Author Topic: How does this feedback network work?  (Read 969 times)

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Offline Red_MicroTopic starter

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How does this feedback network work?
« on: February 28, 2020, 03:18:21 am »
Hello,

In the circuit below, what's the function of D18 and C37 at the junction of R24 and R27? Do R24 and R27 look like they are in series with R26?

 

Offline T3sl4co1l

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Re: How does this feedback network work?
« Reply #1 on: February 28, 2020, 07:47:14 am »
VFB is the AUX winding, and I guess U1 is a quasi-resonant or BCM controller?  Guessing they wanted to adjust the rising and falling timings slightly.  Or maybe D18-R24-C37 serves as a snubber for D28 and the winding, and they're just taking feedback from there for whatever reason.

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Offline Wimberleytech

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Re: How does this feedback network work?
« Reply #2 on: February 28, 2020, 05:38:19 pm »
I don't get it.  R24 appears to be a 205 ohm 1% resistor.   :-//
 

Offline bson

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Re: How does this feedback network work?
« Reply #3 on: February 28, 2020, 05:51:20 pm »
R24 and C37 will make the driver briefly overshoot during start and acceleration.

D18 allows fast discharge of C37 when VFB drops, so it doesn't undershoot when slowing.  Well, maybe a little due to the diode Vf.
 
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Offline Wimberleytech

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Re: How does this feedback network work?
« Reply #4 on: February 28, 2020, 05:53:48 pm »
R24 and C37 will make the driver briefly overshoot during start and acceleration.

D18 allows fast discharge of C37 when VFB drops, so it doesn't undershoot when slowing.  Well, maybe a little due to the diode Vf.

Is there a reason for the 1% value for R24?
 

Offline xavier60

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Re: How does this feedback network work?
« Reply #5 on: February 29, 2020, 12:24:09 am »
Hello,

In the circuit below, what's the function of D18 and C37 at the junction of R24 and R27? Do R24 and R27 look like they are in series with R26?
The IC does some really clever things with the CS pin. http://www.ti.com/lit/ds/symlink/ucc28711.pdf
I still don't understand how it knows where to sample the correct part of the AUX waveform that represents secondary voltage. Anyways the reference is 4.05V which needs the top divider resistance to be close to 90K to get 16V AUX.
So R24 is really the odd value of 205Ω, not possibly 2MΩ.
Although the CS pin is supposed to reliably  block the leakage inductance pulse, the designer of this PSU has decided to RC filter it.
D18 is likely there to quickly discharge C37 during downward swings so that the valleys can be properly detected after demagnetisation.
« Last Edit: February 29, 2020, 02:07:00 am by xavier60 »
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 
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