JP2734837B2 - DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-D which does not destroy a switching transistor even if a load is short-circuited for a long time.
It relates to a C converter.

[0002]

2. Description of the Related Art An example of a conventional DC-DC converter will be described below.

FIG. 2 is a circuit diagram of a conventional DC-DC converter. In FIG. 2, reference numeral 1 denotes a transistor that performs a switching operation, 2 denotes a transformer that stores and releases energy, 3 denotes a resistor that supplies a base current to the transistor 1 at start-up, and 4 denotes a base current to the transistor 1 after start-up. A resistor 5 is an oscillation circuit that causes the transistor 1 to perform a switching operation, 6 is a control circuit that makes the voltage of the output terminal 7 a constant voltage, and 8 is a rectifier / smoothing that makes the energy released by the transformer 2 to the secondary side DC. Circuit.

[0004] With respect to the DC-DC converter comprising the above components, the relationship and operation of each component will be described below.

First, the energy E stored in the transformer 2
Is as follows, where L _{P is} the inductance of the primary side of the transformer 2 and I _{C is} the collector current of the transistor 1.

E = (1/2) · L _P · I _C ² By discharging this energy to the secondary side, a current is supplied to the load. The collector current I _C is as follows, assuming that the base current is I _B , the current amplification factor is h _FE , the voltage applied to the primary winding of the transformer 2 is V _P , and the ON time of the transistor 1 is T _ON. become.

[0007] _{I C = (V P / L} P) T ON ≦ h FE I B Therefore, by increasing the base current I _B and the on-time T _ON, electric power can be supplied to the output increases.

When starting the DC-DC converter, the resistor 3
The base current is supplied to the transistor 1 from the power supply through the
Accordingly, the transistor 1 performs a switching operation.

When a voltage is generated at the output terminal 7, the base current is supplied also through the resistor 4, the energy stored in the transformer 2 increases, and the output terminal 7
And the base current further increases.

[0010] The base current _IB1 at the time of start-up is determined by setting the power supply voltage to
_{If I} , the base-emitter voltage of the transistor 1 is V _BE , and the resistance value of the resistor 3 is R ₃ , the following is obtained.

I _B1 = (V _I -V _BE ) / R ₃ Here, the resistance 3 is sufficient even when the power supply voltage V _I and the current amplification factor h _FE of the transistor 1 are minimum or in the worst condition such as at low temperature. It is determined that the base current I _B1 that can be started flows.

Further, the base current _IB2 after the start is as follows when the output voltage is V _O and the resistance value of the resistor 4 is R ₄ .

[0013] _{I B2 = (V I -V BE} ) / R 3 + (V O -V BE) / R 4 Therefore, if the output voltage V _O rises base current I _B2
Increases, the output voltage V _O rises rapidly.

When the output voltage becomes higher than a predetermined voltage, the control circuit 6 reduces the base current of the transistor 1, shortens the ON time, reduces the energy stored in the transformer 2, and lowers the output voltage. . Conversely, when the output voltage becomes lower than the predetermined voltage, the base current is increased, the ON time is lengthened, the energy stored in the transformer 2 is increased, and the output voltage is raised to a constant voltage.

Therefore, when the control circuit 6 is not provided, the output voltage is set to be higher than a predetermined voltage, and the value of the resistor 4 is determined so that the base current satisfies this even under the worst condition.

[0016]

However, in the above-described conventional configuration, when the output terminal 7 is short-circuited, the base current of the transistor 1 stops being supplied through the resistor 4 and decreases to the same level as at start-up. Since the value of the resistor 3 is determined so that it can be sufficiently started even under the worst condition, the value becomes large under the normal condition.

Further, since the output voltage becomes 0, the control circuit 6
As a result, the on-time of the transistor 1 is increased, the collector current is increased, and the transistor 1 is broken if the short-circuit state continues for a long time.

The present invention solves the above-mentioned problem. Even if the output terminal is short-circuited for a long time, the transistor is not destroyed.
It is intended to provide a C-DC converter.

[0019]

In order to achieve the above object, a DC-DC converter according to the present invention comprises a transformer for storing and discharging energy, and a transistor for switching a current flowing through a primary winding of the transformer. An oscillation circuit that causes the transistor to perform a switching operation, a resistor that supplies a base current to the transistor at startup, and a diode that blocks a current that flows to the transistor through the resistor when the transistor is turned on. A resistor for supplying a base current from the output side to the transistor after startup, a rectifying / smoothing circuit for converting the energy released by the transformer to the secondary side into DC, and a control circuit for making the output voltage constant. Configuration.

[0020]

The DC-DC converter of the present invention supplies the base current of the transistor at the time of starting from the connection point between the primary winding of the transformer and the collector of the transistor. In this case, the base current hardly flows, the collector current does not increase even if the on-time becomes long, and the transistor does not break down.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A DC-DC converter according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a DC-DC according to an embodiment of the present invention.
It is a circuit diagram of a converter. Note that the DC-DC converter of the present embodiment shown in FIG. 1 has basically the same configuration as the conventional DC-DC converter shown in FIG. Detailed description is omitted.

In FIG. 1, 11 is a resistor for supplying a base current to the transistor 1 at startup, 12 is a resistor for supplying a base current to the transistor 1 by a voltage generated at the output terminal 7 after startup, and 13 is a transistor 1 turned on. It is a diode that prevents current from flowing from the output side to the collector through the resistors 11 and 12.

The operation of the DC-DC converter having the above components will be described below.

First, at the time of starting, a base current is supplied to the transistor 1 through the resistor 11, and the DC-DC converter starts. When a voltage is generated at the output terminal 7, the base current is supplied to the transistor 1 through the resistor 12, and the voltage at the output terminal 7 rises and is made constant by the control circuit 6.

The base current I _B1 at the time of starting is the collector-emitter voltage of the transistor 1 as V _CE , the base-emitter voltage of the transistor 1 as V _BE , the resistance of the resistor 11 as R ₁₁ , and the forward voltage of the diode 13 as V _CE . Assuming V _D ,

[0027] _{I B1 = (V CE -V D} -V BE) / R 11 resistor 11 determines to allow sufficient start even in the worst conditions. At the moment when the power is turned on, V _CE is equal to the power supply voltage.

The base current I _B2 after the start is as follows, where the output voltage is V _O and the resistance value of the resistor 12 is R ₁₂ .

I _B2 = (V _O −V _BE ) / R ₁₂ + (V _CE −V _D −V _BE ) / R ₁₁ Therefore, when the output voltage V _O rises, the base current I _B2
, The output voltage rises rapidly. Note that the current represented by the second term in the above equation flows only for an extremely short time when the transistor 1 switches from off to on. Accordingly, the current of the first term of Equation even worst case conditions, predetermined power determines the value R ₁₂ of the resistor 12 so that the base current that can be supplied to the output.

When the output terminal 7 is short-circuited,
Since the base current is only the second term of the above equation, the collector current does not increase, and the transistor 1 does not break down even if the short-circuit state continues for a long time.

As described above, according to the present embodiment, the base current of the transistor 1 at the time of start-up is supplied from the connection point between the primary winding of the transformer 2 and the collector of the transistor 1, so that the short-circuit state occurs. Even if it lasts long, the transistor 1 does not break down.

[0032]

As is apparent from the above description of the embodiment, the present invention provides a transformer for storing and discharging energy,
A transistor for switching a current flowing through a primary winding of the transformer, an oscillating circuit for causing the transistor to perform a switching operation, a resistor for supplying a base current to the transistor at startup, and a transistor for turning on the transistor when the transistor is turned on. A diode for blocking a current flowing to the transistor side through a resistor, a resistor for supplying a base current from the output side to the transistor after startup, and a rectifying / smoothing for converting the energy released by the transformer to a secondary side into a direct current With the provision of the circuit and the control circuit for making the output voltage constant, an excellent DC-DC converter in which the transistor is not destroyed even when the output terminal is short-circuited for a long time can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a DC-DC converter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional DC-DC converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transistor 2 Transformer 5 Oscillation circuit 6 Control circuit 8 Rectification / smoothing circuit 11 Resistance 12 Resistance 13 Diode

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Toshikazu Tsurudome 1006 Kazuma Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-63-274370 (JP, A) 45494 (JP, U)

Claims (1)

(57) [Claims] 1. A transformer for storing and discharging energy, a transistor for switching a current flowing through a primary winding of the transformer, an oscillation circuit for causing the transistor to perform a switching operation, and an output to the transistor after startup. A resistor for supplying a base current from the side, a rectifying / smoothing circuit for converting the energy released by the transformer to the secondary side into DC, and a DC output by detecting the output voltage and controlling the base current of the transistor. A resistor for supplying a base current to the transistor at the time of startup, and a diode for blocking a current flowing to the transistor through the resistor when the transistor is turned on, in series. The connection point between the primary winding of the transformer and the collector of the transistor and the base of the transistor DC-DC converter inserted in between.