Critical Traces

Figure 6-1 Buck from Top to Bottom—Switch ON, Switch OFF, Critical Traces...

Mosfets switch faster than BJTs (bipolar junction transistors). The transition times of a Mosfet can be about 10 to 50ns, as compared to a BJT s typical transition time of 100 to 150ns. But that also makes the spikes far more severe in the case of converters that use Mosfet switches, because of the much higher dl/dt they can generate in the critical trace sections of the PCB. 

Note that while prototyping, it is a bad idea to insert a current probe (through a loop of wire), anywhere in a critical trace section. The current loop becomes an additional inductance that can increase the amplitude of the noise spikes dramatically. Therefore practically speaking, it can often become virtually impossible to measure the switch current or the diode current individually (especially in the case of switcher ICs). In such cases, only the inductor current waveform can really be measured properly. Sometimes we can place a small sense resistor instead of a current loop, because a good resistor will not create inductive kicks at least. 

The first requirement for the designer is to understand the flow of power-related currents in the converter. This leads to an identification of the troublesome or critical traces of the PCB we must pay the closest attention to these traces. We will also see that this identification process is very topology-dependent. So we can t, for example, design the PCB for a buck-boost, the same way we would do it for a buck. The rules change significantly We may thus also realize that very few PCB layout persons out there would understand this too well Therefore it really is a good idea for the power supply designer to do the layout personally, or at the very least, closely supervise the PCB person in the act. 

Figure 6-1 Identifying the Critical Trace Sections for the Three Topologies...

Simultaneous application of standard sequential leaching techniques can be used for geochemical characterization of anoxic, sulfide-bearing sediments in relation to the potential mobility of critical trace metals (Kersten and Fbrstner 1991). 

Crude oil and products of the petroleum industry frequently are subjected to spectroscopic analysis to determine trace metal concentrations. It is important to the industry to know trace element concentrations in crude oil since some trace elements can poison the catalysts used in the cracking process. Some of the particularly critical trace elements are vanadium, copper, nickel, and iron. 

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