help with Lipo powered cordless ratchet

[leeleefocus]

Bought a Sealey cordless ratchet CP2144 which came with 2 ni-cd batteries. It's a bit bulky but comes in very handy for certain jobs.

Today i emptied the ni-cd from one of the packs and fitted 2ah of 20-30c lipo. The packs were 14.4v and i have fitted 4s lipo. The ratchet had a really nice performance boost with more torque and more speed.

Here is where i need the help. Using the ratchet today i heard a pop and the familiar smell of burnt electronics. took it appart to find the fet had cooked itself. The fet is one of these http://pdf1.alldatasheet.com/datasheet-pdf/view/262657/ISC/STP75NF75.html

I was thinking of replacing it with one of these http://pdf1.alldatasheet.com/datasheet-pdf/view/391138/IRF/AUIRF3205Z.html

Or maybe even parralleling a couple of the above fets. Would any of these ideas actually work. If not what would work. I'm at the mercy of the electronics guru's on this forum.

 

[leeleefocus]

Ok going to work tomorrow and will have a look to see what fets are available and maybe have a word with the electronics engineers about the best solution. If all else fails i will experiment myself with what is available and try to get it working.

 

[bigmoose]

I just looked quickly, but the substitution you recommended should work OK. My concern is it "blew for a reason"... as in, it is the new "weak link." So your redesign to a battery with a lower internal resistance did give more power to the tool, but also overstresses the FET's capability. As you said, perhaps time for an upgrade! Parallel two FETs if you can.

Pictures of the tool controller would help diagnose.

 

[leeleefocus]

I've taken a picture but all it is is a PCB with a switch and a Fet. There are no components on the bottom side of the PCB. It does connect to another PCB but that is the LED driver board.

Looks like my only option is to parrallel 2 Fets.

Thank you for the reply as my electronics knowledge is limited.

 

[bigmoose]

I was hoping there was more "guts" in there! Now this is a guy grasping for things in the dark, but what I am thinking is that by using the LiPo with lower internal resistance and a tad more voltage, you have increased the dI/dt when the FET switches off. This causes an inductive spike that overvoltages (perhaps) the FET. Paralleling two FETs will actually slow the transition down (which is likely good) if there is a gate resistor to the FET, as the two FETs in parallel will double the gate capacitance and slow the rise/fall times a bit. This should help to remove the stress from the part. A shot in the dark, but I would give it a try. If you stay with a single FET and it has a gate resistor, increase the value 60%.

 

[liveforphysics]

I would put one of these in:

http://search.digikey.com/us/en/products/IRF1324PBF/IRF1324PBF-ND/2096577

And put a little cap between source to drain. A little poly film or a few ceramic caps should do a good job.


See if that doesn't just fix the problem.

If it doesn't fix it, shorten the battery leads as much as possible, and add a cap at the end of the battery leads as close to the point it connects to the motor control board as possible.


And Dave's suggestion of using a higher resistance gate drive resistor is likely a better and easier fix than the things I mentioned above, which should all help, but Dave's fix actually gets to the root of the issue, excessive dI/dT.

 

[Alan B]

I would sketch out the schematic and review it here. Might be better than blowing a few more parts trying things.

A higher voltage and current rated FET might survive better - the body diode can handle more transient energy. But if the FET is oscillating for a moment it could use a different approach. Is there a gate resistor?

I wonder if a source-drain cap is a good idea as it causes a big current spike at turn-on. Maybe a snubber? A Capacitor across the battery is safer but doesn't do quite the same thing.

 

[leeleefocus]

Just coming back on to say thank you for the help.

I've been extremely busy the last few months but finally got around to having a go at fixing the ratchet.

The route i went with was to parallel two of the fets that i put in the first post. The reason i took this route was i had already purchased 2 of these fets before there were any replies. It was a bit of work to get the second fet in there. I tried to fit a low esr cap on the battery input side but there is absolutely no room once the ratchet is reassembled.

I've used it a few times the last 2 weeks and it's holding up fine.

On a side note i've learnt a few things also by researching what you were all talking about with gate reistors and di/dt.

Thanks again.