Buck Converter Design

Hi

I wish to design a buck converter for charging 12 V and 7 Ah Lead Acid battery from PV panels.

My panels are:
21.1 V: Open Circuit Voltage (Input to buck converter)
4.12 A: Short Circuit Current (Input to buck converter)

I need buck converter of output 4 A, 15 V = 60 W
I am using IRF 540 MOSFET.

I need to know:

How should I check the resistance of my battery when it acts as a load to Buck Converter?
How should I determine Inductor, Capacitor and Schottky diode values of my buck converter?
What should be the frequency of my PWM for driving MOSFET?


In my Proteus design, when I vary load resistance the voltage drop across load also varies. My duty cylce is set to 66.6%. Till 600k ohms (load) the buck output voltage across load increases to 13 V and and if vary load above 600k ohms the voltage drop starts decreasing from 13 V. Is it normal response of buck converter and do I need to implement feedback to regulate voltage to my desired value for changing loads?

I would recommend an integrated buck regulator with input voltage regulation capabilities such as the LT3652. This allows you to track the maximum power point of the PV panel, which will maximize its output power. For larger panels, you can assume that the MPP is nearly constant regardless of solar radiance, so you can configure it with a fixed voltage divider.

QutabBaig said:

I wish to design a buck converter for charging 12 V and 7 Ah Lead Acid battery from PV panels.

My panels are:
21.1 V: Open Circuit Voltage (Input to buck converter)
4.12 A: Short Circuit Current (Input to buck converter)

I need buck converter of output 4 A, 15 V = 60 W

Click to expand...

What is the watt rating of your panels? If it is 40 or 50W, a buck converter will not turn it into 60W.

From seeing your specs I have a hunch your PV panels are able to push maybe 3 A into a 12V battery.

Since your battery is 7 AH, you'll need to watch that you don't charge it so fast as to cause it to bubble.

How should I check the resistance of my battery when it acts as a load to Buck Converter?

Click to expand...

Battery resistance is a fraction of an ohm. Measuring it is not easy. You can attach a massive load and see how much the voltage drops, however you risk ruining the battery when you try to draw its maximum current.

Battery voltage opposes the PV panel's voltage. Net voltage might be a few volts.

Or with the buck converter, the battery immediately gobbles all current coming from it. The output voltage is the same as the battery voltage.

In my Proteus design

Click to expand...

A large capacitor is a usable model of a battery, as long as you charge and discharge at a reasonably slow rate.

Add a small value resistor in series. Probably .2 or .3 ohm.

What should be the value of my Inductor?
What should be the frequency of PWM for driving MOSFET?

QutabBaig said:

What should be the value of my Inductor?
What should be the frequency of PWM for driving MOSFET?

Click to expand...

Values between 100 uH to 1 mH are convenient to work with.

The frequency should be adapted to the inductor. It cannot be too fast or too slow.

Example, suppose your inductor is 500 uH. And suppose you want to charge your battery at a rate of 1A average.

Your inductor needs current peaks of 1.5 to 2 A, per cycle. To build to this peak, takes a few hundred uSec. It's governed by the L/R time constant.

I have a simulation running. The inductor is 500 uH. The frequency is 3 kHz. Duty cycle is about 75 percent.

You can also start by choosing the frequency. 20 kHz is good to start with.