Current limit initial

Rapid reinsertion of the capacitors as soon as the fault coiiditions are removed is an important requirement to retain the stability of the system. This can be achieved with the use of an additional ZnO. non-linear resistance (ZnO being the latest in this field compared to SiC. which was used earlier), across the capacitor banks (Figure 26.12). Generally, the ZnO resistor will be adequate to dampen the fault current without initiating the spark gap. and w ill limit the overvoltage across the capacitors. It will also permit automatic reinsertion of the capacitors as soon as the fault conditions are removed without causing a delay. The spark gap will serve as a backup to the ZnO resistor in the event of very severe faults. 

As shown above, potentiodynamic generation of limiting currents is more rapid and, therefore, preferable in principle to the galvanodynamic technique. However, during a linear decrease of potential to the limiting-current condition, the current density initially rises more rapidly than when a current ramp is used. Therefore, in the case of copper deposition at the cathode, a linear potential ramp tends to yield a rougher deposit and a less well-defined plateau than a linear current ramp (see Section III,F). 

The engineer wanted to know what should be the minimum rating of the bench power supply for a Buck application with 5V at the input and 3.3V of output, delivering 1A of load current. I think his question initially arose because he was trying to make a small converter powered off a USB port for driving a peripheral device. He was worried how the current limit of the USB port was going to affect the load current he could draw. 

With the advent of the genome projects, it now becomes possible to explore how nature has achieved functional diversity from a limited set of structural types by examining structure-function relationships in protein superfamilies. Our ability to do this at a sophisticated level is currently limited by the relatively small number of fold classes that have been structurally characterized, but recent initiatives to determine many more three-dimensional structures can be expected to substantially address this deficit over the next few years (Sali, 1998). 

Fig. 1.10 Trend of the percentage of phenol converted to CO2 (open triangle) and to aromatic compounds (open square) during phenol electrolysis in 1M HCIO4 on BDD under (a) current limited control, initial phenol concentration 20mM, current density 5mA cm-2 (b) mass transport control, initial phenol concentration 5 mM, current density 60 mA cm 2...

To protect the switch (and converter) from excess currents, a current limit is usually required. In this case, the current in the inductor, or in the switch, is sensed, and then compared against a set threshold. If and when that is momentarily exceeded, the control circuitry forces the switch to turn OFF immediately for the remainder of the switching cycle, so as to protect itself. In the next cycle, no memory is usually retained of what may have happened in the preceding cycle. Therefore, every switching cycle is started afresh, with the current being continuously monitored to ensure it is at a safe level. If not, protective action is again initiated, and can be repeated every cycle for several cycles if necessary, until the overcurrent condition ceases. 

We realize that to avoid hitting the initial current limit of the device, we need to ensure that the trough falls below 0.75 x Iclim- So... 

Since r in any case is typically set to about 0.4, we should normally have no trouble with this initial current limit issue. However, note that on finer examination of the electrical tables of the datasheet, this 0.75 x factor is specified only at 25°C. Unfortunately, very few power devices stay at 25°C for long So, the bottom line is that, we, as designers, do not really know the value of the current limit as the device heats up. Yes, we can certainly make an educated guess, possibly leave an additional safety margin when fixing r, and certainly, we may face no problem whatsoever. But the truth is we are on our own now — the vendor has not provided the requisite data (in the form of guaranteed limits within the electrical tables). 

Fig. 9.22. Stepped CC (upper curve) and current-limited CV (lower curve) charge algorithms with a current-interrupt finish. Cl can be initiated anywhere between 80 and 100% charge return. The y-axis is charge current in both cases.

---