JPH0880037A - Switching power supply device - Google Patents

Abstract

PURPOSE: To decrease the number of parts, and to obtain stable output voltage even when there is an ambient temperature change by composing a voltage detector of the series connection of the light-emitting section of a photo-coupler, a passively constituted transistor, a resistor, and a Zener diode. CONSTITUTION: DC voltage V0 by a smoothing circuit 6 increases the current of a light-emitting section 4b when DC voltage V0 is higher than set voltage, and reduces the current of the light-emitting section 4b when DC voltage V0 is lower than set voltage. A PWM control circuit 2 uses a feedback signal from a light-receiving section 4a, and controls a switching element 3 so that the deviation of the voltage V0 and the set value reaches zero. The adjustment of voltage V0 is determined by the Zener voltage of a Zener diode(ZD) 7c, the temperature coefficient of the ZD 7c reaches a large positive value at the time of output voltage of approximately 8V or more, and a power unit having large voltage fluctuation to an ambient temperature is obtained. Accordingly, voltage fluctuation is offset by the negative temperature coefficient of voltage between an emitter and a collector and the negative temperature coefficient of the forward voltage of the light-emitting diode of the light-emitting section 4, thus allowing temperature compensation.

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 a switching power supply device of a type capable of obtaining an output voltage in a relatively wide range.

[0002]

2. Description of the Related Art There are chopper type, RCC (ringing choke converter) type and the like as circuit configuration methods of switching power supply devices. RCC type has a DC power supply with PWM
A switching circuit such as a control circuit is connected, a transformer is provided at the output of this switching circuit, and the AC voltage output to the secondary winding of the switching circuit is rectified / smoothed to obtain a DC voltage. It is composed of a voltage detection circuit and a photocoupler for detecting and feeding back to the switching circuit.

A concrete example of this voltage detecting circuit is described in, for example, "Transistor Technology SPE", CQ Publishing Co., Ltd.
CIAL No. 28 (Special issue: All the latest power supply circuit design technologies) "on page 71.

The circuit is as shown in FIG. 7, in which a resistor 103 and a photocoupler 1 are provided between the outputs of a smoothing circuit composed of a choke transformer 101 and a smoothing capacitor 102.
04 (only the light emitting portion is shown here, the light receiving portion is used as one of the components of the switching circuit), a circuit in which the collector and the emitter of the transistor 105 and the Zener diode 106 are connected in series is inserted. ing. Further, a resistor 107 is connected between the emitter of the transistor 105 and the output of the smoothing circuit, and the voltage Vref applied to the base of the transistor 105 is obtained by comparing the output voltage of the smoothing circuit with the resistor 108, the variable resistor 109 and the resistor 1.
It is obtained from a resistance voltage divider circuit in which 10 are connected in series.

In this voltage detection circuit, the transistor 10
By adjusting the voltage Vref applied to the base of No. 5 with the variable resistor 109, the output voltage Vo of the smoothing circuit can be changed. In this case, the output voltage Vo is the variable resistance 1
If the resistance value on the power supply side of the variable terminal of 09 is R ₁₀ and the resistance value on the ground side of the variable terminal of variable resistor 109 is R ₂₀ , then Vo = 6.9 × {1+ (R ₁₀ / R ₂₀ )} And is suitable for the requirement of obtaining an output voltage of 8 V or more.

Regarding this kind of voltage detection circuit,
In addition to the above documents, JP-A-5-2926 applied to a charger
There is one disclosed in Japanese Patent No. 75.

[0007]

However, in the above-mentioned prior art, the transistor operates as an amplifier, so that a phase delay occurs. For this reason, the number of components is increased, such as requiring the CR circuit 111 for performing phase compensation. As a result, an increase in cost cannot be avoided.

In the device disclosed in Japanese Patent Laid-Open No. 5-292675, the output voltage depends on the voltage of the Zener diode D3. Therefore, when the output voltage is set to 8 V or higher, the temperature coefficient of the Zener diode D3 becomes small. The power supply device has a large positive value and has a large voltage fluctuation with respect to the environmental temperature. Therefore, there is a problem in mounting it on a product that requires high reliability.

An object of the present invention is to provide a switching power supply device which can obtain a stable output voltage even if the environmental temperature changes without increasing the number of parts.

Another object of the present invention is to provide a switching power supply device capable of reducing the fluctuation of the output voltage even when the load changes.

[0011]

In order to achieve the above object, the present invention provides a transformer having a switching element connected to the output of a switching circuit connected to a DC power supply and having a secondary winding as its load. Connecting, detecting the state of the DC voltage obtained by rectifying and smoothing the AC voltage output to the secondary winding by a voltage detection circuit, and feeding back the detected value to the switching circuit via a photo coupler. In the switching power supply device for controlling the output voltage of the secondary winding by means of the above, the voltage detection circuit comprises a light emitting portion of the photocoupler, a semiconductor element with a built-in bias resistor, a resistor, and a Zener diode connected in series. I have to.

In contrast to the above structure, the light emitting portion is disposed on the side of the voltage detecting circuit having the highest potential, and the light emitting portion is disposed between the low potential side and the portion of the voltage detecting circuit having the lowest potential. A capacitor can be connected to. In addition,
Examples of the semiconductor element include a transistor, an IC, and an F
ET or the like is used.

[0013]

According to the above-mentioned means, the light emitting portion of the photocoupler, the passively constructed transistor, the resistor, and the Zener diode are connected in series, and a semiconductor element such as a transistor is used as a component thereof. However, it is not configured to exhibit an amplifying effect during operation. Therefore, a phase delay due to amplification does not occur, a circuit (component) for phase compensation is not required, and since a transistor with a built-in bias resistor is used, the number of components can be reduced and cost can be reduced.

Further, by connecting a capacitor between the light emitting portion and the ground level line, a pulsed current can be passed through the light emitting portion in synchronization with the rising portion of the ripple voltage of the output voltage, and its signal transmission. Since the time can be greatly shortened, it becomes possible to follow a sudden and large load change, and the size and cost of the device can be reduced.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

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

A PWM (pulse width modulation) control circuit 2 as a switching circuit is connected to the DC power supply 1, and a gate of a switching element 3 (hereinafter, referred to as a power FET) is connected to an output terminal thereof. Also, PWM
The light receiving portion 4a of the photocoupler 4 is provided between the control circuit 2 and the ground.
Is connected. Further, the primary winding of the transformer 5 is connected between the drain of the switching element 3 and the positive side of the DC power supply 1. A smoothing circuit 6, a voltage detection circuit 7, and a load 8 (here, a resistance load is shown as an example) are sequentially connected to the secondary winding of the transformer 5.

The smoothing circuit 6 is composed of a rectifying diode (half-wave rectification is shown here, but full-wave rectification using a bridge diode may be used) 6a and a smoothing capacitor 6b. Further, the voltage detection circuit 7 includes a light emitting portion 4b (PC ₁ ) of the photocoupler 4, a transistor 7a (Q ₁ ) with a built-in bias resistor, a resistor 7b (R ₁ ) and a zener diode 7c (ZD ₁ ) connected in series. It is composed of connections.

Next, the operation of the switching power supply device having the configuration of FIG. 1 will be described.

When the DC power supply 1 is turned on and power is supplied to the PWM control circuit 2, the PWM control circuit 2 starts its operation, and the switching element 3 is activated by its output signal.
Starts switching operation, which causes the transformer 5
A rectangular wave current of a constant cycle flows through the primary winding of the. While the switching element 3 is on, the transformer 5 is energized to store energy, and during the off period of the switching element 3, the stored energy is regenerated in the secondary winding of the transformer 5. The voltage generated in the secondary winding is the smoothing circuit 6
Is rectified and smoothed by the rectification, and the resulting DC voltage is applied to the load 8.

DC voltage by smoothing circuit 6 (output voltage V
o) is detected by the voltage detection circuit 7, and increases the current of the light emitting unit 4b when Vo is higher than the set voltage, and decreases the current of the light emitting unit 4b when Vo is low. Light emitting part 4b
Emits light according to the change in the current, and this light appears as a change in the amount of light received by the light receiving portion 4a of the photocoupler 4. P
The WM control circuit 2 controls the switching element 3 so that the deviation ± δV between the output voltage Vo and the set value becomes zero by using the photoelectric conversion value by the light receiving section 4a as a feedback signal.

The output voltage Vo in the configuration of FIG.
It can be expressed by equation (1).

Vo = Vpc ₁ + Vq ₁ + Vr ₁ + Vzd ₁ (1) where Vpc _{1 is} the forward voltage of the light emitting diode of the light emitting section 4 b, Vq _{1 is} the collector-emitter voltage of the transistor 7 a, and Vr _{1 is the} circuit. voltage drop across the resistor 7b due to the current flowing in, Vzd _1: Zener voltage of the Zener diode 7c.

Here, the light emitting portion 4b and the transistor 7a.
Vpc ₁ and Vq ₁ have negative temperature characteristics. Further, Vzd _{1 of the} Zener diode 7c may have a positive or negative temperature characteristic depending on the Zener voltage.

The adjustment of the output voltage Vo in the configuration of FIG. 1 is mainly determined by the Zener voltage of the Zener diode 7c, and when the output voltage is about 8V or more, the temperature coefficient of the Zener diode 7c becomes a large positive value and the ambient temperature. In contrast, the power supply device has large voltage fluctuations. Therefore, the negative temperature coefficient of the emitter-collector voltage Vq ₁ and the light emitting unit 4
If the temperature coefficient is compensated by canceling the large positive temperature coefficient of the Zener diode 7c by the negative temperature coefficient of the forward voltage Vpc ₁ of the light emitting diode of b, the output voltage can be stabilized against the environmental temperature change.

Here, the Zener diode 7c (Z
D ₁ ), the light emitting section 4 b (PC ₁ ), the transistor 7 a (Q ₁ ), and the resistor 7 b (R ₁ ) of the photocoupler 4 can be expressed by the following equation when the voltage relationship based on the temperature coefficient is expressed.

(I) Zener diode 7c (ZD ₁ )
The voltage Vzd ₁ of Vzd ₁ is Vzd ₁ = Vz + γZ · (t-25) (2) where Vz: Zener voltage at 25 ° C. element temperature, γZ: Temperature coefficient of zener diode, t: Element temperature during actual operation (Environmental temperature + element temperature rise).

(Ii) The voltage Vpc ₁ of the light emitting portion 4b of the photocoupler 4 is: Vpc ₁ = Vpc + γpc (t-25) (3) where Vpc: the light emitting diode of PC _{1 at} the device temperature of 25 ° C. Forward voltage, γpc: temperature coefficient of forward voltage of the light emitting diode of PC ₁ .

(Including the temperature coefficient of the current conversion rate of the photocoupler) (iii) The voltage Vq ₁ of the transistor 7a (Q ₁ ) is: Vq ₁ = Vq + γq (t-25) (4) where Vq : collector-to-emitter voltage for Q ₁ in the element temperature 25 ℃, γq: temperature coefficient of the collector-emitter voltage for Q _1.

(Iv) The voltage Vr ₁ of the resistor 7b (R ₁ ) is: Vr ₁ = Vr + γr (t-25) (5) where Vr is the voltage of R ₁ due to the circuit current at the element temperature of 25 ° C. Drop, γr: Temperature coefficient of the voltage drop of R ₁ .

(Including voltage drop due to circuit current changed by temperature coefficient of other components) The closer the total of the temperature coefficient portions represented by the above equations is to 0, the more stable the power supply device against environmental temperature changes. Becomes The total of the temperature coefficient parts is expressed by the following formula.

Σγ = γZ + γpc + γq + γr (6) To bring Σγ represented by the equation (6) close to 0, γq
May be changed, and specifically, the base bias resistance of the transistor 7a may be adjusted.

In FIG. 1, the transistor 7a has a PNP.
However, the present invention is not limited to this, and it is also possible to use an NPN type transistor as shown in FIG.

[Embodiment 2] FIG. 3 is a circuit diagram showing a configuration of a main part of another embodiment of the switching power supply device according to the present invention. Note that, in FIG. 3, the same reference numerals are used for the same elements as those in FIG. 1, and thus duplicated description will be omitted.

In this embodiment, the voltage detection circuit 7 having the configuration shown in FIG. 1 is used.
On the other hand, a capacitor (C ₁ ) 9 is provided between the output side (cold end) of the light emitting portion 4b (PC ₁ ) of the photocoupler 4 and the ground.
It is characterized by connecting. The capacitor 9 is shown in FIG.
As shown in, a function of causing a pulsed current to flow through the light emitting portion 4b (PC ₁ ) at the rising portion of the ripple voltage (generated during the switching operation) included in the output voltage Vo. This current causes the light emitting section 4b to emit a pulsed optical signal in synchronization with the switching operation. In general, the signal transmission time of the photocoupler 4 has a characteristic that it becomes shorter as the current becomes larger. By operating the photocoupler 4 in a pulsed manner as in this embodiment, the transmission time can be made extremely short. In this way, the transmission time is shortened, so that the output voltage V
Error signal due to load fluctuation of o
It becomes possible to transmit to the WM control circuit 2.

Since the ripple voltage increases in proportion to the load current, the pulse current also increases in proportion, and the current in the light emitting section 4b increases as the load increases, so that the output voltage Vo is controlled to decrease. It In this case, the current flowing from the light emitting portion 4b to the transistor 7a → the resistor 7b → the zener diode 7c decreases, and the average current of the light emitting portion 4b decreases, so that the output voltage Vo is stably controlled with respect to the voltage setting value.

When the ripple voltage decreases due to the decrease in load current, the pulsed current also decreases.
The output voltage Vo is stably controlled with respect to the set voltage value by increasing the current flowing from b to the transistor 7a → the resistor 7b → the zener diode 7c and increasing the average current of the light emitting section 4b.

Next, the function and effect of the present invention which can reduce the fluctuation of the output voltage with respect to the load fluctuation will be described.

When the load is suddenly lightened from the heavy load state, the pulsed current increases immediately after the change and the output voltage is prevented from increasing. The details will be described with reference to FIG. 5. When the heavy load, which has been heavy until then, is lightened at time t ₁ , the ripple voltage increases from this point. Then, the pulsed current flowing through the light emitting portion 4b increases, and the PWM control circuit 2 executes control in the direction of lowering the output voltage Vo as described above.

On the other hand, as shown in FIG. 6, when the light load state suddenly changes to the heavy load state at time t ₂ , the pulsed current decreases (or disappears) immediately after the change, and the transistor 7a → resistor Since the current flowing from 7b to Zener diode 7c also decreases, the output voltage recovers within the response time of the PWM control circuit 2.

As described above, according to the embodiment shown in FIG. 3, the output voltage Vo can follow the load variation quickly.

In the above embodiment, the transistor 7a has a built-in bias resistor, but the present invention is not limited to this, and the bias resistor may be externally attached. Further, in a portion where a plurality of components are connected in series, the connection order is not limited to the circuits shown in FIGS. 1 to 3, and any order may be used. Further, instead of the transistor, an IC or FET can be used.

[0043]

Since the present invention is configured as described above, it has the following effects.

According to the first and second aspects of the present invention, the switching element is connected to the output of the switching circuit connected to the DC power supply, and the transformer having the secondary winding is connected as the load, and the secondary winding is connected. The state of the DC voltage obtained by rectifying and smoothing the AC voltage output to the line is detected by a voltage detection circuit, and the detected value is fed back to the switching circuit via a photocoupler, so that the secondary winding In the switching power supply device for controlling the output voltage, the voltage detection circuit has a configuration in which each of the light emitting portion of the photocoupler, the semiconductor element with the built-in bias resistor, the resistor, and the Zener diode is connected in series. Circuits (parts) are no longer required, and this can reduce the number of parts and reduce costs.

In the switching power supply device according to a third aspect of the present invention, the light emitting section is disposed on the side of the voltage detecting circuit with the highest potential, and the low potential side of the light emitting section and the highest potential of the voltage detecting circuit. Since the capacitor is connected between the lower part and the lower part, it is possible to follow abrupt and large load changes, and it is possible to reduce the size and cost of the device.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a configuration example of a voltage detection circuit using transistors of different polarities in FIG.

FIG. 3 is a circuit diagram showing a configuration of a main part of another embodiment of the switching power supply device according to the present invention.

FIG. 4 is a current waveform diagram showing an operation of a photocoupler of the voltage detection circuit shown in FIG.

FIG. 5 is a current waveform diagram showing the operation of the photocoupler when the load is suddenly changed from a heavy state to a light state.

FIG. 6 is a current waveform diagram showing the operation of the photocoupler when the load suddenly changes from a light load state to a heavy load state.

FIG. 7 is a circuit diagram showing a voltage detection circuit in a conventional switching power supply device.

[Explanation of symbols]

 1 DC power supply 2 PWM control circuit 3 Switching element 4 Photocoupler 4a Light receiving part 4b Light emitting part 5 Transformer 6 Smoothing circuit 7 Voltage detection circuit 7a Transistor 7b Resistor 7c Zener diode 9 Capacitor

Claims (3)

[Claims] 1. A switching element is connected to an output of a switching circuit connected to a DC power source, a transformer having a secondary winding is connected as a load, and an AC voltage output to the secondary winding is rectified and In the switching power supply device for controlling the output voltage of the secondary winding by detecting the state of the DC voltage obtained by smoothing by a voltage detection circuit, and feeding back the detected value to the switching circuit via a photocoupler, The switching power supply device is characterized in that the voltage detection circuit comprises a light emitting part of the photocoupler, a semiconductor device with a built-in bias resistor, a resistor and a zener diode connected in series. 2. The semiconductor element is formed of any one of a transistor, an IC and a FET.
The switching power supply described. 3. The light emitting unit is disposed on the side of the voltage detecting circuit having the highest potential, and a capacitor is provided between the low potential side of the light emitting unit and the portion of the voltage detecting circuit having the lowest potential. The switching power supply device according to claim 1, wherein the switching power supply device is connected.