The forward converter does appear similar to the flyback transformer, but it is actually very different. A forward converter transformer transfers its energy instantly (transformer action), unlike the flyback which stores and then releases that stored energy to the SEC side…
General Schematic for a Forward Converter
See below for a general schematic of this forward converter…
As one can see there appear to be two PRI windings, in fact, one of these is what we call a reset winding. It is used to demagnetize the core when the transformer is “off”.
Typically, the reset winding has turns that are the same as the PRI winding, which means the MAX duty cycle is set at 50%.
Now for some basic equations used for this time type of electrical transformer…
For a given input voltage to find the output voltage use:
Or put in terms of the turns ratio…
Now for the MIN number of turns needed to avoid saturating the core. This will be gained from the transformer equation that relates volt-sec’s to turns, flux density and core area…
This is the minimum number of turns required at some MAX input voltage, frequency, and core area to staying at or less than the MAX flux density number you provide for the core you are using.
The reset turns are then simply the same and the SEC turns you find from the turns ratio equation given previously.
Now there is an efficiency factor often used that I am ignoring here, as well as voltage drops across the diodes.
The output inductor should be chosen to ensure continuous conduction mode operation at a 10% load.
There is no upper limit in terms of the inductance you choose but bear in mind a higher inductance means a larger part with more turns and hence higher DCR which means more unwanted power loss.
This is a very basic write-up on a forward converter transformer design but it should get anyone interested in such a transformer in the ballpark for what they need.
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