Design and Implementation of On-board Electrical Power ... - OUFTI-1

There is no data about the case to PBC thermal resistance in the datasheet of theTPS61085. Only the case to air thermal resistance is available but it is useless in vacuum.With no certitude about its capacity to dissipate the produced heat, it was decided to chooseanother IC, i.e. the TPS61087.The TPS61087 is equipped with a “Power PAD”, which allow a good thermal contactbetween the case and the PCB. The junction-air thermal resistance is 30 ◦ C/W when thePower PAD is soldered to the PCB. As the low thermal resistance is essentially due to thejunction-PCB resistance, the thermal resistance will be close to 30 ◦ C/W in vacuum.From “Absolute Maximum Ratings”, the maximum operating junction temperature is150 ◦ C. A margin of 25 ◦ C is taken. The converter is designed to work with a PCB temperatureof -40 ◦ C to 85 ◦ C. The maximum dissipated power isP D = T J − T A 125 − 85= = 1.33W,R ϑJA 30where T J is the junction temperature, T A the air (or PCB) temperature, and R ϑJA thethermal resistance between junction and air.Switching frequencyThe switching frequency f s on the TPS61087 can be chosen to be 650kHz or 1200kHz. Theefficiency can be improved by choosing a f s of 650kHz but the ripple will be more important.The breadboard prototype was designed with f s = 650kHz while the engineering model usesa f s of 1.2MHz. The tests show that the efficiency is practically the same with 1.2MHz as itis with 650kHz.Design of inductor and capacitorA worksheet was written to help with the design of the converters. The inputs are theminimum input voltage V in,Min , the maximum input voltage V in,Max , the output voltage V out ,the maximum output current I out,Max , the expected efficiency η, the switching frequency f s ,the maximum ratio between the current ripple and the average current in the inductor k(∆i L /I av max), and the maximum output voltage ripple ∆v max .There are two steps.In the step 1, the inductance is determined using Eqs. 5.26 and 5.28. The peak inductorcurrent is found with Eq. 5.31 (it is computed for several values of V in in a table and themaximum is kept). The desired series resistance and the output capacitor are also determined.Results for k = 0.3, ∆v max = 10mV, and η = 0.85 are presented in Fig. 5.7. On the basisof these results, an inductor and a capacitor are selected. The inductance and the capacitancewill not exactly correspond to the computed values (real components have standardizedvalues).In the second step, the worksheet is filled up with the values of real components (figure5.9). We obtain the real values of the maximum current in the inductor and of ∆v max .58