UHV voltage stabilizing circuit of multi frequency color display

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UHV voltage stabilizing circuit of multi frequency color display

Traditional CRT displays, monitors and TV sets, without exception, use line scan output stage reverse pulse rectifier to supply power to CRT anode. This power supply mode first has many problems in dual frequency display. With the change of line frequency, the impedance of line reverse transformer and deflection coil will change, and the line scanning current and line reverse pulse amplitude will also change. In order to keep the balance of the scan current of different line frequency, to stabilize the line amplitude and keep the line linearity good, the dual frequency display uses the electronic switch which acts synchronously with the line frequency change to convert the power supply voltage of the line output stage. At the same time, it also synchronously converts the different S correction capacitance and reverse capacitance to keep the line amplitude, line center and line linearity stable. Because the synchronization control is step-by-step, it is impossible to be very accurate, so the change of grating amplitude can still be detected in the display mode of different line frequency of ordinary dual frequency display. This change reflects that the traveling frequency of line scan current still has some changes. The power supply of CRT requires the highest stability of the filament voltage, and the error of the voltage should not exceed 5% of the rated value, otherwise the cathode life of CRT will be greatly shortened. Therefore, the dual frequency display provides a stable power supply voltage to CRT filament from the switching power supply.

The CRT independent power supply mode is adopted in the display. The line scanning circuit only provides scanning current to the line deflection coil, and provides the line reverse pulse as the line frequency reference to the clamp circuit and the blanking circuit. In addition, independent UHV and MV power supply converters with voltage stabilizing function are set to provide voltage to CRT. CRT power supply system consists of PWM pulse control system, reverse conversion system and protection system.

This part of the circuit is shown in figure 4-47. The variable pulse width drive and control system is composed of a pulse driver 7617 (tda8380a). The driving pulse driving power adapter 7604 generated in 7617 controls the magnitude of the energy stored in the pulse transformer 5601 of the converter by controlling the conduction time of 7604, so as to adjust the secondary high-voltage output of 5601.

Tda8380a is a drive controller of other excitation power adapter, which has an independent oscillation circuit inside, and the basic oscillation frequency is set by the external timing capacitor. The oscillator is provided with an external synchronous input terminal. When the input frequency is higher than several negative synchronous signals, the oscillator can synchronize with the external synchronous signal of up to 100kHz. Once the oscillation frequency is set, the duty cycle of the oscillation pulse is controlled by the PWM circuit, so that the duty cycle of the driving pulse changes within 48% (bipolar output). The pulse with variable duty cycle is shaped by the trigger, and the driving circuit outputs two pulse with different timing. In order to increase the flexibility of application, both collector and emitter are open circuit. If the two outputs are in parallel, and the collector of tube a and the emitter of tube B output in parallel, the output is a driving pulse with the same polarity and different time sequence, which can double the duty cycle. This driving mode increases the range of the maximum duty cycle to nearly 98%, which is suitable for driving the single ended switch circuit.

If the two outputs are respectively output by a-tube emitter and B-TUBE collector, the output is the same polarity, different timing and a certain dead time drive pulse, which is suitable for driving push-pull switch circuit. The two driving tubes are powered by the external circuit independently, which makes it easy for the driver to shift the driving level without isolation of the driving transformer. If both a-tube and B-TUBE are output by the same electrode, the driving pulse with opposite polarity will be output. This method is suitable for driving complementary push-pull switch circuit.

Tda8380a is also equipped with a zero crossing detection circuit to sample the induced voltage of pulse transformer. When the induced voltage drops to ov, the magnetic energy of the pulse transformer has been released. The zero crossing detection circuit makes the bistable trigger accept the trigger of the oscillation pulse through the reset of the locking circuit, and outputs the driving pulse of the next cycle. This can avoid the magnetic saturation of the pulse transformer caused by the continuous conduction of the power adapter before the energy of the pulse transformer is released, and the overheating and breakdown of the power adapter caused by the decrease of the inductance.

The in-phase input terminal of tda8380a internal sampling comparator is connected with a reference voltage of 2.5V provided internally, and its reverse input terminal obtains the sampling voltage through the external sampling voltage dividing circuit. A series of protection circuits are also set in dingda8380a, including over-voltage and under voltage protection input, over-current protection input and soft start control of power on. The functions of each pin of tda8380a are as follows:

① and ② pins are emitter and collector of a-way drive tube respectively. When the ② pin is connected with JR VC (·), the ① pin outputs the forward drive pulse. If ① pin is grounded and ② pin is powered by external load resistance, ② pin will output negative driving pulse.

③ the foot is the input of zero crossing detection, and the zero crossing detection pulse is introduced. When the pulse is on the rising edge and during the duration, the bistable flip-flop is closed through the locking circuit, the A and B channels will have no output, and the flip-flop will reset within the pulse falling edge inch.

④ pin is VCC under voltage and over voltage sampling input. The actual circuit is connected in parallel with the power supply end of the ⑤ pin to sample the VCC. The over-voltage and under voltage protection of the input mains can also be realized by sampling the rectifier voltage of the mains through the sampling voltage divider.

⑥ pin is the output terminal of 2.5V reference voltage. As the reference voltage of the internal protection circuit and the error comparator, the external resistance with an error of 1% makes the reference voltage stable.

The ⑦ pin is the reverse input of the sampling comparator. Introduce the secondary sampling voltage of the power adapter. When the secondary output voltage increases, the output terminal ⑧ pin of the comparator outputs electricity.