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PowerLab Notes: AC/DC phone and tablet chargers
It is obvious to most that the smart phone and tablet era has changed almost everyone’s life. However, power supply designers may be the only that noticed manufacturers of these products have also revolutionized the AC/DC adapter. After all, nobody wants to plug their small smartphone into a giant power supply.
Remember those bulky “wall warts” from just a decade ago?
In addition to a small form factor, chargers must be as inexpensive and efficient as possible.
Just a few of years ago, I was asked to design a 5 watt smartphone charger for a large, aftermarket accessory manufacturer. I tried (unsuccessfully) to design the charger using one of our industry-standard, low-cost (very important!), fixed-frequency controllers. The size requirements turned out to be the least of my problems. The power consumption (both at full load and no load) and the overall solution cost required a new type of controller. After the initial request, other similar business opportunities were beginning to pop up regularly. It only took a couple more failed attempts before I gave up.
Thankfully in 2012, TI introduced the UCC28700/UCC28710 family of flyback controllers. Finally, I had the tools that I needed to design an AC/DC charger that is just as good as or better than existing chargers based on size, efficiency and cost. Designing a power supply is so much easier when you are using a controller that is made specifically for your application.
Related: Brian demos the UCC28700 5W Adapter reference design
The features that this new family of parts had that I desperately needed are:
The design of the adapter is not as complicated as it may seem. The term “charger” is a bit of a misnomer in that the device controlling the charge of the battery is actually inside the phone or tablet. The AC/DC charger is really nothing more than a power supply that puts out 5 volts on a USB port.
The trick is making it small. I spent a great deal of design time “shopping” on the internet for input capacitors, FETs and output capacitors that were as small as possible while meeting all of the design requirements.
Maximizing efficiency is not only important for meeting the latest efficiency standards, but also for minimizing the size. If you put 2.7W of dissipation inside a 1-inch cube, you get a 35 degrees Celsius rise from the outside ambient temperature. If your power supply dissipates much more than this, you will need to increase your product size. This is another reason why quasi-resonant control was an important feature.
Carefully selecting an inexpensive FET without much loss is critical. For example, in PMP8363, I chose a 3.5 ohm FET that listed for only $0.21 at a distributor’s website. This was a great starting price and a high-volume customer should be able to get a much lower price by buying direct. What’s even nicer is that this FET dissipates less than 0.25W at maximum load.
Efficiency Graph from the PMP8363
The efficiency graph above illustrates how quasi-resonant control makes it possible to achieve over 80% efficiency, even with a 5V output.
Here are a few smart phone and tablet charger reference designs that me and my fellow engineers have published on PowerLab:
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