SMPS Formula

[microsim444]

Originally Posted by luka

If you want to make money from it, you need to do it by yourself, why would Eva do this for you? She needs money too you know

Take it easy man, I dont want eva help, since its useless for me, I Already have books, Internet, WorkBench to test, and to do what I want.

EVA always gives comments.

[WAJIH]

Hi peoples

im designing a dc to dc converter with a dual polarity (+-30v) (+-6A) output.
i am using an example from Switchmode Power Supply For Car Audio
i just have trouble understanding one thing. please bear with me as i try and explain myself.

The transformer i will wind myself needs two primary and two secondary coils.
i will be applying 12v, 30A(car battery) to center tap of primary and need to get 30v and 6 amps. one for the positive rail and one for the negative rail.

I understand how to get the voltage and current on primary side of transformer. I also understand voltage on secondary of transformer. the only thing im not sure about is how on earth u can get 6A per rail when Faradays transformer equation clearly states:

v1/v2 = i2/i1
so..12A/30V = xV/30A
solving for x i get 12A on secondary.

now 12A cant be spilt to get 6A because i have two secondaries or does it??
then if so does the 30v output split so i get 15v on each rail?????????????

THanks for allowing me to confuse u
any help would be much appreciated

[Eva]

The equation is a simplification and is only valid when one primary and one secondary are used at a time. Also, if the direction of the currents is considered properly, it should be: v1/v2 = -i2/i1

If both terms are multiplied by v2*i1, it becomes: v1*i1 = -v2*i2

Which intuitively means just "input power = output power" (now you can add more windings...)

Anyway, all this is only valid for ideal transformers, which are coupling devices that force the voltage drop per turn to be the same on all the turns, and the sum of the currents flowing on each turn to be zero (considered with proper sign).

A simple aproximation to real transformers is obtained by adding parasitics: A series inductor (leakage inductance) and a series resistor (DC resistance) and then a parallel capacitor for each winding (in parallel with the corresponding winding of the ideal transformer, resulting in various RLC resonances). Magnetizing inductance could be modelled by adding another single turn winding connected to an inductor, but not an ideal one but one that becomes a short above a certain current level (saturation).

[WAJIH]

i think that answers my question thanks Eva

[Krisfr]

Is there a PC board layout available for the presented schematic?

Thanks for your time... this is very interesting as of now.

[WAJIH]

sorry krisfr

there isnt a pcb. id say you can just build it from scratch.

[Krisfr]

What are the specs on the LF and what type and gauge of Litz wire(if used) is used to wind the etd54?

Also, does anyone have a suggestion for a similar pc board layout and parts orientation that I could use as a MODEL to lay out one for this project?

[WAJIH]

if you look at this link Switchmode Power Supply For Car Audio and under Transformer COnstruction details you might find some helpful info.

ALthough they do specify using a ETD39 core whereas u want to use an ETD54 core.

Sorry to not be of much help as i am new to most of this.
if any1 can please attend to Kristfr post i would be grateful

Much Thanks,
Waj