Guys, What Don says is correct; but I don't know if it directly hits where the confusion comes from.
I think that this is the confusion. The inductor is modeled as an ideal inductance in series with its Equivalent Series Resistor (ESR). The voltage across the ideal inductor is Vind=L*(di/dt)...this voltage can be very high, if the rate of change of current is high. This voltage is zero for a DC circuit The voltage across the ESR is simply Vesr= I*Resr...They are using rms values; so they're saying that Vrms = Irms (Resr). This voltage is non-zero whenever any type of current flows through the inductor. The total voltage across the inductor is Vind + Vesr The power dissipated in the inductor is Pind + Pesr. The Pind term is usually purely reactive (no heat); however, there are core heating effects which I'll mention later. The current limit (on the spec sheet) of the inductor usually has more to do with the saturation limits of the core...i.e., the core will saturate at currents above the specified current limit...than with the power dissipation of the inductor. Sometimes, you have to worry about whether you are saturating the inductor...that's where you look at the specified current. Sometimes you have to worry about whether you are overheating the inductor...that's where you look at ESR and calcuate Irms...although the spec sheet that started this thread seemed to specify a Vrms...that's OK, but you have to realize that it's just the voltage across the ESR...not the voltage across the whole inductor "model" which is the ESR and an ideal inductor. As far as peak voltage (based on L*(di/dt)...that's usually a problem for parts around the inductor. The inductor itself won't breakdown until the voltage is so great so as to breakdown the insulation between the windings...however, usually some other component in the circuit that's connected in parallel with the inductor will breakdown first. An example of this is a snubber circuit in a flyback power supply. Without the snubber, the voltage on the primary winding (an a form of inductor) will go through the roof during the flyback phase. The primary winding will be fine with that...but every FET, diode and cap connected to it would blow to bits...if you didn't put a snubber on the circuit. You can also overheat the core of an inductor by generating eddy currents and other heat generating phenomena within the core itself...This is usually proportional to the area within the B-H curve that you would draw for your operating conditions. For a good design, you would have to ensure that you are within the boundaries under all four conditions... 1...staying within the power dissipation limits through the ESR this is usually listed in Watts...... although it could be given as an rms current limit or rms voltage limit...with both assumed to be across the ESR. 2....staying within the peak AC voltage limits so as to not blow up parts around the inductor or breakdown the insulation of the inductor itself 3.... keeping the inductor out of saturation. 4....ensuring that you don't overheat the core due to B-H effects (I've never seen this to be a problem for power supply or filter circuits...mostly for AC power transformers) For most inductors, you've satisfied this if you just stay out of saturation. The usage of the inductor (filter, power supply ...) usually dictates that one of these limits will be bumped into first. To directly answer the original question, they were specifying only the voltage drop across the ESR...not the voltage across the "ideal inductor" part of the inductor model. It's part of the picture; but not the whole picture. So, if you wanted to use this inductor in a filter...the peak voltage across the whole inductor could indeed exceed the voltage drop across the ESR that was specified. Just check all four conditions listed above; and see if you're OK. I hope this helps. Chris Maxwell, M.Sc Electrical Engineer Critical Imaging LLC - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
