Paralleling DCDC converters request

Hi Everyone,

Taking a last stab at this to see if a manufacturer has a controller that has this functionality instilled in it.

I plan on having a battery pack and would like to be able to parallel a few of these and make it somewhat hot swappable. 

The idea is that I can add or remove a pack as needed without disturbing the system and if a pack fails there is redundancy so the whole system doesn't fail.

To do this I want to have the outputs of all the packs be at the nominal same voltage, therefore I plan on boosting the battery voltage which will be between 5.4 ~ 8.4V to up to 12V with at most 3A using a dcdc converter.

Now I have a nominal voltage, although there will be small voltage variations, which cause the dcdc with the highest output voltage to carry the burden of the load. I'm looking for a controller that can accomplish this but also has functionality to trim the output voltage or in some incorporate something to allow for the current to be shared by each dcdc converter.

I really tried to make this as clear as I can, but if not, please dont hesitate to ask.

Any and all help will really be appreciated.

Thanks in advance!

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  • 0
    •  Analog Employees 
    on Feb 22, 2021 8:42 AM

    Hi EngJunkie,

    I assume that for every battery pack, you would want a DCDC boost converter, all regulating at 12V, and loads 3A when combined. Did I pieced that correctly?

    Well, you can interleave multiple converters to share current towards a single load. However, it seems that this would become a complication due to your converters having different sources.

    There are also risks when you hot-swap battery packs in parallel without controller, as the freshly installed battery could pull a large inrush current. (Could also hit input UVLO)  For hot-swapping, there are several solutions ADI offers: e.g. LT4256 and LT1641.

    You would need a set of these solution for every battery pack you add. I think it's better to implement BMS to maximize your battery pack's charge capabilities, and it's more robust and scalable. 

    Regards, 

  • Hi Audison,

    Thanks for the reply!

    You pieced that together correctly!

    "Well, you can interleave multiple converters to share current towards a single load. However, it seems that this would become a complication due to your converters having different sources."

    I think this image will clarify my explanation a lot more.

    As you can see I have an isolated buck converter which connects to a battery charger and BMS. The Battery charger and BMS will protect the batteries, monitor them and keep their voltage between 5.4 - 8.4 However, I want to connect a few of these battery packs together in parallel so I plan on boosting the output to a common bus voltage of 12V.

    My problem is that I have no control over the output voltage of the boost converter by means of a remote sense to trim the output voltage and therefore the DCDC converters wont have the exact same voltage and therefore one converter in the bus will most likely provide the majority of the power. My hope is that analog devices has some form of boost converter that I can use in place of the one I currently have. One that could boost the battery voltage to a common bus level and ensure that several DCDC converters can be connected in parallel with them equally baring the load current.

    I will definitely look into the hot swap controllers you provided. Thank you for the recommendation!

    Hope this helps answer your question?

    Higher resolution version of image:

    PDF

  • 0
    •  Analog Employees 
    on Mar 9, 2021 12:33 AM in reply to EngJunkie

    Hi EngrJunkie,

    That seems an interesting project! However, you should be cautious with paralleling cells. Because you might be wasting a lot of capacity in paralleling cells. Each cell would eventually reach its terminal voltage but the charge in it would be imbalance. Hence the weakest cell will dictate total charge for that parallel pack. 

    Analog BMS products supports series configuration of cells. (Check ADBMS1818, ADMBS6817, and ADMBS6815, LT8584 as Balancer). Every 'physical cell' you connected in parallel will be considered by BMS as single cell with large capacity only. If they're all in parallel, then BMS will not benefit you much.

    Here's what I think you can instead implement. Connect your cells in series (3-12cells) with BMS that constitute as one module. That would give you large voltage, so you can just add Buck converter for high current application. That way, the BMS will handle the only cell balancing you need. This would also saves you of designing parallel converters (multiphase) that would warrant active sharing. If you want to add more cells, you can just add another 'module' in series (More than 1 module is called Stack). Lastly, if there is a faulty cell, you can easily detect it using different algorithm such as Open-cell detect. The series battery will continue to operate.

    If you have  20-100VDC source, then you can use one of ADI's Bidirectional Buck-Boost Battery Charger IC.

    I hope this gives you some insight. Thanks!

    Regards,

  • Audison,

    This was very insightful and has given me some ideas on a path forward.

    I may still want to consider going the route of a hot swappable paralleled pack as I dont really want to build up the voltage on the pack dynamically.

    Could you possibly let me know if analog devices has anything in this line : "This would also saves you of designing parallel converters (multiphase) that would warrant active sharing". Also how would this work?

    Would the LTC4350 be an option for paralleling supplies? Do you have a replacement for it that doesn't use diode oring?

    Thank you for all the help thus far, Starting to feel like I'm moving into a viable direction for my battery pack. :)

  • +1
    •  Analog Employees 
    on Mar 24, 2021 7:51 AM in reply to EngJunkie

    Hi EngJunkie,

    Could you possibly let me know if analog devices has anything in this line : "This would also saves you of designing parallel converters (multiphase) that would warrant active sharing". Also how would this work?

    I'm sorry, I kind of missed the bidirection power path of your design in charging.

    I'm thinking that since each cells are tied directly because of your common charging node, with the separation of hot swap circuit in between, you essentially have 1 source voltage on Boost converter input at discharging (based on your diagram). Since your goal of Boost Converter is to level each of your cell to 12V while sharing equal load, I think you can eliminate the boost converter by using LT8708 as bidirectional buck-boost, which would set your Vdischarge to 12V. The tricky part left is balancing your current for each cell when discharging.

    Would the LTC4350 be an option for paralleling supplies? Do you have a replacement for it that doesn't use diode oring?

    You might want to use LTC4238 for your hot swapping instead (recommended for new design). You would have to implement 1 of these per cell. I dont have any idea for a replacement of load sharing except LTC4370 (operates similar to ORing diode but lower drop for load sharing). I'm thinking that LTC4370 would be in between your cell and charger/discharger circuit.

    And, I'm not sure if your BMS can balance the charges for each cell connected in parallel. Sure, they would be at the same terminal voltage in the end but you will be limited by your weakest cell.

    Thanks!

    Regards,

Reply
  • +1
    •  Analog Employees 
    on Mar 24, 2021 7:51 AM in reply to EngJunkie

    Hi EngJunkie,

    Could you possibly let me know if analog devices has anything in this line : "This would also saves you of designing parallel converters (multiphase) that would warrant active sharing". Also how would this work?

    I'm sorry, I kind of missed the bidirection power path of your design in charging.

    I'm thinking that since each cells are tied directly because of your common charging node, with the separation of hot swap circuit in between, you essentially have 1 source voltage on Boost converter input at discharging (based on your diagram). Since your goal of Boost Converter is to level each of your cell to 12V while sharing equal load, I think you can eliminate the boost converter by using LT8708 as bidirectional buck-boost, which would set your Vdischarge to 12V. The tricky part left is balancing your current for each cell when discharging.

    Would the LTC4350 be an option for paralleling supplies? Do you have a replacement for it that doesn't use diode oring?

    You might want to use LTC4238 for your hot swapping instead (recommended for new design). You would have to implement 1 of these per cell. I dont have any idea for a replacement of load sharing except LTC4370 (operates similar to ORing diode but lower drop for load sharing). I'm thinking that LTC4370 would be in between your cell and charger/discharger circuit.

    And, I'm not sure if your BMS can balance the charges for each cell connected in parallel. Sure, they would be at the same terminal voltage in the end but you will be limited by your weakest cell.

    Thanks!

    Regards,

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