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Author Topic: Digital Power Supply .... Documentation and Help  (Read 3330 times)
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zerro
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« on: June 10, 2007, 10:19:45 22:19 »

http://rapidshare.com/files/36388525/POWER_SUPPLY.ZIP.html

I make this digital power supply (there are 2 projects but are similar) and i have a problem ... the output tension variate, 0.2V up and down ... and move up and down continuously. I discover in the first project a doc file but is in Turkish and i don't understand ... probably there are some explication about my problem ... if somebody could translate this doc i'll appreciate ...
And .. if somebody made this digital power supply and want to share some "tips and tricks" with me and with somebody who is interest to build this good power supply i'll appreciate too.

I must mention that the 2 digital power supply project for who i put the link was posted by gevv in "Sonsivri > Electronics > Hardware and Tools > Pic Archive". Now i must apologize for my English an to wait for an answer .... Thank you!
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bbarney
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« Reply #1 on: June 11, 2007, 12:58:43 00:58 »

the author of this supply pulled it off his website for that same reason,best to find a better supply to build
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mylogin
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« Reply #2 on: June 11, 2007, 03:22:06 03:22 »

I just don't feel confident on the digital power supply especially the MCU controlled.  What will happened when the MCU program went bad/bug, or lost the memory(especially the flash ROM MCU) for any reason,  the power supply output has to be protected from any errors/problems, and should be protected by different unit.   it is dangerous to be controlled only by a single central control unit.    Most of the digital display power supply on the market use traditional power supply with digital meter to read the output.  this is more reliable.  just my own opinion.
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sam_des
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« Reply #3 on: June 13, 2007, 10:22:08 10:22 »

Hi,

I think I can explain your problem. Before that I must say that I haven't seen the design posted by you, but still any Power Supply using Digital Controller (In most cases a MCU) can be explained in same way.

I assume that you have checked that your feedback loop is free from any other problems.
I think that in your design, you are using a PWM with varying duty cycle to regulate the o/p voltage.

If so read on...

Note that varying Duty cycle in any Digital PWM has only finite no. of steps. I will give an example. Lets say that PWM Frequency is 50kHz & PWM resolution is 8-bits (i.e.Duty Cycle control register is 8-bit) than you can have only 256 Duty cycles. Change of 1 step in duty cycle amounts to 78.125nS(0.390625%).
Well, this is too large for any power-supply wishing minimum ripple at o/p.

This is the root of your problem. Finite Resolution of PWM Duty Cycle. Everytime your feedback senses that o/p voltage is not what is desired, it changes the duty cycle. But this change is so larege that it leads the o/p to other way and forcing feedback loop change duty cycle again. This repeats continuously.
If you obesrve your o/p on Scope, you will see that your o/p is like Triangular Wave above & below your Set Point.

Analog systems don't have this problem becauase their PWMs can have infinte Duty Cycle resolution.

There only few solutions - 1) Increase your PWM Frequncy
                                    2) Increase Duty Cycle Resolution
                                    3) Be very careful with your feedback loop.

Well, these are pretty hard requirements on current MCUs. Most current MCUs offer only 8-bit resolution for frequecies upto 500kHz. DSPs do offer PWM with higher Frequencies with Better resolution(16-bit), but DSPs are expensive.

Only few days ago, I've seen ads from TI having digial Controller with PWM resolution 5-10 pS(10e-12).
I am yet to see details, but sounds interesting.

For this reason only, TRUE Digital Power Supplies are rare(well, almost none). Most Digital Power Supplies use MCUs only as Front Panel Controllers. But still this field is evolving rapidly. Microchip / Atmel / Freescale / TI are all working hard. & Advantages of digital Power supply are huge.

All other problems mentioned by mylogin are important, but can be taken care of easily, like most other Digital Control/MCU based designs do.

Regards,
sam_des
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zerro
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« Reply #4 on: June 13, 2007, 01:54:27 13:54 »

Thank you all for the answers ... i make some little experiences and i observe that PWM is the problem ... so .. i take an multiturn potentiometer and I put in schematic to generate a tension between 0 - 5V .... and ... surprise .... my output tension not variate .... it is so stable .... so.... i decide to change the schematic to insert an MAX6150 for  the reference tension (5V) and an MAX5821 (Dual, 10-Bit, Low-Power, 2-Wire, Serial Voltage-Output DAC) and from i2c to command the output between 0 - 5V .... probably this week i'll finish my changes and i'll inform you about results ....

Best regards!
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sam_des
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« Reply #5 on: June 13, 2007, 05:44:43 17:44 »

Hi zerro,

If I guess correct, you are trying to use Pot for reference to DAC & using DAC's o/p as your desired o/p.
Watch out for Current Requirement of circuit(where you are using DAC O/p). You may require an op-amp in between as buffer. Also check the datasheet of Pot & DAC, now they do have thermal, DAC conversion time & other parameters to worry about.

Well, your method may be suitable for 5-10 volts & 5-100 mA max. But for any larger voltages & currents, this method is not feasible. What about 12V/10-15Amp ? You will have to resort to traditional methods only. And with digital method all that I said in my earlier post come back to haunt us !

Any way, I'll be glad if that solves your problem. BTW just out of curiosity what is the application of your power supply ?

regrads,

sam_des
« Last Edit: June 13, 2007, 05:47:02 17:47 by sam_des » Logged

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zerro
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« Reply #6 on: August 16, 2007, 01:28:49 13:28 »

I want to use an MAX5821 (with a MAX6150 for reference voltage) to generate a more stable tension between 0 - 5V for LT1491 references for tension and current. MAX5821 is an 10 bit voltage output ... same like PWM from PIC ....
I hope will be ok ...
I'll keep you inform ...
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« Reply #7 on: August 16, 2007, 02:20:35 14:20 »

lm317 for reference?
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zerro
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« Reply #8 on: August 16, 2007, 03:16:55 15:16 »

I use MAX6150 for 5V reference for MAX5821 ... i think is better ...
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« Reply #9 on: April 18, 2008, 03:51:26 15:51 »

Hi!

And the program source code is good?

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zerro
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« Reply #10 on: April 28, 2008, 08:30:16 20:30 »

the program source code is partialy good ....
I'll still make changes ... but i don't have time finish this project ...
One important thing i observe ... all the bad things for this power supply is from 5V for digital control of the power source ... i'll try to put a separate transformer for this tension and to stabilize and filtrate this 5V tension very good.
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