Need high side logic-level mosfet

[Jan Panteltje]

On a sunny day (Sun, 16 May 2004 14:48:52 GMT) it happened "Anthony Fremont"
<spam@anywhere.com> wrote in <obLpc.102514$NR5.12682@fe1.texas.rr.com>:

"Jan Panteltje" <pNaonStpealmtje@yahoo.com> wrote

I dunno, but lightbulbs have a R cold about 1/10 of wha tyou may
expect,
so the peak current can be 10x that based on the wattage.
That may pull you supply down... If the PIC is on the same supply.

This was my initial thought, but the bulb has a separate 12V power
supply from the PIC's 5V supply. Both supplies have the ability to
deliver more than 20A. Since I'm playing on solderless breadboards,
they share a common ground path. At any rate, it was the resistor that
made the difference so I really feel that it has something to do with
the gate to PIC connection.

Resistor in series gate is goed idea anyways, but I have seen HV
MOSFET
oscillate at many MHz with just a 40 W bulb...
What cured that was a 1nF across drain - source! (at 230V DC).
This oscillation can kill the MOSFET really fast if any real current
flows.

What would cause the oscillation?
This was discussed here some time ago, but don't remember exactly.

In my case there was a small wire wound source resistor that would make
a great coil, add some capacitance and you are in the 20 to 100 MHz range.
Also I had a zener as gate protection in one experiment.
Removing that zener also stopped oscillation.
You need a scope and some time to try a few things, maybe different for
different MOSFETS etc...
One would think from your example that somehow some capacitance between
drain and gate worked back to the PIC.
Perhaps if the PIC was high Z out (power up), and the MOSFET gate charged
up via that capacitance to 12 V, you could 'erase' the PIC?
(When PIC totempole upper FET switches on)?
Realy need to scope this I think.
C drain-gate should be much lower then C gate-source? Help Winfield!
Could even be a ground loop or bad ground connection 'solderless' does
not feel very secure here.
For this kind of stuff you need short wide thick PCB traces, or good old
fashioned thick soldered copper wires.
All I can say with the data at hand..
JP

 

[Jim Adney]

On Sat, 15 May 2004 16:00:56 GMT Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

Given that Vbatt can be quite high, and you do not want Vgs breakdown,
would not Winfrieds circuit recently posted in this group,
with a current source as driver more safely limit Vgs in the top FET?

Vbatt should not go above 15 V, and the Vgs rating on most MOSFETs is
20 V (while the actual breakdown happens at some higher voltage.) I
wouldn't expect this to be a problem, but you could put a resistor and
a transsorb (fast Zener) in there if you wanted to make sure.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Jan Panteltje]

On a sunny day (Sun, 16 May 2004 16:23:52 -0500) it happened Jim Adney
<jadney@vwtype3.org> wrote in <5c6fa09l5ie0n5aufqijmbdsrbhrkr7ji9@4ax.com>:

On Sat, 15 May 2004 16:00:56 GMT Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote:


Given that Vbatt can be quite high, and you do not want Vgs breakdown,
would not Winfrieds circuit recently posted in this group,
with a current source as driver more safely limit Vgs in the top FET?

Vbatt should not go above 15 V, and the Vgs rating on most MOSFETs is
20 V (while the actual breakdown happens at some higher voltage.) I
wouldn't expect this to be a problem, but you could put a resistor and
a transsorb (fast Zener) in there if you wanted to make sure.
But the NPN with only one resistor (and without the bottom MOSFET) is

cheaper.
If Vbat is a car battery .. these things can do funny things, especially
with corroded contacts.
JP

 

[Genome]

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:fa1fa015r665agnabqd80sb7ho1dkd5lpv@4ax.com...
| >What would cause the oscillation?
|
| Inductance in the source.
|
| Best regards,
| Spehro Pefhany

You mean the drain.

DNA

 

[Spehro Pefhany]

On Mon, 17 May 2004 05:13:19 +0100, the renowned "Genome"
<Genome@nothere.com> wrote:

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:fa1fa015r665agnabqd80sb7ho1dkd5lpv@4ax.com...
| >What would cause the oscillation?
|
| Inductance in the source.
|
| Best regards,
| Spehro Pefhany

You mean the drain.

Either, no?

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
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