TEA1751T All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 3 — 10 January 2013 13 of 31
NXP Semiconductors
TEA1751T
HV start-up flyback controller with integrated PFC controller
7.3 Flyback controller
The TEA1751T includes a controller for a flyback converter. The flyback converter
operates in quasi-resonant or DCM 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 TEA1751T flyback controller operates in several modes; see Figure 7.
At high output power, the converter switches to quasi-resonant mode. The next converter
stroke starts after demagnetization of the transformer and detection of the valley. In
quasi-resonant mode switching losses are minimized. This minimization is achieved by
the converter only switching on when the voltage across the external MOSFET is at its
minimum (see also Section 7.3.2
).
To prevent high frequency operation at low loads, the maximum switching frequency is
limited to 125 kHz. When the frequency limit is reached, the quasi-resonant operation
changes to DCM with valley skipping. The frequency limit reduces the MOSFET switch-on
losses and conducted EMI.
A Voltage Controlled Oscillator (VCO) controls the frequency at very low power and
standby levels. The minimum frequency is reduced to zero. During frequency reduction
mode, the primary peak current is kept at 25 % of its maximum level to maintain a high
efficiency. As the primary peak current is low in frequency reduction operation, 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. The flyback maximum
frequency changes linearly with the control voltage on the FBCTRL pin (see Figure 8
).
Hysteresis has been added for stable on and off switching of the PFC. 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
014aaa158
PFC on
