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AN701 Datasheet(PDF) 11 Page  Vishay Siliconix 

AN701 Datasheet(HTML) 11 Page  Vishay Siliconix 
11 / 19 page AN701 Vishay Siliconix Document Number: 70575 16Jan01 www.vishay.com 11 The average input current will be determined by: I DC + P in V in + 17.65 48 + 0.358 A I A + P in V in x d + 17.65 48 x 0.376 + 0.98 A From this equation the RMS value can also be calculated to be approximately 0.475 A. The Marcon TCCR70E2A335 3.3 mF, 100Vdc capacitor has an ESR rating of 20 m Ω at 500 kHz. This type will therefore dissipate P = 0.4752 x 0.020 = 4.5 mW due to the switching current. The ripple produced across this device will be governed by the discharging current of the capacitor less the input dc voltage in accordance with: NV ripple + I C xt C where t + tsw x d and I C + IA–IDC NV ripple + 0.612 A x 2 msx0.376 3.3 mF + 0.14 V Q = i x t = C x V 140 mV of ripple is probably acceptable as a first stage of filtering. If lower ripple is required at the input, then a two stage filter will yield better results. Output capacitor: Cout + DI out 8f DV out where DI out + 0.1 x Iout C out + 0.3 A 8x 500 kHz x 50mV + 1.5 mF DVout = maximum output ripple voltage f = operating frequency The required ESR for obtaining 50 mV of ripple would be defined by: ESRmax + DV out DI out ESRmax + 50 mV 0.3 A + 167 mW In practice, it is impossible to precisely match the value of a capacitor with the required ESR, and the values of the capacitors must often be selected to cover all operating conditions including voltage and temperature. The above equations and calculations are meant to help the designer select the approximate size of the components required, with the final selection based on practical values that meet the minimum required. In designs operating below 500 kHz, the choice of the capacitor is dictated by the ESR, and the best highfrequency electrolytics often require largesize and microfarad values to meet these requirements. When operating at 500 kHz, the choice becomes more based on the practical value closest to the size and voltage rating required. For example, with electrolytics, in order to guarantee the ESR over temperature or age, it might have been necessary to use a radial 1000 mF, 6.3V Aluminum Electrolytic in a 10x16 mm case (1257 mm2) to get an ESR value below 100 m W. It would also be necessary to check the ESR with frequency at 500 kHz, as this data is seldom offered for electrolytics. By comparison, the Marcon TCCR70E1E106 10 mF, 25Vdc is available in 7.5 x 6.3 x 2.75 (130 mm2) and has an ESR of less than 15 m W at 500 kHz. This will be ideal for low output ripple an noise. Recently introduced organic semiconductor electrolytics offer substantial improvements and could also be considered. In this example, it was decided to use 2 x 10 mF capacitors in order to obtain low output ripple. OUTPUT INDUCTOR DESIGN The output inductor limits the rate at which the current flows into the output capacitor when the voltage is applied from the primary through the transformer (Figure 21). Figure 21 Cout Eout iL Lout Ein From simple circuit theory, the voltage applied across an inductor is: V L + L di dt where V L + Ein–Eout and di + DI L then L + E in–Eout xDt DI L In forward converters, at maximum duty cycle, Ein = 2xEout, and: t off + 1 2x F SW In this case, substituting gives: t off + 1mS and L + E out xtoff DI L Therefore L + 5V x 1 mS 0.3 A + 16.7 mH In practice, an inductor between 5 and 10 mH would be an acceptable choice, allowing for manufacturing tolerances and variations. The core selected is the EF12.6, which is identical to the core selected for the transformer design. The EF12.6 is a cheap, lowprofile design available from many suppliers in all parts of the world. A surfacemounted version of this bobbin was selected for a design that could be entirely machine wound and terminated. This implies that larger wire sizes are not possible, due to automated winding restrictions. 
