Howto repair switch mode power supplies #5: Magnetic core materials, ferrite vs steel

In the fifth video in the Switch Mode Power Supply (SMPS) repair series we continue with a bit more background in the magnetic core materials. In this episode we focus on the fact of why in the case of SMPS one need to use high frequency switching circuits paired with ferrite core material. Furthermore, the ferrites are directly compared to the traditional humming transformers with iron cores containing silicon. Silicon is added to change the magnetic properties of the cold rolled steel core material, making it electronically less conductive, and also to shift the material towards to be soft-magnetic. The magnetic loss is explained in SMPS in terms of the magnetic hysteresis loops in hard-magnetic vs. soft-magnetic materials. Namely, in SMPS one must use expensive core materials with soft magnetic properties containing rare-earth metals like cobalt, rhodium and neodymium. This is because the magnetic core loss is directly proportional with the operational frequency. Therefore, old low frequency transformers from linear power supplies operating at 50-60Hz can use cheap steel/iron core material, because they operate at the low frequency regime of the power grid. Via experiments the AC sine wave and the alternating current from the electric grid is explained, together with small experiments and figures. From the experiments with a simple mechanical spring, it is easy to see that the smaller the spring, the higher the frequency will be at which it oscillates. This is a very important design consideration also for SMPS, since the smaller the chopper transformer/inductor, the higher the pulse-width-modulation (PWM) must be. Thus, making smaller and lightweight SMPS means, that one must go to higher switching frequencies. Also, this means, that in the case of SMPS one must use complex electronic circuits to feed the small chopper transformer with high frequency pulses, which is not necessary for humming 50-60Hz transformers.