FSQ110 データシートの表示（PDF） - Fairchild Semiconductor
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FSQ110 Datasheet PDF : 12 Pages
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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.
Figure 19. Equal Loudness Curves
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. 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 a 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 20. Typical Feedback Network of FPS
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
© 2007 Fairchild Semiconductor Corporation
FSQ110 Rev. 1.0.0
10
www.fairchildsemi.com
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