6 Application Example - Super Capacitor Backup

The BQ25856-Q1 also works with super capacitor backup systems. In the following example, the backup capacitor circuits needs to maintain a 10V system load at 4A for 0.9 seconds. This is an extreme case for the BQ25856-Q1 because the maximum discharge current from the capacitors is greater than 15A once the capacitor voltage falls to 2.5V. The design requirements are summarized in Table 6-4.

Supercapacitors offer the best storage for these high energy backup systems. A single cell supercapacitors has a low maximum charge voltage of around 2.5-3.7V. To get a higher voltage and more usable power, multiple supercapacitors have to be used in a series configuration. Two supercapacitors in series allows for a charge voltage of 5V. The equations above can be used to calculate the needed capacitance as 3.84F. Because of the extreme current being pulled from the capacitor, this number is multiplied by 4 to get 11.52F. Two 25F capacitors in series can be used to get a total capacitance of 12.5F with a total voltage rating of 5V.

In Figure 6-1, the BQ25856-Q1 EVM takes 6.78 seconds to charge the capacitors completely. Channel 1 is the input voltage and is blue. Channel 2 is the capacitor voltage and is teal. Channel 3 is the system voltage and is magenta. Channel 4 is the capacitor current and is green. Current into the capacitor is positive.

Figure 6-1 BQ25856-Q1 Charging Up Supercapacitors

In Figure 6-2, the BQ25856-Q1 provides a 4A system load at 10V for 900 milliseconds. Channel 1 is the VAC voltage seen at the charger. Channel 2 is the voltage on the backup capacitors. Channel 3 is the system load current. Channel 4 is current into the capacitor. The capacitor current turns negative as the BQ25856-Q1 begins sourcing power. Note that VAC begins to fall, but VAC stabilizes at 10V.

Figure 6-2 BQ25856-Q1 Provides Backup Power with a 4A Load

Note, that the capacitor voltage comes up after reverse mode turns-off and this indicates that ESR losses present. To maintain a current output of 4A at 10V, the BQ25856-Q1 needs 20A from the capacitor when the capacitor voltage is at 2.5V. This is causing a voltage drop of 1.28V at the capacitor. The ESR of the supercapacitors can be calculated to be 64mΩ and the ESR is dissipating 25.6W at the very end of the discharge. At these large currents, the ESR of the capacitors and IR drop of the wires plays a big role. The available power can be increased by using capacitors with lower a ESR and larger wires. Note, that the maximum discharge current is set by the BQ25856-Q1 to be 20A. This value can be decreased by using the IBAT_REV bits.

Note, that operating the supercapacitors at these high loads decreases the effective capacitance, increases the ESR over time, and reduces the operating lifetime. The lifespan of the supercapacitors can be increased by using supercapacitors with a low ESR to reduce the internal heating. Supercapacitors can also be used in parallel to reduce the current load on each capacitor. An in-depth analysis of these effects is outside the scope of this application note. A few useful research papers are listed in Section 8.

In Figure 6-3, the BQ25856-Q1 takes around 60µs to transfer from charging to discharging when the input power is disconnected. Channel 1 is the current into the 4A system load and is blue. Channel 2 is the /INT pin on the BQ25856-Q1 and is teal. The /INT pin generates an interrupt pulse when the BQ25856-Q1 changes states. Channel 3 is the voltage measured at the 4A system load and is magenta. Channel 4 is switch node 1 and is green. The /INT pin is going low from VAC going low and auto reverse mode turning ON.

Figure 6-3 BQ25856-Q1 Auto Reverse Time with Supercapacitors