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MOSFET specs

Started by Sylvia Else September 20, 2017
I was looking for a high voltage MOSFET with a low drain source 
resistance. I was considering this:

http://au.rs-online.com/web/p/mosfet-transistors/8647925/

I cannot reconcile the information given there. Figure 3 indicates that 
at 25 degrees, an Id of 5.5 amps and Vgs of 10V will give an Rds of 
about 320 milliohms.

So go to Figure 8, and for the same conditions it gives an Rds of 1 ohm. 
So which is right?

Am I missing something here?

Sylvia.
On 20/09/2017 7:00 PM, Sylvia Else wrote:
> I was looking for a high voltage MOSFET with a low drain source > resistance. I was considering this: > > http://au.rs-online.com/web/p/mosfet-transistors/8647925/ > > I cannot reconcile the information given there. Figure 3 indicates that > at 25 degrees, an Id of 5.5 amps and Vgs of 10V will give an Rds of > about 320 milliohms. > > So go to Figure 8, and for the same conditions it gives an Rds of 1 ohm. > So which is right? > > Am I missing something here? > > Sylvia.
Actually, never mind, I've misinterpreted Figure 8. It's expressing Rds(on) as a proportion of its value at 25 degrees. Sylvia.
On Wednesday, September 20, 2017 at 5:00:30 AM UTC-4, Sylvia Else wrote:

FWIW: My present "favorite" FET = IRFP4668
http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf 

Rds = 8.0 mOhm
Id = 130 A
Vdss = 200 V

Later today, I'm planning to make some SSR's out of these.
On 21/09/2017 9:52 PM, mpm wrote:
> On Wednesday, September 20, 2017 at 5:00:30 AM UTC-4, Sylvia Else wrote: > > FWIW: My present "favorite" FET = IRFP4668 > http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf > > Rds = 8.0 mOhm > Id = 130 A > Vdss = 200 V > > Later today, I'm planning to make some SSR's out of these. >
Nice, but I do need the high Vdss unfortunately. Sylvia.
On Thu, 21 Sep 2017 04:52:16 -0700 (PDT), mpm <mpmillard@aol.com>
wrote:

>On Wednesday, September 20, 2017 at 5:00:30 AM UTC-4, Sylvia Else wrote: > >FWIW: My present "favorite" FET = IRFP4668 >http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf > >Rds = 8.0 mOhm >Id = 130 A >Vdss = 200 V > >Later today, I'm planning to make some SSR's out of these.
We're designing some SSRs now too. I was looking at FDD86367, 4.2 mohms, 80V, 100A, cheap. The tricky part is protecting them from customer abuse. We have a simple, cheap, transformer-coupled isolated gate driver circuit, basically a tiny flyback supply. The protection will be a combination of a bit of series resistance and a lot of FPGA code. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 22/09/2017 9:54 AM, Sylvia Else wrote:
> On 21/09/2017 9:52 PM, mpm wrote: >> On Wednesday, September 20, 2017 at 5:00:30 AM UTC-4, Sylvia Else wrote: >> >> FWIW: My present "favorite" FET = IRFP4668 >> http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf >> >> Rds = 8.0 mOhm >> Id = 130 A >> Vdss = 200 V >> >> Later today, I'm planning to make some SSR's out of these. >> > > Nice, but I do need the high Vdss unfortunately. > > Sylvia.
I'm also affected by the high reverse recovery charge that MOSFET body diodes seem to have. For example http://docs-asia.electrocomponents.com/webdocs/14f6/0900766b814f6568.pdf has a reverse recovery charge of 15uC. In a half-bridge situation, at, say 400V, and switching at 20kHz, if my understanding is correct, that creates a loss of over 100W in the high side switch, which is a lot of heat to get rid of even if it's otherwise acceptable. Been looking at IGBTs instead. Sylvia.
On Fri, 22 Sep 2017 13:39:09 +1000, Sylvia Else
<sylvia@not.at.this.address> wrote:

>On 22/09/2017 9:54 AM, Sylvia Else wrote: >> On 21/09/2017 9:52 PM, mpm wrote: >>> On Wednesday, September 20, 2017 at 5:00:30 AM UTC-4, Sylvia Else wrote: >>> >>> FWIW: My present "favorite" FET = IRFP4668 >>> http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf >>> >>> Rds = 8.0 mOhm >>> Id = 130 A >>> Vdss = 200 V >>> >>> Later today, I'm planning to make some SSR's out of these. >>> >> >> Nice, but I do need the high Vdss unfortunately. >> >> Sylvia. > >I'm also affected by the high reverse recovery charge that MOSFET body >diodes seem to have. For example > >http://docs-asia.electrocomponents.com/webdocs/14f6/0900766b814f6568.pdf > >has a reverse recovery charge of 15uC. In a half-bridge situation, at, >say 400V, and switching at 20kHz, if my understanding is correct, that >creates a loss of over 100W in the high side switch, which is a lot of >heat to get rid of even if it's otherwise acceptable. > >Been looking at IGBTs instead. > >Sylvia. >
A fat body-diode reverse recovery spike can blow out the gate, too. -- John Larkin Highland Technology, Inc lunatic fringe electronics
"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:f2jev0FishtU1@mid.individual.net...
> I'm also affected by the high reverse recovery charge that MOSFET body > diodes seem to have. For example > > http://docs-asia.electrocomponents.com/webdocs/14f6/0900766b814f6568.pdf > > has a reverse recovery charge of 15uC. In a half-bridge situation, at, say > 400V, and switching at 20kHz, if my understanding is correct, that creates > a loss of over 100W in the high side switch, which is a lot of heat to get > rid of even if it's otherwise acceptable. > > Been looking at IGBTs instead.
That, or if you can afford to use low enough Rds(on) that Vds(on) < Vf, you can prevent stored charge by synchronous rectification. You can also try with a large SiC schottky in parallel, which isn't quite good enough at DC, but will draw current while the body diode is in forward recovery. Recovery isn't power loss, but it can cause a lot of it. You need some way to deal with it. In the olden days (of SCRs, BJTs and slow diodes), they used saturable reactors to delay switching. Supply inductance is your friend here. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
John Larkin wrote...
> >> Sylvia Else wrote: >> >> FWIW: My present "favorite" FET = IRFP4668 >> http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf >> >> Rds = 8.0 mOhm >> Id = 130 A >> Vdss = 200 V >> >> Later today, I'm planning to make some SSR's out of these. > > We're designing some SSRs now too. I was looking at FDD86367, > 4.2 mohms, 80V, 100A, cheap. The tricky part is protecting > them from customer abuse.
Yes, a nice low Rds(on) for such a small DPak package. I'm also impressed by the FDMS039N08B in a SOIC-8P package.
> We have a simple, cheap, transformer-coupled isolated gate driver > circuit, basically a tiny flyback supply. The protection will be a > combination of a bit of series resistance and a lot of FPGA code.
Did you take a look at AoE III Fig 3.107, page 205? -- Thanks, - Win
On 1 Oct 2017 04:39:33 -0700, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>John Larkin wrote... >> >>> Sylvia Else wrote: >>> >>> FWIW: My present "favorite" FET = IRFP4668 >>> http://www.mouser.com/ds/2/196/irfp4668pbf-937691.pdf >>> >>> Rds = 8.0 mOhm >>> Id = 130 A >>> Vdss = 200 V >>> >>> Later today, I'm planning to make some SSR's out of these. >> >> We're designing some SSRs now too. I was looking at FDD86367, >> 4.2 mohms, 80V, 100A, cheap. The tricky part is protecting >> them from customer abuse. > > Yes, a nice low Rds(on) for such a small DPak package. > I'm also impressed by the FDMS039N08B in a SOIC-8P package. > >> We have a simple, cheap, transformer-coupled isolated gate driver >> circuit, basically a tiny flyback supply. The protection will be a >> combination of a bit of series resistance and a lot of FPGA code. > > Did you take a look at AoE III Fig 3.107, page 205?
Just did. In fig A, you might replace Q1 and Q2 with a dual optocoupler, and eliminate U2. Like this: https://www.dropbox.com/s/pwi91ext9cclhzz/Gate_Driver_3.JPG?raw=1 I considered something like that, but the PVs are wimpy, both voltage and average current. The flyback thing is nice; all the power we could ever want, dirt cheap. https://www.dropbox.com/s/ijc5e6l9i9jws15/V270_SSR.JPG?raw=1 I was going to use some of those self-protecting mosfets, but they have way too much personality. So we're going to measure the voltage across each SSR (which is its own story) and let our FPGA do the protections. I added a wirewound resistor in series with the fets to better define the switch resistance, to limit/measure current, and to take the joules on a fault. We will run a realtime thermal simulation in the FPGA and shut down based on the estimated temperature of the resistor. I'm fiddling with parts placement. https://www.dropbox.com/s/epcc6jpqip5vp5g/V270_Chans_1.jpg?raw=1 There are so many concerns (thermals, currents, voltage clearances) it's easier to do one channel myself than explain it to my layout guy. More fun than trekking out to Loews and repairing the deck, too. -- John Larkin Highland Technology, Inc lunatic fringe electronics