Recommendations for a Flyback Design Challenge

[BarisTaylan]

We are in need of an Isolated Flyback DC/DC Converter design to work with a 24 to 96V battery system. It is to be a 15-140V input to 12V output voltage and, 4-5A nominal 9A peak output current converter. We want to use the Würth transformer "749196547", for its ease on customization. Such as the option of paralleling all the secondary windings in order to reach the output current requirements. 

We would appreciate any suggestions to help us best solve this design challenge. Such as the choice in using either a high voltage supply PWM controller or using one of the primary windings as auxiliary supply for the PWM controller. Notably, we tried finding a suitable design at Webench, but it proved hard since we do not want to use a custom transformer.

The design could be made in a two-stage structure with the voltage range divided in two, if it turns out to be much more convenient that way. But our first choice is that it is done in one stage. 

Thanks in advance for anyone willing to help,

Barış Taylan, Makersan R&D

[Peter BENSCH]

Welcome @BarisTaylan, to the community!

I'm afraid that the transformer is probably not suitable for your application, as the windings have a saturation current of 3.93A. For your peak output current of 9A, this results in an output power of 9A * 12V = 108W, which, assuming an efficiency of 90%, equates to an input power of 120W. At 15V input voltage, this results in a DC current of 8A, with an assumed duty cycle of 1:2 in 16A, which would flow in the primary winding.

Since this 16A is just slightly above the saturation current, the magnetic field would collapse abruptly and your primary side would test the fuse of the supply line for proper function. However, since you probably have charged capacitors there, their stored energy would likely cause something to go up in smoke.

Regardless of the transformer, this is a design that can probably only be implemented with a classic PWM controller. STMicroelectronics is no longer active in such PWM controllers, but is happy to supply the necessary 250V MOSFETs and/or gate drivers.

Hope that helps?

Good luck!
/Peter

[RobK1]

A 24V lead-acid battery should not be discharged to lower than 21V or so. A LiFePO4 is close to empty at 24V.

So, your 15V lower limit is very low, and you might want to reconsider it, especially in light of what Peter wrote.