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UC1901-SP: UC1901: impulse transfo turn ratio

Part Number: UC1901-SP
Other Parts Discussed in Thread: UC1901, UC1843,


I have a question regarding the turn ratio of the impulse transformer used in the following ref design :

I'm actually worried about the turn ratio of the transformer used with the UC1901.

Here is the impulse transfo I’m worry about:




The transformer turn ratio is 1:1, as shown below


But from the UC1843’s datasheet we can see that the error amplifier track 2.5V :


And from UC1901’s datasheet we can see that the output swing of the AM signal, representating the error signal to be feeded in the UC1843, can goes as low as +-1V6 (don’t know under which conditions) :

The output is imply redressed by a diode and a capa at the output of the impulse transfo (AM demodulator)



I'm not 100% sure how to interpret this. of courser, with a ouput swing of the AM modulator at 1.6V , then we will never reach the 2.5V of the error amplifier of the UC1843. Even with the typical value of 2.8V, with the voltage drop of a schotty diode, we are close to the 2.5V of the error amplifier of the UC1843.


So my question, this design is it well done? Shouldn’t we have a turn ratio of 2 in the impulse transfo to get a amplification by 2, so that we are 100% sure to reach the 2.5V after the diode D9 above?


Hope the question is clear,





  • Hey JF,

    On the primary side of the design the transformer is offset by half of the reference voltage in order to help bring it above the needed output voltage.

    This allows it to get to the 2.5 V that is needed.

    There are parts of the design that could be better that are detailed in the design philosophy and a mini section of how to power the uc1901-SP, but the UC1901-SP should be properly giving the error signal back.