flyback design issues

mail2vasu2004 · 17th July 2010, 03:47 AM

HI..
I have designed a flyback which takes in upto 60 volt for a dual output of 12V. It is a self oscillating flyback. I have used transistorised contro circuit. But i have been facing comstant failures, when the circuit is put for use.

I have attched the schematic. Please suggest me the loop holes in the design.

vasu

Sch3mat1c · 17th July 2010, 08:09 PM

Transformer ratio?

Hurtig · 17th July 2010, 08:26 PM

Is this a home-brew circuit??

How about the switch transistor, Q8. Is this supposed to be a small signal transistor?? I would have used a FET instead.

What kind of failures do you see? Will it not start up, or does it burn the transistors??

mail2vasu2004 · 18th July 2010, 01:02 PM

Hi...
The problem is with the reliability of design.
I am using BU-508 at Q8, and BD-139 at q7. As such the circuit works fine and is able to withstand a load upto 600-700mA. But Q7 tends to heat up pretty soon. And fails.

Why do you recommened FET?

Vasu

mail2vasu2004 · 18th July 2010, 01:12 PM

The transformer is 38 turns primary.
I &II secondary-25 turns and III secondary 32 turns.

I would also like to know how is the switching of Q7 controlled

thank you for reading me

Vasu

AP2 · 18th July 2010, 01:43 PM

Hi,
what is..BC565??
Q7 conduction...at inverse circumstance??

Regards

AP2 · 18th July 2010, 04:29 PM

collector of BD139 can not direct on diode,(high current pass when it conduce (this is a mistake)

Regards

AP2 · 19th July 2010, 04:26 PM

Quote:

Originally Posted by mail2vasu2004

Hi...
The problem is with the reliability of design.
I am using BU-508 at Q8, and BD-139 at q7. As such the circuit works fine and is able to withstand a load upto 600-700mA. But Q7 tends to heat up pretty soon. And fails.

Why do you recommened FET?

Vasu

Hi,
have you tried changing?

Regards

mail2vasu2004 · 19th July 2010, 06:15 PM

yes
i changed it to BU-508. It did help, but ocassionaly it still fails.
And Bd139 heats up too. Why is the direct connection to D17 wrong. And how would adding two 470E help.

Also explain me the switching of bd-139.

Thank you for your suggestions and guidance. i m new to power supplies.

vasu

AP2 · 20th July 2010, 12:43 PM

Hi,
I think that you not have try with my correction (cut collector from 2,7K) and add 2x470R. with this modify is not possible heat up BD139.
Not have sense high current of collector (connect direct d17) when it change work point of BU508. (TL431 driving BD139 for regulate Out) it is a "Pseudo" pwm way .
Also not clare in this scheme phase of link feedback (in scheme is inverse).
BTW.
Normally I used BC337 instead of BD139, it work good. (then I.C. current is low)

Regards

17th July 2010, 08:09 PM	#2
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	Transformer ratio? __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

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17th July 2010, 08:26 PM	#3
Hurtig diyAudio Member Join Date: Feb 2005 Location: Denmark - Århus	Is this a home-brew circuit?? How about the switch transistor, Q8. Is this supposed to be a small signal transistor?? I would have used a FET instead. What kind of failures do you see? Will it not start up, or does it burn the transistors??

18th July 2010, 01:02 PM	#4
mail2vasu2004 diyAudio Member Join Date: Jul 2010	Hi... The problem is with the reliability of design. I am using BU-508 at Q8, and BD-139 at q7. As such the circuit works fine and is able to withstand a load upto 600-700mA. But Q7 tends to heat up pretty soon. And fails. Why do you recommened FET? Vasu

18th July 2010, 01:43 PM	#6
AP2 diyAudio Member Join Date: Dec 2009 Location: Italy	Hi, what is..BC565?? Q7 conduction...at inverse circumstance?? Regards

19th July 2010, 06:15 PM	#9
mail2vasu2004 diyAudio Member Join Date: Jul 2010	yes i changed it to BU-508. It did help, but ocassionaly it still fails. And Bd139 heats up too. Why is the direct connection to D17 wrong. And how would adding two 470E help. Also explain me the switching of bd-139. Thank you for your suggestions and guidance. i m new to power supplies. vasu

20th July 2010, 12:43 PM	#10
AP2 diyAudio Member Join Date: Dec 2009 Location: Italy	Hi, I think that you not have try with my correction (cut collector from 2,7K) and add 2x470R. with this modify is not possible heat up BD139. Not have sense high current of collector (connect direct d17) when it change work point of BU508. (TL431 driving BD139 for regulate Out) it is a "Pseudo" pwm way . Also not clare in this scheme phase of link feedback (in scheme is inverse). BTW. Normally I used BC337 instead of BD139, it work good. (then I.C. current is low) Regards

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