University of California Irvine (UCI) researchers have invented a hysteretic charging scheme, which helps extend the upper bound of the capacitance of supercapacitors. It achieves efficient charging operations through hysteretic control, optimal window size, and two stage supercapacitors composition by equipping with a pulse-frequency modulation (PFM) dc-dc boost converter.
Unlike in typical boost dc-dc converters, the present invention utilizes a two-step combination of a smaller charging mode supercapacitor and a large reservoir supercapacitor. In order to overcome the high rate of self-discharge or leakage of the reservoir supercapacitors during charging, the smaller hysteretic charging-mode supercapacitor accumulates energy from the low-power ambient source and then rapidly releases the accumulated energy to the reservoir supercapacitors within the hysteresis window of where charging is still efficient. The released current through the dc-dc converter can offset the leakage current of the reservoir supercapacitor. When the charging current is equal to the leakage current, the charger cannot charge the reservoir supercapacitor anymore, which can be called the boundary of leakage offset current. The charging efficiency can be maximized using pulse frequency modulation (PFM) mode of the dc-dc converter and a hysteretic window of 50mV
While supercapacitors excel in situations requiring rapid charging or discharging, they are not yet a suitable replacement for conventional rechargeable batteries due to their comparatively high rate of self-discharge or leakage. The self-discharge (leakage) rate of supercapacitors increases rapidly near their rated voltage and can be easily similar to or exceed a charging current. Due to this, it is very difficult to charge supercapacitors under low power conditions. In order to charge the supercapacitor with low ambient power sources, the charging power should be larger than the leakage power. Therefore, power transfer efficiency of a power boost converter and the additional overhead of the control circuit are crucial factors to efficiently charge supercapacitors in low ambient energy power source conditions. UCI researchers have invented a charging scheme for supercapacitors that addresses these issues and extends the upper bound on the capacitance of supercapacitors using a hysteretic scheme.
- Boosts charging efficiency of supercapacitor in ultra-low power circuit
- Improves leakage current of super capacitor
State of development
Working prototype developed and tested
Professor Pai H Chou, Department of Electrical Engineering and Computer Science, Henry Samueli School of Engineering, University of California, Irvine.
Source and top image of Professor Pai H Chou: University of California, Irvine