TEA1401T View Datasheet(PDF) - Philips Electronics
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Description
Manufacturer
TEA1401T Datasheet PDF : 20 Pages
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Philips Semiconductors
Power plug for the universal mains
Preliminary specification
TEA1401T
Overshoot protection
Sensing the voltage during the previously mentioned
flyback stroke is also used to signal a voltage overshoot.
A voltage overshoot will delay and minimize the next active
stroke. This is achieved by discharging the capacitor in the
‘track-and-hold’ circuit (see Fig.1). In this way the power
level of the converter is turned down to its minimum
immediately in case of a voltage overshoot.
Minimum output power
Under no-load condition an additional external pre-load
resistor (or Zener diode) is necessary to keep the output
voltage at its nominal value (or at the Zener diode voltage).
This is due to the fact that under no-load condition and also
at voltage overshoot the converter will keep operating
instead of being switched off. Although the converter then
will operate with a short active stroke and a low frequency,
energy is still being converted to the output. To prevent
excessive output voltage this energy has to be dissipated.
The advantage of a pre-load resistor over a Zener diode is
that the converter will stay in regulation, maintaining its fast
response to load variations.
Duty cycle control
The momentary power level required by the I/V control
loop is achieved by controlling the duty cycle of the
converter by two actions. First the peak value of the
primary current is controlled using a cycle-by-cycle current
control. Secondly the pulse frequency is modulated. There
is a broad region in which both regulation principles are
active simultaneously. Both controls have a minimum and
a maximum value which are set by the resistor on the
SOURCE pin and the capacitor on the CPFM pin.
SOPS and PFM
The switching frequency fsw is set by the transformer
demagnetizing time or the frequency control block within
the IC (block ‘single-shot’ in Fig.1).
At a high power level the transformer determines the
frequency. This mode of operation is called Self Oscillating
Power Supply (SOPS), and provides maximum efficiency
(for a non-continuous conducting flyback converter).
In SOPS the next primary stroke is started right after the
previous secondary stroke has ended. Timing information
is collected from the auxiliary winding.
The SOPS frequency will increase when the power level
decreases. The frequency however is limited by the PFM
controller (single-shot). When the PFM controller takes
over, the frequency will be proportional to the required
power level. Thus the frequency is reduced when the
power level decreases. In PFM there is a variable dead
time after the secondary stroke. The next primary stroke is
started after the single-shot time has ended.
Supply
Initially the IC is powered by a high DC input voltage at
pin 1 (Vin). In operation the auxiliary winding takes over.
In the event that the auxiliary winding delivers insufficient
power for the internal circuitry of the IC, this deficit is
supplemented again via pin 1 (Vin).
The supply voltage for the internal circuitry is buffered with
an external capacitor at pin 14 (VIC). When the auxiliary
winding powers the IC, energy is stored during the active
stroke. The rest of the time energy is supplied by the buffer
capacitor.
Protections
The IC has a cycle-by-cycle current regulation, with a
built-in setting for the absolute maximum voltage across
the current sense resistor. Also a maximum time is set for
the duration of the active stroke. A provision for
temperature shut down has been implemented.
1997 Mar 07
6
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