JP2767782B2 - 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, use of a transistor instead of a diode as the rectifying element has been considered. As a switching power supply using such a transistor rectifier, the present inventor has proposed a switching power supply as shown in FIG. 3 in Japanese Patent Application No. 4-105807.

Although the detailed description of the configuration and operation of the circuit shown in FIG. 3 is omitted, the operation of the transistor Q2 as a transistor rectifier is as follows. That is,
When the switching transistor Q1 turns off, the current flowing through the path of the input terminal 1, the choke coil L1, the collector and the emitter of the switching transistor Q1 is cut off. As a result, a flyback voltage is generated in the choke coil L1, and the voltage at the collector of the switching transistor Q1 rises to a voltage obtained by adding the flyback voltage to the input voltage _VIN . When a voltage obtained by adding the flyback voltage and the input voltage V _IN is applied to the emitter of the transistor Q2, the emitter and the base of the transistor Q2 are forward-biased and turned on. Transistor Q
2 is turned on, a high voltage, which is the sum of the flyback voltage and the input voltage _VIN described above, is applied to the capacitor C2.
, And supplied to the load RL via the output terminal 2.

Conversely, when the switching transistor Q1 is turned on, the voltage at the collector of the switching transistor Q1 and the voltage at the emitter of the transistor Q2 decrease, and the forward bias between the emitter and the base is released, and the transistor Q2 is turned off. As described above, the transistor Q2 is turned on when the switching transistor Q1 is turned off and the output voltage of the collector increases, and conversely, the transistor Q2 is turned off when the switching transistor Q1 decreases. That is, in the circuit shown in FIG. 3, the output of the switching transistor Q1 is a signal for turning on and off the transistor Q2.

[0005]

By using a transistor instead of a diode as a rectifying element, it is possible to improve the power conversion efficiency of the switching power supply device.
It is necessary to perform off control. Here, if a PNP transistor is used as the transistor rectifier, a special drive circuit is not required to be provided at the base of the transistor rectifier.
The ON / OFF operation can be performed according to the operation of the switching transistor only by securing a current path between the base and the ground. Therefore, the circuit configuration is simple, and a high-efficiency switching power supply device can be realized while suppressing an increase in cost.

However, a PNP type transistor has its P
From the structure of the N-junction, it can be considered that diodes are formed in the direction from the emitter to the base and in the direction from the collector to the base. Therefore, even if the switching transistor is turned on and the voltage at the emitter of the transistor rectifier drops and the transistor rectifier is turned off, a simple circuit configuration that only secures a current path between the base and the ground requires a transistor. When the voltage of the collector of the rectifier becomes higher than that of the base, a current leaks from the collector toward the base. The leakage current causes an increase in the loss of the transistor rectifier, resulting in a decrease in the power conversion efficiency of the switching power supply device even though the power conversion efficiency is higher than that of a switching power supply using a diode as a rectifier. Accordingly, it is an object of the present invention to provide a switching power supply device using a PNP transistor rectifier, in which a loss generated in the transistor rectifier is further reduced to realize a high power conversion efficiency. .

[0007]

According to the present invention, a flyback voltage is generated in an inductance element when a switching element is turned off, a high voltage obtained by combining the flyback voltage and an input voltage is rectified by a rectifying element, and further smoothed. A switching power supply device for obtaining a predetermined DC voltage, comprising a PNP type bipolar transistor, a transistor rectifying element provided between an inductance element and an output terminal and having a rectifying function, and an NPN type bipolar transistor; A drive transistor that is connected between the base of the rectifier element and a low potential point such as the ground, and that is connected between the connection point of the drive transistor rectifier element and the inductance element and the base of the drive transistor that controls the operation of the transistor rectifier element. To the base of the driving transistor A switching power supply device comprising: a capacitor for guiding an off control signal; and a circuit element connected between a base of the driving transistor and a low potential point such as the ground to form a discharge path of the capacitor. is there.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit diagram of a switching power supply according to the present invention which realizes high power conversion efficiency by reducing the loss generated by a transistor rectifier.
Note that the same components in FIG. 1 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. A capacitor C1 and a capacitor C2 are connected between the input terminal 1 and the output terminal 2 and the ground, respectively, and a resistor R for dividing the output voltage is further connected between the output terminal 2 and the ground.
1 and a series circuit of a resistor R2.

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 the transistor Q2 is connected to a resistor R3.
And a capacitor C3 connected in parallel to the collector of a driving transistor Q3 using an NPN transistor, and the emitter of the driving transistor Q3 is connected to ground. The diode D is further connected to the base of the transistor Q2.
2 and the anode of diode D2 to ground. An RC series circuit comprising a resistor R4 and a capacitor C4 is connected between the emitter of the transistor Q2 and the base of the driving transistor Q3, and the base and cathode are connected between the base and the emitter (earth) of the driving transistor Q3. The diode D3 is connected as described above.

The circuit configuration as described above forms a so-called chop-up type switching power supply. This switching power supply functions particularly as a step-up / step-down converter by the action of the capacitor C4. The operation of this circuit will be described below. 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 switching transistor Q
1, 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. 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 applied to the capacitor C2.
2 will be charged and the energy will be supplied to the load RL. When the switching transistor Q1 is turned on, the voltage at the collector of the switching transistor Q1 and the voltage at the emitter of the transistor Q2 decrease, 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 transistor Q2 is turned off.
Is turned off, the positive bias state between the emitter and the base is released, and the emitter is turned off. 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. If the output voltage V _O increases, the control circuit 3 shortens the ON period of the switching transistor Q1, but does not stop the switching operation. 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 DC component voltage from being applied to the base of the driving transistor Q3, and only the pulsating component due to the ON / OFF operation of the switching transistor Q1 is applied to the base of the driving transistor Q3. Will be done. Therefore, while the switching transistor Q1 is in the off state, the driving transistor Q3 controls the flow rate of the base current of the transistor Q2 according to its switching operation (on duty or output voltage).

As a result, the transistor Q2 operates in the non-saturation region, and functions to keep the output voltage V _O constant by an operation similar to the 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. As can be seen from the above description, in the embodiment of the present invention shown in FIG. 1, when the input voltage V _IN is lower than the required output voltage V _O , the operation of the transistor Q2 is controlled by the driving transistor Q3. Thus, generation of a leakage current based on the PN junction structure of the PNP transistor is prevented. As a result, the transistor Q2
(Rectifier element) loss can be reduced,
A step-up switching power supply device with high power conversion efficiency can be provided. In addition, the step-up switching power supply can be operated as a step-down switching power supply by the action of the capacitor C4, so that a step-up / step-down switching power supply that can easily handle a wide input voltage can be provided.

Here, the parallel circuit of the resistor R3 and the capacitor C3 connected to the base of the transistor Q2 performs the function of limiting the base current of the transistor Q2. May be provided not on the collector side of the transistor Q3 but on the emitter side. The diode D2 forms a discharge path for the electric charge accumulated in the base region when the transistor Q2 is turned off. In the circuit shown in FIG. 1, the cathode of the diode D2 is a transistor Q
2 directly connected to the base of the driving transistor Q
The effect is the same even if the collector is connected to the collector of the driving transistor Q3, and the current may be provided so as to form a parallel current path between the collector and the emitter of the driving transistor Q3. However, this diode D2 may be omitted from the circuit in some cases.

FIG. 2 shows another embodiment of the present invention.
In the circuit of FIG. 2, a series circuit of a capacitor C5 and a resistor R6 is provided between the base of the transistor Q2 and the ground instead of the diode D2 in the circuit of FIG. Other circuit configurations are the same as those of the circuit of FIG. As is well known, there is a slight delay in turning on the transistor element. Similarly, since there is a delay time in turning on the driving transistor Q3, the switching transistor Q3
The time difference from when 1 is turned off to when the transistor Q2 is turned on becomes large. When this time difference increases, the switching transistor Q
Since a high voltage, which is the sum of the input voltage V _IN and the flyback voltage, is applied between the collector and the emitter of No. 1, the loss of the switching transistor Q1 and the voltage load increase.

Therefore, in the circuit shown in FIG. 2, a series circuit of a capacitor C5 and a resistor R6 is provided at the base of the transistor Q2. By doing so, when the voltage at the emitter of the transistor Q2 rises, even if the driving transistor Q3 is off, the transistor Q2
Is positively biased. Therefore, after the switching transistor Q1 is turned off, the transistor Q2
Can be reduced, the loss of the switching transistor Q1 can be reduced, and the power conversion efficiency of the switching power supply can be improved. In the embodiment shown in FIG. 2, one end of the series circuit of the capacitor C5 and the resistor R6 is connected to the base of the transistor Q2, but one end may be connected to the collector of the driving transistor Q3.

[0017]

As described above, the switching power supply according to the present invention uses a PNP transistor as a rectifying element, and the transistor rectifying element is connected via a capacitor from a connection point between the transistor rectifying element and the choke coil. By the driving transistor that receives the control signal,
It is characterized by being controlled on and off. Thereby, the loss of the transistor as the rectifying element can be reduced, and a switching power supply with high power conversion efficiency can be obtained. Furthermore, although the circuit configuration is the same as that of the step-up chopper circuit, a highly efficient step-up / step-down switching power supply device can be easily obtained.

[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 switching power supply device proposed in Japanese Patent Application No. 4-105807.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal 3 Control circuit Q1 Switching transistor Q2 Transistor as a rectifying element Q3 Driving transistor

Claims (3)

(57) [Claims] When a switching element is turned off, a flyback voltage is generated in an inductance element, a high voltage obtained by combining the flyback voltage and an input voltage is rectified by a rectifying element, and smoothed to obtain a predetermined DC voltage. A switching power supply device to be obtained, comprising a PNP-type bipolar transistor, provided between the inductance element and an output terminal, and performing a rectifying action, comprising an NPN-type bipolar transistor, and a base of the transistor rectifying element. A driving transistor that is connected between a low-potential point such as the ground and controls the operation of the transistor rectifying element, is connected between a connection point between the transistor rectifying element and the inductance element and a base of the driving transistor, ON and OFF to the base of the driving transistor Capacitor leads control signals and is connected between the low potential point of the base and the ground or the like of the driving transistor, a switching power supply apparatus characterized by comprising a circuit element which forms a discharge path for the capacitor. 2. The switching power supply device according to claim 1, wherein the circuit element is a diode element having a forward direction from a low potential point such as ground to a base of the driving transistor. 3. The method according to claim 1, further comprising:
3. The switching power supply device according to claim 1, wherein a current path is provided in parallel with a main current path of the driving transistor by a series circuit of a resistor and a capacitor or a capacitor.