JP2835297B2 - 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 synchronous rectification type switching power supply using a transistor instead of a diode as a rectifying element, which promotes a turn-off operation of a switching transistor and reduces a loss.

[0002]

2. Description of the Related Art Since the saturation voltage between the collector and the emitter of an on-state transistor is smaller than the forward voltage drop of a diode, the loss that occurs when a current flows through each element is smaller than that of a transistor. Is known to be small. 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 using such a synchronous rectifier 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 the detailed description of the configuration and operation of the circuit shown in FIG. 2 is omitted, this switching power supply achieves high power conversion efficiency by using a PNP transistor Q2 instead of a diode as a rectifying element. Have been.
Further, this switching power supply device includes a transistor Q
2 is controlled by an NPN-type driving transistor Q3, which receives a control signal via a capacitor C3 at the base, so that a current leaking from the collector to the base or the emitter generated in the structure of the PNP-type transistor is reduced. At the same time, the switching power supply can be operated as a buck-boost converter.

[0004]

By using a transistor instead of a diode as a rectifying element, the power conversion efficiency of a switching power supply can be improved. However, unlike a diode, a transistor element needs to perform on / off control. As is well known, the operation of the transistor element has a delay time due to accumulation or disappearance of charge in its base region. Most of the loss that occurs when the transistor of the synchronous rectifier is turning on and off is caused by the voltage that appears between the collector and emitter of the transistor within the delay time of this operation and the current that flows through it. Yes, of course, the longer the delay time, the greater the loss.

In the switching power supply shown in FIG. 2, a speed-up capacitor C5 is connected in parallel to a resistor R5 for setting a base current in order to reduce the loss of a transistor Q2 as a synchronous rectifying element. Capacitor C between the base of Q2 and ground
6 and a resistor R6 in series. 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 to improve the speed of the turn-on operation, and when the transistor Q2 is turned off, the base circuit of the transistor Q2 is turned off. It functions to form a discharge path for the stored charge, promote the disappearance of the stored charge, and improve the speed of the turn-off operation. Capacitor C5 for speed up
Is a general technique, and the description thereof will be omitted.

However, the series circuit of the capacitor C5 and the capacitor C6 and the resistor R6 passively performs the above-described function with respect to the operation of the transistor Q2 and the base current flowing as a result. There was a limit to the time reduction. 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 rectifier (transistor Q2) increases, and the effect of reducing the loss is considered. Was not done as such. Therefore, the present invention provides a switching power supply device using a PNP transistor as a rectifying element, whereby the accumulation or disappearance of charges in the base region of the transistor as a rectifying element is promptly performed, so that the switching frequency can be increased. It is an object of the present invention to provide a switching power supply capable of obtaining 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, and a voltage in which the flyback voltage and an input voltage are superimposed is rectified through a rectifying element. , A switching power supply for obtaining a predetermined voltage, a PNP
Transistor rectifier element, which is provided between the inductance element and the output terminal and is provided between the inductance element and the output terminal, and which is composed of an NPN type bipolar transistor, between the base of the transistor rectifier element and a low potential point such as ground. And a driving transistor for controlling the operation of the transistor rectifying element by receiving an on / off control signal from a connection point between the transistor rectifying element and the inductance element; and a driving transistor connected to the base of the transistor rectifying element and operating the switching element. And an auxiliary bias circuit that guides a bias voltage to be applied to the transistor rectifier 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 a voltage appearing in the inductance element through the auxiliary bias circuit is The bias voltage is supplied to the transistor rectifier.

[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 a value obtained by adding the flyback voltage to the input voltage _VIN . The drive transistor Q3 is turned on by the increased voltage at 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, and applies a forward bias between the base and the emitter of the transistor Q2. This forward bias causes more base current to flow and transistor Q2 to turn on quickly.

Conversely, when the switching transistor Q1 is turned on, the voltage at 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. Further, a voltage is induced in the choke coil L1 from the point a toward the input terminal 1 by a self-induction action, and the induced voltage is guided by the auxiliary bias circuit 4 to apply a reverse bias between the base and the emitter of the transistor Q2. . Due to this reverse bias, the electric charge in the base region of the transistor Q2 accumulated when the transistor Q2 is on is forcibly removed, and the transistor Q2 is quickly turned off.

[0010]

FIG. 1 is a circuit diagram of an embodiment of a switching power supply according to the present invention, which can reduce a loss occurring in a transistor as a synchronous rectifying element and can obtain a high power conversion efficiency even when a switching frequency is in a high frequency range. It was shown to. The same components as those shown in FIG. 2 in FIG. 1 are denoted by the same reference numerals. In FIG. 1, the switching power supply 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 choke coil L1 (point a)
Is further connected to the emitter of a PNP transistor Q2 as a synchronous rectifier. The collector of the transistor Q2 is connected to the output terminal 2, and a smoothing capacitor C2 is connected between the output terminal 2 and the ground. Further, a series circuit of the resistors R1 and R2 is connected between the output terminal 2 and the ground, and the connection point between 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 connected to the input terminal 1 of a choke coil L1 via a series circuit of a resistor R4 and a capacitor C4. To one end on the side. 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 FIG. 1, C1 is a capacitor, and may be excluded from the circuit in some cases. E indicates an external DC power supply, and RL indicates a load. The operation of the synchronous rectifier (transistor Q2) in the switching power supply having such a circuit configuration is as follows.

When the switching transistor Q1 is on, 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 at the base via the series circuit of the capacitor 3 and the capacitor C3 is turned off. Since the driving transistor Q3 is turned 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 in the direction from the point a to the input terminal 1 by the self-induction action due to the flow of current, 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 the 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 in the direction from the input terminal 1 to the point a by the energy stored so far. As a result, the voltage at 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 a 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, and applies a forward bias between the base and the emitter of the transistor Q2. The transistor Q2 is turned on by 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. In this case, in particular, the transistor Q2
, The turn-on operation speed is improved.

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

Next, when the switching transistor Q1 is turned on, the voltage at point a drops to the ground potential. When the voltage at the point a decreases, the driving transistor Q3
Shifts to the off state, and the current path of the base current of the transistor Q2 is cut off. At the same time, a current flows through 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 a self-induction action. This induced voltage is guided by the auxiliary bias circuit 4, and applies a reverse bias between the base and the emitter of the transistor Q2. The cutoff of the current path by the driving transistor Q3 and the reverse bias of the induced voltage induced by the auxiliary bias circuit 4 turn off the transistor Q2. In this case, particularly, by receiving the reverse bias, the accumulated charge in the base region of the transistor Q2 is forcibly eliminated, and the speed of the turn-off operation is improved.

When the input voltage V _IN is lower than the output voltage V _O , the on-duty of the switching transistor Q1 increases, and as described above, the driving transistor Q3 and the transistor Q2 are synchronized with the switching operation of the switching transistor Q1. Turns on and off. Thereby, the transistor Q2 operates as a synchronous rectifier. 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 off, the driving transistor Q3
In addition, the transistor Q2 is not completely turned on, but is turned on in an unsaturated 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
_The control is performed so that _O is constant. As a result, the transistor Q2 performs an operation similar to a series regulator. Therefore, the switching power supply of the present invention shown in FIG. 1 functions as a step-up / step-down converter.

The switching power supply of the present invention shown in FIG.
2 has been improved. For this reason, even if the switching frequency is increased, an increase in loss occurring in the transistor Q2 can be suppressed. Incidentally, when a comparative experiment of the circuits shown in FIGS. 1 and 2 was performed on a test circuit board, the circuit of the present invention shown in FIG. 1 had a switching frequency of about 160 compared to 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 a switching frequency of about 320 [kHz]. In the switching power supply 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. This also contributes to preventing the number of parts of the circuit element from increasing.

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 the 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 applies also to the case where the choke coil L1 is constituted by a plurality of connected coil components. In the embodiment shown in FIG. 1, the auxiliary bias circuit 4 includes a resistor R4 and a capacitor C
Although the circuit is constituted by the series circuit of No. 4, for example, it may be constituted by only a capacitor or the like, and is not limited to this structure.

[0020]

According to the present invention, in a switching power supply using a PNP 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. A bias voltage (specifically, a voltage appearing in a choke coil) guided by the auxiliary bias circuit is applied to a transistor as a rectifying element. Thereby, the operation 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, passive means (such as a speed-up capacitor) for improving the operation speed of a transistor as a rectifying element, which has been conventionally provided, becomes unnecessary in principle, and an increase in the number of circuit element parts can be prevented. .

[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 a switching power supply device proposed in Japanese Patent Application No. 5-307520 (FIG. 3).

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 output terminal 3 PWM control circuit 4 auxiliary bias circuit Q1 switching transistor Q2 transistor as synchronous rectifier element Q3 driving transistor L1 choke coil C2 smoothing capacitor E external power supply RL load

Claims (3)

(57) [Claims] 1. A switching power supply device for generating a flyback voltage on an inductance element when a switching element is turned off, rectifying a voltage obtained by superimposing the flyback voltage and an input voltage via a rectifying element to obtain a predetermined voltage. A transistor rectifying element which is provided between the inductance element and the output terminal and which performs a rectifying operation; and a low-potential such as a base of the transistor rectifying element and the ground. And a drive transistor connected between the transistor rectifier and the inductance element to control the operation of the transistor rectifier by receiving an on / off control signal from a connection point of the inductor rectifier and the base of the transistor rectifier. And the operation of the switching element An auxiliary bias circuit for guiding a bias voltage applied to the transistor rectifier element according to the following. 2. A bias voltage guided to a base of the transistor rectifier by the auxiliary bias circuit,
The switching power supply according to claim 1, wherein the voltage is a voltage appearing at the inductance element. 3. The circuit according to claim 2, wherein the auxiliary bias circuit comprises a series circuit of a resistance element and a capacitance element connected between the base of the transistor rectifier and one end of the inductance element. 2. A switching power supply device according to claim 1.