Understanding VDC Output Power Supplies and Their Uses in Electronics Design

October 14, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn about VDC output power supply types.

  • Gain a greater understanding of VDC output power supply applications.

  • Understand the advantages and disadvantages of the different types of VDC output power supplies.

 24 VDC power supply output in a control board.

24 VDC power supply in a control board.

As our devices continuously evolve, so do our requirements for more efficient means to power them. Ever since Alessandro Volta's invention of the battery, we have continuously found ourselves consumed by the conservation, use, and efficient production of energy.

With portability at the top of nearly every device's feature list, it is understandable why we find DC voltage in so many applications. Virtually all electronic devices and products utilize direct current (DC), which makes VDC output power supplies the most widely used. Some of the various circuits that rely on DC include AC-DC converters, DC-DC converters, wall warts, and of course, VDC output power supplies.

What Is VDC, and Why Is It Important?

VDC refers to volts of direct current, and it can come from either a battery or a power supply that converts AC (alternating current) into DC. As its name implies, DC flows consistently in one direction, and we typically supply it via conductors (wires). The most apparent advantage of DC is its stability.

This characteristic is ideal for many applications that otherwise would not achieve functionality without the stability of DC. In summary, many devices like PCs, for example, could not function accurately utilizing AC directly.

Though the power grids in the majority of the countries on Earth utilize AC, consumer electronic devices do not—at least not directly. This is a primary example of why VDC output power supplies are vital.

The VDC Output Power Supply

Typically, a VDC output power supply is a simple AC-to-DC converter that has a power supply voltage of 110 or 220 VAC, and it converts this into a 3V, 5V, 9V, 12V, or 24 VDC. Overall, these VDC output power supplies are available in a variety of configurations, sizes, and output levels.

As I am sure you are aware, DC flows at a consistent rate and in a continuous direction. This type of output power supply is a necessity for devices that cannot function properly on AC's variable voltage. One of the best examples of this is the motherboards found in desktops and laptops, as well as other sensitive electronic devices.

Although a typical desktop VDC output power supply will offer 3.3, 5, and 12 volts of DC to meet the various demands of a PC system, not all VDC output power supplies are equivalent. Keeping this in mind, one must take careful consideration when designing a PCB based on power requirements.

VDC Output Power Supply Types

There are two overall types of VDC power supplies: linear and switch-mode. Although they both supply a VDC power output, they utilize different methodologies in the process. In terms of applications, they each have their advantages and disadvantages.

Functionally, a linear power supply conducts current, whereas a switch-mode power supply converts DC to a switch signal. In a switch-mode VDC power supply, a rectifier produces the DC output voltage. Regarding size, a linear VDC power supply is usually larger and heavier. The size differences of these will often determine which is best for your particular design. 

There are also differences in the way each type handles EMI, power regulation, as well as power handling. In the field of electronics, you will encounter some designs that utilize linear VDC power supplies, however, the majority still use switch-mode types.

The Switch-Mode Power Supply

A switch-mode power supply (SMPS) is used in a wide range of applications due to its efficiency and effectiveness as a power source.  An SMPS's advantages include:

  • Smaller size

  • Lighter weight

  •  More cost-effective

  • More efficient

  • Better performance than a linear VDC output power supply

It is understandable why they are the most widely used, considering the ever-increasing reduction in the size of electronic devices and the increasing demand for portability. Overall, an SMPS is a device that utilizes power semiconductors to convert and regulate energy by continuously switching off and on at a high rate.

The regulation in an SMPS is accomplished via a switching regulator. Also, a series-switching element turns the current supply to a smoothing capacitor off and on. In turn, the voltage on the capacitor controls the turning time of the series-element. Finally, it maintains the required voltage level for the application through the continuous switching of the capacitor.

Types of Switch-Mode Power Supplies:

  • Forward converter

  • Self-oscillating flyback converter

  • Flyback converter

  • DC to DC converter

The Linear VDC Output Power Supply

Characteristically, a linear power supply is better suited for low-noise applications since it is not subject to the high-frequency switching of SMPS. They are in use in applications that require excellent regulation, low ripple, low electromagnetic emissions, and superior transient response. In terms of functionality, a linear power supply will only step−down its input voltage to provide a lower output voltage.

By design, a linear power supply utilizes a large transformer to drop the voltage from an AC source to a much lower AC voltage before using a series of rectifier circuits and filters to provide a very clean DC voltage. However, keep in mind that the trade-offs or disadvantages of a linear power supply vs. SMPS include:

  • Larger size

  • Increased weight

  • Decreased efficiency

Typically, we find linear power supplies in use in medical equipment, communications equipment, low-noise amplifiers, sensors, and analog-to-digital converters.

Overall Advantages and Disadvantages of the Power Supply Types

A switch-mode power supply can be as much as 80% smaller in size and weigh considerably less than a linear power supply. The trade-off is that SMPSs produce high-frequency noise that can interfere with sensitive electronic devices. However, SMPS can tolerate small losses of AC power for 10 to 20 ms without interruptions in its output. 

Since a linear power supply utilizes larger semiconductor devices to regulate its output voltage, it generates more heat and is, therefore, less efficient. This equates to approximately 60% efficiency for its voltage output. In terms of efficiency for an SMPS, it is generally 80% or higher for its voltage output.

Concerning transient response times, a linear power supply is up to 100 times faster than an SMPS, which can be very critical in specific applications.

With companies actively seeking ways to mitigate costs, SMPSs are the power supply of choice with their cost-effectiveness, smaller size, and higher efficiency. Furthermore, today's increasing demands for portability and miniaturization only lead to an increase in the usage of the SMPS. The type of VDC output power supply you choose ultimately depends on your specific design requirements. A linear power supply is better suited for sensitive analog circuity, and an SMPS is best suited for smaller, portable equipment.

Measuring VDC power supply output with a digital multimeter.

VDC output power supplies are present in many different electronic devices.

No matter what power supply you choose for your design, having the right set of design and analysis software is the only way to guarantee a successful implementation. OrCAD PCB designer has a full suite of design and analysis features to ensure your board is done right the first time. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

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