AN10881
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© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
34 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
7. PFC
functions
The PFC operates in Quasi Resonant (QR) or Discontinuous Conduction Mode (DCM)
with valley detection to reduce the switch-on losses. The maximum switching frequency of
the PFC is limited to 125 kHz which reduces switching losses by valley skipping. This is
mainly near the zero crossings of the mains voltage and effective at low mains input
voltage and medium/low output load condition.
The PFC is designed as a boost converter with a fixed output voltage. An advantage of
such a fixed boost is that the HBC can be designed to a high input voltage. This makes
the HBC design easier.
Another advantage of the fixed boost is the possibility to use a smaller boost capacitor
value or to have a significant longer hold-up time.
In the TEA1713 system the PFC is always active. The PFC is switched on first when the
mains voltage is present. The HBC is switched on after the boost capacitor is charged to
approximately 90 % of its normal value.
The system can be operated in Burst mode for improved efficiency at low output loads.
During this mode the HBC determines the on/off sequences and the PFC can be made to
burst simultaneously for even better efficiency results.
7.1 PFC output power and voltage control
The PFC of the TEA1713 is time controlled and therefore it is not necessary to measure
the mains phase angle. The on-time is kept constant for a given mains voltage and load
condition during the half sine wave to obtain a good Power Factor (PF) and Mains
Harmonics Reduction (MHR).
With a constant on time, the switching current to the PFC output is proportional to the sine
waveform input voltage.
An essential parameter for the PFC coil design is the highest peak current. This current
occurs at the lowest input voltage with maximum power.
The maximum peak current I
p(max)
for a PFC operating in Critical conduction mode can be
calculated with the following equation:
(5)
Example:
•
Efficiency
= 0.9
•
P
out(nameplate)
= 250 W
•
V
ac(min)
= 90 V
•
I
p(max)
= 8.73 A
•
I
p(max)
+ 10 % = 9.60 A
1
I
p max
2
2
P
in max
V
ac min
------------------------------------------
2
2
P
out nameplate
-----------------------------------
V
ac min
----------------------------------------------------------
=
=