By Pierre Mars • CAP-XX Ltd Coupling a with a small - harvesting source

Supercapacitors store energy and deliver peak power in support of energy har- vesters. Designers should consider several key issues when pairing them with small energy-harvesting sources.

mall wireless are becoming ubiqui- the load decreases, the load current increases to tous. Applications for sensors include building achieve the load power. Referring to Figure 1, designers can control, industrial control, security, location model supercapacitor discharge as tracking, and RFID. It is much more convenient

and cost-effective to autonomously power these VLOAD=VSCAP−ILOAD×ESR; sensors with a small energy-harvesting source PLOAD=VLOAD×ILOAD=(VSCAP−ILOAD×ESR)×ILOAD without expensive wires or batteries that need repeated =V ×I −I 2×ESR, S SCAP LOAD LOAD replacement.

The environment provides infinite ambient energy, where VSCAP is the supercapacitor’s voltage. including piezoelectric, thermal, vibration, and photovoltaic This equation yields the equation for the load current: energy, but at low power, which falls short of the peak power 2 necessary for transmitting data across wireless networks such ILOAD ×ESR−VSCAP×ILOAD+P=0. as IEEE 802.15.4 (Zigbee), 802.11 (WLAN), or GSM/GPRS. A battery or a supercapacitor acts as a power buffer to store Supercapacitor discharge can then be simply modeled in enough energy to provide the power bursts needed to acquire Excel as and transmit data. These energy-storage devices charge at low 2 power and deliver the burst power when necessary. ILOAD(t)=[VSCAP(t)−√(VSCAP(t) −4×ESR×P)]/(2×ESR); V (t)=V (t)−I (t)×ESR; and Sizing the supercapacitor LOAD SCAP LOAD VSCAP(t+dt)=VSCAP(t)−dt×ILOAD/C. Supercapacitor cells typically operate at 2.3 to 2.8V. The most efficient and cost-effective strategy is to limit the supercapaci- This calculation is important if the load current times ESR is tor’s charge voltage to less than the cell-rated voltage and significant compared with the supercapacitor’s final voltage. store enough energy for your application. In this case, a simple energy-balance approach would make A simple approach to sizing the supercapacitor is to cal- the supercapacitor’s value too small. This undersizing is likely culate the energy necessary to support the peak power of to be the case at low temperatures, when ESR is typically two 2 the application, P, and set this value equal to ½C(V INITIAL− to three times higher than at room temperature. 2 2 V FINAL), where C is the capacitance, V INITIAL is the square of The supercapacitor capacitance and ESR should also the supercapacitor’s voltage just before the peak-power burst, allow for aging. slowly lose capacitance 2 and V FINAL is the square of the final voltage. However, this and increase ESR over time. The aging rate depends on equation does not allow for any losses in the supercapacitor’s cell voltage and temperature. Designers should select initial ESR (equivalent series resistance). The load sees a voltage of capacitance and ESR so that the end-of-life capacitance and

VINITIAL−ESR×ILOAD, where ILOAD is the load current. Because ESR can support the applications.

Figure 2 This simple and ef-

VSCAP VLOAD fective charging circuit targets use in cases in which the ESR SOLAR I C ILOAD Figure 1 Ideal supercapaci- CELL PV open-circuit voltage of a solar- tor model; capacitance in cell array is less than the su- series with ESR. percapacitor’s rated voltage.

[www.edn.com ] June 7, 2012 | EDN 39

EDNMS4441 Fig 1.eps DIANE

EDNMS4441 Fig 2.eps DIANE Supercapacitor charging BAT54 diode because it has a low forward voltage at low A discharged supercapacitor looks like a short circuit to an currents—that is, the forward voltage is less than 0.1V at a energy source. Fortunately, many energy-harvesting sources, forward current of less than 10 μA. such as solar cells and microgenerators, can drive into a short Microgenerators are ideal for industrial-control applica- circuit and directly charge a supercapacitor from 0V. ICs to tions, such as monitoring rotating machinery, because by interface energy sources, such as piezoelectric or thermoelec- definition they will vibrate when they are operating. Figure tric energy, must be able to drive into a short circuit to charge 4 shows the voltage-current characteristic of a microgenera- a supercapacitor. tor, which is similar to that of a and which delivers The industry has invested much effort in MPPT (maxi- maximum current into a short circuit. A microgenerator also mum-peak-power tracking) to most efficiently draw power includes a diode bridge, which prevents the supercapacitor from energy-harvesting sources. This approach is applicable from discharging back into the generator, leading to a simple when charging a battery that must charge at constant - charging circuit (Figure 5). age. The is typically a dc/dc converter that is The open-circuit voltage is 8.5V, requiring a dual-cell super- a constant-power load to the energy source, so it makes sense , such as the CAP-XX HZ202, which operates at 5.5V. to draw that power at the most efficient point using MPPT. A shunt regulator provides overvoltage protection, and a low- In contrast to a battery, a supercapacitor need not charge at current active-balance circuit ensures even distribution between a constant voltage but charges most efficiently by drawing the the cells. Linear Technology, with its LT3652, LTC3108, and maximum current the source can sup- ply. Figure 2 shows a simple and effec- D PANEL V 1 tive charging circuit for cases in which OUT BAT54 0 TO 3.78V LOAD the open-circuit voltage of a solar-cell + array is less than the supercapacitor’s VMAX=2.754V rated voltage. The diode prevents the + R3 100k supercapacitor from discharging back R I 1 through the solar cell if it goes dark. If PV 10M R4 the energy source’s open-circuit volt- 1.2M age is greater than the supercapaci- V =2.754V + 3 ON 6 + HS130 tor’s voltage, then the supercapacitor VOFF=2.714V IC 2.4F + 1 1 requires overvoltage protection using TLV3011 26M 4 a shunt regulator (Figure 3). A shunt 2 − I Q 5 PV 1 R regulator is an inexpensive and simple IRLML6401 2 approach to overvoltage protection, 68k R5 and, once the supercapacitor fully 1M charges, it does not matter whether LOAD the excess energy dissipates. – The energy harvester is like a hose with an endless supply of water fill- Figure 3 If the energy source’s open-circuit voltage is greater than the supercapaci- ing a barrel, which is analogous to a tor’s voltage, then the supercapacitor requires overvoltage protection using a shunt supercapacitor. If the hose is still run- regulator. ning once the barrel is full, the water may overflow. This situation differs from that of a battery, which has a limited energy supply and thus would 3000 18 require a series regulator. 16 In the circuit in Figure 2, the 2500 supercapacitor, at 0V, draws short- 14 circuit current from a solar cell. As 2000 12 PMG17 CURRENT the supercapacitor charges, the cur- PMG17 POWER 10 CURRENT 1500 POWER rent decreases, depending on the solar 8 cell’s voltage/current characteristic. (μA) (μA) 1000 6 The supercapacitor always draws the 4 maximum current it can, however, 500 EDNMS4441 Fig 3.eps DIANE so it charges at the highest possible 2 rate. The circuit in Figure 3 uses the 0 0 TLV3011 solar cell because it inte- 01234 56789 grates a voltage reference, draws only MICROGENERATOR OUTPUT VOLTAGE (V) approximately 3-μA quiescent cur- rent, and is an open-drain cell so that Figure 4 The voltage-current characteristic of a microgenerator is similar to that of a the output is open-circuit when the solar cell and delivers maximum current into a short circuit. regulator is off. This circuit uses the

40 EDN | June 7, 2012 [www.edn.com ] DC GENERATOR V =5.25 TO 5.35V VOUT 0 TO 11V IN LOAD +

R8 100k R 7 R R1 10M 4 3.9M 2.2M + 1 R + + 3 3 5 IGN 6 470 IC 4 1 2 IC1 TLV3011 3 4 2 2 HZ202 + − − C1 Q 5 0.09F 1 R MAX447DEVK 1 nF IRLML6401 6 200m 49k R R R 2 9 5 2.2M RL 1.2M

LOAD –

OVERVOLTAGE PROTECTION

Figure 5 A microgenerator includes a diode bridge, which prevents the supercapacitor from discharging back into the generator, leading to a simple charging circuit.

LTC3625 ICs, and Texas Instruments, with its BQ25504, charge in Figure 6 have leakage currents of 0.2 and 0.3 μA after supercapacitors from energy-harvesting sources. 160 hours. Leakage current increases exponentially with temperature. The time it takes to settle to the equilibrium Leakage current value decreases with increased temperature as the ions dif- Because some energy harvesters deliver only a few microamps, more rapidly. Thus, these require a minimum leakage current becomes important. Supercapacitors can have current to charge from 0V. Depending on the supercapacitor, leakage currents of less than 1 μA, making them suitable for this current ranges from 5 to 50 μA. Designers should con- energy-harvesting applications (Figure 6). EDNMS4441 Figsider 5.eps testing DIANE the minimum charging current when selecting When a supercapacitor charges, the leakage current a supercapacitor for an energy-harvesting circuit. decays over time as the ions in the carbon electrodes diffuse into the pores. The leakage current settles to an equilibrium Cell balancing value, which depends on capacitance, voltage, and time. Circuits requiring that the supercapacitor’s terminal volt- Leakage current is proportional to cell capacitance. A rule age is greater than the cell-rated voltage require several of thumb for equilibrium-leakage-current supercapacitors supercapacitor cells in series to reach the rated voltage, at room temperature is 1 μA/F. The 150-mF capacitors such as 5V or 12V. In this case, a cell-balancing circuit is necessary; otherwise, one of the cells could go into an overvoltage condition because the cells all have 40 CAP-XX GZ115 0.15F slightly different leakage currents, CAP-XX GZ115 0.15F 35 CAP-XX HS230 1.2F, 5V with different voltage-to-leakage- CAP-XX HS230 1.2F, 5V current characteristics. Because they 30 MAXWELL PC10 10F MAXWELL PC10 10F are in series, however, they must all POWERBURST 4F 25 POWERBURST 4F have the same leakage current. To POWERSTOR 1F achieve this goal, the cells redis- LEAKAGE 20 POWERSTOR 1F CURRENT NESSCAP 3F tribute charge among themselves; NESSCAP 3F (μA) 15 AVX 0.1F, 5V in doing so, one cell may go into an AVX 0.1F, 5V overvoltage state. Cells at varying 10 temperatures or aging over time at 5 different rates can exacerbate this problem. The simplest balancing 0 circuit is a resistor in parallel across 0204060 80 100 120 140 160 180 each cell. Depending on the leakage TIME (HOURS) current of the supercapacitor and the operating temperature, the resistor’s Figure 6 A rule of thumb for equilibrium-leakage-current CAP-XX supercapacitors at value typically ranges from 1 to 50 room temperature is 1 μA/F. kΩ, but the leakage current through the balancing circuit is too high for

[www.edn.com ] June 7, 2012 | EDN 41 or solar panels in winter sunlight. Supercapacitor ESR at −30°C is typically two to three times ESR at room R1 33.2M temperature, so the device can still deliver peak power even at low temperatures. In contrast, the internal + C1 0.5F + impedance of thin-film batteries may reach several SUPERCAPACITOR MAX4470 kilohms at such low temperatures. CELL

− R3 470 complementing batteries In some applications, supercapacitors are alternatives to batteries; in others, they support them. In some situations, a supercapacitor may be unable to store C3 C4 R2 + C2 10 nF 100 nF sufficient energy, necessitating the use of a battery. 0.5F 33.2M For example, when the ambient-energy source—the SUPERCAPACITOR CELL sun, for example—is intermittent, as it would be at night, then the device must store energy not just for peak-power delivery but also to support the applica- tion for an extended time. If the needed peak power Figure 7 Low-current active-balance circuits target use in exceeds the amount the battery can supply—for GSM energy-harvesting applications. calls or low-power transmission in cold temperatures, for example—then the battery can charge the super- capacitor at low power, and the supercapacitor can most energy-harvesting applications. A better approach deliver the high power bursts. This arrangement also means for energy-harvestingEDNMS4441 applications Fig 7.eps DIANE is to use a low-current that the battery never cycles deeply, extending battery life. active-balance circuit (Figure 7). Supercapacitors store energy by physical-charge storage, not The MAX4470 op amp in the figure has a supply cur- chemically as in batteries, so supercapacitors have an effec- rent of 750 nA and a rail-to-rail input and output. R3 limits tively infinite cycle life. the output current in the event that one cell causes a short When a supercapacitor charges from a battery to supply circuit. The resulting design draws 2 to 3 μA after 160 hours peak-power bursts, there is a critical interval between bursts of balancing a 0.5F CAP-XX HW207 supercapacitor (Figure where if the bursts arrive more often, then it is more energy- 8). To suit a log scale, the absolute value of cell-balancing efficient to always leave the supercapacitor on charge. If the current can be positive or negative. bursts arrive less often, however, it is more energy-efficient to charge the supercapacitor just before the peak-power event. Temperature performance This interval depends on several factors, including the charge A major advantage of supercapacitors for energy-harvesting that the supercapacitor absorbs before reaching equilibrium applications is their wide temperature performance. Examples leakage current, the self-discharge characteristic of the super- include powering location-tracking units using vibration capacitor, and the charge the circuit draws from the superca- transducers, which may be operating in subzero temperatures, pacitor to supply the peak-power event. This choice is avail- able only if you know beforehand when the peak-power event will 0.01 3 occur, and that is not possible if it is in response to an unpredictable 0.001 2.5 event, such as battery failure or an external stimulus.EDN 0.0001 2 CELL VOLTAGE CURRENT 0.00001 1.5 Acknowledgment (A) (V) This article originally appeared on 0.000001 1 EDN’s sister site, SmartEnergy Designline, http://bit.ly/IHmuRf. 0.0000001 0.5

0.00000001 0 Author’s biography 0 1000 10,000 100,000 1,000,000 Pierre Mars is vice president of quality Time (SEC) and application engineering for CAP- XX Ltd. He has both bachelor’s and master’s degrees in electrical engi- ABSOLUTE OP-AMP SUPPLY TOP CURRENT BOTTOM CURRENT POWER CURRENT CURRENT neering from the University of New TOTAL CURRENT BOTTOM CELL VOLTAGE TOP CELL VOLTAGE South Wales (Sydney, Australia) and a master’s degree in business admin- Figure 8 This design draws 2 to 3 μA after 160 hours of balancing a 0.5F CAP-XX HW207 istration from the European Institute supercapacitor. for Business Administration (Paris). Mars is also a member of the IEEE.

42 EDN | June 7, 2012 [www.edn.com ]