controlling Buck module from mcu - Page 1
Forum section: electronics / beginners

author: hsn93
author_rank: Regular Contributor
timestamp: December 10, 2018, 06:19:51 am
content: Hello,i want to fast design a product that has buck converter i want to control from arduinoso what I've chosen300W buck converter modulehttps://www.aliexpress.com/item/300W-20A-DC-DC-Buck-Converter-Step-down-Module-Constant-Current-LED-Driver-Power-Step-Down/32821840536.htmlits based on LM25116anyway, 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 boardhttps://www.aliexpress.com/item/MCP4725-I2C-DAC-Breakout-module-development-board/32821744967.htmlandnpn transistor MMBTA42LT1Ghttps://www.onsemi.com/pub/Collateral/MMBTA42LT1-D.PDFthe 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

author: mikerj
author_rank: Super Contributor
timestamp: December 10, 2018, 10:05:52 am
content: 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.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 10, 2018, 10:22:21 am
content: 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,PierreEnvoyé de mon iPad en utilisant Tapatalk

author: ogden
author_rank: Super Contributor
timestamp: December 10, 2018, 11:00:05 am
content: Quote from: mikerj on December 10, 2018, 10:05:52 amIt 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

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 10, 2018, 11:59:03 am
content: Quote from: ogden on December 10, 2018, 11:00:05 amYep, this can do.Envoyé de mon iPad en utilisant Tapatalk

author: hsn93
author_rank: Regular Contributor
timestamp: December 10, 2018, 01:42:32 pm
content: Quote from: mikerj on December 10, 2018, 10:05:52 amwell its LM25116 module so i would say 1.215V on V-feedbackvery 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 trimmerbut 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?

author: hsn93
author_rank: Regular Contributor
timestamp: December 10, 2018, 01:44:48 pm
content: Quote from: perieanuo on December 10, 2018, 10:22:21 amhmm 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?

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 10, 2018, 10:34:14 pm
content: Quote from: hsn93 on December 10, 2018, 01:44:48 pmYou 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 problemEnvoyé de mon iPad en utilisant Tapatalk