JPS58172974A - Self-oscillation type dc/dc converter circuit with switching transistor protecting function - Google Patents

Abstract

PURPOSE:To safely and effectively obtain an operation of a DC/DC converter by limiting the collector current of a switching transistor to the value not higher than the prescribed value when a self-oscillation is stopped. CONSTITUTION:A starting bias circuit for the base of a switching transistor 13 is composed of a constant-voltage power source 21 and a starting resistor 20. Since a voltage between the base and the emitter of the transistor 13 has constant voltage characteristics, currents which flows through resistors 19 and 20 become constant. Accordingly, the base current of the transistor 13 becomes constant, and the collector current substantially becomes constant. Consequently, even if a trouble occurs, no overcurrent nor overheat occurs, and the transistor 13 can be sufficiently protected.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a self-oscillation type flyback type DC-DC converter having a switch transistor protection function.

A self-oscillation flyback type DC-DC converter using a switching element 1 such as a transistor is well known. Depending on the relationship between input and output voltages, there are boost (up) converters and buck (down) converters.

The basic circuit of a conventionally well-known self-oscillation type flyback type DC-DC converter is as shown in FIG. In this circuit, a current flowing from a DC power supply 1 to a primary winding N of a transformer 2 is interrupted by a switch transistor 3, and a voltage generated in a secondary winding N2 is connected to a diode 4. The output power is rectified and smoothed by a capacitor 5 and supplied to a load 6. A positive feedback voltage is applied to the base of the transistor 3 via a parallel circuit of a diode 7 and a resistor 8, and a resistor 9, thereby performing intermittent control. Further, a starting bias circuit consisting of a starting resistor 10 is connected to the base of this transistor 3. With such a connection, at startup, a sufficient base current for startup is supplied from the DC power supply 1 via the resistor 10, so that the switching operation of the transistor 3 is started.

However, if a fault such as a short circuit on the load side or a short circuit in the winding of the trough 20 occurs in such a circuit, a high voltage equal to the terminal voltage of the DC power supply 1 is applied between the collector and emitter of the transistor 3. In this case, the pace current is more than required for starting, so the power consumption of transistor B is much higher. Such large power consumption causes overheating of the transistor 3, and unless sufficient heat dissipation is performed, serious damage to the circuit elements will result.

Therefore, in order to prevent this, it is necessary to provide a much larger heat sink than is required during normal operation, or to lower the starting bias current. Such failure countermeasures have disadvantages such as increased manufacturing costs, increased device dimensions, and instability in the start of self-oscillation due to variations in element characteristics or changes in outside temperature.

The present invention provides a self-contained system that has a function to prevent destruction of the Suinochi transistor transistor due to thermal runaway even in the event of an unexpected failure such as a short circuit on the output load side of the DC-DC converter or a short circuit in the winding of the converter transformer. An object of the present invention is to provide an excited oscillation type flyback type DC-DC converter circuit.

The object of the present invention is to provide a self-excited excavation type D as described in the claims.
This is accomplished by a C-DC converter circuit.

The present invention eliminates the drawbacks of conventional devices, and achieves a device that has sufficient protection functions and can operate safely and securely at a relatively low cost without increasing the size of the device unnecessarily. can do.

The present invention will be described in detail below with reference to FIGS. 2 and 3 showing embodiments of the present invention. According to these figures, the activation bias circuit for the base of switch transistor 130 is different, but the same elements as in FIG. 1 are designated by the same reference numeral having the same last digit.

Referring to FIG. 2, the starting bias circuit for the base of the switch transistor 13 is composed of a constant voltage power supply 21 and a starting resistor 20.

Since the switch transistor 16 has a constant voltage characteristic between the base and emitter, the current flowing through the resistors 19 and 20 is constant. Therefore, the base current of the switch transistor 13 is constant, and the collector current is also constant. As a result, even if a fault occurs, overcurrent and overheating will not occur, and the switch transistor 13 will be sufficiently protected.

According to FIG. 6 showing another embodiment, a diode 40 is connected to the base of the switch transistor 33 from a constant voltage power supply 41.
Base current for startup through.

flows. In this case, the constant voltage power supply 41 has the emitter potential of the switch transistor 33 connected to the Zener diode 45.
The voltage below the threshold voltage is selected to be a constant potential. Therefore, if the emitter resistor 42 is appropriately selected, a constant collector current will flow at an arbitrary value. Thereafter, when self-oscillation starts and positive feedback from the winding Nf of the transformer 320 occurs to the switch transistor 33, the collector current increases and at the same time, the emitter voltage also increases. When the emitter voltage rises to the threshold voltage of the Zener diode 43, current begins to flow through the diode 43. As a result, a small current, which is part of the collector current, flows through the emitter resistor 42, and most of the increased current flows through the diode 4'5. In addition, Zener diode 43
Instead, the present circuit can be constructed by connecting one or more ordinary diodes so that the forward voltage drop can be regarded as the threshold voltage.

In the circuit according to the present invention shown in FIGS. 2 and 3, the collector power consumption changed in proportion to the square of the DC power supply voltage change, whereas in the circuit according to the present invention, the collector power consumption changed in proportion to the square of the DC power supply voltage change. For voltage changes! It only changes proportionately. Therefore, there is a large difference in heat generation in the transistors, there is no risk of thermal runaway, and the heat sink can be made smaller, achieving the desired purpose.

Although the present invention has been disclosed based on a preferred embodiment of the present invention,
It will be obvious that many modifications and variations are possible within the scope of the invention.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a DC-DC converter circuit according to the prior art. FIG. 2 is a circuit diagram showing one embodiment of the present invention. FIG. 3 is a circuit diagram showing another embodiment of the present invention. 1.11, 31: DC power supply 2, 12, 32: )
Lance 3.13, 33: Transistor
1o: Starting resistance 20: Bias resistance
21, 41: Bias power supply 40: Diode Figure 3 Procedural amendment (h type) % type % Display of case Showa Tough Year Patent Application No. 2/D-Relationship with the person making the amendment No. 7 case Applicant f+ ” WF Name: ``I want to help you.'', Agent address: r'lW, Toranomon 2-8-1, Minato-ku, Tokyo.
Ki Hill) [Telephone 03 (502) 1476 (Representative)
], date of amendment order m - engraving of uniform drawings. (No change in content), details of amendment as attached.

Claims (1)

[Claims] In a self-oscillation type DC-DCC complete circuit, which has a switch transistor that operates intermittently according to a self-oscillation period corresponding to the circuit constant of the constituent elements, and a circuit for flowing a positive feedback current to the base of the switch transistor. , when the self-oscillation stops, the switch transistor (1
In order to limit the collector current of 3, 33) to a predetermined value or less, the switch transistor is connected to a constant current bias circuit (2
0,21:40,41).