JPH0545112Y2 - - Google Patents

Description

[Detailed Description of the Invention] "Field of Industrial Application" The present invention relates to a switching power supply circuit whose purpose is to eliminate unstable operation when the input power supply voltage does not reach the rated value.

“Conventional technology” This type of conventional switching power supply circuit
As shown in the figure, the DC input voltage Vi between input terminals 1 and 2 is connected to a Zener diode 4 through a resistor 3.
In addition, a stabilized voltage Vcc was obtained, which was divided by voltage dividing resistors 5 and 6, and a reference voltage Vf ₁ was applied to one input side of the comparator 8.

In the conventional circuit as described above, the input voltage Vi
is turned on and the voltage rises as shown in Figure 2a. _{At t1} when a certain voltage is reached, a starting pulse is generated from the starting circuit 9 as shown in Figure 2b, turning on the transistor 10 and turning on the transformer 11. A current Id ₁ flows through the primary winding 12 of , and the voltage across the shunt resistor 13 increases.
Vd ₁ increases as shown in Figure 2c. When the potential Vd ₁ across the shunt resistor 13 exceeds the reference voltage Vf ₁ at time t ₂ and an output appears from the comparator 8, this output is applied to the base of the transistor 10, and the transistor 10 is turned off. The flyback magnetic flux of the transformer 11 returns to zero, turning off the transistor 10, but this time, via the tertiary winding 14, the transistor 10 turns on at time _t3 . In this way, self-oscillation continues.

"Problem to be solved by the invention" As described above, in the method of stabilizing the reference voltage Vf ₁ by the Zener diode 4, the input voltage
Only the reference voltage Vf ₁ rises to the target voltage value at a time Tb that is sufficiently earlier than the time Ta at which Vi completely rises. In other words, the reference voltage Vf ₁ rises immediately, but the input voltage
Since Vi does not rise properly and is delayed, the on/off period of the transistor 10 becomes longer and the operation becomes unstable. In addition, when the input voltage Vi is not only applied but also abnormally lowered, the drive current of the transistor 10 decreases and the current flowing through the transistor 10 decreases, making it uncontrollable. There were problems such as damage to the product.

The object of the present invention is to provide a switching power supply circuit that does not operate unstable even when the input power is turned on or when an abnormal drop in input voltage occurs.

"Means for Solving the Problem" This invention converts the current flowing through the main switch of an inverter circuit into a voltage using a shunt resistor, compares this voltage with a reference voltage using a comparator, and uses the output of this comparator to In the switching power supply in which the conduction time of the inverter circuit is controlled by turning on and off the main switch, the reference voltage detection circuit is coupled to the input power supply terminal via a voltage dividing resistor. be.

"Operation" The reference voltage input to one side of the comparator is a voltage corresponding to the input voltage divided by a resistor and applied. That is, the reference voltage is changed in accordance with the input voltage and is input to the comparator. As a result, when the input voltage decreases, the reference voltage also decreases, and the main switch operates to reduce the current. Therefore, the period of the main switch hardly changes even when the input voltage rises or falls. It becomes constant and unstable operation disappears.

"Embodiment" An embodiment of the present invention will be described below with reference to FIG.

The circuit shown in Fig. 1 is basically the same as the conventional circuit shown in Fig. 3, which is a current mode system that controls voltage in current mode every half cycle. The voltage Vf ₂ detection circuit 7 is directly connected to the input terminals 1 and 2 to detect the reference voltage.
While changing Vf ₂ according to the input voltage Vi,
The reference voltage Vf ₂ is applied only when the output voltage Vo is output. Therefore, the reference voltage detection circuit 7 is configured by connecting in series a phototransistor 16 of a photocoupler 15 that is turned on when an output voltage is output, and voltage dividing resistors 5 and 6, and a photodiode 17 of the photocoupler 15 is configured to detect the output voltage. It is connected between output terminals 19 and 20 via a circuit 18. Note that 25 is a secondary winding, 21 is a rectifier diode, and 22 is a smoothing capacitor.

In the above configuration, when the input voltage Vi rises as shown in Fig. 2a, a pulse signal as shown in Fig. 2b is output from the starting circuit 9 at time _t1 , and the transistor 10 as the main switch is activated. Turn on. When this transistor 10 is turned on, a current Id ₂ flows through the transistor 10, which is converted into a voltage by the shunt resistor 13.
It is converted to Vd ₂ and input to one side of the comparator 8. The reference voltage Vf ₂ inputted to the other side is low while the input voltage Vi is low, as shown in Fig. 2d.
It gradually rises in accordance with Vi, and takes approximately the same time as the rise time Ta of the input voltage Vi to rise. This means that while the reference voltage Vf ₂ is low, the comparator 8 immediately
t Reverse at ₁₀ o'clock. Then, the output of the comparator 8 operates to turn off the transistor 10, thereby controlling the on-time of the inverter circuit mainly composed of the transformer 11 and the transistor 10. Thereafter, a base current flows through the transistor 10 through the tertiary winding 14 of the transformer 11, the capacitor 23, and the resistor 24, turning it on, and switching continues in the same manner.

``Effects of the invention'' As mentioned above, this invention takes into consideration that the current flowing through the transistor, which is the main switch, corresponds to the input voltage, and the reference voltage is also changed in accordance with the input voltage, so that the input voltage When the voltage rises or falls abnormally, the reference voltage and the transistor current also change, and the transistor always maintains a constant cycle, resulting in stable operation.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an embodiment of the switching power supply circuit according to the present invention, FIG. 2 is an operating waveform diagram, and FIG. 3 is a conventional circuit diagram. 1, 2... Input terminal, 5, 6... Voltage dividing resistor, 7
...Reference voltage detection circuit, 8...Comparator, 9...Start circuit, 10...Transistor (main switch), 1
1...Transformer, 12...Primary winding, 13...Shunt resistor, 14...Tertiary winding, 15...Photocoupler, 16...Phototransistor, 17...Photodiode, 18...Output voltage detection circuit ,19,
20...Output terminal, 25...Secondary winding.

Claims (1)

[Claims for Utility Model Registration] (1) The current flowing through the main switch of the inverter circuit,
A switching device that converts the voltage into a voltage using a shunt resistor, compares this voltage with a reference voltage using a comparator, and controls the conduction time of the inverter circuit by turning on and off the main switch based on the output of the comparator. A switching power supply circuit characterized in that the reference voltage detection circuit is coupled to an input power supply terminal via a voltage dividing resistor. (2) The switching power supply circuit according to claim 1, wherein the reference voltage detection circuit includes a photocoupler inserted in series with the voltage dividing resistor, and an output voltage detection circuit coupled to the photocoupler.