Most of the parts needed to make this project have been taken from a non functional computer power supply unit. You will easily find the transformer and the fast rectifier diodes from such power supply along with high voltage rating capacitors and heatsink for the MOSFETS

The most important part of getting the output voltage right is by ensuring the correct transformer winding ratio of primary and secondary sides and also to make sure that the wires can carry the required amount of current. I have used an EI-33 core along with bobbin for this purpose. It is the same transformer that you get inside a SMPS. You may also find an EE-35 core as well.

Now our objective is to boost up the input voltage of 12 volts to about 250- 300 volts and for this I have used 3+3 turns in the primary with center tapping and about 75 turns in the secondary side. Since the primary side of the transformer will handle greater current than secondary side, I have used 4 insulated copper wires together to make a group and then wound it around the bobbin. It is a 24 AWG wire that I got from a local hardware store. The reason for taking 4 wires together to make a single wire is to reduce the effects of eddy currents and make a better current carrier. the primary winding consists of 3 turns each with center tapping.

The secondary winding consists of about 75 turns of single 23 AWG insulated copper wire.

Both the primary and secondary winding are insulated with each other using insulating tape wound around the bobbin.

For details of how exactly I made the transformer ,please refer to the video at the end of this instructable.

The SG3525 is used to generate alternate clock pulses which are used to alternatively drive the MOSFETS which push and pull current through the primary coils of the transformer and also for providing feedback control to stabilize the output voltage. The switching frequency can be set by using timing resistors and capacitors. For our application we will be having a switching frequency of 50Khz which is set by capacitor of 1nF on pin 5 and 10K resistor along with a variable resistor at pin 6. The variable resistor helps to fine tune the frequency.

To get more details about the working of the SG3525 IC, here is a link to the datasheet of the IC:

SG3525 datasheet

The 50Khz pulse output from the PWM controller is used to drive the MOSFETs alternatively. I have added a small 22 ohm current limiting resistor to the gate terminal of MOSFET along with a 10K pull down resistor to discharge the gate capacitor. we can also configure the SG3525 to add a small deadtime between the switching of the MOSFET to make sure that they are never ON at the same time. This is done by adding a 33 ohm resistor between pins 5 and 7 of the IC. The center tapping of the transformer is connected to the positive supply while the other two ends are switched using the MOSFETs which periodically connects the path to ground.

The output of the transformer is high voltage pulsed DC signal which needs to be rectified and smoothed out. This is done by implementing a full bridge rectifier using fast recovery diodes UF4007. Then the capacitor banks of 3.3uF each (polar and non polar caps) provide a stable DC output free of any ripples. One must make sure that the voltage reading of the caps is high enough to tolerate and store the generated voltage.

For implementing the feedback I gave used a resistor voltage divider network of 560KiloOhms and 50K variable resistor, the output of the potentiomter goes to the input of the error amplifier of SG3525 and thus by adjusting the potentiometer we can get our desired voltage output.

The undervoltage protection is done using an Operational Amplifier in comparator mode which compares the input source voltage to a fixed reference generated by the SG3525 Vref pin. The threshold is adjustable using a 10K potentiometer. As soon as the voltage falls below the set value, the Shutdown feature of the PWM controller is activated and the output voltage is not generated.

This is the entire circuit diagram of the project with all the previously mentioned concepts discussed. Okay , enough of theoretical part, now let us get our hands dirty !

Before soldering all the components on veroboard, it is essential to make sure that our circuit works and the feedback mechanism works properly.

WARNING: be careful in handling high voltages or can give you a lethal shock. Always keep safety in mind and make sure you don't touch any component while the power is still on. The electrolytic capacitors can hold the charge for quite sometime so make sure it is completely discharged.

After successfully observing the output voltage, I implemented the low voltage cut-off and it works fine.

Now before we begin to start the soldering process, it is important that we fix the position of components in such a way that we have to use minimal wires and relevant components are placed close together such that they can be easily connected suing solder traces.

In this step you can see I have placed all the components for the switching application. I made sure that the traces to the MOSFETs are thick in order to carry higher currents. Also, try to keep the filter capacitor as close to the IC as possible.

It is noteworthy to mention that while soldering care should be taken that the high voltage and low voltage side have good separation and any shorts need to avoided. The high and low voltage side should share a common ground in order for the feedback to work properly.

After about 2 hours of soldering and making sure that my circuit is wired up correctly without shorts, the module was finally complete!

Then I adjusted the frequency, the output voltage and the low voltage cutoff using the three potentiometers.

The circuit works just as expected and gives a very stable output voltage.

I have successfully managed to run my phone and laptop charger with this as they are SMPS based devices. You can easily run small to medium LED lamps and chargers with this unit. The efficiency is also quite acceptable, ranging from around 80 to 85 percent. The most impressive feature is that at no load the current consumption is just about 80-90 milliAmps all thanks to feedback and control!

I hope you like this tutorial. Make sure to share this with your friends and post your feedback and doubts in the comment section below.

Please watch the video for the entire build process and working of the module. Consider subscribing if you like the content :)

I will see you in the next one!