JPH07123704A - Switching power source circuit - Google Patents

Abstract

PURPOSE:To obtain a switching power source circuit which is less in voltage loss and electric power loss. CONSTITUTION:A switching power unit is provided with a switching transistor 14 which transmits or interrupts a first DC voltage from an input terminal, smoothing circuit 15 which smoothes the output voltage of the transistor 14, output terminal 16 upon which a smoothed second DC voltage is impressed from the circuit 5, comparator 17 which compares the voltage impressed upon the terminal 16 with a reference voltage and controls the turning-on/off operations of the transistor 14 in accordance with the compared results, and clamp circuit 19 which prevents the voltage at the terminal 16 from dropping from a prescribed value in accordance with the compared results of the comparator 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chopper type switching power supply circuit, and more particularly to a switching power supply circuit which has a small voltage difference between input and output voltages when DC-DC converting and has a small power loss.

[0002]

2. Description of the Related Art FIG. 2 shows a self-excited switching power supply circuit. A first DC voltage V _IN applied to an input terminal (1) is turned on or off by a switching transistor (2) having a Darlington configuration. And applied to the smoothing circuit (3). The diode (4) is a flywheel diode. The transistor (5) compares the voltage of the output terminal (6) with the reference voltage of the reference power supply (7) and controls the on / off period of the switching transistor (2) according to the result, and accordingly, the output terminal ( 6) Change the voltage value. When the base current of the switching transistor (2) is too large, the off time of the switching transistor (2) becomes long due to the base charge storage effect, resulting in power loss. Therefore, the base current is limited by the resistor (8) and the diodes (9) and (10). or,
When the voltage of the output terminal (6) suddenly drops due to load fluctuation, it is difficult to supply current from the smoothing circuit (3) due to its time constant. Therefore, the diodes (9) and (10)
A current is supplied through the transistor (5) and the voltage of the output terminal (6) is clamped to a predetermined value.

The efficiency of the switching power supply is determined by the power loss in the switching element. This loss includes a loss during switching and a steady loss, and the steady loss is the saturation voltage V when the switching transistor (2) is on.
_It is proportional to the product of _{CE (sat)} and current. The saturation voltage in the case of FIG. 2 is the voltage V _BE (base-emitter rising voltage) of the transistor (11) and the voltage V _{CE (sat)} (collector-emitter voltage at saturation) of the transistor (12). It becomes the sum voltage. Normally, this value is 1 V or more and the current is 1
At A, the steady loss has a relatively large value of 1 W.

Therefore, the circuit of FIG. 3 can be considered as a method of reducing the saturation voltage. In this case, the saturation voltage is only the voltage V _{CE (sat)} of the transistor (13), and the power loss can be reduced. However, in FIG. 3, the voltage difference between point A and the input terminal (1) is 2V.
_Since there is a _BE , three diodes for limiting the base current are required. Then, the maximum output voltage with respect to the input voltage becomes a value lower by the voltage (3V _BE + V _{CE (sat )} ), resulting in voltage loss. In the case of FIG. 2, the voltage is 2V _{B E} + V
_{CE (sat)} ).

[0005]

Therefore, there has been a demand for a switching power supply circuit which has a small power loss and a small voltage loss (voltage difference between input and output voltages).

[0006]

The present invention has been made in view of the above points, and a switching transistor that transmits and cuts off a first DC voltage from an input terminal and an output voltage of the switching transistor is smoothed. A smoothing circuit, an output terminal to which the second DC voltage smoothed by the smoothing circuit is applied, a voltage at the output terminal and a reference voltage are compared, and an ON / OFF operation of the switching transistor is performed according to the comparison result. It is characterized by including a comparator to be controlled, and a clamp circuit for preventing the voltage of the output terminal from dropping to a predetermined value or less according to the comparison result of the comparator.

[0007]

According to the present invention, the voltage at the output terminal is compared with the reference voltage, the switching transistor is turned on / off according to the comparison result, and the clamp circuit is operated.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a switching power supply circuit of the present invention, in which (14) is a switching transistor for transmitting and blocking a first DC voltage from an input terminal,
5) is a smoothing circuit that smoothes the output voltage of the switching transistor (14), and (16) is the smoothing circuit (1
The output terminal (17) to which the smoothed second DC voltage is applied from 5) compares the voltage of the output terminal (16) with the reference voltage of the reference voltage (18) and responds to the comparison result. A comparator (19) for controlling the on / off operation of the switching transistor (14) prevents the voltage of the output terminal (16) from dropping to a predetermined value or less in accordance with the comparison result of the comparator (17). It is a clamp circuit.

The output voltage generated at the output terminal (16) is
It is compared with a reference voltage of a comparator (17) composed of a differential amplifier. That is, when the output voltage is lower than the reference voltage, the current flowing through the transistor (20) increases and the base current of the switching transistor (14) increases, so that the switching transistor (14) is turned on. Then, the output voltage of the smoothing circuit (15) rises. On the contrary, when the output voltage of the output terminal (16) is higher than the reference voltage, the switching transistor (14)
The base current of the switching transistor (1
4) turns off. Therefore, the operating current source (2
The current value of 1) is set to the maximum base current value of the switching transistor (14). By doing so, the off time of the switching transistor (14) can be shortened without requiring any special means (diode) for limiting the base current.

The base voltage of the transistor (22) which constitutes the comparator (17) is maximum at the transistor (2).
It is possible to raise it to the collector voltage of 0). Since the collector voltage of the transistor (20) is the voltage obtained by subtracting the voltage 2V _BE from the input voltage (from the saturation operation of the transistor), the voltage loss on the side of the comparator (17) in FIG. 1 is 2 V _BE .

The power loss in FIG. 1 is determined only by the saturation voltage V _{CE (s at)} of the switching transistor (14). Next, it is assumed that a load fluctuation occurs and the output voltage drops sharply. Then, the current flowing through the transistor (23) of the comparator (17) increases, and the transistors (24) and (25) of the clamp circuit (19) are turned on. Therefore, current is supplied from the emitter of the transistor (25) to the load (26), and the output terminal (16)
Is clamped. The value at this time is a value obtained by subtracting the saturation voltage V _{CE (sat)} of the transistor (24) and the rising voltage V _BE of the transistor (25) from the input voltage, which is smaller than the above-mentioned voltage loss (2V _BE ), so ,
The voltage loss in Fig. 1 is 2V _BE .

Therefore, according to the circuit of FIG. 1, the power loss is determined by the saturation voltage V _{CE (s at) of} one transistor and the voltage loss can be 2V _BE . By the way, the switching power supply circuit of FIG. 1 is suitable for use in a highly efficient amplifier circuit. For example, if the power supply voltage is changed by following the final output level of the amplifier to increase the power efficiency, a signal corresponding to the final output level is applied to the input terminal (1) of FIG. , Reference power source (1
It may be applied to the comparator (17) instead of 8). Then, the optimum power supply voltage level and the optimum clamp level can always be obtained.

[0013]

As described above, according to the present invention, it is possible to obtain a switching power supply circuit with a small power loss and a small voltage loss.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a switching power supply circuit of the present invention.

FIG. 2 is a circuit diagram showing a conventional switching power supply circuit.

FIG. 3 is a circuit diagram showing a conventional switching power supply circuit.

[Explanation of symbols]

 (14) Switching transistor (15) Smoothing circuit (17) Comparator (19) Clamp circuit

Claims (2)

[Claims] 1. A switching transistor for transmitting and blocking a first DC voltage from an input terminal, a smoothing circuit for smoothing an output voltage of the switching transistor, and a smoothed second DC voltage applied from the smoothing circuit. The output terminal, a comparator for comparing the voltage of the output terminal with a reference voltage, and controlling the on / off operation of the switching transistor according to the comparison result, and the output terminal for the output terminal according to the comparison result of the comparator. A switching power supply circuit comprising: a clamp circuit that prevents the voltage from dropping to a predetermined value or lower. 2. The switching power supply circuit according to claim 1, wherein the comparator is composed of a differential amplifier circuit which supplies a current of an operating current source to a base of the switching transistor.