AN10907
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Application note
Rev. 1 — 28 December 2010
56 of 82
NXP Semiconductors
AN10907
TEA1613T resonant power supply control IC
9.7 Lower SUPHS in burst
During idle time SUPHS is not charged.
During normal operation, each time the half-bridge node HB is switched to ground level,
the SUPHS capacitor is charged by the bootstrap function of the external diode between
SUPHS and SUPREG. In burst mode there are periods of non-switching, and therefore no
charging of SUPHS. During this time, the circuit supplied by SUPHS slowly discharges the
supply voltage capacitor. At the moment a new burst starts, the SUPHS voltage is lower
than during normal operation. During the first switching cycles, the SUPHS is re-charged
to its normal level. It is important that, during these first re-charge cycles, SUPREG does
not drop below the protection level of 10.3 V.
9.8 Audible noise
Because the burst mode is normally used when the output power is low, the converted
energy does not contribute much to generate audible noise. The magnetization current
however is still present during low loads and is the dominant energy during burst mode.
Switching on and off the converter sequences continuously at a certain speed and
duration can lead to audible noise. The main mechanism for producing noise is the
interruption of magnetization current sequences leading to a mechanical force. This is
especially the case on the core of the resonant transformer which starts acting as a
loudspeaker.
When burst mode is applied at higher output power conditions, the converted energy also
contributes and leads to an increased chance of audible noise.
9.8.1 Measurements in the resonant transformer construction
To prevent problems with audible noise under specific conditions, it is necessary to adapt
the mechanical transformer construction to allow for this.
One possibility is to adhere the core parts to each other using a material with damping
(vibration absorbing) properties. A combination can be made with the air gap construction.
Other vibration damping measures can also help when audible noise is a critical issue for
a product.
(1) Left-hand transformer with glue to reduce audible noise
(2) Right-hand transformer has standard construction
Fig 44. Transformer construction
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