Electronic Components Datasheet Search

Delete All
ON OFF
 ALLDATASHEET.COM Part No.DescriptionMarking X

## AN701 Datasheet(PDF) 16 Page - Vishay Siliconix

 Part No. AN701 Description reduce the size of energy storage components Download 19 Pages Scroll/Zoom 100% Maker VISHAY [Vishay Siliconix] Homepage http://www.vishay.com Logo

## AN701 Datasheet(HTML) 16 Page - Vishay Siliconix

 16 / 19 page AN701Vishay Siliconixwww.vishay.com16Document Number: 7057516-Jan-01The volume of the EF12.6 core is specified as 384 mm3 or0.384 cm3.Therefore the core loss will be:P = 0.7420.384P = 0.285 W[ 1/4 Wwhich represents less than 2% of the total converter losses.The converter transformer can be calculated by using thefollowing adapted version of Faraday’s equation:N+Vin min xton maxBmax xAeffWhere:Vinmax = minimum input voltage (V)N = number of turns (Integer)Bmax = maximum peak flux (Tesla)Aef = effective area of core (m2)In the case of the EF12.6, the minimum number of turns will be:N+36 V x 1ms85 mT x 12.2 mm2N = 36.64 Turns [ 37 TurnsThis is the calculated minimum number of turns that should beapplied. To determine the turns for a given output voltage, firstdetermine the output voltage (5 V). Then determine all the otherlosses that will exist in series with the output: including thechoke, transformer dc losses, and rectifier forward drop.Assuming these add up to 0.5 V, with a duty cycle of 50%, thetransformer will be required to supply a peak voltage ofVsec = (Vout+Vloss)/DmaxVsec = (5 V + 0.5 V)/0.5 = 11 VThe transformer turns ratio is thus determined: for 36-V input,11-V output is required. The turns ratio is therefore TR=36/11= 3.27.For the 5-V output, the number of turns required is:NS=Np/TR = 11.01Therefore, the number of turns selected is 11. The Si9114A willrequire a few miliamps of current to power itself and drive thepower MOSFET. This power can be taken from a windingwhich peak charges a capacitor through a diode and does notrequire an inductor. In this case, 11 turns can also be taken (asthis was calculated to be sufficient for 11 V).SKIN DEPTHConductors carrying high-frequency ac current are subject toa “skin effect” in which the current has a tendency to flowpredominantly on the surface of the conductor instead throughthe whole cross section. The value at which the current falls to1/e (37%) is called the “skin depth.” Below this depth, very littleusage of copper is made, and multiple strands are required forapplications where higher current is required. In someinstances, it may be necessary to use larger wire thanindicated, due to mechanical assembly constraints and easeof manufacture. The skin depth can be calculated from:XD +66fmmOperation at 500 kHz means that the skin depth will be 0.093mm (or approximately 0.1 mm). Ideally, a conductor with adiameter just over twice the skin depth is recommended. In thiscase, therefore, 0.2 mm or thereabouts will suffice.LEAKAGE INDUCTANCEIn PWM converters, the coupling factor between windingsshould be optimized to minimize the leakage inductance.Leakage inductance is a parasitic element, storing energy thatwill need to be dissipated. This leakage is a measure of thequality of the coupling; the lower it is, the better the transformerand its performance. The leakage inductance can bemeasured by shorting out the 5-V winding and measuring theprimary inductance. Ideally, this should be zero, but in realityvalues of less than 10% of the primary inductance are typical.This value can be minimized by splitting the primary winding intwo halves, and by sandwiching all secondaries in between thetwo primary halves.In this case, the transformer was wound as follows:WindingOrderWindingName#TurnsWire SizeStartPinEndPin1HalfPrimary171x0.3 mm1102BiasWinding113x0.3 mm393Secondary113x0 3 mm4,57 64HalfPrimary181x0.3 mm102Copper wire of 0.3 mm was used to obtain single layer fillacross the surface of the bobbin.The wound transformer had the following characteristics:WindingInductanceDC ResistancePrimary883µH151 mWSecondary87µH25 mWThe leakage inductance was measured at 2.25mH, whichrepresents less than 1/4% of the primary inductance.