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IRU3004 Datasheet(PDF) 11 Page - International Rectifier |
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IRU3004 Datasheet(HTML) 11 Page - International Rectifier |
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11 / 17 page  IRU3004 11 Rev. 1.7 07/16/02 www.irf.com Output Inductor Selection The output inductance must be selected such that un- der low line and the maximum output voltage condition, the inductor current slope times the output capacitor ESR is ramping up faster than the capacitor voltage is drooping during a load current step. However, if the in- ductor is too small, the output ripple current and ripple voltage become too large. One solution to bring the ripple current down is to increase the switching frequency, however that will be at the cost of reduced efficiency and higher system cost. The following set of formulas are derived to achieve the optimum performance without many design iterations. The maximum output inductance is calculated using the following equation: Where: VIN(MIN) = Minimum input voltage Vo = 2.8V , DI = 14.2A Assuming that the programmed switching frequency is set at 200KHz, an inductor is designed using the Micrometals’ powder iron core material. The summary of the design is outlined below: The selected core material is Powder Iron, the selected core is T50-52D from Micro Metal wound with 8 turns of #16 AWG wire, resulting in 3 mH inductance with ≈ 3mV of DC resistance. Assuming L=3 mH and Fsw=200KHz (switching fre- quency), the inductor ripple current and the output ripple voltage is calculated using the following set of equations: In our example for Vo=2.8V and 14.2A load, assuming IRL3103 MOSFET for both switches with maximum on- resistance of 19m V, we have: Power Component Selection Assuming IRL3103 MOSFETs as power components, we will calculate the maximum power dissipation as fol- lows: For high-side switch the maximum power dissipation happens at maximum Vo and maximum duty cycle. For synchronous MOSFET, maximum power dissipa- tion happens at minimum Vo and minimum duty cycle. Heat Sink Selection Selection of the heat sink is based on the maximum allowable junction temperature of the MOSFETS. Since we previously selected the maximum RDS(on) at 125 8C, then we must keep the junction below this temperature. Selecting TO-220 package gives uJC=1.88C/W (from the venders’ data sheet) and assuming that the selected heat sink is black anodized, the heat-sink-to-case ther- mal resistance is uCS=0.058C/W, the maximum heat sink temperature is then calculated as: 1.9 3 1 200000 2.8 + 0.27 5 - 0.27 + 0.27 T = = 5 ms Vsw = Vsync = 14.2 3 0.019 = 0.27V D ≈ = 0.61 TON = 0.61 3 5 = 3.1ms TOFF = 5 - 3.1 = 1.9 ms DIr = (2.8 + 0.27) 3 = 1.94A DVo = 1.94 3 0.006 = 0.011V = 11mV Ts = TJ - PD 3 (uJC + uCS) Ts = 125 - 3.82 3 (1.8 + 0.05) = 1188C L = 0.006 390003 = 3.7 mH 4.75 - 2.8 2 314.2 ( ) L = ESR 3C3 VIN(MIN) - Vo(MAX) 2 3 ∆I ( ) T ≡ Switching Period D ≡ Duty Cycle Vsw ≡ High side Mosfet ON Voltage RDS ≡ Mosfet On Resistance Vsync ≡ Synchronous MOSFET ON Voltage DIr ≡ Inductor Ripple Current DVo ≡ Output Ripple Voltage TON = D 3T TOFF = T - TON DVo = DIr3ESR DIr = (Vo + Vsync)3 TOFF L D ≈ Vo + Vsync VIN - Vsw + Vsync T = 1 Fsw Vsw = Vsync = Io 3RDS DMAX ≈ = 0.65 PDH = DMAX 3 Io2 3 RDS(MAX) PDH = 0.65 3 14.22 3 0.029 = 3.8W RDS(MAX) = Maximum RDS(ON) of the MOSFET (125 8C) (2.8 + 0.27) (4.75 - 0.27 + 0.27) DMIN ≈ = 0.43 PDS = (1 - DMIN) 3 Io2 3 RDS(MAX) PDS = (1 - 0.43) 3 14.22 3 0.029 = 3.33W (2 + 0.27) (5.25 - 0.27 + 0.27) |
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