Power Supply Design for 12V: Challenges and EMI Reduction Tips

March 25, 2020 Cadence PCB Solutions

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

For further discussion, what thermal analysis and regulations could you put in place to regulate your power supply?

 

Power supply on a printed circuit board

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

 

I’m a self-help junkie and digesting motivational quotes in the morning helps to kickstart my day with a bang. One of my favorites that helps me to break an impasse sounds like this ‘if an opportunity doesn’t knock, build a door.’ I try to take this same positive approach in electronics design: often waiting for your boards to tell you what’s wrong with the design will result in a cost-inefficiency and a waste of prototyping runs. 

While I don’t literally build a door on my workstation, I do occasionally need to design a dedicated 12V power supply on the PCB. It’s true that there are ready-made power supply modules, building one alongside the main circuit is often more economical and space-saving. 

Building a 12V Power Supply

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. Most systems convert the incoming AC into 12V DC before stepping down further into 5V, 3.3V or 1.8V

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

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 fluctuation of the voltage. The choice of the DC regulator determines the output load that can be handled by the power supply

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. 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. 

Protecting The 12V Power Supply

The 12V power supply is the first module that comes in contact with the mains. This means that 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.

 

 Printed circuit board on fire

The power supply input is susceptible to transient events.

 

It’s also important to prevent the output of the power supply 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 an SMPS. I once failed a CE test because of radiated emission from a 3rd party switch mode power supply. 

EMI radiations occur 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, it is recommended to place a mirror plane or trace on the opposite PCB layer of the current loop. It’s also important to keep the current loop as small as possible. 

 

Capacitors laying atop a blue printed circuit board

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 and lead 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 it should be shielded against the noisy nodes on the circuit. 

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

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

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