JPS63217973A - Overcurrent protective circuit for switching power source - Google Patents

Abstract

PURPOSE:To enhance the reliability of protecting by providing overcurrent detectors set at respective DC outputs, and so controlling as to increase the current of a light emitting diode when one of them is operated. CONSTITUTION:A DC power source 10 is connected through the primary winding 16 of a switching transistor 15 to a switching transistor (Tr) 20, and a positive pulse positively fed back from the base winding 17 of the transistor 15 is applied to be driven through a diode 30 and a resistor 51 to the base of the Tr 20. The emitter current of the Tr 20 is detected by a resistor 54, and bypassed by turning a Tr 24 ON at the time of overcurrent. A Tr 21 is provided in parallel with a Tr 22, and the collector current of the Tr 21 is controlled by the increase in the quantity of light received by photo Tr 29. Then, predetermined output voltages V01, V02 are obtained at the first, second output windings 18, 19. Thus, when the output voltage V02 is raised, a light emitting diode 38 strengthens the light emitting intensity to operate the Tr 29, thereby controlling to reduce the output voltage V02.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overcurrent protection circuit in a switching power supply for obtaining a DC output from a DC input by switching.

Specifically, it is a DC stabilized power supply using a switching element, and a signal for stabilizing the output voltage is output from F1 to F1.
It is an object of the present invention to provide an overcurrent protection circuit for the output in a circuit that provides feedback by turning.

[Prior Art] A conventional circuit is shown in FIG. 3 and will be described. Here] O is a battery or a DC power source obtained by rectifying an AC power source, 11.12
is the output terminal for the first DC output, 3.14 is the output terminal for the second DC output, 15 is the switching transformer, and 16 is the output terminal for the second DC output.
is the primary winding of the switching transformer 15, 17 is the base winding, 18 is the first output winding, and 19 is the second output winding.

20 is a switching transistor for causing a switching current to flow through the primary winding 16;
The base winding 17 is connected via a diode 30 and a resistor 51 so that the switching voltage from the primary winding is fed back positively. Further, a bias current is supplied to the base of the transistor 20 through a resistor 50, and a transistor 22 is connected between the base of the transistor 20 and the negative side of the DC power supply 10, and the emitter of the transistor 20 is connected to the negative side of the DC power supply 10. When the current increases, it is detected by the resistor 54 and applied to the transistor 22 through the resistor 53, thereby making the transistor 22 conductive, bypassing the base current to the transistor 20, and increasing the collector current of the transistor 20. This reduces the switching current.

The diode 31 and the capacitor 45 are a circuit that receives a DC voltage from the base winding, and this DC voltage is supplied to the collector of the phototransistor 29 via a resistor 52, and the voltage is supplied to the collector of the phototransistor 29 depending on the amount of light applied to the two phototransistors. The emitter current of the phototransistor 29 increases and decreases as the base current is the emitter current of the phototransistor 29.
The collector current of No. 1 is also controlled by the amount of light. Since the collector and emitter of this transistor 21 are connected in parallel to the collector and emitter of the transistor 22, the larger the amount of light is, the more the base current of the transistor 20 is reduced, similar to the effect of the transistor 22, and the switching Reduce current.

The output of the first output winding 18 is rectified and smoothed by the diode 32 and the capacitor 46, and is then connected to the output terminals 11 and 1.
2, the first output is jq.

The output of the second output winding 19 is rectified and smoothed by the diode 33 and the capacitor 47, and an output of 1 is provided at the output terminals 13 and 14. A light emitting diode 38 is supplied with current by a resistor 55 and a programmable shunt regulator 39. The voltage between output terminals 13 and 14 is divided and detected by resistors 57 and 58, and is applied to the reference voltage input terminal of programmable shunt regulator 39. When the voltage between B and 14 rises, the reference voltage also rises, so the shunt current of the three shunt regulators also increases, intensifying the light emission of the light emitting diode 38, and increasing the emitter current of the phototransistor 29 that receives this light emission. This increases the collector current of transistor 21, decreases the base current of transistor 20, and decreases the switching current flowing through primary winding 16.
and 14 output voltages V. I try to keep 2 constant.

Output current Io taken out from output terminals 13 and 14
2 increases, the output voltage V02 tends to decrease, decreasing the shunt current of the programmable shunt regulator 39, weakening the light emission of the light emitting diode 38, decreasing the emitter current of the phototransistor 29, and decreasing the collector current of the transistor 21. transistor 2
0, the switching current of the primary winding 16 is increased, the output voltage of the second output winding 19 is increased, the output voltage is increased, and the output voltage V is increased. 2 is maintained constant.

Therefore, the output current ■. If 2 is increased more and more, the switching current of the collector of the transistor 20 will also be increased more and more, and eventually it will be thermally destroyed. In order to protect the transistor 20 from such an overcurrent, a circuit consisting of the transistor 22 and resistors 53 and 54 is used. When the voltage is exceeded, transistor 22 is made conductive in response to the exceeded voltage. Depending on the degree of conduction of transistor 22, the positive current pulse obtained from base winding 17 through diode 30 and resistor 51 is bypassed by transistor 22, so that transistor 20
The amplitude of the positive current pulse applied to the base of transistor 20 is small, and the switching current in the collector of transistor 20 is limited to not exceed a certain value.

[Problems to be Solved by the Invention] However, as shown in FIG. 3, when multiple outputs are obtained, the outputs obtained from both the first and second windings 18 and 19 are Since the value of the switching current is determined by the sum t-1''C-, for example, when the power capacity of the first winding is small, that is, when the output current I01 during normal operation is small and the output voltage VOI is also small, this Output current I.1
Even if the voltage increases suddenly, if the abnormally increased 101·vol is a small value compared to the product 102·vo2 of the second output voltage VO2 and the output current 102, then the transistor 22 and the resistor 53. The protection circuit will not operate and the diode 32 will be destroyed. To prevent this, the output current I must be increased abnormally. 1
A diode 32 with a capacity that allows the flow of the current is required, and a first output winding with a sufficient winding capacity is required.
This also leads to an increase in the size of the switching lance 1 to the lance 15, which is undesirable. Therefore, in order to prevent this, a series regulator is added separately to the output terminals 11 and 12, and its overcurrent protection function protects the winding that draws only a small amount of power from overcurrent. I was doing it.

As described above, when a plurality of output powers are obtained, there is a problem in that the overcurrent protection of the circuit that obtains small power is extremely insufficient.

[Means for Solving the Problems] The present invention is intended to solve these problems, and is aimed at directing the abnormal overcurrent generated in the output winding to the primary winding side, that is, to the collector of the switching transistor. This was done in view of the problem that the detection was based on an abnormal increase in current.

In the present invention, an overcurrent detection circuit is provided for each DC output that requires overcurrent protection, and when at least one of the detection outputs is activated, the current of the light emitting diode is increased. to control.

[Function] Therefore, since it is now possible to determine the overcurrent value according to each output current value for each output that requires protection from overcurrent, at least one output can be protected against overcurrent. When this occurs, the light emitting diode operates to increase the light emitting output, thereby reducing the switching current, thereby preventing problems such as thermal breakdown.

[Example] A circuit diagram of an embodiment of the present invention is shown in FIG. 1 and will be described.

Here, the same symbols are given to the components corresponding to λ4 shown in FIG. 3.

A DC power supply 10 is connected to the collector of a switching transistor 20 through a primary winding 16 of a switching transformer 15. A positive pulse fed back from the primary winding 16 of the base winding 17 of the switching unit 1 to the lance 15 is applied to the base of the transistor 20 via the diode 3o and the resistor 51. 5
0 so that the base current is supplied from the power supply 10.

]・The emitter current of the transistor 20 is detected by the resistor 54 and applied to the transistor 22 via the resistor 53. When there is an overcurrent in the transistor 20, the emitter current of the transistor 20 becomes conductive and flows to the base of the transistor 20. It bypasses the current flowing into it.

The transistor 21 is also connected in parallel to the transistor 22. A positive power source obtained by the diode 31 and the capacitor 45 is connected to the transistor 29 via the resistor 52, and this emitter current becomes the base current of the transistor 21. Therefore, due to the increase in the amount of light received by Photo 1 ~ Ranjisk;
The collector current of increases and the positive current pulse obtained from the base winding 17 through the diode 30 and the resistor 51 becomes
1 to transistor 21, a smaller positive amplitude current pulse will be applied to the base of transistor 20, reducing the switching current in the collector of transistor 70.

1st output)] The diode 32 and capacitor 46 connected to the winding 1B cause the first DC output or output current to
1. Output voltage V. 1 is obtained at the output terminals 11 and 12.

A diode 33, a capacitor 47, a diode 34, and a capacitor 48 are connected to one second output winding. The voltage power supply is 3T, 7.

This positive power supply is connected to the second DC output from the output width 13.14 through the resistor 60 connected between the base and emitter of the transistor 24, and the output current I02. Output voltage V
. Obtained as 2.

Current for light emitting diode 38 is obtained via resistor 55 and programmable shunt regulator 39. The reference voltage input terminal of this programmable shunt regulator 39 has an output voltage V. A voltage obtained by dividing 2 by a resistor of 57.58 is applied. If the output voltage V. 2
When the current increases, the shunt current of the programmable shunt regulator 39 is increased and the intensity of the light emitted from the light emitting diode 38 is increased. The collector current is increased and the base current of the transistor 20 is decreased to reduce the switching current flowing through the primary winding 16 and the output voltage V. 2
by lowering the output voltage V. I'm trying to keep it constant at 2.

Here, the difference from the conventional example shown in FIG. Capacitor 48. This is the addition of resistors 56, 59, and 60.

If the resistance value of the resistor 60 is R, and the emitter-base voltage of the transistor 24 is VEB, then the output current I is within the range of VEB>■02°R. Since the transistor 24 remains off while 2 is present, the transistors 1 to 23 also remain off, and in this case, the output voltage V is the same as in the conventional example shown in FIG. 2, a constant voltage can be created.

However, when the output current ■.2 such as VEB"02°R flows, the transistor 24 is turned on and the transistor 23 is also turned on, increasing the current flowing to the light emitting diode 38 and intensifying its light emission. Thereby, the switching current of transistor 20 is reduced, so that protection is provided against an overcurrent flowing in excess of the output current I02 such that VFB=IO2°R.

Here, even if the output terminals 13 and 14 are short-circuited, a sufficient negative voltage can be obtained by the diode 34 and the capacitor 48, so the transistors 24 and 23
is turned on and causes the light emitting diode 38 to fully emit light, so that the aforementioned protective effect is not disturbed. Here, the transistor 24 acts as an overcurrent detection means,
The transistor 23 functions as an overcurrent light emission control means that acts to intensify light emission only when there is an overcurrent.

Since it operates in this manner, the maximum value of the output current 102 can be arbitrarily set by selecting the value of R of the resistor 60, completely independently of the value of the output current I01.

FIG. 2 shows another embodiment of the invention.

The difference from FIG. 1 here is that a 25° transistor, diodes 35 and 36, and a resistor 61 are added.

1 - the output current I by the transistor 25 and the resistor 6 connected between its emitter and base. 1 exceeds a certain value, and the transistor 24. resistance 60
It corresponds to the action of Diodes 35 and 36 are
A reverse voltage is not applied to the transistors 25 and 26 to ensure that the overcurrent detection operation is performed.

If one or both of the output currents] and (1) exceeds a certain value, one or both of the transistors 25 and 24 will be turned on, so by the same operation as explained in FIG. Since the switching current of the transistor 20 is limited, the first and second
Both outputs can be protected from overcurrent.

As in the embodiment shown in FIG.
, I> to detect an overcurrent and provide a protective effect, the transistor 22 shown in FIGS.
A protection circuit consisting of resistors 53 and 54 is no longer necessary.

Transistors 24 and 2 in FIGS. 1 and 2
If you replace 5 with an operational amplifier, resistor 6
An even smaller value of 0.61 can be used.

[Effect of the invention] As is clear from the above explanation, in a switching power supply having multiple outputs, it is possible to not only protect from overcurrent of one specific output, but also set the overcurrent of multiple outputs independently. Since it can be protected, the reliability of this type of power supply is improved, and a minimum of 8 Ω diodes and output windings can be used, it has a large impact on cost reduction. Therefore, the effects of the present invention are extremely large.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, FIG. 2 is a circuit diagram showing another embodiment of the invention, and FIG. 3 is a circuit diagram showing a conventional example. 10... DC power supply 11-14... Output terminal 15... Switching transformer 17... Base winding 18... First output winding 19... Second output winding 20- 25...Transistor 29...Phototransistor 30-36...Diode 38...Light emitting diode 39...Programmable shunt regulator 4
5 to 48... Capacitors 50 to 61... Resistance Iol, IO2... Output current Vol, VO2...
Output voltage.

Claims (3)

[Claims] (1) A DC power supply, a switching transistor for passing a switching current, a primary winding connected between the DC power supply and the switching transistor, and applying positive feedback to the base of the switching transistor. a switching transformer having a base winding for obtaining a switching signal for and an output winding for rectifying and smoothing a plurality of DC outputs; and at least one of the plurality of DC outputs.
light emitting means including a light emitting diode that detects the voltage of one DC output and emits light; and a control means for controlling the switching current of the switching transistor to keep the voltage of the DC output constant. A switching power supply comprising: a light-emitting power supply for supplying a current to enable the light-emitting diode to emit sufficient light even when the DC output is short-circuited; and an output current of the at least one DC output reaching a predetermined value. at least one overcurrent detection means for detecting that the switching current has been exceeded; and operation of the at least one overcurrent detection means acts on a light emitting means for causing the control means to operate to limit the switching current. An overcurrent protection circuit for a switching power supply, comprising: an overcurrent emission control means; and an overcurrent protection circuit for a switching power supply. (2) The switching power supply according to claim 1, wherein the light emitting power source is obtained from the output winding for obtaining the at least one DC output with a polarity opposite to that of the DC output. Overcurrent protection circuit. (3) The overcurrent detection means includes a resistor for causing the DC output current to flow, and a transistor having an emitter and a base connected to both ends of the resistor and obtaining a detection signal from the collector. The overcurrent protection circuit for the switching power supply described in item 1.