TDA4862 Datasheet, PDF (5/18 Pages) Siemens Semiconductor Group – Power-Factor Controller PFC IC for High Power Factor and Active Harmonic Filter

TDA 4862
Functional Description (cont’d)
MuItiplier
A one quadrant multiplier is the crucial circuitry that regulates the gate driver with respect
of the DC output voltage and the AC haversine input voltage of the preregulator. Both
inputs are designed for good linearity over a wide dynamic range, 0 V to 4.0 V for the
MULTIN and 2.5 V to 4.0 V for the V_AOUT.
Current Sense Comparator and RS Latch
The multiplier output voltage is compared with the current sense voltage which
represents the current through the MOSFET. The current sense comparator in addition
with the logic ensures that only a single pulse appears at the drive output during a
given cycle. The multiplier output and the current sense threshold are internally clamped
at 1.3 V. So the gate drive MOSFET is protected against critical operating, as they occur
during start up. To prevent the input from negative pulses a special protection circuitry is
implemented. Switch-on current peaks are reduced by an internal RC-Filter.
Zero Current Detector
The zero current detector senses the inductor current via an auxiliary winding and
ensures that the next on-time is initiated immediately when the inductor current has
reached zero. This diminishes the reverse recovery losses of the boost converter diode.
Output switch conduction is terminated when the voltage drop of the shunt resistor
reaches the threshold level of the multiplier output. So the boost current waveform has
a triangular shape and there are no deadtime gaps between the cycles. This leads to a
continuous AC line current limiting the peak current to twice of the average current.
To prevent false tripping the zero current detector is designed as a Schmitt trigger with
a hysteresis of 0.6 V. An internal 5 V clamp protects the input from overvoltage
breakdown, a 0.6 V clamp prevents substrate injection. An external resistor must be
used in series with the auxiliary winding to limit the current through the clamps.
Timer
A restart timer function was added to the IC to eliminate the need for an oscillator when
used in stand-alone applications. The timer starts or restarts the TDA 4862 if the drive
output has been off for more than 15 µs after the inductor current reaches zero.
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2005-02-17