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Driver Circuit Description
MOSFET CURRENT MODE RESONANT COIL DRIVER
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designed by Rob, N3FT; with inspiration from the Duane Bylund article
in Radio Electronics, Sept. 1991, p. 33.
NOTE: The schematic was made long after the circuit was built and
perfected. Sorry. That means I cannot guarantee that it is free of
errors. However, the schematic was compared to the original as best
as I oould.
ERRATTA:
1. The resistor network at pins 6 and 7 of the LM555 is incorrect.
Instead of a fixed 200k and a 500k pot in parallel, it should be the
200k fixed in parallel with a 500k pot and the resulting two in series
with a 1k fixed. Thus the 200k fixed established the maximum duty
cycle, and the 1k pot establishes the minimum duty cycle. You could
try eliminating the 1K, but you might damage the LM555.
CONSTRUCTION:
I advise doing what I did, which results in a pretty easy and
professional looking driver: Go buy some surplus half bridge or
bridge switching supply, cut away the low voltage portions of the pc
board, and then mount the new UC3825 controller on a small daughter
board built out of a plated thru hole + ground plane vector board.
Put the resulting circuit inside an aluminum case of some sort.
The surplus place "Wacky Willies" in Hillsboro, OR where I buy alot of
my stuff still has lots of these NCR 500 watt switching supplies that
I used to build mine. I believe these even have a current mode
controller, though I think its a UC3845 or similar. The supply has
the 4 IRF740's, huge ferrite transformer core, and nice heatsinks-
very, very pretty design. However, I do not know if they will do mail
order, you can try them at 503-642-5111. These supplies were $20-30
last time I was over there. At that price it would be worth it for
someone to make some arrangment with them- only its not going to be
me, sorry, I don't wan't to be in the mail-order business.
Jamesco also sells alot of half-bridge MOSFET 200 watt and up supplies
very cheap.
CIRCUIT DESCRIPTION/OPERATION:
1. The circuit from the AC line up to the MOSFETS is the standard
power portion of any half-bridge 250W or greater switching supply.
Setting jumpers either selects the bridge rectifier for 220v
operation, or the so-called full wave voltage doubler for 120vac
operation.
I added the GE V130LA10A MOV's to keep potentials between ground and
the ac line to a minimum.
The construction info for the common mode filter inductor is my
equivalent to what is actually on the driver, since I have no info on
the actual core material of the on the unit. All the caps are the VDE
and UL rated polypropyene "across the line rated" types.
2. The original power supply was I believe a push-pull type so I had
to cut some traces and add some wires. However, the addition of the
IR 80SQ035 schottkey diodes and the HER 305 ultra fast rectifiers are
my addition to the normally seen type of circuit.
It is absolutely mandatory to disable the internal source to drain
diode of the MOSFETS. Otherwise you may as well buy a bag of a 1000
mosfets. The reason they need to be disabled is that when connected
to a circuit capable of storing huge amounts of energy, like a high Q
coil, the circulating current tends to alternately drive the
source-drain diode of the mosfet to extremely heavy conduction,
storing up large amounts of charge in the juction. Then a hundred
nanoseconds or so later, when the opposite mosfet starts to conduct,
the first mosfet looks like relatively large capacitance- or basically
a short circuit for an instant. One or both mosfets blow. Its
possible to increase the dead time (thats the set off period where no
mosfet can be conducting) by increasing the Ct of the UC3825, but for
high speed operation, there is a practical limitation to this. Also,
the slow diode wastes power even when its not blowing up the fets.
The schottkey diode works by making it impossible to forward bias the
source drain diode of the fets. It only needs to be rated for the
maximum pulse current present- the ones specified are probably
overkill in the ratings. You don't need to worry about the reverse
voltage rating of the schottkey, since the HER305 conducts when the
schottkey gets reverse biased. And schottkey diodes are fast- much
more so than the HER305- so there's another worry gone. The HER305 is
a 3amp 400 volt 50nS reverse recovery time diode. Having it external
to the fets also removes a source of dissipation in them. IR makes
some diodes calle HEXFREDS which are available in even higher voltage
and power ratings but are much more expensive- would be better for a
bigger setup.
3. Output transformer- more attention needs to be paid to its
construction than any other part of the driver.
First obain a core. You don't have to pay a fortune for a new core.
Find some 1kw computer supply and remove the transformer. I have
obtained several of these cores by simply putting the transformer in
boiling water until the shellac softens, and then pulling apart the
core. Then you can take apart the transformer windings to get the
bobbin and if you're lucky, some expensive teflon insulated wire.
Forget any core or transformer that looks like its coated with epoxy
or some other extremely hard enscapsulation.
My core when both E sections are put together has the dimensions:
5.5 x 5.5 x 2 cm. Anything that size or larger will work. The bigger
the core, the less chance with core saturation at low primary turns
count. Thus less secondary turns can be used.
Make certain any used core material has a low resistance. The low
resistivity ferrite core materials are suitable for transformer use.
Any core with unmeasurable resistivity is probably actually from a
choke, and won't work. My core measured about 10k ohms when the VOM
probes were about 1/4 inch apart when touching the core. Type 77
material is good if you can get a brand new core.
After obaining a core, get a hold of some some teflon coated wire-
14-16 gauge. Why cut corners? Drill two holes on the bobbin just the
size to let the primary leads thru and then wind the primary. Secure
the wires, then apply a coating of GE Silicone II sealant around the
primary to there is about 1/8 inch thickness. Use a spatula to make
it perfectly smooth with no ridges. Let it cure overnight.
Then using #30 gauge Belden (again why cut corners, use real Belden
magnet wire, if you can obain any special enamel types for severe use,
but all means use them) Wind 50 turns for the first layer- then coat
with another 1/8 inch of silicone and let dry overnight. Make certain
that the magnet wire going to the spool sticks straight out thru the
silicone, so you can easily start to wind the next layer. Use
gap-filling super glue and accellerator to secure windings as you are
progressing. Then after a night curing, wind the next layer, except
now the windings will be adding so that when this layer is finished
there will be a potential difference of 100 turns of voltage between
the ends of the two windings. This is done only to make winding
easier. If you want only 50 turns of voltage potential between the
two windings, you can put two silicone layers, each half as thick,
with the wire criss-crossing between the two windings. Do this for
the last two layers. Then fit the core together, and add as much
extra silicone as possible to fill the gap between the windings and
the core going aroound the outside. This provides extra safety
against corona and flashovers.
I am sorry but I have not been able to find the exact winding specs
for my transformer. I believe that I used 10 primary turns and 200
secondary turns. However, I can measure the resistance of my
secondary and it measures 7.0 ohms.
I did quite a bit of experimenting. A turns ratio of 10 to 1 puts
much less stress on the mosfets (cw operation is possible), but
streamers of 3 only inches or so. A ratio of 30 to 1 is extreme,
putting much more stress on driver, with really no increase in
streamer length over the 20 to 1 ration At the 20 to 1 ratio
streamers of 8-12 inches are possible.
4. Controller circuit: I used an UC3825 for several reasons. First
its optimized for super fast current limit and shutdown. Secondly, it
operates up to more than 1mhz. Thirdly, it had higher mosfet drive
capablity than any other device at the time I did the design (I
believe 1.5A peak).
Looking at the circuit, the input thru the diode is the on/off singal
from the LM555 duty cycle circuit. When this signal is low, the
UC3825 error amp operates normally and the 10k panel mount control
varies the PWM level- in reality this pot functions more like an on
off switch- I never operate it at any other position than min or max.
When the LM555 drives it high the error amp goes low and the UC3825 is
off.
Pin 5 connects to several fixed resistors and a precision 10 turn
panel mount put for tuning to the coil resonant freq. Socket the
fixed resistors so they can be switched out for coil experimentation.
Pin 6 connects to the 4700pf frequency determining cap and some
additional components that function to stabilize the circuit. I will
refer the reader to the Unitrode Databook for a discussion of slope
compensation for an analytical treatment, but basically, for stability
reasons , one wants to feed some portion of the oscillator signal back
into the ramp pin. The 0.047uF cap is simply a DC blocking cap- so
its really the 4.7k resistor that feeds some current from pin 6 into
pin 7. The 0.001uF cap makes certain there is a sharp rising edge to
the signal. You can empirically adjust the value of the resistor
until the circuit does not squeal (i.e. stable) over the full
revolution of the panel mount PWM control. Socket all these parts.
It is the ramp pin 7 that senses the ouput current to the coil and
adjusts the PWM on a pulse-by-pulse basis to provide as much of a
constant current drive to the coil as possible. The current sense
circuit between the MOSFETS and the tranformer uses a large 0.1ohm
resitor and a small ferrite core to isolate from the HV, meanwhile
feeding back a representation of primary transformer current. This
feedback is what makes the PWM panel mount control pretty much
irrelevant. (except for checking for instabilities)
The sensed signal is also sent into the current limit pin 9 which
shuts off the MOSFET drive on a pulse-by-pulse basis when voltage on
the pin goes beyond about 1 volt.
There are two small pots that adjust the signal back into these pins.
Start them at half of full scale to begin with. Then once you are
sure that the coil is resonant- back them off until the streamer
length hits a maximum.
It is the dynamic current limiting that sets this circuit apart from
all else, especially the Bylund circuit, since my driver can run with
the coil out of resonance indefinitely, since the current mode control
comes into play. (thats as long as these two pots are adjusted
correctly, and the slope compensation is correct)
5. The LM555 Duty Cycle Circuit. With more mosfets the circuit could
function CW. However, more bang for the buck can be obtained with an
impulsive operation. And it looks and sounds like a tesla coil. The
streamers are much longer for a given amount of mosfets when the fets
are pulsed. With this circuit, the duty cycle can be adjusted from
near nothing to a roaring flaming discharge. Its a simple astable
LM555 circuit running around 30hz from the data books.
6. MISC: Power and output voltage sources on the UC3825 have
seperate pins, so I have utilized the feature and isolated both with
ferrite bead choke.
The schottkey diodes on the UC3825 clamp the outputs between 0 and
12V to give crisper drive to mosfets. It is also possible to use a
bidirectional 15v transient suppressor (P6KE15) directly between the
gates of the mosfets to source for additional protection to the
mosfets against gate oxide pucture if any of the other fets fails, but
I didn't include them on mine.
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Thats all I can think of for now. I will try to answer any additional
questions, but now I'm kind of burnt-out on schematics and
documentation and want to get my new fets so I can get this thing up
and running again (don't try to drive any electric fences :) or
anything other than a coil like I did). Also, this driver with the
Bylund coil seems to pack a wallup of low freq AC envelope. I get
quite a jolt just from the leakage on the ground connection. The coil
also seems to put out alot more current than an equivalent size tesla
coil. I have cut thru ceramic tiles with the arc. (was trying to
find an easy way to cut tiles when my roommate was installing new
floor, ha ha)
Good luck. Rob.