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US3004 Datasheet(PDF) 11 Page - UNISEM |
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US3004 Datasheet(HTML) 11 Page - UNISEM |
11 / 14 page US3004/US3005 4-11 Rev. 1.2 12/8/00 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 : L = ESR *C *(Vinm in -Vom ax )/(2* ∆I ) Where : Vinmin = Minimum input voltage For Vo = 2.8 V , ∆I = 14.2 A L =0.006 * 9000 * ( 4.75 - 2.8) / (2 * 14.2) = 3.7 uH Assuming that the programmed switching frequency is set at 200 KHZ , 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 wounded with 8 Turns of # 16 AWG wire, resulting in 3 uH inductance with ≈ 3 mΩ of DC resistance. Assuming L = 3 uH and the switching frequency ; Fsw = 200 KHZ , the inductor ripple current and the output ripple voltage is calculated using the following set of equations : T = 1/Fsw T ≡ Switching Period D ≈ ( Vo + Vsync ) / ( Vin - Vsw + Vsync ) D ≡ Duty Cycle Ton = D * T Vsw ≡ High side Mosfet ON Voltage = Io * Rds Rds ≡ Mosfet On Resistance Toff = T - Ton Vsync ≡ Synchronous MOSFET ON Voltage=Io * Rds ∆Ir = ( Vo + Vsync ) * Toff /L ∆Ir ≡ Inductor Ripple Current ∆Vo = ∆Ir * ESR ∆Vo ≡Output Ripple Voltage In our example for Vo = 2.8V and 14.2 A load , Assum- ing IRL3103 MOSFET for both switches with maximum on resistance of 19 m Ω, we have : T = 1 / 200000 = 5 uSec Vsw =Vsync= 14.2*0.019=0.27 V D ≈ ( 2.8 + 0.27 ) / ( 5 - 0.27 + 0.27 ) = 0.61 Ton = 0.61 * 5 = 3.1 uSec Toff = 5 - 3.1 = 1.9 uSec ∆Ir = ( 2.8 + 0.27 ) * 1.9 / 3 = 1.94 A ∆Vo = 1.94 * .006 = .011 V = 11 mV 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. Dmax ≈ ( 2.8 + 0.27 ) / ( 4.75 - 0.27 + 0.27 ) = 0.65 Pdh = Dmax * Io^2*Rds(max) Pdh= 0.65*14.2^2*0.029=3.8 W Rds(max)=Maximum Rds-on of the MOSFET at 125 °C For synch MOSFET, maximum power dissipation hap- pens at minimum Vo and minimum duty cycle. Dmin ≈ ( 2 + 0.27 ) / ( 5.25 - 0.27 + 0.27 ) = 0.43 Pds = (1-Dmin)*Io^2*Rds(max) Pds=(1 - 0.43) * 14.2^2 * 0.029 = 3.33 W Heatsink 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 °C, then we must keep the junction below this temperature. Selecting TO220 package gives θjc=1.8°C/W ( From the venders’ datasheet ) and assuming that the selected heatsink is Black Anodized , the Heat sink to Case ther- mal resistance is ; θcs=0.05°C/W , the maximum heat sink temperature is then calculated as : Ts = Tj - Pd * ( θjc + θcs) Ts = 125 - 3.82 * (1.8 + 0.05) = 118 °C With the maximum heat sink temperature calculated in the previous step, the Heat Sink to Air thermal resis- tance ( θsa) is calculated as follows : Assuming Ta=35 °C ∆T = Ts - Ta = 118 - 35 = 83 °C Temperature Rise Above Ambient θsa = ∆T/Pd θsa = 83 / 3.82 = 22 °C/W Next , a heat sink with lower θsa than the one calcu- lated in the previous step must be selected. One way to do this is to simply look at the graphs of the “Heat Sink Temp Rise Above the Ambient” vs. the “Power Dissipa- tion” given in the heatsink manufacturers’ catalog and select a heat sink that results in lower temperature rise than the one calculated in previous step. The following heat sinks from AAVID and Thermaloy meet this crite- ria. Co. Part # Thermalloy 6078B AAVID 577002 |
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