JP4136134B2 - DC power supply circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an overvoltage protection circuit for a DC power supply circuit, and is intended to increase the accuracy of an overvoltage operating voltage of a secondary side output voltage and enhance the protection of an output side load.
[0002]
[Prior art]
FIG. 2 is a DC power supply circuit that has been used in the past and detects overvoltage of the supply voltage Vcc of the pulse width control IC 8 to protect the overvoltage. Although it is inexpensive, the accuracy of the overvoltage value is poor, and FIG. 2 is a DC power supply circuit in which an overvoltage detection element 25 is connected to the secondary output of the circuit of FIG. 2, and a circuit that sends a signal to the overvoltage protection terminal of the pulse width control IC 8 by a photocoupler 26 to protect the overvoltage. Is good, but material costs are high.
[0003]
[Problems to be solved by the invention]
As described above, the DC power supply circuit of FIG. 2 that detects the overvoltage of the supply voltage Vcc of the pulse width control IC 8 and stops the pulse width control, which has been conventionally used, is low in material cost, but is secondary. It takes time to overvoltage detection of the supply voltage Vcc of IC8 substantially proportional to the DC voltage V ₀ which side output, there is a drawback that the input voltage fluctuation, can not accurately detect the overvoltage operation value of the DC voltage V0 due to the output load change.
Also been used conventionally, the two via a primary side output detecting photocoupler overvoltage of the DC voltage V ₀ which, in the method of stopping the pulse width control signal the pulse width control IC8 DC power supply of FIG. 3 Although the circuit has good detection accuracy of the overvoltage operating value of the DC voltage V ₀ , there is a disadvantage that the material cost becomes high.
[0004]
[Means for Solving the Problems]
The present invention solves the above-mentioned problem, and the secondary winding 6 induced by applying a voltage to the primary winding 5 of the transformer 4 by turning on and off the transistor 3 by the pulse width control IC 8. The voltage is rectified and smoothed to output the DC voltage V ₀ , the error voltage of V ₀ is detected and fed back to the IC 8, and the supply voltage Vcc substantially proportional to V ₀ is detected from the winding 7 of the transformer 4 for overvoltage detection. In a DC power supply circuit that supplies the IC 8 via the element 9,
The overvoltage of the DC voltage V ₀ is detected by the overvoltage detection element 22, and the impedance between the Vz terminal connected to the error detection element 17 and the secondary winding 6 is lowered by the transistor 20 to reach the overvoltage of Vcc. A DC power supply circuit characterized by accelerating the speed and stopping the pulse width control of the IC8.
[0005]
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, an overvoltage of the secondary output DC voltage V ₀ is detected by an overvoltage detection element (zener diode) 22 and connected to an error detection element 17 for stabilizing the DC voltage V ₀ . by lowering the impedance between the Vz terminal and the secondary winding 6 by transistors 20, to accelerate the overvoltage increase of the DC voltage V ₀ by the equation [equation 1] showing the relationship between the DC voltage V ₀ and the reference voltage Vz, Since the supply voltage Vcc of the pulse width control IC 8 is also increased at an accelerated rate, the overvoltage detection element (zener diode) 9 operates quickly to stop the pulse width control and protect the DC power supply circuit.
[0006]
[Expression 1]
V ₀ = (1 + R ₂ / R ₁ ) × Vz
Here, R ₂ : resistance value of the resistor R ₁ : parallel value of the resistance value of the resistor 19 and the resistance value between the emitter and collector of the transistor 20 V ₀ : DC voltage of the secondary side output Vz: DC voltage Reference voltage of V ₀ error detection element 17
【Example】
FIG. 1 is a basic circuit of an embodiment of the present invention. When the overvoltage operation state of the DC voltage V ₀ on the output side is compared between this circuit and the conventional DC power supply circuit of FIG. 3 using the photocoupler 26, the accuracy of the overvoltage operation value is equal and the material cost is the photocoupler. The price was reduced by 26. In addition, the accuracy of the overvoltage operating value of V ₀ can be remarkably improved as compared with the DC power supply circuit of FIG.
[0008]
【The invention's effect】
The present invention, the transistor 20 by detecting an overvoltage of the DC voltage V ₀ which secondary output Overvoltage detection element (Zener diode) 22 to flow the base current, by lowering the impedance between the emitter and the collector of the transistor 20 Then, the impedance between the Vz terminal connected to the error detection element 17 and the secondary winding 6 is lowered, and the overvoltage rise of the DC voltage V ₀ is accelerated by the above-described equation [Equation 1], and is approximately proportional to V _0. Since the supply voltage Vcc of the pulse width control IC 8 to be increased is accelerated and the overvoltage detection element (zener diode) 9 is operated to stop the pulse width control, the DC power supply circuit can be safely protected. . Therefore, compared to the conventional pulse width control stop method that does not use an overvoltage detection element or transistor, the overvoltage operating value of V ₀ can be detected with high accuracy, and since an expensive photocoupler is not used, the material cost is low. There is an advantage that a circuit configuration can be achieved.
[Brief description of the drawings]
FIG. 1 is a DC power supply circuit showing an embodiment of the present invention.
FIG. 2 shows a conventional DC power supply circuit for overvoltage protection.
FIG. 3 is a conventional DC power supply circuit using a photocoupler for overvoltage protection.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Commercial diode 2, 10, 15 Capacitor 3, 20 Transistor 4 Transformer 5 Transformer primary winding 6 Transformer secondary winding 7 Transformer winding 8 for supply voltage Vcc Pulse width control IC
9, 22, 25 Overvoltage detection element (Zener diode)
11, 14 Diodes 12, 16, 18, 19, 21, 23, 24 Resistors 13, 26 Photocoupler 17 Error detection element Vcc Pulse width control IC supply voltage Vz Error detection IC reference voltage

Claims (1)

The voltage of the secondary winding (6) induced by applying a voltage to the primary winding (5) of the transformer (4) by turning on / off the transistor (3) by the pulse width control IC (8). The DC voltage V ₀ is output by rectification and smoothing, the error voltage of V ₀ is detected and fed back to the IC (8), and the supply voltage Vcc substantially proportional to V ₀ is applied to the winding (7 ) To the IC (8) via the overvoltage detection element (9),
The overvoltage of the DC voltage V ₀ detected by the overvoltage detection element (22), the impedance between the connected Vz terminal and the secondary winding to the error detection device (17) (6) transistor (20) The DC power supply circuit is characterized by accelerating the overvoltage arrival speed of Vcc and stopping the pulse width control of the IC (8).

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