Using a Switching Regulator vs. Linear Regulator for DC-DC Conversion

June 8, 2020 Cadence PCB Solutions

Key Takeaways

  • Linear regulators are simpler regulators that step down the input voltage. The goal is to set the output to a specific DC voltage.

  • Switching regulators provide much higher efficiencies, but they can be more complex and create switching noise.

  • The two types of regulators can still be combined to provide stable power output at a desired voltage and current.

Switching regulator vs linear for motor control

This motor control board could benefit from a switching regulator vs linear regulator.

When most designers talk about power regulation and DC-DC conversion, they focus on efficiency and talk about switching regulators. When a switching regulator vs. linear regulator are compared, this makes sense; for low-level circuits, highly efficient switching regulators are available as ICs. So where do linear regulators fit into the DC-DC conversion landscape, and how do you need them for your power regulation strategy?

In general, whether you’ll use a switching regulator vs. linear regulator, or a combination of cascaded regulators, depends on the nature of the unregulated source. If you design your power regulator specifically to accommodate the behavior of your power source, you can reduce your component count and system complexity. Here’s how each type of regulator plays a role in DC-DC conversion and how you can design circuits to accommodate unregulated power sources.

Switching Regulators vs. Linear Regulators in Power Conversion

Switching regulators and linear regulators are used in a variety of systems, and multiple regulators can be cascaded (i.e., placed in series). Once you have converted to a high DC voltage, it is common to use another switching regulator/VRM to provide the desired output voltage to a specific circuit block. Alternatively, you can provide a stable output with some headroom using a linear regulator on the output stage. This is the typical spot to place a low-dropout regulator. 

This type of regulation strategy, where multiple regulators are placed in series, is quite common and provides several advantages over a single regulator. A block diagram showing this example strategy is shown below.

Switching regulator vs linear for motor control

Example power regulation and DC-DC conversion strategy with switching regulator vs. linear regulator circuits.

Adapting to AC Sources

The above strategy is designed to convert between unregulated DC and regulated DC, but it could also be used with an AC line input. To do so, simply place a full-wave diode bridge at the input to the first switching converter. 

Switching converters can induce harmonic distortion on the unregulated input AC current, which drops the overall efficiency of the regulator. Therefore, a power factor correction (PFC) circuit is used to smooth out the AC current spike and make the input AC current appear sinusoidal with some ripple. The use of PFC circuits is required under European EMC guidelines and helps reduce excess power draw from mains.

Converter Arrangement

The first converter stage in the above diagram is usually a switching regulator. This is used because the converter typically needs to step down a high voltage signal to a moderate or low voltage level. The converter also needs to be configured to have high PSRR in order to provide maximum noise/ripple suppression in the relevant frequency range.

The output converter in the above strategy can be a switching regulator or linear regulator, depending on the exact power requirements, any control mechanism in the first converter stage, and the behavior of the unregulated source. 

In terms of ripple noise, linear regulators tend to provide ripple suppression over a broader range of frequencies, making them useful for suppressing broadband noise from, say, an upstream regulator. This is one reason a linear regulator is often used on the output in the above strategy.

The type of linear regulator normally used on the output is an LDO regulator. These regulators use an op amp to set the regulator’s output to a desired level as long as the input voltage is above the headroom for the regulator. A switching regulator can also be used on the output, again depending on the step-down level needed and whether the signal being input into the final regulator will vary and whether it includes a control circuit.

LDO regulator circuit diagram

Typical LDO regulator circuit. This circuit can be used on the output stage of a power regulator to compensate for drops in the input power level.

Switching Regulator vs. Linear Regulator Comparison

As the first converter typically provides a large step-down in the input voltage, it is best to use a switching regulator in this stage. This is because a switching regulator is very efficient, as shown in the table below. The three common topologies for switching regulators are buck (step-down), boost (step-up), or buck-boost (configurable by adjusting the duty cycle of a PWM signal). As mentioned above, the final regulator stage could be a linear regulator or LDO regulator.

 

Linear regulator

Switching regulator

Efficiency

Low (typically 60% to 70%)

High (typically 95%)

Control method

Passive or active an op amp

PWM signal

Polarity

Same as input voltage

Reversible

Scaling

Step-down

Step-up or step-down

Max. voltage output

Low

Moderate to high

PSRR

Broadband, up to ~70 dB depending on frequency

~50 to 100 dB, depending on frequency

Noise

Low frequency noise that matches input ripple

- 10-1,000 kHz noise due to the PWM signal and switching.

- Ripple on the output.

The flowchart below shows a typical power regulation strategy where an LDO is used on the output of a power converter. The switching converters step down the unregulated input and suppress low frequency ripple. The second switching converter will output a voltage that is just above the headroom of the LDO, and the LDO will output the desired voltage level.

LDO regulator circuit diagram

Typical LDO regulator circuit. This circuit can be used on the output stage of a power regulator to compensate for drops in the input power level.

The feedback circuit on the second regulator can compensate for any upstream changes in the input voltage level. In this case, when the voltage output drops below a chosen level, the feedback circuit increases the PWM duty cycle in the second regulator, which compensates any voltage decrease on the regulator output. This is common when the unregulated source may come from a battery, electromechanical inverter, or other source where the input varies over a large range.

Use the Right Design Software for Power Electronics

Whether you’re designing a power converter from discrete components or using multiple ICs, you should employ verified component models in your schematic design software. 

These models allow you to simulate your new system directly from your schematic with SPICE-equivalent models for components. This gives you a much more accurate view of how your system provides power conversion, as well as a way to see ripple on the output from both types of regulators. You can easily create multiple regulator stages as hierarchical schematic sheets and simulate an entire system or individual regulator subcircuits.

When you’re designing a power conversion system and weighing the use of a switching regulator vs. linear regulator circuit, you can design your schematics and simulate your system with the best PCB design and analysis software. The front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for designing and simulating power systems. Once you’ve created a layout and are ready to examine noise and thermal behavior, you can use Cadence’s suite of SI/PI Analysis Point Tools for post-layout verification and simulation. You’ll have all the features you need to design stable power electronics systems.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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