FET gate waveform
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From: MooseFET on 19 Feb 2010 09:20 On Feb 19, 2:10 am, Raveninghorde <raveninghorde(a)invalid> wrote: > On Thu, 18 Feb 2010 18:40:30 -0800 (PST), MooseFET > > > > <kensm...(a)rahul.net> wrote: > >On Feb 18, 6:55 am, Raveninghorde <raveninghorde(a)invalid> wrote: > >> I'm back looking at an on going project. > > >>http://www.zen88234.zen.co.uk/design/Output-2.png > > >> I am investigating the dissipation of Q2 the high side FET. The > >> designers prototype (20 miles away) has an IRLZ44Z. My prototype gas > >> an 2SK3704 fitted. > > >> The temperature rise is about the same for both FETs though I expected > >> about a 10% improvement with the 2SK3704. > > >> The heatsink is an SW50-4 with a theta of 8.6C per watt. I calculate a > >> FET dissipation of under 2.5W using the methodology in the LM3150 > >> datasheet. > > >> The heatsink rises by about 56C after about 20 minutes at 12V 8A > >> running at 200kHz. This T rise drops to 47C at 140kHz. > > >> So I scoped the gate drive with the scope grounded on the source of > >> Q2. I got this: > > >>http://www.zen88234.zen.co.uk/photos/IMG_0195.JPG > > >> I repeated the measurement with my old Iwatsu analog scope and got the > >> short negative pulse after 100ns but it only goes down to -1V instead > >> of -3V. > > >> As a check I scoped the probe's ground point and there is very little > >> signal there so the probe isn't picking much radiated noise. > > >> The designer (bro in law) has tried the same measurement using his > >> Tektronix scope and gets a pulse down to +1V. > > >> Clearly there is a gate drive issue of some sort. > > >> The IRLZ44Z has a max Rdson of 0.02 at 25C and 5V. The typical gate > >> drain charge is 12nC. > > >> The 2SK3704 has a max Rdson of 0.021 at 25C and 4V. The typical gate > >> drain charge is 10nC. > > >What is the MAX voltage? > > 35V nominal in. I'm allowing 40V max. > > > > >Did you look at the HUF77545S3S? It conducts a lot better with only > >5V > >on the gate. > > Can't find that part number. I assume it's Fairchild. The only similar > part numbers I've found so far are specified at 10V Vgs. http://www.fairchildsemi.com/pf/HU/HUF75545S3S.html
From: Raveninghorde on 19 Feb 2010 10:20 On Fri, 19 Feb 2010 06:20:18 -0800 (PST), MooseFET <kensmith(a)rahul.net> wrote: >On Feb 19, 2:10�am, Raveninghorde <raveninghorde(a)invalid> wrote: >> On Thu, 18 Feb 2010 18:40:30 -0800 (PST), MooseFET >> >> >> >> <kensm...(a)rahul.net> wrote: >> >On Feb 18, 6:55�am, Raveninghorde <raveninghorde(a)invalid> wrote: >> >> I'm back looking at an on going project. >> >> >>http://www.zen88234.zen.co.uk/design/Output-2.png >> >> >> I am investigating the dissipation of Q2 the high side FET. �The >> >> designers prototype (20 miles away) has an IRLZ44Z. My prototype gas >> >> an 2SK3704 fitted. >> >> >> The temperature rise is about the same for both FETs though I expected >> >> about a 10% improvement with the 2SK3704. >> >> >> The heatsink is an SW50-4 with a theta of 8.6C per watt. I calculate a >> >> FET dissipation of under 2.5W using the methodology in the LM3150 >> >> datasheet. >> >> >> The heatsink rises by about 56C after about 20 minutes at 12V 8A >> >> running at 200kHz. This T rise drops to 47C at 140kHz. >> >> >> So I scoped the gate drive with the scope grounded on the source of >> >> Q2. I got this: >> >> >>http://www.zen88234.zen.co.uk/photos/IMG_0195.JPG >> >> >> I repeated the measurement with my old Iwatsu analog scope and got the >> >> short negative pulse after 100ns but it only goes down to -1V instead >> >> of -3V. >> >> >> As a check I scoped the probe's ground point and there is very little >> >> signal there so the probe isn't picking much radiated noise. >> >> >> The designer (bro in law) has tried the same measurement using his >> >> Tektronix scope and gets a pulse down to +1V. >> >> >> Clearly there is a gate drive issue of some sort. >> >> >> The IRLZ44Z has a max Rdson of 0.02 at 25C and 5V. The typical gate >> >> drain charge is 12nC. >> >> >> The 2SK3704 has a max Rdson of 0.021 at 25C and 4V. The typical gate >> >> drain charge is 10nC. >> >> >What is the MAX voltage? >> >> 35V nominal in. I'm allowing 40V max. >> >> >> >> >Did you look at the HUF77545S3S? �It conducts a lot better with only >> >5V >> >on the gate. >> >> Can't find that part number. I assume it's Fairchild. The only similar >> part numbers I've found so far are specified at 10V Vgs. > >http://www.fairchildsemi.com/pf/HU/HUF75545S3S.html I may have missed something but the Rdson is only specified at 10V. And it has Qgd of 43nC which given the drive capabiltiy of the LM3150 looks too high.
From: MooseFET on 19 Feb 2010 11:37 On Feb 19, 7:20 am, Raveninghorde <raveninghorde(a)invalid> wrote: > On Fri, 19 Feb 2010 06:20:18 -0800 (PST), MooseFET > > > > <kensm...(a)rahul.net> wrote: > >On Feb 19, 2:10 am, Raveninghorde <raveninghorde(a)invalid> wrote: > >> On Thu, 18 Feb 2010 18:40:30 -0800 (PST), MooseFET > > >> <kensm...(a)rahul.net> wrote: > >> >On Feb 18, 6:55 am, Raveninghorde <raveninghorde(a)invalid> wrote: > >> >> I'm back looking at an on going project. > > >> >>http://www.zen88234.zen.co.uk/design/Output-2.png > > >> >> I am investigating the dissipation of Q2 the high side FET. The > >> >> designers prototype (20 miles away) has an IRLZ44Z. My prototype gas > >> >> an 2SK3704 fitted. > > >> >> The temperature rise is about the same for both FETs though I expected > >> >> about a 10% improvement with the 2SK3704. > > >> >> The heatsink is an SW50-4 with a theta of 8.6C per watt. I calculate a > >> >> FET dissipation of under 2.5W using the methodology in the LM3150 > >> >> datasheet. > > >> >> The heatsink rises by about 56C after about 20 minutes at 12V 8A > >> >> running at 200kHz. This T rise drops to 47C at 140kHz. > > >> >> So I scoped the gate drive with the scope grounded on the source of > >> >> Q2. I got this: > > >> >>http://www.zen88234.zen.co.uk/photos/IMG_0195.JPG > > >> >> I repeated the measurement with my old Iwatsu analog scope and got the > >> >> short negative pulse after 100ns but it only goes down to -1V instead > >> >> of -3V. > > >> >> As a check I scoped the probe's ground point and there is very little > >> >> signal there so the probe isn't picking much radiated noise. > > >> >> The designer (bro in law) has tried the same measurement using his > >> >> Tektronix scope and gets a pulse down to +1V. > > >> >> Clearly there is a gate drive issue of some sort. > > >> >> The IRLZ44Z has a max Rdson of 0.02 at 25C and 5V. The typical gate > >> >> drain charge is 12nC. > > >> >> The 2SK3704 has a max Rdson of 0.021 at 25C and 4V. The typical gate > >> >> drain charge is 10nC. > > >> >What is the MAX voltage? > > >> 35V nominal in. I'm allowing 40V max. > > >> >Did you look at the HUF77545S3S? It conducts a lot better with only > >> >5V > >> >on the gate. > > >> Can't find that part number. I assume it's Fairchild. The only similar > >> part numbers I've found so far are specified at 10V Vgs. > > >http://www.fairchildsemi.com/pf/HU/HUF75545S3S.html > > I may have missed something but the Rdson is only specified at 10V. > And it has Qgd of 43nC which given the drive capabiltiy of the LM3150 > looks too high. Look in the curves at the current vs drain voltage for given gate drives for the two. I think the part I gave does better.
From: Hammy on 19 Feb 2010 12:18 On Fri, 19 Feb 2010 15:20:51 +0000, Raveninghorde <raveninghorde(a)invalid> wrote: >I may have missed something but the Rdson is only specified at 10V. >And it has Qgd of 43nC which given the drive capabiltiy of the LM3150 >looks too high. TH leaves you with pretty slim pickings but here are a couple speced at 5 or 6 Vgs. http://www.fairchildsemi.com/ds/FD%2FFDP5800.pdf http://canada.newark.com/fairchild-semiconductor/fdp5800/transistor/dp/30M0749 http://canada.newark.com/pdfs/datasheets/Fairchild/FDB14AN06LA0.pdf http://canada.newark.com/fairchild-semiconductor/fdp14an06la0/mosfet/dp/84H4572 http://canada.newark.com/pdfs/datasheets/Fairchild/FDP5645.pdf http://canada.newark.com/fairchild-semiconductor/fdp5645/mosfet/dp/58K8848
From: Joerg on 19 Feb 2010 14:58
Raveninghorde wrote: > I'm back looking at an on going project. > > http://www.zen88234.zen.co.uk/design/Output-2.png > > I am investigating the dissipation of Q2 the high side FET. The > designers prototype (20 miles away) has an IRLZ44Z. My prototype gas > an 2SK3704 fitted. > > The temperature rise is about the same for both FETs though I expected > about a 10% improvement with the 2SK3704. > > The heatsink is an SW50-4 with a theta of 8.6C per watt. I calculate a > FET dissipation of under 2.5W using the methodology in the LM3150 > datasheet. > > The heatsink rises by about 56C after about 20 minutes at 12V 8A > running at 200kHz. This T rise drops to 47C at 140kHz. > > So I scoped the gate drive with the scope grounded on the source of > Q2. I got this: > > http://www.zen88234.zen.co.uk/photos/IMG_0195.JPG > > I repeated the measurement with my old Iwatsu analog scope and got the > short negative pulse after 100ns but it only goes down to -1V instead > of -3V. > > As a check I scoped the probe's ground point and there is very little > signal there so the probe isn't picking much radiated noise. > > The designer (bro in law) has tried the same measurement using his > Tektronix scope and gets a pulse down to +1V. > > Clearly there is a gate drive issue of some sort. > > The IRLZ44Z has a max Rdson of 0.02 at 25C and 5V. The typical gate > drain charge is 12nC. > > The 2SK3704 has a max Rdson of 0.021 at 25C and 4V. The typical gate > drain charge is 10nC. > Since you are in dire need to get a demo prototype out, some more ideas: You could try a snubber from Q2 source to GND. Maybe at least some of the ring-out can be muffled. It may not really boost efficiency much but at least move some dissipation away from the FETs. The snubber resistor needs to be able to stomach that. Ok, I know that is sort of a last resort sledge hammer method in this case. This paper might help a bit: http://etd.uwaterloo.ca/etd/y6chen2005.pdf Other: Does this board have a full ground plane? If not can you kludge at least a local one around the LM3150? Is the line to the SW pin nice and fat and of low inductance? If needs to be so the bootstrap cap has something to "lean onto". -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |