FET gate waveform
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From: Raveninghorde on 18 Feb 2010 09:55 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.
From: Raveninghorde on 18 Feb 2010 10:33 On Thu, 18 Feb 2010 14:55:56 +0000, 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. > It looks like the miller capacitance is driving down the gate as the FET switches on and the LM3150 lacks the drive to prevent this being an issue. I guess the difference between prototypes is the longer turn on delay of the 2SK3704.
From: John Larkin on 18 Feb 2010 10:44 On Thu, 18 Feb 2010 14:55:56 +0000, 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. > > Could be that the substrate diode of Q3 is acting like a step-recovery diode. When Q2 turns on, the Q3 diode looks like a short for roughly 50 ns, then abruptly opens up. That will make a monster positive spike at the D3 drain. It will also cause shoot-through current that will kill efficiency. I had a similar problem with an integrated synchronous switcher: ftp://jjlarkin.lmi.net/SwitcherRise.JPG You can see distinct phases: lower fet on lower fet off, substrate diode conducting upper fet on, fighting substrate diode reverse conduction... lots of current is building in parasitic inductances diode gives up, creating wicked spike Synchronous rectifiers are often not worth the hassle at output voltages and currents like this. Try shunting Q3 with a schottky, or replacing it entirely. John
From: MooseFET on 18 Feb 2010 11:13 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. Try putting a schottky across the lower MOSFET. Replace C23
From: John Larkin on 18 Feb 2010 11:57
On Thu, 18 Feb 2010 08:13:27 -0800 (PST), MooseFET <kensmith(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. > >Try putting a schottky across the lower MOSFET. That might help. But if the schottky drop is enough to let the PN substrate diode conduct a little, it may still snap. John |