Linear Programmable DC source 0.7-12VDC / 530mA

[europus2002]

Hi all,
I am trying to achieve digital control of a DC-DC convertor via microcontroller and DAC. I need variable DC supply from 0.7V to 12.2VDC with current from 0-530mA...The idea is to achieve varible output voltage set through microcontroller and DAC for above spec....I tried Power Mosfet but the charactoristics under load is weird...(fairly nonlinear in the range 3.9-4.8V for FQP12N60 Mosfet) and due to 6W drain through , it heat up tremendously, which would be constraint for a handheld unit.......I am open to ideas...and thanks in advance to all you who help...

 

[itsmoked]

Wow that is a tall order... I can see why you are having problems.. I can also see why you'd like to do it.(handheld)

I would attempt this by using a dedicated microcontroller, minimal pin count one that ran fast and had two analog inputs.

I would then use a buck or boost form depending on what your source is (which you didn't mention shame, shame!)

I would monitor the inductor current with one input and the voltage with another. Then you run the Mosfet digitally based on what voltage you are after while watching the current.

If your device uses a dang analog voltage to request the voltage needed then you use a third analog-in to pick that up.

You want a very tight control loop so ths is why you want a stand-alone microcontroller, plus you can just focus on the task. Note that you can leave this uC off if not needed.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[europus2002]

Hi Itsmoked....we meet again !!.....thanks for your tip...By the way, it would be boost regulator .I will see how i can fit into exhisting unit . Another consideration i had was to use an SMPS boost say from 5V to 12V and then bug the feedback loop to control the Ton/Toff and achieve the required output (0.8-12V). However, the most scary part of this is finding a ferrite core transformer (8W approx) and then control the piece to piece variation ... Does anyone know of any perticular transformer / manufacturer that i can use for this purpose.....and this product is going later for FDA approvallllllll...(.

 

[itsmoked]

Hi europus2002;

Most SMPS's would choke on that service since power-up is a transitory hick-up like function..

Has this controlled supply got to be isolated from the original source? Why do you need a transformer?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Warpspeed]

A boost topology can only boost the incoming voltage. If the output must go down to 0.7 volts, that suggests you must initially have a very low input voltage to work from ?

What is your incoming supply voltage ? And how clean and well regulated does the output have to be ?

One problem you are going to have is feedback loop stabilisation over such a wide operating envelope. For several reasons the buck topology would probably be the easiest to work with.

 

[Yashu]

Try Maxim MAX618 and a digital pot like Analog Devices AD5231.

I have a schematic if you want it for 5V to 20V @ 150ma as a reference.