Switch withstand voltage and leakage requirements
When the input and output voltage of switching power supply exceeds 36V AC and 42V DC, the problem of electric shock needs to be considered. Safety regulations: the leakage between any two accessible parts or any one accessible part and one pole of the power supply shall not exceed 0.7map or DC 2mA.
When the input voltage is 220V of switching power supply, the creepage distance between cold and hot ground shall not be less than 6mm, and the spacing between port lines at both ends must be greater than 3mm.
The withstand voltage between the primary stages of the switching transformer shall be 3000V AC, and the leakage current shall be 10mA. The leakage current must be less than 10mA after one minute test
The input end of switching power supply shall withstand voltage to the ground (shell) with AC 1500V, set the leakage current as 10mA, and conduct withstand voltage test for 1 minute, and the leakage current must be less than 10mA.
DC 500V is used for the withstand voltage of the output end of the switching power supply to the ground (shell), and the leakage current is set as 10mA. Conduct the withstand voltage test for 1 minute, and the leakage current must be less than 10mA.
Requirements for safe creepage distance of switch
The safety distance between the side and the secondary side of the two lines: 6mm, plus 1mm, the slotting should also be 4.5mm.
The safety distance between the side and the secondary side in the third line: 6mm, plus 1mm, the slotting should also be 4.5mm.
Safety distance between two copper foils of fuse > 2.5mm. Add 1mm, and the slotting shall also be 1.5mm.
The distance between L-N, l-gnd and n-gnd is greater than 3.5mm.
Primary filter capacitor pin spacing > 4mm.
Safety distance between primary stages > 6mm.
Switching power supply PCB wiring requirements
Between copper foil and copper foil: 0.5mm
Between copper foil and solder joint: 0.75mm
Between solder joints: 1.0mm
Between copper foil and plate edge: 0.25mm
Between hole edge and hole edge: 1.0mm
Between hole edge and plate edge: 1.0mm
Copper foil line width > 0.3mm.
Turning angle 45 °
Equal spacing is required for wiring between parallel lines.
Safety requirements for switching power supply
Find out the fuse required by the safety regulations from the components of the safety regulations, and the creepage distance between the two pads is > 3.0mm (min). In case of post stage short circuit, the capacitors X and Y shall be in the safety regulation. It considers withstand voltage and allowable leakage current. In subtropical environment, the leakage current of equipment shall be less than 0.7ma, that of equipment working in temperate environment shall be less than 0.35ma, and the general y capacitance shall not be greater than 4700pf. Discharge resistance shall be added to x capacitor with capacity > 0.1uF. After the normal working equipment is powered off, the voltage between the plugs shall not be greater than 42V within 1s.
Switching power supply protection requirements
When the total output power of switching power supply is greater than 15W, short circuit test shall be carried out.
When the output terminal is short circuited, there shall be no overheating or fire in the circuit, or the combustion time shall be within 3.
When the distance between adjacent lines is less than 0.2mm, it can be considered as short circuit.
Short circuit test shall be conducted for electrolytic capacitor. At this time, because electrolytic capacitor is easy to fail, attention shall be paid to devices during short circuit test to prevent fire.
Two metals with different properties cannot be used as connectors because they will produce electrical corrosion.
The contact area between the solder joint and the component pin shall be greater than the cross-sectional area of the component pin. Otherwise, it is considered as faulty welding.
Device affecting switching power supply – electrolytic capacitor
Electrolytic capacitor is an unsafe device in switching power supply and has an impact on the mean time between failures (MBTF) of switching power supply.
After the electrolytic capacitor is used for a period of time, the capacitance will decrease and the ripple voltage will increase, so it is easy to heat and fail.
When the high-power electrolytic capacitor fails to generate heat, it will often cause explosion. Therefore, the electrolytic capacitor with diameter greater than 10mm shall have explosion-proof function. For the electrolytic capacitor with explosion-proof function, a cross groove is opened at the top of the capacitor shell, and an exhaust hole is left at the bottom of the pin.
The service life of the capacitor is mainly determined by the internal temperature of the capacitor, and the temperature rise of the capacitor is mainly related to the ripple current and ripple voltage. Therefore, the ripple current and ripple voltage parameters given by general electrolytic capacitors are the ripple current values under the conditions of specific working temperature (85 ℃ or 105 ℃) and specific service life (2000 hours), That is, under the condition of ripple current and ripple voltage, the service life of electrolytic capacitor is only 2000 hours. When the service life of the capacitor is required to be more than 2000 hours, the service life of the capacitor shall be designed according to the following formula.