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Help Designing a Flyback Inverter

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abhijeet080808

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Hello guys!

I need a little help here. Let me begin with what I am doing. I have a flyback transformer from a generic 400W computer PSU. I am using it in reverse - I am applying 12V to the secondary. And I need 200+V from the primary. Now, when I have no loads on it, the output after the rectifier and capacitor is as I want. The input side is driven by a IRFZ44N MOSFET at around 100kHz by a MCU at 50% duty cycle.

The problem is, if I apply any load (say 10W), o/p goes to 0.xxV and the MOSFET heats up. Any help as to why this is happening?

Thanks for helping!
 

Mr.Cool

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you are shorting out your primary. probably your duty cycle is going to 100%. is this a regulated power supply? or open loop 50%? if open loop, try droping the duty cycle to 45%

post your schematic and we can help better

Mr.Cool
 

abhijeet080808

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It is open loop fixed 50% duty cycle.

The circuit is standard textbook flyback. No snubbers or extra flywheel diodes added anywhere. An Atmega32 MCU drives the MOSFET using a 100khz 5V signal.

One doubt, can a flyback transformer be used in reverse like I did?

p.s. - if anyone still needs a circuit, just let me know.
 

FvM

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If it's actually a flyback transformer, it won't work with 50% duty cycle and no load.
 

abhijeet080808

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^^ Can you please explain? The main transformer in a generic computer PSU is flyback aint it?

At no load (only cap + resistor), I do get the required voltage though.

Edit - if it is not flyback, I guess it can produce similar symptoms due to core saturation, correct?
 

FvM

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How do you know that it's designed as flyback transformer? Did you measure the main inductance or the current waveforms in your circuit?
 

abhijeet080808

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No, I don't know for sure. I was told it is a flyback. And I do not have access to a CRO for some days. Hence this topic! Any qualitative test to find out for sure?
 

rajudp

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i don't think it will work,
most of the Computer SMPS works on half bridge

normally working on 220 volt having min - max volt of about 170-270v , ie at the volt min it should give the required output.

so the primary turns will be very less taking account of duty cycle
 

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