1. The switch tube is damaged by overvoltage
①The mains voltage is too high, the drain voltage provided to the switch tube is high, and the switching pulse amplitude generated by the drain of the switch tube naturally increases a lot. Breaking through the withstand voltage value of the switch tube D-S causes the switch tube to break down.
② There is a problem with the voltage stabilizing circuit. When the output voltage of the switching power supply increases, the induced voltage generated by each winding of the switching transformer is large, and the induced voltage generated by the primary winding is superimposed with the DC working voltage obtained by the drain D of the switching tube. If This superposition value exceeds the withstand voltage value of the switch tube D-S, which will damage the switch tube.
3. There is a problem with the D-protection circuit (spike pulse absorption circuit) of the drain of the switch tube, which cannot absorb the high-amplitude peak pulse of the drain D of the switch tube, resulting in the high voltage breakdown of the switch tube's drain.
④The failure of the large filter capacitor (300V filter capacitor) causes a large number of high-frequency pulses at both ends, which will cause overvoltage damage to the switch after superimposing with the inverse peak voltage during the cut-off period of the switch.
2. Switch tube overcurrent damage
①The heat sink of the switch tube is too small or not fixed firmly.
②The load of the switching power supply is too heavy, which will prolong the conduction time of the switch tube and damage the switch tube. Common reasons are output voltage rectification, poor filter circuit or load circuit short circuit, leakage and other faults.
③The inter-turn short circuit of the switching transformer.
3. The power consumption of the switch tube is large and damaged
There are two common types of turn-on losses and turn-off losses. The large turn-on loss is mainly due to the fact that the switch tube cannot enter the saturation state from the amplifying state within the specified time. The reason for the large turn-on loss of the switch tube is mainly due to insufficient excitation of the switch tube. The large turn-off loss is mainly due to the fact that the switch tube cannot enter the cut-off state from the amplifying state within the specified time. The reason for the large turn-off loss of the switch tube is mainly due to the distortion of the waveform of the gate (base) electrode of the switch tube.
4. The switch tube itself has quality problems
The quality of commercially available power switch tubes varies. If the switch tube has quality problems, repeated damage to the switch tube is inevitable.
5. Improper replacement of switch tube
The power of the field effect switch of the switching power supply is generally large, and it cannot be replaced with a field effect tube with low power and low withstand voltage, otherwise it will be easily damaged. It is also not possible to use BU508A, 2SD1403 and other triodes for replacement. The experiment found that although the power supply can work after the replacement, the triode will overheat after being powered on for a few minutes, which will cause repeated damage to the switching tube.
