[Moved]: Voltage and Current Control for Variable SMPS

[fethiyeli]

I am planning to design smps that has variable output voltage and current. Its voltage will change 0-50V and current will change 0-10A.



Vout comes from output voltage up to 50V. Iout comes from output current up to 10A. Iout will be Rshunt x 10A. Rshunt is 0.05R, so Iout will be 0.5V as voltage. Output pin of 1 and 8 changes between 0 and 2.5V according to Vout and Iout. But Vfb is not changing linearly between 0-2.5V. I am using UC3844 so its non-inverting input is connected to 2.5V, therefore VFB which is connected to inverting input of UC3844, must be 2.5V at most. My question is how can i change the VFB voltage linearly between 0 and 2.5V when i use the potentiometer. These potentiometers are for voltage and current control. Do you have any suggestion to control of voltage and current of SMPS output ?

 

[treez]

To get a variable output voltage (or current) smps, I would use an external error amplifier, and use that to connect into the comp pin of the ucxxxx device, and regulate like that...ground the fb pin on the ucxxxx.

You know the way an opto coupler connects up to the comp pin in an isolated uc3842 type smps, do it on those lines.

The external eror amp will be an opamp, and you have a reference itno one pin, and the other pins input comes from your sensed signal,.

Be aware if you try to regulate output voltage and current at the same time.....because the feedback loops could fight each other.
obviously if you have a 5000 ohm load then you cannot regulate current to 1 amp, so that's ok as the current loop will be railed and out of it, leaving the voltage loop to regulate to 50v.

so you change the ref voltage on the external error amp to get change of voltage/current.

 

[D.A.(Tony)Stewart]

Why using a 5W shunt when you can use a 1W shunt with more gain and care on EMI noise. 75mV shunt is pretty standard max drop for current shunts., not 500mV. (0,5x10=5W)

 

[fethiyeli]

Why using a 5W shunt when you can use a 1W shunt with more gain and care on EMI noise. 75mV shunt is pretty standard max drop for current shunts., not 500mV. (0,5x10=5W)

Yea right but i couldnt find suitable resistor as shunt. Since there is a 1R 1W resistor as the lowest value. But as 5W, there is 0.1R. I used two 0.1R in parallel. I live in Turkey and its not easy to get suitable components for projects.

 

[D.A.(Tony)Stewart]

Yea right but i couldnt find suitable resistor as shunt. Since there is a 1R 1W resistor as the lowest value. But as 5W, there is 0.1R. I used two 0.1R in parallel. I live in Turkey and its not easy to get suitable components for projects.

Can you not make resistor with a small copper wire jumper and a fuse. even 50mV 50mV/10A= 5mΩ 100A²*.005= 0.5 Watt

Then current sense voltage noise must be suppressed by careful common mode routing , shielding, CM choke filtering.

 

[fethiyeli]

Can you not make resistor with a small copper wire jumper and a fuse. even 50mV 50mV/10A= 5mΩ 100A²*.005= 0.5 Watt

Then current sense voltage noise must be suppressed by careful common mode routing , shielding, CM choke filtering.

I couldnt think that. Thnx.