JPH11178332A - Switching regulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching regulator capable of performing overcorrect protection by a specified load current at all times regardless of change of an AC input voltage. SOLUTION: If the voltage of an AC input section 1 becomes larger, even if value of a load current is the same, the peak value of the current of a switching device 19 becomes larger. Thus control is so performed that the drive frequency of the switching device 19 is made higher by a power controlling IC 5, larger the voltage of the AC input section 1 becomes, to cause the peak value of the current of the switching device 19 not to change, even if the voltage of the AC input section 1 becomes large. Consequently, it is possible to detect a specified overcorrect of a load at all times by a current transformer 11, regardless of the change of an AC input voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator, and more particularly to overcurrent protection.

[0002]

2. Description of the Related Art FIG. 7 is a schematic circuit diagram of a conventional switching regulator, and FIG. 8 is a waveform diagram of a current flowing through the switching element. In FIG. 7, 1 is an AC input unit, 2 is a filter unit, 3 is a bridge diode, 4 is a capacitor, 5 is a current control IC, 6 is a resistor, 7 is a capacitor, 8 is a diode, 9 is a capacitor, 10 is a resistor,
11 is a current transformer, 12 is a main transformer, 1
3, 14 are diodes, 15 is a choke coil, 16 is a capacitor, 17 is a voltage detector, 18 is a photocoupler,
19 is a switching element.

[0005] Next, each component and the overall schematic operation will be described.

[0004] First, an AC voltage is input from an AC input unit 1 and input to a bridge diode 3 via a filter 2. The AC voltage is rectified and smoothed by the bridge diode 3 and the capacitor 4. For example, in the case of a 100 VAC input, a DC voltage of about 130 V is output from the bridge diode 3 and input to the main transformer 12.

In the power supply control IC 5, a resistor 6 and a capacitor 7
Is supplied to the switching element 19 at a predetermined frequency set based on the time constant of the above and a pulse voltage having a required duty. With this operation, the main transformer 12 is driven by the switching element 19, and a pulse voltage corresponding to the primary / secondary turns ratio of the main transformer 12 is generated on the secondary side of the main transformer 12. afterwards,
The pulses on the secondary side of the main transformer 12 are rectified and smoothed by the diodes 13 and 14, the choke coil 15, and the capacitor 16, and output as a switching regulator.

[0006] The output voltage of the switching regulator is detected by a voltage detector 17, and the detection signal is input to the power supply control IC 5 via a photocoupler 18. Power control I
In C5, if the signal from the voltage detector 17 is lower than the expected output voltage, the duty of the pulse voltage supplied to the switching element 19 is increased, and conversely, the voltage detector 17
Is higher than the expected value of the output voltage, the duty ratio of the pulse voltage supplied to the switching element 19 is controlled to be small, and the output voltage is controlled regardless of the state of the load connected to the switching regulator. Is kept constant.

Next, a protection operation for detecting the current flowing through the switching element 19 and protecting the switching element 19 and other components when the current value exceeds a predetermined value will be described.

FIG. 8 is a diagram showing a waveform of a current flowing through the switching element 19.

Here, a is a line of a current value obtained by converting a current value supplied to the load into a value on the primary side by a turns ratio of the main transformer 12.

(For example, if the turns ratio of the main transformer 12 is N
1: N2, and the current value supplied to the load is I1, a
Line is I1 * N2 / N1. Also, the slope of b (current change) is
2 is converted to a value on the primary side by the output pulse voltage of the switching regulator, the output voltage of the switching regulator, the inductance value of the choke coil 15, and the turns ratio of the main transformer 12.

(For example, if the turns ratio of the main transformer 12 is N
1: N2, the output voltage of the switching regulator is Vo
If ut, the peak value of the pulse voltage output from the main transformer 12 is Vin, the inductance value of the choke coil 15 is L, and the ON time of the switching element 19 is t, the value of the current change of b is (Vin-Vout). * T *
N2 / (L * N1). (The voltage drop Vf of the diode 13 is ignored.) As for the current flowing through the switching element 19, a current value converted according to the turns ratio of the current transformer 11 is transmitted to the secondary side of the current transformer 11. The current value is converted into a voltage by the resistor 10, rectified and smoothed by the diode 8 and the capacitor 9, and a detection signal corresponding to the current value flowing through the switching element 19 is input to the power supply control IC 5. Here, if the detection signal corresponding to the current value is equal to or higher than a predetermined level, it is determined that the load connected to the switching regulator is abnormal or an overcurrent due to an abnormality of each element in the switching regulator is supplied to the switching element 19. The duty of the applied pulse voltage is extremely reduced to protect each element and the load connected to the switching regulator.

[0012]

However, in the above-described conventional example, when the voltage of the AC input unit 1 changes, the peak point of the current change b shown in FIG. The detection level has also changed, and there has been a problem that the protection is not applied when protection is desired, or the protection is applied when the protection is not applied.

That is, even if the load current is the same, as shown in FIG. 8, when the AC input voltage is low, the peak point of the current change b is also lowered, and when the AC input voltage is high, the current change point b is low. Peak point goes up. Therefore, when detecting the overcurrent of the load with the current of the switching element, if it is detected at the same current level,
A disadvantage arises in that detection is performed using different load overcurrent values depending on the AC input voltage.

The present invention has been made under such circumstances, and it is an object of the present invention to provide a switching regulator capable of always performing overcurrent protection with a predetermined load current regardless of a change in AC input voltage. It is the purpose.

[0015]

In order to achieve the above object, according to the present invention, a switching regulator is configured as in the following (1) to (5).

(1) A switching regulator which receives an alternating current, rectifies it, performs switching by a switching element, and supplies it to a main transformer, wherein current detecting means for detecting a current of the switching element; First control means for switching at a frequency and controlling so as to limit the current of the switching element when an overcurrent is detected by the current detection means;
A second control means for controlling the frequency to increase as the AC voltage increases.

(2) In the switching regulator according to the above (1), the second control means apparently changes a resistance value of a time constant circuit of a resistor and a capacitor for determining the frequency according to the AC voltage. Switching regulator.

(3) A switching regulator that receives AC input, rectifies, switches with a switching element, and supplies the switching transformer with a switching element. The current detecting means detects the current of the switching element. And a first control means for controlling so as to limit the current of the switching element when an overcurrent is detected by the current detection means, and detecting the current detection means as the AC voltage increases. And a second control means for controlling the output to be small.

(4) In the switching regulator according to (3), the second control means bypasses a current flowing to a current detection resistor of the current detection means according to the AC voltage. regulator.

(5) In the switching regulator according to the above (2), the second control means equivalently changes the resistance value by a current mirror circuit whose current changes according to the AC voltage. A switching regulator.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to embodiments of a switching regulator.

[0022]

FIG. 1 is a schematic circuit diagram of a "switching regulator" according to a first embodiment, FIG. 2 is a schematic diagram of a change in a current waveform flowing through a switching element 19, and FIG.
3 shows a schematic circuit diagram of the drive frequency control unit 20.

The same parts as those in the conventional example are denoted by the same reference numerals, and the description thereof will be omitted. Only the characteristic parts of the present embodiment will be described. Further, here, as an example, AC
The case of an input of 85V to 132V will be described. The drive frequency control unit 20 detects a DC voltage obtained by rectifying and smoothing the AC input voltage with the bridge diode 3 and the capacitor 4,
A resistor 10 for setting the drive frequency according to the detection signal
6 acts so as to control the frequency of the pulse voltage supplied to the switching element 19.

FIG. 3A is a diagram showing an internal circuit of the drive frequency control unit 20 shown in FIG. In FIG. 3A,
21 is an operational amplifier, 22, 23, 25 and 26 are resistors, 2
4 is a reference power supply. The non-inverting input terminal of the operational amplifier 21 is connected to a reference power supply 24 corresponding to an AC input of 85 V. Supplied. The output terminal of the operational amplifier 21 is connected via a resistor 22 to
The resistor 106 is connected to a connection point to the power control IC 5.

In the configuration described above, as the voltage of the AC input unit 1 increases, the output of the operational amplifier 21 decreases, and the total current value flowing through the resistors 22 and 106 for setting the driving frequency of the switching element 19 increases. Become.
That is, the value of the drive frequency setting resistor decreases, and the frequency of the pulse voltage supplied from the power control IC 5 to the switching element 19 increases.

Therefore, as shown in FIG. 2, even when the peak portion of the current waveform flowing through the switching element 19 decreases and the AC input voltage increases to, for example, 132 V AC, the overcurrent is maintained at substantially the same load current value as that at 85 V AC. Protection will be applied.

Furthermore, the drive frequency control unit 20 is
It goes without saying that the same operation is performed even if the device is constituted by a so-called current mirror circuit shown in FIG.

(Embodiment 2) FIG. 4 is a schematic circuit diagram of a "switching regulator" according to Embodiment 2 of the present invention, and FIG. 5 is a voltage waveform diagram in which the output of a current transformer 11 is converted into a voltage by a resistor 110 or the like. 6. Current detection resistance control unit 27
FIG. 2 shows a schematic circuit diagram of FIG.

The same parts as those in the conventional example are denoted by the same reference numerals, and the description thereof will be omitted. Only the characteristic parts of this embodiment will be described. Further, here, as an example, AC
The case of an input of 85V to 132V will be described. The current detection resistance control unit 27 detects a DC voltage obtained by rectifying and smoothing the AC input voltage with the bridge diode 3 and the capacitor 4,
In response to the detection signal, the current detection resistor 110 that converts the current flowing through the current transformer 11 into a voltage is apparently varied, and acts to control the output current level of the current transformer 11.

612 is an operational amplifier, 622, 623, 6
25 and 626 are resistors, and 624 is a reference power supply. The AC input 8 is applied to the non-inverting input terminal of the operational amplifier 621.
A reference power supply 624 corresponding to 5 V is connected, and the voltage after AC rectification is connected to the inverting input terminal of the operational amplifier 621 by the resistor 62.
The output divided at 5,626 is supplied.

In the configuration described above, as the voltage of the AC input unit 1 increases, the output of the operational amplifier 621 decreases, the current shunted to the resistor 622 increases, and the current value flowing to the current detection resistor 110 decreases. Accordingly, the peak of the voltage waveform obtained by converting the output current waveform of the current transformer 10 falls.

Therefore, as shown in FIG. 5, even when the peak portion of the current waveform flowing through the switching element 19 apparently falls and the AC input voltage rises to, for example, 132 V AC, the load current is almost the same as that at 85 V AC. The value will cause overcurrent protection.

Further, in this embodiment, an example has been described in which the current flowing through the switching element 19 is detected by a combination of the current transformer 11 and a resistor for voltage conversion. It goes without saying that an example of detection can be implemented with a similar configuration.

[0034]

As described above, according to the present invention,
Irrespective of fluctuations in the AC input voltage, overcurrent protection can always be performed at a predetermined load current value.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a first embodiment.

FIG. 2 is a diagram showing a current waveform of a switching element.

FIG. 3 is a circuit diagram of a drive frequency control unit.

FIG. 4 is a circuit diagram of a second embodiment.

FIG. 5 is a diagram showing an output voltage waveform of a current transformer.

FIG. 6 is a circuit diagram of a current detection resistance control unit.

FIG. 7 is a circuit diagram of a conventional example.

FIG. 8 is a diagram showing a current waveform of a switching element in a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 AC input part 3 Bridge diode 5 Power supply control IC 12 Main transformer 19 Switching element 20 Drive frequency control part 106 Resistance for drive frequency setting

Claims (5)

[Claims] 1. A switching regulator to which an alternating current is input, rectified, switched by a switching element, and supplied to a main transformer, wherein a current detecting means for detecting a current of the switching element; And a first control means for controlling so as to limit the current of the switching element when an overcurrent is detected by the current detection means, and the frequency increases as the AC voltage increases. And a second control means for controlling the switching regulator. 2. The switching regulator according to claim 1, wherein said second control means apparently changes a resistance value of a time constant circuit of a resistor and a capacitor for determining said frequency in accordance with said AC voltage. A switching regulator, characterized in that: 3. A switching regulator that receives AC input, rectifies, switches with a switching element, and supplies the switching transformer with a switching element. The current detecting means detects a current of the switching element. A first control means for switching and controlling the current of the switching element when the overcurrent is detected by the current detection means, and a detection output of the current detection means as the AC voltage increases. And a second control means for controlling the switching regulator to be smaller. 4. The switching regulator according to claim 3, wherein said second control means bypasses a current flowing to a current detection resistor of said current detection means according to said AC voltage. And a switching regulator. 5. The switching regulator according to claim 2, wherein the second control means equivalently changes the resistance value by a current mirror circuit in which a current changes according to the AC voltage. A switching regulator characterized by the following.