Electronic Components Datasheet Search 

AN701 Datasheet(PDF) 16 Page  Vishay Siliconix 

AN701 Datasheet(HTML) 16 Page  Vishay Siliconix 
16 / 19 page AN701 Vishay Siliconix www.vishay.com 16 Document Number: 70575 16Jan01 The volume of the EF12.6 core is specified as 384 mm3 or 0.384 cm3. Therefore the core loss will be: P = 0.742 0.384 P = 0.285 W [ 1/4 W which represents less than 2% of the total converter losses. The converter transformer can be calculated by using the following adapted version of Faraday’s equation: N + V in min xton max Bmax xAeff Where: 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 1 ms 85 mT x 12.2 mm2 N = 36.64 Turns [ 37 Turns This is the calculated minimum number of turns that should be applied. To determine the turns for a given output voltage, first determine the output voltage (5 V). Then determine all the other losses that will exist in series with the output: including the choke, transformer dc losses, and rectifier forward drop. Assuming these add up to 0.5 V, with a duty cycle of 50%, the transformer will be required to supply a peak voltage of Vsec = (Vout+Vloss)/Dmax Vsec = (5 V + 0.5 V)/0.5 = 11 V The transformer turns ratio is thus determined: for 36V input, 11V output is required. The turns ratio is therefore TR=36/11 = 3.27. For the 5V output, the number of turns required is: NS=Np/TR = 11.01 Therefore, the number of turns selected is 11. The Si9114A will require a few miliamps of current to power itself and drive the power MOSFET. This power can be taken from a winding which peak charges a capacitor through a diode and does not require an inductor. In this case, 11 turns can also be taken (as this was calculated to be sufficient for 11 V). SKIN DEPTH Conductors carrying highfrequency ac current are subject to a “skin effect” in which the current has a tendency to flow predominantly on the surface of the conductor instead through the whole cross section. The value at which the current falls to 1/e (37%) is called the “skin depth.” Below this depth, very little usage of copper is made, and multiple strands are required for applications where higher current is required. In some instances, it may be necessary to use larger wire than indicated, due to mechanical assembly constraints and ease of manufacture. The skin depth can be calculated from: X D + 66 f mm Operation at 500 kHz means that the skin depth will be 0.093 mm (or approximately 0.1 mm). Ideally, a conductor with a diameter just over twice the skin depth is recommended. In this case, therefore, 0.2 mm or thereabouts will suffice. LEAKAGE INDUCTANCE In PWM converters, the coupling factor between windings should be optimized to minimize the leakage inductance. Leakage inductance is a parasitic element, storing energy that will need to be dissipated. This leakage is a measure of the quality of the coupling; the lower it is, the better the transformer and its performance. The leakage inductance can be measured by shorting out the 5V winding and measuring the primary inductance. Ideally, this should be zero, but in reality values of less than 10% of the primary inductance are typical. This value can be minimized by splitting the primary winding in two halves, and by sandwiching all secondaries in between the two primary halves. In this case, the transformer was wound as follows: Winding Order Winding Name # Turns Wire Size Start Pin End Pin 1 Half Primary 17 1x0.3 mm 1 10 2 Bias Winding 11 3x0.3 mm 3 9 3 Secondary 11 3x0 3 mm 4,5 7 6 4 Half Primary 18 1x0.3 mm 10 2 Copper wire of 0.3 mm was used to obtain single layer fill across the surface of the bobbin. The wound transformer had the following characteristics: Winding Inductance DC Resistance Primary 883 µH 151 m W Secondary 87 µH 25 m W The leakage inductance was measured at 2.25 mH, which represents less than 1/4% of the primary inductance. 
