Milwaukee Circular Saw MOSFET Failure- Need Replacement - Page 1
Forum section: electronics / repair

author: KDoug
author_rank: Contributor
timestamp: April 07, 2019, 09:00:29 pm
content: Hi,I've recently bought a non working Milwaukee 18V cordless circular saw. I took a chance when I bought it knowing there was a possibility I wouldn't be able to repair it, but I like a challenge so I bought it anyways.I took the saw apart and found two components on the board that have failed, which I later found out are MOSFETs. I've done a little reading online to get a basic understanding of what a MOSFET is, I don't quite understand what it does, but I know it has a drain, gate and source. I guess my biggest question is- what would cause this to fail? And if I replace it, will it fail again?I did a quick google search with the writing on the MOSFET, which reads "F1404ZS". I've found two similar parts and need to know which ones will be the best option for me. Below is a link two the parts I have found:https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-fets-mosfets-single/278?k=F1404ZS&k=&pkeyword=F1404ZS&sv=0&pv7=2&pv1989=0&sf=0&quantity=&ColumnSort=0&page=1&pageSize=25The two parts I have listed are made by Infineon, are there other brands I should be looking at? I didn't know how to filter down my options, because there are a lot of numbers and information on these MOSFETs and I really don't know what most of it means.I have also attached some photos of the broken part. The writing reads F1404ZS, International Rectifier Symbol, 016P, TC, 3W

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2019, 09:10:47 pm
content: Yeah, that's the transistor. IR was picked up by Infineon, so IRF parts are now them.Beware that, when a MOSFET fails, it usually fails as a three-way short. This puts full load voltage on the gate driver, taking it out, and so on...Those things may have survived, since this is merely battery voltage. Mind, 18V may be above the drive/control circuitry's 5 or 10V operating or rated voltage, so who knows. Using a DMM on ohms and diode range, probe around. You're looking for resistors that are obviously not the value they're supposed to be (~ohms to kohms would be nearby), diodes and transistors and ICs that are supposed to have diode drops in one direction but read low-ohms or shorted instead, or completely open.Typically, a toasted switching supply has power parts that failed due to a shorted load, or overheating, or some kind of electrical or mechanical or thermal failure in the circuit. Those power parts in turn take out the drive circuitry, and maybe some supply or limiting or fusing between them and the battery. In all, there may be a dozen or so parts that need replacing. Between that, and the potting probably used?, you can see why module replacement is better...So on that note... I suppose you can't find a replacement module, or they cost the same as a new tool? Yeah, that's pretty standard in the last couple decades... If nothing else, you can wire up the motor to a power supply and use it not-cordlessly. Well, if it's not a BLDC motor and controller. In that case, it's still a few bucks of scrap?...Tim

author: KDoug
author_rank: Contributor
timestamp: April 07, 2019, 09:17:50 pm
content: Below are the photos.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2019, 09:28:46 pm
content: Board doesn't look very healthy either. Can you even solder down anything new without the traces lifting off?Even if you make the connections -- lifted traces havefar, far lesscurrent capacity than bonded traces. Strongly suggest module replacement.Tim

author: KDoug
author_rank: Contributor
timestamp: April 07, 2019, 09:49:35 pm
content: I've already desoldered the two MOSFETs and cleaned the pads up a little with alcohol/acetone mixture. Yeah, one of the pads is coming up a little, not sure if that happened while I was trying to remove them or if it was already like that. Either way, the parts are pretty cheap, if I can get it working for occasional use ill be happy.

author: mr.fabe
author_rank: Regular Contributor
timestamp: April 08, 2019, 02:26:06 am
content: Milwaukee offers a 5 year warranty on their power tools. If you don't have a receipt, they determine warranty coverage based on the serial number date code. If your saw is 5 years or less... you should consider contacting them. I've had good luck having a used CL purchased impact driver replaced with no hassles. They even covered the shipping fees.

author: KL27x
author_rank: Super Contributor
timestamp: April 08, 2019, 03:19:58 am
content: +1 to everything T3sl4co1l posted.It's ok to change just the FET and try again, though. If the thing doesn't work anymore, then work your way back and hope you can find the driver and other parts.I've done a little reading online to get a basic understanding of what a MOSFET is, I don't quite understand what it does,There are a lot of specs on a FET, and most of them don't matter for this use. In this case, it's just a switch to turn the saw on/off. The main reason a physical thumb switch doesn't just turn the saw on/off, directly, is the circuit board has to also be able to cut power when the battery runs low to prevent the battery from being a single use item. And the circuit probably also switches off the power in case the motor draws too much current (like what would happen if you were to stall the blade in something), which could irreversibly damage the battery and/or the motor. For the circuit to do that, it uses the FET as a switch, since it can't flip a thumb switch like you can.If you get the idea that you could just trash the board and replace it with a physical thumb on/off thumbswitch and some common sense, I didn't say it. It would be real easy to overdraw the battery and damage it. It would be somewhat feasible to make your own protection circuit with low voltage cutout and reset as a learning experience, but it's highly unlikely the saw itself is worth that much time in the end.QuoteBrand names don't really matter, so long as you buy it from a regular component retailer and not eBay or Alibaba. Most of the specs don't matter to you other than1. Correct package. Same package as the one you removed from the board, whether that's a TO220 or Dpak or whatever it is.2. Rds on, resistance between drain and source.3. Max voltage of 40V. Don't go too much lower. I bet 30V would work. You could go higher, but it would probably results in worse specs for no reason. 30-40V is fine.4. Peak current draw. The peak is going to be pretty high in any inductive load, like this. Stick with 750A peak minimum.The constant/static current draw is also important, but take that with a grain of salt. What you really care about is the package is the same one that came off the PCB and that the Rds is about the same as the original. If the constant/static current draw is rated higher than the original part, but the Rds is also higher, then it is not going to be "better" and it may die, prematurely.You could go a bit smaller/lower on the Rds than the original without too much issue, but that might affect the cutout voltages of the protection circuitry.Most of the other specs shouldn't really matter for this purpose. I'd ignore every other spec and just sort by price.

author: KDoug
author_rank: Contributor
timestamp: April 08, 2019, 04:59:31 am
content: Yes, I had thought about the warranty. Wasn't sure what it would cost for shipping there and back, which was my only hesitation. A new electronics/switch assembly is $30 for those wandering.

author: KDoug
author_rank: Contributor
timestamp: April 08, 2019, 05:05:07 am
content: Awesome information KL27x!! That's just the kind of input a newbie was looking for. So if replace the MOSFETS and it still doesn't work, what are some other components to start looking at? I believe its a D2PAK, it has three tabs and one of them wasn't soldered to anything. Is that correct?

author: KDoug
author_rank: Contributor
timestamp: April 08, 2019, 06:48:09 pm
content: Okay, so the only difference I see in the two parts I posted above are:Current - Continuous Drain (Id) @ 25°C:IRF1404ZSTRLPBF = 180A (Tc)AUIRF1404ZSTRLCT-ND = 160A (Tc)Can someone explain what this is, and which option would be better?

author: KDoug
author_rank: Contributor
timestamp: April 22, 2019, 09:14:47 pm
content: I forgot to provide an update with what I ended up doing. I got my parts in from Digikey a little over a week ago and soldered the new MOSFETs on. To my surprise the saw actually runs, but I'll have to use it on a few projects before I can confirm it's really fixed and runs like it should. Thanks for everybody that replied and gave me advice.

author: KDoug
author_rank: Contributor
timestamp: April 24, 2019, 02:08:55 pm
content: Here's a picture:

author: CJay
author_rank: Super Contributor
timestamp: April 24, 2019, 02:56:38 pm
content: Nice job!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2019, 06:30:14 pm
content: Cool, a successful fix.Tim