JPH11103246A - Switching power source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely fuse a fuse element at the time of boosting the voltage of a primary-side DC source without using an expensive Zener diode. SOLUTION: A fuse 8 is arranged on the current route of the primary-side DC source and applied to a switching power supply connecting the collector of a control transistor(TR) Q2 for stabilizing output voltage by controlling a base current to the base of a switching TR Q3 for switching a current allowed to flow through a primary coil 2, and when the voltage of the primary-side DC source exceeds a previously set upper limit value, an excess voltage control circuit 1 turns off the TR Q2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply provided with a fuse in a current path of a primary DC source, and more particularly, to a control transistor when a voltage of a primary DC source exceeds an upper limit value. And a switching power supply that promotes blowing of a fuse by turning off the power supply.

[0002]

2. Description of the Related Art In an RCC switching power supply for obtaining a stabilized DC voltage from a commercial power supply, a fuse is provided in a current path of a primary DC source for the purpose of protecting a power supply body and a load when an abnormal situation occurs. Have been. Therefore, 1
If, for example, a commercial power supply of 200 V is erroneously applied to the RCC switching power supply having a configuration in which the input voltage is set to 00 V, the fuse is blown. However, depending on the state of the load and the like, the blowing of the fuse may be delayed or the fuse may not be blown. In such a case, a voltage exceeding the allowable value is applied to the smoothing capacitor for smoothing the primary DC source for a long time, which may cause an explosion of the smoothing capacitor.

In order to prevent such a danger, a conventional technique for ensuring that the fuse is blown when the voltage of the primary-side DC source exceeds a specified value is disclosed in Japanese Utility Model Laid-Open No. 4-11873.
No. 7 is proposed. That is, in this technology,
A zener diode is provided between the positive level and the negative level of the primary DC source. In addition, the Zener voltage of the Zener diode is set as the upper limit of the allowable voltage range of the primary DC source. Therefore, when the voltage of the primary DC source rises beyond the allowable range, a current flows through the Zener diode, and the fuse blows due to the current flowing through the Zener diode.

[0004]

However, in the case where the above-mentioned conventional technology is used, the following problems have occurred.
That is, the Zener voltage of the Zener diode needs to be equal to the upper limit of the allowable voltage range of the primary DC source. For this reason, in the case of a configuration in which a primary-side DC source is obtained by rectifying and smoothing a commercial power supply, the Zener voltage needs to be extremely high, for example, 180 V. Further, in order to blow the fuse, it is necessary to use an element having a large current capacity. That is, it is necessary to use an element having an extremely high Zener voltage and a large current capacity for the Zener diode.
As a result, the price of the Zener diode has become high, and the cost of parts has increased.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a switching transistor when a voltage of a primary DC source exceeds a predetermined upper limit. By turning off the control transistor that limits the base current of the transistor and increasing the current flowing through the switching transistor to promote the fusing of the fuse, the voltage of the primary side DC source can be reduced without using an expensive zener diode at the element price. It is an object of the present invention to provide a switching power supply capable of surely blowing a fuse when it rises.

According to a second aspect of the present invention, in addition to the above object, an overvoltage control circuit for turning off a control transistor when the voltage of a primary DC source rises is configured with an inexpensive low power element. And to provide a switching power supply that can perform the switching.

[0007]

According to a first aspect of the present invention, there is provided a switching power supply having a fuse provided in a current path of a primary DC source, for switching a current flowing through a primary coil. The base of the transistor is applied to a switching power supply connected to a collector of a control transistor that controls an output voltage by controlling a base current of the switching transistor, and a voltage of the primary side DC source is set to a preset upper limit value. , And an overvoltage control circuit that turns off the control transistor when the voltage exceeds the threshold voltage. That is, the overvoltage control circuit turns off the control transistor when the voltage of the primary DC source exceeds the upper limit. For this reason, all of the current that has increased in accordance with the increase in the voltage of the primary DC source flows through the base of the switching transistor without being shunted. Therefore, the maximum value of the collector current of the switching transistor is a current value obtained by multiplying the increased base current value by hfe. This current value is a current value sufficient to blow the fuse. If the switching transistor is short-circuited and broken by an excessive collector current before the fuse is blown, the fuse is blown by the current flowing through the short-circuited switching transistor.

According to a second aspect of the present invention, in addition to the above configuration, the switching power supply is applied to a switching power supply in which an emitter of the control transistor is connected to a minus level of the primary DC source. ,
A first resistor having one terminal connected to the positive level of the primary side DC source, one terminal connected to the other terminal of the first resistor, and the other terminal connected to the minus level; A Zener diode having a cathode connected to a connection point between the first resistor and the second resistor, a base connected to an anode of the Zener diode, a collector connected to a base of the control transistor, And a transistor having an emitter connected to the negative level. That is, the first resistor and the second resistor divide the voltage of the primary DC source. Then, when the divided voltage exceeds the Zener voltage, a current flows through the Zener diode, and the transistor is turned on. As a result, the base level of the control transistor becomes substantially equal to the minus level, and the control transistor is turned off. From this, it is sufficient that the Zener voltage of the Zener diode is a voltage (low voltage) obtained by multiplying the upper limit value of the primary-side DC source by the voltage dividing ratio of the first resistor and the second resistor. Further, only the base current of the transistor flows through the Zener diode, and only a current corresponding to the base current of the control transistor flows through the transistor. Therefore, an inexpensive element for low power can be used for the Zener diode and the transistor.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.
This will be described with reference to the drawings. FIG. 1 is a circuit diagram showing the electrical connection of one embodiment of the switching power supply according to the present invention. In the figure, for example, a 100 V commercial power supply 10
One of them is via a fuse 8 that protects against abnormal conditions.
It is led to one AC input of the diode bridge 9. The other side of the commercial power supply 10 is led to the other AC input of the diode bridge 9.

The positive output terminal of the diode bridge 9 is connected to one terminal of a smoothing capacitor C5 via a resistor R9 for limiting an inrush current. The negative output terminal of the diode bridge 9 is connected to the other terminal of the smoothing capacitor C5. Therefore, one terminal of the smoothing capacitor C5 is at the plus level 5 of the primary DC source, and the other terminal of the smoothing capacitor C5 is at the minus level 6 of the primary DC source.

A positive level 5 is led to one terminal of the primary coil 2 of the transformer 4. A minus level 6 is guided to one terminal of the auxiliary coil 3. The collector of the switching transistor Q3 is connected to the other terminal of the primary coil 2. Further, the emitter of the switching transistor Q3 is connected to the minus level 6. That is, the switching transistor Q3 is an element for switching the current flowing through the primary coil 2. A resistor R3 having one terminal connected to the plus level 5 is a starting resistor that supplies a starting current to the switching transistor Q3.

The two resistors R4 and R4 inserted between the base of the switching transistor Q3 and the resistor R3.
When the switching transistor Q3 is turned on, R5 outputs the switching transistor Q3 from the auxiliary coil 3.
3 is an element for limiting the current supplied to the base. Further, a capacitor C connected between the base of the switching transistor Q3 and the minus level 6
Reference numeral 1 denotes an element that absorbs pulse noise and delays the rise of the base voltage of the switching transistor Q3 when the power is turned on.

A resistor R6 having one terminal connected to a connection point between the resistors R4 and R5 and the other terminal connected to the base of the control transistor Q2 supplies an auxiliary current at the time of start to the base of the control transistor Q2. It is an element to supply. The capacitor C2 connected between the base of the control transistor Q2 and the minus level 6 is connected to a resistor R6
Is an element that delays the rise of the base voltage of the control transistor Q2. Further, the resistor R7 connected in parallel with the capacitor C2 is an element for discharging the charge charged in the capacitor C2.

The photocoupler 7 is an element for feeding back the error of the secondary output voltage (not shown) to the primary side, and the collector of the phototransistor Q5 is connected to one terminal of the resistor R8. Further, the other terminal of the resistor R8 is connected to the other terminal of the auxiliary coil 3. That is, the resistor R8 is an element that supplies a current generated in the auxiliary coil 3 to the collector of the phototransistor Q5 when the switching transistor Q3 is turned on. Further, the emitter of the phototransistor Q3 is connected to the base of the control transistor Q2. Therefore, the base current of the control transistor Q2 is controlled to a current value corresponding to the output voltage error.

The anode of the diode D2 is connected to the other terminal of the auxiliary coil 3. The diode D
The cathode of No. 2 is led to a connection point between the resistors R3 and R4. That is, the diode D2 is an element that supplies the current generated in the auxiliary coil 3 to the base of the switching transistor Q3 when the switching transistor Q3 is in the ON state. Also, the diode D2
The capacitor C3 connected in parallel to the first transistor rapidly discharges the charge stored in the capacitor C1 and the charge stored in the base of the switching transistor Q3 when the switching transistor Q3 shifts from the on state to the off state. For the device.

When the switching transistor Q3 and the control transistor Q2 are viewed from the starting resistor R3 side, the time constant of the base of the switching transistor Q3 is shorter than the time constant of the base of the control transistor Q2. . Therefore, when the power is turned on, first, the switching transistor Q3 is turned on. And
While the collector current of the switching transistor Q3 is increasing, a current flows to the base of the control transistor Q2 to limit the base current of the switching transistor Q3 to a predetermined value. As a result, the switching transistor Q3 transitions from the on state to the off state before the collector current increases to an excessively large current value that may cause destruction.

The overvoltage control circuit 1 in which the plus level 5 is led as an input and the output is connected to the base of the control transistor Q2 is connected to the primary side DC source (the voltage between the plus level 5 and the minus level 6). When the current value exceeds a preset upper limit value, the control transistor Q2 is turned off. To this end, a first resistor R1, a second resistor R2, a Zener diode D1, and a transistor Q1 are provided.

More specifically, one terminal of the first resistor R1 is connected to the plus level 5. Also, the second resistor R
One terminal of the second resistor R2 is connected to the other terminal of the first resistor R1, and the other terminal of the second resistor R2 is connected to the minus level 6. Further, the cathode of the Zener diode D1 is led to a connection point between the first resistor R1 and the second resistor R2. Therefore, the first resistor R1 and the second resistor R2 act as a voltage dividing circuit for dividing the voltage of the primary DC source, and apply the divided voltage to the cathode of the Zener diode D1.

The anode of Zener diode D1 is connected to the base of transistor Q1. The emitter of the transistor Q1 is connected to the negative level 6. The collector of the transistor Q1 is guided to the base of the control transistor Q2. Therefore,
Since the voltage of the primary DC source exceeds the upper limit, when the divided voltage by the first resistor R1 and the second resistor R2 exceeds the Zener voltage of the Zener diode D1, the transistor Q1 is turned on. When the transistor Q1 turns on, the control transistor Q2 turns off.

In the present embodiment, the upper limit of the voltage of the primary DC source is set to 200V. Therefore, the relationship between the voltage dividing ratio of the first resistor R1 and the second resistor R2 and the Zener voltage of the Zener diode D1 is such that when the voltage of the primary DC source becomes 200 V, a base current flows through the transistor Q1. Relationship.

In the present embodiment, the upper limit value is set to 200 V because the voltage of the primary DC source becomes approximately 280 V when a 200 V commercial power supply is erroneously applied. Because there is. However, generally, with respect to the upper limit of the voltage of the primary DC source, when a voltage exceeding the upper limit is applied to the smoothing capacitor C5, a dangerous situation such as explosion of the smoothing capacitor C5 occurs. It is desirable to use a value. Therefore, the upper limit is not limited to 200 V. When the voltage of the commercial power supply 10 is 100 V, another upper limit can be set to, for example, 180 V or 220 V.

The operation of the embodiment having the above configuration will be described. When the voltage of the commercial power supply 10 is near the specified value of 100 V, the voltage of the primary DC source is approximately 140 V. Therefore, the divided voltage by the first resistor R1 and the second resistor R2 is lower than the Zener voltage of the Zener diode D1. Therefore, the transistor Q1 is off. When the overvoltage control circuit 1 is viewed from the control transistor Q2 when the transistor Q1 is off, the connection of the overvoltage control circuit 1 can be ignored.

When the connection of the overvoltage control circuit 1 can be neglected, the base current of the control transistor Q2 is controlled by the phototransistor Q5 to a current value corresponding to the output voltage error. Therefore, the switching transistor Q3
Is turned on, the switching transistor Q3
Is controlled to a value corresponding to the error of the output voltage. As a result, the switching transistor Q3 performs a switching operation so that the output voltage becomes a predetermined voltage.

On the other hand, if the commercial power supply 10
Is 200V. At this time, the voltage between the plus level 5 and the minus level 6 (the voltage of the primary DC source) is about 280V. Therefore, the divided voltage by the first resistor R1 and the second resistor R2 becomes higher than the Zener voltage of the Zener diode D1. Therefore, a base current flows through the transistor Q1, and the transistor Q1 is turned on. When the transistor Q1 is turned on, the base of the control transistor Q2 becomes substantially equal to the minus level 6, so that the control transistor Q2
Is turned off.

On the other hand, when the voltage of the primary DC source is 2
Therefore, the rate of increase of the collector current of the switching transistor Q3 is doubled. Therefore, the voltage generated in the auxiliary coil 3 is doubled. However, the control transistor Q2 is off. Therefore, all the current supplied from the auxiliary coil 3 flows to the base of the switching transistor Q3. Therefore, the base current of the switching transistor Q3 has an excessive value. On the other hand, until the value of the collector current becomes the value obtained by multiplying the value of the base current by hfe, the switching transistor Q3
Does not transition to the off state. Therefore, an extremely large collector current flows through the switching transistor Q3 having an excessively large base current. As a result, the current flowing through the fuse 8 becomes extremely large, and the fuse 8 is blown.

On the other hand, if the blow of the fuse 8 through which an excessive current flows is delayed, an excessive collector current causes short-circuit breakdown of the switching transistor Q3. When short-circuit breakdown occurs in the switching transistor Q3, the current flowing through the primary coil 2 increases further than before the breakdown of the switching transistor Q3. For this reason, the fuse 8 is blown immediately.

From the above, when the voltage of the commercial power supply 10 becomes 200 V due to incorrect connection, the fuse 8 is surely blown in a short time. Therefore, the time during which the voltage exceeding the standard value is applied to the smoothing capacitor C5 is short. Therefore, the applied voltage becomes 0 V before the temperature of the smoothing capacitor C5 rises. Therefore, occurrence of a dangerous situation such as explosion of the smoothing capacitor C5 can be prevented.

The present invention is not limited to the above embodiment, and the configuration of the switching power supply has been described in the case of an RCC type switching power supply that feeds back the error of the secondary output voltage to the primary side. As a configuration, for example, without using the photocoupler 7, based on the voltage generated in the auxiliary coil 3, the switching transistor Q
The present invention can be similarly applied to an RCC type switching power supply having a configuration in which the output voltage is stabilized only by the primary side circuit, such as a configuration for controlling the switching of No. 3.
Further, the present invention is not only applicable to an RCC type switching power supply, but also in the case of a configuration including a switching transistor and a control transistor for controlling a base current of the switching transistor even if the switching power supply is of another type. Then, the same can be applied.

[0029]

According to a first aspect of the present invention, there is provided a switching power supply having a fuse provided in a current path of a primary DC source, and a switching transistor for switching a current flowing through a primary coil. Is applied to a switching power supply to which a collector of a control transistor that controls an output voltage by controlling a base current of the switching transistor is connected, and when the voltage of the primary side DC source exceeds a preset upper limit value. And an overvoltage control circuit for turning off the control transistor. That is, when the voltage of the primary DC source exceeds a preset upper limit,
The control transistor that limits the base current of the switching transistor is turned off, and the current flowing through the switching transistor is increased to promote the blowing of the fuse.
For this reason, the fuse can be reliably blown when the voltage of the primary-side DC source rises without using an expensive Zener diode with an element price.

According to a second aspect of the present invention, in addition to the above configuration, the switching power supply further comprises an overvoltage control circuit comprising: a first resistor having one terminal connected to a positive level of the primary DC source; A terminal is connected to the other terminal of the first resistor, the other terminal is connected to the second resistor connected to the minus level, and a cathode is connected to a connection point between the first resistor and the second resistor. The transistor includes a Zener diode, a base connected to the anode of the Zener diode, a collector connected to the base of the control transistor, and an emitter connected to the minus level. That is, when the voltage divided by the first resistor and the second resistor exceeds the Zener voltage, a current flows through the Zener diode, turning on the transistor and turning off the control transistor. Therefore, the Zener voltage of the Zener diode may be a voltage obtained by multiplying the upper limit of the primary-side DC source by the voltage dividing ratio of the first resistor and the second resistor, that is, a low voltage. Also, only a small current flows through the Zener diode or transistor. Therefore, it is possible to configure the overvoltage control circuit with an inexpensive low-power element.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an electrical connection of an embodiment of a switching power supply according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Overvoltage control circuit 2 Primary coil 5 Positive level of primary side DC source 6 Negative level of primary side DC source 8 Fuse D1 Zener diode Q1 Transistor which is a constituent element of overvoltage control circuit Q2 Control transistor Q3 Switching transistor R1 First resistor R2 Second resistance

Claims (2)

[Claims] 1. A fuse is provided in a current path of a primary side DC source, and a base of a switching transistor for switching a current flowing through a primary coil is controlled by controlling a base current of the switching transistor to stabilize an output voltage. A switching power supply to which a collector of the control transistor is connected, further comprising an overvoltage control circuit that turns off the control transistor when the voltage of the primary DC source exceeds a preset upper limit value. Switching power supply. 2. The switching power supply in which an emitter of the control transistor is connected to a minus level of the primary DC source, wherein the overvoltage control circuit has a first terminal connected to a plus level of the primary DC source. A first resistor, one terminal connected to the other terminal of the first resistor, the other terminal connected to the minus level, a cathode connected to the first resistor and the second resistor, And a transistor whose base is connected to the anode of the Zener diode, whose collector is connected to the base of the control transistor, and whose emitter is connected to the minus level. The switching power supply according to claim 1, wherein