In order to derive the primary current of a Quasi resonant flyback, it is important to know the waveform of the current. Quasi resonant flyback primary current is rectangular in shape. It will rise linearly when the flyback switch is ON while drop immediately to zero once the switch is turned OFF. Below figure do more the explanations. Quasi Resonant
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One of the very important aspect to look at in dealing with quasi resonant flyback transformer is the saturation. A quasi-resonant flyback transformer is not actually a transformer that we usually know. A transformer will transfer energy from the primary to the secondary at real time. However, the flyback converter will store energy on the primary and then transfer that energy to the secondary somehow. This action makes a flyback transformer prone to saturation if not properly been designed.
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Quasi Resonant flyback duty cycle will reach maximum when it operates in the transition mode or boundary region. However, as long as the operation will stay at the discontinues mode (DCM), that is the desired region of operation, the Quasi resonant flyback duty cycle will be variable.
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If you want to make a quasi resonant flyback but do not know where to start, therefore this Quasi Resonant Flyback Step by Step Design Guide is perfect for you. Follow below steps.
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Each of the quasi resonant flyback operating modes discussed above has its strength and weaknesses. It is up to the designer to which mode to select. In my personal experience, I already used both the DCM and TM operating modes. Basically, a quasi resonant flyback will operate in DCM at light load and approaches to TM at very heavy load. During the design stage, the designer will set the operating point in first valley switching for heavier loads for optimum performance.
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A quasi resonant flyback converter is a variant of flyback converter wherein it makes use of the parasitic elements to partially resemble a resonance action. According to dictionary, the word quasi could mean partly, partially, comparatively or relatively. So, a quasi-resonance means a partial resonance, not a total resonance. At light load, Quasi resonant flyback operation will be in DCM. However, it will come closer to the boundary or transition mode while in full load.
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