Hello Antonio,
I’ve recently begun designing a DRSSTC with similar parameters (“high” inductance primary, small capacitance tank, and limited to short burst lengths (3 cycles for current MMC, I can get to 6 by doubling voltage rating). It is my understand that the less than spectacular streamer length is due to the fact that there isn’t enough time for streamers to grow properly. At 3.3uS/cycle a 10 cycle burst is only 33uS. Going up to 20 cycles yields 66uS. Most “large output” DRSSTCs run around 120-500uS bursts and typically have primary peak currents around 400-900A. That being said, the output in your video looks quite good for only 66uS ontime.
The 1MHz “noise”:
You didn’t state what the timing method for the interrupter was, and as there is no evidence of poor performance or the 1MHz being an upper excitation mode in the output of the DRSSTC one has to conclude that it must be from the interrupter or radiating from parasitic oscillations in the wiring.
Your coil’s output:
In order to increase spark length the burst length must be increased. Doing so you will get a higher peak primary current. Increasing Burst Length also increases the voltage across the MMC, so you’ll need to adjust for that as well. Using a higher peak current capacity switch to adjust for the longer burst length is easy; Fairchild’s FDL100N50F MOSFETs will easily tolerate 400A pulsed at low duty cycles.
You can use this formula to find what your MMC voltage will be at a certain peak current.
Vmmc = (Ipk * 2 * Pi * Lpri * fres)
Rearranged to find the peak current your current MMC can support.
I = Vmmc / (2* Pi * Lpri * fres)
For a 13.5uH primary at 294KHz and a Vmmc limit of 4.8kV you can support 192.5Apk. This is of course before losses and loading, and as you noted; loading is hard to calculate.
This is all a rather complicated way of saying that the next step to increasing output would be to increase your MMC’s voltage rating so you can increase your Burst Length to give streamers more time to grow, and more current to support their growth. One reason I decided to reply with all this info is that SSTCs and DRSSTCs are rarely the subject of discussion on the TCML and I hope that others may learn from the info presented here.
Sincerely,
Matt “Sig” Giordano
www.SigurthrEnterprises.com
----------Original Message--------
Message: 2
Date: Tue, 28 Jan 2014 00:11:15 -0200
From: Antonio Queiroz <acmdequeiroz@xxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: [TCML] DRSSTC, driving system, and a strange problem.
Message-ID: <52E711C3.8070907@xxxxxxxxx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi all:
I have built a low-medium power DRSSTC for some studies:
https://www.youtube.com/watch?v=vg9BGXlYXnE
It was designed with my theory at:
http://www.coe.ufrj.br/~acmq/tesla/drsstc.html
I started with a secondary coil that I already had (28.2 mH), an
adjustable top load that resonates with it at ~300 kHz (10.4 pF), and a
22 nF MMC primary capacitor made with 9 22 nF 1600 V capacitors and 10
Mohm bleeding resistors. With mode 37:39:41 this results in primary
inductance of 13.5 uH and coupling coefficient k = 0.102. With
excitation at 294 kHz using a half-bridge powered by +/- 180 V, the
expected maximum output voltage is of 205 kV, and the maximum input
current of 90.5 A.
I made the bridge using two IRFP460 mosfets, assuming that they can
handle this current for brief periods (they can), and other parts taken
from PC power supplies, including the case, blower, bus capacitors, and
a switching power supply for the blower and other uses. The timing
control is a quite overcomplicated circuit made by my students, that
gives a specified number of cycles at specified periods. There is no
feedback, but just a button to adjust the driver frequency manually.
The designed operation required 10-cycles bursts. It worked well in this
setting, producing the expected waveforms, but unimpressive sparks and
streamers. I tried then to increase the burst length, without changing
anything else. The operation is then not so efficient, but the driver
was supporting the increased power. With 20-cycles bursts I obtained this:
https://www.youtube.com/watch?v=vg9BGXlYXnE
The input current didn't increase excessively, starting as in the
10-cycles case, but reaching 170 A in something as a second beat. The
streamer loading turns difficult to calculate if this is the expected.
The driver becomes just noticeably warm.
When I tried to see the driver output voltage with an oscilloscope,
something strange happened. I see the expected bursts, but also a
continuous 1 MHZ oscillation appears. The problem appears to be local to
the driver, and the coil operates normally, as the high-frequency
doesn't transfer energy to it. Some parasitic coupling turning the
driver into an oscillator, but just when I connect the scope (?). Has
someone observed something similar? I have another primary circuit, with
higher impedance, that does not cause this.
Antonio Carlos M. de Queiroz
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