A close-up view of various electronic components arranged on a circuit board, showcasing intricate connections and details.

A Refresher on Basic Power Supplies: Demystifying SEPIC Converters, Part 1 of 4

A power supply is present in every electronic equipment, providing electrical energy at the right voltage and current levels. The primary power source could be a battery, main, wall cube, or something else—but beware: not all power sources are created equal!

When a switched-mode power supply (SMPS) is unstable, it may produce inappropriate voltage or current levels, causing power delivery to fluctuate in response to external conditions. Thus, we need power management blocks (Figure 1) to ensure safe and effective power conversion.

 Power supply block and its benefits

Figure 1 - Power supply block and its benefits

We can use various conversion techniques, each defining the power supply performance in its own way—precision, stability, efficiency, noise, cost, etc. The Demystifying SEPIC Converters blog series will offer a step-by-step guide to designing a single-ended primary inductor converter (SEPIC). Later, we’ll see how this structure is a close cousin to the useful Cuk and Zeta converters.

The journey will be organized into four parts. First, we will refresh the power supply fundamentals by reviewing the three basic types.

The 3 Main Categories of Power Supplies

There are three main types of power supply architectures. Each depends on one of the three components that transfer energy from input to output at a correct and desired quantity: resistor, capacitor, or inductor.

 The elements of energy transfer

Figure 2 - The elements of energy transfer

To obtain the target output (Vout), one must regularly survey the output voltage by comparing it with a stable and precise reference. Adjustments can be made by modifying the resistor R (in the linear category), the charges in the capacitors C, or the current in the inductors L… thus defining the three basic power controller categories.

1. Resistive or Linear Power Supply

We have all the linear power supplies in this category, including low-dropout regulators (LDOs). The linear regulation principle aims to achieve and maintain Vout by reducing the excess voltage from Vin. This excess is dissipated in the control element.

The resistor can be passive or active. If a varying resistor is used, we have a passive structure—a resistance element. If a controlled transistor is used, we have an active structure, such as a bipolar junction transistor (BJT) or a metal–oxide–semiconductor field-effect transistor (MOSFET).

 Linear regulator with BJTs as active element source: ADI

Figure 3 - Linear regulator with BJTs as active element source: ADI

Such a resistive power supply is straightforward to implement, cost-effective, and produces minimal noise since no switching components exist. However, it has poor efficiency because the power lost in the regulator is proportional to the difference between Vin and Vout.

 Low drop-out (LDO) regulator source: ADI

 Figure 4 - Low drop-out (LDO) regulator source: ADI

High-performance linear regulators, like LDOs (Figure 4), are desirable when Vout is close to Vin. However, as with any linear regulator, they can't provide Vout higher than Vin, which is a serious drawback for battery-operated applications.

2. Capacitive or Switched Cap Power Supply

In a capacitive power structure, energy is transferred from input to output by controlling electrical charges in capacitors. This category includes all switched-cap power supplies.

The switch cap principle is illustrated below (Figure 6). The active element is a switch alternately connected to Vin and Vout at a certain frequency. A higher f value lowers the resistance, thus increasing Vout.

 Switch cap principle

Figure 6 - Switch cap principle source: ADI

It can be demonstrated that I = f.ΔQ.(Vin-Vout), where ΔQ is the total charge transferred from Vin to Vout. The association of the switch and the switching frequency form an equivalent resistance of 1/(f.). Being “able” to reproduce a resistor with a capacitor associated with a switch open and close at a certain frequency links with the linear regulator, which is the resistor that regulates the power transfer from Vin to Vout. The advantage of using switches is that such devices have (theoretically) zero power consumption. Figure 7 illustrates a typical example of a Switch cap inverter (Vout=-Vin) without using real resistors.

 Switch-cap inverter block diagram

Figure 7 - Switch-cap inverter block diagram Source:ADI

3. Inductive or Magnetic Power Supply

The inductive category includes over 90% of all power supplies used worldwide. Inductive elements, such as inductors or transformers, control the current stored in them, transferring energy from input to output.

The magnetic mode (Figure 8) is most common. Energy is stored in inductors and released, controlled through switch operations. This method offers superior power performance and efficiency, plus the ability to generate Vout that's either lower or higher than Vin.

 Magnetic SMPS Principle

Figure 8 - Magnetic SMPS Principle

Electrical current is stored in an inductor and collected at the input by an input switching system. Then, another switching system redistributes the energy to the output. The timing of the switches and the structures around the inductor determine how much energy will be transferred. Depending on your objective for Vout, you may choose to configure a buck or boost converter or a buck-boost inverter.

  • Step-down/buck converter: Vout < Vin
  • Step-up/boost converter: Vout > Vin
  • Inverter/buck-boost: |Vout| < or > |Vin|, but opposite polarities

 The 3 basic inductive SMPS

Figure 9 - The 3 basic inductive SMPS

Journey to SEPIC Converters: What’s Next?

Since the SEPIC family falls under inductive SMPS, the rest of the series will focus on inductive switch mode power. Next month, we’ll do a refresher on the 3 fundamental inductive SMPS schemes. 

They are key in our step-by-step journey to our final destination. The intermediate stages will be from inverter to flyback and then from flyback to Sepic.