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What are the reasons for intermittent oscillations in switch mode power supplies?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.19
Switching power supply is a type of power module. He is also a very common type of power supply, and the reason for adding the word "switch" is because its internal structure has a Schottky diode with switching characteristics. This diode has a high switching frequency and can quickly switch off thousands of times in a short period of time. And each time, it can provide power and current output, which plays an important role in general integrated circuits. However, in the use of Switching power supplies, intermittent oscillations may sometimes occur. What are the reasons for this? Next, Kinri will provide a detailed introduction to this issue for everyone.

The reasons for intermittent oscillation in switching power supplies can be divided into three types: overload type, undervoltage type, and no-load type.



1. Overload type

This type of fault is more common among the four types of faults. Usually caused by breakdown of the rectifier diode on the secondary side of the switching transformer, leakage of the filtering capacitor, or damage to the IC components and cooling fans of the load circuit. After removing the corresponding load branch or removing the faulty components of the power supply itself, the power supply will transition from intermittent oscillation to normal output state.

① If it is voltage sampling, such as faults caused by the+5V load branch or the power supply itself (such as leakage of filtering capacitors), measure the maximum output voltage overshoot of other branches, which may slightly exceed the rated voltage. For example, the output of+15V may overshoot to+17V;

② If the fault is caused by a power supply or load branch outside of voltage stabilization sampling, the maximum output voltage overshoot will generally be set at a small value. If the+15V output upstroke is 8V.

Although it belongs to intermittent oscillation, the output voltage has a certain amplitude.

2. Undervoltage type

Usually, it is caused by poor 7-pin power supply branch of 3844, or leakage components in the reverse current absorption circuit of the parallel switching tubes in the primary winding of the switching transformer.

① Due to poor 7-pin power supply branch of 3844, such as inefficient rectifier diodes (good for measuring forward and reverse resistance and conduction voltage drop), and filter capacitor failure, such as changing from 100u to 10u. Unable to provide the working energy required for the oscillation chip (excitation energy of the switching tube), the horse cannot eat enough, and therefore cannot pull the car.

If it is confirmed that the voltage stabilization process of this type of fault is still normal, after providing an additional DC18V power supply to pins 7 and 5 of 3844, the power supply will change from abnormal to normal. If there is no problem with the filter capacitor during this inspection, the rectifier diode should be replaced without hesitation!

② The reverse current absorption circuit of the switch tube in parallel with the primary winding of the switch transformer has leakage components, which greatly reduces the induced voltage of the self powered winding and is not sufficient to provide the working energy required for the oscillation chip and switch tube. After checking that there are no abnormalities in the 7-pin power supply branch of 3844, the focus of the inspection should be shifted to this circuit.

The root cause of the malfunction is the leakage of high-speed high-voltage diodes or voltage regulators, which is the most common. However, the difficulty lies in measuring the diodes or voltage regulators, which feels good. Temporary disconnection of the absorption circuit or replacement of components for testing can often quickly solve problems.
For undervoltage faults, the voltage output of the secondary winding of the switch transformer is low. If it fluctuates within a few volts to a few volts, it feels like the power supply is not producing enough power.

3. No-load type

This is a faulty type. It will only be touched when repairing the switch power supply/driver board separately.

When the load branch is unloaded, due to the inability to form the minimum stable load current, the energy supply side loses the supply target and is at a loss, either allocating too much or too little, resulting in intermittent oscillations.

This is not a fault in itself, but is caused by the suspended load branch. When directly connected to the MCU (especially when connecting to the display panel, the digital tube requires a large working current), the power supply will work normally, and in a sense, there is no need for maintenance.

When it can be repaired separately, it is necessary to "move" the circuit to ensure that the switch power supply can work normally, in order to facilitate the maintenance process.

If it is considered a fault, there is a significant characteristic: when measuring the output voltage of each circuit on the secondary side of the switch transformer, it will be found that the fluctuation range is large, and the voltage overshoot will be much higher than the rated voltage value, such as+5V becoming 8V, or+15V becoming more than 20V. And the downshift value can reach 0V.

The reasons for intermittent oscillations in Switching power supplies mentioned above are shared here. Switching power supplies are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semi conductive cooling and heating, air purifiers, electronic refrigerators, LCD displays, LED lighting fixtures, communication equipment, audio-visual products, security monitoring, LED light bags, computer cases, digital products and instruments, and other fields.
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