FET used for switching capacitive load?
Khoa Du
I've always used a mosfet to drive a resistive/inductive load.
I am now required to switch a capacitor (1600uF, 400V) on/off. I've
connected the -ve terminal of the cap to the drain of the an IGBT. Is this
possible? How will an IGBT work on a capacitive load? Is it the same as a
resistive load??
TIA
Phil Allison
"Khoa Du" <kh...@scanoptics.com.auspam> wrote in message
news:3e3d...@duster.adelaide.on.net...
** What is the application ? What is the IGBT required to do?
Your post makes no sense - as usual.
................. Phil
Khoa Du
> >
> > I am now required to switch a capacitor (1600uF, 400V) on/off. I've
> > connected the -ve terminal of the cap to the drain of the an IGBT. Is
this
> > possible? How will an IGBT work on a capacitive load? Is it the same as
a
> > resistive load??
>
>
>
> ** What is the application ? What is the IGBT required to do?
>
> Your post makes no sense - as usual.
>
Sorry... I'll be more specific.
I'm using an IGBT as a switch to quench a xenon flash discharge. I want to
use an IGBT because I do not know of any MOSFET capable if >400A peak
current.
Please refer to this circuit... (A picture is a thousand word)
http://users.chariot.net.au/~khoa/cct1.gif
Ok.. back to my original question... assuming that C1 has been fully
charged... (ie Q1 is currently switched on)
What will happen when Q1 is turned off? I was thinking of using the IGBT as
'switch' to disconnect the caps from the circuit and hence quench the flash
tube.
Will this work??
Craig Hart
constantly after a single trigger pulse? I gater this circuit
self-oscillates once triggered and flashes the xenon tube at a more-or-less
constant rate?
I'd suggest that your idea is unlikely to work, since the unfiltered DC
would just create a series of higher voltage spikes, not derrive the circuit
of power.
IMHO, if it's a self-oscillating flasher, shorting out the trigger SCR
(short anode to cathode) should turn it off, by removing the 15kv striking
pulse.
"Khoa Du" <kh...@scanoptics.com.auspam> wrote in message
news:3e3df134$1...@duster.adelaide.on.net...
Phil Allison
"Khoa Du" <kh...@scanoptics.com.auspam> wrote in message
** I suspect as soon as you fire the damn thing the IGBT will be fried
along with half the other semis. The peak current will be thousands of amps.
This is serious energy and very dangerous stuff.
You can buy professional photographic flash gear that does this sort of
power ( 120 joules) - and it ain't cheap.
........... Phil
John Crighton
<kh...@scanoptics.com.auspam> wrote:
>Sorry... I'll be more specific.
>
>I'm using an IGBT as a switch to quench a xenon flash discharge. I want to
>use an IGBT because I do not know of any MOSFET capable if >400A peak
>current.
>
>Please refer to this circuit... (A picture is a thousand word)
>
>http://users.chariot.net.au/~khoa/cct1.gif
>
>Ok.. back to my original question... assuming that C1 has been fully
>charged... (ie Q1 is currently switched on)
>
>What will happen when Q1 is turned off? I was thinking of using the IGBT as
>'switch' to disconnect the caps from the circuit and hence quench the flash
>tube.
>
>Will this work??
>
>
Hello Kuoa Du,
what is this device for?
Have you made it up already?
What is the rating of the transformer?
Your question above.
>Ok.. back to my original question... assuming that C1 has been fully
>charged... (ie Q1 is currently switched on)
>What will happen when Q1 is turned off?
I would say C1 stays charged.
> I was thinking of using the IGBT as
>'switch' to disconnect the caps from the circuit and hence quench the flash
>tube.
If Q1 is replaced by a switch that you operate manually, does your
device do what you want it do? If the switch works then try the
transistor.
Tell me more about your device and what it is supposed do?
A bit more info on the the diagram if you can please. Is the
transformer a special type, like a neon sign transformer that can
handle being shorted out briefly.
Regards,
John Crighton
Hornsby Heights
cdb
It is called POWER SEMICONDUCTOR APPLICATIONS and is in 8 chapters.
Chapter 1 MOSFETS and IGBTS in general
2 Designing SMPS
3 Motor Control
4 Televisions
5 Automotive
6 Power Control Thyristors and Triacs
7 Thermal Management
8 Lighting
Each chapter is a separate PDF file. The Philips site is slow there is
University in South AMerica that has them mirrored - A google search will
bring it up.
Colin
andy
news:3e3e4514$0$4279$afc3...@news.optusnet.com.au...
> Philips have a engineers book covering MOSFETS and IGBTs . Worth a
download
>
> It is called POWER SEMICONDUCTOR APPLICATIONS and is in 8 chapters.
>
> Chapter 1 MOSFETS and IGBTS in general
> 2 Designing SMPS
> 3 Motor Control
> 4 Televisions
> 5 Automotive
> 6 Power Control Thyristors and Triacs
> 7 Thermal Management
> 8 Lighting
>
Brilliant book, just what i have been searching for. Many thanks for that
link. Should help th OP as well.
Khoa Du
> what is this device for?
Camera illumination flash application
> Have you made it up already?
Yes... everything works (have not put in Q1 yet as this is where I am
needing help)
> What is the rating of the transformer?
It's a standard trigger module from the supplier of the flash tube. It's
transformer ratio is 1:36
> If Q1 is replaced by a switch that you operate manually, does your
> device do what you want it do? If the switch works then try the
> transistor.
I guess I could replace it with a relay... but I'm not sure if the high
current/voltage would weld the terminals together. I couldn't use a manual
switch as I need to turn it off within 1-4ms after intial discharge.
> Tell me more about your device and what it is supposed do?
> A bit more info on the the diagram if you can please. Is the
> transformer a special type, like a neon sign transformer that can
> handle being shorted out briefly.
Are you asking about the step-up transformer or trigger transformer?
I can tell you everything about the circuit has been prototyped and works
(ie when I apply a signal to the SCR, it creates ~15KV transient pulse to
the anode of the flash tube - hence triggering the tube and gets it started.
Once started the tube continues to illuminate until the caps have been
depleted. However this is sometimes too much light and we need to control
the illumination somehow.
My thinking is that if I disconnect the storage caps (C1) it would allow me
to control the duration of the flash... hence vary the exposure.
Anyway, I've gone ahead and wired it all up and tested it out... and it does
not work as planned. The IGBT does turn off, but the flash tube continues to
discharge. My guess is that you are correct and C1 remains charged
regardless if the IGBT is on or not.
Khoa Du
I'm looking for a supplier of a ~900A, 400V FET, transistor or IGBT (without
the diode). Farnell has one for ~$1500... so forget that one... Thanks.
For those who responded to my question on switching a capacitive load on/off
(refer to to this diag http://users.chariot.net.au/~khoa/cct1.gif) It would
never have worked because...
a) during the discharge cycle, the current is reversed and the
'one-way-switch' wouldn't have worked.
b) the IGBT actually had an internal diode which allowed the caps to
discharge through the flash tube anyway!
I've fixed this now by placing the IGBT in series with the flash lamp's
cathode. However my measurements have come out to ~900A peak current... now
I just have to find the damn thing....
Thanks.