Working Principle and Troubleshooting of Oz965
As the equipment has been discontinued, it is difficult to buy new equipment, and the second mobile phone purchased online has the same fault in a short time. Therefore, the author decided to maintain the display independently. In order to facilitate maintenance, the author draws the circuit diagram according to the real object, as shown in the attached figure. The display backlight adopts cold cathode fluorescent tube CCFL, and the high-voltage driving circuit is composed of a high-efficiency CCFL inverter control chip oz965, a driving chip 4532 and a high-voltage pulse transformer. The high-voltage driving circuit provides a high-frequency AC voltage of about 600V for lighting the CCFL backlight tube and controls the brightness of the tube.
Oz965 is an integrated circuit with 5V power supply and 16 pins. It has two packaging forms: SOP and tssop. Working in a single fixed frequency PWM mode, it provides a driving signal with adjustable pulse width for external n-channel and p-channel FET, and the working frequency is between 30kHz and 200kHz. The chip contains multiple functional modules, mainly including: oscillation signal generation circuit, pulse width modulation control circuit, n-channel / p-channel FET drive signal generation circuit, reference voltage output circuit, soft start control circuit, over-voltage, under voltage, lamp open circuit, over-current and short circuit protection circuit. The functions of each pin are shown in the attached table.
1. Inverter process
5V voltage is added to 16 pin of u701 (oz965) through r711 (22 ) to supply power for it. After u701 receives power supply, its internal oscillator and external timing elements r710 and c712 generate an operating frequency of about 60KHZ, i.e. FOSC = 1.91 / (RT CT). The signal is processed by the pulse width control circuit to form a 60KHZ PWM drive signal, which is output from pin 11 and pin 10. The PWM signal is amplified by the n-channel FET and p-channel FET in the drive block T701 (4532) and added to the primary winding of the high-frequency transformer l701. After conversion, an AC voltage of about 600V is output from the secondary winding of l701 to illuminate the cold cathode fluorescent light-emitting tube CCFL (hereinafter referred to as the lamp tube) connected through the socket con702.
The secondary winding of l701 and capacitor c717 form a resonant circuit. Due to the leakage inductance and secondary resonant circuit of l701, the secondary output voltage and current of l701 are roughly sinusoidal waveform, and the harmonic EMI radiation generated by this sinusoidal wave is small, so a high-efficiency high-voltage inverter is realized.
2. Soft start process
When a high-level (higher than 1.5V) control signal is applied to pin 12 of u701, the constant current source inside u701 charges c708 connected to pin 9. With the increase of voltage at both ends of c708, the duty cycle of pulse signals output by pins 10 and 11 of u701 gradually increases from small to large, and the current flowing through the lamp tube gradually increases until it enters the normal working state to ensure that the lamp tube obtains sufficient ignition time, The service life of the lamp tube is prolonged. During soft start (i.e. when the lamp current sampling voltage is less than 0.6V), the maximum duty cycle of output pulse of pin 10 and 11 is determined by the voltage of pin 3; When the lamp current sampling voltage is greater than 0.6V, that is, the lamp current reaches 0.6 / 0.445 = 1.25ma, the soft start ends, and the maximum duty cycle of output pulse of pin 10 and 11 is determined by the voltage of pin 2.
3. Lamp current stabilizing circuit
The circuit detects the sampling voltage of the current flowing through the lamp tube and adjusts the duty cycle of the output pulse signal of pins 10 and 11 to maintain the stability of the current flowing through the lamp tube, so as to maintain the stability of the brightness of the lamp tube. The specific process is: after the 2.5V reference voltage output by oz9651 pin is divided by r703 and r704, provide 1.25V brightness control reference voltage VC for pin 5, and preset the duty cycle of output pulses of pins 10 and 11, that is, the working current of the lamp. The current flowing through the lamp is rectified by double diode d701, sampled by r714 / / r715 and filtered by C718. A lamp current sampling voltage Va is obtained at point a in the attached figure. The voltage is added to pin 13 through r712 to select the maximum duty cycle of pin 10 and 11 output pulses; The other circuit is connected to pin 6 through r716 and acts together with the brightness control voltage VL and resistor r702 to provide a voltage VB for pin 6. After the voltage VC of pin 5 and the voltage VB of pin 6 are amplified by the differential amplifier inside the chip, an error voltage V is generated. The error voltage is compared by a slope comparator to adjust the duty cycle of output pulses of pins 10 and 11. When the current flowing through the lamp decreases, VA decreases, the potential VB of pin 6 decreases, the duty cycle of output pulses of pins 10 and 11 increases, and the conduction time of n-channel and p-channel FET increases. The current flowing through the lamp increases through the inverter transformer l701. On the contrary, when the current flowing through the lamp increases, VA increases and VB increases successively, and the duty cycle of output pulses of pins 10 and 11 decreases, so that the current flowing through the lamp decreases. In this way, it is continuously adjusted in real time to keep the lamp current constant. By increasing or decreasing the brightness control voltage VL, the duty cycle of pin 10 and 11 output pulses can be adjusted to realize the brightness adjustment of the lamp tube, but the maximum duty cycle of pin 10 and 11 output signals will not exceed the value set by pin 2 voltage. C707 is the compensation capacitance of the differential amplifier in u701.
4. Protection function
When the power supply voltage rises to 3.9v, the circuit starts to work. When the power supply voltage drops to 3.4V, the circuit enters the protection state. When the lamp tube is open or fails, the current sampling voltage Va is very small, the voltage VB of pin 6 also becomes very low, and the output voltage of pin 7 of u701 internal differential amplifier becomes large. Once the voltage is greater than 2.78v, the internal circuit of the chip triggers the lamp tube open circuit protection mechanism, and the chip stops working and enters the protection state. When the voltage of pin 4 is greater than 0.6V, the circuit enters the short-circuit protection state. Pin 4 of this circuit is grounded, so there is no short-circuit protection function. After the chip enters the protection state, pin 1 has no 2.5V reference voltage output.
5. Reference voltage output circuit
The chip contains a 2.5V reference voltage circuit, which can provide 1mA current.
Fault phenomenon: after power on, the display screen flashes and gives out high-frequency squeaking sound. In less than 1 second, the screen goes out and the squeaking sound disappears.
Bao’an District Shenzhen City, China
+86 189 3806 5764
crowberry@laptoprepair.com
www.laptoprepair.com
