12v 250w Car SMPS based off SG3525


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final.fully isolated batt gnd & amp gnd 100% working without noise
best for TDA7294 bridge subwoofer amplifire
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375W RMS (750W peak) tested with 12v (300W) transformer

When we talk of power its best to talk in RMS as this is the equivalent of DC. To really find out its best to test it with a car battery, this will give you plenty of power.


New member
You want to build a 12vdc push pull smps and get around 250W with a +-35vdc output. So lets start gathering some data. First we will assume a poor 60% efficency because of no output inductors.

250W x 1.4 = 350W input power.

Assume 14.7 volts max input.

350W / 14.7vdc = 23.8A, max primary current.

BTW, I am not perfect and i make mistakes. If anyone see’s a mistake speak up!
Hi, I noticed that this is the max primary current @ max primary voltage, like when the car is on, but if the car is off, and maybe battery is a bit low, shouldn't we calculate the max primary current @ minimum primary voltage of around 11.5 to 12.1 V? In this case the current would be in the range of 30.44 to 28.93 A, allowing for a better safe margin

Thank you for all the clear explanation!


New member
Hi, it is great work you are doing here.
I do have a question/topic:
What happens if one would use push pull SMPS (as here are done) and on the secondary use it as single supply voltage - from + to - thus getting single supply out? Would this in some affect the operation of the transformer?
As I see it (it might be wrong) the secondary is floating GND thus it should not be an issue if it is not connected to anywhere?

If used as single supply voltage - do the caps need to be connected towards the middle of the transformer or can the transformer be done without middle point?


Active member
For a push pull topology the primary winding need to be centre tapped.
As for the secondary there is no need for a centre tap. The voltage at the secondary needs to be higher so that the chip can regulate when voltage drops due to load and also for battery voltage variation (11.5v-14v). If for example you need 60v at the output then the secondary voltage need to be x 1.5 higher (90v) this will compesate for any losses and voltage drop in the battery and wiring. Your current in the primary winding will be 20A at 250w with an input voltage of 12v.
The capacitors need to be rated at the peak unregulated voltage. In the example 90v or a little more.
I hope this helps