Troubleshooting Method for Poor Load Carrying Capacity of Frequency Converter Switching Power Supply
This is a single transistor self-excited oscillation shunt regulated switching power supply. The fault is that the output voltage of each circuit is normal when the MCU motherboard is unplugged. Connect the motherboard and switch the power supply to hiccup (intermittent oscillation occurs). The previous repairman checked repeatedly but found no results. After taking over, he followed the circuit and felt a bit strange: it seems that the 3/4 pins of the feedback optocoupler PC1 have fallen? Take a closer look, it's not really upside down.
Firstly, the cause of overload was ruled out. The remaining oscillation and voltage stabilization circuits, combined with only ten or eight components, can be easily checked. I checked everything and found no problem.
The problem lies in the connection of PC1, which is how it works 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 relatively high, and it can be replaced with a 100 ohm resistor to restore normal operation.
Still frustrated with the connection of PC1, I couldn't help but laugh blankly after thinking for a long time: the output side of PC1 is a photosensitive transistor, and the transistor - anyone who has measured its amplification has this experience: the transistor can also pass in the reverse direction, but it is not as good as the forward application. It is also possible to use reverse current to control T2 and achieve shunt voltage regulation control.
Thus, it can be concluded that the poor carrying capacity is essentially due to the excessive value of R2, which leads to a smaller excitation capacity of T1. Therefore, the attempt to "straighten" the 3/4 pin of PC1 is like eating salt when a person is thirsty. This "rectification" approach exacerbates the weak excitation ability of T1, and thus exacerbates the fault performance (the doctor prescribed the wrong prescription and prescribed the wrong medicine, which is a phenomenon).
But the reverse use of PC1 may just be in line with the common saying "hitting it straight". Anyway, the designer also happened to pass the level, and there is really no one who knows about this use of optocouplers.
