# determining the Bmax for inductor design

at this moment of this youtube video following graph is presented for selecting the proper maximum flux density.

the considered proper amount of core loss is 100mW/cm^3. is this a general amount for all cores or does it change according to each material datasheet? should it be selected according to the following information presented in the datasheet of another material about core loss?

Generally the limiting factor is the temperature of the core.

The heat generated within the volume of the core is just one factor. To this we have to add ...

• heat generated in the windings
• ambient temperature
• surface area of the core
• core ventilation, is it fan assisted or natural?

The figure of 100 mw/cm3 is therefore a crude guideline, assuming reasonable defaults for all the other factors. It's a good place to start to see whether a design is feasible.

Once a design looks feasible, then you have to go into the detail of what core material, and do some cooling experiments on the shape and size of transformer you will be using, taking account of your maximum local ambient, and the maximum temperature your core materials, wires, insulation, glues, tapes will tolerate for the lifetime you want to specify.

• finally, what can we get from the presented core loss (Pcv) which is in the range of 400 to 600 kw/m^3? it is already so much higher than the value of 100 even though ignoring the factors you've just mentioned. Jun 17 at 14:40
• Pcv isn't good, or bad, or a target, it is what it is, at that frequency and flux swing. It's the size and cooling of YOUR transformer configuration and materials that will tell you whether it's good or bad. Jun 17 at 15:21
• @KMoradi You've read the graph correctly. For the core material used, 300 gauss at 500 kHz will get you 100 mW/cm3. Now, whether 100 mW/cm3 is right for your transformer configuration, or is too hot, or you could run it hotter, is another matter. But it's probably a good place to start. Jun 17 at 16:55
• How can I determine the amount of temperature increase due to this amount of core loss? Build a core and measure it. There are no tables. Software tools that can do a decent job of simulation are uber expensive. The 80 degrees mentioned is the temperature at which the core material is being tested. The energy dissipated is very temperature dependant. Operate the core at different target temperatures, you will get different heating for the same flux and frequency. Jun 17 at 19:25
• @KMoradi The suggestion of 100mW/cc is a guideline. A thermal analysis is either done experimentally (easiest & most accurate for your conditions) or through FEA modeling. Many MnZn ferrites have an upper operating limit of 100°C and a thermal conductivity ranging from 3.5e-3 to 5.0e-3 W/(mm K°). The max temp point is the middle of the center leg. Don't get hung up on core loss as copper loss can be an overriding factor. Proximity effect loss (one of the copper loss mechanisms) can be a significant source of heating if you're winding geometry is non-ideal.
– qrk
Jun 17 at 19:40