In the world of electronics equipment repair, we always see power supply in almost all equipment. It is like a heart for a human being because you cannot turn on the device without power supply. Regardless the type of power supply is used, there is two major power supply type in the market i.e. Linear Power Supply and Switch Mode Power Supply (SMPS). 

The Linear Power Supply identical with a big bulky transformer and heavy since SMPS lighter and more efficient. It is not a hard job to define the primary and secondary winding of Linear Power Supply because most of it has a label written clearly on each wire. That is not a same case for SMPS because it has many designs out there. Most of step-up SMPS has many secondary winding and it is hard to identify which is which.

Here I will share a little trick to identify the primary winding of the SMPS transformer. It is easy if you know the trick. You can use this trick in any SMPS circuit to make your troubleshooting job easier. The tool needed to check if primary winding still good or not is Ring Tester. The first thing to look for:

B+ supply of Main Capacitor
You can trace this line from the positive side main capacitor; it is usually 22-100uF/450V. You can move forward to find one of the transformer pins.

How to Find SMPS Transformer Primary Winding
Because you are looking for primary winding, just ignore other 3 components (C930, R905, D901) beside the primary winding. That is a safety circuit to save the FET/Transistor from spike voltage.

     Drain (Collector) pins of Mosfet / Transistor 
    You can trace the second primary winding by finding Drain/Collector pins from the power Mosfet or Transistor. It leads to one of SMPS transformer pins which is the primary winding of a transformer. I mark it with red dot as clear as I can. So the primary winding number in the schematic picture I provided is 0 and 3.

For secondary winding, it is a little complicated than primary winding. The method I always use traces the line back from secondary diode as you can see from above picture there are two secondary Schottky diode D902 and D904. If you are interested to repair ATX power supply, this trick could help the repair job easier. Now you can find primary winding of SMPS transformer from any electronics equipment used it. Thanks for coming here.

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  1. The biggest advantage of SMPS is greater efficiency because the switching transistor dissipates little power when it is outside of its active region. SMPS have an efficiency of 68 to 90% regardless of the input voltage, thus reducing heat sink size and cost.
    EI Transformer Manufacturer in India | SMPS Transformer Manufacturer in India


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