TEA1751T_LT_2
?NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 02 23 December 2009
12 of 29
NXP Semiconductors
TEA1751T; TEA1751LT
GreenChip III SMPS control IC
7.3 Flyback controller
The TEA1751(L)T includes a controller for a flyback converter. The flyback converter
operates in quasi-resonant or discontinuous conduction mode with valley switching. The
auxiliary winding of the flyback transformer provides demagnetization detection and
powers the IC after start-up.
7.3.1 Multimode operation
The TEA1751(L)T flyback controller can operate in several modes; see Figure 7
.
At high output power the converter switches to quasi-resonant mode. The next converter
stroke is started after demagnetization of the transformer current. In quasi-resonant mode
switching losses are minimized as the converter only switches on when the voltage across
the external MOSFET is at its minimum (valley switching, see also Section 7.3.2
).
To prevent high frequency operation at lower loads, the quasi-resonant operation changes
to discontinuous mode operation with valley skipping in which the switching frequency is
limited for EMI to f
sw(fb)max
(125 kHz typical). Again, the external MOSFET is only switched
on when the voltage across the MOSFET is at its minimum.
At very low power and standby levels the frequency is controlled down by a Voltage
Controlled Oscillator (VCO). The minimum frequency can be reduced to zero. During
frequency reduction mode, the primary peak current is kept at a minimal level of Ipkmax/4
to maintain a high efficiency. (Ipkmax is the maximum primary peak current set by the
sense resistor and the maximum sense voltage.) As the primary peak current is low in
frequency reduction operation (Ipk = Ipkmax/4), no audible noise is noticeable at
switching frequencies in the audible range. Valley switching is also active in this mode.
In frequency reduction mode the PFC controller is switched off and the flyback maximum
frequency changes linearly with the control voltage on the FBCTRL pin (see Figure 8
).
For stable on and off switching of the PFC, the FBCTRL pin has a 50 mV (typical)
hysteresis. At no load operation the switching frequency can be reduced to (almost) zero.
Fig 7. Multimode operation flyback
discontinuous
with valley
switching
quasi resonant
PFC off
frequency
reduction
f
sw(fb)max
output power
switching frequency
014aaa15
PFC on
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