JPH08322244A - Switching power unit - Google Patents

Abstract

PURPOSE: To provide a switching power unit which can reduce the loss which occurs in a PNP transistor working as a synchronous rectifying element and, especially, can obtain high efficiency even when the switching frequency is raised. CONSTITUTION: One end (point a) of a choke coil L1, the other end of which is connected to an input terminal, is connected to the emitter of a PNP transistor Q2 working as a rectifying element. The base of the transistor Q2 is connected to the collector of a transistor Q3 for drive through a resistor and further connected to one end of the coil L1 on the input terminal 1 side through an auxiliary bias circuit 4 composed of a capacitor C4 and resistor R4. Therefore, the operating speed of the transistor Q2 working as a rectifying element is remarkably improved and the occurrence of losses is reduced. Even when the switching frequency rises, high efficiency is obtained and the increase of the number of parts used in a circuit element can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous rectification type switching power supply device in which a transistor is used as a rectifying element instead of a diode, and the turn-off operation of the switching transistor is promoted to reduce loss.

[0002]

2. Description of the Related Art Since the saturation voltage between the collector and emitter of a transistor in the ON state is smaller than the forward voltage drop of the diode, the loss that occurs when a current is applied to each element is greater in the transistor than in the diode. It is known that there are few. Therefore, as one means for improving the power conversion efficiency of the switching power supply device, it has been considered to use a transistor instead of a diode for the rectifying element provided in the circuit.
As an example of a switching power supply device that uses such a synchronous rectifying element using a transistor, the present inventor has proposed a switching power supply device as shown in FIG. 2 in Japanese Patent Application No. 5-307520.

Although a detailed description of the configuration and operation of the circuit shown in FIG. 2 is omitted, this switching power supply device realizes high power conversion efficiency by using a PNP type transistor Q2 in place of a diode as a rectifying element. Has been done.
In addition, this switching power supply device includes a transistor Q
By controlling the operation of No. 2 by the NPN type driving transistor Q3 whose base receives the control signal via the capacitor C3, the current leaking from the collector generated toward the base or the emitter due to the structure of the PNP type transistor is controlled. At the same time, the switching power supply device can be operated as a step-up / down converter.

[0004]

By using a transistor instead of a diode for the rectifying element, the power conversion efficiency of the switching power supply device can be improved. However, unlike the diode, the transistor element needs to be controlled to be turned on and off, and as is well known, the operation of the transistor element has a delay time due to the accumulation or disappearance of charges in its base region. Most of the loss that occurs when the transistor of the synchronous rectifying device is on and off is due to the voltage that appears between the collector and emitter of the transistor element within the delay time of this operation and the current that flows there. Of course, the longer the delay time, the larger the loss.

In the switching power supply device shown in FIG. 2, in order to reduce the loss of the transistor Q2 as a synchronous rectifying element, a resistor R5 for setting a base current is connected in parallel with a speed-up capacitor C5, and a transistor C2 is further connected. A capacitor C is connected between the base of Q2 and ground.
A series circuit of 6 and the resistor R6 is provided. Here, the series circuit of the capacitor C6 and the resistor R6 draws in the base current of the transistor Q2 when the transistor Q2 is turned on and improves the speed of the turn-on operation, and when the transistor Q2 is turned off, the base region of the transistor Q2 is removed. It functions to form a discharge path for the accumulated charge, promote the disappearance of the accumulated charge, and improve the speed of the turn-off operation. Capacitor C5 for speed up
Since the function of is a general technique, its description is omitted.

However, the series circuit of the capacitor C5 and the capacitor C6 and the resistor R6 passively performs the above-mentioned function with respect to the operation of the transistor Q2 and the base current flowing as a result, and the operation delay of the transistor Q2 is delayed. There was a limit to the reduction of time. Therefore, in the switching power supply device as shown in FIG. 2, when the switching frequency of the switching transistor Q1 is increased, the loss generated in the rectifying means (transistor Q2) increases, which in turn has the effect of reducing the loss. As a result, it was not done. Therefore, according to the present invention, in a switching power supply device using a PNP type transistor as a rectifying element, by rapidly accumulating or eliminating charges in the base region of the transistor as the rectifying element, even when the switching frequency is increased to a high frequency. An object of the present invention is to provide a switching power supply device that can obtain high power conversion efficiency.

[0007]

According to the present invention, a flyback voltage is generated in a choke coil when a switching element is turned off, and a voltage in which the flyback voltage is superposed on an input voltage is rectified through a rectifying element to obtain a predetermined value. In the switching power supply device that obtains the voltage of, a PNP type bipolar transistor is provided between the inductance element and the output terminal, and a transistor rectifying element for performing a rectifying action, and an NPN type bipolar transistor are provided. A drive transistor, which is connected between the base and a low potential point such as ground, and which receives an on / off control signal from the connection point of the transistor rectifying element and the inductance element to control the operation of the transistor rectifying element, a transistor rectifying element Connected to the base of the switching element Is a switching power supply apparatus characterized by comprising an auxiliary biasing circuit for guiding the bias voltage applied to the transistor rectifying element according to work.

[0008]

When the switching transistor Q1 is turned off, a flyback voltage is generated in the choke coil L1 and the voltage at point a rises to the voltage value obtained by adding the flyback voltage to the input voltage V _IN . The driving transistor Q3 is turned on by the increased voltage at the point a, and the current path of the base current of the transistor Q2 is opened. The flyback voltage generated in the choke coil L1 is guided by the auxiliary bias circuit 4 to apply a forward bias between the base and emitter of the transistor Q2. Due to this forward bias, more base current flows and the transistor Q2 turns on quickly.

On the contrary, when the switching transistor Q1 is turned on, the voltage at the point a drops to the ground potential,
The driving transistor Q3 is turned off to cut off the current path of the base current of the transistor Q2. A voltage is induced in the choke coil L1 in the direction from the point a to the input terminal 1 by the self-induction action, and the induced voltage is guided by the auxiliary bias circuit 4 to apply a reverse bias between the base and emitter of the transistor Q2. . Due to this reverse bias, the electric charge in the base region of the transistor Q2 accumulated when it is in the ON state is forcibly removed, and the transistor Q2 is quickly turned off.

[0010]

1 is a circuit diagram of an embodiment of a switching power supply device according to the present invention, which can reduce loss generated in a transistor as a synchronous rectifying element and can obtain high power conversion efficiency even when a switching frequency is in a high frequency region. It was shown to. The same reference numerals are given to the same constituent elements shown in FIG. 1 as those shown in FIG. In FIG. 1, the switching power supply device of the present invention has the following configuration.

One end of the choke coil L1 is connected to the input terminal 1, and the other end (point a) of the choke coil L1 is connected to the collector of the NPN type switching transistor Q1. The emitter of the switching transistor Q1 is connected to the ground, and the base is connected to the pulse output terminal PO of the PWM control circuit 3. The other end of the choke coil L1 (point a)
Is further connected to the emitter of a PNP type transistor Q2 as a synchronous rectifying element. The collector of the transistor Q2 is connected to the output terminal 2, and the smoothing capacitor C2 is connected between the output terminal 2 and the ground. Further, a series circuit of resistors R1 and R2 is connected between the output terminal 2 and ground, and the connection point of the resistors R1 and R2 is connected to the output voltage detection terminal FB of the PWM control circuit.

The base of the transistor Q2 is connected to the collector of an NPN type driving transistor Q3 via a resistor R5, and the base of the transistor Q2 is further connected via a series circuit of a resistor R4 and a capacitor C4 to an input terminal 1 of a choke coil L1. Connect to one end. The auxiliary bias circuit 4 is formed by the series circuit of the resistor R4 and the capacitor C4. The emitter of the driving transistor Q3 is connected to the ground, the diode D1 is connected between the base of the driving transistor Q3 and the ground, and the resistor R3 and the capacitor C3 are connected between the base and the other end (point a) of the choke coil L1. Connect a series circuit. In addition, in FIG. 1, C1 is a capacitor and may be removed from the circuit. E is an external DC power supply, and RL is a load. The operation of the synchronous rectifying element (transistor Q2) in the switching power supply device having such a circuit configuration is as follows.

When the switching transistor Q1 is in the ON state, a current flows through the path of the input terminal 1, the choke coil L1 and the switching transistor Q1, and energy is stored in the choke coil L1. Here, the voltage at point a has dropped to the ground potential, and the voltage at point a is
The driving transistor Q3 received by the base through the series circuit of the capacitor 3 and the capacitor C3 is turned off. Since the driving transistor Q3 is off, the current path of the base current of the transistor Q2 is cut off.
At this time, a voltage is induced in the choke coil L1 from the point a toward the input terminal 1 by the self-induction action due to the current flow, and this induced voltage is guided by the auxiliary bias circuit 4 including the resistor R4 and the capacitor C4. , A reverse bias is applied between the base and emitter of the transistor Q2. Therefore, at this time, the transistor Q2 is turned off.

When the switching transistor Q1 is turned off, a flyback voltage is generated in the choke coil L1 from the input terminal 1 in the direction of the point a due to the energy accumulated up to that point. As a result, the voltage at the point a rises to a voltage value obtained by adding the flyback voltage to the input voltage V _IN . The voltage at the point a is applied to the base of the driving transistor Q3 via the series circuit of the resistor R3 and the capacitor C3, and the driving transistor Q3 is turned on. When the driving transistor Q3 is turned on, the current path of the base current of the transistor Q2 is opened. At the same time, the flyback voltage generated in the choke coil L1 is further guided by the auxiliary bias circuit 4 to apply a forward bias between the base and emitter of the transistor Q2. Due to the opening of the current path by the driving transistor Q3 and the forward bias of the flyback voltage guided by the auxiliary bias circuit 4, the transistor Q2 is turned on. In this case, in particular, the transistor Q2 receives the forward bias.
The base current is increased and the turn-on operation speed is improved.

When the switching transistor Q1 is in the off state, the driving transistor Q3 is kept in the on state by the voltage at the point a which is the input voltage V _IN plus the flyback voltage. Since the driving transistor Q3 is on, the current path of the base current of the transistor Q2 is open. The flyback voltage of the choke coil L1 guided by the auxiliary bias circuit 4 causes the transistor Q2 to receive a forward bias between the base and the emitter. Therefore, at this time, the transistor Q2 is turned on.

Next, when the switching transistor Q1 is turned on, the voltage at the point a drops to the ground potential. Due to the voltage drop at point a, drive transistor Q3
Turns off, and the current path of the base current of the transistor Q2 is cut off. At the same time, a current flows in the path of the input terminal 1, the choke coil L1, and the switching transistor Q1, and a voltage is induced in the choke coil L1 from the point a toward the input terminal 1 by the self-induction effect. This induced voltage is guided by the auxiliary bias circuit 4 and applies a reverse bias between the base and emitter of the transistor Q2. Due to the interruption of the current path by the driving transistor Q3 and the reverse bias of the induced voltage guided by the auxiliary bias circuit 4, the transistor Q2 is turned off. In this case, in particular, by receiving a reverse bias, the accumulated charge in the base region of the transistor Q2 is forcibly removed, and the turn-off operation speed is improved.

When the input voltage V _IN is lower than the output voltage V _O , the on-duty of the switching transistor Q1 becomes large, and as described above, the driving transistor Q3 and the transistor Q2 match the switching operation of the switching transistor Q1. Turns on and off. As a result, the transistor Q2 operates as a synchronous rectifying element. On the other hand, when the input voltage V _IN is higher than the output voltage V _O , the on-duty of the switching transistor Q1 becomes very small, and when the switching transistor Q1 is in the off state, the driving transistor Q3.
Also, the transistor Q2 is not turned on completely, but is turned on in the non-saturated region. Then, the transistor Q2 changes the voltage between the collector and the emitter according to the on-duty of the switching transistor Q1, and the output voltage V
_O will be controlled to be constant. This causes the transistor Q2 to operate like a series regulator. Therefore, the switching power supply device of the present invention shown in FIG. 1 functions as a step-up / down converter.

In the switching power supply device of the present invention shown in FIG. 1, the transistor Q is provided by providing the auxiliary bias circuit 4.
The operating speed of 2 is improved. Therefore, even if the switching frequency is increased, it is possible to suppress an increase in loss generated in the transistor Q2. By the way, when a comparative experiment of the circuits shown in FIG. 1 and FIG. 2 was conducted on a circuit board manufactured experimentally, the circuit of the present invention shown in FIG. 1 had a switching frequency of about 160 as compared with the circuit shown in FIG. It has been confirmed that the power conversion efficiency of the circuit is improved by about 4% at [kHz] and about 30% at the switching frequency of about 320 [kHz]. Further, in the switching power supply device of the present invention shown in FIG. 1, the provision of the auxiliary bias circuit 4 eliminates the need for the speed-up capacitor C5 and the series circuit of the resistor R6 and the capacitor C6 in the circuit shown in FIG. It also contributes to preventing an increase in the number of parts of the circuit element.

In the circuit of the embodiment of the present invention shown in FIG. 1, one end of the auxiliary bias circuit 4 is connected to the base of the transistor Q2 and the other end is connected to one end of the choke coil L1 on the input terminal 1 side. But choke coil L1
Alternatively, a coil component having a tap may be used, and the other end of the auxiliary bias circuit 4 may be connected to the tap. This matter is also applied to the case where the choke coil L1 is composed of a plurality of coil components connected to each other. Further, in the embodiment shown in FIG. 1, the auxiliary bias circuit 4 includes a resistor R4 and a capacitor C.
Although it is configured by the series circuit of No. 4, it may be configured by only a capacitor, for example, and is not limited to this configuration.

[0020]

According to the present invention, in a switching power supply device using a PNP type transistor as a rectifying element instead of a diode, an auxiliary bias circuit is provided in addition to a driving transistor for controlling the operation of the transistor as the rectifying element. The bias voltage (specifically, the voltage appearing in the choke coil) guided by the auxiliary bias circuit is applied to the transistor as the rectifying element. As a result, the operating speed of the transistor as the rectifying element is remarkably improved, the generated loss is reduced, and the power conversion efficiency of the switching power supply device is improved. This effect is particularly remarkable when the switching frequency of the switching element is increased. In addition, the passive means (speed-up capacitor, etc.) for improving the operation speed of the transistor as the rectifying element, which has been conventionally provided, is basically unnecessary, and the increase in the number of parts of the circuit element can be prevented. .

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of a switching power supply device proposed in Japanese Patent Application No. 5-307520 (FIG. 3).

[Explanation of symbols] 1 input terminal 2 output terminal 3 PWM control circuit 4 auxiliary bias circuit Q1 switching transistor Q2 transistor as a synchronous rectifying element Q3 driving transistor L1 choke coil C2 smoothing capacitor E external power supply RL load

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[Procedure amendment]

[Submission date] September 29, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

[0007]

According to the present invention, a flyback voltage is generated in an inductance element when a switching element is turned off, and a voltage in which the flyback voltage and an input voltage are superposed is rectified via a rectifying element. In a switching power supply device that obtains a predetermined voltage, a PNP
Type bipolar transistor, is provided between the inductance element and the output terminal, and is a transistor rectifying element that performs a rectifying function, and is an NPN type bipolar transistor, and is between the base of the transistor rectifying element and a low potential point such as ground. Is connected to the base of the transistor rectifier and the driving transistor that controls the operation of the transistor rectifier by receiving the on / off control signal from the connection point of the transistor rectifier and the inductance element, and the operation of the switching element. It is characterized by comprising an auxiliary bias circuit for guiding a bias voltage applied to the transistor rectifying element according to the above. The auxiliary bias circuit is preferably a series circuit of a resistance element and a capacitance element connected between one end of the inductance element and the base of the transistor rectifying element, and the voltage appearing at the inductance element via this auxiliary bias circuit is The bias voltage is supplied to the transistor rectifying element.

Claims (3)

[Claims] 1. A switching power supply device for generating a flyback voltage in a choke coil when a switching element is turned off, rectifying a voltage superposed on an input voltage by the rectifier element to obtain a predetermined voltage. , A PNP-type bipolar transistor, which is provided between the inductance element and the output terminal and has a rectifying function, and a NPN-type bipolar transistor, which has a low potential such as a base of the transistor rectifying element and ground A driving transistor connected between the transistor rectifying element and the inductance element to control the operation of the transistor rectifying element by receiving an on / off control signal from the connection point of the transistor rectifying element and the base of the transistor rectifying element. Is connected to the switching element according to the operation of the switching element. A switching power supply device comprising an auxiliary bias circuit for guiding a bias voltage applied to a transistor rectifying element. 2. The bias voltage introduced to the base of the transistor rectifying element by the auxiliary bias circuit is:
The switching power supply device according to claim 1, wherein the switching power supply device is a voltage that appears in an inductance element. 3. The switching power supply device, wherein the auxiliary bias circuit comprises a series circuit of a resistance element and a capacitance element connected between the base of the transistor rectifying element and one end of the inductance element.