FET gate waveform
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From: Hammy on 19 Feb 2010 18:12 On Fri, 19 Feb 2010 14:59:59 -0800, Joerg <invalid(a)invalid.invalid> wrote: > >Good. I found that switching times in FET datasheets are often not very >dependable. I've had some that I could swing a lot faster. So it's >always best to try, maybe your FETs will indeed do the trick. I guess >we'll know soon if the UK has parcel service on Saturdays. > >[...] I take them with a grain of salt to:-) But now I am curious about that STP55NF06L. I might test it out after supper. Most of the money for R&D is going into SM parts not to many new TH fets coming out by comparison. Which is to bad because a TO package is cheaper and simpler for dissipating power when you have to.
From: Joerg on 19 Feb 2010 18:23 Hammy wrote: > On Fri, 19 Feb 2010 14:59:59 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > > >> Good. I found that switching times in FET datasheets are often not very >> dependable. I've had some that I could swing a lot faster. So it's >> always best to try, maybe your FETs will indeed do the trick. I guess >> we'll know soon if the UK has parcel service on Saturdays. >> >> [...] > > I take them with a grain of salt to:-) > > But now I am curious about that STP55NF06L. I might test it out after > supper. > > Most of the money for R&D is going into SM parts not to many new TH > fets coming out by comparison. Which is to bad because a TO package is > cheaper and simpler for dissipating power when you have to. That, and the stench from burned up FR-4 is almost worse that Mr.Stinky the skunk who visits our backyard once in a while. I've had cases here where clients asked me to investigate. Even when sent to me more than a week after the circuit board fire the lab and hallway would stink for the rest of the day. A heavily burdened D2PAK is a pretty likely candidate to cause black plumes and even flames. Soot everywhere. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Raveninghorde on 23 Feb 2010 17:14 On Fri, 19 Feb 2010 14:59:59 -0800, Joerg <invalid(a)invalid.invalid> wrote: SNIP > >Good. I found that switching times in FET datasheets are often not very >dependable. I've had some that I could swing a lot faster. So it's >always best to try, maybe your FETs will indeed do the trick. I guess >we'll know soon if the UK has parcel service on Saturdays. > >[...] We are missing something. We have tried 4 different types of FETs wiith no noticable difference. Here are a couple of waveforms. The output stage is now running of the bench PSU. Exhibit 1 running of 15V http://www.zen88234.zen.co.uk/photos/TEK0015.jpg Exhibit 2 running off 30V http://www.zen88234.zen.co.uk/photos/TEK0014.jpg The output is 12V 8A. The scope probe is grounded to positive on the bottom side of the board on the leg of the drain of the high side FET. The probe tip is connected to the source of the high side FET, again on the leg under the board. At 15V there is no significant voltage across the FET when switched on. On 30V you can see there is about 1V across the FET. This would appear to be the cause of the observed dissipation. The voltage drop does not appear to vary with load. It seems to be 1V at 100mA as well as 8A. Gate drive level is consistent at about 5.5V at all input voltages. The voltage drop across the FET seems to vary linearly with input voltage. There is no schottky across the low side FET at the moment. Various diodes have been tried up to a 48CTQ060 which should have a max forward voltage of 0.4V at 10A. The diode does not affect this problem but we will try it again when this is solved. At the moment I hope someone will say DOH, and point out the obvious cause of the problem.
From: Jamie on 23 Feb 2010 18:03 Raveninghorde wrote: > On Fri, 19 Feb 2010 14:59:59 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > > SNIP > > >>Good. I found that switching times in FET datasheets are often not very >>dependable. I've had some that I could swing a lot faster. So it's >>always best to try, maybe your FETs will indeed do the trick. I guess >>we'll know soon if the UK has parcel service on Saturdays. >> >>[...] > > > We are missing something. We have tried 4 different types of FETs > wiith no noticable difference. > > Here are a couple of waveforms. The output stage is now running of the > bench PSU. Exhibit 1 running of 15V > > http://www.zen88234.zen.co.uk/photos/TEK0015.jpg > > Exhibit 2 running off 30V > > http://www.zen88234.zen.co.uk/photos/TEK0014.jpg > > The output is 12V 8A. The scope probe is grounded to positive on the > bottom side of the board on the leg of the drain of the high side FET. > The probe tip is connected to the source of the high side FET, again > on the leg under the board. > > At 15V there is no significant voltage across the FET when switched > on. On 30V you can see there is about 1V across the FET. This would > appear to be the cause of the observed dissipation. > > The voltage drop does not appear to vary with load. It seems to be 1V > at 100mA as well as 8A. Gate drive level is consistent at about 5.5V > at all input voltages. The voltage drop across the FET seems to vary > linearly with input voltage. > > There is no schottky across the low side FET at the moment. Various > diodes have been tried up to a 48CTQ060 which should have a max > forward voltage of 0.4V at 10A. The diode does not affect this problem > but we will try it again when this is solved. > > At the moment I hope someone will say DOH, and point out the obvious > cause of the problem. Hmm., Ringing on the gate.. try driving the gate with a R, something in the neighborhood of 100 ohms or less. Just a guess, who knows. Also lowing the Q of the rails to the FET could also help.
From: Andy on 23 Feb 2010 18:04
It is worth adding that the dissipation is consistent with the voltage across the mosfet being real persisting for the entire on period. It runs approx 20�C hotter on 35V, at the same frequency and with a much shorter conduction time. On 23/02/2010 22:14, Raveninghorde wrote: <snip> > > We are missing something. We have tried 4 different types of FETs > wiith no noticable difference. > > Here are a couple of waveforms. The output stage is now running of the > bench PSU. Exhibit 1 running of 15V > > http://www.zen88234.zen.co.uk/photos/TEK0015.jpg > > Exhibit 2 running off 30V > > http://www.zen88234.zen.co.uk/photos/TEK0014.jpg > > The output is 12V 8A. The scope probe is grounded to positive on the > bottom side of the board on the leg of the drain of the high side FET. > The probe tip is connected to the source of the high side FET, again > on the leg under the board. > > At 15V there is no significant voltage across the FET when switched > on. On 30V you can see there is about 1V across the FET. This would > appear to be the cause of the observed dissipation. > > The voltage drop does not appear to vary with load. It seems to be 1V > at 100mA as well as 8A. Gate drive level is consistent at about 5.5V > at all input voltages. The voltage drop across the FET seems to vary > linearly with input voltage. > > There is no schottky across the low side FET at the moment. Various > diodes have been tried up to a 48CTQ060 which should have a max > forward voltage of 0.4V at 10A. The diode does not affect this problem > but we will try it again when this is solved. > > At the moment I hope someone will say DOH, and point out the obvious > cause of the problem. |