Fairchild Audible Noise Reduction Techniques for FPS Applications - Data Handbook

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Application Note AN4148 Audible Noise Reduction Techniques for FPS Applications

1. Introduction - Capacitor Audible Noise Ceramic capacitors can also produce audible noise, since

In general, switched mode power supplies do not generate

they have piezoelectric characteristics. If there is still too

audible noise when they operate at constant ultrasonic

much audible noise in burst operation after gluing or

frequencies (>20k Hz). However, some switched mode

varnishing the transformer properly, the capacitor in the

power supplies can produce audible noise at certain load

snubber network should be examined.

conditions. Most Fairchild Power Switches are designed to enter into burst switching operation at light load conditions to reduce standby consumption, which can cause audible noise when the fundamental frequency of the burst switching

3. Techniques to Reduce the Audible

bundles is in the range of human hearing as shown in Figure

Noise

1. This application note explains the major sources of audible

noise and offers useful tips to engineers to solve the audible - Varnish the Transformer

noise problem in their Fairchild Power Switch (FPS) The most effective way to reduce the audible noise in the applications. transformer is to remove the possibility of physical

movement of the transformer elements by using adhesive Normal Switching Operation material or by varnishing. This damps mechanical resonance of the transformer elements as well.

- Change the Transformer Design

above 20k Hz

The amplitude of mechanical vibration of the transformer is closely related to the flux swing. Therefore, the audible noise

Burst Switching Operation can be reduced by lowering the flux swing in the burst

switching, which is given by

Lm Ibp

above 20k Hz ∆B 106 (T)×= (1)

20~20k Hz Np Ae Figure 1. Burst Switching Operation

where Lm is the transformer primary side inductance, Ibp is the current peak in burst switching, Ae is the cross-sectional area of the core in mm2 and Np is the number of turns of

2. Sources of Audible Noise

transformer primary side.

Even though the switching frequency of the FPS is above

As can be observed in equation (1), the flux swing can be

the range of human hearing, audible noise can be generated

reduced by increasing Np and/or Ae while keeping Lm

during transient or burst operation. In most flyback

constant, which results in reduced audible noise.

converters, the major noise sources are transformers and capacitors.

- Reduce the Current Peak in Burst Switching

-Transformer Audible Noise

Equation (1) also shows that the flux swing can be decreased Transformers can produce audible noise, since they contain by reducing the current peak (Ibp) in burst switching. By many physically movable elements, such as coils, isolation using slope compensation, the current peak in burst tapes and bobbins. The current in the coils produces electro- switching can be reduced. Unfortunately, this technique can magnetic fields which generate repulsive and/or attractive be applied only to the FPSs below, whose burst switching forces between the coils. This can produce a mechanical levels are determined by the feedback voltage level.

vibration in the coils, ferrite cores or isolation tapes.

- FSDM0465R, FSDM0565R, FSDM07652R, FSDM1265R - FSCM0565R, FSCM0765R, FSD200, FSD210

Rev. 1.0.0

©2005 Fairchild Semiconductor Corporation