12V / 6A Power supply. Help with Buck and Boost converters + OR'ing circuit

[esm.]

12V / 6A Power supply. Help with Buck + Boost controllers + OR'ing circuit

Hello all.
Maybe someone here can give me suggestion of ICs and make considerations about this design.
The picture below states how this power supply should work.

View attachment 64773

Details:

1) IC 1 probably will be a LT3650-8.4 from Linear Technology. It supports up to 32V of input voltage (40V peak). Suggestions of other ICs for this purpose?

2) The power supply INPUT has only two states: (A) Floating (cut-off) and (B) 17 to 30VDC present at the input.

3) The OR'ing circuit at the OUTPUT, as the picture shows, doesn't mean it will be done with diodes. It can be done with MOSFETs, relays, or any kind of logic control like, for example: When the INPUT is connected to a 17 to 30VDC PSU, a logic LOW is present in an ENABLE pin of the BOOST converter, so that this remains in idle state (Output in high impedance). When there is no power supply connected at the INPUT, the same ENABLE pin goes logic HIGH, so the BOOST converter goes to active mode and presents 12V at its output.

4) The intention of using batteries is, case the INPUT be floating, maintain the circuit connected to the 12V OUTPUT working for at least 10 minutes (If more, better), through the BOOST converter

5) Which BUCK and BOOST controllers could I use for this design (IC 2 and IC 3)?

6) How can I do the Or'ing circuit between Buck and Boost controllers? Some suggestions?

7) There are no restrictions if heatsinks are required. No restrictions with the final physical dimensions of the PCB. Just need to work.

8) No problem if external MOSFETs are required, both on Buck and Boost converters

9) Final cost of this PSU is not so important.

10) Preference to SMD packages in all the circuit, if possible.

11) The 12V OUTPUT can be designed to deliver more than 6A. No problem about that.

12) No need of insulation (Flyback / forward topology, for example). The topology can be modified to Flyback / Forward also

Maybe a Power supply like this already exist in the market (a final product I mean), meeting this specifications, or even better than, ready to use. I did some research about but didn't found any models.

For the time, all I found is a Buck converter from Linear Technology, based on a LTC1624. Example of a 12V x 10A output with Vin range of 13 to 28V using this converter:

View attachment 64772

Somebody help me ?
Thanks in advance.

 

[MrAl]

Hi,

Just one note, that when the boost circuit is in operation it will be drawing some 14 amps from the Li-ion batteries. If you increase to 4 batteries this would go way down.

How accurate does the output voltage have to be, can it vary by maybe 0.4 volts perhaps?

 

[esm.]

Hi,

Just one note, that when the boost circuit is in operation it will be drawing some 14 amps from the Li-ion batteries. If you increase to 4 batteries this would go way down.

How accurate does the output voltage have to be, can it vary by maybe 0.4 volts perhaps?

Hi. Thanks for you observation.
I noticed the high current consumption from the battery now.
This was just my initial idea
Yes the output can vary 400mV, this is OK.

There will be 4 devices connected to the 12V OUTPUT, each device as a load of 1.5A, aproximately.

Maybe I will need to limit the OUTPUT of this power supply, to supply power just to one device or maybe to two of these devices when running the Boost converter. So the current consumption would be 50% to 75% lower when powering the circuit with the battery.

I am trying to find a solution for this case yet, its not easy.
I don't even know which Boost and Buck converters to use in such case

 

[crutschow]

If you have a control signal then you can use P-MOSFETs to isolate the converter outputs but with very little voltage drop when ON. Grounding their gates will turn them on and floating the gates to the source voltage (through a resistor) will turn them off. This gate signal can be provided by a small NPN transistor.

To provide proper isolation and prevent the normal drain-source internal diode from conducting when OFF you need to use the MOSFET source as the 12V output. The transistors still works properly since MOSFETS will conduct in either direction when on.

 

[esm.]

Understand.
But which MOSFETs to use? Do you know a standad / not hard to find MOSFET with very low RDSon?

 

[MrAl]

Hi,

When you say "not hard to find" i guess you mean that you are going to look in places you are familiar with and order from them. What you can do is go to one or more of those places and look to see what they have as to their MOSFETs and then go to the manufacturers site and look up the data sheet. When you find the data sheet for a prospective device, you'll see a specification that will tell you the Rds with a given set of parameters. You see what looks good and pick that. If you like after you've found one that looks good you can post it here and someone here can verify if it will work or not.

 

[crutschow]

Understand.
But which MOSFETs to use? Do you know a standad / not hard to find MOSFET with very low RDSon?

Not knowing your location makes it hard to make good suggestions, but if you can order from a supplier such as DigiKey, just go to their website and search for P-MOSFETs.

 

[esm.]

i will buy from digikey, no problem.

actually, i wanted to ask if you had a suggestion, if already had experience with some models tha could meet my requirements

i would like to ask the users from the forum if they already used such kind of controllers, a particular buck or boost IC, similar to the ones I would need to meet the requirements of my circuit, and if the results were satisfactory.

its always good to hear from someone who tested an IC in practice and how were the results. I am trying to get some recommendations.

 

[ronsimpson]

I should draw a picture but:
I would wire the buck and boost together without diodes.
The PWM IC for the buck lives off the 17-30V. Some PWM have a under voltage lockout. Set it so the PWM shuts down if the voltage is below 15 volts.
The boost PWM IC needs to have a enable pin. Maybe through a transistor (inverter) have this PWM stop working if the input voltage (17-30v) is above 12 volts.

The boost supply will have an internal diode that keeps power from flowing backwards so don't add one on the outside.
The buck supply needs a diode to keep the 12V from flowing backwards when the supply is shutdown. I would put the diode on the input of the supply not the output. This allows the feedback to keep the output constant. Also the input current is lower than the input current and there will be less power lost in the diode.

 

[esm.]

Good idea.
I understood all you said.
The problem is to find (choose) suitable (and reliable) buck and boost controllers to make that, and meet the required maximum output current (6A).

Thats why it would be a great help if somebody already have used such kind of controllers in practice (for such high power).
Most of SMPSs i have used before were for a maximum of around 10~12W

 

[ronsimpson]

Look at LT1619 for the boost PWM. The SS pin could be used for shut down.

Buck look at LT3724 or LT3844.

 

[Ubergeek63]

2 things... first you should use a buck boost topology instead of 2 switchers. ... second you would be better off just adding the cells to make it always buck

 

[esm.]

first you should use a buck boost topology instead of 2 switchers.

Let me see if I understood what you mean. You wanted to say that I can use just a buck-boost controller and make a OR'ing circuit between normal input voltage and battery (using P-mosfet(s), for example), at the input of the buck-boost controller?

I think there is no buck-boost controllers available for my circuit needs, in this case. The buck-boost IC would work with a input voltage range of 6V to 30V and deliver 12V @ 6A on the output.

second you would be better off just adding the cells to make it always buck

I didn't understand what you said.
The voltage range of two Li-Ion batteries in series would be 6V to 8.4V (8.4V when both are fully charged and 6V when both are almost discharged). They can't buck to obtain a 12V output
Maybe you wanted to say "always boost"? If yes, What i would do with the normal voltage input?

 

[Ubergeek63]

in the first possibility there is no such thing as a buck-boost controller. there are buck boost topologies, most flyback chips can be used for sepic converters.

the second was to use 4 cells instead of 2 and use a simple buck circuit.

 

[ronsimpson]

in the first possibility there is no such thing as a buck-boost controller. there are buck boost topologies, most flyback chips can be used for sepic converters.

I got 60,000 hits on a search for "buck boost converter".

 

[Ubergeek63]

I got 60,000 hits on a search for "buck boost converter".

well it is nice that the silicon boys are catering to needs, but that particular one is over an order of magnitude to wimpy for the OPs request.

 

[esm.]

most flyback chips can be used for sepic converters.

I never studied the SEPIC topology, i will read about soon, don't have so much time now. I did a quick search on google and i understood it can do the same function of a buck-boost (Vin can be lower and higher than the output). I didnt found info if the SEPIC needs to be "configured" to operate just for buck, or just for boost operation or if it works as buck and also as boost without the "configuration"

the second was to use 4 cellsa instead of 2 and use a simple buck circuit

Using 4 li-ion cells the cost would increase a lot i think. Until now, I just know the "18650" size for li-ion batteries (18650 is a standard format), and they are not so cheap:
Also, the charger IC (LTC3650) is for 2-cell only. I would need to use two LTC3650s or change to a charger IC with support to 4-cell

https://img.alibaba.com/wsphoto/389680361/UltraFire-Li-ion-Rechargeable-battery-18650-3000mAh-3-7V-Li-ion-battery-protected.jpg

My idea was to use two li-ion cells of 18650 size, each one with a PCB holder, like this:
https://elcodis.com/photos/0/10/1044/bk-18650-pc2.jpg

Your idea is to use 4 batteries in series and make an OR between the cells and the normal power supply input? and use just one buck converter?

 

[ChrisP58]

If you have a control signal then you can use P-MOSFETs to isolate the converter outputs but with very little voltage drop when ON. Grounding their gates will turn them on and floating the gates to the source voltage (through a resistor) will turn them off. This gate signal can be provided by a small NPN transistor.

To provide proper isolation and prevent the normal drain-source internal diode from conducting when OFF you need to use the MOSFET source as the 12V output. The transistors still works properly since MOSFETS will conduct in either direction when on.

Just make sure that both mosfets cannot be turned on at the same time, or you will allow current from the higher voltage source to flow back into the lower voltage source, bypassing the battery charger. Not a good thing.

 

[Ubergeek63]

sepic will work buck or boost, requires only a single FET, and disconnects the load on shut down. it has a capacitor in between the input and output, but uses two inductors, but works better if they are on the same core.

Dan

 

[esm.]

sepic will work buck or boost, requires only a single FET, and disconnects the load on shut down. it has a capacitor in between the input and output, but uses two inductors.
Dan

I have a doubt about the SEPIC topology. This topology automatically works like a Buck-Boost (VIN can be lower and higher than the output voltage) ??? Or does it need to be "configured" to work just for buck operation or to be "configured' to work just for boost operation ???

but uses two inductors, but works better if they are on the same core.
Dan

Do you know if I can find this kind of inductors (dual inductors on the same core) on Digikey, Farnell, Mouser or others??? Or if I would need to produce them specifically for my design?