Voltsge Control for Power Supply

[grindylow]

Voltage Control for Power Supply

Hello people,

I have an of-the-shelf SMPS and I want to use it as a battery charger. I will be using a MOSFET driven by a PWM on a microcontroller to control the current. My problem is how to control the voltage while preserving the maximum current of the power supply. The voltage of the power supply is 12V and there would be a parallel load while charging. The voltage(in CV mode) for the battery is 10.8V. The voltage must also be controlled by a microcontroller. I think that resistor divider is not an option here. What device could I use for this application?

Thanks,
Homer

 

[grindylow]

Re: Voltage Control for Power Supply

Okay, will this schematic work?

 

[xaccto]

Re: Voltage Control for Power Supply

what provision does the SMPS have to vary the output voltage ?
A potentiometer ?
If so, I would look at using one of the Digital Potentiometer chips that are available to replace that.

Then also, instead of controlling the current with "MOSFET driven by a PWM",
I would suggest you simply sense/monitor the current, and use the voltage control of SMPS to control the
current with your microcontroller.

ok, re your above circuit,
looks like Q2 will be biased in its linear region and act as a resistor.

Q1, depending on the source capability will simply deliver big current pulses, you want to be sure
the current pulses are not huge, so thats what Q2 is about ?

IMO, its going about it the wrong way.
Especially seeing as there is a standard topology for doing what your attempting.....

essentially replace Q2 with an inductor, and diode. And introduce some measurement of current
for feedback to micro.
Basic elements of a buck regulator actually........

 

[grindylow]

Re: Voltage Control for Power Supply

Thanks xaccto.
There will actually be a hall-effect current sensor to monitor the current while charging and the voltage will be monitored as well. You are saying that I can control the current by controlling the supply voltage? There is a CC and CV mode to charge the battery, will this be affected? This will also be applicable without a parallel load (since the voltage will be varied) to the charging scheme, am I right?