Re: FET coil questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Carl,

On 6 Nov 2001, at 14:31, Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<cwillis-at-guilford.edu>
> 
> 
> This is a set of questions for the SSTC folks.  
> 
> Background: I just lashed up a small coil that is driven from a 555 timer,
> gate driver chip, and IRF740 FET with the body diode isolated with the
> standard schottky and fast diode.  This coil draws about 100 watts and puts
> out 6"+ sparks, with another 555 timer interrupting the main oscillator in
> a staccato fashion to keep duty cycle low.  
> 
> Question #1:  Can I use an oscilloscope to make sure that the FET switching
> transitions look good?  There does seem to be some high-frequency ringing
> when I look at the signal recieved with the 'scope probe dangling near the
> FET gate.  Does anyone know what a good (i. e. the FET is not in danger of
> failing) signal should look like?

You should be able to connect the scope probe to the gate. You *must* 
use mains isolation to run the supply. If you are not sure, DON'T.  
When used as an aerial, the probe will be picking up mainly fields 
from the windings. 
 
> Question #2:  What is the best method for measuring peak primary current,
> and how much peak current should the IRF740 be subject to?  I know the
> specs say it will take 48 A or something on that order, but is it
> permissible to run at 48 A peak on every pulse in this sort of application?
>  I don't know the mechanism by which the FET would fail if the peak current
> is exceeded.  Maybe it just burns up because the localized heat cannot be
> dealt with, although the FET may be staying quite cool in general.

A very low value resistor in series with the source is the usual 
method. You can directly measure the voltage across it (again bear 
isolation in mind if you are doing it with a scope). You can use the 
voltage developed to switch the gate drive off when a target current 
is reached (e.g. a voltage comparator which SMPS control chips 
typicall provide).

     A power MOSFET is not a single device but made up of a lattice 
of hundreds of low power devices in parallel on a single die. If you 
decide to exceed the DC drain current rating, make very sure that you 
are allowed to do this on a repetitive basis or trouble will be 
lurking. It will state this in the data sheets. You must take note of 
the both pulse repetition and pulse width. Peak power dissipation is 
the most likely killer if the electrode voltages are all under 
control. Note that the "on" resistance of the device gives a clue as 
to what is happening. Suppose it is 0.2 Ohms; an 8A current causes it 
to drop 1.6V across its drain-source. If it is around 0.8 Ohms, it 
will be dropping 6.4V! (IRF840 is an example). MOSFETs are not always 
the panacea they appear to be.

> Question #3:  I'd like to isolate the gate with a pulse transformer, but
> all the ones I have made so far seem to decrease coil performance (so I am
> still using direct gate drive from the driver chip).  True, I have no clue
> what type of ferrite I wound the test transformers on.  In the future,
> should I get a bead that is recommended for the HF range? MF range? I may
> have also read something about pulse transformers not being able to handle
> pulse duty cycles different from 50% very well.  Is there truth to this
> claim?  What would be the reason?

It's not really a pulse transformer that you want. You actually want 
a transformer which is as near to ideal as you can get it given the 
on-time of you gate drive (i.e. no sag in the waveform). 

Regards,
malcolm