TEA1753T/N1,518 Datasheet TEA1753T/N1,518 | P16-P18

Product data sheet Rev. 3. — 24 August 2012 16 of 32

NXP Semiconductors

TEA1753T

HV start-up flyback controller with integrated PFC controller

7.3.4 Demagnetization (FBAUX pin)

The system is always in QR or DCM mode. The internal oscillator does not start a new

primary stroke until the previous secondary stroke has ended.

Demagnetization features a cycle-by-cycle output short-circuit protection by immediately

lowering the frequency (longer off-time), thus reducing the power level.

Demagnetization recognition is suppressed during the first t

sup(xfmr_ring)

time of 2 s. This

suppression is necessary at low output voltages and at start-up. It is also required in

applications where the transformer has a large leakage inductance.

If the FBAUX pin is open-circuit or not connected, a fault condition is assumed and the

converter immediately stops. Operation restarts as soon as the fault condition is removed.

7.3.5 Flyback control/time-out (FBCTRL pin)

The FBCTRL pin is connected to an internal voltage source of 3.5 V using an internal

resistor of 3 k. When the voltage on this pin exceeds 2.5 V, the connection is disabled

and the pin is biased with a small current. If the voltage on this pin exceeds 4.5 V, a fault is

assumed, switching is stopped and a restart is made.

If a capacitor and a resistor are connected in series to this pin, a time-out function is

created to protect against an open control loop. See Figure 12

and Figure 13. The

time-out function is disabled by connecting a resistor (100 k) to ground on the FBCTRL

pin.

If the pin is short-circuited to ground, switching of the flyback controller is prevented.

Under normal operating conditions, the converter regulates the output voltage. The

voltage on the FBCTRL pin then varies between 1.3 V for the minimum output power and

2 V for the maximum output power.

Fig 11. Peak current control of flyback

FBCTRL

1.5 V 2.0 V

0.65 V

flyback

frequency

reduction

PFC off

FBSENSE

peak voltage

014aaa747

PFC on

flyback

discontinuous

or QR

flyback

cycle skip

mode

SENSE resistor

peak voltage

FBSENSE

offset voltage

sense(fb)max

0.325 V

Product data sheet Rev. 3. — 24 August 2012 17 of 32

NXP Semiconductors

TEA1753T

HV start-up flyback controller with integrated PFC controller

7.3.6 Soft-start (FBSENSE pin)

To prevent audible transformer noise during start-up, the soft-start function slowly

increases the transformer peak current. This increase is achieved by inserting a resistor

and a capacitor between the FBSENSE pin and the current sense resistor.

An internal current source charges the capacitor to:

(5)

with a maximum of approximately 0.63 V.

The start level and the time constant of the increasing primary current level is adjusted

externally by changing the values of R

SS2

and C

SS2

(6)

The soft-start current I

start(soft)fb

is switched on as soon as V

reaches V

startup

. When

FBSENSE

reaches 0.63 V, the flyback converter starts switching.

Fig 12. Time-out protection circuit

Fig 13. Time-out protection (signals), safe restart in the TEA1753T

014aaa049

FBCTRL

2.5 V

4.5 V

30 μA

3 kΩ

3.5 V

time-out

014aaa050

4.5 V

2.5 V

FBCTRL

output

voltage

intended output

voltage not

reached within

time-out time.

intended output voltage

reached within time-out

time.

restart

start softfb

SS2

=

soft s– tart 3 R

SS1

=

Product data sheet Rev. 3. — 24 August 2012 18 of 32

NXP Semiconductors

TEA1753T

HV start-up flyback controller with integrated PFC controller

The charging current I

start(soft)(FB)

flows as long as V

FBSENSE

is less than approximately

0.63 V. If V

FBSENSE

exceeds 0.63 V, the soft-start current source starts limiting the current.

After the flyback converter has started, the soft-start current source is switched off.

7.3.7 Maximum on-time

The flyback controller limits the on-time of the external MOSFET to 40 s. When the

on-time is longer than 40 s, the IC stops switching and enters the safe restart mode.

7.3.8 Overvoltage protection (FBAUX pin)

An output overvoltage protection is implemented in the GreenChip III series. In the

TEA1753T, the auxiliary voltage is sensed using the current flowing into the FBAUX pin

during the secondary stroke. The auxiliary winding voltage is a well-defined replica of the

output voltage. An internal filter averages voltage spikes.

An internal up-down counter prevents any false OVP detection that occurs during ESD or

lightning events. The internal counter counts up by one when the output voltage exceeds

the OVP trip level within one switching cycle. The internal counter counts down by two

when the output voltage has not exceeded the OVP trip level within one switching cycle.

When the counter has reached eight, the IC assumes a true OVP, sets the latched

protection and switches off both converters.

The converter only restarts after the OVP latch is reset. In a typical application, the

internal latch is reset when the VINSENSE voltage drops below 750 mV and is then raised

to 870 mV.

The latched protection is also reset by removing both the voltage on the V

CC and HV pins.

The demagnetization resistor, R

FBAUX

sets the output voltage V

o(OVP)

at which the OVP

function trips:

(7)

where N

is the number of secondary turns and N

aux

is the number of auxiliary turns of the

transformer. Current I

ovp(FBAUX)

is internally trimmed.

Fig 14. Soft-start up of flyback.

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oOVP

aux

-----------

ovp FBAUX

FBAUX

clamp FBAUX

+=

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30 P31-P32

TEA1753T/N1,518

Mfr. #:

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Manufacturer:

NXP Semiconductors

Description:

AC/DC Converters GreenChip III SMPS control IC

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TEA1753T/N1,518