The power supply circuit of this solution is primarily composed of PFC, standby power, and the main power supply circuit. Integrated circuits like NCP1653A, NCP1207A, and NCP1217A manufactured by ON Semiconductor are commonly used in various LCD TVs such as Hisense, Haier, and TCL. Let’s take Hisense TLM3237D LCD as an example. The TV power supply circuit will be analyzed here, and its circuit composition is illustrated in the accompanying diagram. The power board provides five sets of voltage outputs, including 24V, 12V, 5V_M, and more. The main parameters for each set of voltages are detailed in Table 1. Once the power is turned on, the standby power is activated, and a +5V voltage is delivered to the CPU. Following the second boot, the CPU sends a power-on command to the power circuit, activating the main power circuit. The AC220V mains voltage is first rectified and then raised to approximately 380V (PFC B+) via the PFC circuit. This 380V voltage is split into two paths: one path goes through the double-tube forward circuit, where it undergoes transformer conversion, rectification, and filtering, ultimately outputting 24V, 28V (or 14V) voltages. The 24V voltage is further processed by a DC-DC circuit to output 12V. The other path is sent to the open/standby power supply circuit, where the transformer converts, rectifies, and filters the voltage before outputting a 5v_s voltage. This 5v_s voltage is then reprocessed by a switching circuit to produce a 5v_m voltage. Since the 12V voltage serves as the control voltage for the 5V_M output, the 5V_M can only be output normally once the 12V output is stable. Additionally, the output of the 5v_m voltage is connected to the LED. Thus, as long as the LED is functioning correctly, the 5V_S, 5V_M, 24V, and 12V outputs of the power board are also functioning properly.
First, let's analyze the circuit:
- Main Circuit Analysis:
This circuit is depicted in Figure 2. Upon turning on the power, the AC220V voltage passes through the fuse F801 and the varistor RV801. Common-mode interference is filtered out by C801-C804, while differential mode interference is filtered out by L803 and L804. Finally, VB801 sends the voltage to the full bridge rectifier to generate a pulsating 300V voltage (B+).
- Standby Power Supply:
The standby power supply is mainly composed of the quasi-resonant control chip N803 (NCP1207A), the switch transistor V809 (3A/800V MOS tube FQPF3N80C), and the switching transformer T803, as shown in Figure 3.
- Start Control:
- Voltage Regulation Control:
- Overcurrent Detection:
- Short-circuit Protection for 5V-M:
- Undervoltage Protection:
Upon powering up, the +300V voltage is applied to the 8th pin of N803 via VD811, VZ805, and R826 as the starting high voltage. This high voltage charges the 6-pin (VCC) external capacitor C833 through the DC source circuit within the chip. When the voltage across C833 reaches the chip's start threshold, N803 outputs an excitation pulse from the 5th pin to the gate of V809, and the circuit begins to operate. Initially, the 300V flows through VD801 to T803's 1-2 winding energy group, adding energy to the drain of V809. Once the PFC circuit operates, the PFC B+ voltage (approximately 380V) replaces the 300V to supply the V809 drain. During normal operation, the induced voltage generated in T803's 3-4 windings is output through R833 current limiting, VD810 rectification, and C833 filtering, providing the output 12V voltage to the N803 pin 6. The induced voltage from T803's 3-5 winding is sent to the VCC voltage control circuit.
The 2nd pin of N803 is the regulated feedback terminal. When the 5V voltage of the secondary output increases, the voltage of R855 and R922 is applied to the N808 (TL431) control electrode (R pole), raising the K pole voltage drop. This causes the voltage of the 2nd pin of the optocoupler N804 to drop. Consequently, the light-emitting diodes connected to the 1st and 2nd feet of N804 are enhanced in illumination, and the phototransistor connected to the 3rd and 4th legs is deepened, effectively reducing its c.equivalent resistance. This results in a drop in the voltage of the 2nd pin of N803, decreasing the internal oscillation frequency and ultimately causing the output voltage to drop, achieving voltage regulation. Conversely, when the output voltage decreases, the voltage regulation process reverses.
The 3rd pin of N803 is the overcurrent detection terminal. If the current flowing between the D and S poles of V809 becomes too large, the voltage drop on R832 is fed back to the 3rd pin of N803 through R830, and N803 stops the pulse output.
After the power is turned on, the induced voltage of the T803 secondary is supplied to the CPU via VD812, C842, and 5v_s. After the second power-on, the main power supply circuit outputs 12V, and the 12V voltage through R865 makes the gate of V813 appear high-level, turning on V813 and outputting 5V-M voltage to power USB devices. Components like V812 and VZ816 form a 5V-M short-circuit protection circuit. During normal operation, the 12V voltage is divided by R898 and R870, regulating VZ816. The base voltage of V812 is about 3.3V, and the emitter voltage is 5V. Due to the PN junction reverse bias, V812 is turned off, V813 is turned on, and 5V-M voltage is output. However, if there's a short-circuit fault in the USB device, the 5V-M voltage drops sharply, pulling down the V812 emitter potential, turning on V812 and turning off V813, thus automatically cutting off the USB power supply 5V. Because of the pull-up resistor R865, the 12V voltage remains unaffected, and the +5V-S voltage continues to supply the CPU. Once the short-circuit fault in the USB device is resolved, V812 turns off, V813 turns on, and the 5V-M voltage is automatically restored, achieving independent short-circuit protection and self-recovery functionality for the USB power supply 5V.
When the 300V input voltage is normal, the output voltage of the rectified full bridge is around 310V, and the Zener diode VZ805 turns on, allowing N803 to begin working. If the input voltage falls below a certain threshold, making VZ805 unable to turn on, PNP transistors V808 and V817 turn on due to their base receiving a low-level signal. Once V808 turns on, the voltage at pin 1 of N803 exceeds its overvoltage protection threshold (7.2V), causing the chip to enter a protection state and stop driving pulse output. When V817 turns on, the transistor V807 is cut off, disconnecting the power supply to the PFC and the control chip of the main switching power supply, stopping both circuits. Even though the large-capacity electrolytic capacitor in the primary circuit of the power supply still holds power, its energy cannot be transferred to the secondary, effectively preventing the occurrence of the screen flash phenomenon. Additionally, N803 itself has overvoltage, overcurrent, and overheat protection functions. The pin function and measured data are shown in Table 2.
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