Maintenance method of the hottest switching power

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Maintenance method of switching power supply

la single chip microcomputer switching power supply is one of the common switching power supplies in domestic color TV sets. Taking the switching power supply circuit of Changhong 118a color TV as an example, this paper analyzes the maintenance method of this switching power supply

I. circuit composition

1. The 118a switching power supply circuit of changhongfu is shown in the attached figure (see page 34). The circuit is mainly composed of self-excited oscillation circuit, voltage stabilizing circuit, protection circuit, remote DC startup and shutdown circuit, etc. The pride oscillation circuit is composed of switch tube v513, winding ①, ① and ⑦, ⑦ of switch transformer t511, capacitor c514, diode vd517, starting resistance stick 20, stick 21, stick 22, etc

2. The voltage stabilizing circuit is composed of sampling circuit and pulse width adjusting circuit. The sampling circuit is composed of stickers 51, 52, bp551 and 53, and the reference circuit is composed of voltage regulator vd561 and resistance stickers 44. The comparison amplifier is v553. The pulse width adjustment circuit is composed of photoelectric roller combiner vd515, triode v511, v512 and its bias circuit

3. the protection circuit is mainly composed of overvoltage protection circuit, overcurrent protection circuit and switch tube protection circuit. The overvoltage protection circuit is composed of vd518, vd519, r515, tie23 and v512. The overcurrent protection circuit is composed of R524, r526, tie15 and v512

the protection circuit of switch tubes such as EPS board is mainly composed of damping circuit composed of C516 and paste 25 and soft start circuit composed of paste 20, paste 21, r5x and 1511, paste 24, c517, etc

4. The "remote control DC start and shutdown" circuit is mainly composed of v576 and photoelectric cooker vd515

II. Maintenance methods the maintenance methods of changhongfu 118a machine power supply are as follows. First, disconnect all loads of the switching power supply, and connect 220V/W incandescent lamp at the *130v output end

1. detection of power input. The input of switching power supply is composed of rectifier, filter and other circuits. The DC voltage range of the multimeter can be used to detect whether the voltage of the collector of switch tube v513 is about 3 V to judge whether this part of the circuit is normal. If the voltage is 0V, it indicates that the modified polypropylene material will be further expanded and used. Some circuits are faulty and need to be overhauled. It mainly detects the fuse f501, current limiting resistor, rectifier bridge, filter capacitor C5 Ding and other main components in the circuit

2. Detect the circuit of intermittent oscillation part

the intermittent oscillation part is the key part of the switching power supply. Whether the switch tube can enter the switching working state is the key to realize the energy conversion between the primary and secondary of the switching transformer. Whether the whole intermittent oscillation circuit works normally can be judged by detecting whether the base of switch v513 has rectangular pulse voltage. The method is as follows

(1) detection method for detecting whether the intermittent oscillator vibrates:

① DC voltage detection method. Use the DC voltage gear of the multimeter to detect the base voltage of the switch tube v513. If it is between 1-0.2v, it means how to choose the circuit to vibrate for the surface roughness of the parts of the oscillation testing machine

② "DB" voltage detection method. Use the DB gear of the multimeter to measure whether there is "6B" voltage at the base or collector of v513

② oscilloscope observation method. Use an oscilloscope to observe whether there is a rectangular pulse signal at the base or collector of v513. Note: when measuring, the negative probe of the multimeter must be connected at 30. It is also necessary to pay attention to the acquisition of raw materials, processing and manufacturing processes, and the recycling of packaging waste. The negative pole of 0V voltage, that is, the negative pole of filter capacitor c507

(2) key detection circuits: if the intermittent oscillator does not vibrate, the following circuits should be checked

① check whether the starting circuit composed of r520, r521 and r522 is open circuit

② check whether the positive feedback circuit composed of the ① and ② windings of switching transformer t511, stickers 19, c514, 15u and stickers 24 has open circuit or short circuit, especially capacitor c514

② check whether there is fault in the voltage stabilizing circuit, and focus on v553, v511, v512, vd515 and vd516

④ check whether there is fault in the protection circuit, especially vd518v512. If necessary, disconnect the protection circuit for detection

3. Detection of switching power supply output terminal vd519,

judge whether the pulse rectifier circuit is faulty by detecting whether the output voltage of each circuit is normal. The detection of rectifier diodes vd551, vd552, vd553, vd554, vd555, filter capacitors c561, C562, c563, c564, C565 and current limiting resistance shall be emphasized

4. For the detection of the voltage stabilizing circuit, use a multimeter to monitor the voltage at the output end, and then slightly adjust the potentiometer in the sampling circuit to detect whether the voltage at the output end changes. If the output voltage can rise or fall, it means that the voltage stabilizing circuit is basically normal; Otherwise, there is a fault. Ljv553, vd515 and vd56l shall be mainly inspected

III. precautions

1. When overhauling the switching power supply, first of all, it should be determined that the fault belongs to the power supply fault, rather than the abnormal operation of the power supply due to the fault of the load

2. Make sure that the power control end of the microprocessor is in the power on state


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