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　　Inventory of common noise sources and feasible solutions in flyback power supplies

　　The flyback converter is a commonly used circuit topology for auxiliary power supplies. Its advantage is that it can work in a very wide input voltage range, has a simple circuit and few components, but its efficiency is generally around 75%. Although switching power supplies operate at frequencies well beyond the human hearing range, they can still produce audio noise under certain load conditions. There are many possible sources of audio noise. Noise can be caused by design flaws, such as an oscillating output voltage, or by noisy components such as capacitors or transformers. In some cases, the high-pitched noise or hiss you hear may be caused by a fan oscillating at an unusual frequency, or by the power supply being close to an external EMI source (fluorescent lights or power strips).

　　This article will examine the most common sources of noise in flyback power supplies and describe possible solutions. All 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 may be so low that if the power supply will be used in a sealed enclosure then audio noise will not be an issue.

　　Possible sources of noise

　　The most common source of noise in flyback power supplies are noisy components. This noise is typically produced by ceramic capacitors or ferrite transformer cores. Noise in ceramic capacitors is often caused by the inverse piezoelectric effect. Applying a voltage to a dielectric structure induces mechanical stress or strain, causing the material to deform. When this material deforms, it expels surrounding air, creating noise.

　　Because the reverse piezoelectric effect occurs when large voltage swings occur, designers can focus on looking for ceramic capacitors that exhibit 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 causing noise, replace it with a metal film capacitor of the same capacitance value and the appropriate voltage rating. . If the noise level decreases, you have found the source of the noise in the circuit.

　　If the noise source is a clamping capacitor, you can completely replace it with a metal film capacitor, or try using a ceramic capacitor with a different dielectric material. Another method is to replace the clamping capacitor in use, for example, by replacing it with a zener tube clamp circuit. If the noise problem is due to the snubber capacitor, it can be replaced with a metal film capacitor or the series resistor value can be increased to reduce the dV/dt noise on the capacitor. You can also use ceramic capacitors of other media to see if the noise can be reduced.

　　Managing Transformer Core Noise

　　On the other hand, noise generated by transformer cores is often caused by magnetostriction, which is similar to the inverse piezoelectric effect. Many ferromagnetic materials change shape when affected by a magnetic field. Such materials cause the core to physically vibrate as the magnetic field in the transformer core changes. When the vibration frequency reaches the mechanical resonance frequency of the transformer, the vibration will be amplified and cause greater audio noise. In AC electrical equipment (such as transformers using a 60Hz applied magnetic field), the maximum length changes occur twice per cycle, resulting in the familiar 120Hz noise.

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