JPH07131974A - Switching power supply apparatus - Google Patents

Abstract

PURPOSE:To further stabilize an output power supply by reducing fluctuation in drive conditions for a switching element caused by the fluctuation in an input power supply in a switching power supply circuit. CONSTITUTION:An input power supply is switched by using a switching element Tr1 and switched power supply signals are smoothened by a smoothening circuit portion L1 and C3 and a power supply of a constant voltage is output from output terminals in a switching power supply apparatus 1, detection circuits L1 and L2 outputting signals corresponding to the fluctuation in the input power supply are provided in the smoothening circuit portion L1 and C3 and a drive current of a switching element is controlled on the basis of signals detected in the detection circuit.

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, and more particularly, to an output of the switching power supply device so that the drive current of the switching element for generating the switching power supply pulse is not affected by the fluctuation of the input power supply voltage. Technology to stabilize.

[0002]

2. Description of the Related Art A switching power supply device is used as a device for stably supplying a power supply of a predetermined voltage to an electronic circuit or the like. This switching power supply device receives, for example, a DC power supply of a certain voltage from an input terminal, and intermittently converts this DC power supply by a switching element such as a transistor into a pulse power supply having a specific duty factor and frequency,
The voltage is converted by smoothing the pulse power supply to obtain a desired voltage from the output terminal.

FIG. 3 shows a circuit example of such a switching power supply device which has been conventionally used. The switching power supply circuit 11 of FIG. 3 schematically includes a switching circuit portion 12 that converts an input power supply of a voltage Vin received at an input terminal into a predetermined pulse, and a voltage Vout that integrates and smoothes the converted pulse. Of the smoothing circuit portion 13 for outputting as the output power source. Switching circuit part 12
Is a transistor Tr1 which is a switching element for converting the DC power from the input terminals 15 and 16 into pulses.
And transistors Tr2 and Tr3, resistors R1, R2 and R3, and capacitors C0 and C that form a drive circuit for switching the transistor Tr1.
2 and a control circuit 14 for sending a control signal to the transistor drive circuit in order to control the pulse generated in the transistor Tr1 to a duty factor or frequency according to the output voltage of the switching power supply circuit 11. Here, the capacitor C0 is the transistor Tr1.
Is a speed-up capacitor for increasing the switching speed of. The smoothing circuit portion 13 is composed of a capacitor C3 and a coil L1. The resistors R4 and R5 are voltage dividing resistors for feeding back the output voltage to the control circuit 14. As is well known, the control circuit 14 outputs a drive pulse having a duty factor corresponding to the magnitude of the output voltage Vout, for example.

In the switching power supply circuit 11, the input power supply of the voltage Vin is the switching circuit portion 12
After being converted into a pulse having a specific duty factor and frequency by, the signal is smoothed by the smoothing circuit portion 14, integrated, and output from the output terminals 17 and 18 as a power source of the stable voltage Vout. The switching operation of the transistor Tr1 for converting into a pulse is based on the switching operation of the transistors Tr2 and Tr3. When the transistor Tr1 is turned on, the transistor Tr2 is cut off and the transistor Tr3 is turned on. Is. Therefore, the drive current ID of the transistor Tr1 is from the input terminal 15 to the emitter-base junction of the transistor Tr1, the emitter-collector junction of the transistor Tr3, as shown by the dotted line in the circuit of FIG.
It flows through the parallel circuit of the resistor R3 and the capacitor C0. The base-emitter voltage of the transistor Tr1 is V
_{Assuming BE} and the collector-emitter saturation voltage of the transistor Tr3 is V _CEsat , the transistor drive current ID is expressed as follows. _{ID = (Vin-V BE -V} CEsat) / R3 ··· (1)

In order to obtain a stable output power from the switching power supply circuit 11, the switching circuit portion 12
It is necessary to control the duty and frequency of the pulse generated in 1 above so as to sufficiently correspond to the output voltage Vout of the switching power supply circuit 11. The value of the output voltage Vout is divided by the resistors R3 and R4 and fed back to the control circuit 14, and the switching operation control for determining the duty factor and frequency of the pulse generated from the control circuit 14 to the drive circuit of the switching transistor Tr1. A signal is given. Therefore, in order to control the switching operation of the transistor Tr1 sufficiently corresponding to the fluctuations of the output voltage Vout and the output current, the transistor T1 must be controlled.
It is important to keep the drive current ID, which is one of the drive conditions of r1, constant.

[0006]

However, in the circuit of the conventional switching power supply device, the transistor Tr1 is used.
The transistor drive current ID for driving the transistor is a function of the input power supply voltage Vin as shown in the equation (1), but since the input power supply voltage Vin varies due to various factors, the drive current ID is Fluctuates under the influence of voltage fluctuations. Therefore, the driving condition of the transistor Tr1 is changed and the optimum driving condition is not maintained. Therefore, the duty and frequency of the pulse generated by the transistor Tr1 are not controlled sufficiently corresponding to the output voltage Vout, and as a result, the heat generation of the Tr1 is generated. , Damage and stabilization of output power supply voltage will not be performed completely.

Further, since the transistor drive current ID flows into the ground circuit and is not used as the current of the output power supply, it is used as the drive current ID as the input power supply in addition to the current converted to the output power supply. Required to reduce the efficiency of the switching power supply circuit.

Therefore, an object of the present invention is to keep the optimum driving condition without changing the driving condition of the switching element even if the input power supply voltage fluctuates, and to obtain a stable output power supply voltage. Is to provide.

Another object of the present invention is to improve the conversion efficiency of the switching power supply circuit.

[0010]

In order to solve the above problems, according to the present invention, an input power supply is switched by using a switching element, and the switched power supply signal is smoothed by a smoothing circuit portion and an output terminal is provided. In the switching power supply device that outputs a power supply of a constant voltage from the above, a detection circuit that outputs a signal corresponding to the fluctuation of the input power supply is provided in the smoothing circuit portion, and the switching element is driven based on the signal detected by the detection circuit. It controls the current.

Further, a transformer having a smoothing coil included in the smoothing circuit portion as a primary side is used as a detection circuit for outputting a signal corresponding to the fluctuation of the input power source, and the input is made from the secondary coil of the transformer. It is convenient to configure so as to output a signal corresponding to the fluctuation of the power supply.

[0012]

According to the above structure, the smoothing circuit portion on the output side of the switching power supply unit is provided with the detection circuit for outputting a signal corresponding to the fluctuation of the input power supply, and the fluctuation signal of the detected input power supply signal is supplied to the switching element. By reflecting in the drive current, it is possible to cancel the change in the switching device drive current due to the change in the input power supply voltage. Therefore, the fluctuation of the input power supply of the switching power supply circuit does not affect the drive current of the switching element,
It is possible to maintain the optimum driving condition of the switching element. In this way, by operating the switching element under the optimum driving condition, it is possible to generate a pulse having a duty and frequency that sufficiently correspond to the output voltage of the switching power supply, and thus it is possible to improve the stability of the output power supply voltage of the switching power supply. it can.

A circuit for detecting the fluctuation of the output power supply is
The configuration of the conventional switching power supply circuit is drastically changed by configuring the smoothing coil that constitutes the smoothing circuit part by the transformer with the primary side and obtaining the signal corresponding to the fluctuation of the input power supply from the secondary side coil. The detection circuit can be formed without using a part of the smoothing circuit, and the power supply output can be stabilized and the efficiency of the power supply circuit can be improved with a simple configuration.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a circuit example of a switching power supply device according to an embodiment of the present invention. The switching power supply circuit indicated by reference numeral 1 is roughly composed of a switching circuit portion 2 and a smoothing circuit portion 3 which are surrounded by a chain line. The switching circuit portion 2 has input terminals 5 and 6
The transistor Tr1 is a switching element for intermittently converting the current of the input power source of the voltage Vin from the transistor Tr1 into a predetermined pulse, and the transistor T constituting a drive circuit for switching the transistor Tr1.
r2 and Tr3, resistors R1, R2 and R3, capacitor C
2, a coil L2, and a control circuit 4 for controlling the duty and frequency of pulses generated according to the output of the switching power supply circuit 1. In addition,
The control circuit 4 outputs a drive pulse having a duty factor or frequency according to the magnitude of the output voltage Vout, like the control circuit of FIG.

Further, the smoothing circuit portion 3 is composed of a capacitor C3 and a coil L1 and has a voltage Vou from the output terminals 7 and 8.
The output power of t is output. The coil L1 of the smoothing circuit portion 3 and the coil L2 of the switching circuit portion 2 are inductively coupled to each other via a magnetic core, and each of them is a primary coil and a secondary coil. I am configuring. In addition to the switching circuit portion 2 and the smoothing circuit portion 3, the switching power supply circuit 1 also includes a capacitor C1 acting as a filter, a diode D1 acting for preventing undershoot of the output of the transistor Tr1, and an output power supply voltage. Voltage dividing resistors R4 and R5 for feedback to the control circuit 4 are included.

Next, the operation of the switching power supply circuit 1 will be described. Input voltage V received at input terminals 5 and 6
The input power source of in is converted into a pulse of a certain duty factor having a predetermined frequency, for example, in the switching circuit portion 2. The intermittent operation of converting the input power source into a pulse is directly performed by the transistor Tr1, but the transistor Tr1 includes the transistors Tr2 and Tr3, the resistors R1 and R2 and R3, the capacitor C2, and the coil L2.
It is driven by a transistor drive circuit composed of The operation relationship between these transistors Tr1, Tr2, and Tr3 is that when the transistor Tr2 is cut off and the transistor Tr3 is on, the transistor Tr1 is on, and when the transistor Tr2 is on and the transistor Tr3 is cut off. The transistor Tr1 is cut off. Therefore, the base drive current ID of the transistor Tr1 is, as shown by the dotted line in the figure, the path from the input terminal 5 to the emitter-base junction of the transistor Tr1, the emitter-collector junction of the transistor Tr3, the resistor R3, and the coil L2. It is expressed as a flowing current.

Coil L in the path of this drive current ID
Since 2 is inductively coupled to the coil L1 of the smoothing circuit portion 3, a voltage change across the coil L1, that is, a pulse corresponding to the pulse generated in the switching circuit 2 is induced in the coil L2. Since the pulse generated in the switching circuit 2 is a pulse affected by the fluctuation of the input power source Vin, the pulse induced in the coil L2 is also the input power source Vi.
The variation of n is reflected. Therefore, drive current ID
A signal corresponding to the fluctuation of the input power source Vin is reflected.

The pulse voltage in the coils L1 and L2 is shown in FIG. 1A is the point a in FIG. 1, that is, the pulse voltage at the collector portion of the transistor Tr1, and FIG.
Point b, that is, the pulse voltage at the connection between the resistor R3 and the coil L2 in the path of the drive current ID. In (a), the pulse of the minimum voltage 0 appears at the maximum voltage Vin generated by switching the current of the input power supply voltage Vin received at the input terminals 5 and 6. When the maximum voltage of the pulse in the graph of (a) is Vin, the pulse voltage in (b) is Vin-Vout. Therefore, the coil L2
Acts as a voltage detecting means for detecting a signal corresponding to the fluctuation of the input power supply voltage Vin, and the signal detected by the voltage detecting means is reflected in the drive current ID of the switching transistor Tr1. Therefore, the emitter-base voltage of the transistor Tr1 is V _BE, and the collector-emitter saturation voltage of the transistor Tr3 is V _{BE.
If CEsat} , this switching transistor Tr
The drive current ID of 1 can be expressed as follows. _{ID = {Vin- (Vin-Vout} ) - (V BE + V CEsat)} / R3 = {Vout- (V BE + V CEsat)} / R3 ··· (2)

That is, the drive current ID is the output voltage Vout.
And is not affected by the input voltage Vin. Therefore, even if the input power supply voltage of the switching power supply circuit 1 changes, the driving condition of the switching transistor Tr1 is kept constant. If the driving conditions of these transistors can be maintained at the optimum driving conditions, the transistors can perform the switching operation according to the designed values, and the duty and frequency sufficiently corresponding to the control signal of the control circuit 4 that feeds back the output power supply voltage Vout. Pulse can be generated. Therefore, the output power supply voltage Vou from the output terminals 7 and 8 of the switching power supply circuit 1
The stability of t can be significantly improved.

The collector current IC of the transistor Tr1 is reduced by the drive current ID because a part of the current of the input power source flows as the base drive current ID of the transistor Tr1. Since the coil L2 in the path is inductively coupled to the coil L1, a part of the drive current ID is induced from the coil L2 to the coil L1 side and used as a part of the output current. Therefore, the drive current of the switching element can be effectively used, and the efficiency of the switching power supply circuit can be significantly increased.

Further, in the switching power supply circuit of this embodiment, the coil L2 is connected in series with the resistor R3 so that the capacitor C0 according to the prior art shown in FIG.
It is possible to configure a circuit equivalent to a circuit in which the resistor R3 and the resistor R3 are connected in parallel. Therefore, the conventional speed-up capacitor C0 becomes unnecessary, and the characteristics of the switching power supply circuit can be improved without making the circuit more complicated than the conventional circuit.

In this embodiment, one of many switching power supply circuit systems has been described, but the present invention can be applied to other types of switching power supply circuits.
Moreover, even if it is not used as a power source, it can be applied to an electronic circuit including a similar switching circuit.

[0023]

As described above, according to the present invention, the smoothing circuit portion of the switching power supply circuit is provided with the detection circuit for detecting the signal corresponding to the fluctuation of the input power supply, and the detected signal is driven by the switching element. Since the current is reflected, the drive condition of the switching element does not change due to the change of the input power supply. Therefore, it is possible to generate a drive pulse having a controlled duty factor or frequency sufficiently corresponding to the output voltage, so that the stability of the output power supply voltage can be significantly increased.

Further, since the drive current of the switching element can be used as a part of the output power source by the inductive coupling of the coils L1 and L2, the efficiency of the switching power source circuit can be further enhanced.

Further, by connecting the coil L2 in series to the drive circuit portion of the switching element, an operation equivalent to that of the conventional speed-up capacitor can be achieved, and the characteristics can be improved without increasing the number of parts of the switching power supply circuit. Can be made.

[Brief description of drawings]

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

2 is a waveform diagram showing a temporal change of a pulse voltage at a specific position in a circuit in the switching power supply device of FIG. 1, (a) is a pulse voltage at a point a in FIG. It is the pulse voltage at point b in FIG.

FIG. 3 is an electric circuit diagram schematically showing a circuit example of a conventional switching power supply circuit.

[Explanation of symbols]

 1, 11 Switching power supply circuit 2, 12 Switching circuit part 3, 13 Smoothing circuit part 4, 14 Control circuit 5, 6, 15, 16 Input terminal 7, 8, 17, 18 Output terminal

Claims (2)

[Claims] 1. A switching power supply device which switches an input power supply using a switching element, smoothes the switched power supply signal in a smoothing circuit portion, and outputs a power supply of a constant voltage from an output terminal, wherein the smoothing circuit is provided. A switching power supply device, characterized in that a detection circuit for outputting a signal corresponding to a fluctuation of the input power supply is provided in a part, and a drive current of the switching element is controlled based on the signal detected by the detection circuit. 2. A detection circuit which outputs a signal corresponding to a fluctuation of the input power source includes a transformer having a smoothing coil included in the smoothing circuit portion as a primary side, and the input from the secondary coil of the transformer. The switching power supply device according to claim 1, wherein the switching power supply device outputs a signal corresponding to a fluctuation of the power supply.