SMPS Tranformer design

[WSJ]

What are the considerations regarding the transformer output voltage when using a regulated output with a push pull design. How much higher should the open loop output voltage vs. the closed loop voltage. Obviously if the output voltage is below the set point with the regulator (TL494) at 100% duty cycle the set point can not be reached. What is a reasonable duty cycle at full load and low input voltage.

 

[FauxFrench]

I would say around 90% for the duty cycle. That should translate into 10% higher voltage from the transformer.

 

[jackinnj]

The last few pages of some of the datasheets from Linear Technology have the equations -- its a function of duty cycle, minimum and max current etc. Look at the last few pages of the lt3439 low noise push-pull converter: http://cds.linear.com/docs/en/datasheet/3439fs.pdf

i put it into excel-- but it's not on this computer.

jack

 

[Audio1Man]

Hi WSJ
You need to replace the Schottky with a 60V part as the voltage is > 48V (2x24V).
Duke

 

[jackinnj]

Here's a calculator for the application using the formulas from the LT1683 gate-driver data sheet (in addition to the loss from Ron of the MOSFETs, you have the loss across Rsense).

 

[WSJ]

Thanks for posting the calculator. Is the calculator your design? Could I get a copy?

 

[jackinnj]

PM Me your email address -- it's a little different from the LLTC equations as they circle around after making their initial calculation, and use a "plug" for "L". Read the datasheet for the LT1683 first. Will be happy to take any corrections into consideration.

 

[WSJ]

This is the revised schematic. Input 11 to 14V, output 7.2V @ 15A regulated with current limit.