**Full Analysis of the LED Backlight Drive Circuit in Changhong ITV46920DE LCD TV**
The following figure shows a group of LED backlight driving circuits in the Changhong ITV46920DE LED LCD TV. Each circuit has a similar structure and operates independently, driving a group of LED lamps. The output voltage of each LED driver circuit is 170V at 90mA. Similar to the backlight driver circuit found in the Hisense LED LCD TV, the integrated block HV9911NCJ is used as an LED backlight driver IC. This IC contains multiple internal modules such as a single-switch drive controller, high-side current sensing, closed-loop current control, high PWM dimming ratio, a 9V to 250V internal linear regulator, constant frequency or constant off-time control, VDD=10V output short-circuit protection, overvoltage protection, synchronous lock, programmable MOSFET current limit, soft start, and more.
When the power supply transitions from standby to normal operation, the 130V output is divided by resistors R9006, R9005, and R9004, creating approximately 2.47V on R9004. This voltage turns on ZDT9001, allowing 12V to flow through R9011 and R9010, resulting in about 11.25V on R9010H. This voltage activates Q9002, which then conducts, providing power to the HV9911NG (IC9201, IC9301, etc.) via pin â‘ .
At the same time, the BL_ON signal from the signal processing board is divided by R9013 and R9003, producing 0.56V on the base of Q9003, turning it on. This causes Q9004 to turn off, allowing +5VS to reach pin 13 of IC9101 and other related ICs. Once powered, the internal oscillator of IC9101 starts generating oscillation pulses, which are then sent to the base of Q9102 and Q9103 via pin â‘¢ of the integrated block.
The internal reference voltage of 2.4V from HV9911NG is applied to pin ⑩, and through resistor R9107, it serves as the reference for pin 15. The external resistor R9105 on pin ⑦ sets the oscillator frequency, determining the working frequency of the circuit once set.
The drive pulse from pin â‘¢ of HV9911NG is amplified by Q9012 and applied to the gate of Q9101 via R9101, enabling it to switch. When Q9101 is on, energy is stored in inductor L9101, and when it turns off, a higher voltage is induced across the inductor, charging capacitor C9110 through D9101 and D9105. This results in a DC voltage of 170V/90mA, which powers the LED light bar via R9128 and R9129.
The circuit involving Q9103 and D9102 is a snubber circuit, similar to those in Hisense TVs. It discharges the gate-source capacitance of Q9101 during its off period, preventing charge accumulation that could damage the transistor.
A current feedback circuit composed of R9602, R9603, and the internal circuitry of HV9911NG pin ⑤ Monitors the current through Q9101. If the current increases beyond safe levels, the voltage drop across R9102 triggers a protection mechanism, stopping the drive pulse to prevent damage.
The brightness adjustment circuit includes R9018, R9019, R9003, VR9001, and ICM9001. The 12V is divided to provide a reference voltage to the comparator’s positive input (pin ③). Adjusting VR9001 changes the voltage at pin ①of ICM9001, which affects the current setting on pin 15 of HV9911NG, thus adjusting the pulse width and controlling the backlight brightness.
The current stabilization circuit uses Q9104, R9121, and the internal circuitry of HV9911NG pin ⑯. As the LED current fluctuates, the voltage across R9121 changes, affecting the internal control of the drive pulse duty cycle to stabilize the LED current.
Overvoltage and overcurrent protection are also included. The overvoltage protection circuit uses R9114, R9115, R9116, and the internal comparator on pin â‘« of HV9911NG. If the output voltage becomes too high, the protection circuit stops the drive pulse.
The overcurrent protection circuit, using R9128, R9129, R9123, R9124, R9125, Q9105, and D9104, detects excessive current in the LED light bar. If a short occurs, Q9105 turns on, triggering the main power supply to shut down via the optocoupler PCB803S.
This comprehensive design ensures stable and safe operation of the LED backlight system in the Changhong ITV46920DE TV.
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