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Buck converter IC suggestion for charging supercapacitors

#1

Hey All,

I am looking for a buck converter 8.1v 100A to charge a 3s2p capacitor bank. The capacitors are MAXWELL BCAP0360. Is there any IC that can do this ? I have an HP server power supply laying idle which i could use but it is 12v.

I came across an IC from LTC1629 which is not easy to get what i want from it due to the lack of knowledge of how to use them at those rated currents. There are couple of them that can do 20 amps but based on the formula

T = VC/I
8.1 x 210/20 = 85seconds which is very low. I am looking for at 70A and above. anyone have any suggestions. I am open to suggestions if there is a better solution.

#2

Or i could just use and opamp to set CV as in the Simulation and drive a 200A rated MOSFET.

CV_Sim
CV_Sim1920×1040 46.3 KB

#3

So here are two that i’ve come across from LT, the LT1339 and the LT8390. How can i modify the LT8390 shown here to output 8.1v at 100A. If that would be possible. From the datasheet the output voltage can be adjusted via resistor dividers at pin FB but what about other components like the inductor and sense resistor ?

LT8390_2
LT8390_2884×619 45.9 KB

#4

@DigiKey_Internal

Can anyone assist with this?

1 Like
#5

Hello Anish,

We have seen your request and are researching it. Thank you for your patience!

1 Like
#6

There are several components with an x4 like the first filter capacitor 150uf 36v, the sense resistor. What do they mean ? Are they in series or parallel ?

#7

Hi Anishkgt,

The limiting factor is probably the inductor. You would need an inductor capable of handling at least 150A, and probably in excess of 200A to make this work. We don’t carry single inductor in the range of 3.3uH which can handle this much current. It is conceivable that you could place more than one inductor in series and/or parallel to get an equivalent value somewhere near the inductance needed which, altogether, can handle the necessary current. The only part I could find which might have a chance to work is the 541-4288-ND.

The resistor shouldn’t be a problem, as you can place more than one in series and/or parallel to achieve the resistance and power handling required. However, things like layout are a real problem when dealing with such high power levels.

2 Likes
#8

The capacitors would always be in parallel. The resistors could either one. Look at the values and determine whether series or parallel equal the value specified.

1 Like
#9

Hello @David_1528
Couple of things, we have two Rsense resistors as mentioned in the datasheet one is at the Low Side for buck which connects LSP and LSN and the other at ISP and ISN. One page 19 under the section
‘Rsense Selection & Maximum Output Current’ they talk BUCK and BOOST so i am confused as to which of these should i use in my design for a BUCK. Moreover why is LSP called Low Side when it is connected in series with Vin.

Secondly Calculating the inductance on page 19 . How is Delta iL calculated. I’ve used 30% as the delta IL assuming 30% is always selected to be on the safe side.

Freq (Hz) 150000
Vin(MAX) in Volts 24
Iout(MAX) in AMP 100
Vout in Volts 8.1
Inductor 0.62

Is the inductor unit read as 0.62uH

#10

I had asked if these ICs have a current limit on Stack Exchange and they say single phase are limited to 20A and anything more a multiphase should be considered or parallel the 20A together. Multi phase is something hard for me to understand at this moment as this is my first Buck-converter.

So i am confused about the current rating of the IC now. Is the IC limited to 20A as they say or since its not mentioned in the datasheet like @David_1528 said its only limited by the inductor. Moreover add inductors in parallel, wouldn’t they increase the total inductance also ?

#11

Hi @anishkgt,

Answering your last question first, inductors in parallel are calculated similar to resistors; two in parallel will have half the inductance of a single one, three in parallel will have 1/3, etc.

There is no “current limit” for the LT8390, as the current limit is set by an external resistor, and the high current portion of the circuit will pass through external FETs rather than internally. However, multi-phase topologies are typically a much better way to go for really high output current designs, but they are more complex, and can only be accomplished with IC’s designed for such.

Since you are only using this as a buck, those are the equations you use.

“LSP” refers to inductor (L) sense resistor positive input (assumes current passing from Vin to Vout).

For calculating IL, using 30% for delta IL% is a good value. The Iout(max) they refer to on page19 is the max inductor current rather than load current. The average current should be the ratio of output voltage to the input voltage times the load current. Then, from that and using 30% for delta IL%, you can calculate delta IL. From this you can calculate Iout(inductor max), and from that, you can determine Lbuck minimum for inductor value…

As I stated above, the biggest limiting factor (other than being able to lay out a board which is capable of handling this current) is the inductor. All of the current required to feed the load must pass through this device, and few are spec’d to handle anywhere near what you need.

I must say that what you are trying to accomplish is very difficult. It sounds like you have little to no experience designing switching regulator circuits before. If so, doing an 8.1V, 100A charging circuit is a little like diving off a 30 meter platform and trying to stick the landing. If you have become an expert at diving off the 1 meter, 3 meter, and 10 meter levels, you have a much better chance, but even then, things could easily go very wrong.

However, if doing this as a novice, just as you would likely reach the water when you jump off a 30 meter platform, there’s a pretty good chance you will reach 100A (briefly), but the results are likely to be catastrophic. There are so many places for things to go wrong. I would strongly suggest you start with something a lot less demanding before working your way up to something like this.

1 Like
#12

Thank you @David_1528, I do take your concern seriously but there is first time for everything. I can layout a 2 and 4 layer PCB. The datasheet and couple of Application notes can be of help too. I will be making two boards one double sided (just to see the difference in performance) and a 4 layer pcb with signals on the top and bottom then GND and Vcc in the middle. Keeping GND plane closest to components which would be on the top.

I’ve come up with some numbers here, based on a frequency of 200kHz and 22A per phase, the calculated inductance per phase is 1.49uH round it to 1.5uH and i get this inductor from BOURNS SRP1245A-1R5M. Then coming to current sensing on page 16, RLIM = ILPK x Rds(on) MAX/47 is the formula. What would be unit of resistance applied here ? KiloOhms, MillOhms or just Ohms. How does everyone make sense out of it when its not mentioned like even capacitance or Current in some places.

During my search for some info I came across this evaluation board of AD1850 and it has the schematic for 50A. I could replace R11 with 125K to get 8.1V. Secondly can i add one in parallel to this, that is common Vin and Vout to get 100A ? Or is there a better way of doing it in a way they can work simultaneously as master and salve kinda way.