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

Figure 5.20: Computed Bode diagram of Z D (ω) and Z N (ω) for the 7.2V converter.10dBΩ. The most intriguing difference is that the minimum of the measured Z in occurs ata frequency of 350kHz while the minimum of Z D obtained with Eq. 5.39 is located at 30krad/s (4.77kHz).It is very unlikely that such a difference is due to a measurement error. One possibleexplanation is that the controller design and the switching frequency have an influence onthe input impedance (Middlebrook’s equations do not take them into account). No obviousexplanation was found concerning this difference.The input filter on the engineering model will be designed from the measured curves. Thestability of the converters with these filters will be experimentally verified.5.3.5 Measurements of the input impedance of the convertersMeasurements on first prototypeThe input impedance of the three converters was measured on the “breadboard” prototype.The measurements were made in the conditions under which the converters have the lowestZ in (f), i.e. for the maximum load and the minimum input voltage for the boost converters,and for the maximum load and maximum input voltage for the buck converter. We producedfour measured plots, i.e. for• the 3.3V converter with V in = 2.5V and R = 36Ω (Fig. 5.22).• the 3.3V converter with V in = 4.2V and R = 36Ω (Fig. 5.23).• the 5V converter with V in = 2.5V and R = 24Ω (Fig. 5.24).• the 7.2V converter with V in = 3V and R = 17.2Ω (Fig. 5.25).73