Switching transitions

So if, for example, the source-to-ground trace is a little too long, it can generate a significant inductive kick at the instant of a switch transition that can at best slow down the transition somewhat, or at worst, produce spurious (unintended) turn-on and turn-off of the Fet, leading to its destruction. 

But we may succeed in pushing this roll-off point further and further away by reducing switch transition times. 

D. M. Jonas, X. Yang, and A. M. Wodtke. Axis-switching transitions and the stimulated emission pumping spectrum of HCN. J. Chem. Phys., 97(4) 2284-2298(1992). 

The three simulators have slightly different switch models. The IsSpice model used is the PSW1 switch. This is different from the built-in switch model, which is basically the Berkeley SPICE switch model with hysteresis. The parameters passed are VON = 7 V, RON = 100 Q, VOFF = 2 V, and ROFF = 100 . The PSpice simulation used a model called Sbreak. Like the PSW1 and the Micro-Cap switch models, this switch transitions smoothly between the on and off states and has no hysteresis. 

The conception of GSD microscopy to reversibly switch the fluorophore to a metastable state has led to the consideration of molecular switches between states of even longer lifetimes. As a matter of fact, the ultimate saturable or switching transition occurs between two stable states [38-41]. The advantage of switching between two stable states is obvious since there are no spontaneous interstate transitions, it follows that Ig —> 0. As a result, it should be possible to implement huge values Imax/Is which, following (19.1), should yield very small Ar even at low /max-... 

However, we must be clear that if and when a converter reaches a steady state, voltage reversal will necessarily occur at every switch transition. 

In Figure 5-6, on the left, we have the basic (simplified) model of the mosfet. In particular, we observe that it has three parasitic capacitances — between its drain, source, and gate. These small interelectrode capacitances are the key to maximizing switcher efficiency, especially at higher switching frequencies. Their role in the switching transition needs to be understood clearly. 

Turning our attention to the circuit shown in Figure 5-6, we should be clear that this circuit doesn t really work We know from our discussions in Chapter 1 that we can never hope to achieve a steady state without at least an output capacitor present — to charge up and thereby help stabilize the voltseconds across the inductor. So this circuit is clearly an idealization — it only helps us to perform a paper-analysis of a particular switching transition. 

A switch transition (crossover) occurs when the switch changes from an on-state (switch closed) to an off-state (switch open), or back. It lasts typically less than 100 ns. But most of the trouble starts right here In fact, the noise has little to do with the basic switching frequency of the converter itself — it is the transition that is responsible for most of the... 

Whichever traces have ONLY Switch-ON arrows OR only Switch-OFF arrows have a very high dl/dt during the switch transition, and are therefore critical. They need to be made wide and short ... 

Why do we even bother to connect the enclosure to earth in the first place In some cases, that is not even considered an acceptable level of protection. And besides, we could achieve two-level protection simply by double (or reinforced) insulation. The main reason for using an earthed metal enclosure is that we want to prevent radiation from inside the equipment from spilling out. Without a metal enclosure, whether connected to earth or not, there is very little chance that a typical off-line switching power supply can ever hope to comply with radiated (and possibly conducted) emission limits. That is especially true when switch transition speeds are dropping to a few tens of nanoseconds. Earthing further improves the shielding effect. 

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