Figure 10 OZ960 internal circuit diagram

Table 3 OZ960 pin functions

(1) Drive control circuit

The drive control circuit is composed of U901 (OZ960) and its peripheral components.

The Vdd voltage (usually 5V) generated by the switching power supply is limited by R904 and added to the power supply terminal 5 of OZ960 to provide the voltage required for OZ960 to work.

When the LCD color TV needs to be lit, the ON/OFF signal output by the microcontroller is high level, and through R903, the voltage added to pin 3 of OZ960 is high level (voltage above 1.5V is high level).

OZ960 gets power supply at pin 5 and high level signal at pin 3, then the internal oscillator circuit starts to work, and the oscillation frequency is determined by the value of timing resistor R908 and timing capacitor C912 connected to pins 17 and 18. After the oscillator circuit works, it generates oscillation pulses, which are added to the internal zero voltage switching phase shift control circuit and drive circuit, and after transformation and shaping, PWM pulses are output from pins 19, 20, 12, and 11 to the full bridge drive circuit.

(2) Full-bridge drive circuit

The full-bridge drive circuit is used to generate AC high voltage that meets the requirements to drive the CCFL to work, and is composed of Q904, Q905, Q906, Q907, T901, etc. This is a full-bridge circuit structure with zero voltage switching. Vcc (usually 12V) voltage is added to the source of Q904 and Q906, and the source of Q905 and Q907 is grounded. Under the control of the drive pulse output by OZ960 (whose waveform is shown in Figure 11), Q904 and Q907 are turned on at the same time, Q905 and Q906 are turned on at the same time, and when Q904 and Q907 are turned on, Q905 and Q906 are turned off. In other words, Q904, Q907 and Q905, Q906 are turned on alternately, and symmetrical switch tube drive pulses are output. Through the resonant circuit composed of C915, C916, C917, C918, step-up transformer T901 and backlight tube, a voltage and current approximately in the shape of a sine wave are generated to ignite the backlight tube.

Figure 11 Drive pulse waveform output by OZ960

(3) Brightness adjustment circuit

Pin 14 of OZ960 is the brightness control terminal. When the brightness needs to be adjusted, the brightness control voltage generated by the microcontroller is divided by R906 and R907 and added to pin 14. After being processed by the internal circuit, the duty cycle of the drive pulse output by OZ960 is controlled to achieve the purpose of brightness control.

(4) Protection circuit

①Soft start protection circuit: The 4-pin soft start terminal of OZ960 is connected to an external soft start capacitor C904, which plays the role of soft start timing. After OZ960 works, the circuit inside the 4-pin charges C904. As the voltage across C904 increases, the drive pulse output by OZ960 controls the energy provided by the drive tube to the high-voltage transformer and gradually increases. The soft start circuit can prevent excessive impact current from being generated when the backlight is initially working.

② Overvoltage protection circuit: The overvoltage protection circuit in OZ960 can prevent the secondary side of the high-voltage transformer of the lamp from generating excessively high voltage under abnormal conditions, thereby damaging the high-voltage transformer and the lamp. In the circuit, the high voltage generated by the secondary side of T901 is divided by R930, R932 and R931, R933, and then added to pin 2 as a sampling voltage through D909 and D910. In the startup phase, pin 2 detects the secondary voltage of the high-voltage transformer. When the voltage of pin 2 reaches 2V, OZ960 will no longer increase the output voltage, but enter the stage of stabilizing the output voltage.

③ Overcurrent protection circuit: The overcurrent protection circuit is used to protect CCFL from aging or damage due to excessive current.

In the circuit, R936 and R937 are overcurrent detection resistors. The voltage across R936 and R937 changes with the working current. The larger the current, the higher the voltage across R936 and R937. This voltage is added to the 9th pin of OZ960 through D912 and D914 as the current detection terminal, and the lamp current is stabilized through the internal control circuit. If the working current of CCFL is too large, the voltage of the 9th pin will increase a lot. When the voltage of the 9th pin reaches 1.25V, it will be controlled by the internal processing of OZ960 to stop outputting the driving pulse, so as to achieve the purpose of protection.

3. "BIT3105+ full-bridge structure drive circuit" high-voltage board circuit

The high voltage board circuit composed of "BIT3105 + full bridge structure drive circuit" is shown in Figure 12. BIT3105 is a PWM control chip, and its internal circuit block diagram is shown in Figure 13. The pin functions are shown in Table 4.

Figure 12 "BIT3105+ full-bridge structure drive circuit" high-voltage board circuit

Figure 13 BIT3105 internal circuit block diagram