Datasheet

Transformer is
demagnetized
LM5023
SNVS961D –APRIL 2013–REVISED JANUARY 2014 www.ti.com
Figure 10. The Flyback Drain Voltage Waveform
Transformer demagnetization is detected by sensing the transformers auxiliary winding. When the transformer is
demagnetized the auxiliary winding voltage follows the Drain of the MOSFET and changes from Vout•Naux/Ns to
-Vin•Naux/Np. Internal to the LM5203 QR pin is a comparator with a 0.35 V reference. As the auxiliary winding
voltage falls below 0.35 V, the voltage is sensed and the comparator sets the PWM Flip-Flop turning on the
Flyback MOSFET. Figure 11 shows the QR Converter typical waveforms; the auxiliary winding voltage, primary,
and secondary current waveforms. It is possible to adjustable the delay on the auxiliary winding with a resistor
and external capacitor to ensures that the MOSFET switches when its Drain voltage is at its minimum, refer to
the schematic in Figure 14 and the section on Valley Switching for details. The benefits of QR operation are
reduced EMI, and turn-on switching losses.
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