SMPS not turning on

[Gaber Mohamed Boraey]

hello

i have SMPS circuit, with uc3845B pwm ic, the start up resistor is 100k, the start up capacitor is 47uf, 25v,
this circuit is for charging 6 batteries " lead acid types ", 12v , the batteries are connected in series, so 72v is the total voltage approxoimately, and the charging voltage to be about 85v " aroudn this value "

this circuit should start once i plug the cable in electricity, and should read 300v at the smoothing capacitor which is 47uf, 450v, and that is already happens for now, before this capacitor had bad terminal and ive changed, i thought it will solve the problem, but i found that the problem is still,, whih is no output voltage at all for this circuit

this schematic is similar to the circuit i have , please look

I changed the start up capacitor with good one, checked the start up resistor and its ok, also I've checked every component at the primary stage and the secondary also, nothing bad, except the pwm ic of course as i can not test
so i ve changed with new one, " that maybe still the problem, in other words it may be bad ic as I've heard that some are bad even new , so I am not sure"

I also tried to bias the ic with outside power supply, after taking out the start up resistor, upto 15v and 1 A, but it not even turn on the power supply, " this is all and the mosfet in the circuit,

how you think of this problem?, it still the ic issue?, bad ic , because it not turn on even with outside biasing?, or anything I can do for be sure of the problem?


regards

 

[betwixt]

I'm confused, you say "the start up resistor is 100k, the start up capacitor is 47uf, 25v" but in the schematic they are 300K and 1uF. That's a big difference in start up curve.

Brian.

 

[andre_luis]

I'm confused, you say "the start up resistor is 100k, the start up capacitor is 47uf, 25v" but in the schematic they are 300K and 1uF. That's a big difference

Brian.

Me too, with the statement...

"this schematic is similar to the circuit i have"​

 

[Gaber Mohamed Boraey]

I'm confused, you say "the start up resistor is 100k, the start up capacitor is 47uf, 25v" but in the schematic they are 300K and 1uF. That's a big difference in start up curve.

Brian.

the schematic is similar , just show for the concept not the values, in other words not the circuit i have but similar concept

--- Updated Aug 22, 2024 ---

one more information, that when i measure the vref at the time of applying the threshold voltage , its about 0.3v
and the ic not turn on

if within the resistor installed " start up resistor " , the vcc is 8v , and vref is about 1.3v

 

[andre_luis]

the schematic is similar , just show for the concept not the values, in other words not the circuit i have but similar concept

Sure, I often implement circuits that are similar in concept to the original, but that don't work because of that.
Details make the difference in electronics.

 

[Gaber Mohamed Boraey]

Sure, I often implement circuits that are similar in concept to the original, but that don't work because of that.
Details make the difference in electronics.

Okay, now at least you know
Here is the real one, but sorry I don't have schematic

 

[andre_luis]

Here is the real one, but sorry I don't have schematic

I got it, but sorry, this way I cannot investigate the problem.

--- Updated Aug 22, 2024 ---

BTW, did you note the scrappy soldering close to the U14 IC ?
There may be more behind, that not only component values, but perhaps burnt parts as well.

 

[Gaber Mohamed Boraey]

Ok, maybe other members can help and see something

--- Updated Aug 22, 2024 ---

I got it, but sorry, this way I cannot investigate the problem.

--- Updated Aug 22, 2024 ---

BTW, did you note the scrappy soldering close to the U14 IC ?
There may be more behind, that not only component values, but perhaps burnt parts as well.

I don't think so , everything around this area is good

 

[andre_luis]

Ok, maybe other members can help and see something

I hope so, meanwhile...

I don't think so , everything around this area is good

There are a lot of soldering residues close to the U14. In addition to the solder guts themselves are quite irregular and oversized, giving the impression of the iron solder tip with the wrong temperature.

except the pwm ic of course as i can not test

This is a problem when trying to debug / repair pulsed driven devices.
Didn't you have any cheao handheld Oscilloscope?

 

[Gaber Mohamed Boraey]

This is a problem when trying to debug / repair pulsed driven devices.
Didn't you have any cheao handheld Oscilloscope?

I have oscilloscope, how you want me to use?

 

[andre_luis]

I have oscilloscope, how you want me to use?

You can start by checking all the expected values and waveforms on the UC3842A, as stated on their datasheet, taking extreme care when probing pins 6 and 7, which deals with quite different voltage ranges.

 

[Gaber Mohamed Boraey]

You can start by checking all the expected values and waveforms on the UC3842A, as stated on their datasheet, taking extreme care when probing pins 6 and 7, which deals with quite different voltage ranges.

I have uc3845b, and the problem that it can not turn on the circuit, and not reach the Vref which should be 5v
No output from the IC
The mosfet is ok
I used external power supply and apply. Voltage upto 15v, it show that it consumes some ma's but nothing start
That why im here for help

 

[Easy peasy]

The ckt is for a 3842A, not 3845B - however it may be compatible - you will have to check the pin out, some of the tracks have got really hot from a previous blow up ( or blow ups ) - and may have hairline cracks ( breaks ) - this is going to be a tough one to fix if you don't have the true schematic and the proper parts.

--- Updated Aug 22, 2024 ---

the UC3842A won't start until the Vcc gets to 16V - and sometimes as high as 17.5V - I note the UC3845B has lower turn on threshold about 9V

--- Updated Aug 22, 2024 ---

it would appear to be a good idea to put an 18V zener from Vcc to gnd - else the gate of the mosfets can be taken to 34V or higher and kill the mosfet !

--- Updated Aug 22, 2024 ---

check the mains fuse - and any other fuses supplying the AC line to your psu - be careful !

 

[Gaber Mohamed Boraey]

the UC3842A won't start until the Vcc gets to 16V - and sometimes as high as 17.5V - I note the UC3845B has lower turn on threshold about 9V

I've tried to turn on the IC with external power supply, but it can not turn on, this can be bad ic?, I make separate circuit for testing the IC outside the circuit?

 

[Easy peasy]

It seems likely the IC is fried - yes

 

[Gaber Mohamed Boraey]

It seems likely the IC is fried - yes

Okay, will try different one, thanks
Although they are new