Ok, have you considered just using an agarose gel? You can prepare it denser, with 1.5% agarose (w/v) to better separate the small fragments.

You can use 1% agarose gel to seperate your desire fragment but you have to run longer time

I used to separate large DNA fragments on agarose gels and the key factors are not to overload the gel, use a low agarose concentration (0.5% maximum) and run at a low voltage (2 volts per centimetre) for a long time (12-24 hours).

you can use a low agarose conc. of 0.5- 1.0% and run slowly for a long time for a better result

Thank you all for your kind answer. Your answer is very helpful to me. I will try your suggestions. If I have anything more, I will ask again.

Thank you...