JP2003319641A - Step-up switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent burnout which occurs in a step-up switching power supply when a load is short-circuited or excessively applied. <P>SOLUTION: When a load is short-circuited or excessively applied, a switch element is made to carry out a constant current function. Further, the switching of a main switching element is stopped by the timer latch function of a PWM controller. Thus, the switching power supply loses a boosting function, and an output is reduced down to input a voltage. As a result, the conduction of a switching element is stopped, and an input and an output are cut off from each other. In a normal operation, the switching element is brought into conduction, and a boosted voltage is obtained for the output. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-up switching power supply device, and more particularly to a step-up switching power supply device having a protection function for preventing burnout of a device or an element when an abnormality such as a short circuit occurs. .

[0002]

2. Description of the Related Art Generally, a separately excited switching power supply is a PW.
It is configured using an M control (control circuit) IC. The IC has many auxiliary functions in addition to the basic functions, and the output short-circuit protection function is one of them. The output short-circuit protection function generally prevents burning of the switching power supply when the load of the switching power supply is short-circuited due to some abnormality. Although there are several means for realizing the function, a method called a timer latch type is widely used.

FIG. 2 shows an example of a step-up switching power supply using a PWM control IC adopting such a timer latch system. A main switching element Q _S is connected in series to an inductance element L connected to an input terminal V _IN , a DC output is obtained from the connection point by a diode D, and smoothed by a capacitor to obtain an output from the output terminal V _Out. Is. The DC output is detected by resistance voltage division and is connected to the error amplifier input of the PWM control IC to perform PWM control for controlling the on-duty of the main switching element.

Here, the timer / latch type protection circuit will be briefly described. When the output of the switching power supply is short-circuited and the short-circuited state is not released within the preset time and the output voltage does not return to the predetermined voltage, the timer / latch type short-circuit protection circuit outputs the output of the PWM control IC. fixed to the pulse output signal main switching element Q _S becomes 100% on duty, is intended to prevent the destruction of such main switching element and the rectifier diode. When the output of the switching power supply is short-circuited and the output voltage falls below a predetermined voltage, the protection power supply connected to the SCP terminal from the reference power supply attached to the PWM control IC.
Charging to enable capacitor C _PE begins. If the output voltage does not return to a predetermined value before the terminal voltage of the capacitor reaches a predetermined value set by charging, the short circuit protection circuit is set and the main switching element Q _S
The rest period will be 100%.

During normal operation, the protection enable capacitor C _PE is not charged and the short circuit protection circuit is not set, but when the output of the switching power supply is short-circuited and the output voltage falls below a predetermined value. Therefore, the protection enable capacitor C _PE starts to be charged from the reference voltage attached to the PWM control IC. When C _PE is charged to a preset voltage, the latch circuit is set and the main switching element Q _S
The rest period will be 100%.

In the switching power supply having such a function as shown in FIG. 2, when the output is short-circuited, the main switching element Q _S is turned off by the above operation.
However, the switching element Q _S Despite in the OFF state, V _IN → inductance element → rectifier diode →
Since the ground and the current path are formed, there is a problem that the inductance element L, the rectifying diode D and the like are burnt out.

[0007]

SUMMARY OF THE INVENTION According to the present invention, there is provided a step-up type device having a latch function capable of blocking a current path formed even when a main switching element is turned off and preventing element burnout. A switching power supply is provided.

[0008]

The present invention solves the above problems by adding a switch element having a constant current output function to a current path.

That is, in the step-up switching power supply device for rectifying and smoothing the energy stored in the inductance element connected to the input voltage source by the ON / OFF operation of the switching elements connected in series to obtain a DC output, rectification and smoothing are performed. A DC switching element having a constant current output function is provided between the means and the output terminal, and when the output voltage is lowered for a predetermined time, the protection function provided in the control circuit detects that the boosting function has stopped. The DC switch element is turned off to cut off the input voltage and the output voltage.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In a step-up switching power supply device according to the present invention, a PWM having a timer / latch function in an IC generally used for output control by a PWM system.
The control IC turns on the main switching element.
In addition to controlling the duty, a detection resistor and a switch element (transistor) are connected in series between the rectifying diode and the output terminal so that the current path can be cut off by turning off the switch element.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention, in which a detection resistor R _S and a switching element Q ₂ are connected in series on the cathode side of a rectifying diode D. In this example, the resistor R _S is connected to the emitter of the switch element (transistor Q ₂ ) and its collector is connected to the output terminal. The base of transistor Q ₂ is connected to the collector of transistor Q ₃ via resistor R ₅ , the emitter of transistor Q ₂ is connected to the input terminal, and
Connected to its base via R ₄ .

The cathode side of the diode D and the capacitor C ₂
The transistor Q ₃ and the base of the transistor Q ₃ are connected via a resistor R _3, and the anode side of the diode D is connected to the emitter of the transistor Q ₄ and its collector is connected to the base of the transistor Q _2. The base of Q ₄ is connected to the connection point between resistor R _S and the emitter of transistor Q ₂ .
A switch of the current path is configured by these circuit configurations.

The operation during normal operation will be described. Receiving the pulse output of the PWM controllers, the main switching element Q ₁ is turned on - off. The energy main switching element Q ₁ is stored in the inductance element L during the ON, the main switching element Q ₁ is taken out through the diode D during off. By smoothing this with the capacitor C ₂ , a DC voltage obtained by boosting the input voltage V _IN is obtained across the capacitor C ₂ .

At this time, a current flows into the base of the transistor Q ₃ through the resistor R _3, and the collector and emitter of the transistor Q ₃ are electrically connected. When the collector-emitter of the transistor Q ₃ is turned to open the base current path of the transistor Q ₂ is the transistor Q ₂ is turned on, the output terminal V _Out
A predetermined voltage preset by the resistors R ₁ and R ₂ is output to the load R _L, and the voltage is supplied to the load R _L.

Since the resistance value of the current detection resistor R _S is set so that the forward bias voltage does not drop enough to make the collector and emitter of the transistor Q ₄ conductive,
At this time, the transistor Q ₄ remains off.

Next, the operation when the load is short-circuited and when the load is overloaded will be described. Since a current exceeding the amount defined by the design specifications passes through the detection resistor R _S , the voltage dropped to the resistor R _S becomes a base forward bias voltage sufficient to make the collector and emitter of the transistor Q ₄ conductive.
As a result, the collector and emitter of the transistor Q ₄ become conductive, and the transistor Q ₂ starts the constant current operation.

Then, until now, the output terminal V _Out has a resistance.
When the voltage set by R ₁ and R ₂ falls below a predetermined voltage and the voltage drop time exceeds a preset time, PW
The timer latch function built into the M controller operates. As a result, the pulse output from the PWM controller has an off duty of 100% and stops switching of the main switching element Q ₁ .

When the switching of the transistor Q ₁ , which is the switching element, is stopped, the switching power supply loses its boosting function, and the voltage across the capacitor C ₂ drops to almost the input voltage V _IN . When the voltage across the capacitor C ₂ drops to almost V _IN , the base of the transistor Q ₃ that was positively biased by the resistor R ₃ is released from the positive bias, and the collector-emitter of the transistor Q ₃ becomes non-conductive, and Accordingly, the collector-emitter of the transistor Q ₂ also becomes non-conductive, and the voltage of the output terminal V ₀ is cut off.

Once the timer latch function operates, even if the load short circuit or overload is released, the output voltage does not automatically return and the cutoff state is maintained. An example of the most frequently used releasing method that requires releasing is described. After removing the overload or short-circuit condition, the PWM controller power supply V
Switching power supply will return to normal operation if _CC is disconnected and then reapplied.

The present invention is not limited to the above example, and the transistor Q ₃ is excluded except for the transistor Q ₃ and the resistors R ₃ and R _4.
One end of the resistor R ₅ connected to the collector of the
_May be connected directly to _IN .

[0021]

According to the present invention, in addition to the timer / latch type protection function of the PWM controller, an electronic switch with a constant current function is provided. Therefore, in a step-up switching power supply using a single turn coil, a short circuit occurs. It is possible to prevent burnout of the switching power supply that occurs at the time of overload, including

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional switching power supply.

[Explanation of symbols]

L: inductance element (single-turn coil) _Q 1, _{Q S:} main switching element (transistor) _{Q 2:} switching element (transistor)

Claims (4)

[Claims] 1. A step-up switching power supply device for rectifying and smoothing energy stored in an inductance element connected to an input voltage source by on / off operation of switching elements connected in series to obtain a DC output. The output voltage detection circuit having a plurality of constant current functions is provided between the means and the output terminal, and when the state where the output voltage is reduced continues for a predetermined time, the protection function provided in the control circuit is activated and the output voltage is A boosting type switching power supply device characterized in that the detection circuit operates by detecting that the boosting function has stopped and cuts off the input voltage and the output voltage. 2. A step-up switching power supply device for rectifying and smoothing energy stored in an inductance element connected to an input voltage source by turning on / off switching elements connected in series to obtain a DC output. It has a DC switch element with a constant current output function and an output voltage detection function between the means and the output terminal, and when the output current exceeds a preset value, the output voltage is lowered by the constant current output function. If the lowered state continues for more than the preset time, the protection function provided in the control circuit is activated,
A step-up switching power supply device characterized in that the DC switch element is turned off to cut off an input voltage and an output voltage. 3. The step-up switching power supply device according to claim 2, further comprising a detection resistor connected in series between the rectifying / smoothing means and the output terminal, and a DC switch element having a constant current output function and an output voltage detection function. 4. The step-up switching power supply device according to claim 1, further comprising a detection resistor, a constant current output function, and an output voltage detection circuit connected in series between the rectifying / smoothing means and the output terminal.