U2-U4 MOSFET TO-220 option

[vic7767]

This MOSFET mod is going to replace the poor performing or failed U2 and U4 MOSFETs on the PCAB of a Discovery Roomba with a TO-220 60 volt 12 amp MOSFET. I realize this is a little over the top but with the additional length of some 26 gauge wire. Once the board is in place it should be easy to find a space for the really large MOSFETs. The wire length will be cut down some once locations are found for both replacement TO-220 cases.

This same mod will probably work on the hard working Dirt Dog.

[mfortuna]

That's like putting a V8 in a Yaris!

[vic7767]

Kinda like dropping a Chevy 283 into a 1950 MG-TD Roadster LOL. Further progress in checking for room under the hood of a Dirt Dog has turned up the following location option. At the top far right of the battery form is an area that the two MOSFETs can be situated. Using a couple of 3 pin headers (from Sparkfun.com) and some heat shrink the following pictures in a .gif file will show you the progress so far. Remember to click on the picture so that you can see all three images. Each image will display for 5 seconds.

[vic7767]

Using a stock 1800 mAH iRobot battery that measured a no load voltage of 14vdc, began a charging test using the TO220 case MOSFETs installed into a test PCAB. 22 minutes later the battery was charged. Here is a .txt file of the charging data. The MOSFETs did not generate any heat that could be detected by hand touching.

[Jeff Carver]

Question about the replacement MOSFETS. I have several units that need this job, and I've just taken apart the first one. Looking at the MB, I can't tell if the U2 and U4 MOSFETs are identical, or different. What I bought was 4 each of the 2.5 amp and 10 amp MOSFETS, thinking that I'd need one of each to replace the U2 and U4 chips. Now I'm wondering if what I bought was 4 of the original replacement chips (for either location) and 4 of the higher capacity ones.

Could someone clarify this for me, please? Do the same chips go into both locations?

Also, the power-supply line that comes to the MB does not appear to have an easy disconnect plug to separate it from the MB. Should I just leave it connected and fix the MB with it hanging off the chassis?

[vic7767]

Although this thread is for a TO-220 hack it is about the U2 and U4 MOSFETs used in the Roomba charging circuit so the posted question can be discussed. Both MOSFETs are the same. When looking on the PCB you will notice they are electrically close to each other but on .different sides of the board. Here is a picture of the replacement options I offer The 40 volt 6.4 amp SOT223 MOSFETs are a direct replacement and have all 3 pins available for soldering to the board. These are manufactured in Germany.

[Jeff Carver]

Hm, now I'm even more puzzled. I have 4 of the 60V 10A ones, but the other four I have are 60V 2.5 A, which look just like the ones you show as 40V 6.4A. (I bought these a while back from Protech, and they've been sitting around waiting for me to do the work.)

It's not clear to me how you solder on the 10A guys and get the tab side attached.

[vic7767]

Here's a URL to the procedure using the heat sink as the middle missing lead (similar to what you have)

http://home.comcast.net/~davidtief/room ... grade.html

[Jeff Carver]

Thanks, Vic! That's what I needed.

[Jeff Carver]

Well, maybe what I needed was more skill. I soldered in two original part replacements on the MB of a red, and found it a pretty challenging soldering job. Very close quarters for wielding a soldering iron, especially on the one that had another part right up against the tab I was soldering.

It does not seem to have fixed the Roomba. I reassembled it far enough to have the connections all hooked up, and put a battery in for testing. I get the red pulsing light, but it never goes green, and the battery doesn't seem to be getting any charge.

I can think of two things that might have gone wrong. I might have gotten the new part too hot. (Are those things very heat sensitive?) And when I took the old MOSFETS off, I found a copper strand under each one that was loose at the end where it was under the tab part of the chip. It looked as if it was supposed to be connected to the tab, so I did my best to get it soldered to it when I soldered the tab down. But I'm not sure I got that right (either in concept or execution).

Any hints?

[vic7767]

That's the reason for this thread. It is easier to solder three wires to the circuit board and then connect the TO-220 MOSFET than soldering a SMD into place. Also the MOSFETs can't take much heat. No more that 2 or 3 seconds per connection. Also a little solder flux paste makes the job a bit easier.

[Jeff Carver]

That's the reason for this thread. It is easier to solder three wires to the circuit board and then connect the TO-220 MOSFET that soldering a SMD into place. Also the MOSFETs can't take much heat. No more that 2 or 3 seconds per connection. Also a little solder flux paste makes the job a bit easier.

Gotcha. But what I haven't seen in any of the pictures is the little copper strand going (I think) from the middle pin, under the chip, to the tab. Are that tab and the middle pin supposed to be electrically connected?

[Gordon]

... But what I haven't seen in any of the pictures is the little copper strand going (I think) from the middle pin, under the chip, to the tab. Are that tab and the middle pin supposed to be electrically connected?

Not only "supposed..." but actually connected inside the FET's case, as well as by Cu PCB tracks. That large tab and the smaller middle (typically) lead, opposite side of case, connects to the power-FET's DRAIN electrode.

Here are pix of U4 & U2 mounting areas as seen with larger ckt components dismounted:

U2_pads.jpg (57.08 KiB) Viewed 2822 times

Dismounting this PCA's U2 device was made difficult by iRobot's adhesive bonding (residue to right of "C60" marker, inside of U2's outline mark) of the case to the PCB, which contributed to lifting the FETs' SOURCE pad -- seen missing in the photo.

[Jeff Carver]

Thanks for the clarifying photos, Gordon. I've now attempted a second time. I think I got the U2 on okay, but on the U4, I've pretty well destroyed the mounting points for several of the connections. I don't know if I used too much heat to desolder or what. But I don't have much left to work with.

Is there any recovery from this, or do I need to replace the motherboard now?

[Gordon]

...Thanks for the clarifying photos, Gordon.

Your are quite welcome.

... on the U4, I've pretty well destroyed the mounting points for several of the connections. I don't know if I used too much heat to desolder or what. ...

Not much heat is required to soften the adhesive that bonds copper foil to the PCB's substrate.

...Is there any recovery from this, or do I need to replace the motherboard now?

It might be salvageable. Here is another image to work from. Its view is shifted down to show more tracks, and some markings have been inserted:

The first things to check, visually and with optical magnification, are some of the narrow tracks of copper to ensure they are still connected as needed.

Starting at U4's right, the wide-track through "S" (FET's source pin) branches at its lower end into two narrow tracks. One heads to the right to connect at C86's top terminal; while the other branch heads left and connects to R230's top terminal. If either narrow track is open, it will have to be reconnected (can do by paralleling the broken track with fine Cu wire, soldered at each end). By "fine", I mean small, such as 0.010" to 0.020" diameter. Rip apart some stranded conductors in trashed electrical cables to source such wire. Wire-wrap wire (if you know what that is) would be a good alternate.

U4's "D" (drain pin, at lower edge of its case, has no connections to it -- other than the missing pad and track coming down from the marked "D-tab" pad. Thus, you need only solder the large tab to the large pad. But, I suggest that you first spot-bond the FET's case to the PCB before doing its mounting soldering. Note that with on two soldered connections (S & D-tab) the component would be only weakly (mechanically) attached to the PCB.

At U4's left side, the "G" (gate) pad used to have two narrow tracks fanning out from it. One heads down (under the "Q45" marker to connect to the bottom terminal of R230. A fine wire jumper between the FET's G pin and the resistor will have to be fabricated.

The narrow track, marked "TPvia", is simply a conductor that heads through the PCB, and emerges in the middle of Test-Point #128 (a point that is used during Roomba's mfg process). You may ignore replicating that conductor.

Oh, one other thing: You will be doing more than the usual touching of the FET's gate pin. Due consideration to avoiding electrostatic discharges into / out-of the FET's gate is necessary to prevent killing the device before it gets connected to the PCA copper.

You might google for "ESD safe handling practice", do a bit of reading and see if you can decide how to modify your own device handling procedure to enhance a component's safety. Sorry, it is an overwhelming topic, yet, if the component, and the system it works with are cheap, and no life-support electronics are involved, the tactics can be reduced to safe rate equalization of differing potentials (voltages) between a component's pins and any conductor of electricity they will come close to, or touch.

Good luck Jeff!

[mfortuna]

I found the best way to remove these devices is to carefully clip the small pins. Heat the large tab and pop the transistor off. Then heat the small pads and pluck the remaining pin off the pad.

[Jeff Carver]

Thanks for the detailed feedback (again). It's going to be a couple of weeks before I have a chance to even try again, so silence for a while doesn't mean I haven't read your responses carefully. That said, I may be out of my depth with such fine soldering work. I can solder stuff big enough to see and hold, but at the level of having to squint through a magnifier, I'm on shakier ground. Still, I suppose I've got nothing to lose by making a salvage attempt. I see why Vic came up with the extension wire approach.

[newmom]

I am feeling really intimidated by this process and my husband seems uninterested in attempting this, although I could still possibly convince him to give it a try. If not, is there anyone on this board who does this repair with some confidence for a price, if so, how much? I would be talking 400 series if that makes a difference. I would mail the PCB (not the Roomba) so just wondering about this.

[vic7767]

is there anyone on this board who does this repair with some confidence for a price,

I offer this service, I've sent you a PM.

[Sandman]

This is probably a stupid question but which properties of the mosfets are the ones that matter? Volts, Ohms, Amps? My Roomba doesn't charge the battery properly and the only way to get it to run for at least half an hour instead of five seconds is to reset it when it (falsely) shows that the battery is full and then let it continue charging until it once again shows that the battery is full (which happens a couple of hours later). So I assume that if I do this replacement, it will work normally again or at least it's worth trying. Since a store close to me has some mosfets I could get them quickly and without shipping costs but not exactly the replacements mentioned here or on the page linked to so I need to know what properties can be over/under the original ones. The ones they have are

BUK456-200A - 200V-19AMP-150 WAT
BUK102-50 - 50V.45A-125WATT FET
BUK555-60 - 60V 35AMP-125 WATT M
BUK453-10B - BUK453-100B 100V13A
SUB75N06-08 - N-CHANNEL
IRFZ46NLPBF - MOSFET N-CH 55V 53A
IRFR9024NTR - MOSFET TO252AA
FDC638P P-CHANNEL MOSFET
IPP075N15N3G - 150V,100A. 300watt N Cannel power mosfet Rds6.2mohm
SUP75N06-08 N-CHANNEL MOSFET - 60 VOLT 0.008 OHM 75 Amp 240 WATT
APT5040 - MOSFET 500V16A
BUZ71 - MOSFET 50V14A
SUB75N06-08 - N-CHANNEL MOSFET

Pretty please with sugar on top, can someone tell me which one(s) would presumably work?

As you can probably guess, I'm relatively clueless when it comes to fixing my Roomba but I will ask a friend who is more experienced with electronics for help with doing the actual replacement. I just want to get the right parts first.