Re: PlasmaSonic II Driver Circuit Problems

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

Original poster: dhmccauley-at-spacecatlighting-dot-com 

Chris,

The gate driver chip heating problem has popped up for a few individuals
building the PlasmaSonic II board.  I myself have built a large number of
complete PlasmaSonic systems
so far using identical parts on the parts list and have not had the heating
problems.  Although, my driver chips have gotten slightly warm on a few on
my PlasmaSonic systems,
none have gotten hot to the point where one should be concerned.  I do have
a few thoughts on this . . .

1.  You should be using at a minimum a normal sized muffin fan mounted above
your heatsinks and blowing downward over the heatsinks.  If positioned
correctly, the airflow will also
cool the driver chips.  You may find that this maybe all that is required to
cool your driver chips.  Try this first.  If after air cooling is provided
the chips feel only slightly warm during
operation, than you likely don't have a problem.  Remember, that depending
on the driver frequency and the type FET (especially the larger SOT-227
package FETs which have huge
gate charge), it will require a lot of power to charge and discharge the
associated capacitances seen at the gate terminal of the FETs.

2.  If after air cooling, the FETs *still* feel smoking hot, then there is a
problem somewhere.  This is usually a sign that there is uneven drive on the
gate transformers and a bias has developed,
or something else is not quite right.

3.  What mode are you running in?  Remember, single-ended and push-pull
modes must be configured entirely different.  For audio modulation, you
should be running in push-pull mode.

4.  +/-8V seems too low.  You should be getting at least a 10V at the gate.
If you running 12V into your gate transformer, you should ideally get 12V
output from the gate transformer
provided the turns ratio is 1:1:1.  You are losing 4V which doesn't seem
right.  Also, your risetime of 1us is much too slow.

5.  22 AWG is probably too thick of wire.  What might be happening is that
due to the insulation thickness of this heavier wire, you may not be getting
the best coupling between windings.
Because of this, the leakage inductance is much higher than it should be and
ultimately will cause the slow risetimes, and other problems you are having.
I would try rewinding using either 26AWG or 28AWG wire.

6.  Also, try varying the number of turns on your cores and see if your
results improve or not.

Dan



 > Hi All,
 >
 > I've built and tested a PlasmaSonic II SSTC recently,
 > but it has been plagued by a couple of problems.
 >
 > 1) The gate driver chips have been getting hot. I've
 > fried one set of driver chips already (4 chips) due to
 > excessive heating, but this may be due to the next
 > problem.
 >
 > 2) The voltage at all the FET gates goes only to +/-
 > 8V, and the risetime on the gates is 1us. I don't know
 > what's causing this, but I am sure of the gate
 > transformer construction and phasing. The GDTs are
 > wound on the specified cores with 15 turns of trifilar
 > 22AWG magnet wire. The voltage at the GDT inputs is a
 > 0-12V square wave with approximately the same
 > risetime. I'm quite sure I used the correct parts but
 > I may have to check again.
 >
 > Thanks in advance,
 >
 > Chris Lu
 >
 >
 >
 >
 >