What are the reasons for intermittent oscillation of switching power supply
1. Overload type
This type of fault is more common among the four faults, and the difficulty of maintenance is two stars. It is usually caused by the breakdown of the rectifier diode on the secondary side of the switching transformer, the leakage of the filter capacitor, or the damage of the IC device of the load circuit and the cooling fan. When the corresponding load branch is removed, or the faulty components of the power supply itself are removed, the power supply will turn from intermittent oscillation to normal output state.
① If it is a voltage sampling, such as a +5V load branch or a fault caused by the power supply itself (such as leakage of a filter capacitor), measure the maximum value of the output voltage overshoot of other branches, which may slightly exceed the rated voltage. For example, the +15V output may be Overshoot to +17V;
②If it is a fault caused by a power supply or load branch other than voltage stabilization sampling, the maximum value of the output voltage overshoot will generally be smaller than the rated value. For example, the overshoot of +15V output is 8V.
Although it is an intermittent oscillation, the output voltage has a certain amplitude.
2. Undervoltage type
Usually due to poor power supply branch circuit of 3844 pin 7, or leakage components in the reverse current absorption circuit of the switch tube connected in parallel with the primary winding of the switching transformer, the difficulty of maintenance is three or four stars.
①Because the 7-pin power supply branch of the 3844 is bad, such as the low efficiency of the rectifier diode (measurement of forward and reverse resistance and conduction voltage drop, both are good), and the filter capacitor fails, such as changing from 100u to 10u. If the working energy required by the oscillating chip (the excitation energy of the switch tube) cannot be provided normally, the horse will not be able to pull the car if it cannot eat enough.
For this kind of failure, if it is confirmed that the voltage stabilization link is still normal, after providing an additional DC18V power supply to pins 7 and 5 of the 3844, the power supply will change from abnormal to normal. If there is no problem with the filter capacitor at this time, do not hesitate to Replace the rectifier diode!
②The reverse current absorbing circuit of the switching tube connected in parallel with the primary winding of the switching transformer has a leakage element, which greatly reduces the induced voltage of the self-powered winding, and is not enough to provide the working energy required by the oscillating chip and the switching tube. After checking that there is no abnormality in the 7-pin power supply branch of 3844, the focus of the inspection should be transferred to this circuit. The most common source of failure is the leakage of high-speed high-reverse voltage diodes or voltage regulators, but the difficulty is that it feels good to measure diodes or voltage regulators. The absorption circuit can be temporarily disconnected, or the component can be replaced for testing, and the problem can often be solved quickly.
For undervoltage faults, the voltages output by the secondary winding of the switching transformer are all low. If it fluctuates within a few tenths of a volt to a few volts, it feels that the power supply has no power to output. Ha ha.
3. No-load type
This is a failure profile. Only encountered when overhauling switching power supply/driver boards individually. The difficulty of maintenance is a few stars, so I won't say it. As shown in the power supply in the figure below, ① the output side of the feedback optocoupler is directly connected to the 12 pin of 3844; ② the secondary winding of the switching transformer, especially the filter capacitor of the sampling voltage winding, has a relatively large capacity of 1,000 microamps. When the load branch is unloaded, because the minimum stable load current cannot be formed, the energy supply party loses the supply target and is at a loss. Either the allocation is too much or the allocation is less, and intermittent oscillation is formed.
