AN1729 APPLICATION NOTE

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The Power Mos is turned off when the drain current reaches a threshold that is fixed by the controller depending

on the input and the output voltages. As the secondary winding has run dry of energy, the secondary rectifier

At the point where the drain voltage has a valley (a minimum) we can turn on the Power MOS and we can have

wave flyback (see figure 2). This is Quasi-resonant (QR) operation.

Figure 2. Typical waveforms of QR operation

The main advantages of this kind of control technique are:

2

minimizing this kind of losses.

2) Probably the main benefit concerns the conducted EMI emission. In mains-operated applications, due to the

ripple appearing across the input bulk capacitor, the switching frequency is modulated at twice the mains

frequency, with a depth depending on the ripple amplitude. This causes the spectrum to be spread over fre-

quency bands, rather than to be concentrated on single frequency values. Especially when measuring con-

ducted emissions, with the average detection method, the level reduction can be of several dB

µV.

3) Another important benefit is a high safety degree under short circuit conditions: since the conduction cycles

of the Mosfet are inhibited until the transformer is fully demagnetised, flux run away and, therefore, trans-

former saturation are not possible. Moreover, as during a short circuit the demagnetisation voltage is very

low, the system will be led to work at very low frequency, with a very small duty cycle. As a result, the power

that the converter will be able to carry is very low.

4) Finally, the way the system processes power does not change, thus designer's experience with standard

Flyback can be fully exploited and there is very little additional know-how needed.

Sec

ZVS

t

NO ZVS!

D03IN1463