Re: How to quickly turn on/off N-FET switching high side of 55V?

John Fields wrote:
Winfield wrote:
John Fields wrote:
Winfield Hill wrote:
John Fields wrote:

Here's my second cut for the 55V 30A high side driver:
Version 4 [ snip ]

I'm not sure how useful it is to post Spice analysis
files, it tends to stop or slow down the conversation,
because one can't quickly read it on the spot.

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I like ASCII as much as anyone, but when stuff starts getting
complex to the point where timing is an issue or an ASCII drawing
would just be too busy, I prefer to either run a simulation and post
the schematic list or post a PDF to abse. In any case, how much
bother could it be, if one is interested, to post the thing into
LTSPICE and print out the schematic?

Sorry, but it probably limits the conversation to a
few -- you, me, anybody else? And look at the delay.
You may not have had any answer at all.

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Well, Win, the design was for the OP so I'd expect that he, at
least, would be willing to accept a format that was convenient for
me to work in and to provide a working model of the circuit.

Whether it's inconvenient for you is really immaterial. Play
my way if you want to, else why don't you ASCII-ize the drawing
for everyone else who you think wants to play your way?

NOW, who's being bitchy?

WRT your circuit, John. Whew! A 65-volt gate swing!

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LOL, it's an N channel enhancement mode MOSFET high side
driver switching a 55 volt source into a grounded load!

How else would you propose to do it?

No Sir. Check out the rail-rail current of your
pnp and npn pullup and pulldown transistors, ughh!

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So, since you used the simulator to find that out, It's not _all_
bad, huh? ;)

It's bad, especially if you modify the spice BJT part
model to show the MAXIMUM beta, rather than the minimum
as is usually done in the factory models.

Anyway, thanks for pointing that error out so politely and, if you
can spare the time, see the file at the end of this post for the
easy fix.

Yes, well done.
---

The driver's currents should be referenced to the
MOSFET's source. That's where the capacitance is.

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I prefer to reference the currents to ground for a few reasons:

1. With the load looking like a couple of ohms it won't make any
difference in how long it takes to charge or discharge the gate
capacitance and,

2. The source jumps up to about 50V if the load is disconnected or
opens (whether the switch is on or off) and,

3. With the switch turned off, any current in the driver will have
to return to ground through the load. Not exactly my cup of tea,
but YMMV.
---

BTW, did you notice how the current used to drive the Zener
and establish the gate drive voltage is returned to the
battery instead of being wasted? Kinda slick, I thought...

In the noise level, compared to the mayhem underway.

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My, but you're in a bitchy mood.

Maybe this'll cheer you up:

Version 4 [ snip '

OK, OK, that's MUCH better John. I'll give you
a pass on that version. But I won't make your
ASCII drawing for you. I have work to do! :-)
.

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