© 2007 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSQ211 Rev. 1.0.0

10

Application Tips

Methods of Reducing Audible Noise

Switching mode power converters have electronic and

magnetic components that generate audible noises

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.

Designers can employ several methods to reduce noise.

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 can crack the

core. This is because sudden changes in the ambient

temperature cause the core and the glue to expand or

shrink at a different rate.

Ceramic Capacitor

Using a film capacitor instead of a ceramic capacitor as

a snubber capacitor is another noise-reduction option.

Some dielectric materials show a piezoelectric effect,

depending on the electric field intensity. Therefore, a

snubber capacitor becomes one of the most significant

sources of audible noise. It is possible 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

2~4kHz 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 is perceived as louder, although

the noise intensity level is identical (refer to Figure 22

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

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 23.

Figure 22. Equal Loudness Curves

Figure 23. 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-4138: Design Considerations for Battery Charger

Using Green Mode 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™ Application