Flyback power supply noise source and repair

MOS power IC full range

Passive 3225G 8MHZ 10MHZ

Overview:
Although switching power supplies operate at frequencies far beyond the human hearing range, they still produce audible noise under certain load conditions. The possible sources of audible noise are diverse. Noise can be caused by design defects (such as oscillating output voltage) or by noise components such as capacitors or transformers. In some cases, the sharp, harsh noise or hum that you hear may be like a fan swaying at an abnormal frequency, or because the power supply is close to an external EMI source (fluorescent or power strip).
This article explores the most common sources of noise in flyback power supplies and presents possible solutions. All of the operating procedures described below can be accomplished using a programmable AC power supply or autotransformer and an electronic load. Keep in mind that in some cases, the noise level produced by your power supply can be very low, and if the power supply is to be used in a sealed enclosure, audible noise will not pose a problem.

Possible Noise Sources The most common source of noise in flyback power supplies is the noise component. This noise is usually generated by a ceramic capacitor or a ferrite transformer core. The noise in ceramic capacitors is usually caused by the reverse piezoelectric effect. When a voltage is applied to the dielectric structure, mechanical stress or strain is induced, causing deformation of the material. When this material is deformed, the surrounding air is discharged, causing noise.
Since reverse piezoelectric effects occur when large voltage swings occur, designers can focus on finding ceramic capacitors with higher dV/dt swings. In a typical power supply, these capacitors include snubber capacitors, clamp capacitors, and ceramic output capacitors. To quickly determine if a ceramic capacitor is producing noise, replace it with a metal film capacitor with the same capacitor value and appropriate voltage rating. . If the noise level drops, you have found a source of noise in the circuit.
If the noise source is a clamp capacitor, you can replace it completely with a metal film capacitor, or try a ceramic capacitor with a different dielectric material. Another method is to replace the clamp capacitor being used, for example, by replacing it with a Zener clamp circuit. If the noise problem is due to a snubber capacitor, it can be replaced with a metal film capacitor, or the value of the series resistor can be increased to reduce the dV/dt noise on the capacitor. You can also use ceramic capacitors from other media to see if the noise is reduced.

Figure 1: Method for Repairing High Noise Clamp Capacitors If noise problems exist in ceramic output capacitors, many different strategies can be tried to solve them. One way is to try to switch to an electrolytic capacitor or switch to a capacitor of other dielectric materials. Alternatively, multiple problematic capacitors can be replaced with multiple parallel ceramic capacitors. A reduction in the size of each capacitor will result in a corresponding reduction in its surface area, thereby changing the mechanical resonance of the capacitor.
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