Ferrite Toroid

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One needs to make sure the maximum flux density of the core is not exceeded. If one exceeds this value the permeability will go towards one (the permeability of air) and the transformer will no longer function as a transformer. That is, it will cease transfering power.

So, let's calculate the flux density of our ferrite coupling transformer. This is a Magnetics, Inc P material core. Please refer to the chart below provided by this company.

According to this chart, the maximum flux density is 5000 gauss at 25C. At 100C this number will be closer to 3000 gauss.

Now, we need to calculate the flux density of our core. The cross-sectional area of the ZP48613TC is 1.8cm². We have 8 of them for a total cross-sectional area of 14.4cm². We are using between 19-24 turns depending on the heating application. We'll use 20 for convenience. The voltage is close to a pure square wave with a slight ripple voltage, and the RMS value is near 350V. With use a multiplication factor of 4 for a square-wave; we would use 4.44 for a sine wave.

The formula is B_max = E x 10⁸ / (4 x N x F x A)

This yields: 350 x 10⁸ / (4 x 20 x 100,000 x 14.6) = 300 gauss

So, in a worse-case scenario, we still have a 10x safety margin.