digital potentiometer controlled buck converter

Thread Starter

whtemple1959

Joined Sep 3, 2019
3
As I begin my journey to solve a problem I have, I want to admit that I have only taken an introductory course in electronics.
My situation ..
I need a buck converter to control the voltage to a peltier device. My problem is that all of the buck converters I have seen, have all had manual analog adjustment mechanisms.
My first need is to find out if a digital potentiometer can be attached to a standard buck converter.
I did find this post from 2016 ... https://forum.allaboutcircuits.com/threads/digital-potentiometer-and-power-supply.125512/ ... but I am not sure if a dc-dc booster is equivalent to a buck converter.
My second issue is how to convert the signals from heat sensors into a signal that is usable by the digital potentiometer.
I am thinking of using a microcontroller
So my questions are.
Can a standard buck converter be modified to accept input from a digital potentiometer?
Can a microcontroller query the heat sensors and convert that information into a usable signal for the digital potentiometer?
Any advice and guidance will be greatly appreciated.
 

Alec_t

Joined Sep 17, 2013
10,355
Welcome to AAC!
Can a standard buck converter be modified to accept input from a digital potentiometer?
Buck converters vary. They're not really 'standard'. Doubtless some could be converted for digital control.
Can a microcontroller query the heat sensors and convert that information into a usable signal for the digital potentiometer?
Yes. Assuming the heat sensors are analogue the MCU would need one or more analogue inputs.
 

Thread Starter

whtemple1959

Joined Sep 3, 2019
3
Welcome to AAC!

Buck converters vary. They're not really 'standard'. Doubtless some could be converted for digital control.

Yes. Assuming the heat sensors are analogue the MCU would need one or more analogue inputs.
Alec_t
Thanks for the input.
Do you have any suggestions on which buck converter can be modified?
The sensors are digital.
So since that is a new variable are you saying aMCU will not be able to process the sensor data?
 

crutschow

Joined Mar 14, 2008
23,314
Depending upon the buck converter design, you likely can generate a DAC analog output from the mirco to directly drive the sense voltage input to the converter controller chip (in place of the signal from the pot) to control the converters output voltage.

Note that you will almost certainly need to add some loop compensation in the micro (using Fuzzy logic or PID) to stabilize the control loop.
I like fuzzy logic (discussed here) because it's digital (basically using a series of If-Then-Else statements) and more intuitive to use and optimize than the digital emulation of analog PID, which involves complex transfer functions.
 

Thread Starter

whtemple1959

Joined Sep 3, 2019
3
Depending upon the buck converter design, you likely can generate a DAC analog output from the mirco to directly drive the sense voltage input to the converter controller chip (in place of the signal from the pot) to control the converters output voltage.

Note that you will almost certainly need to add some loop compensation in the micro (using Fuzzy logic or PID) to stabilize the control loop.
I like fuzzy logic (discussed here) because it's digital (basically using a series of If-Then-Else statements) and more intuitive to use and optimize than the digital emulation of analog PID, which involves complex transfer functions.
crutschow,
Thank you for your input
Although I am not at this time certain which buck converter I will be using I am considering this one .... https://www.ebay.com/itm/XL6009-DC-Buck-Boost-Voltage-Converter-Constant-Current-Module-Step-Power-Up/223567868697?hash=item340daef319:m:m5I_bdGY7b_Oq4ELjdwIcrg:sc:USPSFirstClass!43203!US!-1
it will accept 12vdc and has truly variable vdc output so it is perfect for my purpose.
When you say a DAC analog output are you talking about something like this ... https://www.ebay.com/itm/QTY-1x-Analog-Devices-DAC16GS-16Bit-HighSpeed-Current-Output-DAC-24-Lead-SOL-NEW/140913267031?epid=1701591932&hash=item20cf15d157:g:d60AAMXQIBBRD3dw
Or something else?
Of course this project could not be easy.
as if learning about setting up a micro-controller was not daunting enough now I must study DACs.
But nothing worth while comes easy.
But working the problem backward I will study how to make a DAC work with a buck and then how a MC will interface with the DAC
Should be fairly straight forward in those two areas .... I am waiting for the chuckles.
 

MisterBill2

Joined Jan 23, 2018
3,863
The simpler way to adjust the voltage from a "buck converter" is to use a remotely programmable converter, which often has separate sense terminals that allow varying the output through some external voltage or resistance.
But now I have a question about how tightly your application requires the temperature to be controlled, because Peltier cooling devices, being diodes, are fairly non-linear in their heat pumping versus voltage relationship. so the very important question is how precise does the temperature control have to be, and also how fast does it need to respond. There are other ways to control temperature that can be quite accurate and a whole lot simpler while being more efficient, and costing less as well. And with a Peltier device, which has no starting inrush, on/off switching for half a degree temperature change is no problem at all. Also, what kind of temperature sensors are used, and what is their anticipated output signal?
 
Last edited:

crutschow

Joined Mar 14, 2008
23,314
Although I am not at this time certain which buck converter I will be using I am considering this one .... https://www.ebay.com/itm/XL6009-DC-...7b_Oq4ELjdwIcrg:sc:USPSFirstClass!43203!US!-1
The typical diagram for the XL6009 converter used in that module is below.
To directly control the output voltage you would connect the DAC output to the FB pin 5 (when it's inside the feedback loop using the temperature sensors for the feedback).

But that appears to be a boost converter.
You said you needed a buck.

upload_2019-9-3_19-33-53.png
 

Kjeldgaard

Joined Apr 7, 2016
377
Here is a principle schematic for what I have used for Peltier and Blower control respectively.

VregCon_1.jpg

The voltage regulator can be almost anything Linear or Buck Switch Mode, it just needs a Feed Back input which is referenced to GND.

R1 and R2 are the normal resistors to set the desired output voltage.
With R3 you can then pull the output voltage up or down depending on whether Vcon is below or above the voltage regulator reference voltage.

I have used a circuit for Peltier where Vcon = 3.3V equals 0V output and Vcon = 0.0V equals 12V output. Be aware that a higher Vcon gives a lower output voltage.
 

MisterBill2

Joined Jan 23, 2018
3,863
It is still unknown how accurately the temperature must be controlled, and how fast the controlled portion responds to changes in the peltier cooler drive. My reason of asking is that a very simple on/off controller may also be able to provide adequate control while being simpler, more efficient, and cheaper, in addition to providing better reliability. That type of system would use a comparator with a small amount of hysteresis to switch the cooler on and off.
 
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