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

sequently, the k was chosen above 0.4 in our two converters. The more limiting factor wasthe series resistance, which growths quickly when the size of the inductor is decreased (thesection of the wire is reduced).5.3 Design of input filtersAn undesirable feature of switch-mode power converters is their generation of conducted andradiated electromagnetic interference (EMI) on their input at the switching frequency andits harmonics. The power source and other systems supplied by the same power sourcescan be corrupted by EMI currents, if not filtered. Without filtering, EMI will be radiatedby the input line and interference with the operation of near equipments, especially radioequipments.5.3.1 Type of filterThe datasheets of the converters used in section 5.2(TPS63001, TPS61087, and LTC3528)recommend using one 4.7µF to 10µF ceramic capacitor close to the input of the converter.With the resistance of the source and the input line, this makes up a first-order low-pass filter.This may be sufficient in most of the usual applications for these converters, when they aredirectly connected to a battery.In our application, the converters are connected to the batteries bus. The impedanceof the bus seen from a converter is the impedance of the line plus the impedance of all thesystems connected to this bus in parallel (two batteries, five solar panels, two other converters,the EPS2...). As a consequence, the impedance may be quite low and it is difficult to knowthe cut-off frequency of the input filter if it only consists in a capacitor.Therefore, second-order low-pass filters will be used for the converter inputs. Second-orderfilters offer a better attenuation per decade of EMI and their cut-off frequencies can be chosenwith precision.5.3.2 Stability problemAs explained in Chapter 10 of [16], a converter is designed to have an input-to-output transferfunction G vg (s) (the ”audiosusceptibility”) sufficiently small over a wide frequency range. Theoutput voltage is regulated in spite of variations in the input voltage. The introduction of aninput filter will change the dynamics of the converter, often in a manner that degrades theregulator performance. The audiosusceptibility is degraded and there are conditions underwhich the system may even go unstable.The input power of a converter is more or less constant with the input voltage (P in =P out /η), thus one can writeP in = V in I in ⇒ V in = P inI in⇒ dV indI in= −P inI 2 in= −V inI in. (5.35)70