how to switch between DC power and battery

[ilikebbs]

battery backup switch mosfet

the requirement is below:
1. if the DC power exists, the load will be powered by DC
2. If the DC power is removed, the load will be powered by battery automatically.
the power drop is not allowed.

 

[TekUT]

simple battery backup switch mosfet

Just as a idea because I don't know exactly what is your need (voltage in, voltage out, power level and so on), you can use a battery charger able to supply also load connected to the battery (e.g. Ansmann model ALCT 12-3) and a switching DC-DC post regulator (see attached circuit block schema). When the main power is down the battery will supply the DC-DC, if you rate in a right way the battery size with the output current I think that any small drop that you have on the battery side will be controlled by the DC-DC postregulator. About the DC-DC there is TRACO module that can be used for such purpose (e.g. 12 V in - 12 V out such as the TEN 20-2412WI 20 W module).
I've also attached the charger characteristics.

If you don't like to use such type of battery charger there is another way, use the main power supply and a power MOSFET with a voltage monitor (e.g. from Linear Technology), then from the main supply derive also a battery charger for the battery backup. When the main power go down the voltage monitor will turn on the MOSFET and use the battery as power supply for the output DC-DC. In this last way the backup battery will be attached to the output regulator in a fast way trought a sized MOSFET (of course voltage monitor and driver circuit will be feed by the battery itself).

Anyway I'm also searching such type of solution bcz I've a application where I can use main power line, solar cell and I must have also a battery backup such this you need. I'm investigatin some solution, also I've posted some schema on previous post but I've no reply about it, if you like take a look on this post:



Cheers
F.F. a.k.a. Powermos

 

[rikie_rizza]

ltc4412 schematic

Use relay and a huge capacitor to compensate 30ms relay switching period.

 

[saurav_sdpl]

switching 6-36 dc

what is the max load current in both the cases.

if the requirement is of low power load u can simply do with a mosfet
however take into consideration the Rds on of the mosfet.

it should be low to avoid power loss.

 

[ilikebbs]

ideal diode battery relay

The maximum current is 3A.
anyone can share your schematic?

 

[Eugen_E]

If the device has a voltge regulator/ switching supply, use 2 Shottky diodes as a discrete OR "gate". The DC voltage should be a bit higher than battery, so when DC power is connected, the power diode will be forward biased and the battery diode reverse biased, thus switching to DC power. It is very fast and with now power interruption.

 

[GrandAlf]

The crude and easy way is to have a diode in series with the + side of the battery with anode to battery side. You then have a resistor in parallel with the diode. Connect this to you DC PSU. The resistor needs to be of a value that will charge the battery at its rated trickle charge rate. Normally the diode is reversed biased and power is taken from the PSU. If the PSU fails or drops below the battery voltage, the diode becomes forward biased and supplies current to load. There is some voltage drop across the diode, that can be minimised by using a shottky component. No delay though.

 

[TekUT]

[CUT]
You then have a resistor in parallel with the diode.
[CUT]

Hi,
there is somethings that sound not totally clear for me, could you please supply a drawings of the circuit?

Cheers
Powermos

 

[randell_xtian]

hi! you can used the L200CV or even creating a new circuit. hmmm, i think zetex corporation has its pdf format your battery management!


best regards,
randell_xtian

 

[osmanpasali]

you can do it with a simple relay

 

[GrandAlf]

Battery + A--->|---K --- PSU+ (Diode in Series)
| |
Resistor (Charge Resistor)

Battery - ---------------- PSU-

Added after 4 minutes:

Sorry spacing has come out all wrong.
Diode Anode to Battery+
Diode Kathode (Band) to PSU+
Charge resistor goes across diode

Diode need to be able to handle power required.

Added after 4 minutes:

Only real disadvantage with this simple system, is that if your battery runs flat, it will take quite a while to recharge at the trickle rate. Only really suitable if occasionaly power fails.

 

[TekUT]

@GrandAlf

ok tnx now sound good

Bye
Powermos

 

[bobcat1]

Hi

Using shcotky diode in not ideal - you will loase 300mV for etch path

Use the LTC4412 ideal diode IC the best solution for switching between DC power and battery

All the best

Bobi

 

[huzuhong525]

That's easy .
you can place two MOSFET(P &N ) in board for the function .
Using their switch function .
N channel Mosfet :Gate pull high ，Mosfet turn on
gate pull low ,mosfet turn off .
P channel Mosfet has the opposite function .

In our many projects ,to use this for the switch between DC and battery .

 

[bobcat1]

Hi

Can you upload schemtic for the way you suggest ?

All the best

Bobi

 

[eziggurat]

I scanned in this schematic for you to try.

 

[TekUT]

I scanned in this schematic for you to try.

In your schematic you use two P-Channel MOSFET, the complementary action is achieved by means the NPN transistor Q3, just to point because into the previous reply you said use two MOSFET P&N instead here is two P MOSFET.

Thanks.

Bye
Pow

 

[eziggurat]

I scanned in this schematic for you to try.

In your schematic you use two P-Channel MOSFET, the complementary action is achieved by means the NPN transistor Q3, just to point because into the previous reply you said use two MOSFET P&N instead here is two P MOSFET.

Thanks.

Bye
Pow

I think you are referring to huzuhong525 post.

 

[TekUT]

Yes, sorry, I've see only now that the scanned was did from you and not from huzuhong525. During my writing I've not noticed that the post author and the author of the scanned image was different.

Thanks to point it.

Best regards
Pow