Voltage Switching between Main and Battery

Hello,

Have designed a board that requires a supply of 7.4v/3 amps to the circuit.

The board has options to power from mains as well as through the battery (Li Ion)

Purpose of switching circuit is as folllow.

When main power is present, the board is powered through Mains and when its not present, the supply is switch to battery and vice versa.

Concerns

1. The SMPS that I have used is 9.5v/3amps. Consider this to be mains supply and because I need 7.4V, I am dropping it via 2 series Diode (1N5408) but these are heating up when the load takes up 2.5amps plus....

2. During switching between mains and power supply, it all works fine but if I connect the mains power via DC Jack (Normally available for SMPS), the controller board resets. If I keep the DC jack plugged in continuously without removing or plugging it back, it still works fine.

I get controller reset during plugging in the DC Jack/ I am assuming it is some spike that is generated when I plug the DC jack. How can I solve issues like this.


Sachin....

"Ideal diodes" may be useful: https://www.linear.com/products/powerpath_controllers_*_ideal_diodes

Does your power jack have a "break switch" or are you using a series diode for the switching?

Keith

tiwari.sachin said:

Hello,

Have designed a board that requires a supply of 7.4v/3 amps to the circuit.

The board has options to power from mains as well as through the battery (Li Ion)

Purpose of switching circuit is as folllow.

When main power is present, the board is powered through Mains and when its not present, the supply is switch to battery and vice versa.

Concerns

1. The SMPS that I have used is 9.5v/3amps. Consider this to be mains supply and because I need 7.4V, I am dropping it via 2 series Diode (1N5408) but these are heating up when the load takes up 2.5amps plus....

2. During switching between mains and power supply, it all works fine but if I connect the mains power via DC Jack (Normally available for SMPS), the controller board resets. If I keep the DC jack plugged in continuously without removing or plugging it back, it still works fine.

I get controller reset during plugging in the DC Jack/ I am assuming it is some spike that is generated when I plug the DC jack. How can I solve issues like this.


Sachin....

Click to expand...

In your case I think I would prefer to use the battery as an uninterrupted [power supply, and the AC/DC power supply connected through a diode to recharge the battery. No switching or interruption , to avoid resets in the connected load.
The only important requirement for the AC/DC power supply is the voltage regulator adjusted so that no overvoltage can occur as the LiIon battery is sensitive to it.

Maybe this can give you some new idea:

Automatic Voltage Switcher
https://www.insidegadgets.com/projects/automatic-voltage-switcher/



3 and 5V have different grounds!.

Sorry guys.. couldnt reply earlier than this... a small accident and was off duty till today..

Keith... I am using diodes to drop 9.5V to 8.1V. There is drop for sure but when the load (Thermal printer) runs at full capacity (Image printing etc), it takes up around 2.5 amps and with battery charging an additional of 500mA. the diodes heat up quite a bit. I am using 1N5408. Switching reset is a separate issue.

Jeripolivka... We did think about your suggestion earlier and it was sort of better option as the SMPS just needs to supply battery charging current. This would reduce BOM too but then the customer requirement says that if the battery dies up completely, the module should be able to run even with the mains supply through SMPS till he replaces the battery. If your suggestion is considered, then battery charge discharge frequency will be more and so will the life of the battery is also reduced. So we are designing a SMPS that given 9.5V/3amps output

tpeter... we dont need a option like this... to put it simple... need to drop 9.5v to about 8.1V and the load shall minimum draw 3 amps (Not constant but once in a while). when the switching between battery and SMPS happens the board resets and this needs to be addressed.


I hope I have mentioned this earlier...

when SMPS is connected to board and I switch ON/OFF the AC input via wall switch for AC, it works fine.

But when I try to remove and plugin DC jack (Output of SMPS), then there is a reset. Probably some glinch. Need to probe in a CRO and See if there is some variation there but will have to wait as the CRO isnt available right now.

Attached is the schematic that I am using. A bit of details explained in that too..

I am yet to solve this issue. I am somehow missing a small link I guess.

I am infact planning to use a 7.5V SMPS and use it directly to supply to the load and use a boost converter to raise the voltage to 9.5V/500mA from 7.5V and use it as source for battery charger. This I shall probably try if I am unable to solve the first problem.

I am of opinion that using a diode with higher If (Forward current) can probably solve heating issue but the size of the component itself is a problem

Do share your views.

Regards
Sachin Tiwari

Switching reset problem: Q2 swiches of,f when voltage from SMPS reaches about 3,5V. You can delay switch by replacing D11 with resistor or better, completely omit Q2, because SMPS voltage is allways higher then battery voltage, therefore D9, D10, D12 will do the "switching".

I have used SI2333CDS instead of SQ2361EES (For Q2)