**Full Analysis of LED Backlight Drive Circuit Issues in Changhong ITV46920DE LCD TV**
The following diagram illustrates the LED backlight drive circuit used in the Changhong ITV46920DE LED LCD TV. Each section of the circuit follows a similar structure and operates independently, driving a group of LEDs. The output voltage of this LED driver circuit is 170V at 90mA. Similar to the backlight driver circuits seen in other models like Hisense, the HV9911NCJ IC is used as the core component in this design.
This integrated circuit features various built-in modules, including a single-switch drive controller, high-side current sensing, output current closed-loop control, high PWM dimming capability, an internal linear regulator (9V–250V), constant frequency or constant off-time control, VDD=10V short-circuit protection, overvoltage protection, synchronization lock, programmable MOSFET current limit, soft start, and more. These features ensure stable and efficient operation of the backlight system.
When the power supply transitions from standby to normal operation, a 130V output is divided by resistors R9006, R9005, and R9004, resulting in approximately 2.47V across R9004. This voltage turns on ZDT9001, allowing 12V to flow through R9011 and R9010, creating about 11.25V at R9010H. This voltage then activates Q9002, which conducts and supplies power to the HV9911NG IC, along with other ICs such as IC9201, IC9301, etc., ensuring they receive the necessary operating voltage.
At the same time, the BL_ON signal from the signal processing board is divided by R9013 and R9003, producing a 0.56V signal that turns on Q9003. This causes Q9004 to turn off, allowing +5VS to reach pin 13 of IC9101 and others. Once powered up, the oscillator inside IC9101 starts generating pulse signals, which are then sent to the base of Q9102 and Q9103 via pin â‘¢ of the IC.
The internal reference voltage (1.24V) of the HV9911NG is output from pin ⑩ and applied to pin 15 through resistor R9107, serving as a reference for the current setting. Pin ⑦ of the IC is connected to resistor R9105, which sets the oscillation frequency. Adjusting this resistor allows for fine-tuning of the switching frequency.
The drive signal from pin â‘¢ is amplified by Q9012 and applied to the gate of Q9101, enabling it to switch. When Q9101 is on, energy is stored in inductor L9101. Upon turning off, the inductor generates a reverse voltage that charges capacitor C9110 through diodes D9101 and D9105, resulting in a 170V DC output that powers the LED light bar via resistors R9128 and R9129.
A snubber circuit composed of Q9103 and D9102 helps discharge the gate-source capacitance of Q9101 during the off period, preventing potential damage from charge accumulation.
Additionally, a current feedback circuit using R9602, R9603, and the internal circuitry of the HV9911NG provides overcurrent protection. If the current through Q9101 increases due to a fault, the voltage drop across R9102 triggers the protection mechanism, stopping the drive pulse output.
The brightness adjustment circuit includes resistors R9018, R9019, R9003, potentiometer VR9001, and ICM9001. By adjusting VR9001, the reference voltage at the comparator’s positive input changes, affecting the current setting at pin 15 of the HV9911NG and thus controlling the pulse width to adjust brightness.
The current stabilization circuit involves Q9104 and R9121, with the voltage across R9121 feeding back to pin 16 of the HV9911NG. This allows the IC to adjust the duty cycle of the drive pulse, stabilizing the LED current.
Overvoltage and overcurrent protection mechanisms are also implemented. Overvoltage protection uses resistors R9114, R9115, R9116, and internal comparators within the HV9911NG to shut down the circuit if the output exceeds safe limits. Overcurrent protection is achieved through Q9105, which detects excessive current in the LED light bar and triggers the main power supply to shut down, preventing further damage.
This comprehensive analysis highlights the robust design of the backlight drive circuit in the Changhong ITV46920DE model, ensuring reliable performance and safety under various conditions.
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