A variable power supply allows the user to vary and adjust the desired output voltage and current. This type of power supply circuit includes an adjustable voltage regulator that can be set according to a specified input. An adjustable voltage regulator features both line regulation and load regulation. 

Unlike fixed power supplies that provide a constant output voltage, variable power supplies can be adjusted to different output voltages as needed. In contrast, fixed voltage power supplies or battery chargers provide a single, unchanging output voltage and are designed for specific, consistent power requirements. Read on as we discuss variable power supply PCB layout tips.

Major Circuit Blocks for a Variable Power Supply in PCB Layouts 

Component or Circuit Element	Purpose
Transformer	Steps down the AC mains voltage to 12V AC.
Full Wave Rectifier	Converts AC voltage from the transformer to DC voltage with ripple.
Capacitors	Filter out ripples from the rectified DC voltage to produce a smooth DC output.
Main Variable Positive Voltage Regulator	Provides an adjustable positive output voltage. Depending on topology can include (or exclude)PWM Timer ICFixed voltage regulator/referenceFeedback control IC
Pulse with modulation (PWM) / Timer IC	Operates as an astable multivibrator to generate clock pulses for switching speed (can be independent of regulator IC or part of it)
Fixed Voltage Regulator / Reference	It is used to supply a reference voltage. Some designs use a negative voltage regulator at -12V, for example, while others may use a positive version.
Feedback Control Mechanism (e.g.) CMOS Decade Counter)	Activates outputs to control voltage levels. Can also be as simple as a potentiometer.

Creating a Variable Power Supply

There are a couple of different topologies that can be used to achieve a variable power supply. One example involves selecting between preconfigured resistor values to change the gain of the feedback loop. In this setup, the output voltage is compared with a reference voltage. By selecting the desired  resistor combination, the gain of the feedback loop can be altered, resulting in discrete variable power supply outputs. 

Another topology allows for a continuously variable output voltage. This is achieved using a potentiometer that modifies the feedback gain factor in the loop. By adjusting the potentiometer, the feedback gain changes, which in turn adjusts the output voltage continuously over a range.

Variable Power Supply PCB Layout Tips

Depending on your chosen topology, your layout will vary. However, below we’ve summarized the key components of the design, along with individual layout tips for each component.

Transformer

Place the transformer near the edge of the PCB to minimize noise coupling to sensitive areas, and ensure sufficient spacing between the transformer and other circuitry to reduce electromagnetic interference (EMI). Furthermore, use wide traces for the primary and secondary windings to handle high current and minimize voltage drop.

Full Wave Rectifier

Arrange diodes close to the transformer secondary windings to minimize trace length and reduce inductive noise. Ensure adequate thermal dissipation for diodes by using thermal vias and large copper areas if needed. Finally, connect the rectifier’s ground return path directly to the main ground plane to minimize ground loops and noise.

Main Regulating Component

This component will likely produce the most heat on the board. Ensure there is sufficient copper area or a dedicated heatsink to dissipate heat from the regulator, and center it towards the middle of the board for good dissipation.

Place the regulator close to the output filter capacitors to minimize noise and ensure stable output, and place decoupling capacitors close to the input and output pins of the regulator to filter high-frequency noise.

Timer IC and Feedback Control

Route the PWM output trace away from noisy power and ground traces to maintain signal integrity. Generally, the signal paths will not be carrying much current, but it is best to keep them short. Place these components close enough to the main regulator component to minimize signal loss, but in a location where they will be less susceptible to variations in quality due to heating. 

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