JPH09191641A - Self exciting type dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize reduction in size and high efficiency by executing a self-excited oscillation not depending on a feedback winding of a choke coil and moreover by reducing loss generated in a commutating element. SOLUTION: A resistor R1 is connected between the gate of a switching transistor Q1 and the earth, a first driving transistor Q2 is connected between the gate of the switching transistor Q1 and source, and a second driving transistor Q3 is connected in parallel to the resistor R1. The base of transistor Q2 is connected to the drain of the switching transistor Q1 via a feedback circuit 3 and the base of transistor Q3 is connected to the feedback circuit 3 via a capacitor C4. A transistor commutating element Q5 is connected between the drain of the switching transistor Q1 and the earth and the gate of the transistor commutating element Q5 is connected to the collector of transistor Q5 via a capacitor C5.

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-insulation type DC-DC converter which does not require a feedback winding for self-excited oscillation.
The present invention relates to a circuit for using ET and improving its efficiency.

[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. As a circuit of such a DC-DC converter, the present inventor proposed in Japanese Patent Application No. 7-209210 a self-excited DC-DC converter having a circuit configuration as shown in FIG. 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 the same as ground. A diode in which the source and drain terminals of a switching transistor Q1 formed by a P-channel MOS FET and a choke coil L1 are connected in series between the input terminal 1 and the output terminal 2, and both ends of the choke coil L1 are 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, which is a PNP bipolar transistor as a first driving transistor, are connected between sources, and a base of the transistor Q2 is connected to a switching transistor Q1 via a feedback circuit 3 including a series circuit of a resistor R3 and a capacitor C3. Connect to the drain of. The base of the transistor Q2 is the resistor R2.
To the gate of the switching transistor Q1. Between the gate of the switching transistor Q1 and the ground, a transistor Q3 as a second driving transistor by an NPN bipolar transistor, in which a current path between the starting resistor R1 and the resistor R1 and the collector and the emitter is parallel, is provided. Then, 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 resistor R4 and a constant voltage diode DZ, and a base at the voltage dividing point of the output voltage by the resistors R6 and R7. A control circuit 4 is configured by a transistor Q4 having an emitter connected to a connection point of R4 and a constant voltage diode DZ, and a resistor R5 connected between the collector of the transistor Q4 and the feedback circuit 3, and serves to stabilize the output voltage. Carry. The outline of the self-excited oscillation operation of the circuit of FIG. 2 configured as described above 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 across 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 the flyback voltage is generated in the choke coil L1. This choke coil L1
The flyback voltage generated at the bias voltage biases the transistors Q2 and Q3 via the feedback circuit 3,
Transistor Q2 is on and transistor Q2 is on
3 is turned off. This causes the switching transistor Q1 to perform the turn-off operation.

Next, while the switching transistor Q1 is off, the flyback voltage of the choke coil L1 and the voltage across the terminals of the capacitor C3 of the feedback circuit 3 increase, and eventually the base current of the transistor Q2 decreases. Causes the voltage between the collector and the emitter to rise. Along with this, the voltage between the gate and the source of the switching transistor Q1 (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 potential rise biases the transistor Q2 and the transistor Q3 via the feedback circuit 3 to turn off the transistor Q2 and turn on the transistor Q3, respectively. This causes the switching transistor Q1 to perform the turn-on operation.

As can be seen from the above description, the self-exciting DC-DC converter having the circuit configuration shown in FIG. 2 has the driving transistors Q2 and Q3 and the feedback circuit 3 and thus the self-exciting DC-DC converter. The feedback winding provided in the choke coil, which was required in the DC converter, is no longer necessary. Therefore, a single-wound coil component can be used for the choke coil L1, and the DC-DC converter can be downsized and the cost can be reduced. Further, since a driving voltage capable of driving the MOS FET is obtained, there is also 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 order to reduce the size and cost of the power supply circuit, 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 the 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 the bipolar transistor is used for the switching transistor. .

However, the improvement in efficiency by improving the self-oscillation means is almost at its limit, and it has been difficult to achieve higher efficiency. Considering the loss generated in the circuit, there is a forward voltage drop in the diode as a commutation element, and when a current flows through the diode, the power loss corresponding to the product of the current and the forward voltage drop occurs. It will occur in the commutation element. As the flowing current increases, the loss generated in the commutation element naturally increases, which is one of the major causes of lowering the efficiency of the power supply circuit. The present invention causes self-excited oscillation without depending on the feedback winding of the choke coil,
Further, it is an object of the present invention to provide a self-excited DC-DC converter that can reduce the loss generated in a commutation element and realize miniaturization and high efficiency.

[0010]

SUMMARY OF THE INVENTION The present invention is a rectifying / smoothing device including a commutation device, a choke coil, and a smoothing capacitor, which are turned on and off by a switching device and connected to a current output terminal side of the switching device. 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
ET switching element, first driving transistor in which current input terminal is connected to current input terminal of switching element and current output terminal is connected to control terminal of switching element, current output terminal of switching element and first driving transistor A feedback circuit connected between the control terminal of the transistor and a current input terminal connected to the control terminal of the switching element, and a current output terminal connected to the ground, which operates complementarily to the first driving transistor. And a switching element, which is a MOS FET having a polarity opposite to that of the driving transistor and the switching element, and the control terminal of which is connected to the current input terminal of the second driving transistor.

[0011]

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

[0012]

EXAMPLE FIG. 1 shows a circuit of a self-excited DC-DC converter according to the present invention, which does not require a feedback winding for self-excited oscillation, in which loss generated in a commutation element is reduced to improve efficiency. The same components as those shown in FIG. 2 in FIG. 1 are designated by the same reference numerals. A unique point of the circuit of FIG. 1 with respect to the circuit of FIG. 2 is that a transistor commutation element Q5 by an N-channel MOS FET is connected instead of the diode D1 as a commutation element.

That is, the main current path of the transistor commutation element Q5 is connected between the drain of the switching transistor Q1 of the P channel MOS FET and the ground,
A diode D3 for compensating for the operational 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.
The resistor R8 is connected to the connection point of the transistor Q3 and the resistor R9 via the resistor 5, and the resistor R8 forming a charge / discharge path of the capacitor C5 is connected between the gate and the ground. Since the resistor R2 in the circuit of FIG. 2 can be omitted when the control circuit 4 is provided, it is omitted in the circuit of FIG. The resistor R10 in the circuit of FIG. 1 is provided to adjust the base current of the transistor Q3,
It is omitted as shown in FIG. 2 depending on the specifications 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 biases the transistor Q2 and the transistor Q3 via the feedback circuit 3 to turn off the transistor Q2 and turn on the transistor Q3. Therefore, the gate potential of the switching transistor Q1 is lowered and the switching transistor Q1 is kept in the ON state, and the transistor commutation element Q5 is kept in 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 according to the voltage across the terminals of the smoothing capacitor C2, and eventually makes the transistor Q2 conductive. Then, the voltage between the gate and the source of the switching transistor Q1 decreases (the gate potential increases), so that the current passing through the drain decreases. Due to this decrease in current, a flyback voltage is generated in the choke coil L1. The flyback voltage generated in the choke coil L1 biases the transistor Q2 and the transistor Q3 via the feedback circuit 3 to turn on the transistor Q2 and turn off the transistor Q3. 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 off and the transistor commutation element Q5 is on, the choke coil L1 and the smoothing capacitor C2 release energy, so that the flyback voltage and the inter-terminal voltage of the choke coil L1 decrease. The voltage across the terminals of the capacitor C3 of the feedback circuit 3 rises. Then, the base current of the transistor Q2 decreases, and eventually the voltage between the collector and the emitter increases. Along with this, the switching transistor Q
Since the gate-source voltage of No. 1 rises (the gate potential decreases), the switching transistor Q1 starts conducting when its magnitude exceeds the threshold voltage. Then, the drain potential of the switching transistor Q1 rises, and this potential rise biases 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, the switching transistor Q1 turns on and the transistor commutation element Q5 turns off.

As can be seen from the above operation, the switching transistor Q1 and the transistor commutation element Q5 operate in a complementary manner, and the transistor commutation element Q5, like the diode D1 of FIG. 2, has the flyback energy of the choke coil L1. Form a distribution channel. Since the on-voltage (V _DSON ) of the main current path of the transistor is smaller than the forward voltage drop (V _F ) of the diode element, theoretically, the generated loss is small and the efficiency of the power supply circuit is improved. become. However, due to various factors, the operation timings of the switching transistor Q1 and the transistor commutation element Q5 may deviate, which may lead to an increase in loss. Therefore, in the circuit of FIG. 1, the diode D3 is connected in parallel with the transistor commutation element Q5, and the current is bypassed only during the period when the operation timing is deviated to prevent an increase in loss.

However, when there is almost no increase in loss due to the shift of the operation timing due to the circuit specifications, the diode D3 is naturally omitted. Note that the resistors R3, R9, and R10 in the circuit of FIG. 1 may be omitted depending on the circuit specifications (particularly, transistor characteristics). In carrying out the present invention, modifications can be made without impairing the operation and function of the invention portion, 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 formed of the FET, and the base of the first driving transistor is connected to the drain of the switching transistor through the feedback circuit. The main current path (collector, emitter) of the second driving transistor is connected in parallel to the resistor connected between the gate and ground,
It has a self-excited oscillating means in which the base of the second driving transistor is connected to a feedback circuit via a capacitor, and further, a transistor commutation element by a MOSFET is connected as a commutation element, and its gate is connected to the first circuit via a capacitor. 2 is connected to the collector of the driving transistor. According to the present invention, the efficiency of the circuit is improved because the loss generated in the commutation element is reduced. The improved efficiency facilitates heat dissipation measures for the power supply circuit, contributing to downsizing of the power supply circuit and cost reduction. Therefore, highly efficient and compact self-excited DC-DC
The 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]

 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 Commutating Element L1 Single Choke Coil R1 Starting Resistor C4 Capacitor C5 Capacitor

Claims (2)

[Claims] 1. The input DC voltage is turned on and off by a switching element and stabilized via a smoothing circuit composed of a commutation element, a choke coil and a smoothing capacitor connected to the current output terminal side of the switching element. In a non-isolated self-exciting DC-DC converter that supplies a DC output voltage to a load, the switching element by a MOS FET, the current input terminal is connected to the current input terminal of the switching element, and the current output terminal is connected to the switching element. A first driving transistor connected to the control terminal; a feedback circuit connected between the current output terminal of the switching element and the control terminal of the first driving transistor; and a current input terminal via a resistance element A first driving transistor connected to a control terminal of a switching element and having a current output terminal connected to ground; A second driving transistor that operates complementarily, comprising a MOS FET having a polarity opposite to that of the switching element, the control terminal of which is connected to the current input terminal of the second driving transistor, Self-excited DC-comprising a transistor commutation element
DC converter. 2. The input DC voltage is turned on and off by a switching element and stabilized via a smoothing circuit composed of a commutation element, a choke coil and a smoothing capacitor connected to the current output terminal side of the switching element. In a non-isolated self-exciting DC-DC converter that supplies a DC output voltage to a load, a control terminal is connected to ground through a resistor, and a MOS
A first driving transistor having a switching element formed of an FET, 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 to the control terminal of the first driving transistor; a second current circuit whose main current path is connected in parallel to the resistor and whose control terminal is connected to the feedback circuit via a capacitive element. A driving transistor, a transistor commutation element as a commutation element, which is composed of a MOS FET having a polarity opposite to that of the switching element, and whose control terminal is connected to the current input terminal of the second driving transistor. Self-excited DC-
DC converter.