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Designing Products with Power Adapters

Known by a variety of names from adapter brick to wall mount power supply, the ubiquitous small plastic boxes are the most common source for low voltage power.

Power adapter are available in a variety of performance, quality and price levels, and since price can often be the key deciding factor, it is important to understand the differences in power adapters when supplying power to your product, and how the differences affect the overall price.

Power adapters come in two basic varieties AC/AC and AC/DC:

AC/AC ­ AC/AC power adapters are simple step-down low voltage transformers, accepting AC input and providing a certain voltage AC output. There are no other active or passive components. With no line regulation at all, an AC/AC adapters output voltage is proportional to the input voltage. AC/AC do have a small amount of load regulation, with the transformer windings being a significant part of the load.

AC/DC— In its most basic form, a diode is added to the AC/AC adapter circuitry to create an AC/DC power adapter. The lowest cost DC power adapters use half wave rectification, however generally for few cents more you can a full wave rectification.

Common today is the center-tapped design where each half of the secondary transformer winding is idle half of the time. The primary drawback to this design is you have to use a bigger transformer to get the same power output.

The single diode, half wave rectification DC adapter is ideal for use in toy motors, battery adapters and other applications where the 100% ripple and 50% duty cycle won’t affect performance. Simply adding a capacitor can smooth out the ripple, but since it typically needs a larger capacitor at a higher cost than full wave bridge rectification, the design decision is usually in favor of the full wave rectifier.

Full wave rectifiers also have 100% ripple though the DC component of the power frequency spectrum that is much larger than that with half-wave adapters. Adding a capacitor can smooth out the ripple to some extent; however if ripple is important to you, you need to specify it. The reason? As a capacitor gets bigger, it also gets more expensive, and extra expense is what most manufacturers avoid in this highly price sensitive market.

So far none of the AC/DC power adapters mentioned have added line regulation. It is possible to add a three terminal voltage regulator to the adapter to perform volt line and load regulation, but the limited heat sinking capability limits these adapters to low power applications. If you need well-regulated power, you may want to consider using an unregulated power adapter and handle the filtering and regulation in your own product or use a switching power supply that supports a wider range of input voltages. System Connection offers regulation as an option in all our OEM power supplies, linear or switching

The range of output connectors for wall-plug power supplies is unlimited, but the most popular are listed on our Design Your Own Power Adapter form. The most commonly used is the 5.5 x 2.5 mm barrel connector, with center positive. The cost affect is minimal between the various phono style plugs so just select the version that best matches the design requirements of the project, and decide if cable should exit the mold at 90 degrees or 180 degrees.

Wall plug-in power adapters are certified by safety agencies. The adaptors are required to have a thermal fuse to protect against fire if there is a short circuit. Typically, they are soldered-in glass tube type fuses, which are not user replaceable. Glass tube fuses protect the primary side of the transformer and are rated at 240 volts. Don’t get confused by this rating, even a 110 volt input wall power adapters is designed with a fuse rated at 240 volts. The current rating is typically two to three times the rated input current.

Power adapters with UL, TUV, CUL, etc safety approvals will have the input windings of the transformer isolated from the output windings. It is important to note that there is a capacitance between the windings, which could result in a small AC leakage current to ground.

Adapters with safety agency certifications should be properly isolated, eliminating the need for a polarized connector, or a three-pin plug. While it may be useful in certain applications to have a ground connection to reduce hum, such power adapters are rare for commercial applications.

This safety certification is a design advantage because when wall mount power adapters have all the necessary safety agency certifications, you avoid the cumbersome, time consuming and costly safety certification process you would need to endure if you had AC going into your product. For this reason, AC adapters have become so ubiquitous on many of the products we see today.