Turn-off spike

Turn-off EL spikes were found to occur 30-40 nsec after the pulse turn-off and the decay of the stationary EL. Therefore, these two phenomena are well separated in time. The observation is also significant because it indicated that the delay between the voltage turn-off and the EL spike is not an artifact of the scope channel synchronization the disappearance of stationary EL marks the synchronization of the light-readout channel. The time at which the turn-off spike reaches its maximum... 

We recorded the EL decay patterns when a voltage pulse of 10 /u.sec duration was applied to the sample at room temperature (Figure 7.6A). We found that decay patterns of turn-on and turn-off EL spikes are similar initial fast-decay part followed by slower decay. The observed curves were fitted to bi-exponential decay law with characteristic times t = 0.07 xsec and t2 = 1.15 /rsec (Figure 7.6B). The drastic difference in time scales indicates the presence of two different mechanisms playing role in transient EL. Thus we may consider two different time scales for counter-field build-up, which presumably controls the EL spike decay. However, these time scales should be the same for the turn-on and turn-off spikes. 

The turn-off spike intensity as a function of the applied pulse width increases with an increase of the voltage pulse width, and it achieves saturation at a certain value, as shown in Figure 7.8. The dependence was successfully fitted with... 

Where S x is the measured maximum intensity of the turn-off spike and independent variable t is the applied pulse width. The best-fit parameters are A = 150, a = 1.4, b = 0.1, to = 50 nsec, x = 63 nsec and r2 = 1150 nsec. Thus, the dependence reveals two characteristic times, that are very close to those found in EL decay upon the voltage switching. 

FIGURE 7.8. Turn-off spike intensity,, S 2max, dependence on the applied voltage pulse width. The solid line represents a fit to bi-exponent. The characteristic times and relative weights are the same, as for the turn-on spike decay. 

FIGURE 7.12. Plot of the turn-off spike intensity (S2) versus integrated EL during the pulse (Si) for different pulse widths. The solid line is a linear fit to the data. 

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. 

The transformer primary side is powered by the battery and turns off at the same time as the HEI coil. The primary carries 5 to 10 amps, conveyed to the secondary side by its collapsing EM field. At the same time, a HV spike is emitted by the HEI coil, from the collapsing voltage in its primary. Because the transformer secondary is shorted by the HV wire, it doesn t interfere with the primary side induced current. The HV and transformer current are therefore additive, creating a much hotter spark. R = 1 M ohm. 

Figure 37.4 Replicate Pseudo-nitzschia multiseries cultures were grown under 24 h light (100 tmol photons s ) at 15 °C for 4 days as an auxostat (chemostat with growth limited by pump rate) with Si-limited f/2 media. On day -4, the pumps were turned off, forcing the cultures into Si-limitation. On days 0, 4 Si-spikes (10 pM) were added. On day 5, trace-metals (f/2 stock) was added, with no apparent response. Optical density (cell abundance) and variable fluorescence were determined from a PAM fluorometer and are plotted versus time (dashed vertical lines indicate additions of unenriched seawater dashed horizontal line indicates maximal Fv/Fm values in healthy cells). A rapid decline and recovery of variable fluorescence indicates impaired photosynthetic performance, and functionally mimics the response of Fe-limitation, with recovery times dependent on the length of time spent in Si-deprived conditions.

At potentials near the photocurrent onset (roughly Fn,), a spiked response is seen with a characteristic overshoot when the light is turned off. At positive potentials near the plateau regime (again for the specific illustrative case of an n-type semiconductor), the response reverts to a rectangular profile that mimics the excitation waveform. Intermediate response patterns manifest at potentials in between. 

Typical light response to a forward bias (when the ITO serves as anode) voltage pulse is shown in Figure 7.2. Three main features of time-resolved EL can be easily seen (a) EL spike after turn-on of the voltage pulse (Ton) (b) EL spike after turn-off of the voltage pulse (Toff), and (c) a stationary EL appears when applied bias is strong enough to excite a dc emission this stationary EL is usu-... 

FIGURE 7.7. Time-resolved EL upon application of reversed bias pulse at different temperatures. Only EL intensity changes with temperature, not the decay characteristic times. In the inset plot of the turn-on (open circles) and turn-off (closed circles) spike intensity versus temperature. The solid line represents the fitting function. 

We recollect from Figure 1-6 that the spike of induced voltage at switch turn-off occurs only because the current (previously flowing in the inductor) was still demanding a path along which to flow — and somehow unknowingly, we had failed to provide any. Therefore nature, in search of the weakest link. found this in the switch itself— and produced an arc across it, to try and move the current across anyway. 

Note We can ask — since the break point associated with the rise and fall times didn t enter the picture here, does that mean that it doesn t matter how fast we turn-on and turn-off the mosfet Yes from the DM noise viewpoint it really doesn t matter much. However there are parasitics that we have ignored (chiefly the ESL and trace inductances). And since, unlike the ESR, these will produce frequency-dependent voltage spikes, it is in our interest not to keep the mosfet crossover (transition) times too small. 

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