Repair of Switching Transistor Faults in Switching Power Supplies
Repeated damage to power switch tubes (or thick film circuits containing switch tubes) is a key and difficult point in the maintenance of switch power circuits. Below is a systematic analysis.
The switch tube is the core component of the switching power supply, which works in high current and high voltage environments. Its damage rate is relatively high. Once it is damaged, it is often not easy to eliminate the fault by replacing it with a new tube, and it may even damage the new tube. It is difficult to troubleshoot this type of repeatedly damaged switch tube, which often leaves beginners at a loss. Below is a brief analysis of the common causes of repeated damage to switch tubes.
1. Overvoltage damage to the switch tube
① The mains voltage is too high, which results in a high working voltage for the drain of the switch tube. As a result, the amplitude of the switch pulse generated by the drain of the switch tube naturally increases, breaking through the withstand voltage value of the switch tube D-S and causing the switch tube to break down.
② There is a problem with the voltage regulator circuit, which causes the output voltage of the switching power supply to rise. At the same time, the amplitude of the induced voltage generated by each winding of the switching transformer is large. The induced voltage generated by its primary winding is superimposed with the DC working voltage obtained by the drain D of the switching transistor. If this superimposed value exceeds the withstand voltage value of the switching transistor D-S, it will damage the switching transistor.
③ There is a problem with the switch tube drain D protection circuit (peak pulse absorption circuit), which cannot absorb the high amplitude peak pulses of the switch tube drain D, resulting in high drain voltage breakdown of the switch tube.
④ The failure of the large filtering capacitor (300V filtering capacitor) resulted in a large number of high-frequency pulses at both ends, which overlapped with the anti peak voltage during the cutoff period of the switching tube, causing overvoltage damage to the switching tube.
2. Overcurrent damage to the switch tube
① The heat sink of the switch tube is too small or not firmly fixed.
② The switch power supply is overloaded, causing the conduction time of the switch tube to be prolonged and damaging the switch tube. The common reasons are poor rectification and filtering circuits of the output voltage, or faults such as short circuits and leakage in the load circuit.
③ Short circuit between turns of switching transformer.
3. High power consumption and damage to the switch tube
There are two common types: high opening loss and high closing loss. The large opening loss is mainly due to the fact that the switching transistor cannot enter saturation state from the amplified state within the specified time. The main reason for the high opening loss of the switch tube is due to insufficient excitation of the switch tube. The large turn off loss is mainly due to the fact that the switching transistor cannot enter the cutoff state from the amplified state within the specified time. The main reason for the high turn off loss of the switching transistor is due to the distortion of the waveform of the gate (base) pole of the switching transistor.
4. The switch tube itself has quality problems
The quality of commercially available power switch tubes varies greatly. If there are quality problems with the switch tubes, it is inevitable that they will be repeatedly damaged.
5. Improper replacement of switch tubes
The power of the field-effect switching transistor in a switching power supply is generally high, and it cannot be replaced with a low-power, low-voltage field-effect transistor, otherwise it is highly susceptible to damage. It is also not possible to use transistors such as BU508A and 2SD1403 for substitution. Experiments have found that although the power supply can work after substitution, the transistor overheats after a few minutes of power on, which can cause repeated damage to the switching transistor.
