JPH08294283A - Power supply circuit - Google Patents

Abstract

PURPOSE: To protect an electrolytic smoothing capacitor with a simple configuration. CONSTITUTION: A varister 5 is connected in parallel with an electrolytic capacitor C1 for smoothing an output voltage of a power factor improving circuit 2. The varistor 5 is a voltage dependent resistor which suddenly reduces the resistance when an output voltage Vout exceeds a varistor voltage Vv. This varistor 5 is formed for example, by sintering ceramics such as zinc oxide or the like and is usually covered with a scattering preventing member 6. The scattering preventing member 6 is constituted, for example, of insulation tube formed by a material of the flame resistance grade of V0 in the UL standard such as silicone rubber or the like. Since the scattering preventing member closely covers the varistor 5, even if the short-circuited varistor 5 is broken, it maintains the short-circuit condition by preventing the opening condition due to the scattering of material pieces.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit having a power factor correction circuit.

[0002]

2. Description of the Related Art Conventionally, a DC power source obtained by rectifying and smoothing an AC input from an AC power source with a diode and a capacitor is turned on and off at high speed by a switching element, and after being stepped up and down by a transformer, converted to a direct current again. Power supply circuit is used. In such a power supply circuit, the input current flows only while the smoothing capacitor is charged with a voltage, and the current does not flow when the input voltage is lower than the terminal voltage of the smoothing capacitor. For this reason, the input current waveform has a steep pulse shape, and the sine waveform has collapsed to contain a large amount of harmonic current. Therefore, regulations on harmonic currents tend to become stricter worldwide, mainly in Europe.Therefore, in order to reduce the harmonic current, a power supply circuit equipped with a power factor correction circuit of a step-up active filter system is used. There is.

FIG. 3 is a circuit diagram of a conventional power supply circuit having a power factor correction circuit of a boost active filter type.
The power supply circuit 1 includes a bridge diode BD that rectifies an AC input from a connected AC power supply AC, a power factor correction circuit 2, an electrolytic capacitor C1 for smoothing, and a DC-DC converter 3.

The power factor correction circuit 2 is composed of a coil L, a diode D1, an FET 21 and a PWM control circuit 22, and the PWM control circuit 22 inputs an input current Iin and an input voltage V.
By detecting in and outputting a control signal to turn on / off the FET 21, the output voltage Vout is boosted from 140V to 200V, and the waveform of the input current Iin approaches a sine waveform, that is, the waveform of the input voltage Vin. It is like this.

The output voltage Vout smoothed by the electrolytic capacitor C1 is output to the DC-DC converter 3. Then, in the DC-DC converter 3, this output voltage Vout is input to the primary winding of the insulation transformer, and this is switched by the on / off oscillation of the switching element connected in series with this primary winding, and this switching is performed. Thus, the power is supplied to the connected load 4 by the voltage induced in the secondary winding.

In the power supply circuit 1 as described above, the circuit elements constituting the PWM control circuit 22 are short-circuited during use,
If an abnormal condition such as a short circuit test of the circuit element occurs, P
The operation of the WM control circuit 22 becomes unstable. As a result, when the output voltage Vout rises and exceeds the withstand voltage of the smoothing electrolytic capacitor C1, the electrolytic capacitor C1 generates heat, which may be eventually destroyed and the electrolytic solution inside may be scattered.

Therefore, conventionally, the following measures have been taken. As shown in FIG. 4, the crowbar circuit 11 is interposed between the electrolytic capacitor C1 and the DC-DC converter 3, and Vou
When t> V _Z + V _G , the thyristor SCR is turned on and short-circuited to protect the electrolytic capacitor C1. By connecting both ends of the thyristor SCR of the crowbar circuit 11 between the power source of the PWM control circuit 22 and the ground instead of the electrolytic capacitor C1, and short-circuiting the power source of the PWM control circuit 22 to stop the operation, the electrolytic capacitor C1 is connected. Protect. As the electrolytic capacitor C1, for example, one having a high breakdown voltage of 450 V or the like is used.

[0008]

However, since the crowbar circuit 11 and the high withstand voltage electrolytic capacitor C1 are used in the above-mentioned conventional power supply circuit, the space occupied by the circuit components increases and the printed circuit board becomes large. I will end up.
Further, since an excessive current flows at the time of an abnormality such as a short circuit as described above, the FETs 21 and 32 and the SCR having a large rating are required, which further increases the occupied space. Further, these increase the cost of parts.

An object of the present invention is to solve the above problem and to provide a power supply circuit having a power factor correction circuit, which protects the smoothing electrolytic capacitor with a simple structure.

[0010]

In order to achieve the above object, the present invention rectifies an AC input from an AC power source connected via an overcurrent protection member which opens terminals when an excessive current flows. Rectifying means, capacitor for smoothing rectified voltage, and DC-DC for converting smoothed output voltage
In a power supply circuit provided with a converter, the rectifying means and the capacitor, and a power factor correction circuit for making the current waveform of the AC input approach a sine waveform, the power supply circuit is connected in parallel to the capacitor, and the output voltage is Is provided with a varistor formed of a sintered body of ceramics, which has a resistance value lowering when the voltage exceeds a preset voltage, and a scattering prevention member that prevents scattering of fragments when the varistor breaks. (Claim 1).

Further, the scattering prevention member comprises an insulating tube which closely covers the varistor (claim 2).

[0012]

According to the first aspect of the present invention, for example, the operation of the power factor correction circuit becomes unstable and the output voltage rises. When the voltage exceeds a preset voltage, the resistance value of the varistor decreases and the short-circuit state occurs. Will be destroyed. At this time, even if the varistor formed of the ceramic sintered body is broken, the fragments are prevented from scattering, so that the short-circuited state is maintained without being opened. Therefore, an overcurrent flows to the input and the overcurrent protection member is opened.

According to the second aspect of the invention, for example, the operation of the power factor correction circuit becomes unstable and the output voltage rises, and when the voltage exceeds a preset voltage, the resistance value of the varistor decreases. It will be short-circuited and destroyed. At this time, even if the varistor made of a ceramic sintered body is broken, it is closely covered with the insulating tube, so that the fragments are prevented from scattering and the short-circuit state is maintained without opening. To be done. Therefore, an overcurrent flows to the input and the overcurrent protection member is opened.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A is a circuit diagram of an embodiment of a power supply circuit according to the present invention. This power supply circuit 1 has an input terminal 1
When 1 and 12 are connected to AC power supply AC, output terminal 1
A DC current is supplied to a load 4 such as a motor connected between the No. 3 and No. 14, and is composed of a bridge diode BD, a power factor correction circuit 2, an electrolytic capacitor C1, a DC-DC converter 3 and a varistor 5. The power factor correction circuit 2
The waveform of the input current Iin is a sine waveform, that is, the input voltage Vin
Of the coil L, diode D1,
It is composed of an FET 21 and a PWM control circuit 22. The DC-DC converter 3 obtains a direct current by switching an input voltage, and is composed of a transformer 31 including a primary winding L1 and a secondary winding L2, an FET 32, a PWM control circuit 33, and the like.

The AC power supply AC is, for example, a commercial power supply of AC100V. The bridge diode BD and the electrolytic capacitor C1 perform full-wave rectification on the AC input of the AC power supply AC,
For smoothing, the negative electrode of the rectification output terminal of the bridge diode BD and the negative electrode of the electrolytic capacitor C1 are connected, and the coil L and the forward diode are connected between the positive electrode of the rectification output terminal of the bridge diode BD and the positive electrode of the electrolytic capacitor C1. D1 is connected in series. The FET 21 has a drain connected to the anode of the diode D1, a source connected to the negative electrode of the electrolytic capacitor C1, and a gate connected to the PWM control circuit 22. A fuse F as an overcurrent protection member is provided between the input terminal 11 and the bridge diode BD. The overcurrent protection member is replaced by the fuse F,
A breaker or the like may be used.

The PWM control circuit 22 detects the input current Iin and the input voltage Vin and outputs a PWM control signal to turn on and off the FET 21. The waveform of the input current Iin becomes the waveform of the input voltage Vin. P to get closer
Output the WM control signal. This can reduce the harmonic current of the input current Iin. At this time, the output voltage Vout is boosted from 140V to, for example, 200V.

The primary winding L1 of the transformer 31 and the FET 32 are connected in series between both electrodes of the electrolytic capacitor C1, and the varistor 5 is connected in parallel to the series circuit. The FET 32 switches the input voltage to the primary winding L1 by on / off oscillation. P
The WM control circuit 33 has an output terminal connected to the gate of the FET 32 and outputs a PWM control signal to control ON / OFF oscillation of the FET 32.

The secondary winding L2 of the transformer 31 has one end as the anode of the diode D2 and the other end as the electrolytic capacitor C2.
And the output terminal 14. The cathode of the diode D2 is connected to the positive electrode of the electrolytic capacitor C2, and the output terminal 13
It is connected to the.

The varistor 5 is a voltage-dependent resistor whose resistance value sharply decreases when the output voltage Vout exceeds a varistor voltage Vv of a predetermined level described later. This varistor 5
For example, it is formed by sintering ceramics such as zinc oxide, and is covered with a scattering prevention member 6 as shown in FIGS. The anti-scattering member 6 is made of a flame-retardant grade V0 material in the UL standard, such as silicone rubber, and is an insulating tube that closely covers the varistor 5, and when the short-circuited varistor 5 breaks, fragments are scattered. Is to prevent. By fixing the lower end of the insulating tube to the printed board 7,
It is possible to more reliably prevent scattering of fragments.

Next, the operation of the power supply circuit 1 will be described. When the AC power supply AC is connected, the AC input voltage is rectified by the bridge diode BD and smoothed by the electrolytic capacitor C1 to obtain the output voltage Vout. At this time, the waveform of the input current Iin is approximated to the waveform of the input voltage Vin by the power factor correction circuit 2, and the output voltage Vou is increased.
t is boosted from 140V to 200V.

The output voltage Vout is input to the primary winding L1 and is switched by the on / off oscillation of the FET 32, and this switching induces a voltage in the secondary winding L2. And this induced voltage is the diode D
2 is rectified, smoothed by the electrolytic capacitor C2, and electric power is supplied to the load 4 connected between the output terminals 13 and 14.

In such a power supply circuit 1, when a short circuit occurs in a circuit element forming the PWM control circuit 22 or an abnormal state such as a short circuit test is performed, the operation of the PWM control circuit 22 becomes unstable and the output is performed. The voltage Vout increases. At this time, the rising output voltage Vout is Vout> V
The level of the varistor voltage Vv is set in advance so that it becomes v. Then, the output voltage Vout increases and Vout> Vv
Then, the resistance value of the varistor 5 decreases, and when the state of Vout> Vv is maintained, the insulating layer of the varistor 5 is destroyed and the varistor 5 is almost in a short circuit state.

Here, the varistor 5 is formed of a ceramic sintered body, and its shape is destroyed due to a short circuit state and the fragments are opened when they are scattered. However, the varistor 5 is closely covered with the scattering prevention member 6. As a result, the fragments do not scatter and the outer shape is almost kept in the original shape, which prevents the open state and keeps the short-circuited state.

As a result, a short-circuit current flows through the varistor 5, and an excessive current flows through the input terminal 11, so that the fuse F is quickly blown and the electrolytic capacitor C1 is safely protected.

On the other hand, in a silicon-based varistor having a pn junction, the output voltage Vout rises to Vout> Vv, and the resistance value of the varistor 5 decreases to be almost short-circuited, and the state of Vout> Vv is maintained. When it's done, barista 5
The shape of may open without breaking.
Therefore, even if the varistor is tightly covered with the anti-scattering member 6, it is not possible to prevent the varistor from being opened.

As described above, the varistor 5 made of a ceramic sintered body is connected in parallel to the electrolytic capacitor C1 and the scattering of fragments is prevented when the output voltage Vout exceeds the varistor voltage Vv and the varistor 5 is broken. Since the short-circuit state is maintained by the fuse, the fuse F can be quickly melted and opened. Therefore, the electrolytic capacitor C1
Can be safely protected and its destruction can be prevented, so that the scattering of the electrolytic solution inside can be prevented.

The scattering prevention member 6 is shown in FIG.
As shown in (b), it may be formed of an insulating sheet. In FIG. 2, the varistor 5 is covered with the scattering prevention member 6 formed of an insulating sheet, and the lower end of the sheet is covered with the adhesive 81.
The same effect can be obtained by adhesively fixing to the printed circuit board 7.

The same effect can be obtained even if an adhesive is used as the scattering prevention member 6 and the entire varistor 5 is covered and fixed with the adhesive.

[0029]

As described above, according to the invention of claim 1, a varistor made of a ceramic sintered body is connected in parallel to a capacitor, and when the varistor breaks, scattering of the fragments is prevented. Since the operation of the power factor correction circuit becomes unstable and the output voltage rises and the varistor is short-circuited and destroyed, the short-circuit condition is maintained, so open the overcurrent protection member quickly. You can Therefore, the capacitor can be protected safely and reliably with a simple structure.

According to the second aspect of the present invention, the varistor made of a ceramic sintered body is connected in parallel to the capacitor, and the varistor is tightly covered with the insulating tube. Therefore, the operation of the power factor correction circuit is improved. If the output voltage rises due to instability and the varistor is short-circuited and destroyed, scattering of debris is prevented and the short-circuit state is maintained without opening. It can be opened. Therefore, the capacitor is safe with a simple structure,
It can be surely protected.

[Brief description of drawings]

1A is a circuit diagram of an embodiment of a power supply circuit according to the present invention, FIG. 1B is a perspective view of a varistor covered with an insulating tube, and FIG. 1C is a plan view of the varistor.

FIG. 2A is a front view showing another configuration example of the scattering prevention member, and FIG. 2B is a side view of FIG.

FIG. 3 is a circuit diagram of a conventional power supply circuit.

FIG. 4 is a circuit diagram of a crowbar circuit.

[Explanation of symbols]

 1 power supply circuit 2 power factor improvement circuit 21 FET (switching element) 22 PWM control circuit 3 DC-DC converter 31 transformer 32 FET 33 PWM control circuit 4 load 5 varistor 6 scattering prevention member AC AC power supply BD bridge diode C1 electrolytic capacitor D1 diode

Claims (2)

[Claims] 1. A rectifying means for rectifying an AC input from an AC power source connected via an overcurrent protection member which opens terminals when an excessive current flows, a capacitor for smoothing the rectified voltage, and a smoothing unit. DC-DC to convert the output voltage
In a power supply circuit provided with a converter, the rectifying means and the capacitor, and a power factor correction circuit for making the current waveform of the AC input approach a sine waveform, the power supply circuit is connected in parallel to the capacitor, and the output voltage is Is provided with a varistor formed of a sintered body of ceramics, which has a resistance value lowering when the voltage exceeds a preset voltage, and a scattering prevention member that prevents scattering of fragments when the varistor breaks. Power supply circuit characterized by that. 2. The power supply circuit according to claim 1, wherein the anti-scattering member is an insulating tube that tightly covers the varistor.