JPH07213066A - High-power-factor switching power supply - Google Patents

Abstract

PURPOSE:To provide a high-power-factor switching power supply which holds an output voltage which is dropped at a momentary stop of an AC input voltage by a method wherein a capacitor for backup is arranged on the primary side of a transformer for a flyback sine-wave converter circuit. CONSTITUTION:A capacitor C1 charged at the peak value of an AC input is connected across a low-pass filter F for the AC input and a diode bridge. In addition, an AC line monitor which checks an AC input voltage and a switching circuit coupled by the monitor and a photocoupler PC2 and which supplies the electric charge of the C1 to the primary side of a transformer T are arranged. Consequently, when a voltage drop at the short break of the AC input voltage is detected by the AC line monitor, the switching circuit supplies the electric charge of the C1 to the primary side of a flyback converter. As a result, the output voltage is held.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high power factor switching power supply which uses a flyback sine wave converter circuit and can hold an AC input for a long time without interruption.

[0002]

2. Description of the Related Art A flyback type sine wave converter circuit is a converter circuit whose circuit and control are simple, the conventional control circuit of a switching circuit can be used as it is, and the power factor is almost 1.

The converter circuit will be briefly described below. FIG. 2 is a basic structural circuit diagram of a flyback sine wave converter. The peak value of the current flowing during the on-time of the switching element SW is shown in FIG. Ii ′ = I _L is expressed as Ii ′ = I _L = ei × T _on / L (1), where T _On is the on time of the switch and _L is the exciting inductance of the transformer. If the ON time of the switching element is constant during one cycle of the AC input voltage, the peak current Ii 'is proportional to the input voltage ei. Therefore, if the switching cycle T is constant, the average value iav of the input current in the switch is iav = Ii ′ × T _on / 2T, and this current value is proportional to the input voltage.

Since the average current for each switching is proportional to the input voltage as described above, the high frequency component of the switching waveform is removed by a low pass filter by L _F and C _F , so that one cycle of the AC line is shown in FIG. Thus, it becomes similar to the input voltage, and a power factor of almost 1 is obtained.

[0005]

In a conventional switching power supply, the AC input voltage is obtained as a DC power supply by a capacitor input type rectifier circuit, but in a flyback sine wave converter circuit, the AC input voltage is full-wave rectified. However, the smoothing capacitor is not used.
The output power cannot be held when the C input is momentarily cut off, and this holding requires a capacitor with a large electrostatic capacity on the secondary side.

Assuming that the terminal voltage of the charged capacitor C is V, the initial terminal voltage V ₀ decreases as the time t elapses as shown in FIG. 4, and the electrostatic capacitance becomes as shown in FIG. The larger the value, the smaller the terminal voltage drop.

Here, the efficiency of the switching power supply is 100
%, All the energy charged in the smoothing capacitor of the capacitor input type rectifier circuit is supplied to the output capacitor, and when the capacitance is C and the terminal voltage is V, the energy of the capacitor is C.・ Since it is proportional to V ² , the smoothing capacitor capacity of the capacitor input type rectifier circuit is C ₁ , the rectified output voltage is V ₁ , the secondary output voltage is V ₂ , and the output capacitor capacity is C ₂ C ₁ V _From the formula ₁ ² = C ₂ V ₂ ² , C ₂ = (V ₁ / V ₂ ) ² C ₁ (2). Now, C ₁ = 220 μF, V ₁ = 140 V, V ₂ = 5 V
Then, the above formula (2) becomes C ₂ = (140/5) ² × 220 × 10 ^-6 = 172480 μF, and thus it is necessary to arrange such a large-capacity capacitor on the secondary side. However, the volume ratio is about 16 times that of the input side, and there is a drawback that the size cannot be reduced.

The present invention has been made in view of the above problems, and an object thereof is to use a flyback type sine wave converter circuit having a high power factor to reduce the output voltage when the AC input voltage is interrupted. To prevent this, a backup capacitor is inserted in the primary side instead of the large-capacity capacitor on the secondary side to provide a high power factor switching power supply that is intended to be downsized.

[0009]

According to the present invention to achieve the above object, a flyback sine wave converter is used.
High power factor switching power supply using
A backup capacitor provided in the input circuit for charging by rectification of the AC input, an AC line monitor for checking the AC input voltage and outputting the voltage drop, and an output for receiving the output from the AC line monitor. There is provided a high power factor switching power supply including a switch circuit for supplying the electric charge of a capacitor to the transformer primary side of a converter circuit.

[0010]

A capacitor for backup which is charged by the peak value of the AC input voltage is arranged, and a signal from an AC line monitor for checking the drop of the AC voltage is sent to the switch circuit when the AC input is momentarily cut off. Since the switch circuit energizes the charge of the backup capacitor to the primary side of the transformer, a decrease in the output voltage on the secondary side is prevented.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a high power factor switching power supply according to the present invention.

In the figure, the AC input voltage ei is full-wave rectified by a set of diodes D _{1 to} D ₄ through a low-pass filter F and becomes the input voltage of the flyback converter.

A constant voltage circuit is connected to keep the output voltage constant, the constant voltage circuit monitors the output voltage, and an error amount from the reference circuit is detected by a photocoupler PC _1.
Is input to the PWM control circuit to control the on time of the power switch Q ₁ .

A photocoupler-an AC line monitor such as PC ₂ or R _1- is connected to the AC input circuit.
The photo coupler P on the input side of the flyback converter.
A switch circuit having switch elements Q ₂ and Q ₃ connected by C ₂ is connected, and the switch circuit is an AC circuit.
The charge of the capacitor C ₁ charged by the peak of the input voltage is controlled to be supplied to the primary side of the flyback converter.

Note that R ₃ and C ₅ of the switch circuit are Q ₃ when the base circuit of the switch element Q ₂ has a predetermined time constant.
That stabilizes the operation of C, C ₄ is a small capacity that can supply one cycle of high frequency switching power and does not affect the deterioration of the power factor, D ₇ is a photocoupler-PC
_For protection against reverse withstand voltage ₂ , R ₂ , C ₆ and D ₅ are snubber circuits.

[0016] Referring next to the operation of this embodiment having such a configuration, AC input voltage ei is b - pass filter, diode - de D ₁ to D ₄ and through, power - switch Q ₁
Is turned on, it is applied to the primary side of the transformer T of the flyback converter, the output of the secondary side thereof is rectified by D ₆ and input to the constant voltage circuit.

In the constant voltage circuit, the error is compared with a predetermined reference voltage and sent to the PWM control circuit via PC ₁ to control the on time of the power switch Q ₁ to control the flyback converter. The output voltage of is controlled to be a constant voltage.

[0018] Then, the photocoupler -PC ₂ when the AC input voltage is normal is conductive and thus Q ₂ base - scan is turned off at zero potential, and Q ₃ is also turned off.

On the other hand, the capacitor C ₁ is charged to the peak value of the AC input voltage by the diode D ₈ , but when the AC input voltage is normal, Q ₃ is off as described above.
The terminal voltage of the capacitor C ₁ is kept constant and irrelevant to the operation of the flyback converter, and it does not affect the input current waveform of the flyback converter.

Here, when the AC input voltage is momentarily cut off and turned off, for example, a photo coupler which becomes an AC line monitor.
Since PC ₂ is disconnected and Q ₂ and Q ₃ are turned on, the charge stored in the capacitor C ₁ is supplied to the transformer T through Q ₃ and the output voltage on the secondary side is held. Become.

Although the present invention has been described with reference to the above embodiments, various modifications and applications are possible within the scope of the gist of the present invention, and these modifications and applications are not excluded from the scope of the present invention. Absent.

[0022]

According to the present invention as in the above embodiments,
On the primary side of the flyback sine wave converter circuit, a backup capacitor, which is always charged by the AC input voltage, is placed, and a switch circuit that monitors the AC input voltage when the AC input is interrupted Since the charge of the backup capacitor is supplied to the primary side, the output voltage of the secondary side is held, and therefore, the large-capacity capacitor on the secondary side is unnecessary, so that the size can be reduced and a high power factor switching power supply can be obtained. .

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a basic configuration circuit of a flyback sine wave converter.

FIG. 3 is a waveform diagram showing an operation in a flyback sine wave converter.

FIG. 4 is an explanatory diagram and a mathematical diagram of a terminal voltage and time during discharging of a capacitor.

FIG. 5 is a curve diagram showing the relationship between the terminal voltage and the capacitance of a capacitor.

Claims (1)

[Claims] 1. In a high power factor switching power supply using a flyback sine wave converter circuit, a backup capacitor provided in an AC input circuit and charged by rectification of the AC input and an AC input voltage are checked. And an AC line monitor for outputting the voltage drop, and a switch circuit for receiving the output from the AC line monitor and energizing the electric charge of the capacitor to the transformer primary side of the converter circuit. High power factor switching power supply.