Steady Eddie: Learn the Basics How Power Supplies Work Before Designing Your Own
Power supplies form the foundation to all of our electronic devices and provide a consistent flow of pattern where it’s needed most. In today’s modern electronics like computers and other data-sensitive devices, power needs to perform flawlessly, and a single failure can mean lost work and data. But as electronic designers, we typically leave our power supply considerations as an afterthought, often grabbing a pre-made schematic block that we know already works. After all, we just want our 5v output, right? It turns out there’s a lot more going on under the hood.
Power Supplies from 10,000 Feet
Most power supplies will take power from AC mains and convert it into a usable DC for use in electronic devices. During this process, a power supply is filling a number of roles, including:
- Transforming AC from a mains supply into a steady DC
- Preventing any AC from interfering with a DC supply output
- Keeping output voltages at a constant level regardless of changes in input voltages
To make all of this conversion happen, a typical power supply will use several common components including a transformer, rectifier, filter, and regulator.
The process of AC-DC conversion starts with an alternating current that originates at a wall outlet as a sinusoidal wave. This AC waveform fluctuates between negative and positive voltages up to sixty times per second.
The AC voltage is first stepped down by a transformer to satisfy the voltage requirements of the power supply load. Once the voltage is stepped down, a rectifier will then turn the sinusoidal AC waveform into a set of positive troughs and crests.
At this point, there’s still an oscillation in the AC waveform, so a filter is used to smooth out the AC voltage into a usable DC supply.
Now that the AC has been converted into usable DC, some power supplies will further remove any ripples in the waveform with the help of a regulator. This regulator will provide a steady DC output regardless of changes that happen to the input AC voltage.
That’s the process at a glance. Regardless of what power supply you look at, it will always have at least three primary components – a transformer, rectifier, and filter. Regulators may or may not be used depending on if the power supply is unregulated or regulated (more on this later).
Power Supply Components in Detail
As the first line of defense, the transformer has the job of stepping down incoming AC from a mains supply into a voltage level that the power supply load can handle. Transformers can also step up voltages, but for this article, we’ll be focusing on those that step down voltage for low-voltage DC electronic devices.
Within a transformer are two windings of a coil, both physically separated from each other. The first winding takes in AC from a mains supply, and then electromagnetically couples with the second winding to conduct a required AC voltage in the secondary winding. By keeping these two windings physically separate, a transformer can isolate an AC mains voltage from reaching the output of the power supply circuit.
Once AC has been stepped down by a transformer, it’s then the job of the rectifier to convert an AC waveform into a raw DC format. This is accomplished by either one or a set of diodes in a Half Wave, Full Wave, or Bridge Rectification configuration.
Half Wave Rectification
In this configuration, a single rectifier diode is used to extract a DC voltage from half of an AC waveform cycle. This leaves the power supply with half of the output voltage it would get from a full AC waveform at Vpk x 0.318. Half Wave is the cheapest configuration to design, is ideal for non-demanding power uses, and will typically leave the greatest amount of ripple in an output voltage.
Full Wave Rectification
In this configuration, two rectifier diodes are used to extract two half cycles of an incoming AC waveform. This process will provide double the output voltage of Half Wave Rectification at Vpk x 0.637. While this configuration is more expensive to design than Half Wave since it requires a center-tapped transformer, it has the added benefit of improved smoothing of AC ripples.
This configuration uses four diodes arranged in a bridge to achieve Full Wave Rectification without requiring a center-tapped transformer. This will provide the same output voltage as Full Wave at Vpk x 0.637 with diodes that only require half of their reverse breakdown voltage. During each half cycle two opposing diodes conduct, which provides a complete AC waveform at the end of a full cycle.
Now that we have our AC voltage converted, it’s the job of a filter to remove any AC ripples in the output voltage, leaving a smooth DC voltage. Why eliminate the ripples? If they pass into the power supply’s output, they can damage the load and potentially destroy your entire circuit. There are two basic components used in filters, a reservoir capacitor and low pass filter.
A high capacity electrolytic capacitor is used to temporarily store an output current supplied by a rectifier diode. When charged, this capacitor will be able to provide DC output current during the gaps in time when a rectifier diode is not conducting. This allows the power supply to maintain a steady DC output throughout the on/off cycles of a power supply.
Low Pass Filter
You can make a power supply circuit with just a reservoir capacitor, but adding a low pass filter further removes AC ripples that make it through the reservoir capacitor. In most basic power supplies you won’t find low pass filters used, as they require expensive laminated or toroidal core inductors. However, in modern electronics with switch mode supplies, you’ll find low pass filters being used to remove AC ripples at higher frequencies.
When adding both a reservoir capacitor and low pass filter together in a power supply circuit, you’ll be able to remove 95%+ of AC ripples. This will allow you to maintain a steady and clean output voltage that matches the peak of the original AC input wave.
In regulated power supplies, a regulator will be added to smooth out the DC voltage further and provide a consistent output regardless of variations in input levels. With this improved regulation also comes added complexity and cost to power a circuit. You’ll find regulators in two different configurations, either as a Shunt Regulator or Series Regulator.
In this configuration, a regulator is connected in parallel with a load, which ensures that current flows through the regulator at all times before hitting the load. If the load current increases or decreases, the shunt regulator will either reduce or increase its current to maintain a steady supply voltage and current.
In this configuration, a series regulator is wired in series with a load, which provides a variable resistance. This regulator will consistently sample an incoming load voltage using a negative feedback system. If the voltage sample rises or falls, then the series regulator will either lower or raise its resistance, allowing more or less current to flow through the load.
Types of Power Supplies
Typical AC-DC power supplies will employ either some or all of the above components in its circuitry as either an Unregulated or Regulated Power Supply. Which type of power supply you use in your electronics project comes down to the unique requirements of your design.
Unregulated Power Supplies
These power supplies do not have a voltage regulator, and will only produce a set voltage at a maximum output current. Here, the DC voltage output is linked with an internal voltage transformer, and the output voltage will increase or decrease based on the current output of the load. These power supplies are known for being durable and inexpensive but don’t provide enough precision for power-sensitive electronic devices.
Regulated Power Supplies
Regulated power supplies include all of the basic components found in an unregulated power supply with the addition of a voltage regulator. There are three regulator power supply configurations to note:
Linear Power Supply. This configuration uses a semiconductor transistor or FET to control output voltages within a specific range. While these power supplies aren’t the most efficient and generate a lot of heat, they are known for their reliability, minimal electrical noise, and wide commercial availability.
Switch Mode Power Supply. This configuration uses a semiconductor transistor or FET which switches on/off to supply voltage to an output reservoir capacitor. Switch modes are typically smaller and lighter than linear power supplies, offer a high output range, and are more efficient. However, they require complex circuitry, generate more noise, and require interference mitigation for their high frequency operations.
Battery Power Supply. This configuration acts as energy storage and will provide a steady stream of DC to an electronic device. Compared with linear and switch mode power supplies, batteries are the least efficient method for powering devices and are also difficult to match with the correct voltage in a load. However, batteries have the advantage of providing a power source when AC mains is not available and produce no electrical noise.
When considering which power supply to use for your next electronics project, here are the following advantages and disadvantages for Unregulated and Regulated power supplies:
When deciding between linear, switch mode, or battery Regulated Power Supplies, consider the following:
|Regulated Power Supplies|
Power Supply Specifications to Know About
When opting for a pre-made power supply circuit instead of designing your own, there are several specifications to know about. These include:
- Output Current. This is the maximum current the PSU can supply to a load.
- Load Regulator. This defines how well a regulator can maintain a consistent output with a change in load current, typically measured in millivolts (mV) or max output voltage.
- Noise & Ripple. These measure undesired electronic interference and variations in voltage from AC-DC conversion, typically measured in peak-to-peak voltage for switching power supplies.
- Overload Protection. This is a safety feature that will shut off a power supply in the event of a short circuit or overcurrent.
- Efficiency. This is the ratio of power converted from AC mains into DC. High-efficiency systems like switching power supplies can achieve an 80% efficiency rating and will reduce heat and save energy.
Power supplies provide a consistent foundation of power in all our electronic devices, whether that’s your computer, smartphone, or television, the list can go on. Regardless of what type of power supply you use or design, they all include several basic components to convert AC mains into a steady direct current (DC). A transformer first steps down a voltage, which then gets rectified into a raw DC format. This is then filtered and regulated to provide a smooth DC voltage for consistent output. When designing your own power supply circuit, expect to use these primary components along with the unique power specifications for your design to provide a consistent DC output at all hours of the day.
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