Switches duty cycles

Duty types. S, S, and. S5 as discussed in Chapter 3 are normally applicable to crane and hoist motors. For duty types S4 and 5. the duty cycle per unit time is greater than S, . The most important factor is the number of switching operations per hour. A temperature rise in the motor occurs during acceleration, braking and reversing. 

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. 

As seen in Section 4.1, the major types of losses are the conduction and switching losses. Conduction losses are addressed by selecting a better power switch or rectifier with a lower conduction voltage. The synchronous rectifier can be used to reduce the conduction loss of a rectifier, but it can only be used for forward-mode topologies, and excludes the discontinuous boost-mode converters. The synchronous rectifier will improve the efficiency of a power supply about one to six percent depending upon the average operating duty cycle of the supply. For further improvements, other techniques must be pursued. 

What is not clock instability If, for example, you have a Buck with 100% duty cycle, you may find that under transient conditions, the switch will stay completely ON for several cycles. But that is normal. Similarly, on an IC with low-side current sensing, pulses may be omitted entirely under sudden transients. That too is normal. So don t forget to interpret the scope waveforms you see, with due regard to the part s architecture and the applied conditions, not just the topology. 

By the time-sharing principle, we see that in a Buck converter if Vsw is close to VD, the conduction losses do not change with duty cycle or input voltage. But the switching losses progressively increase, and so the efficiency falls off smoothly (almost linearly) with increasing input. See Figure 10-7 for the curve marked Vsw = VD. An example of this is the... 

So in a Buck, even if we have 100% duty cycle (i.e., switch ON for a long time), we will get the output voltage to rise (smoothly). Subsequently, the feedback loop will command the duty cycle to decrease when the required output voltage is reached. 

In our discussion of SMPC circuits, the switching method has been assumed to be hard switching. In hard switching, the current and voltage stresses imposed on the switches and diodes are not considered (or minimized) by the control electronics. The duty cycle or frequency of the system is adjusted solely to maintain the required output voltage, current, or power. Switching losses in these systems can be quite high. 

A limited amount of trapping (either intrinsically or extrinsically) is helpful in that it ensures a deep OFF state of the device during the time at which other lines are addressed that is a (properly adjusted) hysteresis can serve as a memory element over the frame time (Ast, 1982a,b, 1983). This argument is not readily accepted by those who believe that a device must be inherently stable to function in a reliable manner. However, Luo et al. (1983) arrived at an identical conclusion in their analysis of the switching performance of CdSe transistors in matrix-addressed LC displays. It is very important that the trapping be noncumulative. Since the duty cycle of a... 

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