The Turn-off Transition

Since logic level thresholds are steadily getting lower, we can no longer afford to ignore the saturation drop across the pull-down transistor of the driver stage. We call this Vsat (typically 0.2V). 

Vgs falls exponentially. The equation given here satisfies the required boundary conditions. 

Neither Id nor Vd change here because Vgs is still high enough and so the Mosfet is fully conducting. 

During turn-off, the Drain current cannot change unless the switching node (and thus in effect Vd too) goes completely to Vin, and thereby forward biases the diode (ignoring its forward drop), allowing it to start sharing some or all of the Drain current Id. 

Vgs is fixed at Vt+lo/g (Miller region). So current through Cgs is zero. 

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Now, suppose we had ramped up the gate voltage at a rate of 1 V per second as before, but ramped down faster, say, at the rate of 2 V per second. Then the turn-on time and the turn-off transition times would be different. So in that case we need to split up the crossover... 

If we do a similar analysis for the turn-off transition (right side of Figure 5-5), we will see that for the switch current to start decreasing by even a small amount, the diode must first be positioned to take up any current coming its way. So the voltage at the switching node... 

This results in watts for the loss seen only during the turn-on transitions of the power switch. One would add the turn-off and conduction loss to this amount to arrive at the total loss within the power switch. 

Since both ends of the primary winding have a single-ended loaded winding during their respective turn-off transitions, each of the MOSFE R accomplish ZVS turn-off. The output rectifiers gain some efficiency since their current transitions appear more zero-current switching in nature. 

What are the reasons for clock instability High-frequency noise is always generated at turn-on and turnoff in any switcher. This noise can infiltrate into the IC via various pins. It can be very hard to filter out and control. You may need to ultimately simply avoid turning the Fet OFF too dramatically. In most switchers, the turn-on transition is traditionally delayed (or slowed) just a little, so as to allow the output/catch diodes to recover... 

Most calcium channels become activated and inactivated in what appears to be the same way as sodium channels, but in the case of the most common type of cardiac calcium channel (the "L" type), the transitions occur more slowly and at more positive potentials. The action potential plateau (phases 1 and 2) reflects the turning off of most of the sodium current, the waxing and waning of calcium current, and the slow development of a repolarizing potassium current. 

An elementary treatment of a three-level system under pulsed excitation was given in Sec. II.C. Pollack treats the steady-state condition and the turn-off condition as well. These cases are quite interesting and deserve further discussion. Figure 51 shows the three-level system he analyzed. The quantities Ni(f), N2(f), and N3(f)are the electron-distribution functions (populations) and a9 b, and c represent the total transition probabilities per unit time. The boundary condition Nx + N2 + N = JV =constant is assumed. 

Note Here we seem to be indirectly suggesting that the drive resistance is the same for tum-on and turn-off. That need not be so. All the equations we will present can easily take any existing difference in the turn-on and turn-off drive resistances into account. So in general, we will have different crossover times for the turn-on and turn-off transitions. Also note, that in general, within a certain crossover interval (tum-on or turn-off), the actual time it takes for the voltage to transit need not be the same as the time the current takes (unlike the case of a resistive load). 

Finally, with all this background information, we can start looking closely at what actually happens during the turn-on and turn-off transitions. 

Figures 4.8 and 4.9 illustrate the measured and the calculated results of the switching circuit. The switching operation can be simply expressed by the simple model, although the time delay at the turn off caimot be reproduced by the model. The model is accurate enough if the switching frequency is much lower than the transition frequency fj of the transistor.

For the power switeh ae node, the voltage wants to be delayed on the turnoff transition. This provides loading of the magnetie element during the forward reeovery time of the output reetifler. For the output reetifler ae node, the eurrent wants to be delayed at its turn-off. This limits the refleeted eurrent spike eaused by the reverse reeovery period of the reetifler. These teehniques are shown in the following seetions. 

Like other voltage-gated cation channels, Ca2+ channels exist in at least three states A resting state stabilized at negative potentials (such as the resting potentials of most electrically excitable cells) that is a closed state from which the channel can open. The open state is induced by depolarization. Channels do not stay open indefinitely because they are turned off during prolonged depolarization by transition into an inactivated state. Inactivation is driven both by depolarization... 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

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. 

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