DAC Controlled Power Supply

[maurosmartins]

Hello all,

I would like to mount a small power supply ideally capable of supplying up to 2A @12V and must be DAC controlled. I've mounted a linear power supply with a mosfet (I've already tried N and P channel) as series pass transistor and a ampop as the following schematic:



I've calculated the voltage divider to match 12V at the maximum DAC output. It works ok (kinda). With no load voltage regulation works fine, I'm able to change the DAC and set the correct output voltage. as soon as I start to demand some current (as low as 10mA) the output immediately drops.

Some measurements :
Vout@no load = 12V
Vout@10mA = 11.93V
Vout@100mA = 11.48V

to my understanding the ampop should compensate this voltage drop but it doesn't. What am I doing wrong?

Here is my current BOM:
Ampop LM358N
DAC MCP4725
Pchannel mosfet FDD4141
I also have a 100uF at the output that is not shown in the previous schematic.

Both ampop and mosfet source are at 12V R1 = 2k and R2 = 1k.

Thanks in advanced,
Best regards, Mauro.

[Kleinstein]

Both circuit have a chance to oscillate - not guarantied, but well possible with some loads.

The P-Channel version may not be able to fully turn off the FET as the output range of the LM358 is limited. Depending on the FET is may just work. Chances for oscillation are high of there is not capacitor at the output.

The N-Channel Version has quite some voltage drop. Some 2-3 V from the LM358 and some 3-5 V from the FET's gate source voltage.


The noted FDD4141 is not suitable - it may well run too hot.

[Andy Watson]

What is Vin voltage ?

[maurosmartins]

Hello Kleinstein, and Andy Watson, thank you for replying:

@Kleinstein, The FDD4141 was the only P channel mosfetI had lying around I understand that it is not suitable for 2A loads. Also I do have a 100uF capacitor at the output.
@Andy, Vin = 12V

Upon further analysis it was not the circuit that had poor voltage regulation, it were the voltage drops due to poor wiring and bread-boarding. With thicker wires the results are now the following:

Some measurements :
Vout@no load = 12V
Vout@10mA = 12V
Vout@100mA = 12V
Vout@500mA = 11.91V



I'm ok with this, I'll have to get a suitable mosfet but this is the starting point.

Best regards, Mauro.

[Kleinstein]

With Vin = 12 V a piece of wire is working better to get 12 V out. 

So the regulator may not even be active.

[maurosmartins]

 

  you are right, it wasn't the best example but it is working also for other output voltages

Best regards, Mauro.