Ideal buck converter for high current - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 07:53:24 am
content: Hey all,what would be best buck converter for high currents. My power source is a Hp server power supply rated 12v 82A with which i plan to charge a 3s2p supercapacitor bank to 8.1v Would the LTC4824 do it ? Based on the datsheet i've made a spreadsheet but can't seem to get the components based on the sheet.Any help would be appreciated.

author: Warhawk
author_rank: Frequent Contributor
timestamp: August 03, 2020, 08:16:07 am
content: what about LM5170-Q1?

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 08:36:24 am
content: Thanks i am looking for something less to work with because i lack the experience. This is my first buck converter so i'd rather have a couple in parallel. How is the current limit of the IC determined ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2020, 12:41:09 pm
content: Take a look at DC motor controllers and add an inductor.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2020, 01:23:41 pm
content: The typical solution is a multi-phase buck converter using discrete transistors with synchronous rectification.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2020, 01:39:06 pm
content: anishkgt,How big are the super capacitors and how fast do you want (need?) to charge them?Regards,Jay_Diddy_B

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 02:33:22 pm
content: The capacitor is BCAP0360 P270 S18. I am planning on to using them in a spot welder project i've been working on. Charging them at the rated current of 82A would take 26 Seconds initially. A weld drops the voltage to about 6V.

author: CoteRotie
author_rank: Contributor
timestamp: August 03, 2020, 04:35:35 pm
content: 8.1V and 82A is unequivocally in multi-phase synchronous rectified territory. Are you planning to use a soft-start feature of the buck to charge the caps, or would you configure the output as a constant current source? Setting the buck to 8.1V (regulating voltage) and slamming the discharged cap across the output by closing a contactor or big FET might be hard on the cap lifetime. (Depending on other bulk capacitance, series impedance, cap bank esr, duty cycle, etc etc.)

author: wraper
author_rank: Unknown
timestamp: August 03, 2020, 04:46:07 pm
content: Quote from: CoteRotie on August 03, 2020, 04:35:35 pmYeah, to achieve this in one phase would require some ridiculous inductor and diode + MOSFET bank. And efficiency will suck = a lot of heat to somehow transfer away.EDIT: and PWM controller probably won't be able to deal with a gate charge of such MOSFET bank anyway.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 05:06:05 pm
content: This is my first time dealing with a buck converter please bear with me.@CoteRotie How did you confirm this has to be on a multiphase category ? are buck converters limited to specific amps ? is it the peak currents that determines it ? Yea if the IC has a soft-start that would be great. The caps can withstand pulsed current up to 1000A. Should not be a problem in a 3S2P configuration. The total capacitance would be about 260F. The Buck-converter will be switched off via the microcontroller before a weld.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 03, 2020, 08:16:07 pm
content: Hi,So you 360F in a 3S2P configuration.This is 240FAfter the weld the voltage drops to 6V and you need to charge back to 8.1VCV=It240F x 2.1V = 504 AsIf you use 20A charge current the charge time will be about 25 seconds to charge from 6V to 8.1VDesigning a poly-phase high current buck is NOT a good project for somebody who hasn't built one before.Jay_Diddy_B

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 03, 2020, 09:13:30 pm
content: Have you considered another approach ?That is making the HP supply output those 8V instead of building a massive buck converter.Add a series inductor to limit the current spike and it should be OK-ish.Look into the feedback loop of the HP supply and check if there are any safety functions such as undervoltage protection.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 09:26:08 pm
content: Quote from: Pawelr98 on August 03, 2020, 09:13:30 pmNot sure how practical it would be to have an inductor in series.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 03, 2020, 09:53:54 pm
content: Quote from: Jay_Diddy_B on August 03, 2020, 08:16:07 pmThank you mate appreciate your concern.I know the traces has to be routed bearing mind to keep the digital, analog and the inductors traces separated. There is always a first time for everything. To begin with i am looking at the ADP1850. Seems to be built for the kill.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2020, 10:17:26 pm
content: Quote from: wraper on August 03, 2020, 04:46:07 pmIt could be done at the expense of power density and closed loop bandwidth. The higher operating frequency of smaller parallel multiphase converters allows for higher bandwidth for faster response and higher power density.But winding and using high current inductors like that is more like plumbing.

author: CoteRotie
author_rank: Contributor
timestamp: August 03, 2020, 11:03:56 pm
content: Quote from: David Hess on August 03, 2020, 10:17:26 pmAbsolutely, I didn't mean to suggest it couldn't be done in a single phase, only that the (much) better solution from an efficiency, input ripple, input cap size, output response, and ease of design and build would be a multi-phase design. I have seen a 10kW single phase converter, but it was uglyYou can do 30A/phase comfortably for lower voltage outputs but off the top of my head I would probably go with 4 phases for this design. I agree with comments that it's not a good design for a beginner to start with.

author: CoteRotie
author_rank: Contributor
timestamp: August 03, 2020, 11:16:26 pm
content: Quote from: anishkgt on August 03, 2020, 09:53:54 pmWell, if you're happy with 20A of charge current instead of 84A you could do a single phase buck. The dual phase would be fine too. But if you want to charge using the full 84A the ADP1850 says on the datasheet "Output current to more than 25 A per channel". But probably not THAT much more so you would need a different part, likehttps://www.analog.com/media/en/technical-documentation/data-sheets/ltc7871.pdf

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 04, 2020, 06:08:27 am
content: That part is a bit too much with the extra gate drivers. I think i will go with the ADP1850, two in parallel. 25/phase would run them at 23 or 24 per phase give close to 92 to 96A. Inductors rated above 20A is hard to find and are less common.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2020, 07:57:54 am
content: Note that ending voltage of 8V means the average voltage is at 4V, which can be used for a rough loss calculation.So while at 8V you could theoretize going without synch rectification, at 4V average it's almost a must.Indeed, building it in single phase requires massive components and heatsinking, which easily blows your layout loop area up.The more phases you use, the smaller the loops, and the easier cooling becomes; you can safely increase the distance between the phases (not going to extremes, of course, or you lose the benefits of shared DC input bus capacitance) to spread the heat.These smaller transistors in smaller loops enables you to use higher frequency, which enables you to use smaller inductance values; making the off-the-shelf inductor selection current ratings go up!As you have noticed, going below about 20A/phase gets you a wide selection of cheap off-the-shelf inductors.Note you need a control IC that can do unlimited current-mode. Proper CC-CV would be optimal, but assuming you don't need good CC regulation, just pick one which limits current below the ratings of your components and doesn't go in a hickup or latched overcurrent modes.I agree 4 phases would be the minimum. Some 2-phase control ICs have sync outputs phase-shifted by 90 degrees so you can built a 4-phase thing with two such chips. With more than 4 phases, the options for control ICs get more scarce.

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 04, 2020, 01:56:29 pm
content: Quote from: anishkgt on August 03, 2020, 09:26:08 pmThis is in order to stop the thing from tripping OCP and blowing the switch that connects the capacitor bank.If you connect a massive capacitor bank and there is a big voltage difference then massive current will start to flow.We don't want that.A series inductor will limit the current.The only problem I see is the possible kickback which can overvolt the capacitors.Computer power supplies are not equipped with constant current mode.Once the current goes too high they just switch off.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2020, 01:58:51 pm
content: Quote from: Pawelr98 on August 04, 2020, 01:56:29 pmIt would take a very large inductor to approach the energy storage of the capacitors in question.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 04, 2020, 02:10:27 pm
content: Quote from: Siwastaja on August 04, 2020, 07:57:54 amI am looking at the ADP1850 (https://www.analog.com/media/en/technical-documentation/data-sheets/ADP1850.pdf) a dual phase buck-converter. 4 phase does not have gate drivers and the explanation is not very well documented so a beginner with buck-converters like find it hard to follow along. I could use ADP1850 in parallel or interleave, if its possible, to get what i want. I would just need 80A and CC at 100A a soft-start of couple milli seconds.On page 16 Tss is calculated but does not specify the unit of time used. Can somebody please confirm it. On LTC3824 datasheet on page 9 they talk about Css calculation and the unit used is milliseconds so i assume its the same here as well but just not sure.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 04, 2020, 02:26:18 pm
content: Hi,I think that you will find the ADP1850 difficult to use in practice. It is a current mode controller, (which is good), but it measure the voltage drop across the bottom MOSFET to regulate the current.This means:1) the current limit will vary with the junction temperature of the MOSFETs2) It make the layout extra hard, because you need to measure a small voltage in a circuit with high currents and rapidly changing currents.I would suggest looking for a Linear Tech Controller that uses current sense resistors in series with the output.Regards,Jay_Diddy_B

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 04, 2020, 02:46:17 pm
content: Quote from: Jay_Diddy_B on August 04, 2020, 02:26:18 pmIt has an option to use a Sense resistor as well, mentioned on page 17 under the heading of 'ACCURATE CURRENT-LIMIT SENSING'

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2020, 04:00:34 pm
content: Using the Rds(on) sensing isn't necessarily a bad idea; it automagically turns down the current if the MOSFET is heating up!It isn't accurate, obviously, but you don't always need accurate charging current, just ensure power dissipation below maximum ratings. And Rds(on) sensing tracksthatpretty well, if you have a transistor with higher Rds(on) or higher ambient temperature, it's doing the right thing for you. Careful with current sensing layout, of course. With a separate resistor, it's easier to do Kelvin sensing, but OTOH, separate resistor increases the loop area again.ADP1850 doesn't have 90deg phase-shifted SYNC OUT - some ADI parts do have, I remember using one some time ago - so in order to have more phases than two, you would need an external clock generator. Any circuit which provides f_sw*2 with two outputs, 90 deg phase shift to each other would work. You could use them to drive two ADP1850's. ADP1850 internally adds 180 deg phase shift, so this would result in all four phases 0, 90, 180, 270 deg apart.I would likely end up with 4 phases doing this, but well, do the calculation, including efficiency and price. My guesstimate is that with 4 phases, you can use the most popular commercial off-the-shelf parts for a simple BOM, but with 2 phases you may have trouble finding the inductors.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 04, 2020, 05:50:51 pm
content: Quote from: Siwastaja on August 04, 2020, 04:00:34 pmTrue, monitoring the current is not what i intent to achieve just close enough to keep the current within spec of the inductor.Trying to figure how this would be practical, using just one would share the load current of 50A between each phase. So would it make sense to have two AD1850 to share 50A each totaling at 100A for the load ? I just looked at a 4 Phase one and that seems costly and moreover calculations are advanced. The one that i looked were the cheapest among the 4phase ones, the NCP81174 (https://www.onsemi.com/pub/Collateral/NCP81174-D.PDF)

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 05, 2020, 09:06:45 am
content: AD has an evaluation board (https://www.analog.com/media/en/technical-documentation/user-guides/ug-205.pdf). This seems perfect, was wondering if this can be connected in parallel to give 100A but i would be pulling just about 83A though.Is it ok to have it done like that because i don't know a better way to achieve it.

author: Warhawk
author_rank: Frequent Contributor
timestamp: August 05, 2020, 04:31:39 pm
content: Did you look at the LM5170-Q1 i suggested before? This is exactly what you want. Dual-phase constant current controller. You have circuit diagrams for several reference design available.1-% current regulation.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 05, 2020, 04:51:11 pm
content: Quote from: Warhawk on August 05, 2020, 04:31:39 pmAvoided it, costly. I've been looking at the ADP1850.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 05, 2020, 05:18:31 pm
content: Hi,Consider that the ADP1850 demo board is designed for the following conditions:1) Fsw 300kHz2) Vout = 1.09V3) The input power is around 55W, this is 4 to 5AIf you modify for 8V outputThe Inductors need to be 6 to 8 times more inductance 3.3 to 4.7uH at 30A Higher values if you lower the switching frequency.The output capacitors on the demo board 2.5V you need to change to 16V partsAt 40A x 8V = 320WThe input current at 12V is now around 27ARegards,Jay_Diddy_B

author: mariush
author_rank: Super Contributor
timestamp: August 05, 2020, 05:27:36 pm
content: You should be able to change the feedback of the power supply to lower the 12v by 1-2 volts. The psu may even have a trimming potentiometer.If not look for the voltage reference, probably not a LM431 but something similar... you could then change the resistor values to adjust voltage, but you'd also have to look into undervoltage protections.If you don't care about efficiency, you could just get some beefy 30A+ bridge rectifiers and parallel a bunch of them on a good heatsink that will have enough thermal mass to keep them cool for 30s-1m (with a fan it should be doable)... you get 2 diode voltage drops on each rectifier, so you lose 1-2v on each rectifier.For example :https://www.digikey.com/product-detail/en/micro-commercial-co/GBJ3508-BP/GBJ3508-BPMS-ND/2213559That peaks at 35A ... if you use 6 of these in parallel along a heatsink, you'll get around 15-20A per bridge rectifier... you get 1.2v drop per diode so around 2v drop.You could put 2 groups in series for higher voltage drop.They can handle up to 150c ... but basically you could have them on the sides of a heatsink tube and run air through the tube center to dissipate the heat ...scroll down on this page to where they show to-220 packages on a heatsink... think those bridge rectifiers instead:http://www.kerrywong.com/2018/09/17/inside-an-agilent-66312a-two-quadrant-power-supply/

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 05, 2020, 05:41:26 pm
content: Quote from: Jay_Diddy_B on August 05, 2020, 05:18:31 pmNo, not going with the eval board specs, So far this is what i've come up withFreq: 200kHzVinMax: 12VIout per phase: 22AVout: 8.1VDelta iL: 8.8ARMS Delata iL: 2.97APeak iL: 13.2Inductance: 1.49uH Round it up to 1.5uHDuty Cycle: 0.675Rtop: 125kRbot: 10kRds(on) max worst case highest Temp: 0.01OhmRiLIM: 1.8mOhmR_RAMP: 349KCss: 1ufTss: 0.0923msIN CAP I_RMS: 10.3ACin_MIN Capacitance: 100.54ufHaving trouble with calculating input capacitor, I can't seem to find a capacitor based on the value.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 05, 2020, 05:43:43 pm
content: There's nothing advanced in designing in more phases.You just, instead of a 8V, 100A converter, design a 8V, 25A converter and copy-paste it 4 times.The only thing to add is to make them run in sync with phase shifts, for example 180 deg for 2 phases, 120 deg for 3 phases, 90 deg for 4 phases, 72 deg for 5 phases... you get the point. For this, you need to create the sync signal for the ICs with phase shifts, if you can't find a fully integrated controller IC that does 4 phases, or a 2-phase control IC with 90-deg phase shifted SYNC OUT, which would allow a simple single wire from SYNC OUT to another chip's SYNC IN to be used for 4 phases with two chips and no extra clock generators.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 05, 2020, 06:01:10 pm
content: Quote from: Siwastaja on August 05, 2020, 05:43:43 pmThe ADP1850 has a SYNC pin but it only accepts input and no out.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 06, 2020, 07:15:02 pm
content: couple of questions regarding the ADP1850 buck converter.Under ‘Setting the current sense gain’ on page 17 calculating V_CSMAX requires the Rdson_MIN and MAX resistance. Is this RDSon value from the datasheet of the mosfet or something to with the duty cycle. The max is not defined so i presume it is the same like MIN.Then there V_COMMAX where the ton is required. How can i calculate this ?The duty cycle is Vout/Vin = 8.1/12 = 0.675 which is 67.5% of the time the high side mosfet is ON. 67.5% of 200,000Hz is 130,000. So how can i substitute it to the formula as a time.