HOW TO MAKE SMPS FERRITE TRANSFORMER (CHOPPER)

We are going to make a ferrite or chopper transformer in a very simple and more understanding way.

Chopper transformers or ferrite core transformers are popularly called SMPS transformers which are found almost in all current electronic gadgets, mostly in power inverters.

Though chopper transformers or ferrite core transformers makes devices less bulky and more portable, most technicians and up-coming engineers sees it as big challenge because it seems to be complex in their world as beginners.

In this practical work, we are going to learn how to make a ferrite core transformer for DC to DC as well as AC to DC.

I will also give you a part 2 of this, which we will learn how to make a DC to AC ferrite pure sine inverter or High frequency inverter.

FERRITE TRANSFORMER PRACTICAL WINDING 

In this first practicals, we will learn how to wind a ferrite transformer to step down DC voltage and also how to step up DC voltage.

Things needed:

Caliper or Rule, ferrite core, AWG 26 or any, Calculator.


WINDING RULES:

1. Don't use a single thick wire.

2. Don't use bare or uncoated wire.

3. Insulate each section of winding. 

START OF PRACTICAL WORK.

Lets make a 48V DC transformer with two outputs; 15V DC and 12V DC using the circuit diagram below:

At the end of this practicals, we can use the ferrite transformer in our DSPIC30F2010 inverter board in this link


For this work, am going to use a core i took from an old board. I don't know the core area, so i will measure it using my caliper.

First, you have to remove the core from the bubing and remove all old winddings as well. The ferrite core can be separated by applying heat. I separated mine by heating it around 400 degree Celsius. There are times that i have also separated it by putting soldering iron on it for some time.
Now measure length and width of the bubing as shown in image 2.

Image 1



Image 2 

CALCULATING THE NUMBER OF PRIMARY TURNS 

To calculate the primary number of turns for our 48V step down converter, The formula below is used.

From the formula, we need to look for Ae, freq and Vmax. before we can calculate for the number of primary turns. 

Ae has already been measured and calculated as 0.72 cm square.

Vmax is the maximum voltage we can have from a 48V battery and that is 60V

Frequency to be used for this converter is 78000 Hz using the frequency calculation formula on our circuit diagram. That formula is given in the data sheet of the UC3845 IC.

Now that we have all the parameters in the formula, we can calculate the number of turns by putting the values into the formula as below:

Now we know our number of primary turns. Lets calculate for number of secondary turns.

Please note that, if the converter is connected to a 48V battery system, the battery voltage will go close to 60V during charging and will drop to about 40V when battery is in use.

For us to get a continuous stable output even when battery drops to 40V, a built in PWM function of the switching IC is implemented through feedback.

Though PWM will work, the PWM has a maximum width (duty ratio). With our UC3845, the max duty ratio is 50%. This means that when battery level is high, duty ratio will be less than 50%.

On the other hand, as battery is draining, the switching time (duty cycle) will be increased but only to a maximum of 50%.

As the switching IC switches at max duty cycle of 50% and output voltage drops, the switching transistor will overheat and burn as a result of no regulation.

It is therefore necessary to calculate secondary turns in a way that will factor battery draining in the calculation.

SECONDARY TURNS CALCULATIONS

FACTORING BATTERY DRAIN

1. Subtract Min voltage from Max Vmax (60V-40V) = 20V

2. Divide answer by 2 ( 20/2) = 10V

3. Add answer in point 2 to all your expected output voltage (15V+10V) = 25V

4. Add answer in point 2 to all your expected output voltage (12V+10V) = 22V

now because we want to always have a stable output voltages, 10V which we got from point 2 is added to all our output voltages so that we can calculate for secondary turns which will keep our outputs always regulated even when the battery gets as low as 36V.

15V output Number of secondary Turns

NUMBER OF TURNS FOR 12V OUTPUT (AUXILIARY)

HOW TO DO THE WINDING

The winding of our high frequency transformer can now be done by:

1. Using a single 26 AWG copper and wind 71 turns in clockwise on the bubing.

2. When done, insulate the primary windings using heat resistance tape.

3. Double the 26 AWG copper and wind 30 turns in clockwise on the same bubing to get secondary 15V and insulate as done before.

4. Finally, wind the Auxiliary 12V by going 26 turns in clockwise and insulating it using heat resistance tape.

Example 2: lets make a ferrite transformer to step down 180V- 240V AC to 13.2V DC using the circuit below.




To start calculating for our number of turns, we have to convert our input AC range (180V AC - 240V AC) to DC range.

This has to be done because the transformer we are going to design will work on a rectified DC input to produce an output.

To convert our Input AC to equivalent DC (RMS), we multiply our input AC by the square root of two as shown below.

Now our input DC range obtained after rectification will be

255.6VDC to 339.4VDC.

We can now freely calculate for the number of primary turns using same steps as we did for the 48V step down.

CALCULATING NUMBER OF TURNS FOR STEP UP DC TO DC CONVERTER TRANSFORMERS

Step up DC to DC transformer calculations are same as step down calculations. This time a lower voltage is your input and a higher voltage is your output. You can also use the circuit above for step up converters.

Please note that all calculations done here applies to step up converters too.

Don't forget to leave your questions and answers in the comment box.

THANKS TO ALL READERS.