Principle of the power supply circuit of TCL-HiD432/522 rear projection color TV

TCL-HiD432/522 type Rear projection color TV, in order to meet the load power supply requirements, uses two sets of power supply circuits to power different loads, as shown in Figure 1.

Figure 1 Block diagram of the power supply circuit of TCL-HiD432/522 rear-projection color TV

1. Mains input and conversion circuit

After the AC power voltage (AC220 V) is input to the machine, it enters the circuit board through the connector P8005 and is sent to the line filter through the fuse F8001. The filter filters out the symmetrical and asymmetrical interference pulses in the AC power grid and effectively suppresses the high-frequency oscillation pulses generated by the switching power supply. 220 V AC power voltage after low-pass filtering by the line filter:

After passing through the startup impact high current suppression circuit composed of resistors, through the D8003 bridge rectifier, C8018 and L8012 filtering, an unstable DC voltage of about 310 V is established at both ends of C8018.

The other voltage obtained through half-wave rectification by D8007 and filtering by C80ll is used to supply power to the microprocessor.

2. Main power circuit

The main power supply of TCL-HiD432/522 rear-projection color TV adopts a separately excited switching power supply with 1 power control chip MB3759 (IC8004), 2 bridge driver chips IR2112, a switching transformer T8005, and 4 switching tubes (Q8007, Q8008, Q8014, Q8015) as the core, as shown in Figure 2.

Figure 2 Partial circuit diagram of the main power supply of TCL-HiD432/522 rear-projection color TV

1. Excitation signal formation The excitation signal is generated by IC8004 (MB3759) and related circuits

(l) Practical information of MB3759 (pin functions)

MB3759 is a double-ended PWM control chip, which consists of an oscillator, dead time comparator, pulse width modulation comparator (PWM pulse generator), error amplifier 1, error amplifier 2, trigger, and final drive circuit, as shown in Figure 3. It can also be seen from the figure that the internal structure of MB3759 is the same as that of TL494 and KA7500B. In order to help readers understand the working principle of the power supply circuit, the following first introduces the pin functions of MB3759 in combination with the peripheral circuit:

Figure 3 MB3759 internal function block diagram

Pin ① is the non-inverting input terminal of the error amplifier A1.

Pin ② is the inverting input terminal of the error amplifier A1.

Pin ③ is the output of error amplifiers A1 and A2 and the inverting input of comparator B2.

Pin ④ is the rest period (or dead zone) control terminal (when pin ④ is at a high level, there is no excitation voltage output on pins 9 and 10, so pin ④ can be used for soft start or overvoltage protection control).

Pin ⑤ is the oscillator timing capacitor terminal.

Pin ⑥ is the oscillator timing resistor terminal.

Pin 7 is the ground terminal.

Pin ⑧ is the C pole of the final stage driving amplifier Q1.

Pin 9 is the e pole of the final stage driver amplifier Q1.

Pin ⑩ is the e-pole of the final stage driver amplifier Q2.

Pin 11 is the C pole of the final stage driver amplifier Q2.

Pin 12 is the power supply terminal.

Pin 13 is the output mode control terminal.

Pin 14 is the 5 V reference voltage output terminal.

Pin 15 is the inverting input of the error amplifier A2.

Pin 16 is the non-inverting input of the error amplifier A2.

(2) Excitation signal formation

The excitation signal is generated by IC8004 (MB3759) and related circuits.

The controlled voltage VCC is not only directly added to the 12th pin (power supply end) of the power control chip IC8004 to power its internal control circuit, but also added to the ⑧ and 11th pins of IC8004 through R8050 and R8052 to power the final amplifiers Q1 and Q2 in IC8004.

After the control circuit inside IC8004 is powered, the reference voltage generator generates a 5 V reference voltage to power the oscillator and other circuits.

After the oscillator is powered, it oscillates with the timing components C8033 and R8051 connected to the ⑤ and ⑥ pins of IC8004 to obtain a sawtooth pulse signal, which is then processed by the internal circuit of IC8004 and outputs the switching tube excitation voltage from the ⑨ and ⑩ pins of IC8004.