JPH0246232Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a switching regulator, and particularly relates to protection when an overvoltage exceeding a specified value is input.

BACKGROUND ART A switching regulator converts a DC power supply into a pulsating current by switching it, supplies this pulsating current to a transformer, transforms it, and then rectifies it to a desired DC voltage. It is widely used in various electronic circuit devices such as television receivers.

FIG. 1 is a circuit diagram showing an example of a switching regulator used in a television receiver. In the figure, reference numeral 1 denotes a starting transistor that automatically starts the switching regulator when the power is turned on. When power of, for example, 24 VDC is supplied between the power input terminals 2a and 2b, the capacitor 3
By being turned on for a predetermined time determined by the value of the resistor 4 and the resistor 4, the pulse width variable output circuit 7 is output via the resistor 5 and the diode 6 for preventing backflow.
to supply power. The variable pulse width output circuit 7 constitutes a multivibrator circuit by transistors 8 and 9, capacitors 10 and 11, and resistors 12 to 15, and has a constant pulse width determined by the values of the capacitors 10 and 11 and resistors 13 and 14. A periodic rectangular wave signal is output. The rectangular wave output signal of this multivibrator circuit is supplied via a diode 16 and a resistor 17 to a connection point P between a resistor 18 and a capacitor 19 that integrates the power supply. Therefore, the integration circuit performs integration during the period when transistor 8 is off, and when transistor 8 is turned on, the charge in capacitor 19 is rapidly discharged via resistor 17, diode 16, and transistor 8. Become. Therefore, the level of the integrating circuit made up of the resistor 18 and the capacitor 19 gradually increases during the off period of the transistor 8.
When the transistor 8 is turned on, a sawtooth integral signal is generated which is synchronized with the oscillation period of the multivibrator, which rapidly decreases. This integral signal is supplied to the transistor 21 via the resistor 20, so that it is turned on and generates the pulse signal A only during the period in which the integral signal below the set level is supplied. The pulse width of this pulse signal A is varied in accordance with the output level of a switching regulator, which will be described later, and in accordance with the potential of a connection point P between a resistor 18 and a capacitor 19 that constitute an integrating circuit. When the potential at the connection point P is high, the integral output rises steeply, and therefore, as the on period of the transistor 21 becomes longer, the pulse width of the pulse signal A becomes wider. Furthermore, when the potential at the connection point P is low, the integral output rises slowly, so the on period of the transistor 21 becomes short and the pulse width of the pulse signal A becomes narrow. However, since the output of the switching regulator is zero at the start of startup, the pulse width variable output circuit 7 generates a pulse signal A with a predetermined width. The transistor 22 performs on/off operations according to the above. When the transistor 22 is driven by the pulse signal A in this manner, the starting power supplied from the starting transistor 1 is applied to the primary side of the pulse transformer 24 via the resistor 23 to receive the pulse signal A.
The output transistor 25 is turned on and off by its secondary output. When the output transistor 25 performs on/off operations, the power supplied to the power input terminal 2a flows as an intermittent current to the primary side of the output transformer 26, and a primary current flows between both ends of the secondary side. During the ON period of , that is, an alternating current at a level corresponding to the duty of the pulse output signal A generated from the variable pulse width output circuit 7 is output, and after this alternating current output is rectified by the diodes 27 and 28. By being smoothed by the capacitor 29, for example, the voltage between the output ends 30a and 30c is
It will be output as a 22V DC power supply. Therefore, diodes 27, 28 and capacitor 2
9 constitutes a rectifier circuit that rectifies and smoothes the output of the output transformer 26. Further, the output of the intermediate tap on the secondary side of the output transformer is rectified by the diode 31 and output as a 12V DC power source.

Here, the signal is output via the diode 31.
The 12V output power is supplied as power to the variable pulse width output circuit 7 and the pulse transformer 24 via the diode 32. Therefore, the starting transistor 1 supplies the starting power to these circuits only for a short period of time until the output of the switching regulator is supplied as power to the variable pulse width output circuit 7 and the pulse transformer 24. The on-period setting is determined by the values of the capacitor 3 and resistor 4.

On the other hand, the +22V output of the switching regulator is divided by a resistor 33, a variable resistor 34, and a resistor 35, and the divided voltage output is divided by a variable resistor 34.
The output of the transistor 36 is supplied to the connection point P via the resistor 37 by being supplied from the sliding piece to the transistor 36 constituting the error amplifier. The potential at this connection point P is controlled by the transistor 36 in inverse proportion to the output voltage, thereby performing feedback control and making the output voltage constant.

Next, when the load increases, the capacitor 4
Since the overcurrent averaged by 0a and 40b flows, the limiter transistor 39 whose base input is the potential across the microresistance 38 is turned on. Here, when the transistor 39 is turned on, the connection point P is grounded, so the transistor 21 continues to be in the on state, and accordingly, the pulse signal A continues to be in the "H" state. It turns out. As a result, pulse transformer 2
Since the pulse signal no longer flows through the output transistor 4, the on/off operation of the output transistor 25 is stopped and generation of an output is prevented, thereby providing protection against overload. In addition, 40a,
40b is a smoothing capacitor connected between the power input terminal 2a and the ground.

However, in the switching regulator with the above configuration, if an overvoltage of, for example, 48V exceeds the maximum specified value of 24V and is inadvertently input, an excessive current flows to the output transistor, causing damage, and the overvoltage level increases. Depending on the situation, other circuit parts may also be damaged.

DISCLOSURE OF THE INVENTION Therefore, the object of the present invention is to provide a switching regulator that can ensure protection when overvoltage is applied, and in which the protective operation does not operate in case of overcurrent within a predetermined time due to external noise, etc. The goal is to provide the following.

In order to achieve such an object, the present invention provides a series body of a Zener diode and a resistor between the positive input terminal of the DC power supply and the input terminal of the limiter transistor.

Therefore, in a switching regulator configured in this way, when a DC power exceeding a specified value is applied, the Zener diode turns on and its output turns on the limiter transistor, so that the output The transistor will be forced cut off to prevent damage due to overvoltage input.

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 2 is a circuit diagram showing an embodiment of a switching regulator according to the present invention, and the same parts as in FIG. 1 are indicated using the same symbols. In the figure, 41 is a current detection resistor with a minute value of, for example, about 0.1Ω, which is installed between the output transistor 25 and the ground, 42 and 43 are resistors that divide the voltage across the resistor 41, and 44 is a voltage dividing resistor. A diode for level shifting the signal, 45 a capacitor for smoothing and averaging the output signal of the diode 44, and 46 a resistor 4.
This is a limiter transistor connected between the connection point P of the variable pulse width output circuit 7 and the ground via a resistor 48, and has a base input as a smoothed signal by a capacitor 45 supplied via a resistor 48. 49 is a Zener diode whose cathode side is connected to the power input terminal (+) 2a, and 50 is a resistor connected between the Zener diode 49 and the cathode side of the diode 44. The characteristics and the value of the resistor 50 are set so that the Zener diode 49 turns on when an overvoltage exceeding the maximum allowable voltage is input.

In the switching regulator configured in this manner, when the output transistor 25 is subjected to switching control, a sawtooth current flows through the primary side of the output transformer 26, causing the secondary side to flow in accordance with the turns ratio. A transformed output is generated, and this secondary output is connected to diodes 27, 28 and capacitor 2.
9, the signal is rectified and smoothed and supplied as a DC power source of a desired voltage to an electronic device (not shown) from the power source output terminals 30a, 30c. In addition, this power output is connected to a resistor 33, a variable resistor 34, and a resistor 35.
After being divided in voltage at As a result, feedback control for making the output voltage constant is performed.

On the other hand, since the current detection resistor 41 is connected between the output transistor 25 and the ground,
The current flowing through the output transistor 25 itself, that is, the current before being smoothed by the capacitors 40a and 40b is detected. For this purpose, an output transistor 25 is connected across the resistor 41.
Sawtooth current detection signal B corresponding to the current value flowing in
will be generated as shown in FIG. 3a. Then, this current detection signal is voltage-divided by the resistors 42 and 43, so that the signal C shown in FIG.
becomes. Next, this signal C is supplied to the diode 44, whereby the level of V _D shown by the dotted line in FIG. 3b is shifted, and only the peak portion is outputted as signal D as shown in FIG. 3c. This signal D
is smoothed and averaged by the capacitor 45, resulting in the signal E shown in FIG. 3d. Since the signal E extracted in this way is extracted by paying attention to the peak portion of the current waveform flowing through the output transistor 25, the effect of the sawtooth waveform shape is small, and the effect on the Peter value is the strongest. It becomes a signal to be received. Then, when an overcurrent flows through the output transistor 25 and the output signal E exceeds a preset reference value, the transistor 46 is turned on, thereby controlling the pulse width variable control circuit 7.
The connection point P at the terminal is grounded via the resistor 47. As a result, since the transistor 21 continues to be on, the operation of the variable pulse width output circuit 7 is forcibly stopped, and the output transistor 25 is turned off, thereby preventing damage due to overcurrent. .

Next, for some reason, the power input terminals 2a, 2
If an overvoltage exceeding the allowable voltage is input between
The Zener diode 49 is turned on. and,
When the Zener diode 49 is turned on, its output flows to the ground via the resistors 50 and 43, and is divided into resistances and output.
8 to turn on transistor 46. When the transistor 46 is turned on, the connection point P of the variable pulse width output circuit 7 is grounded via the resistor 47, so that the transistor 21 remains on and the operation of the variable pulse width output circuit 7 is forced. will be stopped. As a result, the pulse width variable output circuit 7
Since the output transistor 25, which performs the switching operation, is cut off by the pulse signal A generated from the output voltage, damage caused by overvoltage input is prevented.

In this case, the input power line contains a lot of noise due to the power being turned on and off to other devices, so there is a problem in using this power as a signal for limiter control via a Zener diode. However, in this case, the capacitor 45 absorbs the noise, so there is no problem.

Note that the resistor 50 is inserted to limit the Zener diode current when the Zener diode 49 is turned on, and does not necessarily need to be inserted depending on the fixed allowable power of the Zener diode.

In the above embodiment, a case has been described in which the limiter transistor 47 controls the connection point P of the variable pulse width output circuit 7, but the operation of the output transistor 25 can be forcibly turned off. It goes without saying that any part may be controlled.

As explained above, in the switching regulator according to the present invention, the current flowing through the output transistor is directly detected, and the peak portion of this current detection signal is extracted, smoothed, and averaged. By detecting the input and supplying the output signal of the Zener diode to the limiter transistor, the output transistor is forcibly turned off. This has the effect of preventing damage to the circuits and related circuits.

In addition, the present application does not respond even if the detected current contains external noise and becomes abnormal for one cycle of the current detection signal pulse, but responds when the current detection signal is a continuous abnormal current to a certain extent. Therefore, when using a normal switching regulator, it has the effect of operating only when necessary.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an example of a conventional switching regulator, Fig. 2 is a circuit diagram showing an embodiment of a switching regulator according to the present invention, and Fig. 3 is a circuit diagram showing an example of a switching regulator according to the present invention.
Figures a to d are waveform diagrams of various parts for explaining the operation of the circuit shown in Figure 2. 1...Starting transistor, 3, 10, 11, 1
9, 29, 40a, 40b... Capacitor, 45
...Smoothing/averaging capacitor (capacitor),
4, 5, 12, 13, 14, 15, 17, 18,
20, 23, 33, 35, 37, 41, 42, 4
3,47,48,50...Resistance, 6,16,2
7, 28, 31, 32, 41, 44...Diode, 8, 9, 21, 22, 25, 36, 39, 4
6...transistor, 24...pulse transformer,
26... Output transformer, 49... Zener diode.

Claims (1)

[Scope of utility model registration request] A variable pulse width output circuit that outputs a pulse signal whose pulse width is varied by external control, and an output transistor that supplies intermittent current to the output transformer by opening and closing in response to the pulse signal generated from this variable pulse width output circuit. and a rectifier circuit that rectifies and smoothes the output of this output transistor to output a DC power supply of a target voltage, and a pulse width variable output circuit that controls the pulse width variable output circuit according to the voltage of the DC power output. an error amplifier that controls the pulse width of the pulse signal to be generated; a microresistance connected in series with the output transistor to detect the current flowing directly through the output transistor; A diode that takes out the peak part of the current detection signal by level shifting, a capacitor that smooths and averages the output signal of this diode, and a capacitor that forcibly turns the output transistor when the level of the smoothed signal by this capacitor exceeds a set value. In a switching regulator equipped with a limiter transistor that forcibly turns off a limiter transistor that is turned off when A switching regulator comprising: a Zener diode which is turned on when the above power supply voltage is supplied, and whose output turns on the limiter transistor.