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Flyback supply, translating from LTSPICE to pcb design.

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Newbie level 5
Oct 2, 2011
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I am trying to do a flyback converter. I want to know if the simulation through LTSPICE will translate directly into the voltage supply that I would like. I'm using the LT3574 flyback IC.

The impedance at the output I have derived using based on Ohm's Law, with the voltage I desire (25V), and the current that will be input to the next IC (value of 50mA).

I'm asking because I don't know if there are other factors to consider (that will change components' values) when translating to a real world pcb design.

flyback for edaboard.png

As a first point, you should consider the datasheet. Bias input shouldn't be supplied with > 15V in normal operation.

I presume that at least the RCD snubber has to be adapted to the actual flyback transformer parameters, particularly leakage inductance.

The simulation is for 5V output, but you are attempting 25V?

Thanks for the reply,

Yes the bias would be 1.5V (I believe you are referring to the SHDN/UVLO pin?).

How do I go about designing the RCD only by calculation/simulation? And would it be accurate to just look at the result of the simulation. This is what I can get after simulation, it is below 50V which is what I want:

And I'm sorry, I forgot to remove the title of the simulation. I have already changed components to give me the output of 25V. This is what I can get at output voltage:

With "Bias" I'm referring to the pin with same name. You have tied it to Vin, which shouldn't be done with >15 V input according to datasheet.

If your actual output voltage is 25V, I would find it annoying to have > 1W power dissipation in the RCD snubber. There are simpler topologies for a ruggedized isolated power converter if efficiency isn't an issue.

The datasheet said bias can be connected to Vin, if a third winding is not used, and is Vin < 15V. Both of which applies here, since I'm not using a third winding, and Vin = 12V.

Okay, so the voltage across the snubber would be about 38.5 - 12 = 26.5V. Power would be 26.5^2 / 500 = 1.4W ? Can I then try to increase the resistor value to lower the power dissipation? I don't really have the time to look at other topologies at this point ..

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