# Design a solar charger with MPPT feature that runs in 3 modes

Registered Users Posts: 1

I'm trying to design a solar charger with MPPT feature that runs in 3 modes (bulk, absorption and float) depending on the state of the battery. The logic is implemented in a fast micro controller and the dc-dc converter utilised is a buck converter. After doing my homework (lots of readings and research) I still have a couple of questions that I find confusing.

1. Correct me if I'm wrong on this. Do we only use MPPT during the bulk mode? If yes, we use it only if the charging current generated is less than the current threshold?

2. How, and by what means can I calculate the duty ratio of my transistor during the absorption and float mode in a manner to achieve a constant charging voltage with a maximum threshold charging current? Same thing also for the constant current charging.

3. While digging answers for question #1 I found that TI manufactures solar battery charge controller IC (bq24650 datasheet) that has the capability of regulating the charge voltage and the current voltage with high accuracies. They mention that they use internal loop compensators to do this. Digging in the high level schematic they have I found that the reference voltage feedback VFB and the current feedback SRN & SRP (SR is a current sensing resistor) are inputted to a compensation network. Does anyone know how this compensator network was designed? I would like to implement something similar in software (I'm not asking how to implement it in software).
4. When changing the duty ratio to achieve regulation, the charging voltage and current can't be predicted because changing the duty would take us to another point on the I-V of the PV which is not predictable. Or is it?
Thanks!

• Solar Expert Posts: 1,386✭✭✭✭
edited May 2017 #2

#1: Bulk is typically when you need maximum power and you need to be right at MPP. But can also occur under other conditions (see #4 below).

#2: As far as I know, you measure the output voltage or current and then adjust duty cycle upward/downward until you see the right value. Ie, more trial and error than calculation.

#3: Don't know, but some temporary over/undershoot is not a problem for this application.

#4: Take absorb for example. You know Vmpp and you know the desired Vabsorb. Iteratively adjust duty cycle upwards until the first of a) Vin falls to Vmpp or b) Vout rises to Vabsorb. Keep adjusting to hold it at this point. It will change significantly and quickly if loads switch on and off.

I am available for custom hardware/firmware development

• Solar Expert Posts: 891✭✭✭✭

Correct me if I'm wrong on this. Do we only use MPPT during the bulk mode? If yes, we use it only if the charging current generated is less than the current threshold?

Thanks!
You use MPPT whenever you are input limited and not output limited. This most often occurs during constant-current operation.
How, and by what means can I calculate the duty ratio of my transistor during the absorption and float mode in a manner to achieve a constant charging voltage with a maximum threshold charging current? Same thing also for the constant current charging.
There's about a dozen factors you have to take into account, including ZCS/ZVS operation, transformer/inductor saturation, efficiency etc. In MOST cases you run such that you reduce duty cycle to reduce power to the output - but note that doing the opposite will also work (i.e. you can increase duty cycle and decrease power output if you are operating on the "far right side" of the PV V/I curve.)

1. When changing the duty ratio to achieve regulation, the charging voltage and current can't be predicted because changing the duty would take us to another point on the I-V of the PV which is not predictable. Or is it?
Often that is true. To find out, you can do either small or large sweeps (i.e. change input impedance and see what happens) or make gross assumptions (i.e. MPPT voltage is 10% below open circuit voltage.)