JP2767783B2 - 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 switching power supply device in which loss during conduction of a rectifying element is reduced to improve power conversion efficiency.

[0002]

2. Description of the Related Art Diodes are exclusively used as rectifiers for converting alternating currents and voltages into direct currents. However, when rectification is performed by diodes, the forward voltage V _F existing in the diodes and the current are reduced. It must be kept in mind that a loss corresponding to the product of. If the purpose of which minimize the loss of the rectifying element hand, the collector of the transistor in the on state, the saturation voltage between the emitter is smaller than the forward voltage V _F of the diode, little loss generated It is known that it is necessary. Therefore, as one means for improving the power conversion efficiency of the switching power supply device, it is conceivable to use a transistor instead of a diode for the rectifying element. FIG. 3 shows an example of a circuit of a conventional switching power supply that realizes high power change efficiency by using this transistor rectifier.

In the circuit shown in FIG. 3, a chop-up DC-DC converter is formed by a choke coil L1, a switching transistor Q1, a PNP transistor Q2 as a transistor rectifier, and a smoothing capacitor C2. Switching transistor Q
A control circuit 3 is provided at the base of the control circuit 1 for outputting a control signal whose duty in the on and off periods changes in accordance with the detected value of the output voltage divided by the resistors R1 and R2. Transistor Q2 as transistor rectifier
Has a base current limiting resistor R3 and a capacitor C
The collector of the driving transistor Q3 is connected via the three parallel circuits. The base of the driving transistor Q3 is connected to the collector of the switching transistor Q1 via a series circuit of a resistor R4 and a capacitor C4 to obtain an ON / OFF signal of the switching transistor Q1. Diode D1 forms a current path for discharging the accumulated charge in the base region when transistor Q2 is turned off, and diode D2 forms a discharge path for capacitor C4.

In the circuit having the above configuration, first, the operation of the circuit when the input voltage V _IN is lower than the required output voltage V _O is as follows. When the switching transistor Q1 is turned off by a signal from the control circuit 3, a flyback voltage is generated in the choke coil L1. This flyback voltage is superimposed on the input voltage V _IN , and a high voltage resulting from the superimposition is applied to the collector of the switching transistor Q1, the emitter of the transistor Q2, and the base of the driving transistor Q3 via the series circuit of the resistor R4 and the capacitor C4. Will join. As a result, the driving transistor Q3 receives a positive bias between the base and the emitter, and turns on.

When the driving transistor Q3 is turned on, the transistor Q2 is also turned on with a positive bias between the emitter and the base, and a high voltage, which is the sum of the input voltage V _IN and the flyback voltage, is supplied to the load RL. It will be. Eventually switching transistor Q
1 is turned on, the switching transistor Q1
And the voltage at the emitter of the transistor Q2 decreases, and at the same time, the voltage at the base of the driving transistor Q3 also decreases. As a result, the driving transistor Q3 is turned off, and the positive bias state between the emitter and the base is released and the transistor Q2 is turned off by the turning off of the driving transistor Q3. Thus, when the input voltage V _IN is lower than the required output voltage V _O , it functions as a boost converter.

The operation when the input voltage V _IN is higher than the required output voltage V _O is as follows. When the output voltage V _O increases, the control circuit 3 shortens the ON period of the switching transistor Q1 by the output voltage stabilizing function. Therefore, when the input voltage V _IN is higher than the output voltage V _O , the switching transistor Q1 continues the switching operation whose ON period is very short. Here, the presence of the capacitor C4 prevents the voltage of the DC component from being applied to the base of the driving transistor Q3, and only the voltage of the pulsating component due to the ON / OFF operation of the switching transistor Q1 is applied to the driving transistor Q3. Will be applied to the base. Therefore, the driving transistor Q3 controls the base current of the transistor Q2 according to the switching operation of the switching transistor Q1. As a result, the transistor Q2 performs an operation similar to a series regulator. Therefore, when the input voltage V _IN is higher than the required output voltage V _O , it functions as a step-down converter, and as a whole, it becomes a step-up / step-down switching power supply.

[0007]

The conventional switching power supply shown in FIG. 3 improves power conversion efficiency by using a transistor instead of a diode as a rectifying element, and at the same time, as a buck-boost type switching regulator. I have. Here, in order to obtain a step-up / step-down switching regulator using this transistor rectifier, when the input voltage V _IN becomes higher than the output voltage V _O , the operation of the driving transistor Q3 and the transistor Q2 is performed in an unsaturated region. It is necessary to make the operation of the transistor Q2 like a series regulator.

However, if the operation of the driving transistor Q3 is performed in the saturation region when the input voltage V _IN is higher than the output voltage V _O , the transistor Q2 is turned on and off in the saturation region as a normal rectifier. Will be done.
Then, a high voltage obtained by superimposing the flyback voltage generated in the choke coil L1 on the input voltage V _IN higher than the output voltage V _O charges the smoothing capacitor C2 via the transistor Q2, and is supplied to the load RL. Therefore, the output voltage V _O increases, and eventually exceeds the voltage control range of the control circuit 3. Then, the control circuit 3, which detects the output voltage V _O with the resistors R1 and R2, stops outputting the on / off control signal to the switching transistor Q1. Then, energy is consumed by the load RL, the output voltage V _O returns to a normal value, and the control signal is not output from the control circuit 3 to the switching transistor Q1 until the overvoltage protection function is released.

When the output voltage V _O returns to a normal value and the control circuit 3 outputs a control signal, the switching transistor Q1 is turned on and off again. However, although the driving transistor Q3 operates by the operation of the switching transistor Q1, the operation is performed in a saturation region. Therefore, the transistor Q2 is turned on and off in a saturation region as a normal rectifying element. And again the output voltage V
_O goes high, and the control circuit 3 repeatedly stops the control signal. As described above, when the input voltage V _IN is higher than the output voltage V _O , the driving transistor Q3
If the operation is not performed in the unsaturated region, the control circuit 3 alternately repeats the operation period and the stop period, and this switching power supply device enters an intermittent operation state,
An undesired phenomenon such as an increase in the ripple of the output voltage V _O occurs.

In the conventional circuit shown in FIG. 3, the driving transistor Q3 and the transistor Q2 do not operate in the unsaturated region depending on the characteristics of the transistor element and the constants of other elements constituting the circuit. When the voltage V _IN exceeds the output voltage V _O , an intermittent operation may occur. Further, the on-pulse period for turning on the switching transistor Q1 by the control signal output from the control circuit 3 does not become narrower than a certain period, so that when the input voltage V _IN increases, the input voltage to the base of the driving transistor Q3 is increased. The resulting current increases. Therefore, if the input voltage V _IN rises to a certain level, the device will be in an intermittent operation state, and the conventional circuit shown in FIG. 3 has a problem that the range of the input voltage V _{IN in} which normal operation can be maintained cannot be widened. Was. Therefore, the present invention provides an input voltage V
It is an object of the present invention to provide a switching power supply device in which when the _IN is higher than the output voltage V _O , the operation of the driving transistor Q3 is reliably performed in a non-saturation region, thereby preventing an intermittent operation.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a switching power supply for generating a desired DC power by generating a flyback voltage in an inductance element when a switching element is turned off, and rectifying and smoothing energy by the flyback voltage. , A transistor rectifying element using a PNP type bipolar transistor is used as a rectifying element, and a driving transistor using an NPN type bipolar transistor for operating the transistor rectifying element via a base current limiting resistor is connected to a base of the transistor rectifying element. And the collector of the driving transistor,
A switching power supply device comprising: a feedback circuit provided between bases; and an on / off control signal supplied to a base of a driving transistor from a connection point between the transistor rectifying element and the inductance element via a capacitive element. It is.

[0012]

FIG. 1 shows a circuit of a switching power supply according to the present invention in which the occurrence of an intermittent operation state is suppressed. The circuit configuration of the switching power supply of FIG. 1 is as follows. In FIG. 1, the same components as those in FIG. 3 are denoted by the same reference numerals. In FIG. 1, reference numerals 1 and 2 denote an input terminal and an output terminal on the high potential side, and the input and output terminals on the low potential side are common to the ground. The input terminal 1 is connected to the collector of an NPN-type switching transistor Q1 via a choke coil L1, and the emitter of the switching transistor Q1 is connected to the ground. The collector of the switching transistor Q1 is connected to the emitter of a PNP transistor Q2 as a transistor rectifier, and the collector of the transistor Q2 is connected to the output terminal 2. Connect capacitor C1 between input terminal 1 and ground,
A smoothing capacitor C2 is connected between the output terminal 2 and the ground, and a resistor R1 and a resistor R2 for dividing the output voltage.
Connected in series.

The connection point between the resistor R1 and the resistor R2 is determined by the control circuit 3
And the pulse output terminal (PO) of the control circuit 3 is connected to the base of the switching transistor Q1. The base of transistor Q2 is connected to N through a parallel circuit of base current limiting resistor R3 and capacitor C3.
Connected to the collector of the PN type driving transistor Q3,
The emitter of the driving transistor Q3 is connected to the ground. The base of the transistor Q2 is further connected to a diode D1.
And the anode of the diode D1 is connected to ground. The base of the driving transistor Q3 is connected to the collector of the switching transistor Q1 via a series circuit of a capacitor C4 and a resistor R4. A diode D2 is connected between the base and the emitter of the driving transistor Q3 so as to connect the base and the cathode, and a series circuit of a resistor R6 and a capacitor C6 is connected between the collector and the base of the driving transistor Q3. The resistor R6 and the capacitor C6 form the feedback circuit 4 of the driving transistor Q3.
To form

In the circuit having the above configuration, the operation when the input voltage V _IN is lower than the required output voltage V _O is as follows.
This is the same as the conventional circuit shown in FIG. 3, and the description thereof is omitted here. The basic operation of the circuit when the input voltage V _IN is higher than the required output voltage V _O is the same as that of the conventional circuit shown in FIG. 3, but differs in the following points. In other words, the feedback circuit 4 existing between the collector and the base of the driving transistor Q3 is a negative feedback, which lowers the gain of the driving transistor Q3 and facilitates the operation in the unsaturated region. The feedback circuit 4 works effectively when the on-period of the switching transistor Q1 is short due to the action of the capacitor C6 as a component, and operates the driving transistor Q3 in the non-saturation region, but the on-period of the switching transistor Q1 is long. In this case, the operation in the saturation region of the driving transistor Q3 is not significantly affected. Therefore, only when the input voltage V _IN is higher than the output voltage V _{O and} the ON period of the switching transistor Q1 is short, the driving transistor Q3 operates in the unsaturated region by the feedback circuit 4.

If the driving transistor Q3 operates in an unsaturated region, the base current of the transistor Q2 is limited by the resistance between the collector and the emitter of the driving transistor Q3, and the transistor Q2 is turned on as a rectifying element. , Cannot perform the off operation, and becomes a series regulator operation. Accordingly, the collector of the driving transistors Q3, the circuit shown in FIG. 1, the feedback circuit 4 is provided between the base, compared to the conventional circuit shown in FIG. 3, when the input voltage V _IN is higher than the output voltage V _O The operation of the driving transistor Q3 in the unsaturated region can be ensured, and the intermittent operation state can be prevented. Also, in the circuit shown in FIG. 1, the current flowing to the base of the driving transistor Q3 increases with an increase in the input voltage V _IN , and as in the conventional circuit shown in FIG. 3 because the gain of the driving transistor Q3 is reduced by the feedback circuit 4.
Is less likely to be in an intermittent operation state than the conventional circuit shown in FIG. 1 and a stable operation can be performed for a wide range of input voltage V _IN .

FIG. 2 shows a switching power supply device in which a series circuit of a capacitor C5 and a resistor R5 is connected between the base of the transistor Q2 and the ground instead of the diode D1 in FIG. This is an embodiment to which the present invention is applied. 1 and 2 except that a series circuit of a capacitor C5 and a resistor R5 is connected between the transistor Q2 and the ground instead of the diode D1, and the operation is basically the same. In the embodiment of the present invention shown in FIGS. 1 and 2, the feedback circuit has been described as a series circuit of a resistor and a capacitor. However, depending on various characteristics of a transistor and constants of other circuit components, only a capacitor may be used. A feedback circuit may be configured.

[0017]

As described above, the switching power supply according to the present invention uses a PNP-type transistor as a rectifier, and the transistor rectifier receives a control signal from the collector of the switching transistor. On and off are controlled by an NPN-type driving transistor having a feedback circuit between bases. Due to the presence of this feedback circuit, the gain of the driving transistor decreases, and when the input voltage is higher than the output voltage, the operation of the driving transistor and the transistor as the rectifying element can be reliably performed in the unsaturated region. it can. Therefore, it is possible to prevent the switching power supply from intermittently operating, and to obtain a switching power supply that operates stably with respect to a wide range of input voltage.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of another embodiment of the switching power supply device according to the present invention.

FIG. 3 is a circuit diagram of a conventional switching power supply device.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 input terminal 2 output terminal 3 control circuit 4 feedback circuit Q1 switching transistor Q2 transistor as rectifying element Q3 driving transistor R3 base current limiting resistor

Claims (2)

(57) [Claims] When a switching element is turned off, a flyback voltage is generated in an inductance element, and energy obtained by the flyback voltage is rectified and smoothed to obtain a desired DC power. Using a transistor rectifier element by a bipolar transistor, connecting a drive transistor by an NPN type bipolar transistor for operating the transistor rectifier element via a base current limiting resistor to a base of the transistor rectifier element, A feedback circuit is provided between the collector and the base, and an on / off control signal is supplied to the base of the driving transistor from a connection point between the transistor rectifying element and the inductance element via a capacitive element. To Switching power supply. 2. The switching power supply according to claim 1, wherein the feedback circuit comprises a capacitor or a series circuit of a resistor and a capacitor.