Pros and cons of linear regulators, SMPS

Article By : Henry J. Zhang

Power management design is becoming a challenging task for system designers, many of whom may not have strong power backgrounds.

Today’s designs require an increasing number of power rails and supply solutions in electronics systems. So, it’s important to choose the appropriate solution for the targeted application and to meet specified performance requirements, such as high efficiency, tight PCB space, accurate output regulation, fast transient response, low solution cost, etc. Power management design is becoming a challenging task for system designers, many of whom may not have strong power backgrounds.

Depending on the specific application, a designer can choose either a linear regulator (LR) or a switching mode power supply (SMPS) solution. To make the best choice, it is essential to be familiar with the merits, drawbacks and design concerns of each approach.

In addition to simplicity of use, linear regulators have other performance advantages. A typical 3-pin linear regulator-the LT1083 was developed more than 20 years ago by Linear Technology. It only needs an input capacitor, output capacitor and two feedback resistors to set the output voltage. Almost any electrical engineer can design a supply with these linear regulators. But, a major drawback of using it can be the excessive power dissipation of its series transistor Q1 operating in a linear mode.

For applications that require more power, the regulator must be mounted separately on a heat sink to dissipate the heat. In all-surface-mount systems, this is not an option, so the limitation of power dissipation (1W for example) limits the output current. Unfortunately, it is not easy to directly parallel linear regulators to spread the generated heat.

A quick answer is high efficiency. In an SMPS, the transistors are operated in switching mode instead of linear mode. This means that when the transistor is on and conducting current, the voltage drop across its power path is minimal. When the transistor is off and blocking high voltage, there is almost no current through its power path. So the semiconductor transistor is like an ideal switch. The power loss in the transistor is therefore minimised. High efficiency, low power dissipation and high power density (small size) are the main reasons for designers to use SMPS instead of linear regulators (LDOs), especially in high current applications.

In summary, linear regulators are simple and easy to use. Since series regulation transistors are operated in a linear mode, supply efficiency is usually low when output voltage is much lower than input voltage. In general, LDOs have low voltage ripple and fast transient response. On the other hand, SMPS operate the transistor as a switch, and thus are much more efficient than linear regulators. However, the design and optimisation of SMPS are more challenging, and require more background and experience. Each solution has its own advantages and drawbacks for specific applications.

Read more: Download the full application note

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