JP2522588Y2 - Overcurrent and overvoltage protection circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent and overvoltage protection circuit suitable for use in a high-voltage generation circuit of a television receiver.

[0002]

2. Description of the Related Art An overcurrent protection circuit and an overvoltage protection circuit are provided in a high-voltage generation circuit of a television receiver for the purpose of protecting a switching transistor connected to a CRT or a flyback transformer. The overcurrent protection circuit is provided to prevent the CRT and flyback transformer from burning out when the CRT is overloaded, and FIG. 3 shows a conventional example thereof.

In FIG. 1, reference numeral 12 denotes a flyback transformer, and a primary drive 12 is supplied with a horizontal drive signal from a terminal 14 via a horizontal drive transformer 16. That is, a horizontal drive signal of a horizontal cycle is supplied to the terminal 14, whereby the first control transistor Qa is turned on and off to drive the horizontal drive transformer 16. The output of the horizontal drive transformer 16 is further supplied to a second control transistor (switching transistor) Qb, and the primary coil 1 of the flyback transformer 12 is
2a is switching-controlled by a drive current corresponding to the horizontal period. As a result, a predetermined flyback pulse is output to the secondary coil 12b side of the flyback transformer 12, whereby a predetermined high voltage HV is obtained at the output stage of the rectifier circuit 18. As is well known, the high voltage HV is CR
Applied to the anode of T (not shown). Rectifier circuit 18
Is composed of a resistor and a capacitor as shown in the figure. As is well known, the collector voltage of the second control transistor Qb is used as a drive voltage for a horizontal deflection coil (not shown).

In such a high-voltage generating circuit, the overcurrent protection circuit is configured as follows. A resistor Rs is connected in series with the primary coil 12a of the flyback transformer 12 so that an overcurrent state is detected. That is, when an overcurrent flows and the voltage between both ends of the detection resistor Rs becomes equal to or higher than VBE of the detection transistor Qc, the detection transistor Qc is turned on by this, and the voltage divider provided in the current loop formed by this on is turned on. When the voltage across the voltage dividing resistors R2 and R3, especially the resistor R3 for the circuit becomes higher than the reference voltage Vref, the third control transistor Qd is turned on and the first control transistor Qa is forcibly turned off. Thereby, the flyback transformer 1
Since no horizontal drive signal is supplied to 2, its oscillating operation is stopped, and the high voltage HV output from the rectifier circuit 18 decreases. This can prevent the CRT from being burned due to overcurrent.

FIG. 4 shows a detection resistor Rs for detecting overcurrent.
Is used instead of the current transformer 20, the detection pulse detected by the current transformer 20 is rectified and smoothed by the rectifier circuit 21, and the voltage is compared with the reference voltage Vref. As a result, when the voltage exceeds the reference voltage Vref due to the overcurrent, the third control transistor Qd is turned on by the output of the comparator 15, and therefore, as in the case of the overcurrent protection of FIG. The supply is cut off. Thereby, the CRT and the flyback transformer can be protected.

FIG. 5 shows a conventional example of an overvoltage protection circuit. Since a stabilized voltage is normally supplied to the terminal 13, if the stabilized power supply circuit or the like fails, an abnormal voltage higher than usual is applied to the terminal 13. If this abnormal voltage is applied to the second control transistor Qb of the flyback transformer 12 for a long time, it may be destroyed.

Therefore, this abnormal voltage is applied to the resistors R5 and R6.
When the voltage is detected by the Zener diode 25 functioning as an overvoltage detecting element and a voltage equal to or higher than the reverse voltage determined by the Zener diode 25 is applied, the detection transistor Qc turns on, and the voltage divided by the resistors R2 and R3 is applied. The signal is supplied to the comparator 15. Thus, a comparison operation with reference voltage Vref is performed. Therefore, when the voltage corresponding to the abnormal voltage becomes equal to or higher than the reference voltage Vref, the third control transistor Qd is turned on, and the supply of the horizontal drive signal is cut off as shown in FIG. 3 or FIG. The oscillation of No. 12 stops. Thereby, the destruction of the second control transistor Qb due to the overvoltage can be prevented.

[0008]

As described above, conventionally, there are individual protection circuits for protecting overvoltage or overcurrent. Therefore, in order to protect both the overcurrent and the overvoltage, both the protection circuits shown in FIG. 3 and FIG. 5 must be incorporated, so that the circuit scale is increased. On the other hand, if each protection circuit is mounted, only one of them can be protected, and the protection is not sufficient.

In view of the above, the present invention solves such a conventional problem, and is capable of appropriately protecting both overcurrent and overvoltage without increasing the circuit scale.

[0010]

In the present invention, a resistor for detecting an overcurrent is connected between one end of the primary side of the flyback transformer and a power supply terminal, and the resistor and the flyback transformer are connected to each other. The base of the detection transistor is connected to one end of the primary side, the emitter of the detection transistor is connected between the resistor and the power supply terminal, and the overvoltage detection is connected to the base of the detection transistor. When an element is connected and an overcurrent is detected by the resistor or an overvoltage is detected by the overvoltage detection element, the detection transistor is controlled and a horizontal drive signal is sent to the primary side of the flyback transformer. Is not supplied.

[0011]

[Function] Secondary coil 12b of flyback transformer 12
When an excessive current flows through the primary coil 12a, an excessive current iL also flows through the primary coil 12a. This current i
L is detected by the overcurrent detection resistor Rs. When the detection transistor Qc is turned on by an excessive current, and the resulting divided voltage exceeds the reference voltage Vref, the third control transistor Qd is turned on, and the supply of the horizontal drive signal to the flyback transformer 12 is stopped. . Thus, the CRT and the flyback transformer can be protected from overcurrent.

When an abnormal voltage is applied to the terminal 13 and exceeds the reverse voltage of the Zener diode 25, the detection transistor Qc is turned on, and the divided voltage at that time is changed to the reference voltage Vref.
Exceeds the third control transistor Qd as described above.
Turns on, and the oscillation operation of the flyback transformer 12 also stops. Thereby, the second control transistor Qb and the like can be protected from overvoltage.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an example of an overcurrent and overvoltage protection circuit according to the present invention will be described in detail with reference to FIGS.

FIG. 1 is an example of a main part of the present invention, and the same components as those in FIGS. 3 and 5 are denoted by the same reference numerals, and description thereof will be omitted. In the present invention, as shown in FIG. 1, a resistor Rs for overcurrent detection is connected in series with the primary coil 12a of the flyback transformer 12, and a detection transistor at both ends thereof is connected to a PNP-type detection transistor Qc in this example. The emitter and base are connected. Detection transistor Q
A zener diode 25 functioning as an element for detecting overvoltage is connected in series with the base resistors R7 and R8 connected to the base side of c.

The resistance of the detecting resistor Rs is selected so that the detecting transistor Qc is turned on when an excessive current exceeds a predetermined value. The reverse voltage of the Zener diode 25 connected to its base is The voltage value is selected so that the detection transistor Qc is turned on when a preset excessive voltage is applied to the input terminal 13.

The circuit configuration until the flyback transformer 12 is controlled to the safe side by detecting overcurrent or overvoltage is the same as the conventional configuration.

In this configuration, in the normal operation in which a proper current is supplied to the CRT or when the input terminal 13
Drive voltage is applied to the flyback transformer 1
2 performs an oscillating operation (normal operation), the current iL of FIG. 2A flows through the primary coil 12a, and the voltage Vc at that time becomes that of FIG. 2B. In this state, the overcurrent and overvoltage protection circuits do not operate.

On the other hand, when an abnormality such as an overload occurs, an overcurrent flows through the CRT and, correspondingly, an overcurrent also flows through the primary coil 12a of the flyback transformer 12. When this exceeds the reference value, the voltage across the detecting resistor Rs exceeds the VBE of the detecting transistor Qc, whereby the detecting transistor Qc is turned on, and the predetermined current flows through the voltage dividing resistors R2 and R3. Flows. As a result, the voltage across the resistor R3 becomes equal to the reference voltage Vre.
When f exceeds f, the third control transistor Qd is turned on based on the output of the voltage comparator 15. Thereby, the first
Is forcibly turned off, the horizontal drive signal supplied to the terminal 14 is not transmitted to the flyback transformer 12 side. That is, the second control transistor Qb is turned off, and the oscillation operation of the flyback transformer 12 is stopped, thereby preventing burning of the CRT and the flyback transformer due to an overcurrent.

On the other hand, when an abnormally high drive voltage is applied to the input terminal 13 as compared with the normal case, the Zener diode 25 is turned on and the detection transistor Qc is turned on.
Accordingly, when the voltage between both ends of the voltage dividing resistor R3 becomes equal to or higher than the reference voltage Vref, the voltage comparator 1
The third control transistor Qd is controlled based on the output of No. 5, and the supply of the horizontal drive signal is cut off. As a result, the oscillation operation of the flyback transformer 12 is stopped, so that an excessive voltage is not applied to the second control transistor Qb for a long time.
Of the control transistor Qb can be prevented.

[0020]

As described above, in the present invention, both the overcurrent detecting element and the overvoltage detecting element act on the detecting transistor. According to this, when an overcurrent or an overvoltage is detected by the detection transistor, the detection transistor is controlled in any case, and each state is reliably detected.

Therefore, according to the present invention, the circuit scale is reduced and both overcurrent and overvoltage can be properly detected. Therefore, as described above, the present invention provides a high-voltage generation circuit for a television receiver as described above. It is very suitable to be applied to the protection circuit.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an example of an overcurrent and overvoltage protection circuit according to the present invention.

FIG. 2 is a waveform chart for explaining the operation.

FIG. 3 is a system diagram showing an example of a conventional overcurrent protection circuit.

FIG. 4 is a system diagram showing an example of a conventional overcurrent protection circuit.

FIG. 5 is a system diagram showing an example of a conventional overvoltage protection circuit.

[Explanation of symbols]

 Reference Signs List 12 flyback transformer 15 voltage comparator 16 horizontal drive transformer Qa first control transistor Qb second control transistor Qc detection transistor Qd third control transistor Rs overcurrent detection resistor 25 overvoltage detection zener diode R2, R3 Voltage divider resistor

Claims (1)

(57) [Scope of request for utility model registration] An overcurrent detection resistor is connected between one end of a primary side of a flyback transformer and a power supply terminal, and a resistor is detected between the resistor and one end of a primary side of the flyback transformer. A base of the detection transistor is connected, an emitter of the detection transistor is connected between the resistor and the power supply terminal, an overvoltage detection element is connected to a base of the detection transistor, and When an overcurrent is detected, or when an overvoltage is detected by the overvoltage detection element, the detection transistor is controlled so that a horizontal drive signal is not supplied to the primary side of the flyback transformer. Features overcurrent and overvoltage protection circuits.