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flyback converter not regulating

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bowman1710

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Hi guys,

I've modified this board from linear, I have highlighted the things i have changed on this board, from what I have looked at the controller isn't limited by maximum duty cycle and also it is not going into current limit I have tried tweaking the collector resistor with no effect. The issue i have is that at lower loads it outputs 5.3V fine but when it gets above the 10W range the output drops to 5.1V and if I increase the input voltage above 50V it drops even more, any ideas what else I can look at or change?

modification.png
 

Vbase

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You are trying to get nearly twice the power output, I think it will always be difficult without increasing the size of the transformer and having the right turns ratio.
 

bowman1710

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I think it will always be difficult without increasing the size of the transformer and having the right turns ratio.
if you look at the schematic I have changed the transformer to a PA0478NL which should be ok for the job
 

KlausST

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Hi,

i don´t know what the D4 is for.
The right diode is for DC feedback of output voltage, but what is the left diode good for?
With higher input voltage the left diode of D4 may give a "wrong" feeback.

Klaus
 

bowman1710

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The right diode is for DC feedback of output voltage, but what is the left diode good for?
With higher input voltage the left diode of D4 may give a "wrong" feeback.
I think that on the right is to speed the rate at which the controller turns on so that the compensation can occur faster, without this the controller will be waiting on the voltage charge on the 3 x 100uF caps before it can start to regulate. Without this connected there are start-up issues and control issues start to occur.
 

Vbase

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D4 and the optocoupler are more likely for over voltage and not for normal output.
The feedback for regulating the output could be via R10 if it fitted. In such case R15 has to be different for 5V output.
 

KlausST

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Hi,

now i see.
D4 is for supply of the feedback circuit.

Klaus
 

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