Need a TO220 style FET with the Drain/Source pins swapped - Page 1
Forum section: electronics / projects

author: cravenhaven
author_rank: Contributor
timestamp: July 26, 2014, 02:06:33 pm
content: I've managed to design a PCB with the FET Drain and Source pins swapped. The only solution I can come up with ATM is to cut the tracks and resolder the correct orientation. Is there any case style vaguely similar to TO220 that has Gate-Source-Drain orientation of the pins?.There are 5 FETS and they need to be logic level at 3.3V and handle 4 amps at low voltage.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 26, 2014, 05:43:03 pm
content: It's really the nature of the physical construction that give the standard pin-out. I doubt you'll find anything in TO-220 or similar with a different pin-out. Is there any little "extra" PCB you could make that would solve the problem? That would be easier than cutting traces, the usual remedy for such errors.

author: dannyf
author_rank: Super Contributor
timestamp: July 26, 2014, 06:39:01 pm
content: I haven't seen anything like that for power gets - some RF mosfets do offer swapped pin out arrangements.I would have just used isolation, like plastic tubes, to swap the legs while retaining the PCB.

author: SeanB
author_rank: Super Contributor
timestamp: July 26, 2014, 07:06:21 pm
content: You probably could get the devices with the swapped pins if you went to a manufacturer and paid them to make a new leadframe punch set for it. It will be simple to do at manufacture as they simply move the connection to the back of the package to the side and cut the existing centre to provide the isolation then simply program the wire bonder to do the change. Only hassle would be the initial cost and the MOQ of around 10k units.Other than that all non RF devices I have seen are going to conform to the industry standard pinout. For a RF device they might do that to reduce parasitic capacitance to the connections a little and obviate the need for a thermal insulating washer with an isolated package.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2014, 07:12:42 pm
content: Use a P channel...

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: July 26, 2014, 07:15:36 pm
content: not "logic level" but some Lateral Mosfet used in audio power amps have GSD pinoutyou can also mangle the Q pins rather than the PCB - don't bend the leads right at the package body though

author: AndyC_772
author_rank: Super Contributor
timestamp: July 26, 2014, 07:35:29 pm
content: Crop the source and drain pins, then solder insulated wires between the stubs of the pins and the PCB so they cross over.

author: mikerj
author_rank: Super Contributor
timestamp: July 26, 2014, 07:44:41 pm
content: Is it possible to reverse the direction of the MOSFET on the PCB, i.e. rotate the body 180 degrees? That could get the Drain and Source aligned, with only a short single wire needed for the Gate. Obviously it's not going to happen if it's bolted to a big heatsink.

author: BeltAndBraces
author_rank: Newbie
timestamp: July 26, 2014, 08:02:24 pm
content: I would probably stick with Conrad Hoffman's idea of making a little adapter board to rotate the pinout. You could then just use three pin headers with the standard spacing to connect the two boards. If this only a one-off thing, a piece of perfboard would do just fine.This way you won't sacrifice the clearance of the pinout and the reliability of the package. Because really, soldering wires to component pins is kinda messy...The only issue I see is if you have the TO-220 bolted onto a heatsink. Then the extra 3 or so millimeters spacing might just be a little too much.

author: Smokey
author_rank: Super Contributor
timestamp: July 26, 2014, 08:20:53 pm
content: How many PCBs did you have made before you built up a first article and tested it? If it's a small quantity and you are willing to make an adapter PCB, then you should be willing to make a new revision of the main board fixing the issues that require the adapter board.Finish assembling one PCB with cuts and jumpers to make it work, note all (other) changes and then respin the board fixing all the mistakes.If you are just looking for one working unit for yourself, you can stop here, but it sounds like you want to make a bunch of these.Toss out the original remaining PCBs with the backwards pattern, and move on from there. Everyone that has ever laid out a PCB has gotten a package backwards at some point.

author: cravenhaven
author_rank: Contributor
timestamp: July 26, 2014, 09:28:34 pm
content: Thanks all for replying.Its only a one-off board so I wont be remanufacturing as the PCBs are so expensive to get made, $80 for just one 4"x 3" single sided.I dont have the space to cross the legs over in the physical design so I think the suggestion to use a surface mount FET mounted on some veroboard might be the go.Another alternative I thought of was a FET in a TO92 case if there is one with the current carrying capability. Anyone know of one?.

author: Smokey
author_rank: Super Contributor
timestamp: July 26, 2014, 09:49:33 pm
content: How many watts is it dissipating?Be careful with power device specs on the data sheets. They are usually rated assuming the devices are connected to something like a heatsink that can get the heat out as fast as the device can generate it. Freestanding TO-220s, or even connected to a little freestanding heatsink, usually won't meet their rated spec for peak power. For surface mount like D-PAK you are relying on the PCB as being the heatsink, so layout accordingly.

author: sfiber
author_rank: Regular Contributor
timestamp: July 26, 2014, 10:21:57 pm
content: I recommend to you making modificatin on circuit,it will be ugly but it works if you do it correctly.

author: cravenhaven
author_rank: Contributor
timestamp: July 27, 2014, 08:35:44 am
content: Quote from: Smokey on July 26, 2014, 09:49:33 pmIf I drive it correctly it will be dissipating very little heat. I want it to operate as a switch rather than an amp so provided the device is capable of handling 4 amps continuously and has a low forward resistance it should be fine. The devices I originally intended run perfectly cold.

author: calexanian
author_rank: Super Contributor
timestamp: July 27, 2014, 07:51:08 pm
content: Some parts can be ordered with an "R" after the number for reversed. Try adding that on a parts search.

author: rob77
author_rank: Super Contributor
timestamp: July 28, 2014, 12:45:44 am
content: if it's a one-off, then i have an idea for you1. cut a narrow stipe of double-sided FR4 copper clad2. bend the D & S pins outwards a bit3. slide in the FR4 stip4. solder the leads to the fr45. cut off the excess fr4 strip6. cut the D & S leads7. solder new D & S leads to the correct positionsee the attached picture , i know it's ugly - you should make it much more pretty.. i just wanted to demonstrate the idea (btw... it was a blown mosfet).