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# Current transformer problem

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#### eem2am

##### Banned
current transformer problems

.....i need to make current transformer to act as current sense in this ~150W boost converter (12 to 42V)

however, as i have done it so far.....

.....when the MOSFET is ON..
..if twelve amps flows in the primary of the current transformer....
..then 300mA will flow in the secondary (40:1 turns ratio)
..this 300mA will put 1 volt across the 3R3 resistor
..and there will be 0.7V across the diode....
..therefore there will be 1.7V across the secondary
..therefore , there must in turn be 1.7 * 40 = 68V across the primary

.....but if there is 68V across the primary then there will be minus 56 volts (=68-12) across the boost inductor (?)

How can there be 68 Volts across the current transformer primary?

#### VVV

current transformers problem

You got it backwards. There will be indeed 1.7V in the secondary, but only 1.7/40=0.042V in the primary.

That is why we use current transformers, they allow us to sense the current with little voltage loss, which can be very important, especially at low input voltages, as is your case.

#### FvM

##### Super Moderator
Staff member
Usually, a current transformer would neither need nor have any advanteges from a diode at the secondary and a free-wheeling circuit at the primary.

#### eem2am

##### Banned
VVV:

thankyou for clarification...but..

"You got it backwards. There will be indeed 1.7V in the secondary, but only 1.7/40=0.042V in the primary. "

There are more turns on the primary, so the volts per turn at the secondary, when looking at the primary, the volts/turn will be the same, and therefore primary voltage much bigger....

as i remember....Vp/Np = Vs/Ns

am i incorrect?

#### eem2am

##### Banned
also, sorry again, but don't i need the flyback diode to eat up the magnetising flix kick back?, and the rectifier diode needed or else the IC will get a negative voltage into its input?

#### VVV

Sorry, I only looked carefyully at your schematic now.
Well, your schematic is incorrect. Normally the current transformers have only 1 turn in the primary and 50~300 turns in the secondary. What you are showing is incorrect. The current transformer is a separate transformer, not the main flyback transformer.
Usually, the current transformer is built like a toroid and the 1-turn primary is simply a wire going through the center or the toroid, while the secondary is would uniformly wound on the toroid. That 1 wire can be from the drain or the source connection of the MOSFET.

Regarding your second question, yes, you need the diode to prevent the voltage at the current-sense pin from going negative.

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