Feedback for Flyback converter

[The Al Bundy]

Hello,

I've a little problem with making a feedback from an isolated secondary to
the primary PWM controller.

I plan to use a LM431 as opamp to drive an optocoupler. However the maximum
A-K voltage of this device is 37V.
The output voltage at the secondary will be 60V. Since there will be only 1
secondary output I can not take a lower output to drive this optocoupler and
to keep the Vak lower then 37V.

I saw some supply with a 48V output that had a zener diode in series with
the optocoupler. I wonder how this can go ok if the LM431 is not conducting
and the full 48 (or atleast >37V) is across the A-K of the device. Ok, there
was a series resistor also of course, but in any way the LM431 will be
(shortly) used out of spec.

So my question... is there any good way to make a feedback from one 60V
output? This output must be well regulated and therefore the feedback must
be done from this output.

Thanks
Al

 

[The Al Bundy]

"Joerg" <notthisjoergsch@removethispacbell.net> Wrote
news:is_5d.20464$QJ3.6561@newssvr21.news.prodigy.com...

Hi Al,

How about a couple of resistors as a voltage divider on the 60V side? If
the 1% tolerance is too much you can obtain resistor arrays with much better

relative accuracy.

Best would be to post a schematic on your web space or on a.b.s.e. so we
can all have a look at what you got so far.


Regards, Joerg

Well the problem wouldn't be there if the secondary didn't had to be
isolated from the primary. This means I have to cross that barrier with an
optocoupler. And the only way (afaik) is to do this with a LM431 to 'drive'
the optocoupler.
I will try to make some schematic readable and will post it on a.b.s.e. Will
notify when it's there (can take a day or 2 with my agenda...

Thanks,
Al

 

[Fritz Schlunder]

"The Al Bundy" <the_al_bundy@*^no-spam!^*hotmail.com> wrote in message
news:cj9qh4$ens$1@news3.zwoll1.ov.home.nl...

Hello,

I've a little problem with making a feedback from an isolated secondary to
the primary PWM controller.

I plan to use a LM431 as opamp to drive an optocoupler. However the
maximum
A-K voltage of this device is 37V.
The output voltage at the secondary will be 60V. Since there will be only
1
secondary output I can not take a lower output to drive this optocoupler
and
to keep the Vak lower then 37V.

I saw some supply with a 48V output that had a zener diode in series with
the optocoupler. I wonder how this can go ok if the LM431 is not
conducting
and the full 48 (or atleast >37V) is across the A-K of the device. Ok,
there
was a series resistor also of course, but in any way the LM431 will be
(shortly) used out of spec.

So my question... is there any good way to make a feedback from one 60V
output? This output must be well regulated and therefore the feedback must
be done from this output.

Thanks
Al

Hi there. The last several pages of this datasheet:

http://focus.ti.com/lit/ds/symlink/ucc38500.pdf

Talk about loop compensation of an isolated control loop using the TL431
(Texas Instruments brand of LM431). Figure 13 shows a typical schematic. I
haven't studied it closely enough to be certain but I think Q5 can probably
be replaced with a simple resistor of appropriate value for your
application. D14 would then protect the LM431 from bad stuff.