JP2977482B2 - Self-excited DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-down non-insulated DC-DC converter which does not require a feedback winding for self-excited oscillation.
The present invention relates to a circuit for improving the efficiency by using ET.

[0002]

2. Description of the Related Art A self-excited DC-DC converter has a simple circuit configuration and a small number of circuit elements.
It is used as a relatively low-cost power supply with a small output capacity. The present inventor has proposed a self-excited DC-DC converter having a circuit configuration as shown in FIG. 2 in Japanese Patent Application No. 7-209210 as such a DC-DC converter circuit. In addition, in FIG. 2, 1 and 2 are self-excited DC-DC.
The input and output terminals on the high potential side of the converter and the input and output terminals on the low potential side are shown as being grounded. A diode in which the source and drain terminals of a switching transistor Q1 formed of a P-channel MOS FET and a choke coil L1 are connected in series between an input terminal 1 and an output terminal 2, and one end of each of the two ends of the choke coil L1 is connected to ground. D1 and the smoothing capacitor C2 are connected to form a step-down chopper circuit.

The gate of the switching transistor Q1,
A collector and an emitter of a transistor Q2 as a first driving transistor of a PNP type bipolar transistor are connected between the sources, and the base of the transistor Q2 is connected to a switching transistor Q1 via a feedback circuit 3 of a series circuit of a resistor R3 and a capacitor C3. Connect to the drain of The base of the transistor Q2 is connected to the resistor R2.
To the gate of the switching transistor Q1. Between the gate of the switching transistor Q1 and the ground, there is provided a transistor Q3 as a second driving transistor of an NPN type bipolar transistor in which a starting resistor R1 and a current path between the resistor R1 and the collector and the emitter are parallel. And the base of the transistor Q3 is connected to the feedback circuit 3 via the capacitor C4.
A diode D2 forming a discharge path of the capacitor C4 is connected between the base of the transistor Q3 and the ground.

A series circuit of resistors R6 and R7 connected between the output terminal 2 and the ground, a series circuit of a resistor R4 and a constant voltage diode DZ, and a base connected to a voltage dividing point of the output voltage by the resistors R6 and R7. A control circuit 4 is composed of a transistor Q4 having an emitter connected to a connection point between R4 and the constant voltage diode DZ, and a resistor R5 connected between the collector of the transistor Q4 and the feedback circuit 3, and has a role of stabilizing an output voltage. Carry. The outline of the self-excited oscillation operation of the circuit of FIG. 2 having the above configuration is as follows.

First, when the switching transistor Q1 is on, the smoothing capacitor C2 is charged via the choke coil L1. The control circuit 4 starts to guide the base current of the transistor Q2 in accordance with the voltage between the terminals of the smoothing capacitor C2, and eventually turns on the transistor Q2. Then, the switching transistor Q1 cannot maintain the ON state, the drain current decreases, and a flyback voltage is generated in the choke coil L1. This choke coil L1
Generates a bias to the transistors Q2 and Q3 via the feedback circuit 3,
The transistor Q2 is turned on and the transistor Q2 is turned on.
3 is turned off. As a result, the switching transistor Q1 performs a turn-off operation.

Next, while the switching transistor Q1 is in the off state, the flyback voltage of the choke coil L1 decreases and the voltage between the terminals of the capacitor C3 of the feedback circuit 3 increases, and the transistor Q2 eventually decreases its base current. As a result, the voltage between the collector and the emitter increases. Along with this, the voltage between the gate and the source of the switching transistor Q1 also increases (as the gate potential decreases).
When it rises and its magnitude exceeds the threshold voltage, the switching transistor Q1 starts conducting. Then, the drain potential of the switching transistor Q1 rises, and this rise in potential gives a bias to the transistor Q2 and the transistor Q3 via the feedback circuit 3, thereby turning off the transistor Q2 and turning on the transistor Q3, respectively. As a result, the switching transistor Q1 performs a turn-on operation.

As can be understood from the above description, the self-excited DC-DC converter having the circuit configuration shown in FIG. 2 is driven by the presence of the driving transistors Q2 and Q3 and the feedback circuit 3 so far. The feedback winding provided in the choke coil, which has been required for the DC converter, is not required. For this reason, a single-turn coil component can be used for the choke coil L1, and there is an advantage that the size and cost of the DC-DC converter can be reduced. Further, since a driving voltage capable of driving the MOS FET is obtained, there is an advantage that the efficiency can be improved by using the MOS FET as the switching transistor.

[0008]

Problems to be Solved by the Invention Japanese Patent Application No. 7-20921
As described in No. 0, in reducing the size and cost of the power supply circuit, the heat dissipation measures of the power supply circuit can be cited as a major factor. The efficiency of the power supply circuit is closely related to the amount of heat generated in the circuit, and has a great influence on its shape and cost. Needless to say, the higher the efficiency of the power supply circuit, the better. In the circuit shown in FIG. 2, by using a MOS FET for the switching transistor Q1, the loss generated in the switching element is reduced and the efficiency of the power supply circuit is improved as compared with the case where a bipolar transistor is used for the switching transistor. .

[0009] However, the improvement of the self-excited oscillation means by such improvement of the efficiency is almost at the limit, and it has been difficult to further improve the efficiency. Considering the loss that occurs in the circuit, a diode as a commutation element has a forward drop voltage, and when a current flows through the diode, a power loss corresponding to the product of the current and the forward drop voltage is generated. This will occur in the commutation element. When the flowing current increases, the loss generated in the commutation element naturally increases, which is one of the major causes for reducing the efficiency of the power supply circuit. The present invention allows self-oscillation to be performed without depending on the feedback winding of the choke coil,
It is another object of the present invention to provide a self-excited DC-DC converter capable of reducing the loss generated in the commutation element and realizing the miniaturization and high efficiency.

[0010]

SUMMARY OF THE INVENTION The present invention provides a rectifying / smoothing device comprising a commutating element, a choke coil, and a smoothing capacitor connected to a current output terminal of a switching element for turning on and off an input DC voltage by a switching element. In a non-isolated self-excited DC-DC converter that supplies a stabilized DC output voltage to a load via a circuit, a MOS F
A first driving transistor having a switching element, a current input terminal connected to a current input terminal of the switching element, and a current output terminal connected to a control terminal of the switching element, a current output terminal of the switching element and a first drive A feedback circuit connected between the transistor and a control terminal of the transistor; a current input terminal connected to the control terminal of the switching element; a current output terminal connected to ground; And a transistor commutation element having a control terminal connected to a current input terminal of a second driving transistor, the transistor being composed of a MOS FET having a polarity opposite to that of the driving transistor and the switching element.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A starting resistor is connected between the gate of a switching transistor and ground, and the main current paths (collector, emitter) of a first driving transistor are connected between the gate and source of the switching transistor. .
The base of the first driving transistor is connected to the drain of the switching transistor via a feedback circuit including a resistor and a capacitor. A series circuit of a first resistor and a second driving transistor is connected in parallel with the starting resistor, and the base of the second driving transistor is connected to a feedback circuit via the first capacitor and the second resistor. Connecting. A transistor commutation element is connected between the drain of the switching transistor and the ground, and a diode element is connected in parallel with the main current path of the transistor commutation element. A gate of the transistor commutation element is connected to a connection point between the first resistor and the second driving transistor via a second capacitor, and a third resistor is connected between the gate and the ground.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit of a self-excited DC-DC converter according to the present invention, in which a loss generated in a commutation element is reduced and efficiency is improved, and a feedback winding is not required for self-excited oscillation. Note that the same components as those shown in FIG. 2 in FIG. 1 are denoted by the same reference numerals. The circuit of FIG. 1 is different from the circuit of FIG. 2 in that a transistor commutation element Q5 of an N-channel MOS FET is connected as a commutation element instead of the diode D1.

That is, the main current path of the transistor commutation element Q5 is connected between the drain of the switching transistor Q1 formed by a P-channel MOS FET and the ground,
A diode D3 for compensating for the operation delay of the transistor commutation element Q5 is connected in parallel to the main current path of the transistor commutation element Q5. In order to drive the transistor commutation element Q5, the collector of the transistor Q3 is connected to one end of the resistor R1 via the resistor R9, and the gate of the transistor commutation element Q5 is connected to the capacitor C5.
5 and a connection point between the transistor Q3 and the resistor R9, and a resistor R8 forming a charge / discharge path of the capacitor C5 between the gate and the ground. Note that the resistor R2 in the circuit of FIG. 2 can be omitted when the control circuit 4 is provided, and thus is omitted in the circuit of FIG. The resistor R10 in the circuit of FIG. 1 is provided for adjusting the base current of the transistor Q3.
It is omitted as shown in FIG. 2 depending on the specification of the transistor Q3.

The self-excited DC-DC converter of FIG. 1 having such a circuit configuration operates as follows.
When the switching transistor Q1 is on, the smoothing capacitor C2 is charged via the choke coil L1. At this time, the drain potential of the switching transistor Q1 is high, and this drain potential applies a bias to the transistor Q2 and the transistor Q3 via the feedback circuit 3, thereby turning off the transistor Q2 and turning on the transistor Q3, respectively. Therefore, the gate potential of the switching transistor Q1 decreases and the on state is maintained, and the transistor commutation element Q5 maintains the off state.

As the charging of the smoothing capacitor C2 progresses, the control circuit 4 starts to guide the base current of the transistor Q2 in accordance with the voltage between the terminals of the smoothing capacitor C2, and eventually turns on the transistor Q2. Then, since the voltage between the gate and the source of the switching transistor Q1 decreases (the gate potential increases), the current passing through the drain decreases. Due to the decrease in the current, a flyback voltage is generated in the choke coil L1. The flyback voltage generated in the choke coil L1 applies a bias to the transistor Q2 and the transistor Q3 via the feedback circuit 3, and turns on the transistor Q2 and turns off the transistor Q3, respectively. As a result, the switching transistor Q1 and the transistor commutation element Q5
, The switching transistor Q1 is turned off, and the transistor commutation element Q5 is turned on.

While the switching transistor Q1 is in the off state and the transistor commutation element Q5 is in the on state, the choke coil L1 and the smoothing capacitor C2 release energy, so that their flyback voltage and terminal voltage decrease. The voltage between terminals of the capacitor C3 of the feedback circuit 3 increases. Then, the base current of the transistor Q2 decreases, and the voltage between the collector and the emitter gradually increases. The switching transistor Q
Since the gate and source voltages of No. 1 increase (the gate potential decreases), when the voltage exceeds the threshold voltage, the switching transistor Q1 starts conducting. Then, the drain potential of the switching transistor Q1 rises, and this rise in potential gives a bias to the transistor Q2 and the transistor Q3 via the feedback circuit 3, thereby turning off the transistor Q2 and turning on the transistor Q3, respectively. As a result, the gate potentials of the switching transistor Q1 and the transistor commutation element Q5 decrease, and the switching transistor Q1 is turned on and the transistor commutation element Q5 is turned off.

As can be seen from the above operation, the switching transistor Q1 and the transistor commutation element Q5 operate complementarily, and the transistor commutation element Q5 has the flyback energy of the choke coil L1 similarly to the diode D1 in FIG. Form a distribution channel. Here, since the on-voltage (V _DSON ) of the main current path of the transistor is smaller than the forward drop voltage (V _F ) of the diode element, theoretically the generated loss is reduced and the efficiency of the power supply circuit is improved. become. However, various factors cause a shift in the operation timing of the switching transistor Q1 and the transistor commutation element Q5, which may lead to an increase in loss. Therefore, in the circuit of FIG. 1, a diode D3 is connected in parallel with the transistor commutation element Q5, and current is bypassed only during a period in which the operation timing is shifted, thereby preventing an increase in loss.

However, if there is almost no increase in loss due to a shift in operation timing due to the specifications of the circuit, the diode D3 is naturally omitted. In the circuit of FIG. 1, the resistors R3, R9, and R10 may be omitted depending on circuit specifications (particularly, transistor characteristics). In practicing the present invention, modifications can be made without impairing the operation and function of the invention, and the present invention is not limited to the circuit of the embodiment of FIG.

[0019]

As described above, according to the present invention, the MOS
The collector and the emitter of the first driving transistor are connected between the gate and the source of the switching transistor composed of the FET, the base of the first driving transistor is connected to the drain of the switching transistor via a feedback circuit, Connecting a main current path (collector, emitter) of the second driving transistor in parallel with a resistor connected between the gate and ground;
Self-excited oscillation means in which the base of the second driving transistor is connected to a feedback circuit via a capacitor; a transistor commutation element of MOSFET is connected as a commutation element; 2 is connected to the collector of the driving transistor. ADVANTAGE OF THE INVENTION According to this invention, since the loss which generate | occur | produces in a commutation element is reduced, the efficiency of a circuit improves. The improvement in efficiency makes it easier to dissipate heat in the power supply circuit, which contributes to downsizing and cost reduction of the power supply circuit. Therefore, high efficiency and small self-excited DC-DC
A converter can be provided at low cost.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a self-excited DC-DC converter according to the present invention.

FIG. 2 is a circuit diagram of a self-excited DC-DC converter proposed in Japanese Patent Application No. 7-209210.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 input terminal 2 output terminal 3 feedback circuit 4 control circuit Q1 switching transistor Q2 first driving transistor Q3 second driving transistor Q5 transistor commutation element L1 single-turn choke coil R1 starting resistor C4 capacitor C5 capacitor

Claims (2)

(57) [Claims] An input DC voltage is turned on and off by a switching element, and is stabilized through a smoothing circuit including a commutation element, a choke coil, and a smoothing capacitor connected to a current output terminal of the switching element. In a non-insulated type self-excited DC-DC converter for supplying a DC output voltage to a load, the switching element and a current input terminal of a MOSFET are connected to a current input terminal of the switching element, and a current output terminal of the switching element is connected to the switching element. A first driving transistor connected to a control terminal; a feedback circuit connected between a current output terminal of the switching element and a control terminal of the first driving transistor; A first driving transistor connected to a control terminal of a switching element and a current output terminal connected to ground; A second driving transistor that operates in a complementary manner, comprising a MOSFET having a polarity opposite to that of the switching element, and having a control terminal connected to a current input terminal of the second driving transistor; Self-excited DC- characterized by having a transistor commutation element.
DC converter. 2. An input DC voltage is turned on and off by a switching element, and is stabilized through a smoothing circuit including a commutation element, a choke coil, and a smoothing capacitor connected to a current output terminal of the switching element. In a non-insulation type self-excited DC-DC converter for supplying a DC output voltage to a load, a MOS in which a control terminal is connected to a ground via a resistor.
A first driving transistor having a current input terminal connected to a current input terminal of the switching element and a current output terminal connected to a control terminal of the switching element; a current output terminal of the switching element; A feedback circuit connected between the control terminal of the first driving transistor and a control terminal of the first driving transistor, a main current path of which is connected in parallel to the resistor, and a control terminal of which is connected to the feedback circuit via a capacitive element; A driving transistor, comprising: a MOS FET having a polarity opposite to that of the switching element, having a control terminal connected to a current input terminal of the second driving transistor, and a transistor commutation element as the commutation element. Self-excited DC-
DC converter.