Powering E-ink Displays with Supercapacitors and Energy Harvesters
Electrophoretic or e-ink displays are becoming very widespread, with the best known example the Amazon Kindle. These displays are bistable, that is they consume no power while displaying a page but draw peak power when the page is updated. This power profile, of very low average power with occasional high power peaks, is an ideal profile for a small energy harvester coupled with a supercapacitor. The supercapacitor provides the peak power for display updates and the energy harvester provides the low average power that re-charges the sueprcapacitor between updates. This power scheme can allow battery free e-book readers using solar cells, or can be used to power battery free stand alone or wireless sensors with local displays for industrial and building control applications, using solar, vibration or thermal energy harvesters. For e-books powered with solar cells, the energy harvesting circuit must work over 2 orders of magnitude in input power, ranging from indoor light levels of 300 lux to 50,000 lux in bright sunshine outside. This article is a case study with an e-book reader powered by a supercapacitor and solar cell, showing the power design, and laboratory results for power levels, reading time, re-charge times with discussion of the tradeoffs in optimising the power architecture and component selection.
Electrophoretic or e-ink displays are becoming very widespread, with the best known example the Amazon Kindle. These displays are bistable, that is they consume no power while displaying a page but draw peak power when the page is updated. This power profile, of very low average power with occasional high power peaks, is an ideal profile for a small energy harvester coupled with a supercapacitor. The supercapacitor provides the peak power for display updates and the energy harvester provides the low average power that re-charges the sueprcapacitor between updates. This power scheme can allow battery free e-book readers using solar cells, or can be used to power battery free stand alone or wireless sensors with local displays for industrial and building control applications, using solar, vibration or thermal energy harvesters. For e-books powered with solar cells, the energy harvesting circuit must work over 2 orders of magnitude in input power, ranging from indoor light levels of 300 lux to 50,000 lux in bright sunshine outside. This article is a case study with an e-book reader powered by a supercapacitor and solar cell, showing the power design, and laboratory results for power levels, reading time, re-charge times with discussion of the tradeoffs in optimising the power architecture and component selection.
Characteristics of Electrophoretic or E-Ink Displays
Electrophoresis is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. Positively charged white and negatively charged black particles are encapsulated in a clear fluid and sandwiched between 2 electrodes which control the direction of the field for each pixel, determining whether the pixel is white or black. Electrophoretic displays, more commonly known as e-ink or e-paper displays, are bi-stable, drawing no power when the page is unchanging. They reflect ambient light, so draw no power for backlighting and are easy on the eye for prolonged reading, such as an e-book. However, page refresh time is slow,~1s, making electrophoretic displays unsuitable for video. Peak power during page refresh is high but only short duration, so is ideally supplied from a supercapacitor. These properties make electrophoretic displays ideal for: e-books; electronic shelf labelling where all the prices in a supermarket can be updated wirelessly, automatically and quickly; electronics forms; POS displays, signature pads; industrial controllers; thermostats, HVAC, medical devices, time with minute updates. Figure 1 shows the power profile of a page update for an Amazon Kindle.
Figure 1: Power profile updating a Kindle page
When the page is stable, the power drawn is only 19mW. The peak power during page change was 7.6W, while the average power drawn during the 1s page update duration was 0.56W. At a reading rate of 32s/page, the average power drawn = 34mW. This average power can be supplied by a solar cell with good indoor lighting. We used solar panels made of 4 cells in series that was the same dimension as a Kindle cover, 12.5cm x 16.4cm.
Characteristics of Supercapacitors
CAP-XX supercapacitors are ideal power buffers to store the energy from the harvester delivered at low power and provide high power to the e-ink display for a page update since they have a thin prismatic form factor, suitable for many applications where form factor and industrial design is important, they have very low ESR to deliver high power, they have good energy density to deliver the power for the duration needed, and they have very low leakage current, typically < 1µA, which is important when charging power is low, otherwise you waste a significant portion of the power supplied by the energy harvester. Figure 2 shows a CAP-XX supercapacitor that is 350mF & 70mOhms ESR.
Figure 2: CAP-XX Supercapacitor
Solar Powered Operation of a Kindle
Fig 3 shows how the power architecture of the Kindle was modified to run with a solar cell and supercapacitor. The Kindle’s battery management IC did not know the difference and charged the supercapacitor from the USB port.
Figure 3: Modifying the Kindle to run with an energy harvester + supercapacitor
We placed solar cells on both the inside and outside of a Kindle cover, so the supercapacitor is charged whether you are reading with the cover open or when the cover is closed. Since the solar cell voltage may drop dramatically when you draw current from it, the switch to select the solar cell used must lock its selection to avoid bouncing between cells as one or other cell is selected. The circuit used is shown in Figure 4. If the cover is open, so the inside solar cell has more light and is higher voltage than the solar cell on the outside cover, then T2 is turned ON. This holds the base of T1 to ~0.5V = VCE saturated for T2 + the diode drop across D1, which prevents T1 from turning ON.
Figure 4: Solar cell selection switch
Any supercapacitor charging circuit must have the following characteristics:
1. Must behave gracefully into a short circuit. A discharged supercapacactor looks like a short circuit to the charging circuit.
2. Must start charging from 0V.
3. Must provide over voltage protection to protect the supercapacitor from damage
4. Must prevent the supercapacitor from discharging into the source. For example, if the light goes, the supercapacitor would discharge through the diode across the solar cell without some form of prevention.
5. Should be designed for maximum efficiency.
We selected the LTC3105 for the boost with Maximum Peak Power Tracking since it meets all the above criteria. It has low input startup voltage, 0.25V, so it will start charging the supercapacitor even when there is low light. The part behaves gracefully into a short circuit, limiting output current until the output voltage reaches 1V after which it behaves normally. Very importantly, this IC is reasonably efficient at low power, ~70% at 2mW and regulates the input current it draws from the solar cell to maintain its output near its peak power point. A dual cell supercapacitor with the cells in series is required for our operating voltage and we used an active balance circuit with a very low power op amp, MAX4470, to maintain voltage balance between the 2 cells with minimum leakage current.
With our supercapacitor fully charged, and no light source, we could read 176 pages. The supercapacitor could be charged from the USB port in a few minutes. 176 pages is approximately 1hr 35min reading time. With 1100 lux light, such as an overcast day, you can read the Kindle indefinitely. In a well lit room at 550 lux, the solar cell allows you to read 313 pages, or a 78% increase over no light.
This article, using a Kindle as a case study, shows how an energy harvester coupled with a supercapacitor is an ideal energy source for electrophoretic displays.
