Clamps, active

Only a few controller ICs available at the time of this publication directly support an active clamp drive (for example, Texas Instruments UCC3580). There will be more since its function seems to increase the efficiency of a switching power supply by several percentages. 

Making an active clamp from a controller IC that does not support its direct drive depends upon which signals are brought out from the IC. Figure 4—8 gives a couple of ideas. 

At the present, active clamps are still not broadly used because of the difficulty in implementation. This will change as the industry evolves. 

Figure 4-7 An active clamp used in a one-transistor forward or a flyback converter.

Figure 4-8 Designing an active clamp from an 1C that does not drive an active clamp.

The disadvantage of the quasi-resonant converter compared to the newer lossless snubber and active clamp techniques in addition to the basic PWM converters, is the voltage or current stresses placed upon the power components. The peak voltage or current values that exist within quasi-resonant converters can be two to three times higher than in PWM converters. This forces the designer to use higher-rated power switches and rectifiers which may not have as good conduction characteristics. 

In general, it is probably better for the typical application to use the lossless snubbing and active clamp techniques than the quasi-resonant techniques, because of design time and end product cost. 

Power Supply Cookbook, Second Edition has been updated with the latest advances in the field of efficient power conversion. Efficiencies of between 80 to 95 percent are now possible using these new techniques. The major losses within the switching power supply and the modern techniques to reduce them are discussed at length. These include synchronous rectification, lossless snubbers, and active clamps. The information on methods of control, noise control, and optimum printed circuit board layout has also been updated. 

Dekter, J., N. Machin, and R. Sheehy, Lossless Active Clamp for Secondary Circuits, Proc. 20th IEEE Inti. Telecommunications Energy Conf. 1998, October 4-8, 1998, pp. 386-391. 

Mix 100 g. of active alumina with dry benzene until a suspension or slurry of suitable consistency is obtained, and pour this carefully into the tube. Clamp a dropping-funnel just above the top of the tube and Fig 2 benzene to run slowly down as the alumina... 

Researchers at the MoneU Center (Philadelphia, Pennsylvania) are using a variety of electrophysical and biochemical techniques to characterize the ionic currents produced in taste and olfactory receptor cells by chemical stimuli. These studies are concerned with the identification and pharmacology of the active ion channels and mode of production. One of the techniques employed by the MoneU researchers is that of "patch clamp." This method aUows for the study of the electrical properties of smaU patches of the ceU membrane. The program at MoneU has determined that odors stimulate intraceUular enzymes to produce cycUc adenosine 3, 5 -monophosphate (cAMP). This production of cAMP promotes opening of the ion channel, aUowing cations to enter and excite the ceU. MoneU s future studies wiU focus on the connection of cAMP, and the production of the electrical response to the brain. The patch clamp technique also may be a method to study the specificity of receptor ceUs to different odors, as weU as the adaptation to prolonged stimulation (3). 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

Feld, n. field land, ground pane, panel, compartment sphere, scope, -bau, m. agriculture, farming, -beleuchtung, /, (Microa.) ground illumination, -brand, m. (Ceram.) clamp burning, -chen, n. small area, (Bot.) areole, -dichte, /. intensity of field, -dienst, m. active (military) service, -elektron, n, field electron, feldfrei, a. field-free. 

Figure 4. Effects of dihydro-brevetoxin B (H2BVTX-B) on Na currents in crayfish axon under voltage-clamp. (A) A family of Na currents in control solution each trace shows the current kinetics responding to a step depolarization (ranging from -90 to -I-100 mV in 10 mV increments). Incomplete inactivation at large depolarizations is normal in this preparation. (B) Na currents after internal perfusion with H2BVTX-B (1.2 a M). inactivation is slower and less complete than in the control, and the current amplitudes are reduced. (C) A plot of current amplitudes at their peak value (Ip o, o) and at steady-state (I A, A for long depolarizations) shows that toxin-mOdified channels (filled symbols) activate at more negative membrane potentials and correspond to a reduced peak Na conductance of the axon (Reproduced with permission from Ref. 31. Copyright 1984 American Society for Pharmacology and Experimental Therapeutics).

Other neuronal Cl -channels are Ca " -controlled. Increases in cytosolic Ca enhances the probability of these channels being open [26,27]. These channels stabilize the membrane voltage by clamping it towards the Cl -equilibrium potential. Such channels have been found, e.g., in cultured mouse spinal neurones and in molluscan neurones. They subserve the repolarization phenomena and hence assist Ca -activated -channels. Their conductance is in the small to intermediate range. They are usually gated by depolarization. 

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