All electronic devices require DC power to operate.
Many, who have assembled a modern PC are aware of the component diversity found within. Circuit topology identified by names such as TTL and CMOS, are implemented as parts of circuit function blocks such as CPUs, ROMs, RAMs, EEPROMs, and Hard Drives. Each circuit is known to have its particular voltage demands from the power supply. As well each of these circuit blocks may have its own DC-DC converter to make certain that not only does it receive proper voltage, but also to ensure that this voltage is maintained within close tolerances. Deviation from the nominal voltage may result in damage to the circuit or the loss of proper function causing data corruption. Typically these DC-DC converters are of the small step down mono blocks, which plug into a circuit board and supply a defined local circuit. Levels of power normally available from such DC-DC converters are in the 2 to 10 Watt range and their output voltages may have standard values specified by the manufacturers of the types of integrated circuits they are intended to energize. DC voltages of 3.3V, 5V, 12V, 15V, 24V are common.
As with all DC-DC conversion applications, the characteristics of these small printed circuit board step down converters are not only designed to suit their physical operating environment but also the electrical nuances of their intended functions. Over the decades, industry standard sizes have evolved and many are equipped with supervisory functions. An example of one is the “power OK” pin which signals to load circuitry that the supplied voltage is within limits and that it is safe to turn on. Another is the power interrupt pin, which would reinitiate power up after brownout or power loss.
Let’s consider whence computer-on-board DC-DC converters derive their energy. The main computer power supply, which may have anywhere from 3 to 6 output voltages, takes AC power from the 120 VAC power line and converts it to 170 VDC. This level is too high for circuitry to use directly. Therefore, within the power supply is a central DC-DC converter with a much larger power capacity than the small printed circuit mount types. It supplies bulk power at standard levels on main DC voltage buses for distribution to the various computer circuits. This distribution involves power traveling down networks of wires often in ribbon cables. In the course of this process, at the point that such power arrives at the load, its characteristics may be degraded. One of them is voltage regulation. Voltage regulation is a measure of how constantly the voltage is maintained and is affected by two causes:
a) The mere fact that current is traveling down a conductor to its load causes voltage to drop due to the electrical resistance of the wire. This is analogous to water pressure in a pipe, with flowing water. As the distance from the pressure source is increased, the lower will be the flow from a water tap.
b) Each DC power bus which emanates from the central DC-DC converter, supplies numerous circuits. As each circuit pulses its power from this bus, the effect of this will be imposed on other circuits on that same bus. Again an appropriate water analogy is multiple taps being opened at different points in a house. Often the temporary turning on of one tap will be manifest as a temporary drop in water pressure at another. If any or all of the taps in the building are opened simultaneously, the water pressure at each one will decrease.
Besides the need for voltage conversion to uncommon values (e.g. 3.6V), the latter two factors are why the need for localized DC-DC conversion may be necessary.
The internal environmental restrictions of the computer dictate additional measures that need to be taken to make certain that its DC-DC converters operate reliably. A prime obstacle is heat build up, generated by the computer’s internal workings. One internal heat source are the circuits being powered. The make up of the computer determines the extent of its efficiency and wasted heat from its data processing circuitry. However the greatest generator of heat, is the computer’s central DC-DC converter, which is embedded in its power supply. This is the heat generated as 170 VDC coming from the power supply is stepped down into the main DC voltages. The more efficient the computer circuitry becomes, the less power is drawn from the central DC-DC converter. This results in overall power conservation and a decrease in heat accumulation. Based on today’s technology every computer has a fan for heat evacuation, even laptop varieties.
In our next article we will begin to address critical mobile applications for DC-DC converters. Stay tuned.