STM 300 ENERGY STORAGE — Design Considerations
Contents
1. APPLICATION NOTE 208 Green Smart Wireless enocean Energy management of the STM 300 functioning principle Batteryless EnOcean devices work on the environmental existing energy around them So the available environmental energy sources are various for example light temperature difference and movement depending on the particular application and its surroundings If it is to be used this energy must first be converted into electric charge for instance by ap propriate energy converters like solar cells or Seebeck elements Typically there is not enough ambient energy continuously available e g nighttime so rechargeable longterm energy storage is used to ensure functioning when it is dark The disadvantage is that it needs a relatively long time to reach the startup voltage and the larger the energy storage capacity the longer the charging time To support a fast startup and longterm operation in times when no energy supply is available usually two different kinds of storage are used The small storage fills quickly and allows fast startup The large storage fills slowly but once filled it provides a large buffer for times where no energy is available e g at night Fig 1 The STM 300 therefore provides an extra digital output that allows control of the charging of these two external storage mechanisms The shortterm storage capacitor has a low resistance and also serves as a buffer for the short current spikes for more detai2. is a capacitance larger than 0 1 F Important regarding energy storage is also a very low lea kage current self discharge and a sufficiently wide nominal voltage range ideally up to 5 5 V since the stored energy is E 2 C U Generally longterm storage depending on technology features low nominal voltages between 2 7 and max 5 5 V and also relatively high internal resistance Leakage current depends on the charging time technology tem perature and rises proportionally with physical dimensions and capacitance TH leaded devices usually have much lower leakage current than SMD alternatives Their leakage cur rent is typically specified after the first hundred hours charging time This value tends to fall with time after some charging weeks Like electrolytic capacitors all Supercaps age over time meaning that their capacitance Slowly decreases The rate of aging depends mainly on the maximum operating voltage and ambient temperature The last is the key driver the Ri rate of aging approximately doubles for every 10 C Insulation resistance Ri Ci Background The equivalent circuit of such Supercaps looks like many parallel connected RC combinations WWW 7 see diagram C 2Ci The single resistance values WWW can increase or decrease depending on the individual distance between current collectors contact resis tances etc R1 R2 and Rn are the internal resistance Rn of the single cells while C1 C2 and Cn are their ele
3. NEC Tokin Supercap and Kanthal Max cap for specific requirements are listed in the Alternative energy storage table at the end of this application note together with their key specifications and associated recommended charging circuits Overvoltage Charge switcher e Charge switcher e Overvoltage protection e Undervoltage protection Energy source protection e g solar panel Depending on the selected longterm storage device one or STM 300 more of these function blocks will be needed circuit variants aii i Underveltage A B C see below protection VDDLIM T i iC CRESS SSeS WXODIO l RC delay y i F H F s Shortterm storage Longterm storage Block presentation of the universal fully featured charging circuit with all its function blocks Function blocks selection Depending on the used energy storage device different charging circuits with different function blocks are recommended Below are three representative examples for more de tails refer to the Alternative energy storage table at the end Energy storage Charge Overvoltage Undervoltage Charging switcher protection protection circuit Yes Yes Yes A PAS614L FCOV224ZFTBR24 3 5 V LT055104A 5 5 V p Ys N N B Charge switcher The charge switcher connects both shortterm storage and longterm storage parallel to the energy source as soon as the STM 300 supply voltage reaches the typical Von threshold of 2 45 V
4. Supposing VDD then falls below Von the energy source will be switched back to shortterm storage alone for faster recharging As long as the voltage on longterm storage remains below Von the charge switcher will continuously switch the energy source between shortterm and longterm storage trying to ensure continuous device operation That is be cause of the higher resistance and capacitance of longterm storage which would lead to much too long charging i e non operative time In addition shortterm storage cannot be charged over this threshold until the voltage on longterm storage exceeds Von Overvoltage protection All of these longterm storage solutions have a rated operating voltage that must be not exceeded After reaching this limit the energy source is automatically separated from sto rage to avoid any damage EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 4 7 APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Design Considerations Undervoltage protection deep discharge protection Some forms of longterm energy storage like rechargeable batteries but also PAS capaci tors should not be deep discharged To avoid longterm degradation of their capacity and lifetime an undervoltage protection block can be added Transmission of one complete EnOcean telegram requires a known specific amount of energy Its consumption causes a voltage drop on the stor
5. age element depending on its capacity It should be remembered that E y2 C U As long as the voltage on shortterm energy storage remains higher than Von the stored energy is sufficient to transmit at least one complete telegram before the voltage in the absence of an external energy supply drops below Voff refer to the STM 300 user ma nual For this purpose a typical 470 uF capacitor value is recommended That means as long as shortterm C1 storage is consistently recharged to Von within a pre determined time frame R6 C3 short voltage undershoots below Von as a result of tele gram transmission would cause no separation of longterm C2 storage from a device Only if the supply voltage still remained below Von after this time frame would a new tele gram transmission cause a voltage drop below the Voff threshold In this case longterm energy storage would be automatically separated from the device to avoid deep discharge The major advantages of this feature are protection of the PAS capacitor for example and the shorter recharging time enabled by energy conservation Circuit A universal fully featured strongly recommended for PAS capacitors M1 PMEG2010AEB SOLAR PANEL A Ts Lal L L IA A 4 a K ae gt lt STM300 m T os M2 z RI Q1 oe 2 CCO e BC847B oa ae 1S 100ME A 100nF R6 3 00MEG VDDs So gt s n S voos R pg gt ap Jo oo C2 h 470uF
6. c trostatic capacitance If a voltage V is applied to the equivalent circuit and considering the equivalent circuit 10 of the electric Supercap as having many small capaci tors Cn with various internal resistances Rn then the current that flows through an individual capacitor Cn can be stated by the following equation Cn Charged current A I y t 104 x In R expt a R o 120 240 360 Charging time s EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 2 7 APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Design Considerations So the current within the full capacitor can be regarded as the sum of the currents Ii flowing through each of the small capacitors It can also be seen that if the C x R value is small the charging time will be short Conversely if the time constant C x R value is large the charging time will be long Important It should be noted that if the charging time is limited to only several minutes or the charging source is current limited a Supercap may not be sufficiently charged to provide the required backup energy for the intended time If the capacitor is not sufficiently charged and is called upon to discharge its energy into a load the discharge current will flow from a high to a low voltage level thus causing a sharply sinking terminal voltage A Supercap always requires time to fully charge 3 Findi
7. current of PAS614L lithium technology drops significantly more with charging time than in all double layer capacitor technologies e g FC series Supercaps from NEC Tokin and after a few weeks reaches levels under 60 nA see diagram One of the reasons is what is called the longterm forming effect of these capacitors as a result of their inherent construction PAS capacitors are already charged in manufacture and they keep their charge even after many months of storage until going into use Their output vol tage characteristic looks rather like that of a rechargeable battery unlike conventional ca pacitors where the output voltage falls linearly with their discharge they maintain the out put voltage until near the end of their service life All other capacitor types are discharged from the beginning or will be self discharged within days and thus are not formed or lose their forming The longer capacitors are polarized with a sufficiently high voltage the faster and better is their longterm weeks forming process see also diagrams below PAS61I4L Leak Current ra 4 mo a La 1000 p 100 z i D Current uA Charge Time day Disclaimer The information provided in this document describes typical features of the EnOcean radio system and should not be misunderstood as specified operating characteristics No liability is assumed for errors and or omissions We re serve the right to make changes without prior
8. e threshold voltage corresponding to the selected longterm ener gy storage voltage e g threshold nominally 3 2 V for a 3 3 V capacitor If the selected threshold is too low e g 3 0 V a relatively high amount of energy corresponding to a use ful voltage difference of 0 3 V would be wasted If the nominal threshold is too high e g exactly 3 3 V not forgetting that this could reach 3 4 V as a result of additional manufac turer tolerances it could be critical for energy storage life expectation The S 1000C32 M5T1x voltage detector consequently looks like the best compromise here rated 3 2 V Undervoltage protection is also implemented through M2 In normal operation when Vdd reaches the Von threshold the STM 300 charge control CCO goes high Q2 rapidly dis charges C3 to GND and M2 turns on longterm storage The C3 charge recovers very slowly over R6 so M2 cannot turn off longterm storage immediately Only if Vdd falls below Voff for a longer time does C3 have time to recover and finally to turn off M2 and thus the long term storage path from the STM 300 avoiding deep discharge 4 Charging circuit variants The following looks at two alternative light charging circuit variants Circuit B basic charging circuit alone If a longterm storage C2 with rated vol M1 PMEG2010AEB tage of gt 5 V can be used or the out lane Satan TA put voltage of the energy source e g R3 B ssa gt 23 D4 D2 solar panel is lim
9. ge protection but VDDs a amp Ss having a limited nominal voltage below VDDLIME po R9 Lee L s 5V As mentioned this circuit cannot avoid longterm deep storage discharg 100MEG 100 470pF 0 25F ing meaning a longer recovery time EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 6 7 APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Design Considerations 5 Alternative energy storages key specifications and recommended charging circuits A B C Capacitor Manu Capa Temp i Leakage Recom Max Dis Estim Recom facturer citance range i after mended charge charge dark time mended first voltage dis to 0O V operation charge 100 detector charge allowed after 100 circuit hours current h charge charge SMD up to nA IV TH mA Y N vs PAS DBS ELNA S 1000C35 3R6D334T Dynacap 25 to 13 5 7 5 200 3 6 M5T1G TH 72 Y 85 DXS ELNA 11 5 x 5 0 S 1000C35 3R6V H334U Dynacap 25 to 11 5 x 13 0 200 3 6 M5T1G TH 72 85 DBS ELNA S 1000C35 3R6D224T Dynacap 25 to 13 5x7 5 180 3 6 M5T1G TH Y 110 C 85 DXS ELNA 11 5 x 5 0 S 1000C35 3R6V H224U Dynacap 25 to 11 5 x 13 0 M5T1G 85 Kanthal Not re LT055104A Maxcap 40 to 14 5x 15 5 quired No limits 85 FCOV224ZFT NEC Gl 1000C34 BR24 Tokin 25to 10 5x5 5 M5T1G Supercap 70 Y C Re S 1000C32 PAS614L Tai 0 25 M5T1G VL3 mportant The leakage
10. ited below C2 nominal eae 5 K voltage external overvoltage protection STM300 RI 4 as mentioned above is no longer re COO V VT scare quired In this case the VDDLIM input of TAME the STM 300 connected to VDD is used VDD R gt a i as overvoltage protection up to 5 5 VOOM s V 50 mA This basic circuit B can be t 100MEG 470uF 0 25F consequently sufficient for all standard Supercaps but not PAS However this circuit cannot avoid longterm storage discharge in the absence of energy which means a longer recovery time for empty energy storage elements The circuit first charges shortterm Ci storage directly over D3 till the Von threshold is reached Then the STM 300 starts its charge control output goes high and M1 switches the energy source to longterm storage C2 over D2 The diodes D2 and D3 avoid self discharge of energy storage through the energy source in the absence of input energy D4 ensures a one way direction of energy flow from longterm C2 storage to shortterm C1 storage Circuit C overvoltage protection alone M1 100nF This circuit implements only charge with SOLAR PANEL s 6 overvoltage protection but without un 3 R p3 pe 2 c3 dervoltage protection It is actually the onal B5584 L aya x basic circuit above extended with over STM300 Ri Q1 a voltage protection The circuit can be CCO _ BC847B used for standard longterm storage not 100MEG NJ 6 requiring undervolta
11. l 0 25F T poomeg 100k t lt joomea LA o Charge switcher is the PMOS transistor M1 driven from the STM 300 charge control out put CCO WXODIO over Q1 To start with as long as the Vdd voltage is below the Von thre shold only the small storage C1 is filled over D3 Once the threshold is reached the charge control signal CCO goes high Q2 and M2 are turned on and the longterm storage C2 is filled over M2 Overvoltage protection is implemented by the S 1000C32 M5T1x voltage detector from Seiko SII or the NCP300LSN30T1G series Onsemi which limits the maximum charging voltage to 3 3 V to avoid damaging longterm energy storage In case a different voltage limit is required this device has to be replaced by a suitable voltage variant As soon as the voltage on D2 anode or the voltage detector input exceeds the selected threshold the vol tage detector delivers a High level on its output connected to the Q1 emitter The Q1 base is consequently lower polarized than its emitter and the transistor is turned off That means M1 is turned off too the energy source is switched off and longterm storage is protected EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 5 7 APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Design Considerations The selected voltage detector must have a very low quiescent current in the operating range and an appropriat
12. ls re fer to the user manual The charging voltage can be limited depending on the storage used to its nominal operating voltage So a smaller energy storage capacity can be charged relatively fast but the stored energy would not suffice to ensure the module surviving over a longer time e g weekend without energy On the other hand through the linear voltage increment on the capacitor under constant charging current too large a capacity needs too long a time or too large a solar panel to ensure a positive day night energy balance Energy converter and energy storage example One of the most common examples of converters of ambient energy is the solar cell Its ef ficiency depends on light intensity the spectral radiation of a light source and its technolo gy for more details refer to application note AN 207 Depending on application requirements energy demands and sur roundings different solar cell types and sizes can be chosen Given that most ap plications are in homes and buildings the solar panels used are indoor typically amorphous type One such solar panel consists of a number of serially connected solar cells on the same substrate such as glass Fig 2 _ Overflow Ke Lee ee ee ee ee ee ee ee ee ee ee ee ee e o o o o oa oa oa aa Fig 1 Energy storage example Fig 2 Solar panel APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Desig
13. n Considerations Calculation of the delivered electric charge is simple because for a given light source type the delivered current is directly dependent on the active solar cell area and light intensity while its unloaded output voltage changes very little over a fairly wide light intensity range Energy storage elements 1 Shortterm energy storage buffer typ 470 uF Unlike the STM 1x0 which needs a split solar panel the STM 300 already has integrated energy management with only one power supply input and no longer needs a split solar panel For normal operation and fast startup the STM 300 like the STM 1x0 firstly needs a low resistance and a capacitor rated a few hundred uF that is called shortterm storage This capacitor must have a low leakage current a low resistance and consists typically of a 470 uF SMD tantalum So a good solution is the TAJ series from AVX 2 Longterm energy storage element typ 0 25 F Very much larger about one thousand times longterm energy storage covers energy de mand over the night or bridges long periods without an external energy supply Different energy storage types listed in what follows can be used depending on specific application requirements These are generally Supercaps Goldcaps or Ultracaps Recommended for EnOcean applications depending on functionality sensor smart ack actuator configura tion and available energy e g number and size of solar cells average ambient light
14. ng the right solution for every application Unlike the shortterm storage buffer depending on the specific application the most impor tant requirements and priorities for longterm energy storage can be very different The most common and in part contradictory requirements are listed below Minimal leakage current Minimal dimensions Lowest possible system price energy storage charging circuit Maximum lifetime Device type TH or SMD Widest possible temperature range E Maximum usable energy There is no single best longterm storage device Every solution has its own advantages and drawbacks Furthermore the associated charging circuit is device specific and can include between one and all of the blocks described below So every block of the following charging circuits must be individually designed The block diagram shows the full maximum solution where the function of each single block is explained All the circuits use the STM 300 charge control feature and all known and applicable energy storage EDLCs Electrolyt ics PAS capacitors accumulators can be charged with at least one of the described charg ing circuit variants A B and C Due to the low STM 300 quiescent current compared to the leakage current of the energy storage elements such energy storage would be com pletely discharged over longer periods weeks in the absence of an energy source To avoid this it is recommended that the connection between longterm st
15. notice For the latest documentation visit the EnOcean website at www enocean com EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 7 7
16. orage and the STM 300 power input be cut off as soon as the voltage VDD of the STM 300 drops below Voff as Shown in the full charging circuit Such a procedure also considerably shortens the recovery time after a longer pause by conserving the residual energy in longterm storage This application note will give a short overview of the most competitive and appropriate long term energy storage solutions with their specific charging circuit requirements and help the user to choose the best solution for their specific application A very unique and unconventional solution is the use of PAS polyacenic semiconductor technology This combines the properties of a lithium battery immediately available supply voltage lowest leakage current with energy conservation over years smallest dimensions and a Supercap very long lifetime rechargeable while it is no battery The drawback is that such capacitors unlike all conventional Supercaps must not be discharged to voltages below about 1 5 V Otherwise their capacitance decreases over time So it is essential to observe the manufacturer s handling and soldering instructions A representative example is the PAS614L capacitor from Taiyo Yuden Shoei EnOcean www enocean com Subject to modifications Christian Bach Nov 2011 Page 3 7 APPLICATION NOTE 208 Green Smart Wireless enocean STM 300 ENERGY STORAGE Design Considerations Conventional alternatives from ELNA Dynacap
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