FET gate waveform
Raveninghorde
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.
Raveninghorde
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.
John Larkin
<raveninghorde@invalid> wrote:
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
MooseFET
Try putting a schottky across the lower MOSFET.
Replace C23
John Larkin
That might help. But if the schottky drop is enough to let the PN
substrate diode conduct a little, it may still snap.
John
Joerg
Now don't shoot the messenger. The LM3150 has an internal regulator to
6V, of course minus the drop of the bootstrap diode, minus whatever
other losses in there, see page 6:
http://cache.national.com/ds/LM/LM3150.pdf
That's a bit wimpy for a FET like this. It would be better to drive
those with more gusto, like 10-12V. I will never understand why
manufacturers put such lowish voltages on the gate drivers, makes no
sense at all.
The Schottky MooseFet suggested is also well advised.
Next time I suggest to get a better chip where you can bypass the
<expletive censored> internal LDO and feed it some real VCC. 12V or so.
LTC has some better ones there.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Raveninghorde
wrote:
I've already kicked my brother in law for choosing it. I didn't check
the data myself until we first had problems with a non logic level
FET.
I must admit I felt this was the easy part of the job and was more
concerned with the offline pre regulator.
As you say why does anyone limit the FET drive to under 6V.
Joerg
Don't kick him too often, your sister might not like that :-)
> I must admit I felt this was the easy part of the job and was more
> concerned with the offline pre regulator.
>
> As you say why does anyone limit the FET drive to under 6V.
Even logic-level is deceiving. Yeah, they switch alright under DC loads.
But dynamically, not. Ramp-up is more sluggish, costs efficiency and
some grief with heat. Then when Miller hits you have almost no reserves.
You could demonstrate the improvement if you'd roach in a bootstrappable
dual driver between LM3150 and FETs. Bypass it well enough. But in the
end this looks like a re-layout case :-(
Raveninghorde
wrote:
A re-layout is not a problem.
I'm just trying to get it cool enough so I can get the prototype out
in the box so the customer can evaluate. Once I've got an order for a
couple of hundred I can replace the LM3150 and fix the problem.
Joerg
Then the only option I see other than the Schottky fix is to find the
power FET with the lowest possible threshold and low enough Rdson.
Problem is, most of the modern stuff is going to be borderline for Vds
in your case and doesn't come in TO220 or TO247.
MooseFET
Give or take a foot, the charge in the "charge storage" is
proportional
to the current that flowed in the last storage-time before you try to
stop it. Reducing the current should make an obvious change if that is
the cause of the problem.
>
> John
MooseFET
What is the MAX voltage?
Did you look at the HUF77545S3S? It conducts a lot better with only
5V
on the gate.
Raveninghorde
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.
Raveninghorde
I put a schottky, 48CTQ060, in parallel with Q3. The waveforms looked
better but the input current didn't change from 2.98A for 8A out and
the temperature rise on Q2 was the same.
So I removed Q3 completely and just had the diode. The LM3150 had a
fit and refused to run properly. Looking at the data sheet for the
LM3150 the Rdson of the low side FET is part of the current limit
calculations, so that explains why Q3 can't be omitted.
At the moment I think the diode is not the main problem. So back to
looking at Q2.
Raveninghorde
wrote:
Yep. This coincides with the thread on heat sinking high powered smt
FETS. Bro in law chose TO220 in principle as heatsinking is "easy".
But the controller chip is wimpy and needs a FET with a low gate
charge. And as you say good modern stuff isn't available in through
hole packages.
MooseFET
Raveninghorde
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.
MooseFET
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.
Hammy
<raveninghorde@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
Joerg
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".
Raveninghorde
>On Fri, 19 Feb 2010 15:20:51 +0000, Raveninghorde
><raveninghorde@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
That looks good. Obsolete but Farnell have stock and if it works well
enough it get me out of a corner. Rdson of 14.6 milliohms max at 5V
and only 7.9nC Qgd.
If the LM3150 calculations are correct it should save about a third of
the FET dissipation.
Hammy
<raveninghorde@invalid> wrote:
>On Fri, 19 Feb 2010 12:18:47 -0500, Hammy <sp...@spam.com> wrote:
>
>>On Fri, 19 Feb 2010 15:20:51 +0000, Raveninghorde
>><raveninghorde@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
>
>That looks good. Obsolete but Farnell have stock and if it works well
>enough it get me out of a corner. Rdson of 14.6 milliohms max at 5V
>and only 7.9nC Qgd.
>
>If the LM3150 calculations are correct it should save about a third of
>the FET dissipation.
Newark / Farnell can be a bitch to search so I bookmark anything good
I find while searching around the site.:-)
I found this one STP55NF06L in my inventory and its dirt cheap half
the price of the other one. Not quite as good but not bad. It's not
obsolete; worth looking at particularly if you plan on doing the
supply in quantity.
http://canada.newark.com/stmicroelectronics/stp55nf06l/mosfet/dp/89K1626
Raveninghorde
wrote:
Thanks. I should have the FET Hammy suggested tomorrow. If that
doesn't work well enough I'll try the snubber idea.
>
>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".
Full ground plane for this area. I'll boost the track to the SW and
boost pins.
Joerg
Just don't hold your breath with that FET. I don't know why they state
such long rise/fall and delay times under "Switching Characteristics".
Maybe too much gate path resistance, doesn't look too promising but
definitely worth a shot anyhow.
>
>> 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".
>
> Full ground plane for this area. I'll boost the track to the SW and
> boost pins.
Main concern is between boost cap and pins, the rest is not that
critical. Same for the VCC bypass caps.
Hammy
wrote:
>
>Just don't hold your breath with that FET. I don't know why they state
>such long rise/fall and delay times under "Switching Characteristics".
>Maybe too much gate path resistance, doesn't look too promising but
>definitely worth a shot anyhow.
Those are some pretty beefy FETS. If you want low switching times
your going to have to pay. Even The HUF device you suggested has
similar switching characteristics for almost twice the price.
I'm going test that EL cheapo STP55NF06L and see just how fast it can
switch vs the data sheet.
Joerg
> On Fri, 19 Feb 2010 13:59:38 -0800, Joerg <inv...@invalid.invalid>
> wrote:
>
>
>> Just don't hold your breath with that FET. I don't know why they state
>> such long rise/fall and delay times under "Switching Characteristics".
>> Maybe too much gate path resistance, doesn't look too promising but
>> definitely worth a shot anyhow.
>
> Those are some pretty beefy FETS. If you want low switching times
> your going to have to pay. Even The HUF device you suggested has
> similar switching characteristics for almost twice the price.
>
I didn't suggest those, I suggested to re-do the thing with a better PWM
chip, meaning one that can pounce onto the gates with 10V or more :-)
> I'm going test that EL cheapo STP55NF06L and see just how fast it can
> switch vs the data sheet.
>
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.
[...]
Hammy
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.
Joerg
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.
Raveninghorde
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.
Andy
apparent voltage across it is real. It runs approximately 20�C warmer on
30V.
On 23/02/2010 22:14, Raveninghorde wrote:
<snip>
Jamie
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.
Andy
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>
>
Joerg
Wish I could but this is strange. On the 30V case plot you can see a
distinct run in from 0V drop to 1V, inside the ringout. It appears to be
a 50nsec slope and then "homes in" at 1V. I can't see anything with that
time constant in your circuit. Unless, of course, the inductor L5 sits
in saturation and the small bit of leakage inductance is responsible.
Could L5 be saturating?
Would be interesting to see the time base cranked up around this
ringout, and zoomed a bit. But even more important would be to know:
Does this phenomenon suddenly set one when you crank the input voltage
past a certain level or does it gradually rise to 1V while cranking up
from 15V in to 30V in?
Can you lift the source of Q2, put a few ten milliohms in there and
scope the current? The crank up the bench supply voltage and see if any
sudden pattern change happens there.
My hunch would be inductor core saturation but hard to say from the
distance. I'd check for that first.
Andy
mosfet. This adds ring to other waveforms, but does show a slope during
the on time that is consistent with the inductor being approximately its
specified value. It gets warm, not hot.
The 1V appears proportionally as the primary power supply voltage is
increased.
Unfortunately, the mosfet has been removed and replaced several times,
and further mods may make the pcb fail. I am reluctant to remove it to
add a resistor.
The pcb is 4-layer, so has almost unbroken ground and power planes, so
that supply impedance should not be an issue. There is decoupling with
two low ESR 1000�F electrolytics, and several X7R ceramic capacitors on
each side of the drive chip.
I will try to capture another waveform to show detail of the ringing.
Fred Bartoli
> Hammy wrote:
>> On Fri, 19 Feb 2010 13:59:38 -0800, Joerg <inv...@invalid.invalid>
>> wrote:
>>
>>
>>> Just don't hold your breath with that FET. I don't know why they
>>> state such long rise/fall and delay times under "Switching
>>> Characteristics". Maybe too much gate path resistance, doesn't look
>>> too promising but definitely worth a shot anyhow.
>>
>> Those are some pretty beefy FETS. If you want low switching times
>> your going to have to pay. Even The HUF device you suggested has
>> similar switching characteristics for almost twice the price.
>>
>
> I didn't suggest those, I suggested to re-do the thing with a better PWM
> chip, meaning one that can pounce onto the gates with 10V or more :-)
>
>
>> I'm going test that EL cheapo STP55NF06L and see just how fast it can
>> switch vs the data sheet.
>
> 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.
You mean you've not designed according to the datasheet worst case?
Naughty boy...
--
Thanks,
Fred.
Joerg
> I have also measured the current waveform in the drain of the upper
> mosfet. This adds ring to other waveforms, but does show a slope during
> the on time that is consistent with the inductor being approximately its
> specified value. It gets warm, not hot.
>
A nice linear slope? That really has me puzzled then.
> The 1V appears proportionally as the primary power supply voltage is
> increased.
>
Hmm, can only be two things. A saturating inductor (but you've excluded
that already) or maybe not enough gate drive level and Q2 going into a
wee oscillation your scope can't see.
> Unfortunately, the mosfet has been removed and replaced several times,
> and further mods may make the pcb fail. I am reluctant to remove it to
> add a resistor.
>
> The pcb is 4-layer, so has almost unbroken ground and power planes, so
> that supply impedance should not be an issue. There is decoupling with
> two low ESR 1000�F electrolytics, and several X7R ceramic capacitors on
> each side of the drive chip.
>
There should also be some ceramics across C16 and C21.
> I will try to capture another waveform to show detail of the ringing.
>
That would be good to see. Wish I was there. Then we could fix it and go
to the pub for a McEwan's Heavy afterwards ;-)
--
Joerg
No, that I did but then things stayed a lot cooler than planned, a good
thing.
> Naughty boy...
>
Mom said that to me on occasion ...
Jim Thompson
wrote:
Way back when I was designing off-line switchers (200W), they ran so
cool, I had no heat sinks (MJE13000... something or other)...
I grabbed a flag to see how cool... they WERE cool, but to the great
hilarity of the technicians, they also had around 400V P-P on them :-(
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Joerg
[...]
> Way back when I was designing off-line switchers (200W), they ran so
> cool, I had no heat sinks (MJE13000... something or other)...
>
> I grabbed a flag to see how cool... they WERE cool, but to the great
> hilarity of the technicians, they also had around 400V P-P on them :-(
>
A couple years ago I designed a laser thingie, switchers on there etc.
It ran off 12V and people could think it's innocent but since it
generated HV I had big fat warnings on there. In English and Spanish.
Guess who was bitten by it?
Raveninghorde
wrote:
>Andy wrote:
>> I have also measured the current waveform in the drain of the upper
>> mosfet. This adds ring to other waveforms, but does show a slope during
>> the on time that is consistent with the inductor being approximately its
>> specified value. It gets warm, not hot.
>>
>
>A nice linear slope? That really has me puzzled then.
>
>
>> The 1V appears proportionally as the primary power supply voltage is
>> increased.
>>
>
>Hmm, can only be two things. A saturating inductor (but you've excluded
>that already) or maybe not enough gate drive level and Q2 going into a
>wee oscillation your scope can't see.
>
>
>> Unfortunately, the mosfet has been removed and replaced several times,
>> and further mods may make the pcb fail. I am reluctant to remove it to
>> add a resistor.
>>
>> The pcb is 4-layer, so has almost unbroken ground and power planes, so
>> that supply impedance should not be an issue. There is decoupling with
>> two low ESR 1000湩 electrolytics, and several X7R ceramic capacitors on
There seems to have been some scope artifacts in the measurements
which were misleading.
The problem appears to be cross conduction due to the miller
capacitance of the lower FET switching it on as the upper FET switches
on.
The supply is 35V and the lower FET is logic level which is an
inherent requirement of the LM3150. So the lower FET has a Vgs of 1V
to 3V.
We fitted FAN3100T drivers running at 12V to both FETs and there was a
small reduction in losses. However the faster switching times are
aggrevating the cross conduction due to miller capacitance.
dagmarg...@yahoo.com
> On Tue, 23 Feb 2010 17:07:21 -0800, Joerg <inva...@invalid.invalid>
> wrote:
>
>
>
> >Andy wrote:
> >> I have also measured the current waveform in the drain of the upper
> >> mosfet. This adds ring to other waveforms, but does show a slope during
> >> the on time that is consistent with the inductor being approximately its
> >> specified value. It gets warm, not hot.
>
> >A nice linear slope? That really has me puzzled then.
>
> >> The 1V appears proportionally as the primary power supply voltage is
> >> increased.
>
> >Hmm, can only be two things. A saturating inductor (but you've excluded
> >that already) or maybe not enough gate drive level and Q2 going into a
> >wee oscillation your scope can't see.
>
> >> Unfortunately, the mosfet has been removed and replaced several times,
> >> and further mods may make the pcb fail. I am reluctant to remove it to
> >> add a resistor.
>
> >> The pcb is 4-layer, so has almost unbroken ground and power planes, so
> >> that supply impedance should not be an issue. There is decoupling with
> >> each side of the drive chip.
>
> >There should also be some ceramics across C16 and C21.
>
> >> I will try to capture another waveform to show detail of the ringing.
>
> >That would be good to see. Wish I was there. Then we could fix it and go
> >to the pub for a McEwan's Heavy afterwards ;-)
>
> >> On 23/02/2010 23:12, Joerg wrote:
> >>> Raveninghorde wrote:
Time for a little dead-time. An r-c + diode network could do that.
--
Cheers,
James Arthur
Raveninghorde
wrote:
>On Mar 16, 7:26�pm, Raveninghorde <raveninghorde@invalid> wrote:
>> On Tue, 23 Feb 2010 17:07:21 -0800, Joerg <inva...@invalid.invalid>
>> wrote:
>>
>>
>>
>> >Andy wrote:
>> >> I have also measured the current waveform in the drain of the upper
>> >> mosfet. This adds ring to other waveforms, but does show a slope during
>> >> the on time that is consistent with the inductor being approximately its
>> >> specified value. It gets warm, not hot.
>>
>> >A nice linear slope? That really has me puzzled then.
>>
>> >> The 1V appears proportionally as the primary power supply voltage is
>> >> increased.
>>
>> >Hmm, can only be two things. A saturating inductor (but you've excluded
>> >that already) or maybe not enough gate drive level and Q2 going into a
>> >wee oscillation your scope can't see.
>>
>> >> Unfortunately, the mosfet has been removed and replaced several times,
>> >> and further mods may make the pcb fail. I am reluctant to remove it to
>> >> add a resistor.
>>
>> >> The pcb is 4-layer, so has almost unbroken ground and power planes, so
>> >> that supply impedance should not be an issue. There is decoupling with
>> >> two low ESR 1000�F electrolytics, and several X7R ceramic capacitors on
There is about 100ns dead time and the gate of the bottom FET is 0V.
When the top FET turns on the gate of the bottom FET rises to about 5V
for about 15ns and then drops back to 0V.
We added an extra pnp pull down with schottky diode from base to
emitter between the driver and FET. The 5V signal comes from the FET
not the driver. Also the faster the gate rise time on the top FET the
bigger the signal on the lower FET gate.
Google found a few article sto explain the issue, such as:
http://powerelectronics.com/mag/507PET22.pdf
So I don't believe the dead time is the issue.
dagmarg...@yahoo.com
> On Wed, 17 Mar 2010 07:17:33 -0700 (PDT), dagmargoodb...@yahoo.com
I'm familiar with the problem but can't see the waveforms, so I
thought (hoped?) the lower FET's gate mightn't have enough time to
discharge fully. But, you say the low-side gate spikes up from 0v, so
I'm convinced--it's Miller, not dead time.
That article you linked covers the options: slow the slew rate,
stiffen the low-side driver, or split the low-side driver supply and
drive the low-side FET gate negative (so the Miller spike still can't
turn the FET on).
Given your external gate-driver, you could also switch low-side FETs
for one with a higher Vth, or lower Cdg, or both.
The low-side drive could also be stiffened with a emitter-follower
(ah, I see you did that. Did it work?), or a beefier FAN3xxx, I
s'pose.
Me? I'd shop low-side FETs first, and consider slowing the high-side
slew.
--
Cheers,
James Arthur
Raveninghorde
wrote:
We are modifying the board to drive the gate to -5V as shown in the
article. So a 5V spike will leave the FET gate at 0V. The FET worked
OK from the 6V drive of the LM3150. The FAN3100T is running on 12V so
even with the gate at -5V it will turn on well at +7V.
I'm on the road tomorrow so I'll know if it solves the problem or not
on Friday.
dagmarg...@yahoo.com
> On Wed, 17 Mar 2010 14:39:47 -0700 (PDT), dagmargoodb...@yahoo.com
Your fix sounds solid, but the glitch bothers me--I don't like
unexplained problems. The FAN3100 looks pretty husky. You'd think
it'd keep the low-side FET's gate low while the common FET node slews
high, Miller or no...
If the common FET node slews 1 V/nS, and the low-side FET has, say,
Cdg=1nF, that's only an amp, which the FAN3100 should handle.
Is the gate connection tight with the driver? Inductance in that
trace would explain your spike.
James Arthur
Joerg
Driving a gate negative is very good policy but here it's a bit like
sweeping the spike under the rug :-)
Some day a new batch of FETs might show some more Miller and the problem
creeps back in. The FAN3100 appears to be a bit wimpy, page 6 list its
output at 2.5A but at that current it only pulls down to VDD/2:
http://www.fairchildsemi.com/ds/FA%2FFAN3100C.pdf
Personally I prefer drivers with staunch CMOS outputs that really hold
things down. The MIC44xx series, for example.
> I'm on the road tomorrow so I'll know if it solves the problem or not
> on Friday.
Careful, everyone is driving on the wrong side of the road over there :-)
dagmarg...@yahoo.com
> Raveninghorde wrote:
That might help. OTOH, his emitter-follower should've been pretty
stiff. That's why I asked about trace inductance.
Any which way, Mr. Horde should have it whipped soon.
--
Cheers,
James Arthur
Joerg
Yeah, he knows his way around switchers, he'll get it going. I think the
first order of the day was to get this one prototype working so it can
be demo'ed, then he'd be free to re-design the thing with a better
switcher chip and so on.
I almost lost this thread. I changed the newsreader roll-off from 30
days to 60 days but somehow that didn't stick :-(
Raveninghorde
wrote:
At last the dissipation is looking reasonable. 22C rise on a 10C/W
heat sink for the high side FET. It was nearly 60C. I think there is
another 5C to be found but that's for the next version.
The low side FET gate is -5V when the FET is off. Switching to +7V
when on. This didn't completely fix it. The high side FET seems to
also suffer from miller capacitance switching it on when the low side
switches. So the high side FET is now held off with -4V.
These fixes meant the +12V supply feeding the FAN3100 FET drivers was
current limiting so we had to beef that up.
I had a look at the specs on the Micrel parts and while they had
higher drive currents than the FAN3100 I didn't spot a current drive
figure for holding down the GATE at near zero. The FAN3100 has FETS
across the bipolar drivers to hold down the gate turn on due to miller
capacitance - obviously they are not good enough.
Joerg
Great! So I take it that the demo was a success, you could take an order
for the next five million, swing by the LearJet dealer to put in an
order ...
> The low side FET gate is -5V when the FET is off. Switching to +7V
> when on. This didn't completely fix it. The high side FET seems to
> also suffer from miller capacitance switching it on when the low side
> switches. So the high side FET is now held off with -4V.
>
> These fixes meant the +12V supply feeding the FAN3100 FET drivers was
> current limiting so we had to beef that up.
>
> I had a look at the specs on the Micrel parts and while they had
> higher drive currents than the FAN3100 I didn't spot a current drive
> figure for holding down the GATE at near zero. The FAN3100 has FETS
> across the bipolar drivers to hold down the gate turn on due to miller
> capacitance - obviously they are not good enough.
>
It's a popular driver chip architecture these days but often the FETs
are a bit wimpy. I prefer the MIC44xx series, something with just FETs
in there but lots of gusto.