Assistance needed in determining correct mosfet to use - Page 1
Forum section: electronics / projects

author: aries1470
author_rank: Regular Contributor
timestamp: December 03, 2016, 02:57:25 pm
content: Hi everyone, I just wanted to confirm if the following mosfets will work, since I want to do a “one fits all” design, using just replacement parts.I want to build an “input agnostic” power supply.It will be based on the following IC from Linear:LT4320Input supply from 7V – 36V AC/DC using a 2.1mm barrel plug.So, can someone please confirm if the following mosfets are suitable:The design will be made in two variations, one SMT (MSOP) at some point in the future, and the other THT.It will be for my own personal use and then I will share it once I have it workingThe SMT will only be used for up to 1Amp continuousThe mosfet I chose, is theIRLML0040(SOT23). Will this work?For the voltage output, the following chip will be used:LT3988It should not reach the total of 2Amps, i.e. 1Amp each rail, but around 100mA – 500mA tops on each rail.The THT will use any of the following, depended on power need (Iout) :IPP040N06Nor similar but cheaperhereIPP060N06NIRFIZ24NIf someone can suggest a TO-92 package for up to 1-2amps… or other TO220 with less amperage, that would be great, as I am not sure at what characteristics I should be looking for.2SK975TZ-Eor the same one instockwould this work?or will it be a. If this TO-92 does work, that mean I will have one less component to worry about chaning overbut will I need aTO-92 heatsinkfor it?As for the voltage regulator, it will be either of these:Single output options will be 5V and then a 3V3 linear.LTC3118MAX5090MAX50351A THT availableLM2574500mA PDIPLM25751A PDIPLM25763A TO-220-5Come to think of it, I just want the “beefy” parts for a “just in case” future scenario, so up to 1A is adequate for me at the moment, and in the future, I will consider upping it more :-)Just thinking of using this method instead of 4 diodes, of which shouldn't be an issue for low output current, but if I need more power or make what I want to use it for in the end, like for driving motors, and not worrying about any power adaptor if it will be suitable and what type it is and polarity etc, just thought to use this design.Thank you to all in advance.

author: madires
author_rank: Super Contributor
timestamp: December 03, 2016, 03:36:08 pm
content: The IRLML0040's V_DSS of 40V might be a tad low for the input voltage. For the THT I'd choose a TO-220 because the TO-92s usually got a high R_DS_on.

author: aries1470
author_rank: Regular Contributor
timestamp: December 05, 2016, 12:03:17 am
content: Quote from: madires on December 03, 2016, 03:36:08 pmThank you for pointing me in the right directionI re-read the datasheet and got a better understanding, and I believe I have found the best "comprimise" between price/ performance, that is theToshiba TK100A06N1. It has a RDSON of 0.0027 ?.So, IF I understood p.7 correctly, doing the math that would mean:30mV/1A = 30m?. So this one being 27m? will suit the bill perfectly for up to 1 Amp.Also being a 60V / 100A part, means I have plenty of headroom to wiggle.Now, if I remember math correctly ( yeah, I didn't like the teachers much that delivered math except save for one ), that would mean1A * 30 m? = 30mV. That would be the drop out per mosfet. Now, since it is 27m?, that would mean 27mV per Amp. So if I go to 2Amp in the future, it will 54mV or 3A would be 81mV, so still way less voltage drop than using even Schottky diodes.Going more, means I will need to change my regulators... but then, it can be used as a moduleIf anyone sees any errors or something that I may have missed for choosing this mosfet, please speak up, as I will be creating a board for use with itOnce I do, I will post here again to share it and get scrutinisedSo, front end - independant board will consist of:4xTK100A06N11xLT4320and then it will output the DC voltage in a fixed manner in regards to polarity.Thank you.

author: madires
author_rank: Super Contributor
timestamp: December 05, 2016, 11:47:15 am
content: It's R = V / I, i.e. V = R * I. With R = 0.003 and I = 1 you get V = 3mV per MOSFET. But there's another point to pay attention to. To drive the MOSFET it needs charge on the gate, and the driver chip has a limited current source/sink capability. When the driver chip can't provide enough current for the gate, the MOSFET won't turn on (or off) fast enough. Beefy MOSFETs got a high Q_g (total gate charge), so they need more drive current for switching. In your case it would be better to go for a smaller one, maybe something like the IRFZ44N. There are a lot of application notes about calculating the drive current for MOSFETs.

author: digsys
author_rank: Unknown
timestamp: December 05, 2016, 01:23:46 pm
content: I wouldn't worry about gate capacitance. We make 100s bridges using the LT4320, many up to 25A (max 40V). I use the AUIRFS-8409-7P.It has 13K5pF+ gate capacitance and the IC runs it fine. I've even connected 2 in parallel and it still drove it ok. The FETs are soldered directly to a 4oz PCB,with NO other heat-sink. The Flir says they're lucky to get to 60oC on a bad day :-) These have an rds_on of 0.55mR. On some units I use a clip on Al heat-sink,which is plenty.www.pbase.com/digsys/image/153385878www.pbase.com/digsys/image/153385879

author: aries1470
author_rank: Regular Contributor
timestamp: December 09, 2016, 01:49:29 pm
content: Thank you for your reply guys, very much appreciated.I will be using it for DC to 50/60Hz applications.I will be using THT initially and also as an option so it is easily serviceable by novices too.It is for a project I hope I can get off the ground@digsys Thank you for your designs, they have showed me other options and layouts too.@madires. I did just that, that is the reason for the delay, reading, and then some more reading, and I hope I have understood it right, as I read plenty of primers, especially from T.I.To follow: pcb and schematic for sharing in the next day or two at most, time permitting.Thank you.

author: madires
author_rank: Super Contributor
timestamp: December 09, 2016, 02:59:00 pm
content: I agree, zero crossing switching with 50/60Hz AC is quite forgivable regarding Q_g. If the MOSFETs stay a little bit longer in resistive mode it doesn't have a huge impact, since the voltage needs some time to rise too.

author: aries1470
author_rank: Regular Contributor
timestamp: December 10, 2016, 01:36:51 pm
content: Thank you for your patience everyone.Here is V1.0They are in PDF Format and PNG of the Schematic and Board.Let me know what you think

author: jram
author_rank: Contributor
timestamp: May 16, 2018, 10:12:32 pm
content: How did it turn out?