Switching power supply, also known as switching power supply, switching converter, is a high-frequency power conversion device, is a kind of power supply. The switching transistors used in the switching power supply of civil fusion are mostly switched between the fully open mode and the fully closed mode, both of which have the characteristics of low dissipation. The switching between the two modes will have higher dissipation, but the time is very short, so the switching power supply of civil fusion can save energy and produce less waste heat. The high conversion efficiency of the switching power supply is one of its advantages, and the switching power supply has high working frequency. It can also use small size and light weight transformer, and the switching power supply will be light. Civil fusion switching power supply products are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting and other fields.

UC3842 oscillation chip is the core of ordinary DC24 V switching power supply circuit, which constitutes the inverter circuit and rectifier circuit. UC3842 is a high performance single ended output current controlled pulse width modulator chip. Using common mode filter L1 to introduce AC 220 V power supply can better suppress the high-frequency interference radiated from power grid and power supply itself. The AC voltage is filtered by bridge rectifier circuit and capacitor C4 to form an unstable DC voltage of about 280v, which is used as an inverter circuit composed of oscillation chip U1, switch transistor Q1, switch transformer T1, etc. The following small class mainly shares the maintenance points of UC3842 switching power supply. In fact, a good switching power supply maintenance is very convenient, the switching power supply below is a good example.

Maintenance methods and basic knowledge of switching power supply

Maintenance points of UC3842 switching power supply.

1. Burn the fuse or blow up the pipe

It mainly checks the large filter capacitor, rectifier bridge diode, switch tube, etc. on 300V. The problems in the anti-interference circuit will also lead to the black fuse. It should be noted that the fuse burned due to the breakdown of the switch tube will generally burn the current detection resistance and the power control chip. The negative temperature coefficient thermistor, rectifier bridge and fuse were burned together.

2. No output, fuse is normal. This phenomenon indicates that the switching power supply does not work or has entered the protection state.

First of all, it is necessary to measure whether there is starting voltage on the starting pin of the power control chip. If there is no starting voltage or the starting voltage is too low, check the starting resistance and the external components of the starting pin for leakage. At this time, if the power control chip is normal, the above inspection can quickly detect the fault. If there is start-up voltage, measure whether there is high and low level jump at the output end of the control chip. If there is no jump, it means that the control chip is broken, and the peripheral oscillation circuit or protection circuit is faulty. You can replace the control chip first, and then check the peripheral components. If there is tripping, the switch tube is generally bad or damaged.

3. There is output voltage, but the output voltage is too high

This kind of fault usually comes from voltage stabilized sampling and voltage stabilized control circuit. In DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler and power control chip constitute a closed-loop control loop. Any fault will lead to the rise of output voltage.

4. The temperature of no-load power on switch increases continuously

This failure is rare. Once found, should immediately cut off the power supply, because with the rise of the switch temperature, the switch will burn out, will bring unnecessary trouble.

In this case, check whether the diode of RCD absorption circuit has soft breakdown and whether the resistance changes, mainly the diode. If the measurement is normal, it can be eliminated by substitution. Some manufacturers have made false marks for the benefit of others, but the actual withstand voltage of diodes can not meet the standard, so it is necessary to replace diodes of the same model and different manufacturers.

5. Low output voltage

In addition to the low output voltage caused by the voltage regulator control circuit, there are also some reasons for the low output voltage

B. Fault of rectifier diode and C filter capacitor. C. The performance of the switch will inevitably lead to the failure of the switch to conduct normally, thus increasing the internal resistance of the power supply and reducing the load capacity. D. The bad switch transformer not only causes the output voltage to drop, but also causes the switch tube excitation insufficiency, thus repeatedly damages the switch tube. The poor e-300v filter capacitor leads to poor load capacity of the power supply, and the output voltage drops after the load is connected.

The above is the summary of the maintenance skills of switching power supply shared in small class today. After sharing in Minrong small class, everyone should have a certain in-depth understanding.

Editor in charge ajx

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