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

[sententia]

Hello everyone,

I have this charger where the fuse was blown. I found one of the two main power fets shorted as well one of the smd diodes beneath the heatsink (STP9NK50ZFP and MURS160T3G).

I connected a lamp at the fuse and switched on the power to see that the lamp is pulsating. Upon further inspection i noticed four 1R smd diodes open so i replaced those as well. The charger still does the same thing.

Output on bridge rectifier is pulsating from 295V to 305V.
VCC on power IC (UC3845) is 8V steady, although according to datasheet it should be around 32V or 15V and above in order to get started.

I am not sure if the diode i installed is in correct direction as i did not note it before removing and there was no print under it...

Those two powerfets are interconnected..

If i reverse the smd diode and is in wrong direction afterwards will that kill more components??

I am totally stuck! Any suggestions greatly appreciated!

[CapLeaker]

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

Most new chargers won't work until they see a battery connected to the terminals. They are not like the linear ones that have 12V out as soon as you plug it in. I can't see which direction the diode is in. Is it the same direction than the other one?

[sententia]

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

Yes that is the case for this charger as well. However, according to manual, there is a switch for float charge, that when on the charger can be used as a power supply.

At the moment kathode sides on both diodes are facing towards the outer board.

I am trying to make a small schematic of how the fets and diodes are connected....

[sententia]

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

i am "almost" positive that this is how they are connected...

[budm]

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

Is the positive leg of that cap to the right of the SMD diode connected to one of the Diode leg? or see where the right pad of the diode is connected to, it should go to a filter cap some where.

[sententia]

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

The right pad of the diode in circle is connected to the main filter capacitor thus getting 300v.

If you mean the blue c82 cap, it does show a connection with the anode of the diode on the left with 2kr resistance...

[rievax_60]

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

Is it possible that the original diode is one of these? https://en.wikipedia.org/wiki/Transi...pression_diode

[budm]

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

This power supply has PFC Voltage booster, you can see the PFC inductor ELY0704, and next to it are the POWER MOSFET and the PFC Diode mounted on the heatsink.
So the DCV on that main filter rated at 450VDC should have steady >360VDC on the filter cap if the booster circuit is working.
BTW, so the Main power MOSFETs and the Diode you replaced are the one that drive the power transformer? If the MOSFET had shorted between the Drain and Gate , then for sure the IC that drive the MOSFET will be damaged, including the Gate drive R/D network.

[sententia]

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

Rievax the diode was replaced with exactly the same part number so it is definitely a diode.

Budm,

You observation is spot on!!!

There is a booster circuit running on a TDA4863-2 and the other so8 is a TL431.

The mosfet is indeed the one driving the transformer and was shorted gate-drain. 4X 1R resistors where burnt as well as was a 2R2 on the other mosftet.

The main filter capacitor is 400V 220uf rating and not 450V! It measures a bit lower at 198uf but i doubt this is causing the problem.

So do you think the main power ic UC3845B would get damaged?? There is definitely not enough VCC to run it at 8V.

Another observation is that i measure on the red circled zener diode D84 208V on its legs that might have to do with the VCC supply of the power IC.

Many thanks for the help!!

[rievax_60]

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

As well as the parts Budm mentioned, look for a small resistor usually 1K that connects between the MOSFET's source and the Current Sense pin on the UC3845. If this resistor has been blown open, the same damage will reoccur.

[sententia]

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

Current sense pin 5 of power IC connects through the 100R resistor to source (check image).

Resistor is good. The 4X 1R resistors forced current to go through there maybe??

I am still concerned about the voltage measurement of D84 zener diode. I cannot see the voltage of the diode but if i measure 200V there doesn't that mean that the zener is not conducting? As a diode it measures ok.

[budm]

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

If you are only getting 8VDC at the VCC pin of the SMPS IC, the IC is more likely to be damaged when the MOSFET shorted out.
You should take the resistance reading between GND pin of the IC against all other pin of the IC to see if any of them show low resistance, especially the VCC pin.

[sententia]

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

VCC and ground have 1.9K resistance. All other pins are ok compared to VCC.

Please check previous post with D84 zener which shows 200V on its terminals.
It connect through a 150K resistor straight to + of bridge rectifier and i think it might be part of the power IC supply. Is it possible that it is not conducting?

[budm]

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

VCC and ground have 1.9K resistance. All other pins are ok compared to VCC.

Please check previous post with D84 zener which shows 200V on its terminals.
It connect through a 150K resistor straight to + of bridge rectifier and i think it might be part of the power IC supply. Is it possible that it is not conducting?

I expect more than 1.9K on the VCC pin. Is there a ZENER directly connected between the VCC pin and the Ground pin?
As you can see, there is also internal 34V Zener diode protection on the VCC pin 7 which can be damaged, they may add an external lower Voltage Zener for protection.

[sententia]

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

Yep 1.9K does sound very low for an IC with 32V imput....
The ic is not expensive but i have to order it from abroad.

The zener does not connect straight to VCC of the IC nor to ground.

I will order the IC but i am trying to see if there are any other parts i should order for this as this will be my third time

Thanks for the suggestions!!!

[budm]

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

"Please check previous post with D84 zener which shows 200V on its terminals.
It connect through a 150K resistor straight to + of bridge rectifier and i think it might be part of the power IC supply. Is it possible that it is not conducting?" See the App notes: the IC gets the kick start Voltage from the main bridge through high Ohms resistor, once the IC starts running then it gets the Volage from the AUX winding of the transformer that it is driving to keep itself running (just think of it as a car, you start the car with the batteries, once the car is running, it gets the power from the alternator, then you can disconnect the batteries from the car and the car will keep on running), that is typical setup of the SMPS IC, The Voltage from the AUX winding is also used to supply the VCC for the PFC Controller IC.

[sententia]

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

Very informAtive post. It does make clearer about how the ic operates.
I will be away for few days.. I expect the order to arrive by the end of next week so I will get back with feedback.....

[sententia]

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

I am having a hard time locating the ON UC3845B ic.
I can find TI UC3845D but i cannot understand from datasheets if they are interchangeable. The only difference i have noted is that one has max supply of 36V (ON) and the other has max supply 30V(TI).

EDIT!

Just noticed that the supplier lists the UC3845D as ON semi. The only place i found a datasheet for that is at mouser where datasheet is stamped as "obsolete". Is this a previous version of the IC??

[CapLeaker]

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

Not sure what your problem is. Went to mouser put in UC3845B and they got thousands in stock from ON semi and STM.

[sententia]

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

At the moment i am trying to avoid mouser because i have a very small order and shipping cost is 20e. Also i am almost positive that i won't be able to proceed with payment, even if i wanted to, due to capital controls in Greece....