AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
20 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
5.3.3.1
SUPIC and SNSOUT by auxiliary winding
The SNSOUT input provides a combination of four functions:
•
Overvoltage protection: SNSOUT > 3.5 V, latched
•
Undervoltage protection: SNSOUT < 2.35 V, protection timer
•
Hold HBC: SNSOUT < 1.1 V, stop switching HBC (for Burst mode)
•
Hold HBC + PFC: SNSOUT < 0.4 V, stop switching HBC and PFC (for Burst mode)
Remark:
A more detailed explanation of the SNSOUT functions can be found in
and
.
Often, a circuit is used which combines SUPIC and the output voltage monitoring by
SNSOUT, with one auxiliary winding on the HBC transformer. But an independent
construction for SUPIC and SNSOUT is also possible. This could be in a situation where
SUPIC is supplied by a separate standby supply and an auxiliary winding is only used for
output voltage sensing. It is also possible not to use SNSOUT for output sensing but as a
general-purpose protection input. See
for more information.
In a combined function of SUPIC and SNSOUT by an auxiliary winding on the HBC
transformer, some issues must be addressed to obtain a good representation of the output
voltage for SNSOUT measurement.
The advantage of a good coupling/representation of the auxiliary winding with the output
windings is also that a stable auxiliary voltage is obtained for SUPIC. A low SUPIC voltage
value can be designed more easily for lowest power consumption.
5.3.3.2
Auxiliary supply voltage variations by output current
At high (peak) current loads, the voltage drop across the series components of the HBC
output stage (resistance and diodes) is compensated by regulation. This results in a
higher voltage on the windings at higher output currents due to the higher currents
causing a larger voltage drop across the series components. An auxiliary winding supply
shows this variation caused by the HBC output.
5.3.3.3
Voltage variations by auxiliary winding position: primary side component
Due to a less optimal position of the auxiliary winding, the voltage for SNSOUT and/or
SUPIC can contain a certain amount of undesired primary voltage component. This can
seriously endanger the feasibility of the SNSOUT sensing function.
Fig 7.
Auxiliary winding on primary side (left) and secondary side (right)
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