Isolated Offline flyback with no auxiliary coil

[drillz]

Hello,

My offline flyback auxiliary coil voltage wanders too much with changes in load. So i am going to try and power the PWM controller from isolated "off-the-shelf" DCDC converters connected to the 24V isolated secondary of the flyback.

I've never seen this circuit in a book or anywhere....and wonder if there is some subtlty i have overlooked, very grateful if readers could cast an eye and give any comments......

 

[MacGyverS2000]

This sounds a lot like someone who was just asking a lot of PWM and SMPS questions... what is this for?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VisiGoth]

Flybacks are for dirt cheap volume. Flybacks that power the back circuit are even cheaper. They are for very high volume experts that can trade NRE for RE. If you are going to put in a power module you probably could have had a safer easier to design to start with, or just buy modules if this is a low volume application.

 

[LionelHutz]

I don't see how this will improve the output regulation or really improve the power supply in general.

You need some voltage feedback to help with the regulation.

Why don't you have a third coil on the transformer to power the drive circuit once it begins running? Sometimes, this coil can be used to power the driver circuit and provide the feedback for voltage regulation. And, the circuits never use relays to switch from the "bootstrap resistor" to the running power supply.

Try doing some more research for reference designs. A proper design usually involves a circuit with an opto-isolator in the feedback path and are much simpler than what you are proposing. Look for Topswitches as one possibility to do what you want.