The Key Fundamentals of Power Supply Design for Circuit Boards

September 12, 2019 , in Blog

One of the most fundamental laws of physics is the Law of Conservation of Energy, which can be summarized as follows:

“Within a closed system, energy cannot be created or destroyed, but only change form.”

Basically, this can be interpreted as an isolated system, which does not interact with any outside force, retains a constant level of internal energy. This premise has been the catalyst for many schemes to build self-sustaining energy systems that could last perpetually. So far, completely isolating a system so that no energy is gained or loss has proved to be difficult. That means that systems that require energy need to be periodically recharged, just as we do.

Energy-depleted man at work

In need of recharging

Power supply circuits are the source of recharging for electronics systems and circuit boards. Some boards contain power supply sub-circuits; however, it is common for PCBs to serve as power supplies, as well. These boards are actually converters, as they convert an input energy source to an output that meets the requirements of a load, system or circuit. Irrespective of source and load requirements, it is always important to make the building of your board an integral part of the PCB layout design for your design. First, let’s discuss the different types of power supply circuits and then we will define the fundamentals of power supply design that should be applied for their development.

Types of Power Supply Circuit Boards

Being converters or bridges between an input electrical source and an electronic load, power supply circuits can be classified into one of the groups of the table below.

Types of Power Supply Circuits

Outputs

AC OutputDC Output
AC Input Isolation, Frequency ConverterRectifier
DC InputInverterDC-DC Converter

As shown above, power supply circuits are basically used to change energy from one state to the other, AC to DC or vice-versa, to change levels, raise or lower voltage, or frequency. AC-AC power supplies may also be used to isolate input circuits from outputs. In addition to the types above, power supply circuits can be categorized as either regulated or unregulated. Regulated power supplies include devices to maintain the output voltage level. These voltage regulators are not present in unregulated power supplies and the output varies with the input and changes in load current.

Power supply circuits are also classified according to their operation. The two basic operational types are linear and switching or switch-mode.

Linear Power Supply

schematic of regulated linear power supply

Linear power supply schematic example

The Linear power supply above is used to convert a mains AC input, the primary side of transformer TR1, into DC for distribution. This circuit includes a voltage regulator, IC1, that will provide a constant voltage irrespective of the load, R1. This linear power supply demonstrates the basic operation of these circuits, which may have many different configurations. Linear power supplies are typically used in lower power systems. The advantages are their simplicity, low cost, reliability, and low noise; however, they are inefficient which becomes more of a concern in higher power applications.

Switch-Mode Power Supply

The alternative to using a linear power supply is a switch-mode power supply or SMPS, shown in the figure below.

switch-mode power supply schematic

SMPS power supply schematic example

An SMPS power supply contains switching circuitry; such as transistor T1 above, that converts the rectified DC from the bridge circuit, B1, into high-frequency AC. The frequency level is determined or set by the control signal that turns the transistor on and off. In the circuit above, the output is smoothed or regulated by the LC filter before being applied to the load, R1. Typically, SMPS circuits are more complex than linear power supplies and the switching introduces noise that can create EMI which may affect your trace routing during PCB layout. However, these power supplies are more efficient and can utilize smaller components than linear power supplies. SMPSs are most often for digital systems.

Fundamentals of Power Supply Design

When designing an SMPS or a linear power supply circuit board, there are common areas of concern. These include thermal considerations, EMI or noise and depending on power level copper weights. Another important consideration is the power supply filter design. Although your specific design requirements will dictate specific design choices, there are general fundamentals of power supply design for PCBs that should always be followed, as listed below.

  • Optimize your filtering design

The performance of your filtering circuitry depends upon selecting the appropriate component values for filter components, inductance, capacitance, and resistance. As actual component values available may not match calculated values you should the combination of component values that provide the best response as determined through simulations.

  • Choose appropriate copper weights

Power supply currents may be quite high; therefore, it is imperative to ensure that trace widths and copper thickness or weights are able to carry the needed currents. It is also important to ensure that your layout adheres to clearance tolerances as stipulated by your contract manufacturer’s (CM’s) DFM rules.

  • Match your material selection to board type

For high power circuits, make sure that your board can withstand the temperature levels that will be generated by choosing materials with an appropriate coefficient of thermal expansion (CTE). For SMPSs, if a high-speed design then properties such as dielectric constant, dk, dissipation factor, df, dielectric loss, conductor loss, Ploss, become important and should guide your material selection.

  • Ensure that your board includes adequate thermal dissipation

One, if not the greatest, concern for power supply boards is removing excess heat. It is critical that your design includes adequate techniques for thermal dissipation. For example, the use of thermal pads and heat sinks. In contrast, it is also important for PCB assembly that your board have adequate thermal resistance in order that good solder joint quality can be achieved.

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By instituting the fundamentals listed above into power supply design process you will be able to identify and address areas that significantly the manufacture of your boards. Tempo Automation, the industry leader in fast, high-quality PCB prototype and low-volume manufacturing, can assist you in ensuring that design meets all the requirements for the best construction of your boards.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the fundamentals of power supply design for circuit boards, contact us.

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