JP2003289668A - Feedback circuit for power supply unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a feedback circuit for a power supply unit by which gain is kept almost constant without being affected by temperature characteristics of conversion efficiency of a photocoupler, even if ambient temperature changes. <P>SOLUTION: Stability of an output voltage Vo is attained by transmitting through the photocoupler 16 a feedback signal meeting fluctuation of the output voltage Vo. The feedback circuit 11 has a thermistor 31 which compensates a change in conversion efficiency CTR by temperatures Ta into a phototransistor 16B from a photodiode 16A of the photocoupler 16, by which even if the conversion efficiency CTR of the photocoupler 16 varies according to a change in the ambient temperature, variation of the conversion efficiency CTR is compensated by the thermistor 31. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a feedback circuit of a power supply device provided for stabilizing an output voltage, and more particularly to a photocoupler for insulating and transmitting a feedback signal. The present invention relates to a feedback circuit of a power supply device provided with: [0002] In a so-called insulated power supply device provided with an insulating transformer for transmitting power from an input side to an output side, a so-called insulated power supply is used as a feedback circuit for stabilizing an output voltage. In general, a photocoupler combining a light emitting element and a light receiving element is used for transmitting a feedback signal between primary and secondary. FIG. 6 is a circuit diagram showing an example of such a power supply device. In the figure, reference numerals 1 and 2 denote a DC input voltage Vi.
And an input terminal for connecting an input power supply (not shown) for supplying a power supply. The primary winding 3A of a transformer 3 for insulating a primary side and a secondary side between the input terminals 1 and 2 and a MOS type FET. A series circuit with the switching element 4 is connected. On the other hand, the secondary winding 3B of the transformer 3 is connected to a rectifying / smoothing circuit 5 composed of, for example, a diode, a cheek coil, and a capacitor. By switching the switching element 4, the primary winding 3A of the transformer 3 is switched.
The DC input voltage Vi is intermittently applied to the output terminal, whereby the voltage induced in the secondary winding 3B of the transformer 3 is rectified and smoothed by the rectifying and smoothing circuit 5, and a predetermined DC output is applied between both ends of the output terminals 7 and 8. The voltage Vo is obtained. On the other hand, a feedback circuit 11 stabilizes the DC output voltage Vo from the main converter circuit.
Here, the feedback circuit 11 amplifies the resistors 12, 13 for dividing the output voltage Vo, and a detection signal of the output voltage Vo obtained from the connection point of the resistors 12, 13 with the reference voltage of the reference power supply 14 for amplification. An operational amplifier 15 as a comparator,
A photocoupler 16 that electrically insulates the secondary feedback signal corresponding to the variation of the output voltage Vo obtained by the operational amplifier 15 and transmits the signal to the primary side of the transformer 3;
A control IC 17 as a control circuit for controlling the conduction width of the pulse drive signal to the switching element 4 based on the primary feedback signal insulated and transmitted by the control device 16 is provided. The feedback circuit 11 further includes a series circuit of a capacitor 21 and a resistor 22 as a phase compensation circuit for preventing abnormal oscillation of the operational amplifier 15.
Is connected between one input terminal and the output terminal. Further, a series circuit of a photodiode 16A, which is a light emitting element of the photocoupler 16, and a resistor 23 includes an operating voltage Vcc2 (or output voltage Vo) line on the secondary side of the transformer 3 and an operational amplifier 15a.
Connected to the output terminal of Note that the resistor 23 sets the amplification factor of the operational amplifier 15. Further, a series circuit of a phototransistor 16B, which is a light receiving element of the photocoupler 16, and a resistor 24 forms an operating voltage V on the primary side of the transformer 3.
A feedback signal is supplied to the control IC 17 from the connection point of the phototransistor 16B and the resistor 24, which is connected between the cc1 line and the ground line. When the output voltage Vo increases during the switching operation of the switching element 4, for example, the resistance 1
The voltage level of the detection signal obtained from the connection point between 2 and 13 also increases, and the output terminal voltage Vamp of the operational amplifier 15 as a result of comparison with the reference voltage of the reference power supply 14 decreases. This allows
The current If of the secondary feedback signal flowing through the photodiode 16A, and thus the amount of light emitted from the photodiode 16A, increases, and the current Ic of the primary feedback signal flowing through the phototransistor 16B also increases.
The feedback terminal voltage Vfb of C17 rises and the control I
C17 performs control to reduce the conduction width of the pulse drive signal to the switching element 4 in order to lower the output voltage Vo.
On the other hand, when the output voltage Vo decreases, the resistance 1
The voltage level of the detection signal obtained from the connection point between 2 and 13 decreases, and the output terminal voltage Vamp of the operational amplifier 15 increases.
As a result, the current If flowing through the photodiode 16A and thus the light emission amount of the photodiode 16A decrease, the current Ic flowing through the phototransistor 16B decreases, and the feedback terminal voltage Vfb of the control IC 17 decreases. Therefore, in this case, in order for the control IC 17 to increase the output voltage Vo, control is performed to increase the conduction width of the pulse drive signal to the switching element 4. The photocoupler 16 constituting the feedback circuit 11 has a conversion efficiency CTR (= (Ic /
If) × 100) has a characteristic that varies with the ambient temperature. The conversion efficiency CTR of the photocoupler 16 indicates the ratio of the current Ic flowing through the phototransistor 16B on the output side to the current If flowing through the photodiode 16A on the input side, and is expressed as a percentage (%).
Is represented by For example, as shown in FIG.
If the conversion efficiency CTR of the photodiode 16A decreases as the temperature Ta increases, the current Ic flowing through the phototransistor 16B increases as the temperature Ta increases, even if the current If flowing through the photodiode 16A is the same. Is reduced.
Therefore, in a region where the conversion efficiency CTR is low and the temperature Ta is high, a variation (ΔIc / ΔVamp) of the current Ic flowing through the phototransistor 16B with respect to a variation of the output terminal voltage Vamp of the operational amplifier 15 depending on the output voltage Vo. Decreases,
As a result, the variation of the feedback terminal voltage Vfb with respect to the variation of the output terminal voltage Vamp (ΔVfb / ΔVamp)
And the gain of the feedback circuit 11 decreases. As described above, the conversion efficiency CT of the photocoupler 16 is
When R changes according to the temperature Ta, the gain of the feedback circuit 11 decreases at a specific temperature Ta, and the load (not shown) connected between the output terminals 7 and 8 changes suddenly or the input voltage Vi changes suddenly. , The response performance of the output voltage Vo deteriorates (in the example of FIG.
Response performance deteriorates). SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problem, and has as its object the advantage that even when the ambient temperature changes, the gain is not affected by the temperature characteristics of the conversion efficiency of the photocoupler. Is to obtain a feedback circuit of a power supply device capable of keeping the constant substantially constant. According to the present invention, there is provided a feedback circuit for a power supply device, which achieves the above object.
In a feedback circuit of a power supply device for stabilizing the output voltage, a feedback signal corresponding to a change in the output voltage is transmitted by a photocoupler combining a light receiving element and a light emitting element. It is provided with a temperature compensating element for compensating for a change in conversion efficiency of the element due to temperature. In this case, even when the conversion efficiency of the photocoupler fluctuates as the ambient temperature of the power supply changes, the fluctuation of the conversion efficiency can be compensated for by the temperature compensation element provided in the feedback circuit. it can. Therefore, the gain of the feedback circuit can be kept substantially constant without being affected by the temperature characteristics of the conversion efficiency of the photocoupler. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In each embodiment, the same parts as those of the conventional example are denoted by the same reference numerals, and description of common parts will be omitted because they are duplicated. FIG. 1 to FIG. 4 show a first embodiment of the present invention. In FIG. 1 showing a circuit configuration of the power supply device,
In this embodiment, as a temperature compensating element for compensating a change in the conversion efficiency CTR of the photocoupler 16 from the photodiode 16A to the phototransistor 16B due to temperature, a thermistor is provided between both ends of a resistor 23 forming a series circuit with the photodiode 16A.
31 are connected in parallel. Other configurations are completely the same as those of the conventional FIG. In the circuit configuration shown in FIG.
If the conversion efficiency CTR of the thermistor 31 has a temperature characteristic that decreases with an increase in the temperature Ta as shown in FIG. 7, for example, the resistance-temperature characteristic of the thermistor 31 becomes as shown in FIG. A so-called negative characteristic in which the resistance value decreases as the temperature Ta increases is selected. Conversely, photocoupler 16
If the conversion efficiency CTR has a temperature characteristic that decreases as the temperature Ta decreases, a thermistor 31 having a positive characteristic in which the resistance value increases as the temperature increases as shown in FIG. 3 is selected. do it. Next, the operation of the above configuration will be described. In the main converter circuit, the switching element 4 is switched by a pulse drive signal from the control IC 17, and the DC input voltage Vi is intermittently applied to the primary winding 3A of the transformer 3. Is applied to As a result, a voltage corresponding to the turn ratio with respect to the primary winding 3A is induced in the secondary winding 3B of the transformer 3, and this voltage is rectified and smoothed by the rectifying and smoothing circuit 5 so that the output terminals 7, 8 A predetermined DC output voltage Vo is applied between both ends.
Occurs. On the other hand, the feedback circuit 11 forming the feedback loop of the main converter circuit divides the output voltage Vo by the resistors 12 and 13 on the secondary side of the transformer 3, and outputs the divided voltage from the divided detection signal and the reference power supply 14. The reference voltage is compared with an operational amplifier 15 and amplified. Then, the secondary feedback signal based on the comparison result is insulated and transmitted via the photocoupler 16 to the control IC 17 on the primary side of the transformer 3. In this case, if the output voltage Vo increases, the currents If and Ic of the secondary and primary side feedback signals increase, the feedback terminal voltage Vfb of the control IC 17 increases, and the control IC 17 When the output voltage Vo decreases, the currents If and Ic of the secondary-side and primary-side feedback signals decrease, and the feedback terminal voltage Vfb of the control IC 17 decreases. Then, the control IC 17 performs control to increase the conduction width of the pulse drive signal to the switching element 4. On the other hand, the conversion efficiency CTR of the photocoupler 16 is
For example, in a case where the photocoupler has the temperature characteristics shown in FIG.
The conversion efficiency CTR of 16 gradually decreases. However, since the resistance value of the thermistor 31 connected in series with the photodiode 16A also gradually decreases, even if the output terminal voltage Vamp of the operational amplifier 15 is the same, when the temperature Ta rises, the secondary side flowing through the photodiode 16A The current If of the feedback signal increases. That is, as the temperature Ta rises, the variation of the current If flowing through the photodiode 16A with respect to the variation of the output terminal voltage Vamp of the operational amplifier 15 (Δ
If / ΔVamp) increases, and the current Ic flowing through the phototransistor 16B with respect to the variation of the output terminal voltage Vamp
(ΔIc / ΔVamp) is kept almost constant without lowering. Therefore, the variation of the feedback terminal voltage Vfb with respect to the variation of the output terminal voltage Vamp (Δ
Vfb / ΔVamp) is also substantially constant, and the response performance of the output voltage Vo does not deteriorate when the load or the input voltage Vi changes suddenly. When designing the feedback circuit 11 having the above configuration, the feedback circuit 11 must be designed to operate within the operating temperature range of the power supply device (for example, the range of temperatures Ta1 to Ta2 in FIG. 7).
When the conversion efficiency CTR of the photocoupler 16 is the highest (the highest point Po in FIG. 7), a margin is provided to some extent so that the feedback system does not become unstable. For example, a capacitor as a phase compensation circuit of the operational amplifier 15 is provided. It is preferable to determine the constants of the resistor 21 and the resistor 22. In addition, the conversion efficiency CTR of the photocoupler 16 within the operating temperature range Ta1 to Ta2.
The temperature range in which the temperature decreases (in FIG. 7, the high temperature side toward the temperature Ta2) may be compensated by the thermistor 31. In this case, the gain of the feedback circuit 11 is set to the operating temperature range Ta1.
It becomes almost constant in all of the range from Ta2 to Ta2, and a desirable response performance can be obtained. The conversion efficiency CTR of the photocoupler 16 exhibits various temperature characteristics depending on each element. For example, in the case of the temperature characteristic as shown in FIG. 4, the highest point Po of the conversion efficiency CTR of the photocoupler 16 is in the middle of the operating temperature range Ta1 to Ta2. The feedback system is designed so that the feedback system does not become unstable at the temperature Ta of the highest point Po at which the highest point is obtained, and the temperature range where the conversion efficiency CTR of the photocoupler 16 is reduced by the thermistor 31 (in this example, mainly the highest point Low temperature side from Po)
Should be compensated for. As described above, in this embodiment, the output voltage Vo
A feedback signal corresponding to the fluctuation of the photodetector 16 is transmitted by a photocoupler 16 in which a photodiode 16A as a light receiving element and a phototransistor 16B as a light emitting element are combined to stabilize the output voltage Vo. The feedback circuit 11 includes a thermistor 31 as a temperature compensating element for compensating a change in the conversion efficiency CTR from the photodiode 16A of the photocoupler 16 to the phototransistor 16B due to the temperature Ta. In this case, even if the conversion efficiency CTR of the photocoupler 16 fluctuates as the ambient temperature of the power supply changes, the thermistor provided in the feedback circuit 11
The fluctuation of the conversion efficiency CTR can be compensated by 31. Therefore, the gain of the feedback circuit 11 can be kept almost constant without being affected by the temperature characteristics of the conversion efficiency CTR of the photocoupler 16. Further, in this embodiment, as the conversion efficiency CTR of the photocoupler 16 decreases with the temperature Ta, the temperature compensating element or thermistor 31 having the characteristic of increasing the current If of the feedback signal flowing through the photodiode 16A is provided.
It is connected in series with the photodiode 16A. In this case, even if the level of the output voltage Vo is constant, if the conversion efficiency CTR of the photocoupler 16 decreases due to the temperature Ta, the thermistor 31 operates so as to compensate for the reduction in the conversion efficiency CTR. The current If of the feedback signal flowing through 16 A is increased. Therefore,
The gain of the feedback circuit 11 is kept almost constant irrespective of the change in the temperature Ta. Note that, instead of connecting the thermistor 31 in parallel with the resistor 23 as in this embodiment, the thermistor 31 is connected instead of the resistor 23, and the detection signal of the output voltage Vo and the reference voltage are compared and amplified. May also be used as an element for determining the amplification factor of the operational amplifier 15 for performing the operation. By doing so, the number of elements in the feedback circuit 11 does not need to be increased. Next, a second embodiment of the present invention will be described with reference to FIG. The same portions as those in the first embodiment are denoted by the same reference numerals, and the description of the common portions will be omitted because they are duplicated. In this embodiment, a thermistor 31 as a temperature compensating element is connected in parallel with a resistor 24 forming a series circuit with the phototransistor 16B. In the circuit configuration shown in FIG. 5, the conversion efficiency CTR of the photocoupler 16 is
For example, as shown in FIG. 7, when the temperature characteristic is such that the temperature decreases as the temperature Ta increases, the thermistor
As shown in FIG. 3, a resistance-temperature characteristic of 31 is selected such that the resistance value increases as the temperature Ta increases. Conversely, when the conversion efficiency CTR of the photocoupler 16 has a temperature characteristic that decreases as the temperature Ta decreases, as shown in FIG. 2, the resistance value decreases as the temperature increases. What is necessary is just to select the thermistor 31 of the characteristic. In this case, the conversion efficiency CTR of the photocoupler 16
However, for example, when the temperature characteristic shown in FIG. 7 is provided, the conversion efficiency CTR of the photocoupler 16 gradually decreases as the ambient temperature of the power supply increases. However, the thermistor 31 connected in series with the phototransistor 16B
The resistance of the phototransistor 16B with respect to the variation of the output terminal voltage Vamp of the operational amplifier 15 gradually increases.
Although the variation (ΔIc / ΔVamp) of the current Ic flowing through the feedback terminal becomes small, the variation of the feedback terminal voltage Vfb with respect to the variation of the output terminal voltage Vamp (ΔVfb / ΔVamp)
Is kept almost constant without lowering, and the photocoupler 16
Is substantially compensated for. Therefore, also in this embodiment, the response performance of the output voltage Vo does not deteriorate when the load or the input voltage Vi changes suddenly. As described above, also in this embodiment, the photocoupler
16 photodiodes 16A to phototransistors 16B
Is provided with a thermistor 31 as a temperature compensating element for compensating the change in the conversion efficiency CTR due to the temperature Ta, so that as the ambient temperature of the power supply device changes, the
Even when the conversion efficiency CTR changes, the thermistor 31 provided in the feedback circuit 11 can compensate for the change in the conversion efficiency CTR. Therefore, without being affected by the temperature characteristics of the conversion efficiency CTR of the photocoupler 16,
The gain of the feedback circuit 11 can be kept almost constant. In this embodiment, as the conversion efficiency CTR of the photocoupler 16 decreases with the temperature Ta, the control IC
A temperature compensating element or thermistor 31 having such a characteristic that the voltage level applied to
B is connected in series. In this case, even if the level of the output voltage Vo is constant, if the conversion efficiency CTR of the photocoupler 16 decreases due to the temperature Ta, the thermistor 31 controls the conversion so as to compensate for the reduction in the conversion efficiency CTR. The voltage level applied to the IC 17 is increased. Therefore, the gain of the feedback circuit 11 is kept almost constant irrespective of the change in the temperature Ta. The present invention is not limited to the above embodiment, but can be variously modified. For example, the light receiving element of the photocoupler in the present embodiment is a phototransistor, but the present invention can be applied to other photocouplers using a light receiving element such as a thyristor or a triac. As the temperature compensating element, a non-linear element such as a varistor can be used in addition to the thermistor.
Further, the power supply device can be applied not only to various converters having an insulation transformer such as a forward type or a flyback type but also to a non-insulated power supply device in which an insulation transformer does not exist in a main converter circuit. According to the feedback circuit of the power supply device of the present invention, even if the ambient temperature changes, the gain of the feedback circuit can be substantially reduced without being affected by the temperature characteristics of the conversion efficiency of the photocoupler. Can be kept constant.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a power supply device according to a first embodiment of the present invention. FIG. 2 is a graph showing resistance-temperature characteristics of a thermistor used as a temperature compensating element according to the first embodiment; FIG. 3 is a graph showing resistance-temperature characteristics of a thermistor used as a temperature compensating element according to the first embodiment; FIG. 4 is a graph showing the relationship between the conversion efficiency of a photocoupler and temperature. FIG. 5 is a circuit diagram of a power supply device according to a second embodiment of the present invention. FIG. 6 is a circuit diagram of a power supply device showing a conventional example. FIG. 7 is a graph showing the relationship between the conversion efficiency of a photocoupler and temperature. [Description of Signs] 11 Feedback circuit 16 Photocoupler 16A Photodiode (light-emitting element) 16B Phototransistor (light-receiving element) 31 Thermistor (temperature compensation element)

Claims (1)

Claims: 1. A power supply device for stabilizing an output voltage by transmitting a feedback signal corresponding to a change in an output voltage by a photocoupler combining a light receiving element and a light emitting element. A feedback circuit for a power supply device, wherein the circuit includes a temperature compensating element for compensating a change in conversion efficiency of the photocoupler from a light emitting element to a light receiving element with temperature.