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Old 17th July 2010, 03:47 AM   #1
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Default flyback design issues

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
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Old 17th July 2010, 08:09 PM   #2
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Transformer ratio?
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Old 17th July 2010, 08:26 PM   #3
Hurtig is offline Hurtig  Denmark
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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??
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Old 18th July 2010, 01:02 PM   #4
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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
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Old 18th July 2010, 01:12 PM   #5
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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
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Old 18th July 2010, 01:43 PM   #6
AP2 is offline AP2  Italy
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Hi,
what is..BC565??
Q7 conduction...at inverse circumstance??

Regards
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Old 18th July 2010, 04:29 PM   #7
AP2 is offline AP2  Italy
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collector of BD139 can not direct on diode,(high current pass when it conduce (this is a mistake)

Regards
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Old 19th July 2010, 04:26 PM   #8
AP2 is offline AP2  Italy
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Quote:
Originally Posted by mail2vasu2004 View Post
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
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Old 19th July 2010, 06:15 PM   #9
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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
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Old 20th July 2010, 12:43 PM   #10
AP2 is offline AP2  Italy
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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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