Burn insurance or frying tube

Mainly check the large filter capacitor on 300V, the diodes of the rectifier bridge and the switch tube. The problem of anti-interference circuit will also lead to burn-in and blackout. It should be noted that the burnout will generally burn the current sense resistor and the power control chip due to the breakdown of the switch tube. The negative temperature coefficient thermistor and rectifier bridge are also burned out together with the fuse.

2. No output, normal fuse

This phenomenon indicates that the switching power supply is not working or has entered the protection state. Firstly, it is necessary to measure whether the starting pin of the power control chip has a starting voltage. If there is no starting voltage or the starting voltage is too low, it is necessary to check whether the starting resistor and the external component of the starting pin are leaking. At this time, if the power control chip is normal, the above check is performed. You can quickly find the fault. If there is a starting voltage, then whether the output of the control chip has a high or low transition at the instant of power-on, if there is no jump, it means that the control chip is bad, the peripheral oscillator circuit component or the protection circuit has a problem, and the control can be replaced first. Chip, then check the external components; if there is a jump, the switch is generally bad or damaged.

3. There is output voltage, but the output voltage is too high. This fault is usually from the voltage regulation sampling and voltage regulation control circuit. In the DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits together form a closed control loop, any problem will lead to an increase in output voltage.

4. The output voltage is too low, except that the voltage regulator control circuit will cause the output voltage to be low. There are also some reasons below that cause the output voltage to be low:

a Switching power supply load has a short circuit fault (especially DC/DC converter short circuit or poor performance, etc.). At this time, all the loads of the switching power supply circuit should be disconnected to distinguish whether the switching power supply circuit or the load circuit is faulty. If the voltage output of the load circuit is disconnected, it indicates that the load is too heavy or not normal, indicating that the switching power supply circuit is faulty.

b Output voltage terminal rectifier diode, c filter capacitor failure, etc., can be judged by the substitution method.

The performance of the c-switch is degraded, which inevitably causes the switch tube to be unable to conduct normally, which increases the internal resistance of the power supply and reduces the load capacity.

D switch transformer is not good, not only causes the output voltage to drop, but also causes the switch tube to be insufficiently excited to damage the switch tube.

The e300V filter capacitor is poor, resulting in poor power supply load capacity, and the load output voltage will drop when connected.

No-load power switch tube temperature rises continuously

This type of fault is rare. Once it is found, it is necessary to immediately cut off the power, because as the temperature of the switch tube rises, the switch tube will burn out, causing unnecessary trouble. In this case, check whether the diode of the RCD absorbing circuit is soft-punched, and whether the resistance is damped, mainly the diode. If the measurement is normal, it is excluded by the substitution method. Some JS are falsely identified for the benefit, and the actual withstand voltage of the diode. It does not meet the standard, so it must be replaced by a diode of the same model and different manufacturers.

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