Duty cycle limiting

MOSFET driver output (totem-pole) 50 percent duty cycle limiting... 

Selecting the SMPS controller IC. The important factors within this application that affect the choice of switching power supply controller IC are MOSFET driver needed (totem-pole driver), single-ended output, 50 percent duty cycle limit desired, and current-mode control desired. The popular industry choice that meets these needs is the UC3845B. 

Note however, that this is the operating DMAX.When we power down our converter for example, the duty cycle will actually increase further in an effort to maintain regulation (unless current limit and/or duty cycle limit is encountered along the way). Then depending upon the number of missing ac cycles for which we may need to ensure regulation (the holdup time specification), we will need to select a suitable input capacitance and also the maximum duty cycle limit, Dlim, of our controller. Typically, DLim is set around 70%, and the capacitance is selected on the basis of the 3 ptF/W rule-of-thumb. For example, for our 74 W supply with an estimated 70% efficiency at low line, we will draw an input power of 74/0.7= 106 W. Therefore we should use a 106 x 3 = 318 p,F (standard value 330 iF)... 

The second consideration is that even if, for simplicity, we assume zero forward voltage drops across both the switch and the diode, we still may not be able to deliver the required output voltage — because of maximum duty cycle limitations. So for example, in our case, what we need is a (theoretical) duty cycle of Vo/Vin = 14.5 V/15 V = 0.97, that is, 97%. However, most buck ICs in the market are not designed to guarantee such a high duty cycle. They usually come with an internally set maximum duty cycle limit ( Dmax X typically around 90 to 95%. And if that is so, D = 97% would be clearly out of their capability. 

Combinations of magnetic and electrostatic analyzers have been used to make instruments with high resolution capable (with appropriate calibration) of accurate mass measurements. Because they are scanning instruments, magnetic sector analyzers suffer from duty cycle limitations, as do quadrupole analyzers. Furthermore, because the scan speed of the magnetic field is proportional to resolution, additional time is required to obtain a spectrum as resolution is increased. Scanning the... 

For general purpose instruments (scenario 2), the most important parameters are resolution and accurate mass measurement for the identification and characterization of unknowns. Typical are MS/MS systems based on TOE or FT (orbitrap or ICR). The major categories here are TOF/TOF, QTOF, LT-TOF, LT-orbitrap, and LT-FT-ICR. For analytes of >1 kDa, TOF/TOF systems in combination with MALDI sources are straightforward and should be evaluated where multiple users are involved. QTOF, LT-TOF, and LT-orbitrap are all suited to LC operation at regular or nanoflow rates. The resolution of orbitraps is much higher (250,000) than that of TOF instruments (20,000 to 60,000), although available resolution on the former is subject to duty cycle limitations. Table 3.13 lists representative examples of the types of instruments one might consider for common applications. 

The DMA is a narrow band mobility filter that needs to be scanned to cover a desired mobility range. It therefore has a duty cycle limitation similar to that of a gated IMS instrument coupled to a steady ion source. For a pulsed ion source such as MALDI,(5 ) however, pulsed IMS would use the available ions more efficiently than the DMA, and the duty cycle would be determined by the repetition rate of the source. For a given mobility, however, the DMA has near unit transmission and is far more efficient than gated IMS with steady ion sources. 

Duty cycles limited by hysteresis of magnetic field... 

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