Power Adapter Chino No Working, Failed VCC IC UC3842

[SoporteHR]

Hello everyone, receive a warm greeting.

I present a new case and I appeal to your help.

I have this Chinese laptop adapter. Model: PA-130017. output of 20v x 4.5Amp.

I received it unarmed and practically destroyed.

Visually a diode in the primary stage exploded
Mosfet in short.

Test each component of the primary stage, the diodes are fine, zener and rectification.
replace the Original Mosfet K2761 with a 07N60 mosfet. I am aware that it is of a lower magnitude than the original.

replace diode D03, I extracted it from another adapter that I have for spare parts, with the inscription "A2" as it is a 1N4148.
I measured the resistance that is parallel to that diode and is open, SMD 220, replace it with a conventional resistance of 22ohm 1 / 4w (Here in the city where I live, SMD components are not available).

I turned on the adapter through a series with 100W bulb and I had no output voltage or the zener diode that powers the KA3842 IC. Pin 7.

Check the feed that reaches that pin and is 0vdc. Test the resistances and they are correct. R8 to R11.

I removed the IC and connected the adapter and the voltage in pin 7 was set to 25 vdc.

Probe continuity in pins 7 and 5 (VCC and GND) of the IC and had low resistance.

I bought a new IC. UC3842N was the one I found. I installed it and connected the adapter to the electric power. I measure the voltage at Pin 7 and it ranges from 10vdc to 15vdc and it does not stay stable, replace the capacitor and it stays the same.

The voltage on the output pin to the Pin6 mosfet. Oscillates between 0 and 0.50 vdc

At the outlet of the adapter I do not have any tension.

I have not plugged it directly into the electrical current without the series, for fear of damaging the IC.

Annex images of the adapter.

What else can I check or do?

Thank you.

[R_J]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Did you check R01? it should be low Ω

[SoporteHR]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Hi R_J

Thanks for your help, you were right the R01 was open. I just replaced it with one of the same value. 0,39ohm and I already have in Mosfet 0vdc, before a voltage oscillated in that line.

But the tension on pin 7 of the UC3842 is still not stable.

What else could it be?

Thank you.

[R_J]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Check if there is a resistor that goes from R01 to pin 3 (CS) of the KA3842, it may have been damage when R01 opened.

Looks like it might be R02
Also check the transformer winding connected to R12, check that R12 is good and that the diode (D01?) connected to r12 is good, that winding supplies the run voltage for the ic.

[SoporteHR]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842 (Solved)

Hi R_J.

Thank you very much for the help, you guided me to the solution of the problem, although I had measured with the tester those resistances, it was when I took them out of the circuit and I tested them with a voltage source that I realized that they were not operating well, they passed some voltage but it was not complete.

Example: My power source was on 4vdc and the other end of the resistor had a voltage that ranged from 2.5 to 3vdc. First time I see that behavior.

Then when reviewing R02, it was open, replace it with an equivalent non-SMD resistor (it did not have that format).

R03 is fine.

After these changes, the pin7 voltage of the IC stabilized at 15.7vdc. And I got voltage on the secondary, stabilized at 20.4vdc. according to what the adapter label indicates.

I attach photos of the tracks side with the voltages I got once the adapter started working.

Again, thank God and you R_J for the help.

I consult you, is there any way to test the output signal on the pin that controls the gate of the Mosfet ?, I put the tester at that point and the screen goes crazy.

Case closed.

[petehall347]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

i guess you might mean the chopper fet ? i wouldn't worry if output at secondary is good

[R_J]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Glad to here it is working.

To check the drive signal from the ic to the gate of the fet you would need a scope and an isolation transformer to isolate the scopes ground because you would be checking the HOT primary circuit.

[SoporteHR]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Thank petehall347 and R_J for your comments.

If it's working, it's time to reassemble it, test it with load and keep the output voltage stable.

Do you know if you are at risk for having installed a 7-amp mosfet instead of 10 amps ?. The output of the adapter is 4.2amp according to the label.

I do not think it affects, if I'm not mistaken I would have a margin of 3 amps, I remain attentive to your opinion.

You will have some scheme of the isolation transformer that you mention.

It's just curiosity to know how to measure that signal.

Greetings and thanks.

[eccerr0r]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

It's probably okay. Remember that this is primary side and should have lower average current flowing - though peak currents is a lot higher. Question is, how much current are you actually using on the output side, which would determine the frequency of how often peak current draw would happen. Not an easy correlation.

The isolation transformer is one of the basic safety devices, basically it's a 1:1 transformer, so that the two sides no longer have a direct connection between them and thus no current can flow. Then it will be safe to use an oscilloscope and you won't get line power shorted to ground of the oscilloscope.

[SoporteHR]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

Hello Eccerr0r.

Thank you very much for the explanation of the isolation transformer.

Then test the consumption of the laptop and see if it is safe to work with this adapter as well as with the 7 Amp Mosfet.

regards

[budm]

Re: Power Adapter Chino No Working, Failed VCC IC UC3842

You will plug the DUT (Device Under Test) into the isolation transformer, so you can use your scope on the hot side of the DUT.