Small 5/12V dual output power supply from battery

[jfsimon]

Hi, I need some support eventually, if someone can be kind enough to provide advices on this design:

I'm developping a beagle board module which requires 2 regulagted voltage (+/-5%) 12V and 5V for screen and beagleboard respectively. It will run on battery. I found that 4 LiPo 3.7V 3 Ah in serie can do the job.

The issue is battery voltage will be between 13.2 and 16.8 V and I need to regulate and make efficient outputs at 5/12V.

I have a quite complex solution on theory and working on the bench:

For the first stage: battery to regulated 12V, i designed a custom circuit with ampli op/voltage ref driving an emiter follower transistor

The output of this stage feeds a TBA 1211 DC-DC 12V/5V power suuply (switched mode with good efficiency).
The output of this 5V stage needs another stage of regulation, because it works only with >10% load, hence when the circuit is in stand-by, it gets 6,5V out. Therefore the TBA output goes into the same custom circuit as above with an ampli-op, vref and emiter follower transistor.

I could'nt use 7812 nor 7805 regulator since the first one isn't available with minimum input 13.2. We can find it with min voltage 19V, or some at 14.5V, which would wasted battery reserve.

The second one, for the same reason, I can't use it. The Vin can be up to 6.5V, but nominally about 5V.

This solution, though it works, seems very complex for just going from V_Bat = 13.2...16.8V to 2 outputs 12V + 5V regulated +/-5%. I have the simple question, has someone worked out this type of power supply with a more elegant and simple design ?



Thanks & regards

Jean-François Simon

[mariush]

You'll have to decide on some things...
If you make your battery use 4  lipo 3Ah batteries, you add complexity to your battery charging / management circuit - you need to balance the batteries when charging and you need a higher input voltage to power your charger ( 4 x 4.2v + at least 1-3v, so you'd probably need at least 18v power adapter).

A circuit with just one or at most two batteries in series would be much simpler and cheaper, and your product would also work from a potentially much cheaper and simpler 9v..12v power adapter.

You should also consider the amount of power you're actually gonna use on the two voltages.
The display is probably gonna use the 12v just for led backlight - depending on the size of the display it's probably gonna be at most 2-3 watts, but even if it's 10 watts or so it's not a big deal. It may make more sense to BOOST 3.5v...4.2v (1s) / 7..8.4 (2s) / 5v..9v..12v (dc in) to 12v
With 2s battery, you could use a very efficient buck regulator to produce 5v
 
For example ... MCP73213 (with T or not at end, t just means tape and reel) is a decent and cheap 2s charger and available in a bunch of sub-versions : https://www.digikey.com/en/products/detail/microchip-technology/MCP73213T-G6XI-MF/7898434

It could work with 9v power adapter just fine but 12v would be safer.

You can easily get a step-down regulator that would work from either 6v..12v to 5v for the beagle board and a 6v..8.4v / 12v dc in to 12v a cheap step-up regulator would work.

[jfsimon]

Thanks for highlights.

The 12V load (the screen) can work with 7.5 ... 12V, it has 700 mA and 400 mA consumption at respective voltage.
The beagle needs 5V, its consumption is varying 240-400 mA.

Jean-François

[mariush]

Even better ... so you could power it directly with 9..12v  and have a step-up regulator to boost battery voltage to 9v..12v
Look at the display and what it does with that wide input voltage - does it have a linear driver or a buck led driver for the backlight? If it's a linear led driver, it would be a waste to boost to 12v only to have the linear led driver drop the voltage down in the form of heat.

You could also have 2S2P (series of two batteries in parallel for higher capacity)...

[DavidAlfa]

Why choose 4x LiPo in series when 7.5V is enough for all your needs?
I would use 2 LiPo in series, they'll give 7.4-8.4V. If you want longer battery, use 2S2P.

The display can be connected directly without regulator, then use a small 5V buck converter, like these:

www.aliexpress.com/item/4001054875390.html
This one uses MP2315 (datasheet).
I tested minimum Vin for different loads (5Vout): 5.25V(200mA),  5.5V(500mA), 6.1V (2A).
So for 7.5Vin minimum, it should provide all the Beaglebone needs.

www.aliexpress.com/item/1888459743.html
This one uses MP2307 (datasheet), less efficient (100mΩ fet and rectifier)

[mariush]

I'd still add a step-up regulator to around 9v or more, just for safety and less stress. 
It would allow the batteries to go to lower voltage before the voltage is considered too low. (ex 3.2v x 2 = 6.4v which would be below the screen's 7.5v lower limit)
It would also allow for some variation in screen quality (maybe some screens would need 7.8v to work and not minimum 7.5v, or maybe the minimum voltage is higher at some temperatures ex minimum 8v required at 0..10c)
The battery voltage may also drop below 7.5v from current spikes, for example when turning on screen backlight, the battery goes from around 0.2-0.4A of load to around 1A of load, so there could be some momentary voltage drop and also if the current goes up to 0.7A at 7.5v then you need thicker wires between the battery and the display which may not be possible or desired.

A 9v 1A adapter would be able to power the display and the 5v with some small amount of current left for charging the battery (let's say 500mA for display, 150-200mA for 5v, you'd still have around 300mA for charging)
They also make charger ICs with passthrough system to disconnect batteries from system when input is detected,  but they're a bit more expensive, and you can do that from software or hardware (ex 9v from dc jack can go in a mosfet or a relay or something to connect the circuit directly to the dc in.