AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
68 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
9.7 Lower SUPHS in burst
During the idle time SUPHS is not charged.
During normal operation, each time the half-bridge node HB is switched to ground level,
the bootstrap function of the external diode between SUPHS and SUPREG charges the
SUPHS capacitor. 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. When a new burst starts, the SUPHS voltage is lower than in
normal operation. During the first switching cycles, the SUPHS is recharged to its normal
level. It is important that, during these first recharge 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 the converter sequences on and off 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 during higher output power conditions, the converted energy
also contributes and leads to an increased risk of audible noise.
9.8.1 Measurements in the resonant transformer construction
It is necessary to adapt the mechanical transformer construction to prevent problems with
audible noise under specific conditions.
SNSFB - output power
Frequency - output power
Fig 46. Normal mode output power characteristics (Adapted for easy implementation of Burst mode comparator
level detection)
Pout (W)
0
100
80
40
60
20
001aal055
4.8
5.6
6.2
SNSFB
(V)
4.0
burst
level =
5 V
Pout (W)
0
100
80
40
60
20
001aal097
120
160
200
f
(kHz)
80