March 6, 2014

Engineering with wall plug-in power supplies and AC adapters

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How to design products with wall plug-in power supplies.

Basics

Call them wall mount, AC adapters, wall bumps, power cubes, wall adapters, wall warts, or wall plug transformers, linear wall mount power supplies are the most common source of low voltage power. They are the small plastic boxes that plug directly into the wall. They come in a variety of performance and quality levels, and generally you get what you pay for.

How to design with wall warts and AC/DC Adaptors

Most design engineers are mostly concerned with the cost of a wall transformer, and indeed, as AC adapters they can get very very cheap. It is important, however, to know what you are getting with a wall mount power supply, and even circuit designers make mistakes in this area.

AC/AC -- First, there is the AC/AC wall mount power supply. This is a simple step-down low voltage transformer, with AC in and AC out. There are no other active or passive components. The transformer is designed to give a certain voltage at a certain current. AC/AC adapters have no line regulation at all, so their output voltage is proportional to the input voltage. Wall transformers have a small amount of load regulation, since the transformer windings are a significant part of the load.

AC transformer wall plug-in power supply, as used in an AC adaptor

AC/DC-- Add a diode and you have a simple AC/DC wall mount power supply. The cheapest DC wall transformers use half wave rectification, literally a few cents more gets you a full wave. At one time a two diode design that used a center tapped transformer was popular, but since the price of diodes is lower than the price of center tapped secondaries there isn't much point. In the center tapped version each half of the secondary transformer winding is idle half of the time, so you have to use a bigger transformer to get the same power output, which is a lot more expensive than a tiny speck of silicon rectifier.

The single diode, half wave rectification DC adapter is used for the cheapest of battery chargers, toy motors, battery adapters and other places that the 100% ripple and 50% duty cycle won't matter. Adding a capacitor can smooth out the ripple, but since it needs a larger capacitor than full wave bridge rectification, the cost trade off usually leans toward the full wave rectifier.

Half Wave Power Transformer as used in a DC Adaptor Waveform from a half wave power transformer wall plug power supply adaptor
Center Tapped Full Wave Rectifier Power Transformer, used in older adaptors    Waveform from a full wave rectified plug-in AC/DC transformer adaptor
AC/AC Power Transformer used in modern adaptors 

Of course the full wave rectifiers also have 100% ripple, though the DC component of the power frequency spectrum is much larger than with half-wave adapters. Add a few cents and you can get a capacitor to smooth out the ripple to some extent. If ripple is important to you, you had better specify it, because the bigger the capacitor the more expensive it gets, and the manufacturers are well tuned to saving money at every opportunity. (See below for more on ripple suppression trade-offs).


None of the DC wall warts mentioned so far have any line regulation. It is possible to add a three terminal voltage regulator to the adapter to perform volt line and load regulation, but because of the limited heat sinking capability these are limited to low power.


Even if you need well regulated power it often makes sense to use an unregulated wall wart and do the filtering and regulation in your own box. This is because the wall mount power supply can be obtained with all the necessary safety agency certifications, which you would have to do yourself if you had AC going into your box. Many famous products use AC adapters for this reason. (See below for more on voltage regulation trade-offs)


More on ripple in DC adapters

The lowest frequency ripple from a linear wall plug-in power supply which uses a full bridge rectifier is at twice the line frequency–120 Hz or 100 Hz depending on where you live. The capacitance for the filter capacitor can be approximated by

C1 (F) = [ ( ILoad ) /(120* Vp-p ripple) ] Farads

(multiply by one million go get microfarads). A quick insight into this formula is that at 1 mV ripple a 1 amp current will need to flow into the capacitor for 1/120th of a second, and out of the capacitor for 1/120th of a second. The amount of charge that goes in and out of the capacitor is delta V times the Capacitance, so C = ILoad/(120*Vp-p ripple). This doesn't have the finesse of a tuned filter, and so requires higher capacitance than, say, a pi filter, but it is the cheapest way to solve the problem.

This capacitor can be either in the AC/DC adapter, or in your own equipment. The voltage rating of the capacitor should be above that of the ripple of the raw signal, or 1.414 times the desired DC voltage. A good rule of thumb is to kill noise where it originates, so it is good to have the filter capacitor in the wall-mount power supply rather than let all those harmonics radiate down the cord and filter them in your box.

There will be a lot of frequency harmonics above 120 Hz, but this capacitor should take care of them, too. It is less practical to try to use a capacitor to filter out a half-wave rectified wall bump.

Regulation in DC Adapters

Most wall-mount power supplies have no active regulation. They are designed so that the voltage will be X when the current is Y, just like the label says. Many engineers are confused by this, thinking that a 12 volt, 1 amp power supply can be substituted for a 12 volt 500 mA power supply. This might be true, but the voltage at 500 mA will be higher than the voltage at 1 amp, maybe 14V. How much this varies depends on the load line of the transformer. A load-line is a graph of voltage versus current. PowerStream can supply load lines for our products, not every manufacturer does. Cheaper transformers have fewer windings, and wilder voltage swings with load.

Another problem is the line regulation. As the input (or line) is varied, a simple transformer adapter will vary its voltage proportionally. Since in the USA the line voltage varies between 105 and 125 volts, your 115 volt nominal input wall wart will vary by 10 % in output (12 volts varies between 13.2 and 10.8 volts) depending on how close you are to the mains transformer and how much current your neighbors are drawing.

Active Regulation in DC Adapters

Active regulation of wall mount power supplies solves the regulation problem, and takes care of ripple at the same time. It is surprising how seldom this is done in the wall bump itself, unless the wall mount is a switching power supply. Switching power supplies are always regulated. PowerStream offers regulation as an option in all our OEM power supplies, linear or switching. In transformer based wall mount power supplies regulation is done with a diode chain, a zener diode, a zener plus a bipolar transistor, or a three terminal regulator, depending on the price of components and the output current required.

Internal fuses

Those wall plug-in power supplies certified by safety agencies are required to have an internal fuse to protect against fire if there is a short circuit. They are almost always soldered-in glass tube type fuses, and are almost never user replaceable. They are protecting the primary side of the transformer, so are typically rated at 240 volts. Don't get confused by this rating, even a 110 volt input wall mount transformer will use a fuse rated at 240 volts. The current rating is typically two to three times the rated input current.

Isolation of Transformer Adapters

Again, if a wall plug transformer has safety agency approvals (UL, TUV, CUL, etc.) the input windings of the transformer will be isolated from the output windings. However, there is a capacitance between the windings which could result in a small AC leakage current to ground.

Grounding.

Because the wall wart should be properly isolated there is no need for a polarized connector, or a three pin plug. However, in certain applications it is useful to have a ground connection to reduce hum. Such wall plug power supplies are rare, however.

Output connectors

The range of output connectors for wall-plug power supplies is unlimited, but the most popular are listed on our web page Power Adaptor Connector Page The most popular of all is the 5.5 x 2.1 mm barrel connector, with center positive, shown below.

A common connector for wall plug power supply  


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