This means the inductance also rises by 25%. Warming the core will change the permeability and alter the inductance value: -Īt 25 degC the relative permeability is 2000 and if, through core losses, the temperature rises to 50 degC, then the relative permeability rises to 2500. However, for an inductor, to remain stable, I wouldn't operate it at a frequency greater than about 300 kHz and this is to avoid warming the core too much. For this material I would want to operate a power application at no greater than 1 MHz - there will be significant warming of the core at this frequency but it should be OK. The solid line is the real permeability and the dotted line is effectively the losses. Take for instance 3F3 material (one I've recently worked with):. You also have to take into account the core material's ability to handle the operating frequency.
So if you have a core that has an ungapped relative permeability 900 and you insert a gap of 1% of the mean length, the gapped permeability would become 90. \$\mu_e\$ and \$\mu_r\$ are the gapped and ungapped permeabilities.
This formula applies to quite small gaps that don't cause much fringing. Where \$l_g\$ and \$l_e\$ are the gap and mean lengths respectively.
A reduction of ten means you need more windings to obtain the same inductance but you only need the \$\sqrt\$ Sometimes (quite often in fact), it is necessary to make a small gap to reduce the permeability by say 10 to 1. It's all about maximising the power throughput for a given core size and operating frequency. However, with EE ferrites you can easily sand down the centre limb (or put thin spacers in) and make fairly accurate gaps but, why might you want to do this I hear people say. One good reason for staying away from toroids is that you can't put gaps into them because they are one solid lump.
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