JP3240773B2 - 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 / DC converter, and more particularly to a separately-excited non-insulated type using a control integrated circuit (IC) having a built-in protection circuit for stopping oscillation when a load is short-circuited. It relates to a DC / DC converter.

[0002]

2. Description of the Related Art A control IC used in a separately excited non-insulated DC / DC converter generally has a built-in short circuit protection circuit of a voltage detection type. The protection circuit constantly monitors the output voltage, detects that the output voltage has dropped when the load is short-circuited, stops the oscillation of the switching transistor, and turns off the switching transistor.

FIG. 3 shows such a separately excited non-insulated DC / D.
FIG. 3 is a basic circuit diagram of a C converter, which shows an output voltage V when a DC voltage of, for example, 12 V is input as an input voltage Vi;
As o, the voltage is reduced to, for example, 5V. FIG.
, An input voltage Vi is applied to a power supply input terminal Vcc of the control IC 1 and a ground terminal G is grounded. The oscillation output terminal OUT is connected to the base of the switching transistor Q1.
The input voltage Vi is applied to the emitter of the switching transistor Q1, and the collector of the switching transistor Q1 is connected to one end of the coil L and to the cathode of the rectifying diode Di. The other end of the coil L is connected to the output terminal, and the input terminal IN of the control IC 1 is connected via the resistor R1.
It is connected to the. The anode of the rectifying diode Di is grounded, and a smoothing capacitor C1 is connected between the other end of the coil L and the ground. Further, the input terminal IN of the control IC 1 is grounded via the resistor R2,
A capacitor C2 is connected between the CP terminal and the ground. The capacitor C2 is charged when the load is short-circuited, and when the charging is finished, the output voltage V is output via the resistor R1.
If o is not output, the oscillation output from the oscillation output terminal OUT of the control IC 1 is stopped.

In the conventional DC / DC converter thus configured, when an input voltage Vi of, for example, 12 V is input, the control IC 1 outputs an oscillation output such that the output voltage Vo becomes 5 V to the base of the switching transistor Q1. And the switching transistor Q1 performs a switching operation. This switching output is rectified by the rectifying diode Di, and the coil L and the smoothing capacitor C
1 and the DC voltage of 5 V is output voltage Vo.
Is output as When the load is short-circuited, the capacitor C2 is charged, and when the charging is completed, if the output voltage Vo is not output, the control IC1 stops the oscillation output to the switching transistor Q1.

FIG. 4 shows another example of a conventional DC / DC converter. For example, for an input voltage Vi of 5 V,-
It is a circuit diagram which outputs the output voltage Vo of 12V. In FIG. 4, except that the rectifying diode Di is connected to the connection position of the coil L and the coil L is connected to the connection position of the rectifying diode Di, the input voltage V
When 5 V is input as i, the output voltage Vo is -1.
The operation when 2 V is output and the load is short-circuited is the same as in FIG.

FIG. 5 shows still another example of the conventional DC / DC converter, which is a step-up type that outputs a +12 V DC voltage when a 5 V DC voltage is input, for example. In FIG. 5, except that the coil L and the rectifying diode Di are connected in series, the connection point of the coil L and the rectifying diode Di is connected to the collector of the switching transistor Q1, and the emitter of the switching transistor Q1 is grounded. It is configured in the same manner as. The DC shown in FIG.
When energy is stored in the coil L by the input voltage Vi and the switching transistor Q1 switches according to the oscillation output of the control IC 1,
The energy stored in the coil L is converted into a rectifying diode Di.
, And the input voltage Vi is boosted and output as the output voltage Vo.

[0007]

The DC / D shown in FIG.
In the C converter, when the load is short-circuited, the oscillation output of the control IC 1 is not supplied to the base of the switching transistor Q1, and the switching operation of the switching transistor Q1 is stopped, as in FIG. However, as long as the input voltage Vi is applied, as shown in FIG. 6, a short-circuit current flows from the coil L via the rectifying diode Di, and the coil L and the rectifying diode Di may be destroyed. .

As shown in FIG. 7, the control IC 1
Connect switch 2 between control terminal CTL and input power supply
When the switch 2 is turned off, the system enters a standby state,
Oscillation from control IC 1 only when switch 2 is turned on
Apply output to base of switching transistor Q1
Can be controlled as follows. However, the switch
2 is turned off, the oscillation of the control IC 1 is stopped, and the
State, the input voltage Vi is supplied,
At the end, a voltage Vo = Vi−V according to the load current_D(V _DIs negative
Changes depending on the load current), and the load side
Malfunctions such as malfunctions of devices or equipment connected to the
Create a match.

[0009] Therefore, a main object of the present invention is to provide a D such that the output current can be cut off when the load is short-circuited or during output standby.
The object is to provide a C / DC converter.

[0010]

According to the present invention, there is provided a coil and a rectifier diode connected in series between an input and an output, a switching element connected between a connection point of the coil and the rectifier diode and ground, In a step-up DC / DC converter using an integrated circuit including a protection circuit capable of stopping oscillation of a switching element when a load is short-circuited or in a standby state, an output current cutoff transistor connected between a rectifier diode and a load in a boost DC / DC converter When, with only the second resistor connected between the first resistor and preparative La <br/> control terminal and the ground Njisuta connected between the input terminal and the control terminal of the transistor, the load A control voltage is applied to the transistor so that the transistor can be turned off when oscillation of the switching element stops during a short circuit or in a standby state.

[0011]

[0012]

In the DC / DC converter according to the present invention, the oscillation of the switching element stops when the load is short-circuited or during standby.
When the load is short-circuited, each element constituting the DC / DC converter can be prevented from being destroyed because the transistor for cutting off the output current is made non-conductive.

[0013]

FIG. 1 is an electric circuit diagram of an embodiment of the present invention. In FIG. 1, an emitter-collector of an output current interrupting transistor Q2 is connected between a cathode of a rectifying diode Di and an output terminal, and a base of the transistor Q2 is grounded via a resistor R3 as a second resistor. At the same time, it is connected to the emitter of the transistor Q2 via the resistor R4, which is the first resistor . Other configurations are shown in FIG.
Is the same as

In normal operation, the switching transistor Q1 is switched by the oscillation operation of the control IC 1, and the energy stored in the coil L is output via the rectifying diode Di.
The voltage VA at the cathode of i, that is, the point _A is the following (1)
It can be represented by an equation.

V _A = V _O + V _CES ≒ V _O (1) When the load is short-circuited, the voltage V _A can be expressed by the following equation (2).

V _A = Vi−V _D ≒ V _i (2) where V _CES is a voltage between the collector and the emitter of the transistor Q2, and V _D is a voltage drop generated between the cathode and the anode of the rectifying diode Di. Minute voltage. In the above equations (1) and (2), V
_CES, ignoring because it is V _D << Vi, Vo, may be placed in normal operation V _A ≒ Vo, when a load short-circuit the V _A ≒ Vi. Therefore, the ratio of the voltage _VA during the normal operation and when the load is short-circuited is Vo / Vi.

The base-emitter voltage V _BE of the transistor Q2 is V _BE = V _A (R3 / (R3 + R4)), and the ratio of V _BE between normal operation and load short-circuit is DC / D
The ratio Vo / of the input voltage Vi and the output voltage Vo of the C converter
It becomes the same as Vi.

On the other hand, the threshold voltage of the on-state and off-state of the bipolar transistor is generally V _{BE (on)} MIN.
= Approximately 0.6 V and V _{BE (off)} MAX. = About 0.4V
And the voltage ratio is V _{BE (on)} / V _{BE (off)} = about 1.5
Becomes

The actual base-emitter voltage V _BE of the transistor Q2 is V _BE = V _A (R3 / (R3 + R4)), and is obtained by dividing the voltage V _A by the resistors R3 and R4. Now, when the input voltage is Vi and the output voltage is Vo,
The resistors R3 and R4 are set according to the output voltage Vo so that the emitter-to-emitter voltage V _BE is turned on (V _BE = 0.6 V). The resistor R4 has a value that allows a base current to flow through the transistor Q2 according to the load current. Next, when the resistances of the resistors R3 and R4 are constant, the input / output voltage ratio (step-up ratio) becomes V
If o / Vi = 1.5 or more, the voltage V _BE between the base and the emitter of the transistor Q2 when the load is short-circuited is V _BE = 0.
The voltage becomes 4 V or less, and the device can be turned off.

However, when the step-up ratio is 1.5 or less, V _BE becomes 0.4 V or more when the load is short-circuited, and it cannot be turned off. That is, the base-emitter voltage V _BE
Can be applied only when the boosting ratio is larger than the on / off threshold voltage. Therefore, during normal operation, the transistor Q2 is turned on, and the output voltage V
o can be output, and when the load is short-circuited, the transistor Q2 is turned off, and the output voltage Vo can be cut off.

FIG. 2 is an electric circuit diagram showing another embodiment of the present invention. The embodiment shown in FIG. 2 uses a MOS FET Q3 instead of the bipolar transistor Q2 of FIG. 1, and the other configuration is the same as that of FIG.
Only the set voltage _VGS applied between the gate and the source of the OS FET Q3 is different. Thus, the MOS FET Q3
The same effect as in FIG. 1 can be obtained by using

[0022]

As described above, according to the present invention, the oscillation of the switching element is stopped when the load is short-circuited or during standby.
Since the transistor for cutting off the output current is made non-conductive when the load is shorted, it is possible to prevent an excessive load current from flowing when the load is short-circuited or in a standby state, thereby preventing the electronic components constituting the DC / DC converter from being damaged.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram of one embodiment of the present invention.

FIG. 2 is an electric circuit diagram of another embodiment of the present invention.

FIG. 3 is a basic circuit diagram of a conventional separately-excited non-insulated DC / DC converter.

FIG. 4 is a circuit diagram showing another example of a conventional DC / DC converter.

FIG. 5 is an electric circuit diagram of a conventional step-up DC / DC converter.

FIG. 6 is a diagram for explaining an operation in which a short-circuit current flows in the DC / DC converter shown in FIG.

FIG. 7 is an electric circuit diagram illustrating an example of a conventional standby type DC / DC converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Control IC 2 Switch Q1, Q2 Transistor Q3 MOS FET Di Rectifying diode L Coil R1-R4 Resistance C1, C2 Capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-91727 (JP, A) JP-A-4-125710 (JP, A) JP-A-4-21362 (JP, A) 86418 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H02M 3/155

Claims (1)

(57) [Claims] 1. A coil and a rectifying diode connected in series between an input and an output, a switching element connected between a connection point of the coil and the rectifying diode and ground, and In a step-up DC / DC converter using an integrated circuit including a protection circuit capable of stopping oscillation of a switching element, an output current cutoff transistor connected between the rectifier diode and a load is provided. with only the second resistor connected between the control terminal of the first resistor and before Symbol transistor connected between the input terminal and the control terminal and the ground, of the switching element in case of load short-circuit or standby Applying a control voltage to the transistor so that the transistor can be turned off when oscillation stops. C converter.