Analysis and overhaul NCP1217A+NCP1653A program LCD TV power supply circuit (on) - power supply circuit - circuit diagram - Huaqiang Electronics

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The power supply design of this solution primarily consists of three key circuits: PFC (Power Factor Correction), standby power, and main power supply circuits. Integrated circuits such as NCP1653A, NCP1207A, and NCP1217A from ON Semiconductor are widely used in brands like Hisense, Haier, and TCL. As an example, let’s examine the Hisense TLM3237D LCD TV's power supply circuit. The overall circuit composition is illustrated in the accompanying diagram. The power board provides five sets of voltage outputs, including 24V, 12V, and 5V_M. The main parameters for each voltage output are listed in Table 1. Upon turning on the power, the standby power is activated, producing a +5V voltage supplied to the CPU. After the second boot, the CPU sends a signal to the power circuit, initiating the main power circuit. The AC220V mains voltage is first rectified and then boosted to approximately 380V (PFC B+) through the PFC circuit. From there, the 380V voltage splits into two paths: one path goes through the dual-tube forward converter, where it is transformed, rectified, and filtered to produce 24V, 28V (or 14V), and 12V. The 24V is further converted through a DC-DC circuit to produce 12V. The other path is directed to the on/off power supply circuit, where it undergoes transformation, rectification, and filtering to yield the 5V_S voltage. This 5V_S voltage is subsequently converted through a switching circuit to produce 5V_M. Given that the 12V voltage serves as the control voltage for the 5V_M output, the 5V_M can only function properly once the 12V output is stable. Additionally, the 5V_M output powers the LED, meaning that as long as the LEDs are functioning correctly, the 5V_S, 5V_M, 24V, and 12V outputs of the power board are all operational.

Let us now analyze the circuit in detail: 1. **Main Power Input Circuit**: Shown in Figure 2, when the power is switched on, the AC220V voltage passes through fuse F801 and varistor RV801. Common-mode interference is filtered using L803 and L804, while differential-mode interference is filtered using C801-C804. The VB801 then feeds the full-bridge rectifier, generating a pulsating 300V voltage (B+). 2. **Standby Power Supply**: This section is primarily made up of the quasi-resonant control chip N803 (NCP1207A), the switching transistor V809 (a 3A/800V MOSFET FQPF3N80C), and the switching transformer T803, as shown in Figure 3. - **Startup Control**: When the power is turned on, the +300V voltage is applied to the 8th pin of N803 via VD811, VZ805, and R826, providing the initial high voltage. This voltage charges the external capacitor C833 at pin 6 (VCC) through the internal DC source circuit within the chip. Once the voltage across C833 reaches the startup threshold of the chip, N803 begins to output an excitation pulse from pin 5 to the gate of V809, thus activating the circuit. - During the initial power-on phase, the 300V voltage energizes the 1-2 winding set of T803 and applies it to the drain of V809. After the PFC circuit is operational, the PFC B+ voltage (approximately 380V) replaces the 300V to power the V809 drain. During normal operation, the induced voltage from the 3-4 windings of T803 is output through R833 current limiting, rectified by VD810, and filtered by C833, ultimately delivering a 12V output to the N803 pin 6. The induced voltage from the 3-5 windings of T803 is sent to the VCC voltage control circuit. - **Voltage Regulation Control**: Pin 2 of N803 is the regulated feedback terminal. When the secondary 5V output rises, the voltage across R855 and R922 is applied to the control electrode (R pole) of N808 (TL431), raising its K pole voltage. This results in a drop in the voltage at pin 2 of the optocoupler N804, increasing the illumination of the LEDs connected to pins 1 and 2, and deepening the conduction of the phototransistor connected to pins 3 and 4. This is equivalent to reducing the equivalent resistance at c.e., causing the voltage at pin 2 of N803 to drop, decreasing the internal oscillation frequency and ultimately lowering the output voltage to achieve regulation. If the output voltage drops, the regulation process reverses. - **Overcurrent Detection**: Pin 3 of N803 is the overcurrent detection terminal. When the current flowing through the D and S poles of V809 becomes too large, the voltage drop across R832 is fed back to pin 3 of N803 via R830, causing N803 to stop pulse output. - **Short Circuit Protection for 5V-M**: After the power is turned on, the induced voltage from the secondary of T803 is supplied to the CPU via VD812, C842, and 5V_S. After the second power-on, the main power supply circuit outputs 12V, which, through R865, creates a high-level gate voltage for V813, turning it on and producing the 5V-M voltage to supply power to USB devices. Components such as V812 and VZ816 form a 5V-M short-circuit protection circuit. During normal operation, the 12V voltage divided by R898 and R870 regulates VZ816. The base voltage of V812 is approximately 3.3V, and the emitter voltage is 5V, so V812 is turned off due to reverse bias of the PN junction, allowing V813 to turn on and output 5V-M voltage. If a short-circuit fault occurs in the USB device, the 5V-M voltage drops sharply, pulling the emitter potential of V812 low and turning it on, thereby turning off V813 and automatically cutting off the 5V power supply. Since there is a pull-up resistor R865, the 12V voltage remains unaffected, continuing to supply the +5V-S voltage to the CPU. Once the short-circuit fault is resolved, V812 turns off, V813 turns on, and the 5V-M voltage automatically restores itself, enabling independent short-circuit protection and self-recovery functionality for the USB power supply. - **Undervoltage Protection**: When the rectified full-bridge output voltage is about 310V, the Zener diode VZ805 turns on, and N803 begins to operate. If the input voltage falls below the threshold required to turn on VZ805, PNP transistors V808 and V817 turn on due to the low base voltage. Once V808 turns on, the voltage at pin 1 of N803 exceeds its overvoltage protection threshold (7.2V), causing the chip to enter a protective state with no drive pulse output. After V817 turns on, the transistor V807 is cut off, disconnecting the power supply to the PFC and the main switching power supply control chip, stopping both the PFC and the main switching power supply circuits. Even if the large-capacity electrolytic capacitor in the primary circuit retains some power, its energy cannot be transferred to the secondary, effectively preventing the occurrence of screen flickering. Additionally, N803 itself includes overvoltage, overcurrent, and overheat protection features. The pin functions and measured data are detailed in Table 2.