[SOLVED] Difference between an MPPT solar charge controller and an ordinary charge controller

[BradtheRad]

A resistor develops a voltage across it, due to current going through it. This voltage can be detected by an op amp (or ADC in a microcontroller).

It's just a convenient way to measure current through equipment.

The sense resistor usually has a low ohm value, so that it has only minimal effect on the performance of the equipment.



- - - Updated - - -

As for how to design an MPPT, a microcontroller can be programmed to try a different level every few seconds, and use the level that is greater than previous.

I did not undertake to make an MPPT, and I'm not sure I would be able to do so by using simple analog circuitry (eg., op amps).

I made a simple On/Off charge controller for my PV panels and battery bank. It had an op amp which sensed battery voltage, and a mosfet which controlled current from the panel.

 

[Akashmd]

Thank you ... i will post my progress regularly and my doubts as well....

 

[Akashmd]

Guys I am really concerned that I may not be able to complete the project ...Does my sepic converter posted before work ..?I am concerned about the values of the components ...and the design of inductors...please help!!

 

[BradtheRad]

A sepic converter is a complex project for you to take on at this stage.

Suppose you were to start simple instead?

Have you designed a simple charge controller? Its principles of operation are essential to know, in order to move on to a buck (step down) converter, or flyback, etc.

 

[Akashmd]

I have started building sepic converter and have simulated in proteus also...
The inductor is designed for 64khz,2A with ferite core..and i have decided to use IRF540 mosfet...but which driver to use ?
ucc2701,ir2110,ir2184,uln2803 or ucc27322...?
Is using FR304 diode ok for this circuit..?
My pwm is 62.5 - 64 khz...
Which capacitor to use ...?

 

[BradtheRad]

Which capacitor to use ...?

The presence of a battery changes a lot of things. When charging a battery the capacitor value is not critical.

However if you ever run the converter with no battery connected, then it's a good idea to leave a resistive load in place, and probably a capacitor. You may need to experiment to find suitable values.

 

[Akashmd]

However if you ever run the converter with no battery connected, then it's a good idea to leave a resistive load in place, and probably a capacitor. You may need to experiment to find suitable values.

resistive load in the sense,a Bulb? and probably a capacitor ?

- - - Updated - - -

If my sepic converter is designed to give 14v output at 8v input when duty cycle is set to 64%,can i measure the open circuit output voltage....?
or for 25v input is designed to give 14v output at 36% duty cycle?can i test open circuit voltage?
Or else,how to find out my circuit is working?

 

[BradtheRad]

If my sepic converter is designed to give 14v output at 8v input when duty cycle is set to 64%,can i measure the open circuit output voltage....?
or for 25v input is designed to give 14v output at 36% duty cycle?can i test open circuit voltage?

It is risky to run open output on any switched-coil converter, because it can result in unpredictably high volt levels (possibly in the hundreds or thousands of volts).

This can happen because the inductor has current running through it, then it is switched off suddenly so that it sees high impedance. A high voltage spike develops. It can be high enough to create a spark. If a spark occurs inside a lead-acid battery, it can ignite hydrogen gas, possibly causing an explosion.

If sparks travel between the inductor windings, then it can arc-weld the wires and cause shorts, which will make the inductor useless.

A simulation without any load connected will give you an idea what levels the output could reach.

resistive load in the sense,a Bulb? and probably a capacitor ?

A bulb will let you confirm proper operation. Even an led with safety resistor may be sufficient to prevent too high a voltage from developing.

And by adding a capacitor, you reduce the high voltage spikes.

How to choose the values? It will require experimentation. It depends on whether you want to leave the resistive load connected at the same time the battery is charging.

Or you might want to disconnect them so the battery can get maximum current. However the changeover should always ensure that some load is present, for reasons stated above.

It will help to run simulations on a variety of configurations.

 

[Akashmd]

Thanks for the reply,but I have another doubt....
In the sepic converter ,does the ground point be same as the microcontroller ground....The sepic converter ground is the -ve of the solar panel....and ,to measure the voltage of the solar panel ,I have used voltage divider network where the solar panel -ve and microcontroller ground are tied together....In short all the ground points of microcontroller ,solar panel -ve and dc-dc converter ground are tied together.....the problem is the mosfet is getting very hot.....and the output voltage is not there.....the voltage across inductor L2 in fig is showing 0v....it means the current is returning back from the inductor to the solar panel ......What am I doing wrong ....I am setting pwm as 20% for mosfet switching ...

 

[BradtheRad]

Thanks for the reply,but I have another doubt....
In the sepic converter ,does the ground point be same as the microcontroller ground....The sepic converter ground is the -ve of the solar panel....and ,to measure the voltage of the solar panel ,I have used voltage divider network where the solar panel -ve and microcontroller ground are tied together....In short all the ground points of microcontroller ,solar panel -ve and dc-dc converter ground are tied together.....the problem is the mosfet is getting very hot.....and the output voltage is not there.....the voltage across inductor L2 in fig is showing 0v....it means the current is returning back from the inductor to the solar panel ......What am I doing wrong ....I am setting pwm as 20% for mosfet switching ...

I guess I don't know the answer. Maybe someone else does.

This is a project with several obstacles to success. You'll have an easier time if you start simple.

As you can see, it is a difficult job to design a workable MPPT charge controller.
Because:

a) The power supply is not an unchanging voltage (whether you use the PV panel or the battery)

b) The battery, as a load, creates problems you don't get with a resistive load. It is possible for the battery voltage to be too high for your controller/converter to push amperes into it.

c) You must avoid exposing the battery to unsafe current or voltage levels. Your charging schedule must be safe for battery parameters.