Laying Out the Printed Circuit Board

Sometimes it is desirable to turn off a seetion of the produet but leave other portions operating. This is typieally the ease in laptop eomputers for power savings or eonsumer applianee applieations, sueh as CRT monitors, televisions, and stereo equipment. 

Many eonsumer applianees must plaee themselves in the standby mode when the user has walked away from the produet. This type of operation is ealled green eertified. This eertifieation dietates that the produet must use less than 5 watts of power while in the standby mode. The switehing power supply alone would have quieseent operating eurrent greater than 5 watts, so a very low power switehing power supply, with mueh lower quieseent eurrents, ean be used to power only that portion of the produet needed to reaetivate the produet upon demand. 

In the ease of power faetor eorreeted ae power supplies, the main eonverter ean be turned off and the eireuit to remain aetive eould use the auxiliary voltage on the power faetor transformer. The power faetor eireuit would run at a mueh redueed pulsewidth and/or frequency. 

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In a practical implementation, we have to design the converter, select the switch, and then lay it all out on a printed circuit board (PCB) with great care — to ensure in particular that there is no voltage spike that can kill the switch (nor any other semiconductor devices present on the board). Occasionally, we may therefore need to add an external snubber or clamp across the switch, so as to truncate any remnant spikes to within the voltage ratings of the switch. 

The bus usually looks like a printed circuit board covered with connectors. Most of the lines are parallel and run from one connector to the other. Adapting resistors exist at the line extremities for a correct line termination. The processor, memory, and I/O board plug into these connectors and are maintained by a chassis. Indirect connectors are preferred for industrial applications, but PCs still use direct connectors, which allows for a reduced board height for the same complexity. Indirect connectors have up to 5 rows of pins, suitable for wide buses (many data/address lines) and a dense array of ground lines in between. The card size depends on the bus objective 32-bit buses need a lot of interface circuitry, and large boards are more efficient. Personal computer buses have narrow boards that are difficult to lay out (Fig. 12). 

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