Power Supply Design for 5V: A Historical Motivation
Setting the stage with early logic families.
TTL and the adoption of the 5V standard.
How to design a 5V power supply, from component selection to calculations.
Power supply design for 5V focuses on the ubiquitousness of the voltage in modern electronics
Even casual users are likely to notice the preponderance of 5V ratings on their electronics. After main voltage levels, it is probably the most common voltage users are likely to encounter, and considering the market saturation of devices containing transistor-transistor logic (TTL) ICs, it could be thought of as an international voltage standard. Although 5V has been adopted and enshrined in this position for an extended period, this was not always the case; it wasn’t until the introduction of TTL that the unofficial standard took off.
Unless developing PCBs at extremely high or low voltage levels, it’s a near-certainty that engineers and designers will encounter power distribution networks comprised partially (and sometimes solely) of 5V. Power supply design for 5V is therefore a critical resource to understand both from advancements in digitization as well as active layout.
Logic Families at the Dawn of Digitization
As the electronics industry began the arduous switch from analog components, resistor-transistor logic (RTL) represented the first steps into a new paradigm. As the name might suggest, RTL utilizes a resistor on the input and a bipolar junction transistor (BJT) as the switching mechanism. As IC development was only beginning to take shape, RTL minimized the usage of individual transistors that could be significant cost adders while laying the groundwork for gate-level logic with single- and multi-input NOR gates (one of the two universal logic gates). However, BJTs would offer poor switching speeds and high current demands (and therefore high heat dissipation needs) that would render its relevance short-lived.
Next in the digital electronics lineage would be diode-transistor logic (DTL). As discrete components, DTL is powered by two rails spaced 12V apart (6V and -6V for NPN and 0V and -12V for PNP), but this is reduced to a single rail for ICs. Similar to RTL, DTL can be used to construct a universal NAND gate for gate-level logic design and had many advantages over the preceding RTL format. Additionally, DTL generally had greater interfacing capabilities on I/O than RTL. However, difficulties remained: the switching action was still slow to recover from the two input high state due to a buildup of charge on the transistor base that had to be dissipated. Notably, DTL would be the base case for combination with a Schottky diode to form the Schottky transistor: the Schottky diode prevented saturation of the base by minimizing bias on the collector-base junction and would later be applied more generally to additional logic family ICs.
TTL Introduces the 5V Standard
While RTL and DTL represented important digital milestones, transistor-transistor logic (TTL) was the breakthrough the industry had been building towards. Unlike earlier logic families that relegated transistors to only amplification, TTL utilized a transistor on input as the logic element; along with other design considerations, this led to marked improvements:
Faster switching speeds - The input transistor and amplification transistor are tied together between the former’s collector and the latter’s base. On logic high outputs, the input transistor is forward active simultaneously and conducts the built-up charge on the base of the amplification transistor. This prevents the saturated condition that plagues RTL and DTL switching cycles.
Low power consumption - Both of the input pins require only a small, constant current when active to keep the input transistor running in reverse for a logic low output.
Modularity - An exclusion of the collector-resistor on the amplification transistor yields open-collector output, which allows designers to connect many transistors with a single pull-up resistor to create custom logic circuits whereby a single low TTL gate pulls the combined output low. Another variant is the totem pole, which utilizes a push-pull amplifier to lower the resistance at the output, improving the fanout ability of the IC at the cost of a reduced operating voltage range.
For these reasons and more, TTL technology quickly became the driving development force within electronics of the era, and sure enough, its supply voltage was established at 5V. The exact choice for the value may be attributable to a range of factors, but in terms of logic, it represents the maximum output voltage from a high logic level output. Even with newer technologies like CMOS effectively displacing TTL in performance, TTL’s cheap and reliable nature sees its continued usage as an interface between mismatched IC logic families, further enshrining the 5V as an irreplaceable power rail.
Power Supply Design for 5V From Start to Finish
The importance of a 5V rail even in today’s electronics is clear; therefore, designers need to be able to furnish it to power networks where necessary. Assuming main as a source, a 5V power supply design needs to perform four sequential steps:
Transform the voltage to step it down close to its target of 5V.
Rectify the signal to convert to DC from AC, removing the negative lobes of the sinusoidal wave.
Filter to reduce the effect of ripples on the output power.
Regulate the output at a consistent voltage level.
These four steps typically can be accomplished with a handful of components: a transformer, diode network, capacitor network, and regulator IC. From here, the design will want to begin at the ends, as they represent the overall system I/O. The regulator should be selected to produce the desired voltage level, but the input range will naturally constrain the possibilities of the components occurring before the input. In addition, the operating current may be a factor depending on how large or strenuous the output load is expected to be. Much like the output voltage, the current rating will also be limited by the regulator parameters.
Now, circle back to the transformer. The current should match that of the regulator, as it’s impossible to pass more than what’s available at the output. Additionally, the expected operating current should be well below that of the current rating. The voltage, however, will need to account for drops across the diode, typically 0.6V per diode. The voltage out of the transformer must meet the minimum input range of the regulator plus any diode losses, and designers may want to leave a bit of breathing room by increasing the output voltage of the transformer slightly more (within reason and feasibility).
Lastly, capacitor values need to be selected such that they are rated properly for the input and output voltage of the regulator while providing the necessary capacitance. Design can have a goal of maximum ripple factor at the regulator output, which represents an inefficiency in the AC to DC conversion and detracts from the output voltage. With this value, it is possible to calculate the capacitance using the filter equation.
Plugging in some chosen and calculated values give a target capacitance for filtering.
It’s important to note that this calculated value is unlikely to match that of commonly available values; engineers should choose the closest value approximation.
Cadence Offers a Wealth of Tools to Simplify Power Design
Power supply design for 5V has grown, and despite advancements in technology and reductions in voltages associated with more power-conscious devices, engineers should be confident in their ability to design a functional power supply given a handful of values to target. Fortunately, the details can be left to the computers: Cadence’s PCB Design and Analysis Software suite has all the supporting tools, from simulation to manufacturer information, to assist engineers in optimizing power solutions. Alongside the quick and powerful OrCAD PCB Designer layout environment, development teams can more rapidly move from the conceptual to the physical board.
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