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I'm considering making my "Big 3kW SSTC" into a DR coil since I'm rebuilding the primary side and driver after a near-fatal flashover when the old primary overheated last month.
Is there a formula for determining Peak Primary variables? I'd like to figure out how long of a pulse width I can use to stay under a certain peak primary current, and if possible, determine what the peak primary voltage will be so I can properly design the MMC.
Here's data on my coil: J A V A T C version 13.2 - CONSOLIDATED OUTPUT Wednesday, January 22, 2014 6:22:30 PM
-----------------------------------------
----------- Secondary Outputs: -----------------------------------------
----------- 172.12 kHz = Secondary Resonant Frequency 90 deg° = Angle of Secondary 17.75 inch = Length of Winding 84.8 inch = Turns Per Unit 0.00177 inch = Space Between Turns (edge to edge) 1773 ft = Length of Wire 3.94:1 = H/D Aspect Ratio 181.4684 Ohms = DC Resistance 63705 Ohms = Reactance at Resonance 0.54 lbs = Weight of Wire 58.906 mH = Les-Effective Series Inductance 60.636 mH = Lee-Equivalent Energy Inductance 58.794 mH = Ldc-Low Frequency Inductance 14.515 pF = Ces-Effective Shunt Capacitance 14.101 pF = Cee-Equivalent Energy Capacitance 23.583 pF = Cdc-Low Frequency Capacitance 7.23 mils = Skin Depth 10.1 pF = Topload Effective Capacitance 261.8198 Ohms = Effective AC Resistance 243 = Q
-----------------------------------------------
----- Primary Outputs: -----------------------------------------
----------- 157.6 kHz = Primary Resonant Frequency 8.44 % high = Percent Detuned 90 deg° = Angle of Primary 16.36 ft = Length of Wire 25.99 mOhms = DC Resistance 0.099 inch = Average spacing between turns (edge to edge) 1.075 inch = Proximity between coils 0 inch = Recommended minimum proximity between coils 21.434 µH = Ldc-Low Frequency Inductance 0.0394 µF = Cap size needed with Primary L (reference) 0.266 µH = Lead Length Inductance 104.31 µH = Lm-Mutual Inductance 0.093 k = Coupling Coefficient 0.131 k = Recommended Coupling Coefficient 10.75 = Number of half cycles for energy transfer at K 33.93 µs = Time for total energy transfer (ideal quench time)
I messed around with the JavaDRC calculator, but I'm unfamiliar with it, and it doesn't seem to take in account the bus voltage, half or full bridge inverter, etc, which all seem to be rather important for determining peak currents and voltages. Here are the outputs anyway.
J A V A D R C - CONSOLIDATED OUTPUT Wednesday, January 22, 2014 6:47:40 PM
Data Inputs: 0.047 [uF] = Single Cap Capacitance value 1 [qty] = Number of Caps in Series String 1 [qty] = Number of Strings in MMC 1414 [Vac] = Single Capacitors AC Voltage Rating 22 [uH] = Primary Inductance 170000 [Hz] = Secondary Resonant Frequency 90 [Amps] = Expected Peak Current 350 [uS] = Silicon Pulse On Time 100 [pps] = Silicon Pulses per Second
------------------------------------------
---------- Data Outputs: 0.047 [uF] = Total Cap Bank Capacitance 2000 [Vp] = Cap String Rated Peak Voltage 1947 [Vp] = Expected Peak Voltage 97 [%] = Percentage of Cap String Rating to Expected Peak Voltage 0.09 [joules] = Peak Energy at Expected Current 21.64 [ohms] = Tank Surge Impedance 156516 [Hz] = Primary Resonant Frequency 170000 [Hz] = Secondary Resonant Frequency 8 [%] = Primary to Secondary Percent Detuned 14 [Amps] = RMS Current Per Silicon On Time and PPS
I'll be using a Half-Bridge fed by full wave rectified and filtered 120V mains with variac control. My interrupter will be set for 100Hz and I'll have precision variable ON pulse width. I'll be using FDL100N50F MOSFETs so I want to stay under 100Apk. I know my Fres will be 170KHz, so if I can figure out the current and voltage rise per cycle of ON TIME then I can determine the ON TIME limit to set. For example, at 5.88uS/cycle then 350uS ON will yield 60 cycles. I just need to ballpark how many cycles (how long of ON TIME) to set the interrupter for, for the first light. I'd hate to blow $30 of silicon in the first mS of run time because I was off by a factor of ten by arbitrarily picking a value.
I'll be happy if I can achieve 6" breakout as that's how much spark length I got running this coil ISSTCC at 1.5kW.
Forgive my confusion, but it seems that calc assumes you know your peak current. It is nice that it gives you the peak voltage across each cap and dV/dT though! How do you determine how much your primary current rings up per cycle in order to determine what the peak current will be for a given on time?
Basically you'll add the switching voltage to your MMC voltage every time you switch. In detail: With a mains voltage of 120V and a half bridge you switch between -170V and +170V. Initially you start off with 0V. When voltage is first applied, the MMC voltage will rise to 170V after one half cycle, i.e. 3us. Then you switch input to -170V. After 3us MMC voltage will be 170V + 340V. You have added the switching amplitude (340V) to the initial voltage. After another 3us MMC voltage wil be 170V + 2*340V and so on.
The MMC voltage determines the current: I = V / (2* Pi * Lpri * fres) with primary fres. For 21uH and 160kHz, 100A will be reached, when the MMC voltage is 2100V, i.e. after about 6 half cycles or about 18us.
This is very much an upper limit for the current. Losses in the primary tank and even more the loading due to the secondary will limit the current. If you have a scope, I'd monitor it and go from there. I believe these fets can stand much more current for the short burst times you are aiming at.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
For (Q)CW coils I have always estimated the primary current by the Q of the primary tank circuit. For the CW coils the lowest vaule seems to be around 5, I usually calculate for 10 and then detune to get the required current. For a pulsed DR, the Q might reach around 10-15(?) during the first peak.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Sigurthr wrote ...
Forgive my confusion, but it seems that calc assumes you know your peak current. It is nice that it gives you the peak voltage across each cap and dV/dT though! How do you determine how much your primary current rings up per cycle in order to determine what the peak current will be for a given on time?
I read your thread on the phone at breakfast, missed half of the question in my eager to post a answer before rushing out the door for work :)
Yep I plan on scoping it out but I don't have a storage oscilloscope, just an old analog CRT one, so I typically can't see pulsed or transient effects that happen less frequently than 200Hz or so.
I'm going to have to enlarge my MMC if voltage rings up that fast. These fets can handle either 400 or 500amps pulsed but my mmc was only rated for 2kV! 18uS sounds like an incredibly short burst length, I usually hear of people running hundreds of uS.
18uS sounds like an incredibly short burst length, I usually hear of people running hundreds of uS.
A 170V, 100A half bridge can supply more than 10kW of power, while a 2kV 47nF MMC only stores about 0.1J. It is not surprising, that you can run only short bursts unless the secondary draws a significant part of the power. It might be possible to tune the coil, so that 100A won't be exceeded in longer burst. But I have doubts about this, since the arc would have to break out very fast, i.e. faster than 18us. Also without an OCD this is hazardous, since a ground arc throws the secondary out of tune and will allow almost unlimited current rise.
Yeah. I was hoping for a quick and easy conversion to DR; add MMC and special interrupter to limit burst length. Unless I can reliably limit burst length to ~15uS (a tall order in of itself) and make sure ouput stays under 20" I'm going to need a complex controller with OCD and synchronized turn off (to prevent a hard switch), and I have no idea how to implement such a synchronized stop, haha. Hell, I'm not sure I can get my slow MCU to do accurate 15uS pulses.
Basically you'll add the switching voltage to your MMC voltage every time you switch. In detail: With a mains voltage of 120V and a half bridge you switch between -170V and +170V. Initially you start off with 0V. When voltage is first applied, the MMC voltage will rise to 170V after one half cycle, i.e. 3us. Then you switch input to -170V. After 3us MMC voltage will be 170V + 340V. You have added the switching amplitude (340V) to the initial voltage. After another 3us MMC voltage wil be 170V + 2*340V and so on.
Bear with me if you would be so kind please, I want to verify the timeline here so I know I have a good grasp of it. I'm not quite following the wording of the mathematical function.
Based on "adding switching voltage to MMC voltage every time you switch" I originally thought: 1) Voltage first applied; MMC voltage: 0V. Time Since Start: 0uS 2) First half cycle complete. MMC voltage: 170V. Time Since Start: 3.125uS 3) Second half cycle complete. MMC voltage: 340V. Time Since Start: 6.25uS. 4) 3rd half cycle complete. MMC voltage: 510V. Time Since Start: 9.375uS. 5) 4th half cycle complete. MMC voltage: 680V. Time Since Start: 12.5uS
With each new line being a half cycle: 170 170 170 170 170 170 170 170 170 170
With the timeline I spelled out based on beginning of your description it seems like each cycle 2x bus voltage is added to whatever the MMC was at prior. However, at the middle and end of your description it seems there is an offset to my timeline; You say that at 3.125uS the voltage will be at 170V, then at 6.25uS the voltage will be 510V, then at the 9.375uS mark the voltage will be 850V.
This shows that each half cycle after the first we are adding 2x the bus voltage to what the MMC voltage was the previous half cycle. Is this right? If so then we can extrapolate:
V= Vin x 1 = Vin x (N + 0) V= Vin x 3 = Vin x (N + 1) V= Vin x 5 = Vin x (N + 2) V= Vin x 7 = Vin x (N + 3) V= Vin x 9 = Vin x (N + 4) V= Vin x 11 = Vin x (N + 5)
which yields the following formula for MMC voltage at a given On Time at a given Input Voltage:
V = Vin x (N + (N-1)) where N = number of half cycles
Math is regrettably my weakest foundation, so if this looks horribly wrong I do apologize and humbly ask that the correction be spelled out in painfully clear detail. My sincerest gratitude for your time, patience, and help.
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DRSSTC Peak Primary Current formula?
