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The demag(netization) sense is realized by an internal circuit that guards the voltage
(Vdemag) at pin 4 that is connected to V
winding by resistor R1 (R3522).
CC
The Figure below shows the circuit and the idealized waveforms across this winding.
Figure: Demagnetization protection
Over Voltage Protection
The Over Voltage Protection ensures that the output voltage will remain below an
adjustable level. This works by sensing the auxiliary voltage via the current flowing into
pin 4 (DEM) during the secondary stroke. This voltage is a well-defined replica of the
output voltage. Any voltage spikes are averaged by an internal filter.
If the output voltage exceeds the OVP trip level, the OVP circuit switches the power
MOSFET 'off'.
Next, the controller waits until the 'under voltage lock out' level (UVLO = ± 9 V) is
reached on pin 1 (V
). This is followed by a safe restart cycle, after which switching
CC
starts again. This process is repeated as long as the OVP condition exists. The output
voltage at which the OVP function trips, is set by the demagnetization resistor R3522.
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