The power supply circuit of this solution is mainly composed of PFC, standby power supply and main power supply circuit. The integrated circuits NCP1653A, NCP1207A and NCP1217A produced by ON Semiconductor are mainly used in some LCD TVs such as Hisense, Haier and TCL. The following is Hisense TLM3237D LCD. The TV power supply circuit is analyzed as an example, and its circuit composition is as shown in FIG.
The power board has 5 sets of voltage output such as 24V, 12V, 5V_M, etc. The main parameters of each group voltage are shown in Table 1. After the power is turned on, the standby power is turned on, and the +5V voltage is output to the CPU. After the second boot, the CPU sends a power-on command to the power circuit, the main power circuit is turned on, the AC220V mains is rectified first, and then the rectified voltage is raised to about 380V (PFC B+) through the PFC circuit. 380V voltage is divided into two ways: one way through the double-tube forward circuit, after transformer conversion and rectification and filtering, it outputs 24V, 28V (or 14V) voltage, 24V passes through DC-DC circuit output 12V; the other way is sent to open/standby power supply circuit After the transformer is converted and rectified and filtered, the 5v_s voltage is output. The 5v _s voltage is again output by a switching circuit with a voltage of 5v_m. Since the 12V voltage is used as the control voltage for controlling the 5V_M output, 5V_M can be output normally only after the 12V output is normal.
In addition, the output of the 5v_m voltage is connected to the LED. Therefore, as long as the LED is normally illuminated, the 5V_S, 5V_M, 24V, and 12V outputs of the power board are normal.
First, the circuit analysis
1. Mains input circuit This circuit is shown in Figure 2. After the power is turned on, the AC220V voltage passes through the insurance F801 and the varistor RV801, the differential mode interference is filtered out by C801-C804, the common mode interference is filtered out by L803 and L804, and finally the VB801 is sent to the full bridge rectification to generate the pulsating 300V voltage. (B+).
2. Standby power standby power supply is mainly composed of quasi-resonant control chip N803 (NCP1207A), switch tube V809 (3A/800V MOs tube FQPF3N80C) and switching transformer T803, as shown in Figure 3.
(1) Start control After the power is turned on, the +300V voltage is applied to the 8th pin of the N803 through VD811, VZ805, and R826 as the starting high voltage. The high voltage is charged to the 6-pin (VCC) external capacitor C833 through the DC source circuit inside the chip. When the voltage across the C833 reaches the chip start threshold, the N803 outputs an excitation pulse from the 5 pin to the gate of the V809, and the circuit starts to work. .
At the beginning of the power-on, 300V through the VD801 to T803 1-2 winding energy group is added to the drain of V809. After the PFC circuit is operated, the PFC B+ voltage (about 380V) replaces the 300V to supply the V809 drain. During normal operation, the induced voltage generated in the T803's 3-4 windings is output through the R833 current limit, VD810 rectification, and C833 filtering, and the output 12V voltage is supplied to the N803 pin 6 of the N803. The induced voltage of the T803's 3-5 winding output is sent to the VCC voltage control circuit.
(2) Voltage regulation control
The 2 pin of N803 is the regulated feedback terminal. When the 5V voltage of the secondary output rises, the voltage of R855 and R922 is applied to the N808 (TL431) control electrode (R pole), and the voltage of the N808 is raised. K pole) voltage drop, that is, the voltage of the 2 pin of the optocoupler N804 drops, then the light-emitting diodes connected to the 1st and 2nd feet of the N804 are enhanced in illumination, and the phototransistor connected to the 3rd and 4th legs is deepened, which is equivalent to its c. When the e-equivalent resistance becomes smaller, the voltage of the 2 pin of the N803 drops, and the internal oscillation frequency decreases, eventually causing the output voltage to drop, achieving the purpose of voltage regulation. If the output voltage of 5V is lowered, the voltage regulation process is opposite to the above.
The 3 pin of the N803 is the overcurrent detection terminal. When the current between the D and S poles flowing through the V809 is too large, the voltage drop on the R832 is fed back to the 3 pin of the N803 through the R830, and the N803 stops the pulse output.
(3) 5V-M voltage short-circuit protection 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, 12V voltage through R865 to make the gate of V813 appear high level, V813 is turned on, output 5V-M voltage, supply power to USB device.
Components such as V812 and VZ816 form a 5V-M short circuit protection circuit. During normal operation, since 12V voltage is divided by R898 and R870, VZ816 is regulated. The base voltage of V812 is about 3.3V, the emitter voltage is 5V, V812 is turned off due to PN junction reverse bias, V813 is turned on, and output is 5V- M voltage. When the USB device has a short-circuit fault, the 5V-M voltage drops sharply, that is, the V812 emitter potential is pulled low, V812 is turned on, V813 is turned off, and the USB power supply 5V is automatically cut off. Due to the presence of the pull-up resistor R865, the 12V voltage is not affected at this time, and the +5V-S voltage supplied to the CPU. After the USB device short-circuit fault is removed, V812 is turned off, V813 is turned on, and the 5V-M voltage is automatically restored, thereby achieving independent short-circuit protection and self-recovery function of USB power supply 5V.
(4) When 300V undervoltage protection works normally, the output voltage of the rectified full bridge is about 310V, the Zener diode VZ805 is turned on, and the N803 starts working. When the input voltage falls below enough to make VZ805 turn on, PNP transistors V808 and V817 turn on due to the base low level. After V808 is turned on, the voltage of pin 1 of N803 is higher than its overvoltage protection threshold (7.2V). , the chip enters the protection state, no drive pulse output; after V817 is turned on, the triode V807 will be cut off, the power supply of the PFC and the control chip of the main switching power supply is cut off, and the PFC and the main switching power supply circuit are stopped, even if the switch The large-capacity electrolytic capacitor in the primary circuit of the power supply contains power, and its energy cannot be transmitted to the secondary, thereby effectively avoiding the occurrence of the screen flash phenomenon.
In addition, N803 itself has overvoltage, overcurrent and overheat protection functions. The pin function and measured data are shown in Table 2.
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