A quasi-resonant controlled flyback converter operates by storing energy in the transformer's primary during the on-time of the MOSFET. During the on-time (t_ON) V_IN is applied across the primary of the transformer. The primary current starts out at zero and ramps toward a peak value (I_PEAK). When the peak-primary current reaches the feedback compensation error voltage the PWM comparator resets the output drive, turning off the MOSFET. Due to the phasing of the transformer, the output diode is reverse-biased during the MOSFET on-time.

During the off time of the MOSFET, the output diode is forward biased and the stored energy in the transformer primary inductor is transferred to the output. The voltage seen on the secondary winding is V_OUT plus the forward voltage drop of the output diode, V_F. The current in the secondary winding linearly decreases from I_PEAK × Np/Ns to zero, refer to Figure 6-4.

When the current in the secondary reaches zero, the transformer is demagnetized, and there is an open circuit on the secondary, and with the primary MOSFET also turned off, there is an open on the primary. A resonant circuit is formed between the transformers primary inductance and the MOSFET output capacitance. The resonant frequency is calculated by Equation 11.

During the resonant period the drain voltage of the MOSFET rings down toward ground, refer to Figure 6-3. When the drain voltage is at its minimum the flyback MOSFET is turned back on. The point where the voltage is at its minimum is calculated by Equation 12.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 ${\mathrm{V}}_{\mathrm{OUT}}\times {\mathrm{N}}_{\mathrm{AUX}}/{\mathrm{N}}_{\mathrm{S}}$ to $-V_{\mathrm{IN}}\times N_{\mathrm{AUX}}/Np$. Internal to the LM5203 QR pin is a comparator with a 0.35V reference. As the auxiliary-winding voltage falls below 0.35V, the voltage is sensed and the comparator sets the PWM flip-flop turning on the flyback MOSFET. Figure 6-4 shows the QR converter typical waveforms; the auxiliary winding voltage, and primary and secondary current waveforms. It is possible to adjust the delay on the auxiliary winding with a resistor and external capacitor to confirms that the MOSFET switches when its drain voltage is at its minimum. Refer to the schematic in Figure 7-3 and the section on Section 7.2.2.3 for details. The benefits of QR operation are reduced EMI, and reduced turn-on switching losses.