 is turned off. While the second switching transistor Q2 is in the ON state, the choke coil L1 is provided at the base of the second switching transistor Q2.
A current flows through the capacitor C3 and the resistor R2, and the capacitor C3 is charged so that the connection point (b) side has a low potential.

Due to the reduction of the flyback voltage generated in the choke coil L1 and the progress of the charging of the capacitors C3 and C6, the base current of the second switching transistor Q2 is reduced and the reverse bias of the transistor Q4 is reduced. 2 switching transistor Q2
Is turned off and the transistor Q4 is turned on. Since the second switching transistor Q2 is turned off and the transistor Q4 is turned on, the transistor Q1 is provided at the base of the first switching transistor Q1.
The current again flows in through the resistor 4 and the resistor R1, and the first switching transistor Q1 shifts to the ON state. Self-excited oscillation is performed by repeating such an operation process.

As can be seen from the above description of the operation, the second switching transistor Q2 and the transistor Q4 are
Complementary operation is performed according to the change in voltage at the connection point (b).
Therefore, the transistor Q4 supplies a current to the resistor R1 in order to supply a base current to the first switching transistor Q1 when the second switching transistor Q2 is in the off state, but supplies a current to the resistor R1 when the second switching transistor Q2 is in the on state. Do not supply current to. Therefore, in the circuit shown in FIG. 1, the unnecessary loss generated in the resistor R1 when the second switching transistor Q2 is in the ON state in the conventional circuit is reduced.

The control transistor Q4 in FIG.
Originally, the base current of the first switching transistor Q1 is controlled in order to perform constant voltage control of the output voltage. Here, the control transistor Q4 is turned off by the voltage at the connection point (b) led by the capacitor C6 when the second switching transistor Q2 is on, and the constant voltage diode DZ2 is turned on when the second switching transistor Q2 is off. The base current of the first switching transistor Q1 is controlled by the signal according to the output voltage. That is, the control transistor Q4 of the circuit of FIG.
Has a function as an original control transistor and a function as an auxiliary switch in the present application.

In the circuit diagram shown in FIG. 1, the impedance element for supplying a current to the base of the first switching transistor Q1 is the resistor R1. However, for example, a constant current element (circuit) using a transistor element may be used. Good. In the circuit diagram shown in FIG. 1, the first switching transistor Q1 and the second switching transistor Q2
Although both are shown as the same junction type bipolar transistor, other types of transistor elements such as MOSFET may be used. When using a MOSFET, a constant voltage element may be used as the impedance element in addition to the resistor R1. Further, the resistor R2 in FIG.
May be omitted in some cases, and the power supply device to which the present invention is applied is not limited to the circuit having the configuration of FIG.

[0021]

As described above, according to the present invention, the impedance element for bias supply, which is connected between the base of the first switching transistor and the input terminal of the power supply device or the reference potential point, is provided as the second element. The switching transistor is characterized in that auxiliary switches that perform complementary operations are connected in series. With this configuration, a current flows through the impedance element that supplies a bias to the first switching transistor when the second switching transistor is in the off state, and a current is cut off when the second switching transistor is in the on state. .
As a result, unnecessary loss that has occurred in the impedance element when the second switching transistor is in the ON state is reduced, and a self-excited switching power supply device with high conversion efficiency can be obtained.

[Brief description of drawings]

1 is a circuit diagram of an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional self-excited switching power supply device.

[Explanation of symbols]

1: Input terminal 3: Output terminal (polarity inversion side) C2: Smoothing capacitor C3: Capacitor for trigger D1: Rectifying diode DZ2: Constant voltage diode L1: Choke coil Q1: First switching transistor Q2: Second switching transistor Q4: Control transistor R1: resistor as impedance element for bias supply

Claims (3)

(57) [Claims] 1. A first switching transistor and a choke coil are connected in series between an input end and a reference potential,
The second switching transistor and the impedance element are connected in series between the input terminal and the reference potential, and a signal is supplied between the first switching transistor and the choke coil to the control terminal of the second switching transistor.
A signal is supplied between the switching transistor and the impedance element of the first switching transistor to the control terminal of the first switching transistor, the two switching transistors perform complementary operation and self-excited oscillation, and the output voltage from the first switching transistor and the choke coil. In the self-excited switching power supply device, the auxiliary switch that operates complementarily with the second switching transistor is connected in series with the impedance element. 2. The self-excited switching power supply device according to claim 1, wherein the auxiliary switch is a transistor whose control terminal is connected to a connection point between the first switching transistor and the choke coil via a capacitive element. 3. The self-excited switching power supply device according to claim 1, wherein the impedance element is a resistor.