Power Supply Design for 12V: Challenges and Tips

Key Takeaways:

• Discuss the components and factors for building a DC power supply
• Understand what threats there are to your power supply
• Minimize potential threats to your 12v power supply such as EMI

There are formidable challenges in building a power supply design for 12V.

If you’re working on commercial products like alarm systems, data acquisition, or audio interfaces, you’ll often have a 12V supply connected to the primary control modules. While there are ready-made power supply modules, making one alongside the main circuit is often more economical and space-saving. The table below lists the components you’ll need for power supply design for 12V products.

Building a 12V power supply may seem straightforward, but you could run into some unexpected issues if you overlook some important points. Generally, you could classify a 12V power supply by its linear or switch-mode operation.

• Linear 12V Power Supply: A linear 12V power supply is easy to build, as it involves a transformer, diode bridge circuit, a 12V DC regulator, and bypass capacitors to minimize voltage fluctuation. The choice of the DC regulator determines the output load that the power supply can handle. While DC power supplies for 12V are easy to build, they can be incredibly inefficient when driving a large load. The regulator gives off tremendous heat when the load current increases. 

• Switch-Mode 12V Power Supply: This is where a switch-mode power supply is often a better alternative. As the name implies, a switch-mode power supply (SMPS) features a power management IC that alternates the state of the DC voltage with high-frequency switching. Therefore, a 12V SMPS power supply is more efficient but at the cost of an increased risk of EMI emission. 

Power Supply Design for 12V: Surge Protection

The 12V power supply is the first module that comes in contact with the mains. This means it stands at a high risk of being damaged in the event of a transient surge if left unprotected. A fuse and varistor are often placed at the mains input of the power supply for overcurrent and overvoltage protection.

The power supply input is susceptible to transient events.

It’s also essential to prevent the power supply output from being damaged by a load short circuit. A fuse is handy in breaking up the connection should the current exceed the maximum limit of the output. 

Minimizing EMI from a 12V Power Supply

You don’t have to worry about EMI if you’re designing a linear 12V power supply. However, EMI is a concern when you’re dealing with a SMPS.

EMI radiation occurs on the current loop where the voltage switches between the on and off state. This is usually traced to the input pin of the power management IC, the ground pin, and the capacitor that connects both. To minimize radiation, placing a mirror plane or trace on the opposite PCB layer of the current loop is recommended. It’s also important to keep the current loop as small as possible.

Capacitors filter high-frequency noise from the output.

A fast rise time of the switching voltage leads to higher radiation. Adding a resistor in series with a capacitor on the boot pin of the power management IC can increase the rise time, thus reducing EMI radiation. However, you shouldn’t overdo this measure, as the SMPS’s efficiency will be compromised, leading to thermal issues.

Making good use of capacitors at the output of the 12V power supply will stabilize the voltage and reduce the chances of noise creeping into the load circuit. The feedback pin on the switching IC is extremely sensitive to noise and should be shielded against the noisy nodes on the circuit. 

Cadence OrCAD X Can Help with Power Supply Design

Your task in a power supply design for 12V is easier with simulation and layout tools from a considerate and advanced suite like Cadence. OrCAD X helps eliminate uncertainties and reduce the chances of post-prototype problems.