controlling Buck module from mcu

[hsn93]

Hello,

i want to fast design a product that has buck converter i want to control from arduino

so what I've chosen


300W buck converter module
https://www.aliexpress.com/item/300W-20A-DC-DC-Buck-Converter-Step-down-Module-Constant-Current-LED-Driver-Power-Step-Down/32821840536.html
its based on LM25116

anyway, there is CV which is variable resistor that goes to the buck controller and i wanted to control it so this is what i though of:

to use:
MCP4725 I2C DAC Breakout module development board
https://www.aliexpress.com/item/MCP4725-I2C-DAC-Breakout-module-development-board/32821744967.html
and
npn transistor  MMBTA42LT1G
https://www.onsemi.com/pub/Collateral/MMBTA42LT1-D.PDF


the plan is to connect npn transistor like this:

Note: R1 is inside the buck module so i will make jumper wire.

do you think this is good idea? will it work effectively? accurate (+/- 50mV)?
any other ideas?

i've seen this also which seems good it has 64 values need accuracy right?
https://www.analog.com/media/en/technical-documentation/data-sheets/AD5171.pdf

[mikerj]

No, this will not work.  I'm assuming terminal 2 of R1 provides feedback to the switchmode regulator device?

You DAC output will do nothing until the voltage is high enough to start turning on the transistor, so maybe 0.0-0.55v does nothing.  Then there will be a small voltage range over which the transistor switches on and saturates maybe 0.55-0.7v and then any further voltage will make no difference.  This means you have a tiny range of your DAC output that will do anything, when when operating in this range it will be very temperature sensitive, so will drift.

What is the reference voltage of the switchmode device, and what is the reference voltage of your DAC?  Typically you would wire this so the DAC sinks or sources current into the feedback node (terminal 2 of R1) which may be as simple as a resistor between the DAC and R1, but you need to calculate this this.

[perieanuo]

Hi,
Just like this no.
First you need a precise mechanism to start the dcdc module with zero output or disabled.this means controlling enable pin of dcdc converter if he has one.
Then put your reference made with the DAC on dcdc reference module, wait some ms then enable the dcdc.
Any other way is risky and amateur-like.
Best regards,Pierre


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[ogden]

No, this will not work.

It won't work indeed. DAC you mention is buffered meaning you may build control circuit like this:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/818

[perieanuo]

No, this will not work.

It won't work indeed. DAC you mention is buffered meaning you may build control circuit like this:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/818

Yep, this can do.


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[hsn93]

No, this will not work.  I'm assuming terminal 2 of R1 provides feedback to the switchmode regulator device?

You DAC output will do nothing until the voltage is high enough to start turning on the transistor, so maybe 0.0-0.55v does nothing.  Then there will be a small voltage range over which the transistor switches on and saturates maybe 0.55-0.7v and then any further voltage will make no difference.  This means you have a tiny range of your DAC output that will do anything, when when operating in this range it will be very temperature sensitive, so will drift.

What is the reference voltage of the switchmode device, and what is the reference voltage of your DAC?  Typically you would wire this so the DAC sinks or sources current into the feedback node (terminal 2 of R1) which may be as simple as a resistor between the DAC and R1, but you need to calculate this this.

well its LM25116 module so i would say 1.215V on V-feedback
very helpful yes i was thinking about saturation also but dont know how sensitive so i choose low Hfe npn transistor for that and thought i will put high resistance series with Vbase.
i didnt think Vbe would be a problem as DAC can output more than what i need.

anyway, from pictures of the product it has W104 on CV which is 10K trimmer
but i can see:

which i dont know what it is ... (capacitor filtering?)

im not sure what is the resistors configuration (above/below/parallel with the 10k) in the schematic.

if i replaced it with digital pot with 64 steps for a (0-32V) it will be (0.5V stepping) so not accurate.

if i put DAC i would need to know the circuit schematic right?

[hsn93]

Hi,
Just like this no.
First you need a precise mechanism to start the dcdc module with zero output or disabled.this means controlling enable pin of dcdc converter if he has one.
Then put your reference made with the DAC on dcdc reference module, wait some ms then enable the dcdc.
Any other way is risky and amateur-like.
Best regards,Pierre


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hmm i understand what you say but isnt controlling enable pin will disable the output (hence the feedback) ?.. i can just disable DAC (0V) and have the module running with feedback and then control the DAC right?

[perieanuo]

Hi,
Just like this no.
First you need a precise mechanism to start the dcdc module with zero output or disabled.this means controlling enable pin of dcdc converter if he has one.
Then put your reference made with the DAC on dcdc reference module, wait some ms then enable the dcdc.
Any other way is risky and amateur-like.
Best regards,Pierre


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hmm i understand what you say but isnt controlling enable pin will disable the output (hence the feedback) ?.. i can just disable DAC (0V) and have the module running with feedback and then control the DAC right?

You decide here.
I had just some idea of organizing startup sequence (first 0V than after some time wait for transient to settle down activating dcdc, with DAC already with correct/stable value).
If you prefer other way, no problem


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