Linear regulators operation

If the system eannot handle the heat dissipated by this loss at its maximum speeified ambient operating temperature, then another design approaeh should be taken. This loss determines how large a heatsink the linear regulator must have on the pass unit. 

A quiek estimated thermal analysis will reveal to the designer whether the linear regulator will have enough thermal margin to meet the needs of the produet at its highest speeified operating ambient temperature. One ean find sueh a thermal analysis in Appendix A. 

This drive loss ean beeome signifieant. A driver transistor ean be added to the pass transistor to inerease the effeetive gain of the pass unit and thus deerease the drive eurrent, or a power MOSFET ean be used as a pass unit that uses magnitudes less de drive eurrent than the bipolar power transistor. Unfortunately, the MOSFET requires up to 10 VDC to drive the gate. This ean drasti-eally inerease the dropout voltage. In the vast majority of linear regulator applieations, there is little differenee in operation between a buffered pass unit and a MOSFET insofar as effieieney is eoneerned. Bipolar transistors are mueh less expensive than power MOSFET and have less propensity to oseillate. 

These types of linear regulators were eommonly built before the advent of operational amplifiers and they ean save money in eonsumer designs. Some of their drawbaeks inelude drift with temperature and limited load eurrent range. 

The operation of switehing power supplies ean be relatively easy to understand. Unlike linear regulators whieh operate the power transistor in the linear mode, the PWM switehing power supply operates the power transistors in both the saturated and eutoff states. In these states, the volt-ampere produet aeross the power transistor is always kept low (saturated, low-U/high-/ and eutoff, Hi-T/No-T). This El produet within the power deviee is the loss within all the power semieonduetors. 

The transistor is operating as a starved (highly eurrent-limited) linear regulator. Here the eolleetor resistors dissipate the bulk of the power. The transistor, though, should be able to handle about one watt of dissipation at an ambient temperature of -i-50°C. This dietates that a TO-220 paekage should be used. It must also handle 400 VDC in breakdown voltage. A TIP50 would be more than suffleient for this purpose. 

This circuit is a bridge rectifier followed by a filter capacitor to produce a DC voltage with ripple at Vin. Connected to Vin is a linear regulator made from a Zener voltage reference and an NPN pass transistor. We will first run a Transient Analysis to see the operation of the circuit at room temperature (27°C). To set up a Transient Analysis, select PSpice and then New Simulation Profile from the Capture menus, enter a name for the profile and then click the Create button. By default the Time Domain (Transient) Analysis type is selected. Fill in the parameters as shown in the Time Domain dialog box below ... 

The operation of the buek regulator ean be seen by breaking its operation into two periods (refer to Figure 3-2). When the switeh is turned on, the input voltage is presented to the input of the L-C Alter. The induetor eurrent ramps linearly upward and is deseribed as... 

Ion traps, ICR eells as well as QITs, are best operated with the number of trapped ions elose to their respeetive optimum, beeause otherwise ion trajectories are distorted by eoulombie repulsion. Henee, external ion sourees in eombination with ion transfer opties eapable of eontrolling the number of ions injeeted are ideally attaehed to ion traps. Currently, MALDI [207] and ESI (Fig. 4.54) [192-194,199,208] ion sourees are predominating in FT-ICR work. The ion produetion may either be regulated by the souree itself, or alternatively, by some deviee to eolleet and store the desired amount of ions from that source until injection into the ICR. For that purpose, linear RF multipole ion traps are often employed (Chap. 4.4.6), [118,209] but other systems are also in use. [195] RF-only multipoles are eommonly used to transfer the ions into the cell (Chap. 4.4.4). For the injeetion, it is important to adjust the conditions so that the ions have low kinetic energy in z-direction in order not to overcome the shallow trapping potential. 

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