mkogan wrote:
> > >
> >>> > >>>>> > >>>>>>> I have a MOSFET consistently blowing up. 550V part, 230V in, buck
> >>>>>>> configuration, transformer drive.
> >>>>>>> The symptom is always the same: g-s measures =A0short. What may b=
e
>>>>>>> causing it? Is it gate drive overvoltage?
> >>>>>>> Thanks
> >>>>>> If with 230V you mean it's running off of a 230VAC directly rectif=
ied
>>>>>> mains circuit then a 550V device is IMHO below marginal.
> >>>>>> --
> >>>>>> Regards, Joerg
> >>>>>>
formatting link
> >>>>>> "gmail" domain blocked because of excessive spam.
> >>>>>> Use another domain or send PM.
> >>>>> It's running off 230V DC, there is no AC in sight....- Hide quoted =
text -
>>>> Is there anything else running off the same 230V DC supply? It only
> >>>> takes one narrow switching spike to blow the gate oxide.
> >>>> --
> >>>> Bill Sloman, Nijmegen
> >>> It's a good question. There is a spark discharge generated by the sam=
e
>>> board. It sends electrical "tremors" everywhere (I have seen TDS3054
> >>> blink). I do not think that it causes the failure as I have been
> >>> working on this circuit for a while and have never seen shorted gate
> >>> before. The only recent change I can think of is using four (instead
> >>> of two) =A0MOSFETs (STP20NM50, if it matters) in parallel. One of the=
m
>>> develops g-s short very quickly. I do not see Vg-s coming even close
> >>> to (maximum) 20V. So I wonder what it takes to short gate oxide...
> >> =A0Does each one have its own gate resistor? =A0High-voltage
> >> =A0mosfets love to oscillate at 15 to 25MHz RF when they're
> >> =A0in the linear region. =A0Especially mosfets in parallel. =A0100V
> >> =A0drains swings are not uncommon during oscillation. =A0This
> >> =A0can happen during the 100ns, etc, of slow switching. =A0RF
> >> =A0oscillation can cause high voltages across the source
> >> =A0bond-wire inductance, and expose a mosfet gate-source
> >> =A0to damaging voltages. =A0Solutions are to switch faster, or
> >> =A0to add ferrite beads to the source pins, etc.
>
> > The MOSFET drive looks like Fig 2 here:
formatting link
o/appnotes/an-950.pdf
> with 1us (0.1uF, 10Ohm) HPF on the transformer primary side. No gate
> > resistors. The switching is indeed slow (~120ns rise-fall times). I
> > see nothing suspicious (like high frequency ringing) .... My records
> > show 10-20ns sampling rate, I would notice some waveform irregularity
> > caused by oscillation, wouldn't I?
> > I'll try to add ferrite beads to sources...
>
> --
> Regards, Joerg
>
>
e
formatting link
>
> "gmail" domain blocked because of excessive spam.
> Use another domain or send PM.> On Aug 6, 2:17 pm, Winfield Hill
>
>
>
> wrote:
> >> >
> >>> > >>>>> > >>>>>>> I have a MOSFET consistently blowing up. 550V part, 230V in, buck
> >>>>>>> configuration, transformer drive.
> >>>>>>> The symptom is always the same: g-s measures =A0short. What may b=
>>>>>>> causing it? Is it gate drive overvoltage?
> >>>>>>> Thanks
> >>>>>> If with 230V you mean it's running off of a 230VAC directly rectif=
ied
>>>>>> mains circuit then a 550V device is IMHO below marginal.
> >>>>>> --
> >>>>>> Regards, Joerg
> >>>>>>
formatting link
> >>>>>> "gmail" domain blocked because of excessive spam.
> >>>>>> Use another domain or send PM.
> >>>>> It's running off 230V DC, there is no AC in sight....- Hide quoted =
text -
>>>> Is there anything else running off the same 230V DC supply? It only
> >>>> takes one narrow switching spike to blow the gate oxide.
> >>>> --
> >>>> Bill Sloman, Nijmegen
> >>> It's a good question. There is a spark discharge generated by the sam=
e
>>> board. It sends electrical "tremors" everywhere (I have seen TDS3054
> >>> blink). I do not think that it causes the failure as I have been
> >>> working on this circuit for a while and have never seen shorted gate
> >>> before. The only recent change I can think of is using four (instead
> >>> of two) =A0MOSFETs (STP20NM50, if it matters) in parallel. One of the=
m
>>> develops g-s short very quickly. I do not see Vg-s coming even close
> >>> to (maximum) 20V. So I wonder what it takes to short gate oxide...
> >> =A0Does each one have its own gate resistor? =A0High-voltage
> >> =A0mosfets love to oscillate at 15 to 25MHz RF when they're
> >> =A0in the linear region. =A0Especially mosfets in parallel. =A0100V
> >> =A0drains swings are not uncommon during oscillation. =A0This
> >> =A0can happen during the 100ns, etc, of slow switching. =A0RF
> >> =A0oscillation can cause high voltages across the source
> >> =A0bond-wire inductance, and expose a mosfet gate-source
> >> =A0to damaging voltages. =A0Solutions are to switch faster, or
> >> =A0to add ferrite beads to the source pins, etc.
>
> > The MOSFET drive looks like Fig 2 here:
formatting link
o/appnotes/an-950.pdf
> with 1us (0.1uF, 10Ohm) HPF on the transformer primary side. No gate
> > resistors. The switching is indeed slow (~120ns rise-fall times). I
> > see nothing suspicious (like high frequency ringing) .... My records
> > show 10-20ns sampling rate, I would notice some waveform irregularity
> > caused by oscillation, wouldn't I?
> > I'll try to add ferrite beads to sources...
>
> Where did you place the TVS in there?
>
> --
> Regards, Joerg
>
>
formatting link
>
> "gmail" domain blocked because of excessive spam.
> Use another domain or send PM.
It's not pretty.... I have four TO220 mounted on the heat sink (2"x1.5"x0.25") plate. The contraption is attached to the PCB as close as possible to the D2PAK footprint. The TVS is attached here (at the footprint) - I assumed that drive signal is the killer (as opposed to drain/source ringing which I have never seen ). Source connection is 1/4" wide Cu strip1"-2.5" long (four TO220s are longer than 1"). The gate is thin wire parallel to the source strip. I should move the TVS to the heatsink assembly...... Grrrrrrrrr