[introduction]Reserve task key equipment is confined to before power source solution, but industry, commerce applies with the extensive electron in consuming kind of end item now need to reserve power source. Although have a few kinds of choices, but use super capacitor to be able to achieve an appearance the solution with the most compact, the most concentrated energy, the harbour when be being interrupted as town report can device. For example, be interrupted when town report or change when batteries.

Reserve task key equipment is confined to before power source solution, but industry, commerce applies with the extensive electron in consuming kind of end item now need to reserve power source. Although have a few kinds of choices, but use super capacitor to be able to achieve an appearance the solution with the most compact, the most concentrated energy, the harbour when be being interrupted as town report can device. For example, be interrupted when town report or change when batteries.

However, super capacitor brought design challenge, because every super capacitor is highest,can offer the voltage of 2.7 V only. This may be meant offer regulated power supply to need many super capacitor to course of 5 V power source, and every capacitor has relevant batteries balance and step up or step-down converter. This forms a complex and subtle circuit, relatively costly and need more circuit board space.

The article will compare batteries and super capacitor, explanation why latter is compact model application of low voltage electron brought a certain number of technology advantages. How does the explanation design the article subsequently a kind of simple and clever solution, combine a reversible step-down with a capacitor only / step up converter is power supply of course of 5 V power source.

Batteries and super capacitor

Whether can contemporary electron equipment provide satisfactory user experience, the UPS already became a crucial factor. If do not have constant to decide power source, electronic product can quit the job not only, and lose important information possibly still. For example, if produce power failure, the personal computer that receives town report repeatedly will be lost maintain presence its lose the data in sexual RAM easily. Perhaps changing when insulin pump when batteries, may lose the important blood sugar reading in breaking sexual memory easily.

A kind of method that prevents this kind of circumstance happening is to use reserve batteries, use labor above all can, release electric energy when town report breakdown next. Lithium is ionic (the technology that Li-ion) batteries is a maturity, its energy density is very high, can offer for long relative to more compact batteries bulk implementation reserve power source.

But no matter its are basic chemical composition how, all batteries have respective striking feature, the problem may be caused below certain circumstance. For example, batteries is opposite heavier, need is relatively longer charge time (if often power cut, this may be a problem) , charge the number is limited (can increase maintain cost) and uses chemical may endanger safety and environment.

Reserve another kind of solution of power source is to use super capacitor. The formal name of super capacitor is capacitor of electric double deck (EDLC) . This kind of parts of an apparatus stabilizes by electrochemistry function, have the symmetrical carbon electrode of negative pole is formed. These electrode permeate segregator to separate by insulation ion, the park inside this kind of segregator is full of organic salt / in the container of electrolyte dissolvent. Electrolyte aims utmost land increases ionic conductivity and electrode wetability. Electrode of activated carbon of high table area and dinky charge depart are united in wedlock, make the capacitance far outclass of super capacitor traditional capacitor (graph 1) .

Graph 1: Of super capacitor use semmetry losing carbolic electrode, be isolated from mediumly in electrolyte through immersing ion permeates segregator to separate electrode. Detached photograph union brought about electrode of big list area and dinky charge high electric capacity. (picture source: Maxwell Technologies)

Charge stores with electrostatic means below the reversible sorption of electrolyte big surface accumulates carbolic electrode to go up. In electrode / charge happens to depart below the polarization action of electrolyte interface, produce homonymic double deck. Height of this kind of mechanism is reversible, make super capacitor can be endured tens of charge discharge 10 thousand times, although the elapse as time, capacitance value can appear a few decrease.

Because rely on electrostatic mechanism to store electric energy, the electric function of super capacitor compares batteries to be able to be forecasted more, and uses structural data makes his more reliable, suffer the effect that temperature changes not easily. In safe respect, the volatile material in super capacitor is less than batteries, and can discharge completely, implementation safety is carried.

Super capacitor excel 2 another respect of batteries is to charge rate is rapidder, and won’t produce overcharge. Accordingly, be in first when before long power source disappears again after breakdown, can throw reserve power source. Super capacitor still has more charging periodic, reduced thereby maintain cost.

In addition, super capacitor is had get much power density higher than batteries (can store inside quantity unit time or carry how many power with Yu Heng) . This ensures not only charge quickly, and still allow to undertake when need abrupt of big electric current is sent, make super capacitor can use as in more application reserve power source (graph 2) . In addition, the resistor of effective series connection of super capacitor (ESR) is lower than batteries much. This allows super capacitor to be able to provide power source effectively, and without overheat danger. The power changeover efficiency of super capacitor is more than normally 98% .

Graph 2: Can charge batteries can leave long-term power supply in proper electric current, but need charges for a long time. Contrary, super capacitor discharges quickly with big electric current, also can charge quickly. (picture source: Maxwell Technologies)

With can charge batteries photograph is compared, the main drawback of super capacitor is his energy density is relatively inferior (the electric energy that measures the memory inside every unit volume with Yu Heng) . The harbour of technology of batteries of current lithium ion can be to be the same as bulk 20 times of super capacitor. As use new material to undertake improvement to super capacitor ceaselessly, this one difference is narrowing, but this may be made in future old inside very remarkable still. Another of super capacitor marked weakness is batteries of ion of lithium of its cost prep above.

The design note of super capacitor

If product of some kind of electron wants to rely on super capacitor to regard as,reserve power source, so the architect must understand how to choose optimal component, in order to realize reliable harbour can, carry and prolong service life.

The temperature effect to capacitance and resistor is checked in data table, this is principal check one of item. Good design way is, the choice can work in the anticipation of terminal product the parts of an apparatus with the very little change inside temperature limits, and reserve in need when power source, can offer stable voltage and transport electric energy effectively.

The life of super capacitor basically is decided by the combined influence of working voltage and temperature (graph 3) . Super capacitor produces disastrous breakdown rarely. Contrary, the capacitance of super capacitor and internal resistance can produce change as the elapse of time and reduce performance gradually, till this component cannot satisfy the normative requirement of end item quite. Function drops in the beginning of end item life normally level is compared big, and as the ageing of end item drop off.

Graph 3: Higher temperature and the voltage that bring to bear on can shorten the life of super capacitor. (picture origin: Elcap, CC0, pass Wikimedia Commons, revise by the author)

Should use as reserve when power source, super capacitor will maintain for a long time fall in working voltage, can need to release the electric energy that its store now and then only. This can affect performance finally. Data table can give out to fall in typical job voltage and different temperature, capacitance lets time drop circumstance. For example, is a super capacitor in 25? Be in below C temperature 88, 000 hours (10 years) inside the voltage that maintains 2.5 V, its capacitance may decrease 15% , internal resistance increases 40% . Designing for the end item with longer service life reserve when power source, should consider this kind of function to drop.

The time constant of capacitor is to show capacitor is achieved fill completely 63.2% or discharge comes to be filled completely 36.8% when needs time. The time constant of super capacitor is 1 second about, this than electroanalysising capacitance weak point is gotten much. Because this time is invariable very short, the architect should ensure reserve power source is super capacitor won’t get the influence of successive ripple electric current, can cause damage otherwise.

Super capacitor can be mixed in 0 V its work between the largest nominal capacity. Although be in inside the widest voltage range when the job, can use the electric energy of the effective energy of super capacitor and memory effectively, but most electron component has lowermost voltage threshold value. Voltage of this kind of lowest asked to restrict the electric energy that can get from inside capacitor.

For example, store is the energy in capacitor E = ? CV2. Can calculate from inside this relation piece, if the system is in,capacitor moves below the half of rated voltage (arrive from 2.7 V for example 1.35 V) , can achieve the effective electric energy of about 75% .

Use the design challenge when many super capacitor

Although the advantage of super capacitor makes its suit to be extensive electronic product to offer,reserve power source, but architect must the design challenge that vigilance brings from this. To the engineer without experience, implementation reserves power source circuit may be a main job. Crucial complexity depends on, business is 2.7 V about with the rated voltage of super capacitor, when wanting the power supply of course of 5 V power source to the model accordingly, must establish ties two super capacitor (graph 4) .

Graph 4: Business is 2.7 V about with the rated voltage of super capacitor, when wanting the power supply of course of 5 V power source to the model accordingly, must establish ties two super capacitor. (picture source: Maxim Integrated)

Although this job solution is satisfactory, but because need to have battery of active or passive type,balance, because this meeting increases cost and complexity. The ESR that because capacitance tolerance, different discharge leakage current is mixed,differs, two or more name is photogenic with and the capacitor voltage that fills completely may be different. Lopsided meeting causes this kind of tension reserve the another of voltage prep above that one of circuit super capacitor offers. As temperature lift and / or the ageing of super capacitor, lopsided meeting increases this kind of tension, the rated voltage threshold value that can make the voltage of a super capacitor exceeds this equipment affects moving life.

To low occupy sky comparing for application, it is shunt-wound on every batteries normally its come to resistor of a bypass balance of batteries of implementation only part. When the resistance that chooses by-pass capacitor, should make any electric current metropolis prep above is super the leak electric current of capacitor. This kind of technology ensured any change of the equivalent shunt resistance between super capacitor are negligible effectively not plan. For example, if reserve,the average discharge leakage current of super capacitor in circuit is A of 10 μ , a 1% resistor will allow the by-pass current of A of 100 μ , electric current of will average leak raises A of 110 μ . Such, this resistor can change the leak electric current between super capacitor effectively from hundred reduce have hundred only.

In all shunt resistance implement match good case to fall, any super capacitor that have taller voltage will pass its shunt resistance, with prep above inferior voltage is super the speed discharge of capacitor. This makes total voltage even on the super capacitor that ground allocation goes to to establish ties entirely. To tall laden applying, need undertakes more complex super capacitor is balanced.

Use individual and super capacitor to realize 5 V power source

If use a super capacitor only, is not two or many, so reserve power source circuit is likely so won’t complex, what need a space fewer also. This kind of layout need not have a balance to super capacitor. However, the 2.7 V output of individual and super capacitor needs to rise through step up manostat, form enough voltage to overcome diode to press thereby fall, offer 5 V voltage for the system. Super capacitor through charging device charges, discharge of step up converter can be passed when need. Diode allows advocate power source or super capacitor provide power source for the system (graph 5) .

Graph 5: In reserve individual and super capacitor is used to need not undertake batteries is balanced in power source circuit, but the output voltage that need promotes super capacitor through step up manostat. (picture source: Maxim Integrated)

Cleverer solution is to use individual capacitor, and complementary with special voltage converter, be like manostat of step up of reversible step-down of the MAX38888 of Maxim Integrated or MAX38889. Former offer 2.5 V to be outputted with what be as high as 2.5 A to 5 V, and latter offers 2.5 V to come 5.5, 3A is outputted (graph 6) .

Graph 6: Should reserve with power source of capacitor of Yu Chao class when circuit, MAX38889 (or MAX38888) reversible manostat need not parts of an apparatus of alone charger, step up and diode. (picture source: Maxim Integrated)

MAX38889 is agile store can capacitor or capacitor group reserve manostat, can transmit electric energy effectively. Exist when town report and when the power source voltage of system of threshold value of lowest of its voltage prep above, manostat is charging the job below mode, it is super capacitor to charge with electric current of average inductance of electric current of peak value of the biggest 3 A, 1.5 A. Super capacitor is in as reserve the need before power source moves is achieved fill position completely. After super capacitor is charging, circuit uses up electric current of A of 4 μ only, maintain super capacitor to be in condition of be in order at the same time.

Should move except advocate when power source, manostat can be in of set highest below the electric current of inductance peak value that can amount to 3 A, voltage of will super capacitor promotes wants system voltage, work with preventing a system to drop to reserve to the system of set voltage is the following. Reversible manostat can maintain move, till voltage of super capacitor power source is 0.5 V only, thereby utmost favourable geographical position uses the electric energy that store.

Reserve the accident of time depends on the electric energy reserve of super capacitor and systematic power comsumption. The characteristic of Maxim Integrated product is to be able to pass super capacitor of individual 2.7 V to obtain the biggest reserve power source, because do not need alone charger, step up,install buy and diode at the same time, because this reduced the amount of yuan of parts of an apparatus of circuit.

Epilogue

If those need to change often,be in in the specific application of batteries, super capacitor has a certain number of advantages than 2 batteries, can use as reserve power source. With can charge batteries photograph is compared, super capacitor charges rate is rapidder, can repeat often use and power density is higher. However, when seeking the 5 V power source that backs a typical case, its are the biggest 2.7 V output brought challenge of a few designs.

If plan institute is shown, reversible step-down / step up manostat offerred a kind of clever solution, it allows individual and super capacitor to support 5 V circuitry, at the same time utmost ground reduces the amount of needs space and component.

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