Author Topic: Ideal buck converter for high current  (Read 7031 times)

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Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #25 on: August 04, 2020, 05:50:51 pm »
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 tracks that pretty 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.

True, 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)
 

Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #26 on: August 05, 2020, 09:06:45 am »
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.
 

Offline Warhawk

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Re: Ideal buck converter for high current
« Reply #27 on: August 05, 2020, 04:31:39 pm »
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.

Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #28 on: August 05, 2020, 04:51:11 pm »
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.

Avoided it, costly. I've been looking at the ADP1850.
 

Offline Jay_Diddy_B

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Re: Ideal buck converter for high current
« Reply #29 on: August 05, 2020, 05:18:31 pm »
Hi,
Consider that the ADP1850 demo board is designed for the following conditions:

1) Fsw 300kHz

2) Vout = 1.09V

3) The input power is around 55W, this is 4 to 5A



If you modify for 8V output

The 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 parts

At 40A x 8V = 320W

The input current at 12V is now around 27A

Regards,
Jay_Diddy_B

 

Offline mariush

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Re: Ideal buck converter for high current
« Reply #30 on: August 05, 2020, 05:27:36 pm »
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/2213559

That 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/

 

Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #31 on: August 05, 2020, 05:41:26 pm »
Hi,
Consider that the ADP1850 demo board is designed for the following conditions:

1) Fsw 300kHz

2) Vout = 1.09V

3) The input power is around 55W, this is 4 to 5A



If you modify for 8V output

The 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 parts

At 40A x 8V = 320W

The input current at 12V is now around 27A

Regards,
Jay_Diddy_B

No, not going with the eval board specs, So far this is what i've come up with
Freq: 200kHz
VinMax: 12V
Iout per phase: 22A
Vout: 8.1V
Delta iL: 8.8A
RMS Delata iL: 2.97A
Peak iL: 13.2
Inductance: 1.49uH Round it up to 1.5uH
Duty Cycle: 0.675
Rtop: 125k
Rbot: 10k
Rds(on) max worst case highest Temp: 0.01Ohm
RiLIM: 1.8mOhm
R_RAMP: 349K
Css: 1uf
Tss: 0.0923ms
IN CAP I_RMS: 10.3A
Cin_MIN Capacitance: 100.54uf

Having trouble with calculating input capacitor, I can't seem to find a capacitor based on the value.
 

Online Siwastaja

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Re: Ideal buck converter for high current
« Reply #32 on: August 05, 2020, 05:43:43 pm »
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.
 

Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #33 on: August 05, 2020, 06:01:10 pm »
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.

The ADP1850 has a SYNC pin but it only accepts input and no out.
 

Offline anishkgtTopic starter

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Re: Ideal buck converter for high current
« Reply #34 on: August 06, 2020, 07:15:02 pm »
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.
 


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