JPH10243646A - Excessive current protective circuit for switching regulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a change in an excessive current detecting point against a change in input voltage, by setting the excessive current detecting point of a switching regulator high when the input voltage is low, and setting it low when the input voltage is high. SOLUTION: The input voltage is detected by a constant voltage diode 10 and resistors 11, 12. When the input voltage is low, the constant voltage diode 10 is in an off-state and therefore a transistor 13 is in an off-state, too, and an on-state threshold value of a transistor 6 is the sum of the VBE voltage of itself and the forward voltage of the diode 14. On the other hand, when the input voltage is high, the constant voltage diode 10 is in an on state and therefore the transistor 13 is in an on state, too, and the on-state threshold value of the transistor 6 is the VBE voltage of itself. This provides the same effect as setting the excessive current detecting point lower when the input voltage is high than when the input voltage is low.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit for a switching regulator, and more particularly, to a secondary side output voltage of a transformer, and on / off control of a switching transistor in accordance with the detected output to control the primary side of the transformer. The present invention relates to an overcurrent protection circuit for a switching regulator that controls a supply pulse.

[0002]

2. Description of the Related Art FIG. 3 shows a circuit example of a switching regulator having an overcurrent protection circuit. In FIG. 3, the input voltage VIN applied to the input terminal 1 is the primary winding L of the transformer 2.
It is applied to P1 via a switching transistor 3 and a current detection resistor 4.

The output of the secondary winding LS of the transformer 2 is converted into a DC voltage by a rectifier diode 8 and a smoothing capacitor 9 to become a circuit output VO, which is supplied to a load (not shown).

The output voltage VO is output from an output voltage detection circuit 15.
, And is transmitted to the control circuit 7 via an insulation circuit 16 that provides insulation between the primary and secondary of the transformer.

The control circuit 7 changes the duty of the on / off pulse of the switching transistor 3 according to the output voltage detection output and the induced voltage of the auxiliary coil LP2 wound to have the same polarity as the secondary winding LS. Control. As a result, the output voltage VO is controlled to be constant.

[0006] As an overcurrent protection circuit, resistors 4 and 5 and a transistor 6 are provided. That is, by inserting the resistor 4 in series with the switching transistor 3, the peak value of the pulse current on the primary side of the transformer 2 is detected, and this detected voltage is supplied to the base of the transistor 6 via the resistor 5. ing. Thus, when the overcurrent occurs, the transistor 6 is turned on to limit the conduction of the switching transistor 3, thereby performing the overcurrent protection.

FIG. 4 shows a circuit disclosed in Japanese Patent Application Laid-Open No. 2-119529, and the same parts as those in FIG. 3 are denoted by the same reference numerals. In FIG. 4, the voltage of the primary side auxiliary coil LP2 is rectified by a rectifier diode 20, smoothed by a capacitor 21 and converted to a direct current (auxiliary voltage), thereby generating a voltage corresponding to the output voltage. Supply to
This voltage is applied to the current detection output by the current detection resistor 4 via the resistors 12 and 18.

Thus, when the output voltage on the secondary side of the transformer 2 is increased, the DC voltage of the auxiliary voltage source (winding LP2, diode 20, capacitor 21) that changes in proportion to the output voltage becomes overcurrent detected. The voltage of the overcurrent detection section (the resistor 4 and the capacitor 19) increases at the terminal. Therefore, even if the current flowing through the overcurrent detection resistor 4 is small by the amount of the rise, the voltage is such that the overcurrent protection circuit operates.

That is, the operating point of the overcurrent protection circuit has shifted in proportion to the output voltage. By appropriately selecting the value of the resistor 18, the overcurrent protection circuit is controlled so that the output voltage becomes constant. It can be operated.

Note that the switching transistor 3 is a bipolar transistor, and therefore has a base resistor 17.

[0011]

The technique shown in FIG. 4 is used for a switching regulator which uses the output voltage of the secondary side of the transformer 2 by widely varying the output voltage. This is for detecting. Therefore, for a wide range of input voltage fluctuation,
Since the primary-side pulse current fluctuates, the width of the overcurrent detection point is widened, and expensive parts with large power and excessive margin are required. In particular, the transformer 2 is an important component of the switching regulator, and hinders downsizing of the transformer having a larger shape than other components.

In the technique shown in FIG. 3, the value of the resistor 4 is set in a low input voltage state in which the primary side pulse current at the time of overcurrent detection is maximum. On the contrary, at a high input voltage, the value of the primary side is inversely proportional to the voltage. The pulse current becomes smaller. However, since the resistance value of the overcurrent detection is fixed, the output power at the time of the overcurrent detection on the primary side becomes excessive.

As described above, in the output overcurrent detection on the primary side, the overcurrent detection point greatly changes depending on the magnitude of the input voltage, and particularly at a high input voltage, a considerably large output current with respect to the rated output current value. As a result, the stress on the parts increases. In addition, the overcurrent detection point may reach near the core saturation point of the transformer when the input voltage is high, and therefore, it is necessary to enlarge the core and shape of the transformer, which is disadvantageous in mounting.

It is an object of the present invention to provide an overcurrent protection circuit for a switching regulator which can reduce the width of an output overcurrent detection point for a wide range of input voltage fluctuations and can downsize a transformer. .

[0015]

According to the present invention, a secondary-side output voltage of a transformer is detected, and a switching transistor is turned on / off in accordance with the detected output to control a supply pulse to the primary side of the transformer. An overcurrent protection circuit for a switching regulator, wherein the current detection means detects the primary side pulse current, and has a predetermined threshold, and the switching is performed when a detection result of the current detection means reaches the threshold. Control means for controlling the conduction of the transistor; voltage detection means for detecting the voltage on the primary side; and threshold control means for changing a threshold value of the control means in accordance with a detection result of the voltage detection means. Thus, an overcurrent protection circuit for a switching regulator characterized by the following is obtained.

The operation of the present invention will be described. An input voltage detection circuit that monitors the input voltage is added to the primary side of the switching regulator, and the overcurrent detection point fluctuates in response to input voltage fluctuations. At times, the overcurrent detection point is switched to a lower level to reduce the fluctuation of the overcurrent detection point due to the input voltage fluctuation.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, and the same parts as those in FIGS. In FIG. 1, only different parts from FIG. 3 will be described.

The input voltage VIN supplied to the primary side of the transformer 2
The constant voltage diode 10 and the resistor voltage dividing circuit (resistors 11 and 12)
Are provided. The divided voltage output of the resistance voltage dividing circuit is supplied as a control input of a MOS transistor 13 as a switching element, and ON / OFF control of the transistor 13 is performed.

The transistor 13 controls the short-circuit of the diode 14 for determining the threshold value of the switching transistor 6, and the switching of the threshold value of the transistor 6 is controlled according to the input voltage level.

The operation will be described below. The constant voltage diode 10 and the resistors 11 and 12 are an input voltage detecting circuit for detecting an input voltage. At a high input voltage, the constant voltage diode 10 turns on, a current flows through the resistors 11 and 12, and the transistor 13 turns on. Conversely, at a low input voltage, the constant voltage diode 10 is turned off and the transistor 13 is also turned off.

The value of the resistor 4 serving as an overcurrent detection resistor is set in a low input voltage state where the primary-side pulse current at the time of overcurrent detection is maximized. When the primary-side pulse current increases at a low input voltage and the voltage across the resistor 4 exceeds the sum of the base-emitter on-voltage of the transistor 6 and the forward voltage drop of the diode 14 (first threshold), the transistor 6 turns on. I do.

Then, the voltage between the gate and the source of the transistor 3 is reduced to limit the ON state, the power supply to the transformer 2 is limited, and the output voltage is reduced. Although the output overcurrent continues to increase even after the output voltage drops, the ON operation of the transistor 3 is further restricted, so that the overcurrent is protected.

When the input voltage is high, the constant voltage diode 10 and the resistors 11 and 12 operate.
3 turns on and short-circuits the diode 14. When the primary-side pulse current increases at a high input voltage, the voltage across the resistor 4 also increases. However, since the transistor 13 is turned on, the drain-source voltage becomes almost zero and the diode 14 is short-circuited. Is almost determined by the base-emitter ON voltage (second threshold).

When the voltage across the resistor 4 becomes equal to or higher than the ON voltage of the base-emitter voltage of the transistor 6, the transistor 6 is turned on similarly to the overcurrent protection operation at the time of low input, and the gate-source voltage of the transistor 3 is turned on. Therefore, the transistor 3 is restricted from being turned on, restricting the power supply to the transformer 2 and lowering the output voltage on the secondary side. At this time,
The overcurrent detection voltage of the resistor 4 becomes smaller by the amount of the change in the voltage applied to the diode 14 compared to the overcurrent detection at the time of the low input voltage.

FIG. 2 shows the output current-output voltage characteristics during the overcurrent protection operation. In the conventional circuit, the overcurrent protection characteristic at low input voltage is A in FIG. 2, and the overcurrent protection characteristic at high input voltage is FIG.
Was C. According to the present invention, when the overcurrent is detected at the time of high input voltage, the detection point is advanced by switching the overcurrent detection voltage to a lower voltage, and the overcurrent protection characteristic at the time of high input voltage is shown in FIG.
B, and the overcurrent can be suppressed as compared with the related art.

[0027]

According to the above operation, the overcurrent is suppressed to the high input voltage, the stress of the parts can be reduced, and the use of large parts of large power with an excessive margin is not required.
In addition, the transformer can be downsized.

[Brief description of the drawings]

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

FIG. 2 is a diagram for comparing characteristics between a circuit of the present invention and a conventional circuit.

FIG. 3 is a circuit diagram illustrating an example of a conventional switching regulator.

FIG. 4 is a circuit diagram showing another example of a conventional switching regulator.

[Explanation of symbols]

 2 Transformer 3 Switching transistor 4 Current detection resistor 6, 13 Transistor 7 Control circuit 8 Rectifier diode 9 Smoothing capacitor 10 Constant voltage diode 11, 12 Voltage dividing resistor 14 Thresholding diode 15 Output voltage detection circuit 16 Insulation circuit

Claims (4)

[Claims] An overcurrent of a switching regulator for detecting a secondary-side output voltage of a transformer and controlling a switching transistor on / off according to the detected output to control a supply pulse to a primary side of the transformer. A protection circuit, comprising: current detection means for detecting the primary-side pulse current; and control means for controlling a conduction limit of the switching transistor when the detection result of the current detection means has a predetermined threshold value and reaches the threshold value. An overcurrent of a switching regulator, comprising: voltage detection means for detecting the voltage on the primary side; and threshold control means for changing a threshold value of the control means in accordance with a detection result of the voltage detection means. Protection circuit. 2. The control device according to claim 1, wherein the control unit includes a first switching element provided between the control electrode of the switching transistor and a reference potential point on the primary side, and a series of a threshold value determining element for determining a threshold value of the switching element. 2. The switching regulator according to claim 1, further comprising a connection circuit, wherein the threshold control unit includes a second switching element that performs short-circuit control of the threshold value determining element according to a detection result of the voltage detection unit. 3. Overcurrent protection circuit. 3. The method according to claim 1, wherein the first switching element is a transistor, and the threshold value determining element is a diode, and the diode is connected between an emitter of the transistor and the reference potential point. An overcurrent protection circuit for a switching regulator according to claim 2. 4. The voltage detecting means is a series connection circuit of a constant voltage diode and a resistance voltage dividing circuit provided between the primary side lines, and the threshold value controlling means is a voltage dividing output of the resistance voltage dividing circuit. 4. An overcurrent protection circuit for a switching regulator according to claim 2, wherein the transistor has a control input as a control input.