4 BOM Evaluation

Several BOMs have been evaluated for efficiencies and losses. Each found that efficient systems can be designed using the BQ2575x family of devices. A summary of the highest efficiencies for each BOM is included in Table 4-1.

The operating condition shows the output current, the bus voltage, switching frequency, input voltage, and external gate drive voltage.

The following graphs show the efficiency and losses for each of these BOMs.

In Figure 4-1 and Figure 4-2, the efficiency is highest when V_IN is approximately equal to V_OUT. Big differences from V_IN to V_OUT reduce the duty cycle and make the charger work harder. Buck-Boost mode is the most efficient mode because the buck-phase and the boost-phase operate in a low duty cycle and the switching losses are minimized.

In Figure 4-2, BOM1 is the same as the BQ25758BOM and BOM1 can cover the full range of USB-EPR voltages.

In Figure 4-3, BOM2 is designed to work with the 100W USB-PD and fit the components into a small area.

Figure 4-1 BOM1 Efficiency and Loss With V_IN=48V

Figure 4-2 BOM1 Efficiency and Loss With V_IN=20V

Figure 4-3 BOM2 With V_IN=20V

In Figure 4-4, BOM3 is designed for automotive applications to work with 12V LiFePO₄ batteries.

In Figure 4-5, BOM4 was designed to work with 140W USB-PD charging. In the graph, BOM4 is using a external gate drive supply of 7V.

In Figure 4-6, BOM5 is designed for automotive applications with a 48V LiFePO₄ battery.

Figure 4-4 BOM3 Efficiency Graph With V_IN=12V

Figure 4-6 BOM5 Efficiency With V_IN=48V

Figure 4-5 BOM4 Efficiency Graph With V_IN=20V

The key takeaway here is that the end application can inform the operating conditions and BOM selection. Efficiency is highest when VIN is close to VOUT. These decisions can largely determine the overall efficiency of the system.

Next, a closer examination of different gate drive voltages can be performed. Now, compare cases with the same input voltage, output voltage, and BOM but with different gate drive voltages:

Figure 4-7 BOM2 With V_IN=20V and V_OUT=15V

Note, the device has an internal LDO that provides the gate drive voltage for the switching converter. Having an external drive omits the losses caused by the internal LDO, REGN. The higher the input voltage, the higher the LDO losses. This effect can be observed in the above figure.

To make the graph easier to read, Figure 4-8 is the gate drive supplies compared at only 5A.

Figure 4-8 BOM2 With V_IN=20V, V_OUT=15V, and I_CHG=5A

In this case, 7V was found to be the voltage at which the switching losses and conduction losses are lower. For 10V, the switching losses increase to offset the reduced FET conduction gained. Figure 4-9 shows the increased efficiency with a 7V external gate drive.

Figure 4-9 BOM2 With V_IN=20V and V_{DRV_SUP}=7