Author Topic: Different approaches to power supply reference voltage  (Read 1682 times)

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Offline Torrentula

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Different approaches to power supply reference voltage
« on: February 08, 2014, 02:23:55 pm »
I am currently working on a modular power supply/front end that will allow me to get use it as an adjustable constant current source/constant voltage supply with current limiting general purpose lab supply. I settled for the ATX supply not because I really need oodles of power (proper technical term here) but I want to hook a couple of those front end boards to a single supply.

Those front ends will be connected to one front panel control board so I don't need two expensive ten turn pots per output channel.

In order to set my current limit/output current and output voltage I would prefer a DAC over simple PWM control. I'm currently looking at the STM32F100 value line devices as those are pretty cheap and they include a 12-bit ADC and DAC. They will communicate with the front panel controller over I2C.
Throwing a 32-bit ARM at this task might look ridiculous at first but the truth is it's cheaper than having a PIC and an external DAC, plus I get 12-bit resolution also for the ADC.

Unfortunately the low pin count LQFP-48 package of the STM32F100s don't have separate voltage reference inputs, so I would like to use the controller's supply voltage as the reference voltage for ADC and DAC.

The devices can't operate down to 2.048V so I chose 3.072V as the next base 2 divisible reference voltage which should give me a resolution of 750┬ÁV/bit.

And now for the actual question: how should I go about supplying 3.072V to the microcontroller? I have thought about three options:

1. Using an adjustable voltage reference (output current might be too low to run the microcontroller
2. Whipping up my own regulator with a voltage reference, an op amp, a trim pot and an NPN transistor and adjust the pot until the output reaches 3.072V
3. Using a bog standard LM317 two resistors, a trim pot and some caps to generate the reference voltage

I don't know if option 1 is viable so I currently tend towards using option 3 because based on my experience with building Dave's constant current source it might be hard to get the loop stability for option 2 right.

The only reason I think of why the LM317 might be problematic is that the LM317's internal voltage reference might be complete crap, but on the other hand the temp coefficient of the trim pot probably would be much more of a concern.

Any advice that some more experienced people might have?

Cheers,
Elia
 

Offline han

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Re: Different approaches to power supply reference voltage
« Reply #1 on: February 08, 2014, 03:00:04 pm »
Using uControler for controlling Voltage reference is not recommended:
1. Lot of digital noise
2. The voltage is affected by ucontroler activity, communication, lcd, ...


i recommended, using analog part for voltage/current generation and ucontroler for display only, since adc is can be made precise better than dac..


you can use 4 pots (2 for each V and I) and coarse/ fine.
example series of 50k and 5 kohm pots instead one 50k precision pots


but if there is a need to make it programmable, find a external DAC like MCP4922
so you can attach good Voltage reference
 

Offline Torrentula

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Re: Different approaches to power supply reference voltage
« Reply #2 on: February 09, 2014, 09:09:33 am »
but if there is a need to make it programmable, find a external DAC like MCP4922
so you can attach good Voltage reference

Yep it should be programmable as I want to get away with as few knobs/buttons on the front panel as possible because the two pot solution would still require 4 knobs per channel, so ideally I want just a rotary encoder and buttons for selecting between voltage/current setting and a button for switching between CC/CV modes (and maybe some LEDs to indicate current limiting mode).

The display could be a graphic display like on Dave's Rigol supply which is controlled by the master controller which then sends the info to each channel controller.

Could I attach the DAC output to an RC filter in order to get rid of some of the noise induced by microcontroller operation?
 


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