TEA1742T_1

© NXP B.V. 2009. All rights reserved.

Objective data sheet

Rev. 01 — 10 February 2009

7 of 17

NXP Semiconductors

TEA1742T

GreenChip PFC controller

7.2.2 Valley switching and demagnetization (PFCAUX pin)

The PFC MOSFET is switched on after the transformer is demagnetized. Internal circuitry

connected to the PFCAUX pin detects the end of the secondary stroke. It also detects the

voltage across the PFC MOSFET. The next stroke is started if the voltage across the PFC

MOSFET is at its minimum in order to reduce switching losses and ElectroMagnetic

Interference (EMI) (valley switching).

If no demagnetization signal is detected on the PFCAUX pin, the controller generates a

Zero Current Signal (ZCS), 50 ms (typ) after the last PFCGATE signal.

If no valley signal is detected on the PFCAUX pin, the controller generates a valley signal

4 ms (typ) after demagnetization was detected.

To protect the internal circuitry, for example during lightning events, it is advisable to add a

5k

Ω series resistor to this pin. To prevent incorrect switching due to external disturbance,

the resistor should be placed close to the IC on the printed circuit board.

7.2.3 Frequency limitation

To optimize the transformer and minimize switching losses, the switching frequency is

limit, the system switches over to discontinuous conduction mode. Also here, the PFC

MOSFET is only switched on at a minimum voltage across the switch (valley switching).

7.2.4 Mains voltage compensation (VINSENSE pin)

The mathematical equation for the transfer function of a power factor corrector contains

the square of the mains input voltage. In a typical application this results in a low

bandwidth for low mains input voltages, while at high mains input voltages the Mains

Harmonic Reduction (MHR) requirements may be hard to meet.

To compensate for the mains input voltage influence, the TEA1742T contains a correction

circuit. Via the VINSENSE pin the average input voltage is measured and the information

is fed to an internal compensation circuit. With this compensation it is possible to keep the

regulation loop bandwidth constant over the full mains input range, yielding a fast transient

response on load steps, while still complying with class-D MHR requirements.

In a typical application, the bandwidth of the regulation loop is set by a resistor and two

capacitors on the PFCCOMP pin.

7.2.5 Soft start-up (pin PFCSENSE)

To prevent audible transformer noise at start-up or during hiccup, the transformer peak

The start level and the time constant of the increasing primary current level can be

τsoftstart

3

R

SS1

C

SS1

×

×

=