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FSQ0370RNA Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FSQ0370RNA Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 17 page © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FSQ0170RNA, FSQ0270RNA, FSQ0370RNA Rev. 1.0.2 11 Application Information Methods of Reducing Audible Noise Switching-mode power converters have electronic and magnetic components, which generate audible noise when the operating frequency is in the range of 20~20,000Hz. Even though they operate above 20KHz, they can make noise, depending on the load condition. The following sections discuss methods to reduce noise. Glue or Varnish The most common method of reducing noise involves using glue or varnish to tighten magnetic components. The motion of core, bobbin, and coil and the chattering or magnetostriction of core can cause the transformer to produce audible noise. The use of rigid glue and varnish helps reduce the transformer noise. Glue or varnish can also can crack the core because sudden changes in the ambient temperature cause the core and the glue to expand or shrink in a different ratio. Ceramic Capacitor Using a film capacitor instead of a ceramic capacitor as a snubber capacitor is another noise reduction solution. Some dielectric materials show a piezoelectric effect, depending on the electric field intensity. Hence, a snubber capacitor becomes one of the most significant sources of audible noise. Another possibility is to use a Zener clamp circuit instead of an RCD snubber for higher efficiency as well as lower audible noise. Adjusting Sound Frequency Moving the fundamental frequency of noise out of the 2~4kHz range is the third method. Generally, humans are more sensitive to noise in the range of 2~4kHz. When the fundamental frequency of noise is located in this range, the noise sounds louder although the noise intensity level is identical (see Figure 19). When the FPS acts in burst mode and the burst operation is suspected to be a source of noise, this method may be helpful. If the frequency of burst mode operation lies in the range of 2~4kHz, adjusting the feedback loop can shift the burst operation frequency. To reduce the burst operation frequency, increase a feedback gain capacitor (CF), opto-coupler supply resistor (RD); and feedback capacitor (CB), and decrease a feedback gain resistor (RF), as shown in Figure 20. Figure 19. Equal Loudness Curves Figure 20. Typical Feedback Network of FPS Other Reference Materials AN-4134: Design Guidelines for Off-line Forward Converters Using Fairchild Power Switch (FPS™) AN-4137: Design Guidelines for Off-line Flyback Converters Using Fairchild Power Switch (FPS™) AN-4140: Transformer Design Consideration for Off-line Flyback Converters using Fairchild Power Switch (FPS™) AN-4141: Troubleshooting and Design Tips for Fairchild Power Switch (FPS™) Flyback Applications AN-4147: Design Guidelines for RCD Snubber of Flyback AN-4148: Audible Noise Reduction Techniques for FPS™ Applications |
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