[introduction]UPS (UPS) is acting important role in a lot of application, for example, independent disk is redundant array (the data protection in RAID) memory, car remote supervising system that is used at safe operation, and the appliance giving drug such as the insulin pump of Medical Protection industry.

  

UPS (UPS) is acting important role in a lot of application, for example, independent disk is redundant array (the data protection in RAID) memory, car remote supervising system that is used at safe operation, and the appliance giving drug such as the insulin pump of Medical Protection industry.

However, design UPS may have challenge sex, fall in the circumstance with limited space especially. In addition, to a lot of cannot bear the electric energy application from power source of storage system regurgitate, still need to undertake elaborate design.

The consideration uses compositive method, mix many converter charge circuit is replaced individual yuan of parts of an apparatus, can reduce the difficulty of this kind of design. This kind of compositive method can simplify circuit design, more ensure easily in reserve there is voltaic regurgitate power source during power source moves.

The article summarized UPS to designed difficulty to introduce a kind of traditional solution. Subsequently, the article introduced a kind of simplifying compositive type to replace a method, the step-down that this method is based on Analog Devices / manostat of step up switch.

Use super capacitor to regard as store can implement

Graph the 1 UPS that showed a kind of tradition devises a method. Give typical examples is here medium, UPS is power supply of 24 VDC sensor. The circuit of this sensor needs 3.3 V and 5 V input. When systematic voltage is usable, UPS uses linear manostat to charge for super capacitor. If systematic voltage drops, the electric energy in capacitor can raise the level of power supply tension that requires to place through step up manostat.

Graph 1: This UPS charges for super capacitor when systematic voltage is normal, in the derive when systematic voltage drops among them electric energy. (picture origin: Analog Devices)

If 24 V power source still is used at be the power supply of other circuit element beyond sensor, answer and capacitor of class of unfavorable balance of trade, make its are sensor circuit power supply only, and do not be the power supply of other electron equipment related to 24 V circuitry. Be in when circuit reserve when mode, diode ” D ” can prevent to produce this kind of situation.

Although movement of this kind of system is favorable, but the likelihood is carried out hard, because it needs,use many voltage converter. In addition, if the space is limited, the likelihood also is a challenge. Pursue 2 showed another kind replaces a method. This method is used individual reserve manostat will replace a plan a many manostat in showing circuit, can save a space not only, still can simplify design.

Graph 2: Compositive type reserves manostat can make the design of UPS is mixed more simply compact. (picture origin: Analog Devices)

Compositive type reserves solution

The designs a concept to be able to pass Analog Devices MAX38889 step-down that the graph shows 2 times / manostat of step up switch will come true. This is agile and compact store can capacitor or capacitor group reserve manostat, can transmit electric energy effectively. Dimension of this parts of an apparatus is 3 X 3 Mm, input from the 0.5 super capacitor to 5.5 V (VCAP) , in 3 A the biggest electric current (the 2.5 output to 5.5 V can generate below ISYSMAX) (VSYS) (graph 3) . The working temperature limits of this manostat is – 40 ° C comes + 125 ° C.

Graph 3: To be based on the UPS of MAX38889, the ISYSMAX that gives VSYS depends on VCAP. (picture origin: Analog Devices)

When advocate power source puts through and tension of power supply of system of its voltage prep above is the least when threshold value, this manostat can feel with the average report of the peak value electric current of the biggest 3 A and 1.5 A electric current charges for super capacitor. After super capacitor fills report, when holding position of be in order, static electric current has A of 4 μ only. Super capacitor is in as reserve before power source moves, must achieve fill position completely.

When advocate power source disconnects and when super capacitor fills report, this manostat can prevent a system to fall to reserve to the system of set working voltage (VBACKUP) is the following. This parts of an apparatus comes through making super capacitor spark voltage rises VSYS (namely voltage of stabilized voltage system) will achieve this goal. In reserve during power source moves, MAX38889 is used suit oneself guide a time and modulation of frequency of current limliting pulse (PFM) dominates program.

Of manostat exterior cite a base controallable all sorts of settings, for example voltage of the biggest super capacitor (VCAPMAX) , VSYS, and peak value inductance charges and discharge electric current.

MAX38889 is had involve a function really, can disconnect SYS and CAP, prevent SYS short circuit when VCAP is more than VSYS. Maintain low n through making ENC and ENB cite a base respectively, can ban with charge to use a function with equipment (graph 4) .

Graph 4: Those who pass MAX38889 is exterior bring a foot to be able to install VCAPMAX of voltage of the biggest super capacitor, VSYS, and peak value inductance charges and discharge electric current; Reserve systematic condition can undertake monitoring through RDY mark. (picture origin: Analog Devices)

Reserve systematic condition can undertake monitoring through two state output: Condition of be in order (RDY) indicates (indicate super capacitor charge time) and reserve condition (BKB) indicates (the directive reserves power source runs state) .

Super capacitor type selecting

Graph 5 showed those who be based on the UPS of MAX38889 to simplify applied circuit. During charging, VCAPMAX makes decision of resistor voltage divider by drive FBCH. Give typical examples is here medium, Ω of Ω of resistance R1 = 1.82 M, R2 = 402 K and Ω of R3 = 499 K can ensure VCAPMAX setting is 2.7 V. Super capacitor feels with the average report of the peak value of the biggest 3 A and 1.5 A electric current charges. During discharge, electric current of peak value inductance is 3 A.

Graph 5: Graphic representation is those who be based on the UPS of MAX38889 to simplify applied circuit. Super capacitor feels with the average report of the peak value of the biggest 3 A and 1.5 A electric current charges. During discharge, electric current of peak value inductance is 3 A. (picture origin: Analog Devices)

In to reserve power source runs an option when super capacitor, need to notice all the more. When advocate when power source occurrence breakdown, laden power is offerred by MAX38889, this parts of an apparatus uses super capacitor to regard energy as origin, in reserve or move below step up mode. Super capacitor is in the power that the power that its can provide below voltage of canzonet section power supply most must be more than systematic place to need.

When the working voltage of MAX38889 is close to VCAPMAX, this parts of an apparatus can offer constant power load for super capacitor, bring about from which the voltaic decrescent of derive. But, discharge when super capacitor (and voltage drop is small) when, from which the electric current of derive can increase, in order to keep laden constant power. In reserve during power source moves (TBACKUP) , reserve the product that the energy that requires below mode uses power for continuous equipment (VSYS X ISYS) .

Energy of the practicable in capacitor of class of 1 calculable favorable balance of trade is worth use formula (with joule [J] is an unit) (CSC) :

 Formulary 1

Use formula 2 calculable piece finish reserve the energy cost that power source moving place requires:

 Formulary 2

Among them ISYS is the load current during reserving.

Because reserve,the energy that laden place requires during incident is offerred by super capacitor, change efficiency suppose (η ) the TBACKUP that gives place to need, use formula the 3 CSC that can need certainly are worth (with farad [F] is an unit) :

 Formulary 3

The applied circuit that to in an attempt to shows 5 times is exemple, hypothesis system load is 200 MA, average efficiency is 93% , reserve time is 10 S, the least value is the super capacitor that needs:

 Formulary 4

Graph 6 showed the graph shows those who use circuit 5 times to charge with discharge curve.

Graph 6: The graph shows those who use circuit 5 times to charge with discharge curve. VSYS = 3.6 V, VCAP = 2.7 V, VBACKUP = 3 V. (picture origin: Analog Devices)

Begin to use evaluate board

Management of power source of MAX38889AEVKIT# capacitor charger is evaluated board supply agile circuit, can evaluate step-down / step up reserves manostat, check the UPS that is based on MAX38889 and super capacitor. Exterior support of yuan of parts of an apparatus all sorts of systems and super capacitor voltage and charge and discharge electric current.

Should evaluate board include 3 spreader: ENC (enable charge) , ENB (enable reserve) with LOAD (graph 7) . Set below the circumstance of positional 1-2 in ENC spreader, charge when VSYS prep above enable when threshold value charge. Set below the circumstance of positional 1-2 in ENB spreader, fall when VSYS to reserve enable when threshold value is the following reserve. LOAD can be set check mode in order to enter for positional 1-2, among them laden bond of 4.02 Ω arrives VSYS, and ground connection puts electric field view with imitate. If spreader joins one cites a base only, should evaluate board will enter regular job mode.

Graph 7: MAX38889AEVKIT supplies agile circuit, can evaluate MAX38889 step-down / step up is super capacitor reserves manostat. (picture origin: Analog Devices)

When advocate the voltage that batteries offers charges more than when needs lowermost system voltage, MAX38889 manostat charges for super capacitor with the average electric current of 1.5 A, VFBCH = 0.5 V, when Ω of Ω of Ω of resistor R1 = 499 K, R2 = 402 K, R3 = 1.82 M, VCAPMAX = 2.7 V.

EVKIT VBACKUP by Ω of resistor R5 (1.21 M) with Ω of R6 (1.82 M) the setting is 3 V, VFBS = 1.2 V. This makes clear when advocate batteries disconnects and VFBS falls to 1.2 V when, MAX38889 can adjust VSYS from electric energy of super capacitor derive to VBACKUP.

MAX38889A EVKIT offers RDY to check a dot to come monitoring is super capacitor charges condition. The FBCR voltage threshold value that brings crural voltage to exceed 0.5 V when FBCR (install by R1, R2 and R3) when, RDY checks a dot to be tall n. This is meant when VCAP exceeds 1.5 V, RDY will be elevatory. Same, spare circumstance is offerred to fall in super capacitor, the voltage that offers when super capacitor is low at 1.5 V, RDY mark can be reduced.

EVKIT still offers BKB to check a dot to come supervisory system reserves condition. Offer when the system reserve when power source, BKB is pulled low; When charging when the system or be in unwanted position, BKB is pulled tall.

Resistor (R4) can install ISET and ground connection (the electric current of peak value inductor between GND) . According to the following formula, the resistance of 33 K Ω is setting of electric current of peak value inductance 3 A: Peak value charging current (Ω of ILX_CHG) = 3 A X (33 K / R4) (graph 8) .

Graph 8: Graphic representation is evaluated for MAX38889 board principle graph; This parts of an apparatus uses 11 F super capacitor to move, offer test site to come monitoring VCAP, VSYS, RDY and BKB. (picture origin: Analog Devices)

Summary

Super capacitor can use as of UPS store can component. Conventional UPS pictures attacks use many manostat, these manostat need to take up a large number of spaces, it is very intractable to because this is designed,rise. Use compositive type step-down / step up manostat can mix many converter charge circuit is replaced individual and compact model yuan parts of an apparatus, reduce the difficulty of this kind of design thereby.

(author: Steven Keeping)

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