JPH0134453Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a power supply circuit used in a television receiver.

BACKGROUND TECHNOLOGY AND PROBLEMS Conventionally, in a power supply circuit using a switching regulator of a television receiver, the characteristic frequency _O of the oscillator that constitutes a part of the switching regulator is set slightly lower than the horizontal scanning frequency _H. I try to choose. In the case of a receiver configured to perform remote control, the power supply circuit is operated when the receiver has stopped receiving operations and is waiting for an operation command from a remote control device. At this time, since the horizontal scanning frequency deviates from _H ,
By operating the oscillator at the frequency _O , the receiver may generate harsh sounds.

In order to prevent this sound, conventionally, when the receiver is in the standby state, the frequency of the oscillator is switched to a frequency outside the audible band. A circuit shown in FIG. 1 has conventionally been used for this purpose.

In FIG. 1, during the reception operation of the receiver for remote control, the command signal input terminal 1 is at a low level, and the transistors Q ₁ , Q ₂ and transistor Q ₃ that constitute the frequency switching circuit 7 are off. . With these, the oscillator 2 has a resistance
_R1 and resistor _C1 are connected, and the oscillation frequency is selected to be a value lower (eg, 13 to 14 KHz) than _H (eg, 15.75 KHz) by the time constants _of R1 _{and C1} . PWM (pulse width modulation) is performed by the output of this oscillator 2.
The circuit 3 is activated and its output pulse P switches the regulator transistor _Q9 at _O. Further, _{the H} output signal obtained from the horizontal oscillator 9 switches the horizontal drive transistor _Q4 . This switching output is applied via the horizontal drive transformer _T1 to the base of the horizontal output transistor _Q5 to switch this transistor _Q5 . A DC voltage +B obtained as the output of the regulator is applied to the primary coil 4 of the flyback transformer _T2 , and this DC voltage +B is switched by the transistor _Q5 . This switching output is rectified and smoothed by diodes D ₁ and D ₂ and capacitors C ₂ and C ₃ connected to the secondary coils 5 and 6, respectively, and then supplied to a predetermined load circuit. At the same time, the switching output is taken out from the coil 5 as a flyback pulse FB, which resets the oscillator 2 and synchronizes its oscillation output with the horizontal output. This synchronization prevents the beat disturbance of the screen due to the frequency of the oscillator 2 and _H. The pulses output from the regulator transformer _T3 are rectified and smoothed by a diode _D4 and a capacitor _C4 . This output DC voltage +B is added to the PWM circuit 3 as a control voltage,
The width of the output pulse P is controlled.

By the above operation, regulator transformer T ₃
The DC voltage +B obtained from the secondary side output is controlled to be constant.

Next, in the above operating state, when a high-level command signal CM instructing to stop reception is applied from the remote control device (not shown) to input terminal 1,
Transistor Q ₃ turns on and transistor Q ₄ turns on.
is turned off. Therefore, the switching operation of transistor _Q5 is stopped. Along with this, the transistor
Q ₁ and Q ₂ turn on, which causes resistance R ₂ to change to R ₁
connected in parallel. As a result, the time constant of the oscillator 2 becomes small, and the oscillation frequency is changed to a frequency outside the audible band, such as 20 KHz or more. According to the above, generation of harsh sound by the oscillator 2 can be prevented when the receiver is in a standby state.

However, this circuit
The configuration becomes complicated, and it is necessary to apply a command signal to both the switching circuit and the horizontal circuit. Therefore, when the power supply circuit and the high voltage circuit are configured on separate printed circuit boards, wiring materials are required to supply command signals to the two circuits.

Purpose of the invention The present invention eliminates the above-mentioned drawbacks and provides a power supply circuit that can switch the frequency of an oscillator with a simple circuit configuration.

Summary of the invention The invention uses flyback pulses to perform the switching operation of the frequency switching circuit of the oscillator. This allows the circuit configuration to be simplified.

Embodiment FIG. 2 shows the first embodiment of the present invention.
Portions corresponding to those in FIG. 1 are given the same reference numerals.

In FIG. 2, the frequency switching circuit 7 includes a switching transistor Q ₁ , resistors R ₁ , R ₂ and a capacitor.
It is composed of _C1 etc. as shown in the figure. In this embodiment, the command signal CM is input to input terminal 1.
, so that it is added only to transistor _Q3 . During reception, a negative flyback pulse FB obtained from the coil 5 is applied to the base of the transistor _Q1 to turn the transistor _Q1 on. This connects the resistor _R2 in parallel with the capacitor _C1 , and further connects the resistor _R1 between the connection point and the power supply voltage _E2 . At this time, the combined resistance becomes large and the frequency of the oscillator 2 becomes lower than _H. In addition, the negative polarity flyback transformer FB is a transistor.
When added to Q ₁ , this transistor Q ₁ is momentarily turned off, but this poses no practical problem.

Next, in the reception state described above, when a high-level command signal CM is applied to input terminal 1, transistor Q ₃ is turned on, and therefore transistors Q ₄ ,
_Q5 turns off and switching operation stops. Therefore, the flyback pulse FB disappears and transistor _Q1 is turned off. As a result,
Resistor R ₂ is separated, leaving only R ₁ , and the capacitor
The combined resistance seen from C ₁ becomes small. Therefore, the frequency of the oscillator 2 is changed outside the audible band.

FIG. 3 shows a second embodiment of the switching circuit 7, and the other parts have the same structure as FIG. 2.

This switching circuit 7 is configured to apply a flyback pulse FB of positive polarity from a terminal 8.
During reception operation, when flyback pulse FB is applied, diode _D3 turns on and resistor _R2 turns on.
Connected to the time constant circuit of R ₁ and C ₁ . This causes the frequency of the oscillator 2 to be lower than _H. In addition, the flyback pulse FB is also added to oscillator 2,
This oscillator 2 is synchronized with the horizontal output. Next, when the command signal CM is applied and the receiving standby state is entered, the flyback pulse FB disappears, the diode _D3 is turned off, and the resistor _R2 is disconnected. This switches the frequency of the oscillator 2 out of the audible band. In addition, resistor _R3 is flyback pulse FB
Set the cathode potential of diode _D3 when there is no current.

FIG. 4 shows a third embodiment.

The switching circuit 7 according to this embodiment is configured to apply a negative flyback pulse FB to the terminal 8 during reception operation, and is provided with a current mirror circuit including transistors Q ₆ and Q ₇ .

When a command signal is applied and the device is in a reception standby state, there is no flyback pulse FB, so diode _D3 is off. At this time, the current I ₁ determined by the resistor R ₁ of the current mirror circuit becomes large, and as this current I ₁ charges the capacitor C ₁ , the frequency of the oscillator 2 is outside the audible band. During reception operation, a flyback pulse FB is applied to turn on diode _D3 , which causes current _I2 to flow through resistor _R2 . Therefore the current I ₁
decreases and the frequency of oscillator 2 becomes lower than _H.

Figure 5 shows the fourth embodiment, in which a positive flyback pulse FB is applied during reception operation to turn on transistor _Q8 , which connects resistor _R2 to create a current mirror circuit. The frequency is made lower than _H by decreasing the current I ₁ of .

Effects of the invention: The configuration of the oscillator frequency switching circuit is simplified, and the wiring is simplified because the command signal only needs to be applied to the high voltage circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional example, Fig. 2 is a circuit diagram showing a first embodiment of the present invention, and Figs. 3 to 5 are main parts showing the second to fourth embodiments of the present invention, respectively. It is a circuit diagram. In addition, in the symbols used in the drawings, 2...oscillator, 3...PWM circuit, 7...frequency switching circuit, _Q1 ...switching transistor, _Q5 ...switching transistor, _T2 ...flyback transformer. be.

Claims (1)

[Claims for Utility Model Registration] Switching that generates a DC voltage of a predetermined value by performing a switching operation based on the oscillation output of a switching oscillator capable of oscillating a frequency slightly lower than the horizontal scanning frequency and a frequency outside the audible band. a regulator; a horizontal output transistor that switches the voltage supplied to the primary side of the flyback transformer to generate a flyback pulse on the secondary side of the flyback transformer; A horizontal drive circuit for switching the horizontal output transistor; and a frequency switching circuit provided for switching the oscillation frequency of the switching oscillator and performing the switching operation using the flyback pulse given from the flyback transformer. The oscillation frequency of the above-mentioned switching oscillator is switched to a frequency slightly lower than the horizontal scanning frequency to prevent beat interference while the TV receiver is receiving, and to prevent noise generation while waiting for a command signal from the remote control. 1. A power supply circuit for a television receiver, characterized in that it switches to a frequency outside the audible band in order to transmit the signal.