For any harvester, the power curve goes first up, achieves his maximum power point and goes down with the voltage until reaching the open-circuit voltage – Voc. Looking at this power curve, we can see that a maximum power point exists at a certain voltage called Maximum Power Point Voltage – V MPP. This MPP Voltage is a certain percent of the open-circuit voltage – Voc. This ratio varies depending on the harvester : MPPT ratio = VMPP / Voc.
To resume, it defines the best voltage to work with in order to harvest the maximum power out of the harvester.
- For a PV cell, the ratio varies from 70% to 90% depending on the PV cell material.
- For a piezoelectric generator, the ratio depends on the power curve of the piezo including the rectifier.
- For a thermoelectric generator, the ratio is most of the time around 50%.
- For RF, using the integrated rectifier in the AEM30940, the optimal ratio is around 65%.
- For the RF, using an external rectifier with the AEM30940, the optimal ratio of our design is around 50%.
For any harvester, it might be interesting to draw its power curve in the targeted conditions.
Note that the P-V curve might vary under different luminosity for PV cell for example, but the ratio will remain the same.
The AEM10941 performs a Voc measure each 5 s during 81 ms to adapt the VMPP voltage to work at. The ratio is defined by the SELMPP0-1 pin-configuration (See page 12 in the datasheet).
The AEM20940 performs a Voc measure each 21 s during 330 ms to adapt the VMPP voltage to work at. The ratio is defined by the SELMPP0-1 pin-configuration (See page 12 in the datasheet).
The AEM30940 performs a Voc measure each 0.33 s during 5 ms to adapt the VMPP voltage to work at. The ratio is defined by the SELMPP0-1 pin-configuration (See page 12 in the datasheet).