Switching power supply output voltage low cause and check method
1,The reason for low output voltage of switching power supply
(1) The 220V AC voltage input circuit and rectifier filter circuit do not provide enough working voltage to the switching tube, which is beyond the control range of the pulse width modulation circuit.
(2) Load circuit overcurrent caused by the switching power supply load aggravation and lead to a drop in output voltage.
( (3) The on/off interface circuit is in standby mode, causing the switching power supply to operate in a low-frequency oscillation state whose output voltage is the degree of the standby state. This type of fault only should only be used in models without a standby power supply, where the operating voltage in the CPU standby state is provided by the switching power supply.
(4) The end of the switch-on/switch-off interface circuit works in the state between switch-on or standby for some reasons, which causes the switching power supply to work in the operating frequency between standby and switch-on states, causing the output voltage of the switching power supply to be in the degree of the standby state. frequency, causing the switching power supply output voltage to be higher than the standby value and lower than the power-on value.
(5) The protection circuit terminal works in the conduction state due to a fault, which causes the power supply to enter the weak vibration narrow pulse power supply, causing the output voltage of the switching power supply to drop.
(6) The diode and filter capacitor in the rectifier output circuit and the current limiting resistor are damaged, causing the output voltage to become low.
(7) Pulse width modulation circuit has a problem, can not make a tangent response to changes in the output voltage of the switching power supply, the power supply switching tube base voltage adjustment direction is not the right size. Incorrect, resulting in low output voltage of the switching power supply.
(8) The positive feedback resistance in the positive feedback circuit becomes large, the performance of the discharge diode deteriorates, and the amount of positive feedback is insufficient, resulting in a longer oscillation period. The oscillation frequency decreases. This causes low output voltage of the switching power supply.
(9) It excitation type switching power supply because not get line inversion and work below the low frequency state, resulting in low output voltage.
2, judge the fault methods and steps
(1) Measure the collector voltage of the line output tube to determine the fault.
(2) Measure the voltage of each output terminal of the switching power supply to determine the fault.
(3) Output voltage drop ratio column is large, some output voltage drop ratio column is small.
