Non-Isolated Switching Power Supplies — Topologies & Use Cases

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Introduction

In our previous article, we explored the fundamentals of power conversion in An Insight into Power Topologies and Design Considerations, where we covered the core concepts behind switching power supplies.
Building on that foundation, this article will discuss the further classification of Switching Mode Power Supplies (SMPS). Based on the method of energy flow, SMPS topologies are categorized into various types — each designed to suit specific application needs, performance goals, and design constraints.

In this section, we focus on Non-Isolated Switching Power Supplies, which are widely used in scenarios where input and output share a common ground. These topologies — including Buck, Boost, Buck-Boost, Cuk, and SEPIC — offer flexible solutions for efficient DC-DC conversion across a wide range of voltage requirements.

Non-Isolated switching power supplies

Non-isolated switching power supplies do not use transformers and are ideal for application when input and output share a common ground.

These are classified into several topologies based on the desired voltage conversion:

1. Buck Converter (Step-Down)

  • Function: only step-down voltage (output voltage is lower than input voltage) and are used in DC-DC conversion
  • Typical Use: Converting 12V to 5V to power microcontrollers.
  • Key Components:
    • Inductor and capacitor at the output.
  • Current Behavior:
    • Input current is discontinuous since the switch is in series with the input.
    • When the input current is discontinuous, an input capacitor is required to handle:
      • Compensation for line inductance.
      • High-frequency components of the input current.

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2. Boost Converter (Step-Up)

  • Function: Boost topologies, on the other hand, only step-up voltage (output voltage is higher than input voltage)
  • Typical Use: Boosting 3.7V from Li-ion battery to 5V for USB power.
  • Key Components:
    • Inductor at the input, capacitor at the output.
  • Current Behavior:
    • Since the inductor is in series with the input, the input current is continuous over most of the operating range.
    • The output current is discontinuous, so the output capacitor must:
      • Handle worst-case ripple current.
      • Meet hold-up time and stability needs.

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3. Buck-Boost Converter (Step-Up / Step-Down)

  • Function: can both step-down and step-up voltage, allowing the output voltage to be either higher or lower than the input voltage.
  • Typical Use: Stabilizing output (e.g., 5V) from a variable input (e.g., 3V–12V).
  • Design Note:
    • The Buck-Boost converter is a DC-DC converter that is equivalent to a flyback converter but uses a single inductor instead of a transformer.

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4. Cuk Converter

  • Function: Cuk converters can both step-down and step-up voltage.
  • Unique Feature: Output polarity is reversed compared to input.
  • Typical Use:
    • Bipolar supplies, e.g., audio amplifiers.

5. SEPIC (Single-Ended Primary Inductor Converter)

  • Function: The SEPIC allows the output voltage to be equal to, lower than, or higher than the input voltage without polarity reversal
  • Typical Use:
    • Battery-powered devices such as LED drivers.

For more on different Switching Power Supply topologies, please check out our next article.

Related Article

An Insight into Power Topologies and Design Considerations