Most circuits and systems now run on DC-voltage power rails. That simple statement glosses over the wide range of voltages, issues of tolerance and ripple, designations of these rails, and management of both single and multiple rails. This FAQ will explore those uses.
Q: Why are DC rails so important?
A: They are the pathways used to deliver energy to the system, analogous to the flow of blood through arteries, capillaries, and veins within a body. Most electronic functions operate from DC, not AC, with the DC rails derived from other DC supplies (such as batteries) or from the AC line.
Just as with the blood-distribution system, the power rails deliver different amounts of voltage (pressure) and current (volume of flow) via different sized conductors (blood vessels); of course, the total current flow (blood volume) leaves and then returns via a loop to the source. DC rails which are not steady at the right voltage are the equivalent of low or fluttering blood pressure, and will cause system weakness, poor or erratic performance, or even shutdown.
Q: What are some of the common DC-rail voltages in use?
A: Older systems used 48 V and 24 V, both of which are below the human-safety threshold. As time went on, the DC rail voltages dropped to 15, 12, 5, and 3.3 V, and now are at the low single digits such as 1.8, 1.2, and even 0.8 V. The impetus for lower voltages is two-fold: to reduce losses in the power-conversion process, and to allow use of lower-power ICs and circuits.
Q: Are the higher voltages still in use?
A: Absolutely, for both legacy reasons, and due the mandatory needs of some circuits and components. When a system must do real work and deliver real power as defined by physics, the higher voltages allow for use of lower current, and thus lower I2R losses. It’s much more efficient to drive a motor from higher voltage, for example.
Q: So there will be a high, medium, and low voltages in a system?
A: Maybe, or maybe not. Even if a system does not need to use the relatively higher voltages, it may still have many different lower-voltage rails, each one tailored to the needs of specific components and subsections. Also, even a single IC may need multiple rails, such as 3.3 V for its I/O and 1.2 V for its internal operation, and perhaps yet a third voltage for another specialized internal or external function.
Q: How is a lower-voltage DC rail developed? What if there are multiple rails?
A: This is a question with many answers. It is possible to convert higher-voltage DC, such as from an AC/DC converter, into an intermediate value such as 48 VDC, then convert this down 15 or 12 VDC via an intermediate bus converter (IBC), and finally down to lower, final-rail voltages using point-of-load (PoL) converters, Figure 1.
In recent years, due to both efficiency demands and IC advances, there’s been a recent trend to go directly from the higher-voltage DC, such as 48 V, down to the final low-voltage rail, and not use an IBC, Figure 2; this is called “direct conversion (not to be confused with RF-to-baseband direct conversion).
Q: So what is the best topology?
A: There is no “best: the “best” topological solution, requires careful analysis of stage-by-stage losses, number and type of low-voltage DC rails, and physical location of the loads, among many issues. The final decision is a function of various tradeoffs and compromises with respect to system priorities, plus cost, size, and thermal considerations.
Q: How are the various DC rails designated? What nomenclature is used?
A: The answer depends on the point of view. On a full-system schematic with multiple rails, they will likely be labeled simply as V1, V2, and so on, often supplemented with functional callouts such as VBat or VPA. However, on the data sheet for an individual component or subcircuit, there will be designations such as VCC, VDD, VEE, plus others, as well as their lower-case versions, Vcc, Vdd, Vee, and so one.
Q: So what does it all mean?
A: It can be very confusing, for several reasons: first, there are industry standards which define which designations should be used, some for process technology reason (bipolar versus CMOS) and some as holdovers from earlier days of solid-state electronics and transistors. Second, there is a lot of sloppiness and non-adherence to these standards, sometimes even from a single vendor who is not careful about consistency in usage. Third, when engineers talk to each other about voltage rails, they also are often casual about the designations and names they use for the rails. Thus, over time, the naming-convention boundaries have become blurred.
Q: So what are the common designations, according to the standards and common usage?
A: Vcc and Vee are used for bipolar transistors, while Vdd and Vss are used for field-effect transistors (FETs), with Vcc and Vdd for the positive supply and Vee and Vss for the negative supply. These came about due to power connections to collector, emitter, drain, and source terminals.
According to 1963 IEEE standard 255-1963 “Letter Symbols for Semiconductor Devices” (IEEE Standard 255-1963), subscript letters in upper case mean DC (constant) voltage values but not necessarily a supply rail, while lower case indicates a varying, non-constant voltage. Thus, DC supply rails should have upper-case subscripts, but that is often not how it is done in practice. Further, the standard says that supply voltages – not signal voltages – should have a double-letter subscript, such as the very common VCC.
In summary, it is a very confusing topic, with many rules to follow but which are often not followed. When you encounter a designation such as VCC or VDD, the best thing to do is read the data sheet or circuit description, define the required voltages, and be internally consistent in designations and terminology within all your schematics and documentation.
Part 2 of this FAQ will look at the accuracy and ripple goals on lower-voltage DC rails, as well as the management of single- and multiple-rail systems.