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TEA1401T Datasheet(PDF) 5 Page - NXP Semiconductors

Part No. TEA1401T
Description  Power plug for the universal mains
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Maker  PHILIPS [NXP Semiconductors]
Homepage  http://www.nxp.com
Logo PHILIPS - NXP Semiconductors

TEA1401T Datasheet(HTML) 5 Page - NXP Semiconductors

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1997 Mar 07
5
Philips Semiconductors
Preliminary specification
Power plug for the universal mains
TEA1401T
FUNCTIONAL DESCRIPTION
The TEA1401T is the heart of a compact flyback DC-to-DC
converter, with the IC placed at the primary side.
An auxiliary primary winding of the transformer is used for
indirect feedback to control the isolated output. This extra
winding also powers the device.
Control of the converted power is carried out by current
mode control and Pulse Frequency Modulation (PFM), as
illustrated in Fig.1. The primary current is sensed by a
comparator. The frequency is determined by the maximum
of the transformer demagnetizing time and the time of the
voltage controlled monostable multivibrator (single-shot).
The TEA1401T senses signals at the primary side of the
transformer to reconstruct the current and voltage which
are present at the secondary side. Comparison of these
reconstructions with the internal reference leads to
adaptation of the turn-off current level for the primary
switch and also to adaptation of the single-shot time.
Current control (see Fig.3)
The current through the main switch is measured by the
peak detector shown in Fig.1. The timing block generates
a signal ‘secondary stroke’ which is logic 1 when the
voltage of the auxiliary winding is negative.
The measured peak current, multiplied by the ratio of the
resistors connected to pins 4 (SOURCE) and 7 (RI), is
integrated by a capacitor during the secondary stroke.
In this way a reconstruction is made of the secondary
charge transfer. The charge estimation Q-pulse’
(see Fig.3) is drawn from the capacitor at pin 8 (CI) for
each pulse. Also this capacitor, the charge error memory,
is continuously charged with the reference current. In this
way the real (reconstructed) current is compared with the
reference yielding the voltage VCI at pin 8. The VCI level
provides the turn-off current level for the main switch and
the single-shot time.
Input from the voltage part of the loop is used to improve
the current reconstruction, resulting in a lower output
conductance of the complete converter. In the block
diagram this is denoted as ‘GOUT compensation’.
The block ‘IPEAK correction’ is able to increase the output
from the peak detector to improve line regulation.
Voltage control
The voltage from the auxiliary winding is sensed as a
measure of the secondary voltage. During the secondary
stroke the auxiliary winding delivers a negative voltage.
This voltage is converted into a current by an external
resistor at the RV pin between the transformer winding and
virtual ground. This current is compared with a reference
current.
The difference between the reconstructed voltage and the
reference is integrated during the secondary stroke by a
capacitor on the CV pin. The voltage on the CV pin is
transferred, via a ‘track-and-hold’ circuit, to the connection
point of the current and the voltage loop.
The ‘track-and-hold’ output provides the turn-off current
level for the main switch and the single-shot time.
The ‘track-and-hold’ circuit itself is present for loop
stability.
Input from the current part of the loop is used to improve
the voltage reconstruction, resulting in lower output
impedance of the complete converter (analog to the
current control). In the block diagram this is denoted as
‘ROUT compensation’.
Combined control
The two loops, I loop and V loop, each request their own
turn-off current level for the main switch and single-shot
time. The block ‘minimum’ in the block diagram outputs the
lowest value of the two, preventing the output voltage or
current from exceeding its nominal value. The output
characteristics of the power plug are displayed in Fig.4
(with enabled foldback option).
Optional foldback (see Fig.4)
The optional foldback feature of the TEA1401T is
performed by sensing the voltage of the auxiliary winding
at the end of the flyback stroke. It is actually not a voltage,
but the current through pin 12 (RV) that is measured. When
this voltage is low, the reference current in the current
control loop is set to the low level Jref/3.
The steep foldback enables a turn-down of the converter
by short-circuiting the output on the secondary side, for
example by a switch-transistor.


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