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Rob's Circuit

Please note projects described here use very high voltages and should only be attempted by experience contructors who understand the dangers.

This is not mine - Rob Lytle designed this 375V half bridge. Sorry the circuit is the best quality available, if you click on the diagram with your left mouse button you can save the image file and blow it up with any image handling software.

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MOSFET CURRENT MODE RESONANT COIL DRIVER

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 ¼ 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. -----------------------------------------

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.

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