Heat buildup in a ferrite is self-limiting. Once temperature reaches the material's Curie point, it looses its magnetic properties, and heat buildup essentially stops. Mind you, this temperature can be as high as 200°C for some power ferrites, so it may get rather toasty.
There is no actual "rating" as such, that I am aware of because there are far too many variables. Hysteresis heating can be estimated from the usual curves of specific power loss-vs-excitation frequency-vs-flux density that most manufacturers provide for each material. But this only tells you how much heat is being generated due to hysteresis losses. It still won't tell you the heat generated by "resistive" losses in a lossy type of ferrite (which is what is commonly used for this purpose). It also won't tell you what the thermal resistance of the ferrite is (i.e. how easily it can lose the heat being generated). The latter is sometimes available for ferrite E-E cores (and other transformer shapes), but for small beads, I suspect there is nothing available. Finally, a lot depends on the harmonic content you are hitting the ferrite bead with, and this is obviously impossible for a ferrite manufacturer to know. The answer, I suspect, is to try it. At least, the thing will not blow itself to "smithereens", but it might get mighty hot before it hits the self-limiting Curie point. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: shbe...@rockwellcollins.com [mailto:shbe...@rockwellcollins.com] Sent: June 5, 2002 10:09 AM To: emc-p...@ieee.org Subject: RE: ferrite transient voltage/current response Sorry that I wasn't clear; I typically try to keep my questions general so not to get too detailed about the specific application. And thanks to Bob, Chris and Mike who have responded ... putting it into Chris's words ... I was just trying to find out if ferrites had ratings to prevent them from "j ust plain blowing the ferrite to smithereens". Also, I was looking for a shortcut if someone else had faced this question rather than reading through all of the vendor web sites. I understand and have used ferrites quite often for typical EMI suppression; the ferrites typically being rated for the application currents, voltages, etc. In this case, the program is trying to protect a power supply input from the DO-160 waveform 5B pin injected lightning pulse of 300 volts open circuit & 300A short circuit. If the Gas Discharge Tube is located past (closer to the supply which was done for packaging limitations) than the "T" EMI filter, a question was raised as to whether the ferrite properties would be altered by the lightning pulse. Most of the standard literature on the use of ferrites does not address these types of transients. Susan Beard "Robert Wilson" <robert_wil...@tirsys.com>@majordomo.ieee.org on 06/04/2002 02:16:48 PM Please respond to "Robert Wilson" <robert_wil...@tirsys.com> Sent by: owner-emc-p...@majordomo.ieee.org To: <shbe...@rockwellcollins.com>, <emc-p...@ieee.org> cc: Subject: RE: ferrite transient voltage/current response Your question is not all that clear. It appears to imply that transients have an affect on the ferrite beads, but it is the other way around (maybe that is what you meant). But in general, small ferrite beads have little effect, except at very high frequencies (hundreds of MHz), unless they are no longer "beads" (i.e. they are very large). Have a look at the various magnetics vendors data sheets and app notes. Magnetics Inc: www.mag-inc.com Fair-Rite Inc: www.fair-rite.com (whoever came up with THAT name should be shot! Steward Inc: www.steward.com Ferroxcube: www.ferroxcube.com Epcos (was Siemens): www.epcos.com Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: shbe...@rockwellcollins.com [mailto:shbe...@rockwellcollins.com] Sent: June 4, 2002 8:57 AM To: emc-p...@ieee.org Subject: ferrite transient voltage/current response Could someone point me to some good App Note information on the response of and affect on ferrite beads to transient voltage & current waveforms? The waveforms are based on the indirect lightning pulses specified in Section 22 of DO-160. Thanks in advance, Susan Beard ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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