MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

[budm]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

That diode and the 10 Ohms resistor is probably connected to the AUX winding of the transformer to provide the running Voltage once the circuit is running. See the app circuit that I told you to look at.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Yep it connects to primary winding of the transformer....

But i replaced the 22R resistor that i missed and the charger IS WORKING!!!

Thank you VERY VERY much for your time, its really appreciated!

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Hello again!

Yesterday i noticed that this charger i repaired failed again!

The same mosfet that was shorted is shorted again and the 4X 1R0 resistors are blown.

HOWEVER fuse is intact and rectifier.....

Any ideas if this is an isolated incident due to power outage or whatever, or is it something i have missed..... The charger operated for 7-8 months without problems....

I am also worried about the 22R resistor i replaced with a through hole one did not have a very good contact connection (soldered on the smd one) and maybe caused the problem? Although i used some hot glue on it, it looked brittle.

Thank you all!

EDIT: I have also noticed that the clip of the mosfet sits just above those resistors and looks a bit like something arced there??!

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I went ahead and replaced 1 out 2 mosfets (the second one measures good, not shorted and internal diode) the 4x 1R0 resistor and the 2R2 0805 and the second mosfet.

I powered it up but no go ...

I replaced the UC3845 just in case the output pin was damaged but same result... UC3845 gets 8.1V which is within the no power up threshold.

[Agent24]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Just because one measures OK doesn't mean it's working.
You may also have some other bad part in the control circuitry which caused the failure again.

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

Just because one measures OK doesn't mean it's working.
You may also have some other bad part in the control circuitry which caused the failure again.

That is very truth.... I removed and measured it on my transistor tester and showed open/no part although new mosfet detected correctly.

So i installed the fet and it worked for a second or two (led blinked) and died again.... I must remove and check the smd rectifier diodes behind the heatsink to check them as well....

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

That diode and the 10 Ohms resistor is probably connected to the AUX winding of the transformer to provide the running Voltage once the circuit is running. See the app circuit that I told you to look at.

Hello!

I got this charger out of the pile,and removed the heatsink to get another look...

The situation is that the UC3845 gets in undervoltage lockout and it gets 8.0V on VCC so it won't start. I checked with an oscilloscope and it does output a pulse every couple of seconds but does not start.
If i supply VCC the chip outputs a signal and operates (without the mains connected).

Budm mentioned to check the application diagram... This design is a bit different as it uses 2X Mosfets ..... Also the gate of the second mosfet is driven by a transformer winding!? through a 2R2 resistor....

I would be happy to hear suggestions as of what to troubleshoot next!

[sententia]

Re: MasterVolt IVO 12/25-3 Marine charger - replaced power fet but now pulsating

I have removed the transformer to try to understand the windings and how the mosfets connect. The pins marked 1-2 are the Aux winding. Primary is pins 3-4 but on pin 4 there are 3x wires connected where the 3rd one goes to pin 5 of the transformer. That pin and winding is used to drive the 2nd mosfet.
Now on pins 6-7 there is a separate winding which seems to connect straight to the secondary's ground and circuits (i have striped a pad there but easy fix...)

It is not a standard application of the UC3845...