[Forward Converter][Calculations of transformer inductances]

Thread Starter

Doppler2902

Joined Sep 15, 2020
37
Hi guys, I'm doing a simulation on PSpice of a Forward Converter with the following parameters:

1663990031238.png
I calculated the values of output inductance and capacitor and the simulation give me the specified requeriments, but the current on the primary winding of the transformer is too high:

1663990291436.png

Making Pin = Pout the current in the primary winding wouldn't be 187.5 mA?

Iin = 22.5 W / 120 V = 187.5 mA but I get almost 24 A in the simulation

This is the circuit:

1663990428327.png

Someone could help why happen this?

I think that the problem is the calculation of the primary and secondary winding of the transformer but I'm not sure, I did this for calulate it:

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1663990579180.png
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1663990603192.png
1663990611779.png

Someone could guide me with this calculations?, I only have the Ns/Np ratio, thanks a lot for the help
 

LowQCab

Joined Nov 6, 2012
2,501
You can only glean a rough starting point with the calculations,
then there's what happens in the real-World.

You would be way ahead in the game by using a Push-Pull-Controller-Chip and
a generic, oversized Toroidal-Transformer designed for the AC-Mains.
It will also be far more stable and quiet,
and have Voltage and Current-Feedback built-in.

https://www.digikey.com/en/products/detail/microchip-technology/SG2524N/380224
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Push-Pull Transformer .png
 

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Ian0

Joined Aug 7, 2020
6,286
I'm not sure why you think that the impedance of the secondary should equal the resistance of the load.
The secondary inductance doesn't exist in isolation - it is coupled to the primary inductance.

For a single-ended forward converter the primary and secondary inductances are of little relevance.
You should calculate the flux and determine firstly if it exceeds the saturation flux of the core, and secondly, optimise the flux for the lowest transformer loss.

@LowQCab 's circuit is just an example - designed for a much lower voltage - don't use TIP41 transistors on a 120V supply!
 

Thread Starter

Doppler2902

Joined Sep 15, 2020
37
I'm not sure why you think that the impedance of the secondary should equal the resistance of the load.
The secondary inductance doesn't exist in isolation - it is coupled to the primary inductance.

For a single-ended forward converter the primary and secondary inductances are of little relevance.
You should calculate the flux and determine firstly if it exceeds the saturation flux of the core, and secondly, optimise the flux for the lowest transformer loss.

@LowQCab 's circuit is just an example - designed for a much lower voltage - don't use TIP41 transistors on a 120V supply!
For find the Flux I need the turns on the both coils, If I have the Ns/Np relationship can I assume for example Np and find Ns?
 

Ian0

Joined Aug 7, 2020
6,286
You must calculate Np, and then you can find Ns.
Assume maximum flux density of 0.2T for a ferrite core.
You can work out the number of primary turns from:
Np=V*ton/(Bmax*Ae)

Core losses are proportional to Bmax^(5/2) the datasheet will give the constant of proportionality
You can calculate copper losses from I^2.R by calculating R from the length and diameter of the wire.
Try various values for Bmax until you get the lowest total loss.
 

LowQCab

Joined Nov 6, 2012
2,501
Yeah but .............
It can be really satisfying to build something yourself, and have it perform better than expected.

Aside from that,
I would be willing to bet that that Wall-Wort won't
put out an actual ~30-watts RMS for very long, if it can actually do it at all.

Also, will it accept an Input of ~120-Volts DC ?

Some SMPS blobs are quite amazing, but not necessarily the ~$3.99 variety.
Even better is ~$2.99 for a LapTop-Power-Supply from the Thrift-Store, usually ~60+ Watts.
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