TEA1750T/N1,518【PDF 9/29 页】IC PFC CONTROLLER DCM16SOIC

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Sheet目录46

5页 6页 7页 8页 [9页]10页 11页 12页 13页

TEA1750_2

Product data sheet

Rev. 02 15 December 2008

9 of 29

NXP Semiconductors

TEA1750

GreenChip III SMPS control IC

For applications with high transformer ringing frequencies (after the secondary stroke),

the PFCAUX pin should be connected via a capacitor and a resistor to the auxiliary

winding. A diode must than be placed from the ground connection to the PFCAUX pin.

7.2.3 Frequency limitation

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

limited to f

sw(PFC)max

. If the frequency for quasi-resonant operation is above the f

sw(PFC)max

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 inuence, the TEA1750 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

current, I

, is increased slowly by the soft start function. This can be achieved by

inserting R

SS1

and C

SS1

between pin PFCSENSE and current sense resistor, R

SENSE1

An internal current source charges the capacitor to V

PFCSENSE

= I

start(soft)PFC

SS1

. The

voltage is limited to V

start(soft)PFC

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

adjusted externally by changing the values of R

SS1

and C

SS1

The charging current I

start(soft)PFC

ows as long as the voltage on pin PFCSENSE is

below 0.5 V (typ). If the voltage on pin PFCSENSE exceeds 0.5 V, the soft start current

source starts limiting current I

start(soft)PFC

. As soon as the PFC starts switching, the

start(soft)PFC

current source is switched off; see

Figure

?/DIV>

SoftStart

3 R

SS1

?/DIV>

SS1

?/DIV>

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