How to Drive a Relay with a MOSFET

It is very common application to drive a relay with a MOSFET. From household applications to automotive are using this circuit. There are some aspects to consider like the stress on the MOSFET, the level of driving voltage to guarantee MOSFET saturation, the level of the actual voltage applied to the relay coil if enough to meet the requirement and

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Transformer saturation waveform

Quasi Resonant Flyback Transformer Saturation Analysis

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 Operation Mode

Quasi Resonant Flyback Operating Modes

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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