Resistive loads startup

Test your switcher s phase margins with something very close to the final load as early as possible. As mentioned previously, even simple startup problems of switchers do not usually show up with resistive loads. For these, you really need to test using an electronic load, placed at least in constant current (CC) mode. 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

In tray columns the first mechanism is dominant. This can lead to a large number of different steady state solutions for a given set of operating conditions. If N is the number of steady states (typically an odd number). Then (N + l)/2 of these steady states are stable. This can lead to complex multi-stable dynamic behavior during column startup and set-point or load changes. These phenomena were observed for vanishing as well as for finite intra-particle mass transfer resistance. An example with a total number of six trays (two reactive and two non-reactive trays plus reboiler and condenser) is shown in Fig. 10.16 for the well-known MTBE process. In contrast to the previous section, the column is now operated in the kinetic... 

Once an acceptable, stable open circuit potential is observed, the external load is reduced methodically. We generally reduce the resistor by half its resistance for each change. A period of time is required to allow the system to reach a steady-state potential after each load change. This period of time may vary between systems. In this example, the equilibration time ranged from 6 days in the beginning of startup... 

Likewise, SiC has been considered a suitable material for the coolant channels of the blankets of fusion reactors created from a SiC composite (Ward and Dudaev, 2008), and also as a low-activation structural material to protect against the excessive heat loads of the metal first wall of a potential fusion reactor (Hopkins, 1974). The key figure of merit for the latter application is a high thermal shock resistance, R", which is necessary to withstand the stresses introduced by startups and plasma disruptions, together with the thermal cycling associated with normal pulsed mode operation. In this case, Rf = Ob k (l - t lE-a, where CTj, is the flexural strength, k the thermal conductivity, v the Poisson ratio, E the modulus of elasticity, and a the coefficient of thermal expansion. 

Some of the attractive features of PEMFC compared to the other types of fuel cells are (i) compactness and lightweight owing to the use of solid polymer electrol5de membrane (PEM) (ii) operation at relatively lower temperatures (80°C), making them suitable for operations involving frequent cycles of startups and shutdowns, and more responsive to load variations (iii) use of thin membrane resulting in lower ohmic resistance and operations at higher current and power densities and (iv) ease in fabrication of solid polymer in thinner membrane. 

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