AN966 Datasheet, PDF (5/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966 APPLICATION NOTE
This dynamic OVP, with its combination of soft and sharp braking, is effective to handle most of load
changes but does not provide a complete protection. In fact it is sensitive to output voltage variations
(whence the appellative "dynamic") and cannot reveal a steady overvoltage, which is likely to occur in
case of load disconnection.
The above mentioned concept of the E/A saturation is effective to achieve a "static" OVP. If the overvol-
tage lasts so long that the output of E/A goes below 2.25V (the E/A is in linear dynamics up to 2.5V), the
protection is activated. Besides turning off the output stage and the external MOSFET, it disables some
internal blocks reducing the quiescent current of the chip to 1.4mA (typ). The operation of the device is
re-enabled as the E/A output goes back into its linear region.
Fig. 4 illustrates the combined action of dynamic and static OVP.
ZERO CURRENT DETECTION AND TRIGGERING BLOCK (see fig. 5)
The Zero Current Detection (ZCD) block switches on the external MOSFET as the voltage across the
boost inductor reverses, just after the current through the boost inductor has gone to zero. This feature
allows TM operation.
Figure 5. Zero Current Detection, Triggering and Disable Block.
+Vi
ZCD 5
200µA
5.7V
-
-
+
+
0.15V 1.6V
PWM
R
Q
S
7
DRIVER
GD
DISABLE
STARTER
D97IN674A
As the circuit is running, the signal for ZCD is obtained with an auxiliary winding on the boost inductor.
Of course, a circuit is needed that turns on the external MOSFET at start-up since no signal is coming
from the ZCD. This is realized with an internal starter, which forces the driver to deliver a pulse to the
gate of the MOSFET, producing also the signal for arming the ZCD circuit.
The repetition rate of the starter is greater than 70 µs (≅ 14 kHz) and this maximum frequency must be
taken into account at design time.
DISABLE BLOCK (see fig. 5)
The ZCD pin is used also to activate the Disable Block. If the voltage on the pin is taken below 150 mV
the device will be shut down. As a result, its current consumption will be reduced. To re-enable the de-
vice operation, the pull-down on the pin must be released.
MULTIPLIER BLOCK (see fig. 6)
The multiplier has two inputs: the first one takes a partition of the instantaneous rectified line voltage and
the second one the output of the E/A. If this voltage is constant (over a given line half-cycle) the output
of the multiplier will be shaped as a rectified sinusoid too. This is the reference signal for the current
comparator, which sets the MOSFET peak current cycle by cycle.
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