WFCO Camper Converter need help understanding KA3846 SMPS Controller

[Shane711]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

You have bad capacitors causing the problem. Can you send me pictures so I can tell you which capacitors to change?

If the KA3846 goes bad the unit will blow up so you have bad capacitors.

All the capacitors within 1 inch of the 2 white resistors need to be changed except for the 2 BIG ones, only replace those if they are bulging. If your unit looks like my pictures then change the same capacitors. The heat from the resistors destroys those capacitors.

[shadetreetrader11]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Its not just the capacitors that are near the large white resistors. I have noticed the ALL of the Capxon brand caps seem to have issues. Start with replacing all the caps and tell us how it turned out for you.

[Shane711]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Hi Shane711
I have a WFCO -9875. When off load I can adjust to 13.60 volts (with Vr1). As soon as I put a battery the voltage drop to the battery level. If I leave it connected it it will charge the battery very slowly over many days then stabilize at 13.2v. When in my trailer the voltage keeps on reducing to the point that the fridge doesn't work anymore. Do you know which IC controls the amperage and voltage output ? Is this the mini controller or the KA3846. I have a new 9875 but would like to fix this one.

Thanks

Claude,

You also have some bad capacitors that need to be replaced. If your KA3846 was bad the unit would not work at all.

[connorcc2005]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Hi Badcaps readers,
I'm new to the forum and stumbled onto this site today after a great deal of research and troubleshooting of SMPS'. I was looking for detail on the KA3846 SMPS controller when I came across Shane711 post and repair of WFCO supplies.
I have a WFCO WF-8735-P, 2011 vintage, that I'm working to repair. It is supposed to charge the 12v battery and run lights in the camper when plugged into shore power. After a week of camping in Colorado, it quit charging and the 12 volt output is zero.
Having repaired electronics for along time, I wanted to take a shot at fixing it. Removing it from the camper, I found R27, a 100 ohm ceramic resistor, had come unsoldered from the circuit board due to heat and was laying in the supply. I thought, well this is going to be easy, soldered it back in and my learning began.
Needless to say I'm learning a lot about SMPS' but still don't have this unit repaired yet.
The thing is, I have yet to find any faulty or burnt components.
Here is what I do know:
120VAC input is being rectified by the bridge and filtered by the 1000uf/200v electrolytic. I'm getting 170VDC at the cap. This part seems to be working correctly.
It has 2 MOSFET drivers on the input side. They have been tested out of circuit and test good.
It has 2 Shottky diodes on the output side. They test fine as well, out of circuit.
This is a slightly different design than the unit Shane711 detail in his post as this unit has only 1 cap on the input side and the newer designs use 2.
I can send pics if needed.

At this point, I've ordered replacements for all electrolytic caps. They should arrive tomorrow or Monday and I'll install them. So far, I haven't found a defective cap but with the detail above, will look at the small one next to the two large ceramic resistors as they get extremely hot. I was told by the WFCO tech support team that those resistors are used to get the 170vdc down to a usable level for the KA3846 controller 15v VCC.
I'll report back once all the new caps are in. In the mean time, if you have any advice, I'm open to all ideas.
Thanks,
Chris

[Shane711]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

All the capacitors around that resistor got too hot and need to be replaced. You need to test the mosfets on the large heat sink, if they are bad then the KA3846 needs to be replaced. But the KA3846 was discontinued because it has problems and replaced with UC3846 so I would replace it anyway. I never solder these chips to the board, I always use a IC Socket and then I plug the chip in.

[PeteS in CA]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

The UC3846 is part of the same PWM family as the ubiquitous 3842/3/4/5. The 3846/7 is usable with half and full bridge inverters. TI is the current mfr, and it looks like Mouser and Digikey carry the part.

[connorcc2005]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Hi Shane711 and PeteS,
Thanks for your replies!
The new caps arrived today and installed all of them. The issue was definitely the caps.
Most measured fine but the 22uf by the two ceramic resistors was definitely bad.
Post cap install, the supply fired up first time and was putting out 13.65vdc.
I'm still waiting and wondering when it will drop to 13.2vdc as that hasn't happened yet. I'll see what it looks like in the morning.
Thanks again for your input.
Do you see any benefits to swapping the 3846 at this point or wait till it needs caps/service the next time?
Thanks,
Chris

[R_J]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

I see no reason to replace the ic if it is working, The KA3846 was made by Fairchild/Onsemi, and is dicontinued the UC3846 is the replacement

[Shane711]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

I repaired about 50 or so of these converters and in my experience they are faulty and need to be replaced with the new version. The originals have a fault that causes them to overload and it's best to replace it before it fails.

[PeteS in CA]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Wellllll ... KA3846 is a Samsung part number. Samsung sold the rights to a bunch of their linear parts to Fairchild when Fairchild was spun back out from National Semiconductor. Fairchild was later bought by On Semiconductor (and Nat Semi was bought by TI). The entire UC384x family was created by Unitrode, which was bought by TI.

If you followed that and are still awake, you're more patient than me.

[redwire]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

There also have been many lawsuits over SMPS IC's between chip makers and that might be part of the brand shuffle.

[connorcc2005]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

GM and thank you all for the detail on the updated controller as I am certainly considering the swap to the updated controller!
The supply output is at 13.65vdc consistently, charging the battery and running accessories as it should. There is still a small issue in that the output voltage never switches to 13.2 volts to maintain charge and trickle charge the battery. The constant 13.65v on the battery is cooking the battery. Maybe it is my battery as it is aged at this point?
I know this unit should charge at 13.6 as it is and trickle charge at 13.2 and every 48 hours or so go to 14.4v for up to 4 hours. I'm not so concerned about the 14.4 but it would be nice if it would go to 13.2 to stop cooking the battery. I suspect the PIC 12F675 micro controller is making these logic calls on timing and what voltages to put out?
Earlier in this thread, Shane711 talks about reading the program in the PIC 12F675. Did that ever happen?
Any ideas or troubleshooting tips as to why this unit isn't going to 13.2?
I appreciate your input.
Thanks!

[R_J]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

I would try it on a new(er) battery first.

I suspect the PIC has it's Code protection bit set.

[redwire]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

13.65V is not high enough to cook or boil a lead-acid battery, unless it has a failed (shorted) cell. Both AGM and flooded types don't mind 13.65V at all.
Smart chargers will not leave and go on to the next charging phase until the charging current has dropped off or they timeout say 8-24hrs.
If the camper has loads switching on and off, or the battery has a problem, it will stay stuck in that charging phase. I'm not sure how sophisticated the PIC firmware is.
You might just leave it charging for a long time and see if it does an equalize (14.4V) and then goes to a low float (13.2V) phase, if the water levels are good no excessive gassing and the camper is not drawing current.

I would replace the 3846 for good measure it sounds like the KA IC is weak.
If the charger still behaves weird, we'd need pictures to see if there is another bad part. The 100R burning up seems unusual.

edit: other brands of chargers get fooled going into low float 13.2V despite camper loads turning on and off, the charger never notices and kicks up to 13.8V. So people have problems with the batteries having a short life ~2 years, because they are being chronically undercharged. Low float is only good for saving energy/water over really long periods i.e. weeks.

[connorcc2005]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Thanks for your replies RJ and redwire!
I've removed the camper from the circuit and have the charger connected directly to the battery only at this point and the charger stays at 13.65v and never switches to 13.2, even after 24 to 36 hours. I do want to see what the current level is as well. What should the current drop to for the charger to cycle to the next level of 13.2?
I plan to try another battery soon but this project is on hold as a family member is having some health issues.
For some more detail and clarification, the lead acid cells are bubbling consistently and somewhat aggressively which is why I used the "cooking" word. I realize the battery will degas as it charges but as they charge my experience has been the bubbling subsides. A new battery may correct this as mentioned and is my #1 priority as next steps go.
In regards to the 100 ohm ceramic, it didn't burn up, it just got hot enough over time that it unsoldered itself and came off of the circuit board. It still reads 100 ohms and was re-soldered into the circuit.
I'll report back once the new battery is installed. Any thoughts on the charge current question are appreciated.
Thanks,
Chris

[R_J]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

I have seen these larger 5~10 watt ceramic resistors come unsoldered over time in lots of equipment, the resistors get warm/hot that's why they use a large watt resistor and not a small 1/2 watt as it would just burn up. When the boards are made it is hard to wave solder the board and have the small lead components get soldered the same as large lead components like transformers or large watt resistors, so after time, heat and vibration, the solder becomes weak and the component can come loose.

The bubbling in the battery is normal, How old is the battery? You could take the battery to a shop that can test it using a battery analyzer (not just a load tester) and it only takes one weak cell to throw off the charging.

[connorcc2005]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

GM, here is the latest on the WFCO 8735 repair.
I took the original battery in and had it load tested. It failed the load test. Not much other detail was provided on why, so I went with another battery. This battery was new June 2021 and has had little use and was on a battery tender when idle.

With the June 2021 battery installed, the WFCO SMPS turns on and goes to 13.6vdc charging the battery. After a couple of hours, I measured the current draw by the battery at 23.4ma. After 12 hours or so, the charger did start slowly dropping the current and voltage until it gets to 13.2v and 8ma as I expect it should.
Thanks to all who responded with your ideas and suggestions for this repair!
Till next post, take care,
Chris

[PeteS in CA]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

The history I gave of Fairchild and its purchase of rights to some of Samsung's linear ICs had no lawsuit in it that I'm aware of. Fairchild had been spun back out of Nat Semi, and bought parts from multiple companies, parts that those companies were willing to be rid of and which rebuilt Fairchild's catalog. Fairchild was allowed to use Samsung's IC die artwork for several years after the purchase - Samsung's die artwork included Samsung's logo (I saw the purchase agreement online). This gave Fairchild reasonable time to re-spin the die, possibly shrinking it to get more dice from each wafer.

On Semi is another growth-by-acquisition company, and later purchased Fairchild, again, no lawsuit I'm aware of. I believe TI bought Unitrode back in the 1990s. Unitrode had been around quite a while, having done rectifiers and power MOSFETs, but their most enduring legacy is the 384x PWM controller family.

On another tack, a power resistor really shouldn't scorch a PCB or de-solder itself - provided it is mounted sufficiently off the PCB and is properly de-rated (= not operating at >70-80% rated power). Back in ancient times, Boschert used ceramic beads threaded onto resistor leads for resistors 1/2 watt and greater to ensure they were lifted above the PCB.

[Shane711]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Wow, this is a lot of information. I was told the new IC were made because the originals were defective and discontinued but as long as we get these units up and running is all that matters. And I assume the IC gets weak over time.

I don't remember if I was able to read the PIC firmware or not, that was some time ago. If anyone feels it to be necessary I'll give it a try.

Those BIG resistor solder joints break loose mainly because they use lead free solder which has no flex to it and cracks, resoldering with leaded solder is recommended and put a good amount on there.

Also I add heat sinks to those resistors and I add an extra fan when possible. I add a thermal switch to prevent the fan from running constantly, I also try to add heat shields when possible. The heat from those large ceramic resistors will destroy ever capacitor nearby and you won't know it because the heat doesn't make the capacitor pop, all small caps within 1 inch should be changed. The small cap next to the ceramic resistor I always solder to the bottom of the board to keep it away from the heat.

[kiloamp]

Re: WFCO Camper Converter need help understanding KA3846 SMPS Controller

Hello, I was given a WFCO-8845 for salvage but could not resist fixing it. And yes it was just the one electrolytic cap (C10) that caused all the trouble. It works fine now but the two power resistors sure get HOT, even with only a 2.5 amp load. Many thanks for your sharing.
I have one question, can someone tell me what the 3 pin connector is for. I never saw the board installed. Thanks.