Forward converter can have max duty cycle of 50%

[Vicky]

Hi,
i would like to know why is that there is a constarint on the forward
converter topology that the MAX duty cycle cannot be graeter than
50%.I have read that it is due to the core saturation,but how come it
is not applicable to other topologies.
Regards
Vicky

 

[analog]

Vicky wrote:

I would like to know why is that there is a constraint on the
forward converter topology that the MAX duty cycle cannot be
greater than 50%.

There is no such absolute constraint. This is a practical limit
that works well both with simple current mode control and when
either no separate transformer reset winding is used or the reset
winding has the same number of turns as the main winding. This
limit also is very easy to produce in a control IC via a toggle
flip-flop in the pulse stream.

I have read that it is due to the core saturation, but how come
it is not applicable to other topologies.

All topologies must be arranged so that there is no net dc voltage
applied to all the inductances in the circuit, including the
transformers. Do you have a specific circuit under consideration?
If so, consider where the transformer core's magnetizing current
flows during the off time of the transistor switch(es), consider
the voltage into which it must flow, and consider how long it will
take this magnetizing current to decay to zero (and why it must do
so).

If knowledge is your goal, you really should study some of the
papers on magnetics by Lloyd Dixon from the TI/Unitrode power
supply design seminar series instead of trying to beg someone on
usenet to do your thinking for you. These papers are available
on their web site (search for Lloyd Dixon). Have you seen them?