Due to the short circuit of the load, the failure of the self-oscillation circuit and the loss of control of the protection circuit, the switching power supply will cause the base of the switch tube to be unable to get the switching pulse and intermittent operation, which makes it difficult to locate the shortcomings of SMPS. The difficulty of the repair operation is improper, and the improper method will expand the defects and constitute the phenomenon of continuous burning of the switch pipe, so the author thinks that the inspection of SMPS should isolate the circuit and check the intermittent defect location step by step.
Independent power supply, remove protection, check the shortcomings caused by the loadSMPS and linear regulated power supply are different, no-load situation is easy to break down the switch, therefore, SMPS damage intermittent independent repair # disconnect the load and protection circuit together # must be on the main load power supply group power supply with a false load. The selection of false load according to the power supply power and test conditions, can be used incandescent lamp or electric soldering core, some people have written that the incandescent lamp as a fake load due to the cold resistance is very small, the switch tube starting current is large, it is easy to constitute the switch tube damage A short test and theory support, the test and theory prove that the SMPS voltage regulation scale is wide, there will be no switch tube damage, and the light and dark of the incandescent lamp has the function of an indicator, which is worth promoting. If the power supply can operate normally after the virtual load is connected, the load size or input voltage can be appropriately changed, and the power supply can have a stable output, it can be judged that the power supply is defective, and the disadvantage lies in the load; On the contrary, the shortcomings are investigated in the second step of the power supply.
Independent oscillation circuit, check the shortcomings caused by the voltage regulator circuit module At present, SMPS does not deal with the "hot bottom" problem, many use parallel optocouplers to control the voltage regulator SMPS, when there is a problem with the voltage regulator circuit and SMPS is interrupted, the two control pins of the hot ground end of the optocoupler can be disconnected # and a 10k variable resistor is connected at the original pin, so that the oscillation circuit is independent, and the influence of the voltage regulator circuit on the switching circuit is cleaned, but the safety method of low-voltage power supply should be used, When the SMPS can start normally and there is a voltage output, it can be cleaned The shortcomings of the oscillation circuit may be, the shortcomings are in the regulated circuit, most of the problems are compared with the damage caused by the damage of the optocoupler, and the damage caused by the optocoupler will cause damage together, assuming that there is no voltage output, it indicates that the oscillation circuit part has a problem.
Gradually restore the protection circuit, check the shortcomings caused by the protection circuit module, and introduce over-current protection, over-voltage protection, under-voltage protection, and over-temperature protection for SMPS
In addition to protecting the SMPS itself safely, it also prevents damage to other operating circuits caused by the shortcomings of the SMPS. Most of the overcurrent sampling points of the overcurrent protection circuit are at the emitter potential of the main oscillating power tube. According to the voltage after communication rectification filtering or the power supply voltage of the main load, the sampling point of the overvoltage protection circuit is intermittently sampled and judged by the Zener diode & stable voltage tube.
The sampling point of the short-circuit protection circuit is generally intermittently discriminated sampling of the low-voltage group power supply output by the diode, and a "latching circuit" is arranged inside the power supply adopted by some equipment, and the "latching circuit" is in practice a protection execution circuit, and the intermittent control of the signal intermittent circuit transmitted from each sampling point is based on the signal transmitted from each sampling point. After cleaning and normal operation of the defects of the oscillation circuit and the stable circuit, one protection is added before the protection starts, and the defects of the power supply can be determined by what kind of protection circuit is introduced.
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