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Topics - Jesperb123

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1
Hi everyone!

I've been trying my best to understand the schematic for Steve Wards 1.3b driver, but i'm having a hard time understanding the output stage to the GDT. I've used LTspice with some some similar mosfets (they're rated for Vds of -60V and 60V and have a similar gate charge - I couldn't find the L/W/Kp/lambda for the actual mosfets).



These are my conclusions so far (please tell me if i'm wrong):
The additional mosfets connected externally to the UCC2743 are used to switch a higher output voltage than what the UCC can provide. There are two push/pull pairs that work together so we can send current both ways through the GDT - which creates alternating voltages on the GDT secondaries.

I simulated the top pair with this schematic:


Here are the traces:


From this I've gathered that C1 and R2 are used to generated the pulse (the top most trace). The green trace is the voltage source that goes from 9V to 0V after 1ms. Initally the voltage at the node n1 is 24V (when the supply is high) since no current flows through the cap/resistor, but when the supply goes low the voltage at this node drops since the C1 begins charging through R1 (and R1s voltage drop brings down V(n1)). M2 briefly starts to conduct when n1 goes below some thresholdvoltage, and stops conducting when C1's charge is high enough to pull the n1 voltage above M2s threshold voltage. This genereates the positive pulse (while the other part of the circuit generates the negative pulse). Here comes a million questions :D

What purpose does the diode have? is it to limit the voltage at n1 to less than 24+a diode drop?
Another thing I don't understand is the capacitors in the output path (C3 and C2). They seem to increase the fall time of the purple output trace (compared to running the simulation without them). Do I need to choose values for these?

Also, Steve's schematic show mosfets with body diodes across the drain and the source of the mosfets. I presume these are important when we are driving an inductive load but the suggested mosfets (STB16NF06 Nfet and SPB18P06 G Pfet) doesn't seem to have these. Is this some intrinsic property of all mosfets? What am I missing?

Last question: Why does the UCC state that it can provide 4A of output current, but it doesn't mention the output voltage at which it can do this?

Best regards
Jesper Bolin

2
Hi!
I'm planning on building a small DRSSTC, but my progress has slowed to a halt. I've tried my best to follow Mads Barnkob's guide on kaizerpowerelectronics.dk to determine some of the parameters:

Secondary design:
75mm diameter secondary
375mm tall
1700 turns (0.2mm cablediameter)

Topload
shape: toroid
Rminor = 3.75cm
Rmajor = 18.75cm

Primary
shape:flat spiral coil
4 turns
86mm inner diameter
10mm inbetween turns
6mm copperpipe with 1mm thickness
tapped at around 3 turns if 0.45uF capacitance is used (to match fres of secondary LC-circuit)


This should give a resonantfrequency of 202.5kHz and a reactance of 50k at resonantfrequency. I've read that 0.15uF to 0.45uF capacitance is suitable for a coil  of this size (http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/mmc-tank-capacitor/). Here's my problem: I need to know how large the current (peak and RMS) in the primary LC-circuit will be to determine which capacitors to use, but I have no idea how to come up with these values. Since i'm on a relatively small budget I figured that the transistors i use will put an upper limit on this value. Some people seem to go about it this way: select a peak current and then design a MMC (and buy transistors) that can handle this current. Since i've got no prior experience with DRSSTCs I don't know what kind of currents I can expect to run in a system like this, hence why i'm stuck.

 If anyone could point me in the right direction I would be very grateful.
//Jesper




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