![]() ![]() 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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