How to switch from SMPS to battery?

[blapcb]

how to switch on smps

I am planning to power a motherboard with a 12vdc regulated power supply (which feeds a ATX power supply, going to the board). I also want to include a 12v battery that will act as a backup. My questions are:

1. What type of a circuit is used to switch from the SMPS to the battery?
2. What type of circuit is used to charge the battery and how does it fit in to all this?

I am sure that this is basically a simple question, but I do not know the proper terminologies for this so would appreciate some help. Once I know what to look for I can Google further etc.

Thanks!

 

[snafflekid]

smps powersuply 13.7v

A computer motherboard also uses +/- 5V and 3.3V supplies. The 12V is lightly utilized in today's computers. A UPS is the simplest solution.

N/M. I see you want to power the ATX supply. There are many different designs, some even tolerate DC input, but you likely need higher input voltage than 12 V.

 

[blapcb]

battery supply switch

A computer motherboard also uses +/- 5V and 3.3V supplies. The 12V is lightly utilized in today's computers. A UPS is the simplest solution.

You did not read the question. I said that board is powered from a ATX power supply (which has all the voltages you mentioned) but is fed from a 12VDC supply (unlike most ATX PSs that are fed from mains power). It is this 12VDC PS that I want to switch over to a bettery if it fails.

 

[IanP]

constant voltage constant smps

One simple method of switching over power from two sources is by using a relay with NO (normally-open) and NC (normally-closed) contacts ..
The relay’s coil is activated by mains-powered power supply and the same power flows to load through NO-Common contacts ..
When the mains-power is not present, the relay’s coil is de-energized, and the power flows from the battery and NC-Common contacts ..
On top of that, you can add a circuitry that charges battery when the mains power is present ..
Here is an example of similar circuit:
https://www.siliconchip.com.au/cms/A_109625/article.html

Of course, you can find special ICs + power MOSFETS + this and that, but that depends on the complexity of the circuit you are trying to design ..

Rgds,
IanP

 

[FvM]

mains battery switch

As the most simple solution, you can operate the battery as it's done in a car. Have a 14.4V constant voltage/constant current supply connected in parallel to the battery. Switchover is achieved automaticly without additional circuitry. As far as I know, most simple UPS systems are operated in this way.

 

[TekUT]

smps with battery backup

I've similar trouble but my source can be either main line and a 80W 12 V solar cell, my load is a 12 V @ 3A systems.
Because the system is build around a embedded PC every transition must be did in zero transfer time.

Regards
Pow

 

[FvM]

connecting battery in smps

If direct parallel circuit, as the sketched backup battery solution isn't suitable, (schottky) diodes can be used to combine the sources. Each source must have it's own regulator, the preferred source (e.g. solar panel) is set to a higher voltage. Considering allowed voltage tolerances, this solution should be suitable for most applications. I have used it e.g. for uninterruptable 24V PLC supply. If tighter voltage tolerances are required, you can use lower voltage drop electronic switches. Or design a two-way voltage regulator, supplied from two sources. Furthermore, many SMPS are suited for direct parallel circuit, without diodes.

 

[GrandAlf]

parallel smps schottky

If the power supply can be a few hundred millivolts higher than the battery, you could connect a battery with a series diode across the supply. Anode to battery, cathode to psu, grounds common. Normally if the psu were on, it would reverse bias the diode, hence no current from battery. If supply fails, diode would be forward biased and deliver current from the battery, pretty instant. For low voltage drop, high current shottky diode would be best. If the battery is rechargeable, you could wire a resistor across the diode to give an appropriate trickle charge.

 

[FvM]

supply switch schematic battery

If the power supply can be a few hundred millivolts higher than the battery

Doesn't work with a few hundred millivolts. If the battery is intended to be charged (most likely), 13.7V with sealed batteries up to 14.4 with standard lead-acid cells are required. Discharge end voltage must be calculated with 10.5 up to 12 V, depending on the load current. So the voltage variation in standby operation is always in a several volts range.

 

[GrandAlf]

lead acid smps solar cells

In practical terms you are right of course. I probably did not explain very well. What I meant was that the PSU should always be a few hundred millivolts higher than the battery. Ideally for any reasonable charge you would need around 15v. This may of course be perfectly ok as an input to a smpsu. I have used this method to add backup a battery supply to systems that do not have it as standard, not the best way, but works fine. Just throwing a few ideas around anyway.

 

[TekUT]

operation of a smps

Here below I've sketched a theoretical possible solution. About the main line power supply there is several commercial power supply available. About the solar cell charger I've some doubt about right behaviour of the charger, every type I've see the battery backup is connected directly to the charger and the loa take another output, in these system the battery will be switched to the load only when the main power is lost but I need that the battery is always connected to the load and also in this situation the charger must act also as a power supply.
I've two battery on the drawing because I need to sustain the system for (minimum) 48 h then is not a current reason but mainly to have more time when the system will be powered only from the battery backup.
To increase the efficiency I can use a ideal diode controller

https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1142,D14177

instead conventional Schottky diodes between the power source and the charger.

What is your think about this stuff? I'll really appreciate every idea, suggest, info on this subject.

Thanks in advance
Pow

 

[FvM]

3.3v smps

As said, the battery voltage is varying between load and discharge. So as first point, you have to clarify, if the connected equipment can accept these variations. Most devices intended for battery operation, shoud be O.K. with e. g. 10-15 V.

Normally, you won't need separate SMPS and battery charger, if the SMPS has a regular constant current/constant voltage characteristic. This is the case with many SMPS. Because of the rather high battery capacity, there shouldn't be a problem when using the regular current limit in charge operation, if the SMPS can source it unconditionally. Also a diode isn't needed, if the SMPS tolerates backward supply and doesn't consume a high idle current.

Solar panels are usually operated with shunt regulators and isolated by series diodes. The panel regulated voltage should be adjusted slightly above SMPS voltage.

 

[TekUT]

ups status: smps not ok

@FvM
Thanks for your reply.
Regarding the battery voltage variation there is no problem because the auxiliary power supply which is after the battery will take in account a input voltage variation between 10 V and 15 V. I think that shunt regulator for the solar charger have a lot of waste power when the battery is fully charged then I'm thinking to use a series regulator type (may be switching with MPPT algo, I've to see if there is around some commercial model). After that I've to see also if commercial model is able to tollerate a load directly connected to the battery.
Regarding the power supply connected from the main line I'm thinking to use a PFC but I've to see also if there is commercial model with programmable constant-current / constant-voltage characteristics, do you know some model?
Do you've some other suggest?

As usual thanks for your help.

Best regards
Pow

 

[mclnsharma]

There are 2 ways in which simple power transfer switch can be designed. 1. Two diodes D1 and D2 have to be connected in such a way that anode of D1 is connected to SMPS DC output and anode of D2 should be connected to battery positive. Cathodes of both diodes should be shorted. Keep SMPS output voltage about 1V higher than battery voltage. This way, when SMPS is ON, load is driven by it and when SMPS is off battery would be delivering power through D2. The disadvantage of this circuit is that if SMPS output is regulated and is always less than battery voltage then it wouldn't work.
2. Two diodes D1 and D2 anodes should be connected to battery positive and SMPS positive respectively. D1 cathode should be connected to cathode of a Zener diode. Anode of Zener diode connected to a resistor. Other end of resistor connected to cathode of D2. A power MOSFET gate should be connected to junction of zener anode and resistor. Connect MOSFET drain to a current limiting resistor. Other end of this resistor to cathode of D1. Connect Source of MOSFET to cathode of D2. Connect load to drain of MOSFET. The values of D1 and D2 depend on the power rating of battery and SMPS. Suppose your battery is 12V 1Ah then diode should be 30% higher than 1A i.e. 1.3A rated. Zener value depends on battery voltage. For 12V battery choose a Zener of about 8V or 10V and gate resistor value should be adjusted such that enough bias voltage is applied to MOSFET. Value of drain resistor is limited by current to load. MOSFET would be generally rated for watts. For e.g. you may have 20W MOSFET. You would need heat sink for MOSFET. For any clarification write to me at mclnsharma@indiatimes.com

Regards
Sharma M.C.L