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.


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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!