Understanding battery charger features and charging topologies

In my previous blog “Selecting the right battery charger for industrial applications”, we discussed the standalone vs. host controlled chargers and external vs integrated switching FETs.  Now let’s take a look at different charging topologies.

First of all, we have to better understand the battery charger features, Dynamic Power Management (DPM) and Dynamic Power Path Management (DPPM). These two features are closely related to charging topology and are equally important. The different topologies determine the DPM and DPPM capability as well as the total cost associated with different components selected.  For low power applications, NVDC charger has generated a lot interests for its lower cost and DPM/DPPM features.   For higher power applications, the traditional charging topology is selected to minimize the power loss.

Adapters with higher output ratings are generally more expensive. To reduce costs, you may want to use a lower-rated adapter, but doing so requires a charger with a current-based dynamic power management (DPM) function to prevent adapter overload.  This protection is in place in case the combined system load and battery load exceed the total power that the adapter can provide. For example, chargers with current based DPM such as the bq24133 can handle wide input power sources without overloading (Figure 1).

Figure 1: Current-based DPM

For peak system performance, you will also need a dynamic power path management (DPPM) function so that the charger can work in supplement mode, which enables the battery to provide power to the system through the battery FET instead of having to be charged (Figure 2). You should consider the trade-off between performance and cost during the design.Higher performance is normally associated with higher cost.  A charger controller such as TI’s bq24610 has both DPM and DPPM control that can support up to 10A charging current.

Figure 2: An example of DPPM Current Path

With better understanding on the DPM and DPPM, we can then looking into charging topology. The three most common charging topologies are traditional, hybrid and narrow VDC (NVDC).

Traditional topology chargers such as the bq24170, synchronous switch-mode stand-alone battery charger and the bq24725A SMBus charge controller, the system rail can go as high as the maximum adapter voltage. If operating from the battery, the system voltage can go as low as the minimum battery voltage. A high-voltage input source can cause a wide swing on the system rail (Figure 3). The benefit of using this topology is the system gets maximum power from input source.  The downside of this is that the total solution cost is high since the components need to handle high power and are more expensive.

Figure 3: Traditional charging topology

In some applications, the system only requires peak power delivery. An adapter designed for normal operation cannot meet peak power requirements, and a traditional charging topology does not allow batteries to work in supplement mode to provide additional power. The solution is a hybrid charging topology, as shown in Figure 4.

Figure 4: Hybrid charging topology

In a hybrid charging topology, the battery can provide additional power to the system in boost mode for peak power delivery. Devices such as the bq24735 and bq24780S battery charger ICs fall into this category.  The hybrid charging topology is also called “turbo boost” mode. This topology is very popular in laptop applications.

Both traditional and hybrid charging topologies require the system rail to handle a high voltage at the same level as the input source. However, in some applications, the system rail needs to have lower rating components to reduce the cost. In such cases, consider an NVDC topology included in such products as the bq24770 or bq24773 to align the system voltage very closely to the battery voltage by controlling the battery FET, as shown in Figure 5.

Figure 5: NVDC charging topology

When designing a charging system, the balance among performance, features and solution cost has to be balanced.  Choosing the right topology and device can achieve higher efficiency while maintain the lowest solution cost. For more information on choosing the right battery charger for your design, please visit the battery charger solutions page.

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