SMPS || Charge Your Laptop in a Car





Introduction: SMPS || Charge Your Laptop in a Car

In this instructable, I have made a SMPS circuit which can be used to charge your laptop in a car or from any 12V battery. You can plug your existing charger in the circuit.

It is not an inverter. Basically it's a DC DC converter, which takes 12V DC as an input and generated 220V DC as an output. The output power is 200W . Now you might be thinking that output is 220V DC and not an AC, while in our home, the grid supply is an AC voltage so how will laptop charger operate on 220V DC?

Well, i must say that in any SMPS based charger, the first stage is rectification stage, after that it is processed to low voltage, so it doesn't matter whether you provide either AC voltage or DC voltage it will be rectified to DC only.

Remember, this is a DC DC converter so, don't try to connect any inductive load on it.

Parts and Tools Used:


IRFZ44 Mosfets

ETD 34 core and Bobbin

Heat sink

SG3525 IC



Soldering Rod

12 V Battery

Step 1: Designing the Circuit

The heart of the circuit is SG3525 IC, It is a pwm ic with feedback controller. The IC is configured here to generate 200KHz square wave. The value of Rt and Ct is used to set the frequency of the IC.

The formula for calculating the frequency can be found in the datasheet of the IC. I have also calculated the values in the above video.

Now this high frequency square wave will be used to drive the ferrite core transformer which i will discuss in later steps .

Step 2: Designing the Power Stage

As the current supplied by the IC is not sufficient, so it is amplified by the power stage, In power stage i have used IRFZ44 mosfets in push-pull configuration.

I have used heat-sink to dissipate the heat generated by the power mosfets. Also don't try to build the power stage on breadboard because it deals with high currents and the breadboard won't be able to handle this much amount of current and will melt. Use some thick wire to make the power stage as shown in the image.

Step 3: Designing the Ferrite Core Transformer

For designing the transformer i used ETD34 core as it can easily handle the load of 200 Watt without getting saturated.

I used this formula to calculate the primary turns for the transformer

N = (V* 10^8) / (4 *F*Bmax*Ac)

After calculating I got primary turns to be 2+2 and secondary to be 42

I wound the transformer followed by securing with miller tape as shown in the image. Ensure that winding and taping is done tightly so as to avoid leakage inductance.

Step 4: Testing the Circuit

After connecting all the stages, i.e driver stage, power stage and transformer, the circuit was finally ready to be tested.

I connected the 12V battery to the circuit and measured the output voltage. The output voltage was 220 V after that i connected my laptop charger to the output and the laptop started charging as you can see in the image

By this time, the project was successfully completed.



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    Why is 100khz frequency chosen for the square waves output? Can I use say 50kHz or any other frequencies? What are the effects?

    How can i calculate perfectly for a 2.5kva transfer

    Given that all laptops operate off DC input from the power supply, could you not have simply converted to the DC charging voltage of the laptop and achieved higher efficiency? I would like to use this type of conversion to power a remote internet lab/cafe that runs off solar cells without incurring the cost and losses of an inverter. Any comments would be appreciated.

    1 reply

    No. Almost all power adapters have thrid pin other than vcc and ground. It looks like a small pin at the center of dc jack. Every manufacturer have their own unique signal. This is why you can't charge DELL laptop with HP power adapter. After reading forums, I think that certain manufactures used constant dc volt (using resistor). It may have changed now to ICs. Without this pin laptop refuses to charge battery.