Hi all
my goal is to design a MPPT solar charge controller for my home use.The circuit will charge a 12v/150AH gel battery(My current inverter battery).The max output current can be around 20-25A(more is appreciable).I will use 24v panels for this design.
The system will run the load from mains when battery voltage falls below set point.In the presence of adequate lighting,the system will charge the battery from the solar panels using MPPT.Upon charging completion(If adequate sunlight remains),the system will start to run the appliances from solar panel.Previously I successfully built a low capacity(1.5A) buck converter which can be found here.(I had to shut down the project due to some of my personal problems)
The voltage of the solar panels will change from 21v(min) to 41v(max).So my query here is,how to select the parts(mainly inductor,i/p & o/p caps) for this kind of wide range varying input?I previously built an excel about calculating the values of the parts(I took help from various links from the internet).I will be highly obliged if you guys take a look at that excel file.

 

Practical Design of Buck Converter
http://ewh.ieee.org/r10/malaysia/ie_ia_pel/pecon2008/taufik_tutorial.pdf

 

the system will charge the battery from the solar panels using MPPT.

How is the MPPT point controlled?

 

Practical Design of Buck Converter
http://ewh.ieee.org/r10/malaysia/ie_ia_pel/pecon2008/taufik_tutorial.pdf

Thanks a lot.....i will go through the link.That is useful!
But did u check my excel?

 

How is the MPPT point controlled?

The MPPT will be controlled using a bare Atmega328p.PWM frequency will be 62.5 KHz.

 

Hi guys,
I came up with two sets of values(For Min & Max PV output)

For Min PV output
Vin - 21
Iout(Max) - 25A
Di - 7.5A(30% of Iout)
Duty - 64.76
Fsw - 62.5 KHz
Inductor - 10.22uH
Min Input Cap - 1333uF
Min Output Cap - 1323uF.

For Max PV output
Vin - 41
Iout(Max) - 25A
Di - 7.5A(30% of Iout)
Duty - 33.17
Fsw - 62.5 KHz
Inductor - 19.39uH
Min Input Cap - 1333uF
Min Output Cap - 1323uF.

For both the cases,the Min output current will be 3% of battery capacity(150AH) at the end of float charging i.e - 4.5A

Now my query is,which Inductor value to choose between the two??Also,please cross check my calculations if it is right.....Please,please reply guys as my whole project is stuck here!!

 

Please guys....reply!!

 

94 hits,still no reply.....strange!!

 

94 hits,still no reply.....strange!!

Your spreadsheet calc looks ok, I plugged in the values for a mppt controller I'm currently designing for a 50v Lifepo4 bank (150 vdc in, 50v out at 60 amps) and the LC values are similar to what I'm using.

The L value at constant frequency is determined by your smallest duty cycle and highest expected PV input, so use that value 20uH or slightly larger as a start. The output C is not so important here as your battery bank is an infinite capacitor, just make sure it has an extremely low esr, I make use of small double sided PCB's with approx 40 of 250v 1uf ceramic caps in parallel immediately following the output inductor.

Just make sure your inductor never saturates at the 25 amps, allow a good margin or make your self a saturation tester if not using off the shelf components.

If using synchronous rectification be vary wary of your circuit going into discontinuous mode, the buck circuit also works as a boost when a battery is connected.

Cheers
Mike

 

Thanks a lot for your reply.First of all,i will choose the inductor & post the link here if that is suitable.Also,suggest me some good Mosfet Driver.I was using IR2104 for my last setup.

If using synchronous rectification be vary wary of your circuit going into discontinuous mode

Please be more elaborate about this part.

 

....Also,suggest me some good Mosfet Driver.I was using IR2104 for my last setup.
Please be more elaborate about this part.

As you haven't posted your circuit, if you are using synchronous rectification (see p74 of your pdf) and the cpu detects that the battery is getting near charged then it will reduce the "on" pwm period or even stop it, this will increase the on time of any sync mosfet, if this period gets too long the inductor will store excessive amounts of reverse energy from the battery, then when the sync mosfet turns off a huge voltage spike will blow things up.

I suggest a mosfet driver that can supply 2-4 amps drive to the high side mosfet. look at the various data sheets.

Cheers
Mike

 

I am using synchronous buck topology.I don't have a diagram ready,will post soon.
My circuit will work like this:
1)First,charge the battery using Boost mode(14.2v,current limited to max 15A for 150ah...right?)This time load(Inverter in my case)will be running from supply.
2)Now,whenever the current drawn by the battery will drop to 3%(3% of total capacity150AH i.e 4.5A),the CPU will switch the charging voltage to 13.6v for topping charge.Now,it will also connect the load so that the load can be driven from solar panel.In this condition,the circuit will monitor the max current drawn by the load(Here in my case,20/25A.)
So,in this scenario,the ON pwm period will never be that low cause the system will maintain 13.6v in worst case scenario.Right?