Need help with a simple BUCK

[crazifunguy]

I have a few push pull SMSP's that output around 90VDC. I would like to leave them as is and add a buck to the transformer output. I am looking to do 48V 6A. I have used some calculators to provide me with the required component values but I do not know what controller to use. I would like to use a TI controller but I cannot find an application for a DC/DC buck that uses only an inductor and not a transformer. I am still new to most of this.

EDIT:
The supply also has a +-15 volt supply that is fed through some LM7815/LM7915 LDO regulators. The secondary tap is ~35V at the diodes.

This will allow me to use the primary push pull without any modification and have the required output that I need.

This is the controller I was looking at.
http://www.ti.com/product/ucc38c40

 

[gman76]

Not sure what your input is, can you provide more detail? 35v AC? Or the other secondary that drives the 90v DC output? A buck would imply that Vin is > 48v.

 

[KX36]

The implication is that the input voltage of the buck is a presumably unregulated 90V dc from the push pull.

That's not a bad general purpose peak current mode control IC. The difficulties you'll find with a buck are that the switch FET is on the high side, so will need some mechanism to drive it such as a gate drive transformer or high side gate driver IC. At lower power and input voltage, you could have used a p channel FET or bipolar transistor. And that depending on the exact configuration of the secondary, you probably need a high side current sense to use CMC.

There are probably better ICs to use although many buck specific ICs will only work for low power/voltage.

I've used a SG3525 equivalent part with a high side gate driver IC before and that should work although it's not the cheapest or most elegant solution. Sorry I don't have a clear answer for you.