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Power Supplies 11 skills you need to design switch-mode power supplies

From Emmanuel Odunlade

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Switch-mode power supplies can be certified, ubiquitous. Their high-power efficiency and small form factor, among other advantages, have made them the go-to power supply type for designers creating all kinds of devices and equipment.

Tiny DC to DC converters and regulators serve as the backbone of nearly every piece of electronics.
Tiny DC to DC converters and regulators serve as the backbone of nearly every piece of electronics.
(Bild: Adobe Stock)

While switch-mode power supplies have these fantastic features and benefits, one disadvantage typically associated with them is the complexity of their design and implementation. They require optimal attention to detail, be it when designing them.

Tracking all these decisions requires a complete understanding of the elements and can be quite the drag. This article looks at some of the basic things to consider when designing or purchasing a switch-mode power supply.

11 must-know facts about switch-mode power supply

1. Decide on Topology

While choices are usually between the three popular SMPS topologies, Buck, Boost, and Buck-Boost, there are about 13 other possible SMPS configurations, each of which has specific advantages (and disadvantages) that make them good(or bad) for your project. Thus, the first decision you must make is to determine which of these topologies best suits the needs of your project. You must envisage the scenarios around the device to be powered and determine if boost, buck, buck-boost, or any of the other topologies will be the best over time.

2. Output Current

This determines the amount of current that can be sourced through the regulator, and it is important to confirm that the value matches the requirements of the load. In buy situations, ensure to check the datasheet for the values. While it is usually very precise, it is smart to add a safety margin, so your max current draw is always below what is specified in the datasheet.

In design situations, you must ensure the ratings of the components being used match the output current demands.

3. Output Voltage

Switch-mode power supplies usually have variable output voltages. But whatever the case is (fixed or variable), always ensure the power supply is rated for the needs of your application. A lower, higher, or unstable voltage could damage whatever device or circuit that is connected to the power supply.

4. Input Voltage

Whether Buck, Boost, or Buck-Boost, your power supply must be designed with a specific input voltage range in mind, and the effect of that voltage on the output voltage must also be considered. For example, for boost converters, the regulators provide a stable voltage at the output even when the voltage at their input is lower, however, for buck converters, this is not the case as the voltage at the input must be higher than the desired output voltage. This is closely related to the decisions you will have to make as regards topologies.

5. Power Efficiency Requirements

Certain applications have low power efficiency expectations and could be ignored, while others, especially in battery/ Inverter based situations, require optimum power efficiency levels. From component topology to component selection and layout, you must ensure every decision made adds to the efficiency of the system, rather than reducing it.

6. Decide on Switching Frequency

Another important factor to consider when designing an SMPS is the Switching Frequency. It refers to the rate at which the pass transistor is switched "on" and "off" during the pulse width modulation process behind the efficiency of SMPS. It is a fundamental parameter that is intertwined with every part of the Power supply from functionality to size, components selection, and layout.

Typical switching frequencies usually range between 100 kHz to 2 MHz+, with the high side being the most common, thanks to the desire for small form factors and high efficiency. This, however, comes with side effects that include Noise, Potential EMI troubles, and increased occurrence of periodic variations(ripples) in the output. As such, all these effects should be taken into consideration before a switching frequency is selected.

7. Operating Temperature

The temperature of the environment where the device will be used, and the amount of heat that the device generates are key parameters that should be considered when buying or designing a Switch Mode Power Supply.

Knowing the amount of heat the device will generate will come in handy when selecting the heat sink. It will also provide design guidance, so the PCB and device enclosure is designed in a way that allows for maximum ventilation. Some of the electronic components (e.g., Capacitors) used in the power supply also have an operating temperature range. By pre-determining the operating temperature range of the power supply, you will be able to select components that have similar characteristics.

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8. Components Selection

Selecting the right component is critical for the development of a reliable Switch mode power supply. Just like every engineering decision, it is usually a game of tradeoffs, so it's important that all necessary design constraints, like temperature, efficiency, etc., are established before the components are selected.

It's important to also hammer on the value of the components. For example, the capacitance value of capacitors varies as a function of temperature and frequency, so using capacitors with slight differences in value could affect the operations of the power supply.

9. Power and EMI Verification

Switch Mode Power Supplies (and the devices in which they are embedded) have to undergo certification processes like EMC, where the amount of EMI being generated in and around them, is measured to ensure compliance with regulatory standards. This presents one of the biggest challenges with SMPS design as their inherent switching nature means they generate Noise and Electromagnetic interference that require extra design efforts to tackle, to ease product certification.

The process of designing your SMPS to minimize EMI encompasses activities like component selection, PCB design, and even enclosure design. The fact that actual EMI levels can only be monitored after your PCB has been manufactured doesn't really help matters. But paying attention to things like; component selection, component placement/positioning, PCB layout, etc., during the design phase will help you reduce the chances of failing the EMC tests.

10. PCB Area, Layout, and Footprint

While the PCB area and footprint affect the overall size of your project, they are even more critical in the fight against EMI.

To prevent/reduce EMI Propagation, the design layouts and component placement need to be thoroughly considered to reduce current loops and eradicate stray impedance/capacitance that could radiate or conduct EMI. The desired size of the project must also be taken into consideration when making final decisions on things like component selection and switching frequency since higher frequency means a smaller device and vice-versa.

11. Design for Worst-Case Scenarios

Lastly, design for worst-case scenarios. This means evaluating the conditions under which your SMPS will fail or cause the device to which it is connected to fail, and designing the system with a factor of safety that allows the device to continue to perform, even beyond the worst-case scenario. Simulations can be employed to examine how the design will behave under different scenarios.

In conclusion, designing (or buying) an SMPS that suit your specific need is going to be a game of tradeoffs. You must ensure the flip side of every decision you take is properly considered, so it doesn't create unplanned problems.

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