The switching power supply controls the time ratio of the switching tube on and off in the circuit, and maintains a stable circuit voltage output. It is a very common power supply design. But those who have been engaged in the design of switching power supply know that in the process of testing the switching power supply, some whistling sounds are often heard, similar to the leakage sound when the high voltage is bad, or the sound like the high voltage arcing. So when these phenomena occur, how should they be addressed?

Generally speaking, the reasons for the whistling of switching power supply generally have the following incentives.

Poor dipping of transformer

Includes unimpregnated varnish water. The whistling causes the waveform to have spikes, but the load capacity is generally normal. Special instructions: the higher the output power, the stronger the whistling, and the less powerful the performance is not necessarily obvious. A 72W charger product has had bad loading experience, and found in this product that there are strict requirements on the material of the magnetic core. In addition, when the design of the transformer is not good, it is also possible that the vibration will produce abnormal noise during operation.

PWM IC ground trace error

Usually, some products can work normally, but some products cannot be loaded and may not be able to vibrate. Especially when some low-power ICs are used, they are more likely to fail to work normally. For example, the SG6848 test board, because the performance of the IC was not thoroughly understood at the beginning, the layout was hastily based on experience, and the result was that the wide voltage test could not be performed during the test.

Optocoupler working current point wiring error

When the position of the working current resistor of the optocoupler is connected before the secondary filter capacitor, there is also the possibility of howling, especially when the load is more.

Ground Wire Error of Reference Voltage Regulator IC TL431

The grounding of the same secondary reference voltage regulator IC has the same requirements as the grounding of the primary IC, that is, they cannot be directly connected to the cold ground and hot ground of the transformer. If they are connected together, the result is that the load capacity is reduced and the howling sound is proportional to the output power.

When the output load is large, close to the power limit of the power supply, the switching transformer may enter an unstable state. The duty cycle of the switch tube in the previous cycle was too large, the conduction time was too long, and too much energy was transmitted through the high-frequency transformer; the energy of the energy storage inductor of DC rectification was not fully released in this cycle, and it was judged by PWM that in the next cycle There is no drive signal to turn on the switch, or the duty cycle is too small. The switch tube is in the off state in the entire cycle thereafter, or the on time is too short. After the energy release of the energy storage inductor for more than one whole cycle, the output voltage drops, and the duty cycle of the switch tube in the next cycle will be larger... This cycle will cause the transformer to generate a lower frequency (regular intermittent full cut-off). period, or the frequency with which the duty cycle changes drastically), producing lower-frequency sounds that can be heard by the human ear.

At the same time, the output voltage fluctuation will be larger than normal operation. When the number of intermittent full cut-off cycles per unit time reaches a considerable proportion of the total number of cycles, it will even reduce the vibration frequency of the transformer originally working in the ultrasonic frequency band, enter the audible frequency range, and emit sharp high frequencies. whistle". At this time, the switching transformer is working in a serious overload state, and there is a possibility of burning it at all times - this is the origin of the "screaming" before many power supplies burn out. I believe that some users have had similar experiences.