Switching power supply uses electronic switching devices to make the electronic switching devices "on" and "off" continuously through the control circuit, so that the electronic switching devices pulse modulate the input voltage, so as to realize voltage conversion and voltage stabilization.
If the output voltage of switching power supply is lower than the nominal voltage in actual work, there are the following possibilities:
(1) The newly installed switching power supply may have insufficient output power. Sufficient margin shall be reserved for the power of the selected power supply to avoid "small horse pulling cart"; It is also possible that the output of the power supply does not meet the design index, which is the quality problem of the product itself. In addition, at high temperature, the output power will be reduced. To reduce the rating, the manufacturer will usually provide output characteristic curves at different temperatures.
(2) The working voltage provided by 220V AC voltage input and rectifier filter circuit to the switch tube is not enough, which is beyond the control range of pulse width adjustment circuit.
(3) There is overcurrent in the load circuit, which causes the load of switching power supply to increase and the output voltage to drop.
(4) The output of the original power supply suddenly decreases. First, check whether the load changes. If the load is normal, it is very likely that the internal components are damaged. If possible, you can check whether the switching device of the product is damaged, otherwise you can only send it for repair or replace it with a new one.
(5) The on / off switching is wrong. At the moment when the line scanning circuit starts working, the switching power supply is in standby state. This kind of fault is applicable to the machine without standby power supply. CPU power supply is from the same power supply and not provided by auxiliary power supply.
(6) The terminal of the on / off interface circuit is in the state between startup and standby due to fault, resulting in the output voltage of the switching power supply being lower than the normal value and higher than the standby value.
1. Power load short circuit fault
When we suspect that the load is short circuited, we should disconnect the power load to check whether the power circuit or the load circuit is faulty. After disconnection, the voltage output is normal, which indicates that it is due to the load. If it is not normal, it must be the fault of the switching power supply circuit, which needs further inspection and repair.
(7) The end of the protection circuit enters the on state due to fault, which makes the power supply enter the weak vibration state, resulting in the decrease of the output voltage of the switching power supply.
(8) The diode, filter capacitor and current limiting resistor in the rectifier output circuit are damaged, resulting in low output voltage.
(9) Pulse width modulation circuit failure can not make correct response to the change of switching power supply output voltage, and the adjustment direction of switching tube base voltage is wrong, resulting in low switching power supply output voltage.
(10) The positive feedback resistance in the positive feedback circuit changes, the performance of the freewheeling diode deteriorates or the constant current source fails, resulting in insufficient positive feedback, resulting in longer oscillation period and lower oscillation frequency, resulting in low output voltage of the switching power supply.
(11) The non excitation switching power supply works at low frequency without reverse pulse, resulting in low output voltage.
(12) The filter capacitor or rectifier diode at the output end is broken (the filter capacitor is bulging, or leakage, etc. the bulging can be seen directly. If the leakage or diode is broken, use a multimeter to detect it, or use the replacement method)
(13) Leakage or performance degradation of 300V filter capacitor (the detection method is the same as above)
(14) Poor performance of switch tube (very common, teachers should see more)
Common faults and troubleshooting of unstable output voltage of switching power supply.
1. Switching power supply load short circuit fault (especially DC / DC converter short circuit or poor performance)
2. The failure of the filter capacitor or rectifier diode at the output voltage end can be judged by the replacement method
3. The performance of the switch tube is degraded, resulting in the switch tube unable to conduct normally, increasing the internal resistance of the power supply and reducing the load capacity
4. Poor switching transformer not only causes the output voltage to drop, but also causes insufficient excitation of the switch tube, thus damaging the switch tube
5. The 300V filter capacitor is poor, resulting in poor load capacity of the power supply, and the output voltage will drop as soon as it is connected to the load
6. High output voltage generally comes from regulated sampling and regulated control circuit.
