JP3118424B2 - 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 high-efficiency power supply circuit using a field-effect transistor, specifically, a switching transistor formed of a MOSFET, in a self-excited oscillation type switching power supply.

[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, there is a switching power supply whose circuit is configured as shown in FIG. In FIG. 4, reference numerals 1 and 2 denote input terminals and output terminals on the high potential side, and both the input terminals and the output terminals on the low potential side are considered to be the same as the ground because both are connected to the ground. is there. Here, the ground is not necessarily grounded, but indicates a reference potential point of the circuit. The specific configuration of the circuit shown in FIG. 4 is as follows.

A first switching transistor Q5 of PNP type and a primary winding N1 of a transformer T are connected in series between an input terminal 1 and the ground, and both ends of a secondary winding N2 of the transformer T are connected to an AC output terminal. 3a and 3b. A diode D1 is connected between the connection point (A) between the primary winding N1 and the first switching transistor Q5 and the DC output terminal 2, and a smoothing capacitor C is connected between the DC output terminal 2 and the ground.
3 is connected. In this configuration, the first switching transistor Q5 and the transformer T (the primary winding N1, the secondary winding N
2) forms a single-type inverter circuit, and at the same time, the first
The switching transistor Q5, the primary winding N1, the diode D1, and the smoothing capacitor C3 form a polarity inversion type DC converter.

Further, a PN is connected between the input terminal 1 and the ground.
The P-type second switching transistor Q2 and the resistor R9 are connected in series, and the connection point (B) between the second switching transistor Q2 and the resistor R9 is connected to the base of the first switching transistor Q5. The base of the second switching transistor Q2 is connected to the connection point (A) via a series circuit of the resistor R2 and the capacitor C2, and the resistor R1 is connected between the bases of the first switching transistor Q5 and the second switching transistor Q2. . And the resistance R
3, a control circuit 4 formed by R4, R5, a constant voltage diode DZ, and a control transistor Q3 is provided for the DC output terminal 2, and the control signal is supplied to a connection point between the resistor R2 and the capacitor C2 via the resistor R6. It is configured to apply. The self-excited oscillation operation of the circuit having such a configuration was roughly as follows.

When an input voltage is applied to the input terminal 1, a current flows through the path between the input terminal 1, the emitter and the base terminal of the first switching transistor Q5, and the resistor R9, and the first switching transistor Q5 receives a forward bias. It turns on. At this time, the portion between the base and the emitter of the second switching transistor Q2 cannot be subjected to a forward bias necessary for turning on, and is turned off. When the first switching transistor Q5 is turned on, a linearly increasing current starts flowing through the primary winding N1 of the transformer T. Here, the upper limit value of the collector current of the first switching transistor Q5 (hereinafter referred to as the collector saturation current) is determined by the value of the current flowing to the base at that time, and is set substantially constant by the resistor R9 in the circuit having the configuration of FIG. Have been.

When the value of the current flowing through the primary winding N1 reaches the collector saturation current of the first switching transistor Q5, the current flowing through the primary winding N1 becomes constant. There is no potential difference between them. At this time, current flows from the base of the second switching transistor Q2 through the path of the resistor R2, the capacitor C2, and the primary winding N1, and the second switching transistor Q2 conducts.
Then, the base current of the first switching transistor Q5 decreases, and the first switching transistor Q5 suppresses the flow rate of the collector current thereof, so that a flyback voltage is generated in the primary winding N1. The flyback voltage generated in the primary winding N1 guides a current to the base of the second switching transistor Q2 via the capacitor C2 and the resistor R2, and turns on the second switching transistor Q2.

When the second switching transistor Q2 is turned on, the potential at the connection point (B) rises substantially to the input voltage, so that the first switching transistor Q5 is released from the forward bias state and turned off. Second
While the switching transistor Q2 is in the ON state, the resistor R is connected to the base of the second switching transistor Q2.
2. A current flows through the path of the capacitor C2 and the primary winding N1, and the capacitor C2 is charged so that the resistor R2 has 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 from the base of the first switching transistor Q5 toward the resistor R9, and the first switching transistor Q5 shifts to the on state. The self-excited oscillation operation is performed by repeating the above operation.

[0008]

When a bipolar transistor and a MOSFET are compared, the power consumption of the MOSFET is smaller than that of the MOSFET. Therefore, in order to improve the efficiency, a MOSFET is used instead of the bipolar transistor as the switching transistor of the power supply circuit. Sometimes. But,
In using a MOSFET, a voltage of about 3 [V] or more is required to drive the MOSFET,
Driving voltage of bipolar transistor (about 0.7 [V])
Note that it is much larger than FIG.
In the circuit shown in FIG.
Type first switching transistor Q5 is simply MOS
When the FET is replaced with the FET, the following problems are caused due to the driving voltage.

That is, immediately after the input voltage is applied to the input terminal 1, the second switching transistor Q
A current path is formed by the path of the two emitters, the base terminal, the resistor R1, and the resistor R9. Then, a voltage substantially equal to the input voltage divided by the resistors R1 and R8 is applied to the gate of the MOSFET (Q5). This resistor R1,
A voltage difference in which the divided voltage by R8 is higher than the lowest drive voltage (so-called gate threshold voltage)
5) If it can be given between the gate and source, MO
The SFET (Q5) conducts, and the circuit shown in FIG. 4 starts operating by self-excited oscillation. However, when the power supply device of the circuit of FIG. 4 is realized by using a MOSFET as the first switching transistor Q5, the input voltage is determined from the minimum drive voltage of the MOSFET and the voltage division ratio of the resistors R1 and R8, although it depends on the specifications of the circuit. The circuit does not operate unless it is about 8 [V] or more.

In the case of an electronic device in which the power supply source is a battery or a battery, the voltage supplied to the power supply circuit is often about 5 [V]. For such a device, a MOSFET shown in FIG. It could not be used for switching transistors. Therefore, the present invention provides a self-excited switching power supply that can reliably start and operate a switching transistor using a MOSFET even when an input voltage is low, and that can obtain high power conversion efficiency over a wide range of input voltages. With the goal.

[0011]

According to the present invention, there is provided an inductance component comprising first and second switching transistors which operate in a complementary manner and connected in series to the first switching transistor by the first switching transistor. A self-oscillation type switching power supply configured to turn on and off a flowing current and to obtain a desired output voltage by utilizing a voltage appearing in an inductance component, wherein the first switching transistor comprising a field effect transistor; An auxiliary transistor having its main current path connected to a connection point between the gate of the first switching transistor and the main current path of the second switching transistor, and a voltage signal generated at a connection point between the first switching transistor and the inductance component. Capacitive element input to the control terminal of the transistor , Characterized by comprising a connecting bias means between the gate and the control terminal of the main current path of the connection point with the auxiliary transistor of the second switching transistor of the first switching transistor.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A MOS is provided between an input terminal and ground.
A first switching transistor using an FET and an inductance component are connected in series, and a second switching transistor is connected between an input terminal and a gate of the first switching transistor. The control terminal of the second switching transistor is connected to the connection point (A) between the first switching transistor and the inductance component through a series circuit of the first resistor and the first capacitor, and further connected to the control terminal of the second switching transistor and the second terminal. A second resistor is connected between the switching transistor and the gate of one switching transistor. An auxiliary transistor is connected between the connection point (B) between the main current path of the second switching transistor and the gate of the first switching transistor and ground. A third means as a bias means between the control terminal of the auxiliary transistor and the connection point (B).
And a second capacitor is connected as a capacitive element between the control terminal of the auxiliary transistor and the connection point (A).

[0013]

FIG. 1 shows a circuit of a first embodiment of a self-excited switching power supply according to the present invention capable of obtaining high efficiency over a wide range of input voltages. The structure of the circuit shown in FIG. 1 is as follows. A first switching transistor Q1 composed of a P-channel MOSFET and a primary winding N1 of a transformer T are connected in series between the input terminal 1 and the ground, and the input terminal 1 and the first switching transistor Q
The emitter and collector terminals of a PNP-type second switching transistor Q2 are connected to the first gate. Second
The base of the switching transistor Q2 is connected to the first switching transistor Q2 via a resistor R2 and a capacitor C2.
1 and the connection point (A) between the primary winding N1 and
A resistor R1 is connected between the base of the second switching transistor Q2 and the gate of the first switching transistor Q1.

The collector and emitter terminals of an NPN type auxiliary transistor Q4 are connected between the collector (B) of the collector of the second switching transistor Q2 and the gate of the first switching transistor Q1 and ground. A resistor R7 is connected between the base of the auxiliary transistor Q4 and the connection point (B).
And a capacitor C4 is connected between the base of the auxiliary transistor Q4 and the connection point (A). The other circuit configuration is the same as the circuit of FIG. The self-excited oscillation operation of the circuit of FIG. 1 having such a configuration is as follows.

When an input voltage is applied to the input terminal 1, first, a current path is formed by the path of the emitter and base terminals of the second switching transistor Q2, the resistors R1 and R7, and the base and emitter terminals of the auxiliary transistor Q4. . When the auxiliary transistor Q4 becomes conductive, the gate potential of the first switching transistor Q1 decreases, and the first switching transistor Q1 becomes conductive. In this case, by setting the transistor element used for the auxiliary transistor Q4 to have a larger current amplification factor than the transistor element used for the second switching transistor Q2, the operation of the first switching transistor Q1 can be reliably started. Will be able to do it.

When the first switching transistor Q1 conducts, the potential at the connection point (A) rises, and the second switching transistor Q2 whose base is connected to the connection point (A) via the resistor R2 and the capacitor C2 is turned off. The auxiliary transistor Q4 whose base is connected to the connection point (A) via the capacitor C4 shifts to the ON state.
As a result, the first switching transistor Q1 is completely turned on, and a linearly increasing current flows through the primary winding N1.

When the value of the current flowing through the primary winding N1 reaches the drain saturation current of the first switching transistor Q1, there is no potential difference between the terminals of the primary winding N1. Then, a current path is formed from the base of the second switching transistor Q2 to the path of the resistor R2, the capacitor C2, and the primary winding N1, so that the second switching transistor Q2 conducts, and at the same time, the auxiliary transistor is charged by the charging voltage of the capacitor C4. Q4 has a state of having an electric resistance between the collector and the emitter. At this time, the gate potential of the first switching transistor Q1 increases, and the drain current of the first switching transistor Q1 decreases, so that a flyback voltage is generated in the primary winding N1. The flyback voltage generated in the primary winding N1 is equal to the value of the capacitor C
2 and the second switching transistor Q via the resistor R2.
2 and simultaneously applies a reverse bias to the auxiliary transistor Q4 via the capacitor C4 to turn on the second switching transistor Q2 and turn off the auxiliary transistor Q4, respectively.

The operation of these two transistors turns off the first switching transistor Q1. As the charging of the capacitor C2 and the capacitor C4 with the connection point (A) side to a low potential progresses, the second switching transistor Q2 is turned off and the auxiliary transistor Q4 is turned on. Then, the gate potential of the first switching transistor Q1 decreases, the first switching transistor Q1 conducts again, and the potential at the connection point (A) increases. Thereafter, the above operation is repeated, and the self-excited oscillation operation is continuously performed.

As can be seen from this operation, in the circuit of FIG. 1, at the time of start-up, the resistor R7 applies a forward bias to the auxiliary transistor Q4 to make the auxiliary transistor Q4 conductive. The auxiliary transistor Q4 conducts to lower the potential of the connection point (B) by conducting, so that even when the input voltage is low, for example, when the input voltage is 5 [V], the MOSFET is turned on.
As a result, a voltage higher than the lowest drive voltage of the MOSFET is applied between the gate and the source of the first switching transistor Q1. Therefore, it is possible to reliably start and operate the switching transistor including the MOSFET even when the input voltage is low.

In the circuit shown in FIG. 1, the auxiliary transistor Q4 is turned on and off complementarily with the second switching transistor Q2 during operation, so that the second switching transistor Q2 is turned on when the second switching transistor Q2 is on. Cut off the current flowing through. Therefore, the loss caused by the current flowing through the second switching transistor Q2, specifically, the loss of the resistor R9 in the conventional circuit of FIG. 4 is reduced in the circuit of FIG. Therefore, according to the circuit of the present invention shown in FIG. 1, higher power conversion efficiency can be obtained as compared with the conventional circuit shown in FIG. 4, and it can be applied to a wide range of input voltage.

FIG. 2 shows a second embodiment of the self-excited switching power supply according to the present invention. The circuit in FIG. 2 obtains a DC voltage from the secondary winding N3 provided in the transformer T, and it is necessary to insulate the input side (primary side) and the output side (secondary side) with respect to the DC output. Applies to certain specifications. The circuit shown in FIG. 2 is used only for DC output, and is a circuit in a case where AC output is not performed. In general, when the input side and the output side are insulated, the output capacitance of the power supply circuit often becomes relatively large. In the circuit shown in FIG. 2, the auxiliary transistor Q4 is not damaged by the voltage appearing at the connection point (A). , A resistor R8 is connected in series with the capacitor C4. 1 except that a DC output is obtained from the secondary winding N3 of the transformer T and that a resistor R8 is connected.
2 is the same as the circuit configuration of FIG. 2, and the self-excited oscillation operation of the circuit of FIG. 2 is the same as the self-excited oscillation operation of the circuit of FIG.

FIG. 3 shows a third embodiment of the self-excited switching power supply according to the present invention. In FIG. 3,
Components that perform the same functions as the components of the circuit shown in FIG. 1 are given the same reference numerals. The circuit of FIG. 3 includes a choke coil L connected in series between the input terminal 1 and the ground.
1, a first switching transistor Q1 formed by an N-channel MOSFET, a connection point (A) between the choke coil L1 and the first switching transistor Q1, and a DC output terminal 2
And a smoothing capacitor C3 connected between the DC output terminal 2 and the ground, to form a step-up chopper type DC converter. Here, the first switching transistor Q
The collector and emitter terminals of an NPN-type second switching transistor Q2 are connected between the gate of the first switching transistor Q1 and the ground, the base of the second switching transistor Q2 is connected to a connection point (A) via a capacitor C2, The resistor R1 is connected between the gate of the switching transistor Q1 and the base of the second switching transistor Q2.

Further, in order to enable the use of a MOSFET, a PNP type auxiliary transistor Q4 is connected between a connection point (B) between the gate of the first switching transistor Q1 and the collector of the second switching transistor Q2 and the input terminal 1. Of the auxiliary transistor Q4 and a connection point (A) between the base of the auxiliary transistor Q4 and the connection point (A).
4 and a resistor R7 is connected between the base of the auxiliary transistor Q4 and the connection point (B). Since the circuit configuration of FIG. 3 is equivalent to a configuration in which the polarities of the respective elements of the circuit of FIG. 1 are inverted, the operations of the circuits of FIG. 1 and FIG. 3 are basically the same. Omitted.

In the circuits of FIGS. 1, 2 and 3 described above, one end of the capacitor C4 is connected to the connection point (A) as an optimum example.
Connected to Here, considering that the signal input to the base of the auxiliary transistor Q4 via the capacitor C4 is an AC component of a voltage appearing at the connection point (A), one end of the capacitor C4 is connected to the second switching transistor. It may be connected to the base of Q2 or the connection point between the resistor R2 and the capacitor C2. The present invention is not limited to the circuit configurations shown in FIGS. 1, 2 and 3, and various modifications can be made without departing from the spirit of the present invention.

[0025]

As described above, according to the present invention, the connection point (B) between the gate of the first switching transistor and the main current path of the second switching transistor is used so that the MOSFET can be used as the switching transistor. ) Is provided with an auxiliary transistor connected to the main current path, a resistance element is provided as a bias means between the connection point (B) and the control terminal of the auxiliary transistor, and a connection point (B) between the first switching transistor and the inductance. A capacitor is connected between the control terminal of the auxiliary transistor and the control terminal. According to this invention, even if the input voltage is low,
It is possible to reliably operate the switching transistor using the MOSFET. Further, a loss caused by a current flowing through the second switching transistor is:
It is reduced by the off operation of the auxiliary transistor. This makes it possible to obtain a self-excited switching power supply that exhibits high power conversion efficiency over a wide range of input voltages.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of a self-excited switching power supply according to the present invention.
FIG.

FIG. 2 shows a second embodiment of the self-excited switching power supply according to the present invention.
FIG.

FIG. 3 shows a third embodiment of the self-excited switching power supply according to the present invention.
FIG.

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

[Explanation of symbols]

1 Input terminal 2 DC output terminal 3a, 3b AC output terminal 4 Control circuit C2 Capacitor C3 Smoothing capacitor C4 Capacitor as capacitance element D1 Diode DZ Constant voltage diode L1 Choke coil N1 Primary winding N2 Secondary winding Q1 Use MOSFET First switching transistor Q2 Second switching transistor Q4 Auxiliary transistor Q5 First switching transistor using bipolar transistor R7 Resistance as bias means T Transformer (A) Connection point (B) Connection point

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/338 H02M 3/28 H02M 7/48 H02M 7/5383 H03K 17/732

Claims (2)

(57) [Claims] A first switching transistor that operates in a complementary manner, and turns on and off a current flowing through an inductance component connected in series to the first switching transistor by the first switching transistor. A self-oscillation type switching power supply device configured to obtain a desired output voltage by utilizing a voltage appearing in the inductance component, wherein the first switching transistor comprising a field effect transistor; An auxiliary transistor having a main current path connected to a connection point between a gate of the transistor and the main current path of the second switching transistor; a voltage signal generated at a connection point between the first switching transistor and the inductance component; A capacitive element input to a control terminal of Self-excited switching power supply, characterized in that it comprises a connected biasing means, between the control terminal of the connection point and the auxiliary transistor of the main current path of the gate and the second switching transistor of the switching transistor. 2. A semiconductor device comprising: first and second switching transistors operating in a complementary manner; and turning on and off a current flowing through an inductance component connected in series to the first switching transistor by the first switching transistor. A self-oscillation type switching power supply device configured to obtain a desired output voltage by utilizing a voltage appearing in the inductance component, wherein the first switching transistor comprising a field effect transistor; An auxiliary transistor having its main current path connected to a connection point between the gate of the transistor and the main current path of the second switching transistor; and a connection point between the connection point between the first switching transistor and the inductance component and a control terminal of the auxiliary transistor. A capacitor connected between the first switch and the first switch; Self-excited switching power supply, characterized in that it comprises a resistive element, connected between the control terminal of the main current path of the connection point between the auxiliary transistor gate and the second switching transistor of the ring transistor.