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It does not have to be 50%
However, you need to understand that the primary of the transformer is applied the entire input voltage during the transistor on time and the voltage is allowed to "fly-back" during the off period, or flyback action.
But during the two intervals the volts*seconds applied to the primary MUST be equal, or the core will saturate.
During the flyback period (off time), the voltage across the primary is the voltage reflected back to the primary from the secondary with the turns ratio, by transformer action. Hence, you have control over this flyback voltage by adjusting the turns ratio. So you could make it higher and the off time shorter, until you exceed 50% duty-cycle.
But, remember that the volts*seconds has to stay the same, whatever it was during the on time. Thus, if you make the duty-cycle too high, the flyback voltage will increase. But the transistor must withstand this voltage, added to the input voltage. This means that you will be stressing the transistor more, so you want to keep the maximum DC below 50% in order to keep the voltage across the transitor at acceptable levels.
Another reason has to do with the secondary current. Depending on the mode (continuous or discontinuous current), the secondary current is a trapezoid of a triangle. However, its average MUST always equal the output DC current. That means that if you make the duty-cycle larger, the off time needs to get shorter, but the peak secondary current has to get higher, for the same output current. You can end up with very high peak secondary currents. Flybacks already suffer from high peak secondary currents (especially the DCM one) so there is little advantage to making the duty-cycle much larger than 50%.
All that being said, there are situations when you must allow the duty-cycle to increase over 50% a(not much, though) and you design everything accordingly.
may be it is very old post, but gives me very good sense while reading it.
I need small clarification regarding above discussion.
When flyback is used as buck converter, then above analysis is crucial regarding dutycycle < 50%.
When flyback is used as boost converter, then above analysis is not that much crucial to limit dutycycle< 50%.
Reason behind this is, in boost topology secondary ratio is more compare to primary. So, induced back EMF is less in boost topology.