sub milli-ohm MOSFET or IGBT - Page 1
Forum section: electronics / projects

author: VintageNut
author_rank: Frequent Contributor
timestamp: December 26, 2017, 06:35:08 pm
content: I am performing a survey looking for a power device to switch high currents at low voltages. The idea is to be able to create a pulse in the 10 uS to 1 mS range where the voltage to the device under test will be 10V or less at several hundred amperes.So far, the only sub milli-ohm switching devices I have found are modules intended for three phase motor control.

author: wraper
author_rank: Unknown
timestamp: December 26, 2017, 06:44:45 pm
content: It's impossible for IGBTs which have over 1V forward voltage drop by their nature. You cannot measure IGBT in ohms, it's basically BJT on the output. Therefore they are good for high voltages only.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 26, 2017, 07:14:03 pm
content: Use a lot of MOSFETs in parallel. They're fairly cheap since they're commonly used in processor power supply circuits.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2017, 07:21:23 pm
content: Visit any reputable distributor, such as Digikey, RS Components or Farnel and use the parametric search function to sort MOSFETs in ascending RON.

author: parasole
author_rank: Contributor
timestamp: December 26, 2017, 08:24:36 pm
content: Sounds like a spot welding use case? As stated by NiHaoMike, a number of paralel mosfets should fit the bill... unless you want to make it small and fancy

author: matseng
author_rank: Frequent Contributor
timestamp: December 26, 2017, 09:10:12 pm
content: Quote from: VintageNut on December 26, 2017, 06:35:08 pmWouldn't any minor inductances in the wiring be a real problem when trying to switch a few hundred amps at speeds corresponding to 50 KHz?

author: slugrustle
author_rank: Frequent Contributor
timestamp: December 26, 2017, 09:50:46 pm
content: The Vishay SIRA20DP-T1-RE3 might be a good starting point. 0.58mOhm max Rds at Vgs = 10V. Max current for a 100us pulse is 500A with Vds = 10V, single shot, at 25 deg. C. This drops to just over 100A for a 1ms pulse under the same conditions.http://www.vishay.com/docs/76212/sira20dp.pdfhttps://www.digikey.com/product-detail/en/vishay-siliconix/SIRA20DP-T1-RE3/SIRA20DP-T1-RE3CT-ND/The IXYS IXTN660N04T4 has a much more generous SOA due to its packaging, hitting ~750A for a single 1ms pulse at 25 deg. C ambient. Man, this thing is a monster.http://ixapps.ixys.com/DataSheet/DS100728A(IXTN660N04T4).pdfhttps://www.digikey.com/product-detail/en/ixys/IXTN660N04T4/IXTN660N04T4-ND/If you end up paralleling smaller mosfets, I'm curious to know how one handles synchronization of the gate drives. Separate driver per mosfet?

author: VintageNut
author_rank: Frequent Contributor
timestamp: December 26, 2017, 10:20:38 pm
content: Quote from: slugrustle on December 26, 2017, 09:50:46 pmThank you for the very thoughtful reply. These are both great devices.The application is to characterize non-linear devices in the 10 milli-ohm range at up to 400A.The module looks very good at 0.85 milliohms. At 400A, the module would waste only 0.34V which is nice if using 4v across the 10 milli-ohm DUT. A 5V super-capacitor might be a good cap for powering the pulse. Have to look into the surge current capability of supercaps.

author: tszaboo
author_rank: Super Contributor
timestamp: December 26, 2017, 10:46:38 pm
content: Quote from: slugrustle on December 26, 2017, 09:50:46 pmNo, in fact, you would place the gates in parallel, with a small resistor on each one. And use a giga gate driver, like FAN3121 (9A) to switch them.

author: filssavi
author_rank: Frequent Contributor
timestamp: December 26, 2017, 10:51:49 pm
content: Quote from: matseng on December 26, 2017, 09:10:12 pmdepending on the desired rise/fall time it goes from not even a problem (assuming a half decent PCB layout and minimum wire/bussbar lenght) at low speeds (say anything from 0.5 uS upwards)to a real nightmare as rise and fall times decrease down to almost uncharted territories where modules and through hole packages are useless due to pin and bond wire inductances

author: texaspyro
author_rank: Super Contributor
timestamp: December 26, 2017, 11:09:49 pm
content: My CD spot welder uses 18 IRFP2907 TO-247 MOSFETs in parallel. Cap bank is 3.6F / 500 micro-ohm ESR. It can theoretically do around 20,000 amp pulses but is limited by lead inductance.The MOSFETS were matched for the gate voltage Vth where the channel resistance was 100 ohms.Gate driver is an obsolete DS0026 driver through a single 100 (?) ohm resistor. Another topology that works well is a one series resistor from the gate driver driving separate gate resistors to each FET. It was necessary to limit the gate rise time otherwise the switching current slope exceeded 9 giga-amps / second.https://endless-sphere.com/forums/viewtopic.php?f=2&t=2633&hilit=megadonkey&start=550#p280216https://endless-sphere.com/forums/viewtopic.php?f=2&t=2633&hilit=megadonkey&start=575#p282544

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2017, 11:03:46 pm
content: Quote from: matseng on December 26, 2017, 09:10:12 pmYes and that is a good reason to use multiple devices even if doing so is not strictly required. Package parasitics are roughly similar between different parts in the same package so using multiple lower power parts in parallel is advantageous as long as it does not conflict with a layout that has low parasitics. Even then, it might be possible to take advantage of a distributed transmission line layout.

author: VintageNut
author_rank: Frequent Contributor
timestamp: December 26, 2017, 11:16:31 pm
content: Quote from: filssavi on December 26, 2017, 10:51:49 pmThe initial idea is to use copper bus bars for the drain and source connections. The supply capacitors will be attached to the bus bars close to the FET. No PCB. The pulse width will be in the 1mS range to start so rise and fall time are not terribly important. Output pulse shape is also not terribly important. As long as current and voltage can be measured, then other issues are not important.

author: texaspyro
author_rank: Super Contributor
timestamp: December 26, 2017, 11:19:23 pm
content: Quote from: VintageNut on December 26, 2017, 11:16:31 pmWanna bet?

author: VintageNut
author_rank: Frequent Contributor
timestamp: December 26, 2017, 11:42:11 pm
content: Quote from: texaspyro on December 26, 2017, 11:19:23 pmRather than bet, I will lash it up and see how it behaves. I will be using a sourcemeter to drive the gate.

author: jbb
author_rank: Super Contributor
timestamp: December 27, 2017, 01:29:57 am
content: Quote from: blueskull on December 26, 2017, 11:56:17 pmOoh, that is a monster. But be careful with the gate drive...

author: texaspyro
author_rank: Super Contributor
timestamp: December 27, 2017, 01:48:16 am
content: Quote from: blueskull on December 27, 2017, 01:40:13 amAlas, many TVS diodes were turned to plasma when I was building my welder... Oh, and for some strange reason high power LEDs don't seem to like being driven 20,000 amp pulses.

author: filssavi
author_rank: Frequent Contributor
timestamp: December 27, 2017, 09:02:29 am
content: Quote from: blueskull on December 27, 2017, 01:40:13 amGaN hemt are fantastic devices they have truly astonishing switching characteristics, but not they are fragile they blow up by simply looking at them the wrong way li-poli/ion batteries are extremely reliable and easy to work with in comparison.As already said the gate is very very sensitive to an kind of overvoltage, also soldering can be quite a challenge and they are easily killed if soldered the wrong way (especially by hand)As for the layout if you want to use them as intended with extremely high switching frequencies is at the edge of black magic RF territory (a 1 ns edge has frequency content up to 350 MHz)