Notes on the design of self-excited switching power supply
1. Insulation of primary side and secondary side
Attention must be paid to the insulation of the AC side and the secondary side. Each country has its own regulations on this issue. For example, the spacing (planar distance) between adjacent printed circuits on the primary side and secondary side is 3mm, and the space distance between adjacent components on the primary side and secondary side is 5mm, etc., and the transformer is used for electrical insulation.
There are three components connecting the primary side and the secondary side, namely the transformer T1, capacitor C12, and optocoupler IC2 in the figure, and they must meet their respective safety specifications. For transformer T1, the main concern is the insulation layer between the primary and secondary stages. C12 is used to remove the noise from the power line, need to choose AC capacitor. And there is enough withstand voltage, and its capacity cannot be too large. Otherwise, the leakage current will increase.
2. Primary side components must not have short circuit hazards
The surge voltage absorbers SAl, Rl, Cl, C2 and the rectifier bridge Dl on the primary side directly bear the AC mains power, if a short circuit occurs, it may cause a fire. C1 and C2 are used to reduce common mode noise, and AC capacitors should be used. And ensure that its withstand voltage is greater than twice the AC mains voltage, such as 400V AC withstand voltage for 220V voltage.
3. The protective measures in the circuit cannot be omitted
Fuse F1, when the circuit is short-circuited, it will be blown in time to cut off the AC source to avoid fire, and the fast-fusing type should be selected; while F2 is a temperature fuse to protect the switch tube Trl. Should be installed next to Trl.
also. ICl has a built-in over-current limiting circuit, which detects over-current through the voltage drop on R12; SA1 can absorb external over-voltage.
4. Pay attention to the safety regulations of export products
Switching power supplies for export should be designed and produced according to the safety standards of the importing country. In addition to the detection of insulation and leakage. It is necessary to ensure that other parts will not be damaged when the capacitor is short-circuited, and that a metal rod inserted into the case will not cause electric shock and short-circuit.
