Understanding the Fundamentals of Power Supply Design

Key Takeaways

• Find out why it’s important to get the fundamentals of power supply design right.

• Explore key components of a power supply.

• Learn how to mitigate common challenges in a power supply.

Forget about fancy frying pans, high-tech mixers, or expensive kitchen knife sets. If you don’t know the fundamentals of cooking, for example, how to slice onions or crack an egg, navigating complex recipes will be challenging and possibly result in failure.

Similarly, understanding the fundamentals of power supply design is essential to avoiding failures and ensuring high-speed microcontrollers, memory chips, and logic ICs meet design expectations. In this article, we’ll explore the basic principles of power supply design to equip you with the knowledge you need to secure the reliable performance of your electronic circuits.

Why is it Important To Get The Fundamentals Of Power Supply Design Right?

Power supply design fundamentals are important for load reliability and stability

The power supply is less exciting than a powerful 32-bit microcontroller and other fancy ICs. Yet, if it fails to adhere to fundamental principles, you’ll be in for various rude surprises. For example, a power supply that fails to provide steady voltage can put a microcontroller into a brownout reset. This causes disruption to a microcontroller’s operation, as it keeps resetting whenever the voltage dips below the brownout threshold.

A switching power supply is commonly used in electronics. It features a PWM controller that turns the voltage on and off to provide the required voltage level. The noise from high-speed switching can affect the load circuit if it’s not filtered out properly. 

You may also end up with issues that affect the performance of the power supply itself if you’re not getting the fundamentals of power supply design right. Failing to address thermal management of the power supply may result in overheating. Transient protection, if ignored, leads to damage from surges. To prevent these issues, let’s first review the basic components of a power supply.

The Key Components Of A Power Supply

When designing a power supply, you’ll often come across the following components.

Transformers

A transformer is used in an isolated power supply. It steps down the main AC voltage to a lower value, which is then fed into a rectifier circuit. When designing an isolated power supply, it’s important to choose a transformer with the right turn ratio and current rating. 

Bridge Rectifiers

You’ll need a bridge rectifier to turn AC voltage into DC voltage. A bridge rectifier is made of 4 diodes in the arrangement shown in the image below. A capacitor is placed in parallel to the bridge rectifier output to smooth the DC ripple. 

The bridge rectifier turns AC voltage to DC voltage

PWM Controllers 

If you’re working on a switch-mode power supply, you’ll need to include a PWM controller in the design. The PWM controller is connected to a power MOSFET, which acts as a switch that operates at a specific duty cycle. The PWM controller can be used in a buck, boost, or buck-boost power supply configuration. 

A PWM-driven power supply is more efficient and generates less heat. The output of the MOSFET is a series of high-frequency DC pulses, which are smoothed by an LC filter. The LC filter also acts as a low-pass filter that prevents high-frequency noise from creeping into the load circuit. 

Mitigating Challenges In Power Supply Design

Heatsinks help to dissipate heat from a power MOSFET

Thermal management is important whether you’re designing a linear or switching power supply. While the latter does generate less heat, it’s still important to ensure that the heat dissipates efficiently. Thermal vias and heat sinks are usually used to drive heat off the power MOSFET. 

Ensuring a switching power supply produces minimal switching noise is also a challenge. Besides choosing the right cut off value of the LC filter, it’s also important to reduce parasitic inductance on the connecting traces. This can be done by keeping the traces as short as possible.

The power supply must be designed to operate reliably on the rated current. One important aspect that you need to pay attention to is the trace width. The trace width must be large enough to handle the maximum current without degenerating. The minimum trace width can be derived from the charts found in the IPC-2221 standard.

If you’re armed with sophisticated PCB design and analysis software, you’ll sort out most fundamental power supply design issues easily. With OrCAD, you’ll have a range of handy tools at your disposal. If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.