Infrastructure underpinning the information economy such as data centers and telecoms systems operates 24 hours a day: unplanned loss of power is not acceptable. Energy-storage devices are therefore needed to provide a source of back-up power.
An energy-storage device collects and stores energy by charging itself from an electrical power source, and then supplying the stored power to the loads by discharging itself. The charge and discharge processes need to be precisely managed to ensure the safety, reliability and long life of the storage devices. In most applications, the charge and discharge functions are controlled by two separate power trains to implement the different control specifications, such as a smaller charge current in contrast with a larger discharge current for a lithium-ion battery.
However, a fast charge-to-discharge or fast discharge-to-charge transition is needed for some applications. For example, a DC battery back-up system used in a data center server needs to implement a fast charge-to-discharge transition to provide seamless power delivery. A motor drive’s braking operation, on the other hand, calls for a fast discharge-to-charge transition.
A single powertrain on-the-fly bi-directional charge and discharge converter is needed for these applications to achieve a seamless fast transition between charge and discharge operations. By combining the charge and discharge powertrain, the designer can realize a compact design and the system cost can be minimized. This approach can also be of benefit in applications which do not require a fast charge and discharge transition.
The ISL81601 four-switch buck-boost controller provides an easy and reliable solution for on-the-fly bi-directional DC-DC power conversion. Figure 1 shows two different systems for bi-directional operation controlled by the ISL81601:
- 1a is a battery charge/discharge bi-directional operation system
- 1b is a supercapacitor back-up system
Fig. 1a: Battery pack with a charge/discharge bi-directional DC-DC converter
Fig. 1b: Supercapacitor DC back-up system
In a battery charge/discharge system, the bi-directional DC-DC converter can be a four-switch buck-boost converter when the output voltage is close to the battery operating voltage; or it can be a boost converter when the output voltage is always higher than the battery voltage; or it can be a buck converter when the output voltage is always lower than the battery voltage.
The four-switch buck-boost converter achieves the best efficiency because it uses power devices with a lower voltage rating and lower operating current. It can also ensure safe battery operation because of its ability to provide full over-current and short-circuit protection on both charge and discharge operations.
Implementation in a battery pack
In a charge/discharge bi-directional DC-DC converter for a battery pack, as shown in Figure 1a, Constant Current/Constant Voltage (CC/CV) control is needed in both directions. Figure 2 shows the battery pack DC-DC converter block diagram. The battery pack is connected to the input end of the DC-DC converter. In the forward battery-discharge direction, the output voltage’s CV control is performed by Gm1, and the output current’s CC control is performed by A2 and Gm4.
When charging the battery in the reverse direction, the CC and CV charge control can also be implemented by adding two op amp circuits (A3 and A4) to the ISL81601 controller.
Fig. 2: How the ISL81601 implements CC/CV control of battery charge/discharge operations
Figure 3 shows the waveforms generated by battery charge and discharge operations. When the DC source is removed at T1, the battery starts to discharge to output the power to the load immediately on the fly. When the DC source is reapplied at T2, the DC-DC converter immediately changes its direction on the fly to charge the battery. The battery is charged in CC mode when the voltage is lower than the input-voltage regulation set point, and in CV mode when the voltage reaches the set point.
Fig. 3: Waveform generated by battery charge/discharge operations
The Renesas ISL81601 is a highly integrated bi-directional buck-boost PWM controller. The device’s unique system architecture makes it easy to control both voltage and current at the output and input ends in forward and reverse directions in a four-switch buck-boost DC-DC converter.
This capability provides a simple, reliable and flexible solution for on-the-fly bi-directional DC-DC power conversion.