PowerLab Notes: AC/DC Phone and Tablet Chargers

It’s clear that the smartphone and tablet era has changed everyone’s life. However, power supply designers may be the only ones who have noticed that the manufacturers of these products have also started a revolution in AC/DC adapters. After all, no one wants to plug their small smartphone into a huge power supply.

Remember those huge " wall plugs " from ten years ago?

In addition to being compact, the charger must also be as low-cost and efficient as possible .

Just a few years ago, I was asked to design a 5W smartphone charger for a large aftermarket accessory manufacturer. I tried to design the charger using a low-cost fixed-frequency controller that was standard in our industry, but was unsuccessful. Size requirements turned out to be just one of the challenges I faced. Power consumption at full load and no-load conditions and overall solution cost required a new controller. After this initial request, other similar opportunities began to appear regularly. After a few unsuccessful attempts, I gave up.

Fortunately, in 2012, Texas Instruments (TI) introduced  the UCC28700 / UCC28710  family of flyback controllers. Finally, I had the tools I needed to design an AC/DC charger that matched or exceeded existing chargers in size, efficiency, and cost. Using a controller built specifically for your application can greatly simplify power supply design.

Brian demonstrates the UCC28700 5W Adapter Reference Design

This new series of parts has the features I really want, including:

• Supports quasi-resonant control for higher efficiency;
• Burst mode operation;
• Low startup current to minimize no-load power consumption;
• Primary-side regulation eliminates the need for an optocoupler, reducing cost and size.

The design of this adapter is not as complicated as it looks. The term "charger" is probably a bit of a misnomer, since the components that control the battery charging are actually located inside the phone or tablet. An AC/DC charger is really just a power supply that delivers 5 volts to the USB port.

The trick is to make it small . I spent a lot of design time "shopping" on the Internet for the smallest possible input capacitor, FET, and output capacitor that would meet all of the design requirements.

Maximizing efficiency is important not only to meet the latest efficiency standards, but also to minimize size . If you put 2.7W of dissipation in a 1-inch cube, the external ambient temperature will increase by 35 degrees Celsius. If the power supply dissipates more than that, the product size needs to increase. This is why quasi-resonant control is an important feature.

Careful selection of low-cost FETs  without too much loss is critical . For example, in  the PMP8363  , I selected a 3.5-ohm FET listed at $0.21 from a distributor's website. This is a good starting price, and high-volume customers should be able to get lower prices by purchasing directly. Even better, this FET dissipates less than 0.25W under maximum load conditions. 

 Efficiency graph of PMP8363

The above efficiency graph shows that even at 5V  output voltage, quasi-resonant control can achieve over 80%  efficiency.  

 Here are a few smartphone and tablet charger reference designs that my colleagues and I  published at PowerLab :

• PMP8363  — 10W Square Tablet Charger;
• PMP8286  — 10W square tablet charger with universal device detection;
• PMP4351  — 10W tablet charger built on a single PCB;
• PMP8757  — 5W square smartphone charger;
• PMP4335  — 5W smartphone charger on a single PCB;
• PMP4344  — 6W smartphone charger on a single PCB.

*Note: If you are interested in car chargers, my colleague did a post about car charger design here .