JPS60223470A - Dc/dc converter with overcurrent protecting circuit - Google Patents

Abstract

PURPOSE:To reliably protect a circuit against a load overcurrent by providing a control transistor for turning ON a light emitting element which becomes ON at the normal time in response to a load current. CONSTITUTION:A transistor Tr3 is started through a resistor 2 from a DC power source, the Tr3 is then oscillated by a voltage of the auxiliary winding 4-2 of a transformer 4, and a periodic repetitive voltage is induced between the secondary windings 4 and 3. This is rectified by a diode 6 and a capacitor 7, and the output of the DC voltage V0 is applied to a load 16. A shortcircuit of the load 16 is detected by a resistor 8, and the Tr3 is turned OFF through a Tr9. A light emitting diode 11 is fired to turn OFF a thyristor 15 at the normal time. In this case, a resistor 17 and Tr18 are added to the rectified output side. When the load current flowed to the resistor 17 slightly exceeds the maximum using current in this manner, the Tr18 is turned ON to turn OFF the diode 11, and the oscillation of a DC/DC inverter is stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a DC-DC converter with an overcurrent protection circuit used as a power source for switching regulators and the like.

[Prior art]

In general, in this type of DC-DC converter, if the load side through which the output current flows is overloaded or short-circuited, it may not only malfunction, but also cause damage to eight parts. It is well known that it becomes unusable. In order to eliminate such inconveniences, DC-DC converters are usually provided with an overcurrent protection circuit for the load.

A typical example conventionally used as a DC-DC converter with an overcurrent protection circuit will be described with reference to the circuit diagram shown in FIG. In the figure, when a starting current flows from the battery 1, which is a DC power source, through the resistor 2 and between the pace and emitter of the transistor 3, the collector current flows through the primary winding 4-1 of the transformer 4 between the collector voltage of the transistor 3 and the gap. flows. The sitransr 4 is excited by this current, a voltage is induced in the auxiliary winding 4-2, and the pace current of the transistor 3 is strengthened. As the base current is strengthened, the collector current further increases, causing the transistor 3 to suddenly become saturated (turned on). Then, when the collector current attempts to flow more than 8 (DC current amplification factor) times the base current which should be limited by the resistor 5, the transistor 3 is abruptly cut off. By repeating this phenomenon, the secondary winding 4-3 of the transformer 4
A periodic repeating voltage is induced in , which is rectified by a diode 6 and a capacitor 7, and then becomes a DC voltage ■. is given to the load 16 as an output.

Under the above operating conditions, the output current. In order to prevent the current from flowing beyond the set value due to a short circuit or overload in the load 16, a resistor 8 is connected in series with the battery 1, and the voltage drop caused by the current flowing through the resistor 8 is transferred to the transistor 9. Pick up between pace and emitter. When this picked-up voltage exceeds a certain value, transistor 9 is no longer "on", its collector voltage is applied to the base of transistor 30, and transistor 3 is cut off. As a result, the output voltage V. decreases, and the photocoupler light emitting diode 11 connected between the output terminals via the resistor 10 stops operating. In this way, the phototransistor 12 on the light receiving side is cut off, and the voltage of the capacitor 14 connected to its output side via the resistor 13 is controlled. the result,
Thyristor 1 connected to the pace of transistor 3
5 is "'on"', the oscillation operation of the converter is completely stopped. Note that at the time of startup, the output voltage 9 does not appear and the two light emitting diodes 11 are not operating, so the output voltage is ■. Thyristor 15 does not operate until this occurs.

The capacitor 14 has a delay effect.

FIG. 2 is a graph showing the operating characteristics of the conventional example shown in FIG. In this graph.

The output load current i1 is set as a reference value for operation. now.

In characteristic A, load current engineering. When il increases to i2 and further exceeds i2, transistor 9 operates to increase the output voltage V. decreases. Then, when o becomes vl, the nine light emitting diodes 11 become inactive, and as a result, the oscillation stops as described above. Note that in this graph, when the battery voltage is increased, the characteristics become as shown in B.

In the above characteristics, if the load current i1 is the maximum current expected in the load 16, then 2 is normal between the current i1 and the current value i2 or i2' before oscillation stops.

The range in which this circuit is not used is shown, but it is an operating range in which the probability is very low when this circuit is in use.

(5) Transistor 3. Trance 4. It is necessary to leave room for diode 6, etc. Therefore, the structure becomes large and uneconomical, as components with large capacities must be used and the heat sink must be designed to be large.

[Purpose of the invention]

An object of the present invention is to eliminate the above-mentioned conventional drawbacks and to provide a small and economical DC-DC converter with an overcurrent protection circuit that does not require allowances for breakage or overheating of used parts.

[Structure of the invention]

The configuration of the present invention is to connect a transistor in series to the primary winding of the transformer, connect the pace of the transistor to the auxiliary winding of the transformer via a current limiting resistor, and connect the transistor to the auxiliary winding of the transformer through the transistor. A DC voltage is applied to the winding, a feedback current obtained from the auxiliary winding causes self-excited oscillation, and a DC output is obtained from the secondary winding of the transformer via a rectifier circuit (6). In a DC-DC converter, a light emitting element is connected to the output side of the rectifier circuit and operates "on" during normal operation, a resistor is connected in series to a circuit through which a load current flows on the output side, and the input side is connected to the resistor. connected so that there is a potential difference from both ends.

a control transistor connected to the light emitting element so as to turn off the light emitting element when the potential difference exceeds a predetermined threshold on the output side; The oscillation operation of the oscillation transistor is stopped when the light receiving element becomes 1 OFF via a light receiving element optically coupled to the light emitting element and a control circuit connected to the output side of the light receiving element. shall be.

[Embodiments of the invention]

Next, two embodiments of a DC-DC converter with an overcurrent protection circuit according to the present invention will be described with reference to one drawing.

FIG. 3 shows a circuit diagram of a first embodiment according to the present invention. Note that in FIG. 1, elements using the same reference numerals as in the conventional example of FIG. 1 should be understood to have the same functions. According to this example, a resistor 17 is connected to the rectified output side in FIG. 1 to cause a voltage drop due to the load current, a pace and an emitter are connected to both ends of the resistor 17, and the collector side is connected to a resistor lO. By configuring the circuit like a light emitting die, when the load current flowing through the resistor 17 reaches a value if slightly exceeding the maximum operating current il, referring to FIG. 4 which shows the characteristics corresponding to the embodiment, If the transistor 18 is set to operate,
At that point, the light emitting diode 11 becomes inactive and can stop oscillation.

Thus, as seen in the characteristics of Fig. 4, the load current B
It is no longer necessary to consider damage or overheating of a single component even for power that can operate in the range from i2 to i'2, and as a result, inexpensive components and heat dissipation mechanisms can be used.

FIG. 5 shows a circuit diagram of a second embodiment of the present invention. The differences between this example and the first example are as follows:
Resistance on the input side8. The transistor 9 and the thyristor 15 are removed and an NPN type transistor 19 is provided in their place, the base of which is connected to the output side of the phototransistor 12, and the collector side of the transistor 19 is connected to the base of the transistor 3. Further, FIG. 6 shows a circuit diagram of a third embodiment according to the present invention. This example is different from the second example in that the input side is changed as shown in the figure, and a PNP type transistor is used as the transistor 21. In these second and third embodiments, as in the first embodiment, the operation of the oscillation transistor is controlled by the overcurrent detection circuit provided on the output side via the photocoupler. There is no change in the fact that it was controlled. −
[Effects of the Invention] As is clear from the above explanation, according to the present invention, the parts used are free from damage and overheating.
It is possible to reliably protect the circuit against overcurrent flowing through the load, and as a result, a compact and economical C-DC converter can be obtained, which is a great effect.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional DC-DC converter with an overcurrent protection circuit, and FIG. 2 is a graph showing characteristics for explaining overcurrent protection operation in the conventional example of FIG.
FIG. 3 is a circuit diagram showing a first embodiment according to the present invention, FIG. 4 is a graph showing characteristics for explaining overcurrent protection operation in the embodiment of FIG. 3, and FIG. FIG. 6 is a circuit diagram showing a third embodiment according to the present invention.

Claims (1)

[Claims] 1. A transistor is connected in series to the i-th winding of a transformer, and the base of the transistor is connected to the auxiliary winding of the transformer via a current-limiting resistor. A DC voltage is applied to the secondary winding through the transistor, a feedback current obtained from the auxiliary winding causes self-excited oscillation, and a DC output is generated from the secondary winding of the transformer through a rectifier circuit. The DC-DC converter is connected to the output side of the rectifier circuit, and under normal conditions
A light emitting element that operates when it is on, a resistor connected in series with a circuit through which a load current flows on the output side, an input side connected so that a potential difference is received from both ends of the resistor, and an output side connected so that the potential difference is determined in advance. a control transistor connected to the light-emitting element so as to turn off the light-emitting element when the threshold value exceeds the threshold; Motoko and
A DC-DC converter with an overcurrent protection circuit, characterized in that the oscillation operation of the oscillation transistor is stopped when the light receiving element is turned off via a control circuit connected to the output side of the light receiving element. .