FET gate waveform

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.

Thanks. I should have the FET Hammy suggested tomorrow. If that doesn't work well enough I'll try the snubber idea.

Full ground plane for this area. I'll boost the track to the SW and boost pins.

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.

Main concern is between boost cap and pins, the rest is not that critical. Same for the VCC bypass caps.

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.

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 :-)

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.

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

Exhibit 2 running off 30V

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.

It is worth adding that the temperature of the mosfet suggests that the apparent voltage across it is real. It runs approximately 20°C warmer on

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.

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.

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.

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 1000µF 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 r> Raven>> >>

You mean you've not designed according to the datasheet worst case?

Naughty boy...

A nice linear slope? That really has me puzzled then.

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.

There should also be some ceramics across C16 and C21.

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 ;-)

No, that I did but then things stayed a lot cooler than planned, a good thing.

Mom said that to me on occasion ...

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

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?

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.