Modifying UE24W3-SPAZ PSU from 12V to 5V

[metal]

Hi,

I am trying to modify this hHuawei PSU. From the schematic attached, I did the following:

620R -> 330~470R
10K -> 2.5K
1K -> 750R

It worked and produced 5.0V, but I noticed it was fluctuating between 4.5V and 5.0V in a uniform manner which renders this PSU astable, I don't know why, what am I supposed to do in this case?

thanks

[sadman]

Hi,

I am trying to modify this hHuawei PSU. From the schematic attached, I did the following:

620R -> 330~470R
10K -> 2.5K
1K -> 750R

It worked and produced 5.0V, but I noticed it was fluctuating between 4.5V and 5.0V in a uniform manner which renders this PSU astable, I don't know why, what am I supposed to do in this case?

thanks

only adjust the 10k and 2.1k for 5V and kept the rest same

[metal]

I did that for the first time, the output voltage was fluctuating, this is why I replaced the rest, still the same.

[sam_des]

Hi,

I guess TL431 compensation is the issue. Are you getting a stable +2.5V? If not See the attached ON Semi app-note.
Also attaching feedback network of an 5V SMPS designed by me.

Hope that helps.

sam_des

[titi]

Metal,
In the original version the current through the 620 Ohms was approximately (12.7-1.1-2.5)/620=0.01467 A
If you want have the same current at 5v you need to replace the 620 by a (5-1.1-2.5)/0.01467=95.3 Ohm =>100 Ohms
  1.1v is the voltage drop by the IR LED of the OPTO
  2.5v is the voltage drop by the TL431
To simplify the calculation I neglected the 1K.
So use 100 Ohms instead of 330~470k.
Also I am not sure to reduce the 1K is a good idea (I will lets the 1k value).

If the power supply is not stable, it is possible to adapt the value of the cap between 2.1k and the TL431, this change the gain of the loop and adapt also the value of the 1k across the LED to maintain a correct gain to avoid the power supply to oscillate.

Sometime you need to have a minimal load at the output of the power supply else it oscillates so try with a by example 100 Ohms to see if it becomes stable or not.
Take a look at page 7 of the document in attachment this can help.

Best regards.

[metal]

Non of this has worked, I changed many values, I tried two PSUs that produce 12V but have different resistor values, both behaved the same. There must be an equation for these PSUs when deciding on the output voltage.

[pickit2]

can you not add a 3 pin regulator to get the 5V.

[metal]

@2A what do you suggest

[pickit2]

There are a few used on the X-Box

http://www.ti.com/lit/ds/symlink/lp3966.pdf

[vern]

it is a switch mode power supply, the output voltage is determined by the on/off time but also by the ratio of the build in transformer, which in this case is designed for the 12V output.
If you try to get 5V out of a 12V power supply the on-time of the power switch has to be shorter, you need to change the compensation of the TL431 and maybe on the primary side to keep it stable. You probably also need more output capacity to keep it stable.
If there is no load at all it might not be possible for the SMPS Circuit to produce a pulse that is short enough, the output might get to high and the regulation tries to keep it down thus resulting in a voltage fluctuation.

Do you know what switch mode chip this thing has?

[metal]

it is 6 pin chip, no numbers in on this one.

This is what I thought too, may be the tranfo ratio is kinda fixed for 12V, also I could not figure what should be changed on the other side, I am not a SMPS expert.

PICKit2, this is a linear regulator, the load is 5V @2A, the input voltage is 12V, dropout is 7V, dissipation is 14 watts :S

[dennis78]

Can you attach quality photos of board(both sides)? Maybe someone can recognize type of IC or...

[metal]

The IC is similar in circuitry/concept to XN1049, RT7737, OB2262, LD7537, NCP1251A, SG6848TZ1. Note that pin 3 is left unconnected, thu I have not checked the other side. One of the closest schematics I found is on this page but the resistor values are not similar, I have two adapters and they use exact same schematics with different resistor values. The trick appears to be the resistors calculations around TL431. I coundn't use the formulas found in ONsemi's document as it needs the resistor value for the pullup on the feedback pin, which is unknown for me.

images: https://mir.cr/ZKTDAPE5

[CocaCola]

@2A what do you suggest

There are 3A DC-DC step-down modules on Ebay for under $1 delivered...

A few examples...

https://www.ebay.com/itm/DC-DC-Adjustable-Buck-Converter-5V-12V-3A-Voltage-Step-Down-Regulator-Module/302473175701

https://www.ebay.com/itm/1PCS-DC-DC-LM2596-power-Supply-Buck-Converter-step-down-module-NEW-CA/112719012100



Dial it into 5v measure the pot value and replace it with resistor(s) and you should have a reasonably solid 5V out module...

[solutions]

Unwind the secondary of the transformer until you get the right number of turns for 5V...that'll mean unwinding it completely so you can get the exact number of turns you need to make when you put the winding back.

THEN play with the reference circuit.

[metal]

aha this is what I did not want to hear :D:D I was thinking while going to work, what if I had to unwind the tranfo!!?? and here you go :D

[bigtoy]

Do you have a reasonable load on that 5V output rail, or are you measuring it unloaded? It's common for switch mode PSUs to require a minimum load to regulate properly.

[dennis78]

Yes, "reasonable" load can help to detect problem. Unloaded psu (with "simple" psu controler) works with small duty cycle (<100ns) and this can be problem for psu controller, switching transistor (on/off times, delays,...). In your case, transformer ratio increase these problems. Smart psu controlers have pulse skipping option (or similar) and this problem is much smaller.    

I think, in this case feedback compensation will be very hard (maybe impossible) to setup correctly over full range of load.

Also, problem can be in self supply circuit because of lower output voltage (12V-> 5V ~ 2.5x smaller than normal specification). In try to detect problem if you have chance supply psu controller with external source.

[fpgaguy]

step 1) toss the huawei supply
step 2) ebay for line to 5V
step 3) buy bottle of favorite liquor and enjoy while waiting for ebay part arrival and pat self on back for saving a few days time

[metal]

That's what I did, I tossed both of these adapters, whatsapped many friends for a tablet charger (that's 2A), one called me yesterday and offered a 2A Samsung charger coz the tablet was broken

[vern]

smart move!