How 1.2% more efficiency can help you charge faster and cooler

With portable products packed with new features and an embedded lithium-ion battery inside, one key design consideration is how to optimize the end-user experience with fast and cool charging. Most higher-current (>1A) portable electronics have adopted a high-efficiency synchronous-switching battery charger with integrated MOSFETs. These chargers provide an efficient charging solution with less heat and longer battery life.

As smartphones, tablets and other portable gadgets feature bigger and higher-capacity batteries, switching charger efficiency is the key to charging them quickly. While consumers may not notice a 1% or 2% charging efficiency difference, they will certainly sense an additional 5 or 10 degrees of heat on the outer surface of their handheld device. Therefore, any portable products where space between the printed circuit board (PCB) and exterior case is tight must have a certain power budget for the heat that fast charging generates.

Figure 1 is an example of a power-loss budget limited at 1.1W. The charger IC is 43°C and the case temperature is 34°C, which is linearly proportional to the power loss in an enclosed case. Thus, consumers will not be able to feel the heat generated inside.


Figure 1: (a) IC 43°C with 1.1W loss (b) case 34°C with 1.1W loss                                          

Let’s compare two of TI’s switching chargers: the bq25898 vs. the bq25890, with 9V input and 3A charging. The bq25898 can achieve 1.2% higher charging efficiency compared to the bq25890, as shown in Figure 2.

Figure 2: bq25898 vs. bq25890 Charge Efficiency

What does a 1.2% charger-efficiency improvement mean to you? With the same 1.1W loss budget, the bq25898 can deliver 380mA more (13% higher) charging current. And yes, you can charge >3A using the bq25898 charger maintaining the same temperature as the bq25890. The charging current for the bq25890 is about 2.99A with an efficiency of 91.2%; the charging current for the bq25898 is about 3.37A with an efficiency of 92.1%.

Many studies have shown that lithium-Ion batteries suffer capacity degradation over time when exposed to high temperatures. High efficiency is not only good for fast charging but also good for the long-term life of the battery, as well as a consumer’s impression of product quality. Using a high-efficiency battery charger is one of the best ways to minimize heat generation in a portable system and charge up the battery fast.

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