FSDH321, FSDL321
13
Application Tips
1. Methods of Reducing Audible Noise
Switching mode power converters have electronic and
magnetic components, which generate audible noises when
the operating frequency is in the range of 20~20,000 Hz.
Even though they operate above 20 kHz, they can make
noise depending on the load condition. Designers can
employ several methods to reduce these noises. Here are
three of these methods:
Glue or Varnish
The most common method 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.
But, it also can crack the core. This is because sudden
changes in the ambient temperature cause the core and the
glue to expand or shrink in a different ratio according to the
temperature.
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. It is considerable 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 2~4 kHz
range is the third method. Generally, humans are more sensi-
tive to noise in the range of 2~4 kHz. When the fundamental
frequency of noise is located in this range, one perceives the
noise as louder although the noise intensity level is identical.
Refer to Figure 14. Equal Loudness Curves.
When FPS acts in Burst mode and the Burst operation is
suspected to be a source of noise, this method may be help-
ful. If the frequency of Burst mode operation lies in the
range of 2~4 kHz, adjusting feedback loop can shift the
Burst operation frequency. In order to reduce the Burst oper-
ation frequency, increase a feedback gain capacitor (C
F
),
opto-coupler supply resistor (R
D
) and feedback capacitor
(C
B
) and decrease a feedback gain resistor (R
F
) as shown in
Figure 15. Typical Feedback Network of FPS.
Figure 14. Equal Loudness Curves
Figure 15. Typical Feedback Network of FPS
2. Other Reference Materials
AN-4134
: Design Guidelines for Off-line Forward Convert-
ers Using Fairchild Power Switch (FPS
TM
)
AN-4137
: Design Guidelines for Off-line Flyback Convert-
ers Using Fairchild Power Switch (FPS)
AN-4140
: Transformer Design Consideration for Off-line
Flyback Converters using Fairchild Power Switch
(FPS
TM
)
AN-4141
: Troubleshooting and Design Tips for Fairchild
Power Switch (FPS
TM
) Flyback Applications
AN-4147
: Design Guidelines for RCD Snubber of Flyback
AN-4148
: Audible Noise Reduction Techniques for FPS
Applications
