AN4148 Datasheet, PDF (1/5 Pages) STMicroelectronics – Using the SPIRIT1 transceiver under ARIB STD-T93 in the 315 MHz band

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Application Note AN4148
Audible Noise Reduction Techniques for FPS
Applications
1. Introduction
In general, switched mode power supplies do not generate
audible noise when they operate at constant ultrasonic
frequencies (>20kHz). However, some switched mode
power supplies can produce audible noise at certain load
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
bundles is in the range of human hearing as shown in Figure
1.
This application note explains the major sources of audible
noise and offers useful tips to engineers to solve the audible
noise problem in their Fairchild Power Switch (FPS)
applications.
Normal Switching Operation
- Capacitor Audible Noise
Ceramic capacitors can also produce audible noise, since
they have piezoelectric characteristics. If there is still too
much audible noise in burst operation after gluing or
varnishing the transformer properly, the capacitor in the
snubber network should be examined.
3. Techniques to Reduce the Audible
Noise
- Varnish the Transformer
The most effective way to reduce the audible noise in the
transformer is to remove the possibility of physical
movement of the transformer elements by using adhesive
material or by varnishing. This damps mechanical resonance
of the transformer elements as well.
above 20kHz
Burst Switching Operation
- Change the Transformer Design
The amplitude of mechanical vibration of the transformer is
closely related to the flux swing. Therefore, the audible noise
can be reduced by lowering the flux swing in the burst
switching, which is given by
20~20kHz
above 20kHz
Figure 1. Burst Switching Operation
2. Sources of Audible Noise
Even though the switching frequency of the FPS is above
the range of human hearing, audible noise can be generated
during transient or burst operation. In most flyback
converters, the major noise sources are transformers and
capacitors.
-Transformer Audible Noise
Transformers can produce audible noise, since they contain
many physically movable elements, such as coils, isolation
tapes and bobbins. The current in the coils produces electro-
magnetic fields which generate repulsive and/or attractive
forces between the coils. This can produce a mechanical
vibration in the coils, ferrite cores or isolation tapes.
©2005 Fairchild Semiconductor Corporation
∆B = -L---^m----I--^b---^p- × 10⁶ (T)
(1)
N_pA_e
where L_m is the transformer primary side inductance, I_bp is
the current peak in burst switching, A_e is the cross-sectional
area of the core in mm² and N_p is the number of turns of
transformer primary side.
As can be observed in equation (1), the flux swing can be
reduced by increasing N_p and/or A_e while keeping L_m
constant, which results in reduced audible noise.
- Reduce the Current Peak in Burst Switching
Equation (1) also shows that the flux swing can be decreased
by reducing the current peak (I_bp) in burst switching. By
using slope compensation, the current peak in burst
switching can be reduced. Unfortunately, this technique can
be applied only to the FPSs below, whose burst switching
levels are determined by the feedback voltage level.
- FSDM0465R, FSDM0565R, FSDM07652R, FSDM1265R
- FSCM0565R, FSCM0765R, FSD200, FSD210
Rev. 1.0.0