Changhong ITV46920DE LED LCD TV Backlight Driver Circuit Analysis
The following image shows the LED backlight driver circuit in a Changhong ITV46920DE LED LCD TV. Each section of the circuit has the same structure and operates independently, driving a group of LEDs. The LED driver circuit provides an output voltage of 170V at 90mA. Similar to the backlight circuits seen in Hisense LED TVs, this circuit uses the integrated chip HV9911NCJ as the LED driver IC. This IC includes features such as a single-switch drive controller, high-side current sensing, closed-loop current control, high PWM dimming ratio, internal linear regulator (from 9V to 250V), constant frequency or constant off-time control, VDD = 10V short-circuit protection, overvoltage protection, synchronization lock, programmable MOSFET current limit, soft start, and other essential modules.
When the power supply transitions from standby to normal operation, the 130V power supply voltage is divided by resistors R9006, R9005, and R9004, creating a 2.47V voltage on R9004. This voltage turns on ZDT9001, allowing 12V to pass through R9011 and R9010, resulting in approximately 11.25V on R9010H. This voltage triggers Q9002 to enter saturation, allowing 12V to flow through its collector-emitter junction and reach the first pin of the HV9911NG IC (also connected to IC9201, IC9301, IC9401, IC9501, and IC9601), providing the operating voltage for these ICs.
Meanwhile, the BL_ON signal from the signal processing board is divided by R9013 and R9003, generating a 0.56V voltage that turns on Q9003. This causes the collector voltage of Q9003 to drop, turning off Q9004. The +5VS voltage then flows through R9016 and R9017 to the 13th pin of IC9101 and the corresponding pins of IC9201, IC9301, IC9401, IC9501, and IC9601. Once powered, the oscillating circuit inside IC9101 starts working, generating pulse signals that are sent to the base of Q9102 and Q9103.
The internal reference voltage of the HV9911NG generates 2.4V at pin 10, which is used as a reference for pin 15 via resistor R9107. Pin 7 of the HV9911NG is connected to an external resistor R9105, which sets the oscillator frequency. Once set, the oscillator continues to operate at that frequency.
The drive pulse from pin 3 of the HV9911NG is amplified by Q9012 and applied to the gate of Q9101 via R9101, switching it on and off. When Q9101 is on, energy is stored in inductor L9101, creating a positive voltage on one side and negative on the other. When Q9101 turns off, the inductor’s voltage reverses, charging capacitor C9110 through D9101 and D9105, resulting in a DC voltage of 170V/90mA, which powers the LED strip via R9128 and R9129.
The circuit with Q9103 and D9102 serves as a bleed-off circuit, discharging any charge accumulated on the gate of Q9101 during its off period, preventing damage due to voltage buildup.
A current feedback circuit using R9602, R9603, and pin 5 of the HV9911NG is also included. This circuit detects abnormal currents through Q9101 and stops the driver pulse if an overcurrent condition occurs, protecting the switch transistor.
The brightness setting circuit includes R9018, R9019, R9003, VR9001, and ICM9001. A 12V voltage is divided across R9018 and R9019, and further divided by R9003 and VR9001 before being applied to the positive input of ICM9001. This controls the current setting pin (pin 15) of the HV9911NG, adjusting the duty cycle of the drive pulses to regulate backlight brightness.
The current stabilization circuit uses Q9104, R9121, and pin 16 of the HV9911NG. As the LED current changes, the voltage across R9121 reflects this change, adjusting the pulse width to maintain stable LED current.
Overvoltage and overcurrent protection circuits are also present. Overvoltage protection involves resistors R9114, R9115, R9116, and pin 12 of the HV9911NG. If the output voltage becomes too high, the protection circuit stops the driver pulse. Overcurrent protection is implemented through resistors R9128, R9129, R9123, R9124, R9125, Q9105, and D9104. In case of a short circuit, Q9105 turns on, triggering the main power supply to shut down.
This detailed analysis highlights the complexity and safety mechanisms built into the backlight driver circuit of the Changhong ITV46920DE TV.
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