AC-DC Power Supply Guidelines for Beginners

September 23, 2020 Cadence PCB Solutions

Key Takeaways

  • Learn what an AC-DC power supply is.

  • Explore the different types of AC-DC power supplies.

  • Find out key challenges in AC-DC power supply design.

I take pride in being good at assembling furniture. After buying a new chair, I threw the instructions in the trash and started to put the various pieces together. However, after 20 minutes, I swallowed my pride and picked up the crumpled guidelines from the trash. Doing so helped me realize that I could have assembled the chair in half of the time. 

In electronics design, there are times where I rely on experience, and there are times where I accept that having good guidelines helps. Designing an AC-DC power supply is one of those occasions where you’ll want to reference a manual or some other guidelines. There’s just too much at stake to make mistakes with an AC-DC power supply. So, let’s take a look at the guidelines I have laid out for you. 

What Is an AC-DC Power Supply?

A switch-mode AC-DC power supply

A switch-mode AC-DC power supply.

Almost every low-voltage device needs a power supply to function. These devices operate on DC voltage which isn’t directly available from the AC sockets in buildings and our homes. An AC-DC power supply is designed to convert and lower AC mains to a safe and suitable level of DC for low-voltage components. 

An AC-DC power supply can exist as a separate module or be included within the appliance itself. Either way, they operate by converting the AC voltage to the required output level, rectifying the sinusoidal voltage, and smoothing the peak to provide a linear, stable DC output. 

Types of AC-DC Power Supplies

Before you design an AC-DC power supply, you have to know the differences between the numerous variations. 

Isolated vs. Non-Isolated

isolated ac-dc power supply circuit

This isolated power supply uses a transformer to keep AC and DC apart. 

Using an isolated power supply means that the AC input is physically separated from the DC output. However, a non-isolated or offline power supply is connected between the input and output. If safety is a concern, you’ll want to opt for an isolated topology. 

Note that the inclusion of a transformer in an isolated design is known to drastically reduce the efficiency of the power supply. The non-isolated power supply offers increased efficiency but at the cost of reduced safety. 

Some systems, like medical devices, require mandatory use of isolated power supplies. Non-isolated power supplies can be found in less-critical applications, such as lighting and various sensors. 

Linear vs. Switch-Mode

Linear and switch-mode power supplies refer to how the output voltage is generated. In a linear power supply, the AC waveform is rectified, smoothed, and supplied to the DC load. Meanwhile, a switch-mode power supply generates high-frequency voltage pulses with PWM.

If you need a low-cost and ‘clean’ power supply, you’ll want to go for a linear one. However, linear power supplies are not efficient and heat up easily when powering heavy loads. Switch-mode power supplies are more efficient in transferring power to the load but are notorious for the noise generated by the high-frequency switching. 

Buck vs. Boost

You’ll often hear the terms ‘buck’ and ‘boost’ associated with power supplies. The buck topology indicates a step-down power supply, while a ‘boost’ power supply converts the input to a higher output voltage.

If you’re stepping down from AC mains to power low-voltage components, you’ll need a buck AC-DC power supply. 

Challenges in AC-DC Power Supply Design

AC-DC power supply cable on fire

Power surges are a concern in AC-DC power supplies.

It’s a mistake to underestimate the challenges in AC-DC power supply design. Here are some of the common ones that you’ll need to address. 

Switching Noise

For switch-mode power supplies, noise will inevitably be introduced by the regulator. You’ll need to contain the noise and stop it from affecting sensitive traces or components that are powered by the supply. A low pass filter will suppress the high-frequency noise.

Surges

An AC-DC power supply is prone to surges or transients from the AC mains. There are some standards (EN61000-4-4, EN61000-4-5) that regulate how the power supply responds to such events. To protect the power supply, you’ll need to use components like a metal-oxide varistor (MOV) and a gas discharge tube (GDT).

Overcurrent

AC-DC power supplies are designed to operate up to a maximum limit of current. Yet, there are occasions where the load could draw more current than is allowed, whether intentionally or not. You’ll need to ensure that the voltage is reduced to zero in the event of a short circuit or overcurrent. Some power supplies are designed to be driven to exceed the limit for a short duration before the overcurrent protection kicks in. 

You’ll have to make sure you are thinking about safety and compliance when working on an AC-DC power supply, and using simulation-enabled software helps. Cadence PSPICE allows you to simulate the power supply’s behavior in different conditions to help you optimize your design for the desired application. Having the best tools is essential for creating the best, safest designs.  

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

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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