Voltage Regulator Circuit Types and Simulation Tools

July 28, 2020 Cadence PCB Solutions

Key Takeaways

  • Voltage regulator circuits come in a variety of topologies and contain different features.

  • Everything, from basic circuit functions to additional features, needs to be simulated to evaluate functionality before you create your layout.

  • If you want to create a complex voltage regulator circuit, it helps to have your circuit creation tools alongside simulation tools for design evaluation before PCB layout.

Voltage regulator module

This power supply module contains multiple voltage regulator circuits.

Every new electronic device needs some level of power regulation. Whether a new product runs on batteries, an external power supply, or AC mains, you need to devise a regulation strategy for your new system. This can involve multiple power regulation circuits, often with feedback to provide high-efficiency power conversion. You may need a variety of supporting components and features that help you regulate your power output, especially when the system runs at high power.

Everything from VRMs for CPUs and GPUs to simple linear DC regulators needs some level of evaluation before layout, and a SPICE-based simulation package in your circuit design tools will help you spot design problems. There are some fundamental simulations you should perform for your voltage regulator circuits before you create your layout. Here’s what to watch for in your voltage regulator circuit simulation steps and how you can spot circuit design problems.

Voltage Regulator Circuit Types and Topologies

A voltage regulator circuit topology will determine how the device functions and defines where components are placed in the system. Each type of voltage regulator circuit type and topology provides different advantages and disadvantages. Before you begin simulating your voltage regulator circuits, you’ll need to decide which type of regulator you need, the topology, and any other features needed to maintain stable power with high conversion efficiency.

Types of Voltage Regulators

There are two fundamental types of voltage regulators you’ll find in electronic systems:

  • Linear regulator. These regulators use a linear resistive element (e.g., a potentiometer) or a nonlinear element operating in the linear regime (e.g., a MOSFET) to provide stable voltage output. The common types of linear regulators are series, shunt, and low-dropout (LDO). The efficiency of these regulators may range anywhere from 60% to 80%.

  • Switching regulator. These regulators use a switching FET with discharging reactive elements to step up (boost converter) or step down (buck converter) the output voltage as needed for a specific application. These regulators provide high-efficiency DC-DC power conversion with efficiency easily exceeding 90%. A VRM for a GPU or CPU is a common type of switching regulator.

Although these different types of voltage regulator circuit designs offer different efficiencies and require different components, they can be used together in a multi-stage regulator strategy. This type of arrangement can provide higher efficiency power conversion than when a linear voltage regulator circuit is used alone, and it allows the designer to provide power to multiple circuit blocks running at different voltages.

Linear Regulators vs. Switching Regulators

The simplest type of linear regulator is a shunt regulator. This voltage regulator circuit uses a Zener diode to provide feedback between the low end of the load and the high end of the output, thereby providing stable power output. A series regulator is a variation on this, although a bipolar transistor or MOSFET is used on the high side of the regulator to provide stable output. The base/gate of the transistor is connected to the Zener diode in the feedback loop, which then modulates the base/gate current to a stable value.

Linear voltage regulator circuit diagrams

Three common linear voltage circuit diagrams: (Left) shunt, (Center) series, and (Right) LDO.

Switching regulator designs have buck, boost, or buck-boost topologies. The common theme among these circuits is their use of switching transistors to regulate the output voltage and current to specific levels. This requires sourcing the transistors with a PWM signal to modulate the output current. Take a look at this article to learn more about switching regulator design and some important points to examine during a simulation.

Switching regulators and complex linear regulators often include a feedback loop as part of the regulation strategy. For an LDO, the feedback loop consists of a silicon bandgap reference and an op-amp (called an error amplifier in LDO circuit diagrams). For a switching regulator, the feedback loop and control strategy can be as simple as an ADC and MCU to control a PWM signal, or a programmable sensing amplifier for higher power applications.

Feedback and Safety Ratings in Voltage Regulators

In a voltage regulator circuit, feedback is about more than just providing gain to reach the desired output power. It is also about controlling the regulator should the output voltage and current start to deviate from the desired value. A common strategy in high power switching regulators is to use a sensing amplifier and an ADC to quantify the output voltage. This can then be used with a programmed MCU to adjust the PWM signal’s duty cycle that modulates the switching FET.

In a simulation, you typically do not need to examine what happens in the feedback loop directly unless you’re using active components. The key here is to check that the voltage/current through the feedback loop does not exceed component specifications. Components like current sense amplifiers (in switching regulators) or an error amplifier (in an LDO) will have absolute maximum ratings that cannot be exceeded. You should compare these ratings against the simulated voltage/current in the feedback loop and apply the appropriate safety margin in your design.

Making these comparisons is as simple as using DC sweeps and checking when the input voltage places unacceptable loading on your components in your voltage regulator circuit. If your absolute maxima are not reached up to the maximum DC voltage you intend to use, then the components you chose will likely be safe during operation.

For more complex power delivery and regulation systems that will run on AC power may need a power factor correction (PFC) circuit, particularly when a switching regulator is used. This additional circuit is placed after the rectifier and smooths out the stepped-up AC current from the input. If the current draw into the switching regulator stage has unacceptably high total harmonic distortion (THD), then you need a PFC circuit on the input according to the European IEC 61000-3 standard.

PCB circuit waveforms

Waveforms you can expect with low and high power factor. A PFC circuit is meant to take the waveform on the left and smooth out the current drawn by the switching regulator. This produces the graph on the right, although the current has some superimposed ripple.

Power delivery circuits can be complicated in many systems, but you can ease the design, analysis, and layout process for a voltage regulator circuit with the best PCB design and analysis software. The front-end design features from Cadence integrate with the powerful PSpice Simulator for voltage regulator circuit design and simulation, followed by schematic capture and PCB layout. Cadence also has a suite of SI/PI Analysis Point Tools for post-layout verification and simulation. You can easily create and simulate your circuits with design tools from Cadence.

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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