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Two current sense transformers needed for unknown SMPS current?

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Flyback

Well-Known Member
Hi,
Customer has sent us 150W , 37vout offline SMPS (100-265VAC). They are told it is very special. They gave it to us to check it.

We have no BOM or schem and no gerbers or layout. We only have the PCB.(i think 4 layer). It uses a PQ26/25 core.

There is no output inductor. It kind of looks like a One transistor forward with high voltage reset. One pri and one sec coil. Sec coil is capacitively coupled to the output rectifiers. Neither end of the sec coil has a direct connection to output ground. (the mentioned series capacitors are “In the way”)

We want to scope the power currents. However, we don’t know if the current , in whichever branch, is unidirectional, balanced bidirectional, or unbalanced bidirectional. As such, do you agree, we will get best depiction of the power currents by using two current sense transformers as in the following attached? (LTspice and jpeg)

In the attached , the current is unbalanced bidirectional…..so using the two CSTs as shown , we are able to piece together what the actual current looks like. Would you agree? This is the cheapest way?..we cant afford a pricey high BW current clamp.

Best to add the series cap..then no DC average component can flow in the secondary, and the CST can thus never saturate
 

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  • Two tran forward with CST.jpg
    Two tran forward with CST.jpg
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  • Two tran forward_CST.asc
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