TEA1750T/N1,518【PDF 13/29 页】IC PFC CONTROLLER DCM16SOIC

TEA1750_2

Product data sheet

Rev. 02 15 December 2008

13 of 29

NXP Semiconductors

TEA1750

GreenChip III SMPS control IC

In frequency reduction mode the PFC controller is switched to burst mode operation and

the yback maximum frequency changes linearly with the control voltage on the FBCTRL

pin (see

Figure

8 ). For stable on-off switching of the PFC burst mode, the FBCTRL pin

has a 50 mV (typ) hysteresis. At no load operation the switching frequency of the yback

can be reduced to (almost) zero.

7.3.2 Valley switching (HV pin)

Refer to

Figure

9. A new cycle starts when the external MOSFET is activated. After the

on-time (determined by the FBSENSE voltage and the FBCTRL voltage), the MOSFET is

switched off and the secondary stroke starts. After the secondary stroke, the drain voltage

shows an oscillation with a frequency of approximately

where L

p

is the primary self inductance of the yback transformer and C

d

is the

capacitance on the drain node.

As soon as the internal oscillator voltage is high again and the secondary stroke has

ended, the circuit waits for the lowest drain voltage before starting a new primary stroke.

Figure

9 shows the drain voltage, valley signal, secondary stroke signal and the internal

oscillator signal.

Valley switching allows high frequency operation as capacitive switching losses are

reduced, see

Equation

1. High frequency operation makes small and cost-effective

magnetics possible.

(1)

Fig 8. Frequency control of yback part

f

sw(fb)max

V

FBCTRL

1.5 V

discontinuous

with valley

switching

quasi resonant

frequency

reduction

PFC burst mode

switching frequency

014aaa026

PFC on

1

2 ?/SPAN>

?/DIV>

L

p

C

d

?/DIV>

(

)

?/DIV>

(

)

--------------------------------------------------- -

P

1

2

-- - C

d

V

2

?/DIV>

f

?/DIV>

=

?/DIV>

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