Introduction to troubleshooting methods for poor load capacity of inverter switching power supply
This is a single tube self-excited oscillation shunt stabilized switching power supply. The fault is that the output voltage of each circuit on the MCU motherboard is normal after unplugging it. Connect the motherboard and turn on/off the power supply, causing intermittent oscillation. The previous repairman searched all over but found no results. After taking over, he straightened the circuit and felt a bit strange: the 3rd and 4th pins of the feedback optocoupler PC1 seemed to be inverted? Upon closer inspection, it's not really collapsed.
Firstly, the cause of overload was ruled out. The remaining oscillation and voltage stabilization circuits, combined, only consist of ten or eight components, so it's not difficult to check them all. I have checked everything and there are no issues.
The problem lies in the connection method of PC1, which is just like this when it leaves the factory. It shouldn't be a problem to try it upside down, but the hiccups are even more severe. Obviously not the problem.
Compared to similar circuits, the R2 value is too high. Replace it with a 100 ohm resistor to restore normal operation.
Still frustrated with the connection of PC1, I couldn't help but laugh after thinking for a long time: the output side of PC1 is a photosensitive transistor, and the transistor - anyone who has measured its amplification factor has this experience: the transistor can also work in reverse, but not as well as in forward applications. The use of reverse current to control T2 and achieve shunt voltage regulation control is also valid.
The conclusion drawn is that the poor load-bearing capacity is essentially due to the R2 value being too large, resulting in a lower T1 excitation ability. Therefore, attempting to "correct" the 3/4 pin of PC1 is like eating salt when thirsty. This "rectifying" approach exacerbates the weak T1 excitation ability, and thus the fault performance is exacerbated (the doctor prescribed the wrong prescription and medication is the same phenomenon).
But the reverse use of PC1 may just be in line with the saying "get it right". Anyway, the designer has also passed the test, and there is really no one who knows how to use optocouplers in this way.,
