adjustable power supply circuit help

[aruna1]

Hi
I want to build a adjustable power supply system that can adjust output voltage using a pwm channel of a micro-controller.

output requirement is 0-15v and about maximum 3A.
and should be a linear regulator (as I'm still learning switching regulators and have no good idea about them)

basic idea is adjust the output by varying pulse width and feedback output voltage to the same micro-controller to keep output constant.

Any idea how to achieve this? any regulator IC I can use for this?

thank you

 

[MikeMl]

One problem I see is that if you start with PWM, there will always be some residual feedthrough of the PWM frequency into the control loop of the linear regulator. Depending on what you are using the supply for, this could be a problem.

You will probably have to filter the PWMed control voltage very aggressively.

If you want to control the supply's voltage from a uC, why not get a $2 serial DAC and use that to control the linear regulator's loop?

 

[aruna1]

One problem I see is that if you start with PWM, there will always be some residual feedthrough of the PWM frequency into the control loop of the linear regulator. Depending on what you are using the supply for, this could be a problem.

You will probably have to filter the PWMed control voltage very aggressively.

If you want to control the supply's voltage from a uC, why not get a $2 serial DAC and use that to control the linear regulator's loop?

I was thinking of adding RC low pass filter to the pwm output.

And uC i was going to use is a 8 pin uC so i have to go for a higher pin count uC if i use a DAC and also since i dont know how to use I2C its going to be a problem.

can you tell me how to control regulators control loop with external supply? with an example diagram?

Thank you

 

[MikeMl]

Look at the "applications section" of the National LM317 data sheet. I remember seeing a circuit where an opamp controlled the ADJ terminal. No reason it wouldn't work with one of the 3A regulators...

Depending on how high your PWM rate is, a single pole RC filter won't even come close to providing enough filtering. You will likely need an active filter with four to six poles, meaning two or three opamps.

 

[aruna1]

in that case what type of soluton do you suggest? a SMPS?

 

[MikeMl]

in that case what type of soluton do you suggest? a SMPS?

Back up and say what you need the device to do. You apriori jumped to the conclusion using a UC and PWM was the way to build a power supply; it may not be.

 

[ronv]

PWM Control

I think something like this might be what your after.