Regulators linear

The linear power supply finds a very strong niehe within applieations where its ineffieieney is not important. These inelude wall-powered, ground-base equipment where foreed air eooling is not a problem and also those applieations in whieh the instrument is so sensitive to eleetrieal noise that it requires an eleetrieally quiet power supply—these produets might inelude audio and video amplifiers, RF reeeivers, and so forth. Linear regulators are also popular as loeal, board-level regulators. Here only a few watts are needed by the board, so the few watts of loss ean be aeeommodated by a simple heatsink. If dielee-trie isolation is desired from an ae input power souree it is provided by an ae transformer or bulk power supply. 

In general, the linear regulator is quite useful for those power supply applieations requiring less than 10 W of output power. Above 10 W, the heatsink required beeomes so large and expensive that a switehing power supply beeomes more attraetive. 

All power supplies work under the same basie prineiple, whether the supply is a linear or a more eomplieated switehing supply. All power supplies have at their heart a elosed negative feedbaek loop. This feedbaek loop does nothing more than hold the output voltage at a eonstant value. Figure 2-1 shows the major parts of a series-pass linear regulator. 

The majority of linear regulator applications today are board-level, low-power applications that are easily satisfied through the use of highly integrated 3-... 

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. 

Linear regulators ean be designed to meet a variety of eost and funetional needs. The design examples that follow illustrate that linear regulator designs ean... 

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. 

Three-terminal regulators are used in the majority of board-level regulator applieations. They exeel in eost and ease of use for these applieations. They ean also, with eare, be used as the basis or higher funetionality linear regulators. 

Figure 2-8 A high voltage floating linear regulator.

Floating linear regulators are partieularly suited for high-output voltage regulation, but may be used anywhere. This regulator ean be seen in Figure 2-9. 

Although pulsewidth modulated (PWM) switehing power supplies have been around for a long time, it wasn t until the mid-1970s that they beeame more aeeepted and broadly applied. Switehing power supplies offer many advantages over linear regulators. 

Switehing power supplies are more effieient and are smaller in size than linear regulators of similar ratings. They are, however, more diffieult to design and radiate more eleetromagnetie interferenee (EMI). 

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. 

Figure 3-47 The high-voltage linear regulator bootstrap start-up circuit (used only at startup and foldback periods).

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